The spider with emerald jaws

As well as mystery cocoons, breaking up some old fence panels for firewood dislodged numerous scuttling invertebrates. Plenty of woodlice, barklice and so on, and many small spiders, but also a splendid specimen of Segestria florentina. This is the largest species of the familt Segestriidae in Europe, with females reaching up to 22mm in length (excluding legs and other appendages). The family is distinguished by having 6 eyes arranged in a semi-circle (most spiders have 8) and the first three pairs of legs directed forwards (most have the first two forwards and the other two backwards). Although this species can bite (it's apparently painful, a bit like a bee sting or sharp jab with a pin, but not dangerous to humans), when disturbed, they curl up or flee to find a crevice to hide in - being nocturnal hunters using tunnel-webs with radiating threads. First found in Britain in the 19th century, this is a circum-Mediterranean/continental species that most likely arrived with ships to ports in southern England and has since spread slowly northwards - a likely candidate to increase its range as mean temperatures rise with climate change.

A large female Segestria florentina, characteristically curled up when disturbed during daylight.

This photo shows the first three pairs of legs pointing forwards very clearly. They are generally a fairly uniform black in colour with some faint paler marks such as the median line seen here, although this has been enhanced by the camera flash - to the eye, this was a very dark spider. However, the chelicerae (jaws, bearing the fangs) are an iridescent green. She needed a little gentle persuasion to show these, then was allowed to scuttle away under the shed - we do after all run a spider-friendly household...

The iridescent green chelicerae of S. florentina - the arrangement of eyes is also just about visible. Note that I am not testing her ability to bite.

No seeds, no fruit, no fungi - November!

It's been an odd year in the UK (and elsewhere) - an unusually warm early spring, then a cold, wet early spring and summer and a very variable autumn with frosts and warm periods. Unsurprisingly, this has seriously impacted some of the UK's wildlife negatively. For example, the Big Butterfly Count found declines in many common species, with 11 of the 21 target species decreasing in abundance by more than a third since 2011. There are a number of reasons - low temperatures clearly affect cold-blooded groups such as insects, which in turn reduces the food available for insectivores such as birds and bats. Plant growth is also affected by cold and water-logging, so less sugar is produced, fruit growth is reduced (and what does grow may be affected by moulds and blights and fall early or rot on the plant), leading to less seed production - bad for plant reproduction and seed-feeders such as many winter birds. This is already having visible effects as more unusual species appear in gardens - so, it is even more important to keep bird-feeders full. Poor fruit/seed growth in Scandinavia has already meant that at least a couple of thousand waxwings have flow across the North Sea to NE Britain. There are also less obvious effects. For example, the reduced sugar production means that ectomycorrhizal (externally root-associated) fungi grow poorly despite the damp conditions.

However, not all is doom and gloom. Some species have taken advantage of warm late summer and autumn temperatures to grow and breed - I have certainly seen late bird-nesting activity, and while in south Devon last week (the SW is the warmest part of the country), while some trees were losing their leaves, others were budding as seen here.

Hazel coming into bud in mid-November in south Devon

Late flowering has provided at least some extended nectar availability (apart from ivy which is always a winter source) as seen here where a small Panurgus calcaratus bee is feeding on a yellow composite flower, again in mid-November. Found in a band across SE England it is also known all round the SW coast as you can see on the map here; clearly a species needing warm temperatures as it is absent further north.

Panurgus calcaratus feeding on a yellow composite flower in warm sunny mid-November conditions in south Devon

Warm damp autumn weather has meant that some fungi have done well eventually, such as those living on damp deadwood and leaf-litter, while in our new garden pond, the pond-skaters have bred very successfully and are highly active. Slugs and snails have also had an excellent year, though this is not popular with gardeners and allotment-holders, not to mention those of us with old houses that have little holes where slugs can gain access in the middkle of the night...

Overall, there are sadly probably more wildlife losers than winners, but what does the future hold? Well, nothing is certain, but an important study by Overland et al. (2012) does give some indications. Firstly, as many people have suggested, it isn't just 2012 when the summer has been cold and wet - this is a pattern that seems to have begun in 2007 when there was what appears to be a sustained shift in early summer Arctic winds. This change is linked to increased North American atmospheric blocking which ultimately leads to the southward movement of the jet stream that has been mentioned in TV weather forecasts. The study also looked at why this has happened and has unsurprisingly concluded that climate change is a likely candidate - in particular the melting of Arctic ice (particularly around Greenland, remembering that Greenland is politically European but geographically North American) which highlights the potential connectivity between Arctic climate and mid-latitude weather i.e. the Arctic heats up, the UK gets bad summers.

This is of course an ongoing story - research is undoubtedly ongoing to finesse some of the findings and explanations. As an academic, I find this fascinating but as someone interested in wildlife and environmental issues, I also find it deeply troubling, especially when the UK government seems to be trying to pull back from its low-carbon committments. However, I'll stop there lest the Ecology Spot becomes my political ranting zone!

Reference

Overland, J. E., Francis, J. A., Hanna, E. & Wang, M. (2012). The recent shift in early summer Arctic atmospheric circulation, Geophysical Research Letters 39, L19804 (6pp.)

Dragon Hunt 2012

Nope, this is not about Harry Potter or video-gaming, so if that's what brought you here, sorry - but do feel free to stay and read on... instead, it's about Hampshire Conservation Volunteers' annual pilgrimage to see the dragonflies in one of the most Odonata-friendly parts of the New Forest.

This year's event which took place last Sunday was of particular interest as it follows an exceptionally wet early to mid-summer in Britain which has led to dire predictions about the fate of many invertebrate species, a topic I've looked at briefly here. Of course, the Odonata (damselflies, demoiselles and dragonflies - all of which are illustrated below) are aquatic as juveniles, and require water to breed as adults, so it is less clear what effect wet conditions might have than, for example, on butterflies. However, with heavy rains come lower-than-average temperatures, so some of the same impacts might be seen in the adults, such as difficulties finding food and mates.

For those not familiar with Odonata, the damselflies and demoiselles (suborder Zygoptera) are slender-bodied and hold their wings closed lengthways at rest, while the dragonflies (infraorder Anisoptera) are more robust 'typical' Odonata and hold their wings open at rest. There is another infraorder of dragonflies, the Epiophlebioptera, but most of these are extinct with just three Asian species currently known (the third was discovered recently in China by Li et al., 2012) - I won't worry about these here, so back to the Dragon Hunt...

Our first stop was Hatchet Pond, a well-known Odonata-watching spot where it is possible to track down some botanical rarities too (more on that another time) and which provided us with good views of familiar species such as Common blue damselfly (with various female colour forms, and both pale-immature and blue-mature males, plus egg-laying behaviour seen) and an Emperor dragonfly patrolling a section of the large pond (it's the largest in the New Forest). However, although we visited a couple of the small nearby ponds as well, the best views of the day - and the most interesting records - were around the nearby stream at Crockford, where one of the first things we noticed (and we were helped by the sunnier-than-forecast conditions) was a number of very active Beautiful demoiselles, including males trying to entice females with a fluttering wing display.

A male (note the claspers at the tip of the abdomen) Beautiful demoiselle Calopteryx virgo.

We also couldn't help but notice a much larger species flying rapidly back and forth along a section of stream, the typical male territorial behaviour of the Golden-ringed dragonfly. The length of stream patrolled depends on the density of males present as they turn back to fly the other way when another male is encountered. Although territories are generally not guarded very vigorously, aggressive encounters between males will sometimes occur when they meet - there were 3 males along this stretch and they did encounter each other more than once. They are especially active during this period of territorial behaviour and hence can be difficult to photograph, but as they occasionally pause on vegetation, patience is generally rewarded.

Golden-ringed dragonfly Cordulegaster boltonii hanging from a twig.

Of the 14 species recorded (the list, including scientific names, is below), one is a rarity in Britain, associated largely with central southern England (though it is widespread in continental Europe), and that is the Southern damselfly. It is found mainly in slow-flowing base-rich ditches and streams in acid heathlands (and also in some of Hampshire's chalk streams, one of which is about 500m from where I live despite being urban-based, lucky me!), and seems to be limited by needing water which is stable in temperature, rarely dropping below 10^oC, clearly a potential limitation during winter. However, at Crockford, numerous individuals were seen, including (again, a little patience was required) some that would tolerate a human presence near enough to see the 'mercury' mark that distinguishes it from other blue damselfly species.

Southern damselfly Coenagrion mercuriale showing the identifying 'mercury' mark near the front of the abdomen.

More importantly, active breeding behaviour could be seen - I have witnessed this before in the Southern damselfly, but never in quite such large numbers. The photograph below shows three pairs at different stages of mating. On the left, a pair which is unattached (and may simply be resting on vegetation), on the right, a male has gripped a paler female with his claspers, and at the top, the typical 'mating wheel' arrangement seen during copulation. It's not often an opportunity for a shot like this comes around...

Three pairs of Southern damselfly Coenagrion mercuriale

Of course, it's easy to get a little spoiled and forget that despite the numbers seen during the Dragon Hunt, this is actually a rare species in Britain and is covered by a UK Biodiversity Action Plan as well as being listed in Schedule 5 of the Wildlife & Countryside Act 1991 which prohibits handling without a licence.

Looking at the day's records as a whole, what does it say about the Odonata community in the area we visited? Well, it's only a snapshot, but a fair number of species were seen even though a few that I expected to see were not noted (this doesn't mean they weren't there of course) such as Emerald damselfly (Lestes sponsa) and Red-eyed damselfly (Erythromma najas). On the whole, the blue damselfly species were reasonably abundant along with the darters and skimmers (Libellulidae), but the red damselflies were sparse, as were the large hawker dragonflies (Aeshnidae) with just two Emperor dragonflies and a single Migrant hawker seen. Is this a real effect of poor weather? Well, without a more systematic study, it is of course impossible to tell for certain, but as Corbet & Brooks (2008) note using various examples (and noting the need for targeted study), Odonata are temperature-limited, so it seems likely that some impact might occur. However, volunteer records such as those here can only help to build up a picture of the current status of Britain's Odonata populations.

The day's Dragon List

Azure damselfly (Coenagrion puella)
Beautiful demoiselle (Calopteryx virgo)
Black-tailed skimmer (Orthetrum cancellatum)
Blue-tailed damselfly (Ischnura elegans)
Common blue damselfly (Enallagma cyathigerum)
Common darter (Sympetrum striolatum)
Emperor dragonfly (Anax imperator)
Golden-ringed dragonfly (Cordulegaster boltonii)
Keeled skimmer (Orthetrum coerulescens)
Large red damselfly (Pyrrhosoma nymphula)
Migrant hawker (Aeshna mixta)
Ruddy darter (Sympetrum sanguineum)
Small red damselfly (Ceriagrion tenellum)
Southern damselfly (Coenagrion mercuriale)

More about dragonflies...

If you are interested in identifying the Odonata of Britain and Europe, the following books are all excellent (there are others, but these are my personal favourites):

Brooks, S. (2004). Field Guide to the Dragonflies and Damselflies of Great Britain and Ireland (revised edition). British Wildlife Publishing, Gillingham.
Cham, S. (2007). Field Guide to the Larvae and Exuviae of British Dragonflies. Volume 1: Dragonflies (Anisoptera). British Dragonfly Society, Peterborough.
Cham, S. (2009). Field Guide to the Larvae and Exuviae of British Dragonflies. Volume 2: Damselflies (Zygoptera). British Dragonfly Society, Peterborough.
Dijkstra, K.D. (2006). Field Guide to the Dragonflies of Britain and Europe. British Wildlife Publishing, Gillingham.
Smallshire, D. & Swash, A. (2004). Britain's Dragonflies. WildGuides, Old Basing.

...and the New Forest

Brock, P.D. (2011). A Photographic Guide to Insects of the New Forest and Surrounding Area. Pisces, Newbury.
Tubbs, C.R.. (2001). The New Forest: History, Ecology & Conservation. (2nd ed.). New Forest Ninth Centenary Trust, Lyndhurst.

References

Corbet, P. & Brooks, S. (2008). Dragonflies. HarperCollins, London.
Li, J.-K., Nel, A., Zhang, X.-P., Fleck, G., Gao, M.-X., Lin, L. & Zhou, J. (2012). A third species of the relict family Epiophlebiidae discovered in China (Odonata: Epiproctophora). Systematic Entomology 37(2): 408-412.

After the rains and beyond the pale...

...or 'from floods and the 2012 wildlife apocalypse to meadow creation via the wordy worlds of genetics and biochemistry'.

If you've been in the UK during the middle of 2012, you will have noticed that it's been raining a bit. Well, I say 'a bit', I actually mean a lot. Really a lot - following an unusually hot spring, it was wet from April to mid-July, including the wettest April for a century, and the wettest June on record. It's well understood that this was because the jet stream looped south of the UK and stayed there, but what is less well known is why this happened - this is an area of active research (including links to climate change) and there is an excellent summary here.

It has also been widely reported in both national and local media (e.g. here and here) that this unusually lengthy and heavy rainfall has had a huge impact on British wildlife - some species such as ringlet (Aphantopus hyperantus) butterflies (which breed in damp grassy areas) can do well in wet conditions, but current predictions are that 2012 will be the worst year on record for British butterflies overall. Volunteer recording will be hugely important in determining the effects on this group and there is still time to join in with the Big Butterfly Count which runs until 5th August. Although slugs and snails have done well (much to the annoyance of gardeners and vegetable growers), winged insects (including those most eseential of pollinators, bees) have fared poorly, being unable to feed or find mates effectively in cold, wet conditions and therefore are also unable to reproduce successfully. Although some mollusc-feeders may have plenty of food, this may well not lead to a good year for amphibians as the hot spring dried up breeding ponds and the April rainwater was too cold for reproduction. Thus, despite a few winners, the effect has largely been an overwhelmingly negative one e.g. many birds have been unable to keep chicks warm and fed, while many species, such as puffins, have seen nests flooded.

A pair of common garden snails Cornu aspersum (often known as Helix aspersa)

With widespread breeding failures, local extinctions have been predicted and conservationists are rightly worried about how severe the impacts will be, especially as the summer of 2011 was also poor. However, with the weather changing to become hot and sunny about a week ago, hopes for a good late summer and autumn have been fuelled, and I have to wonder to what extent various species can make up for lost time while conditions are suitable. Certainly my own observations, and those of other naturalists I've been in contact with, suggest that there is currently a rapid burst of invertebrate adult emergence, including species usually encountered earlier in the summer. Some moths which have emerged late have been reported as unusually pale, although this is from a fairly small number of observations, so its importance remains unclear. Delayed moth emergence related to temperature is well documented (e.g. DuRant 1990) but there does not appear to be any mention in the literature of aberrant colouration due to such delays. Certainly, a pale colour does not suggest an adaptive response as darker colours are generally associated with cooler temperatures as they permit more rapid warming by absorption of solar radiation. So, my question is whether delayed emergence can lead to paler colouration through some effect on the mechanisms related to pigmentation. Although this idea is somewhat speculative, two possibilities (which are not mutually exclusive) spring readily to mind:

1. Delayed emergence affects the mechanism of pigment production.
2. A longer period as a pupa means energy reserves become depleted and less essential material (such as pigments) is metabolised to enable emergence to be postponed during unfavourable conditions.


Looking at pigment production, some relevant research has been undertaken by Sawada et al. (2002) who looked at the expression of an mRNA-encoding guanosine triphosphate-cyclohydrolase I (GTP-CH I), the first key enzyme in the synthesis of pteridines during pigment formation in the wings of the butterfly Precis coenia. The biochemical details are not important here, but the key result was that gene expression was strongest one day before butterfly emergence. So, if pigment production is timed to peak around emergence, could a delay lead directly to reduced pigmentation? From this example, I have to say that it doesn't appear so - the onset and duration of gene expression appears to be controlled by the decline in the ecdysteroid hormone 20-hydroxyecdysone (linked to moulting and metamorphosis and usually called 20E), and a short delay in emergence simply led to a later peak in expression such that pigmentation and emergence remained synchronised. It is still possible that low temperatures prevent pigment formation though this implies the retention of higher levels of 20E and/or one of more effects elsewhere in the chain of biochemical processes involved. Essentially it seems that no-one knows if such effects occur.

Moving onto pigment breakdown, I'll stick with pteridines as there are a number of other pigment groups and I want to keep this at least reasonably simple. However, as Watt (1967) shows, the pteridine pathway is anything but simple, which in turn means that there are many points at which it might be disrupted, and as above, there is no indication that anyone has looked at the effects of delayed emergence on pigmentation, including the breakdown of pigment compounds.

So, although this has been a somewhat limited look at possible pigmentation effects, it has at least shown that it is an area where future research is needed. However, this does not mean that there is no every-day or real-world relevance here. Coming back to the 'volunteer' aspect mentioned earlier, it is not just recording that is needed, but habitat creation - in particular, the small-scale improvements that anyone with a garden can make (or indeed councils who own open spaces) by cultivating more-or-less natural 'meadow' areas rather than ecologically sterile mown lawns. Our garden is not large but it does include a meadow patch with scabious, bird's-foot trefoil, clovers, meadow clary, cornflower, lady's bedstraw, knapweeds and others. Not only is it more interesting and attractive than a billard-table lawn, but it has been a haven for flying insects throughout the wet summer - I have still identified around 25 bee species alone this year, with flowers being used during even the briefest of lulls in rainfall, and very actively during genuinely warm conditions. Also, you don't have to be an expert/experienced entomologist, botanist or gardener to do this. Wild flowers are in fashion at the moment (let's hope they stay that way rather than gravel, paving-for-parking and swathes of decking) with thoughtful gardening writers and TV presenters such as Sarah Raven promoting this important subject, including easy how-to guides if you want a garden meadow - and this means that garden centres and plant nurseries are likely to be well-stocked with native insect-friendly species. If you do this, not only will invertebrates reap the benefits, but so will you.

The bee Andrena flavipes on white clover during the wettest summer on record.

References

DuRant, J.A. (1990). Influence of temperature on spring emergence of European corn borer moths (Lepidoptera: Pyralidae). Journal of Agricultural Entomology 7(3): 259-264.

Sawada, H., Nakagoshi, M., Reinhardt, R.K., Ziegler, I.& Koch, P.B. (2002). Hormonal control of GTP cyclohydrolase I gene expression and enzyme activity during color pattern development in wings of Precis coenia. Insect Biochemistry and Molecular Biology 32: 609–615.

Watt, W.B. (1967). Pteridine biosynthesis in the butterfly Colias eurytheme. Journal of Biological Chemistry 242(4): 565-562.

Lovely local longhorns

In Britain there are approaching 4,100 species of beetle and some of the most attractive and charismatic of these are the longhorns (Cerambycidae) known for their large size, bright patterns and long antennae. However, although some are strikingly coloured (such as the yellow and black Rutpela maculata), many are actually more sombre blacks and browns. One such species is the 'tawny longhorn' Paracorymbia fulva which I have found at three nearby locations during the last week or so, including my back garden.

Paracorymbia fulva

As you can see, it has yellow-brown elytra with black cut-off tips and is also otherwise black. It is very similar to male Anastrangalia sanguinolenta but has a pronotum wider than it is long and with rounded sides (in A. sanguinolenta the pronotum is more slender and less rounded), is 9-14mm long (excluding appendages) and is found in the adult stage between June and August. P. fulva is also interesting for a number of reasons.

Firstly, it is generally described as being associated with broadleaved woodland (e.g. Duff 2007a), but observations suggest that its habitats are more diverse than this. For example, my three recent sightings have been in a suburban garden, rough trackside vegetation and woodland edge while Michael Darby (in Wright 2011) reports that it is associated with chalk grassland in Wiltshire without any woodland or fallen timber.

Secondly, it is considered 'Rare' in the UK as it is listed as a Category 3 Red Data Book species (again e.g. Duff 2007a), but observations by mant recorders suggest that it is more common than this, even if still mainly associated with central and southern England. More widely, it is found across most of Europe, except the north and Turkey (Hoskovec & Rejzek 2007). Given that the northern limit is likely to be due to temperature, its expansion in the UK may be another example of a species spreading due to climate change.

Thirdly, no-one knows what it feeds on - or much else about it. This may seem surprising, and in some ways it is - as Martin Rejzec says in Wright (2011), P. fulva is one of the few remaining European species which has an unknown host plant, a larva that is completely undescribed, and an unknown life history. However, he goes on to explain that this may also be because, unlike most other longhorns, it does not develop in timber; instead it might do so in the underground parts of trees or shrubs, and the larvae may even be free-living in the soil, feeding for example on fungi. Whatever the case, it is clear that this is a species where there is clear opportunity for significant gains through targeted study and research, and as it is now more common in the UK, there may be a greater chance that this will happen.

If you are interested in longhorns in the UK, I strongly recommend acquiring a copy of Duff's excellent illustrated guide (2007, b) which will help you become familiar with this fascinating family of beetles.

A reminder that many longhorns are colourful, Rutpela maculata (sometimes placed in the genus Strangalia)

References

Duff, A. (2007a). Longhorn beetles: Part 1. British Wildlife 18(6): 406-414.
Duff, A. (2007b). Longhorn beetles: Part 2. British Wildlife 19(1): 35-43.
Hoskovec, M. & Rejzek, M.(2007). Paracorymbia fulva (De Geer, 1775). Cerambycidae. [accessed 19/07/2012].
Wright, R. (2011). Paracorymbia fulva - further information received. Beetle News 3(3): 6.

From loofahs to LEDs

I've been a bit quiet on the blogging front over the last couple of weeks - partly due to having too many assignments to mark and partly because the unusually wet weather has tended to keep me from doing much in the way of field ecology (this amount of rain is not good for most British terrestrial invertebrates). However, yesterday was the first dry day for a while and happily coincided with a wildlife walk I was due to lead for the Southampton Natural History Society.

We visited a conservation-friendly farm and the event provided an opportunity to record mainly invertebrates, but also anything else of interest that we came across - given the weather (and climate change links which I probably won't write much about here - other bloggers will be doing so) and the huge negative impact expected to be seen on British wildlife, records such as these are of great importance, especially when there is so little funding for professional survey work.

As it happens, we did see a reasonable diversity of invertebrates, though only in small numbers, and those that were numerous tended to be associated with wet or at least damp conditions (e.g. molluscs, woodlice, millipedes). One of these was the slime-mould Stemonitopsis typhina...

One of several clusters of Stemonitopsis typhina.

Just as one of my recent posts focused on a slime-mould that looked like a cluster of tiny loofahs, this species reminds me of little pearly-silvery LEDs. Each of the stalked sporangia is about 3-4 mm tall with a black stalk that is no more than half the overall height. Fresh specimens like this are silvery, but can become more lilac-grey with age. It is superficially similar to some of the more cylindrical species and forms within the genus Comatricha (e.g. the uncommon cylindrical form of the usually globular C. nigra) but can easily be separated by the presence of a silvery sheath around the stalk - here this is clearly visible as a pair of pale longitudinal lines running up either side of each stalk, although it actually goes all the way round but is translucent and the stalk shows through. S. typhina is a common and widespread species found on wet, rotten wood, usually of broad-leaved trees, and this example was found (along with several similar clusters) on fragments of sodden and well-rotted willow wood around the base of an old willow tree. A common, if unfamilar, example of the importance of dead wood habitats - more wet-summer observations here soon...

Wandering wool-carder?

One of Britain's most spectacular bees (in my opinion at least) is not a bumblebee, but the wool-carder bee Anthidium manicatum, one of the Megachilinae which includes the leaf-cutters. So, when I saw one basking on a Geranium leaf during a welcome burst of sunshine a few days ago (it then moved onto common bird's-foot trefoil Lotus corniculatus), I had to take a photo - however, when I looked more closely, it was almost, but not quite, as I expected.

The wool-carder bee Anthidium manicatum

At first glance, it looks much like any other individual of this species, however the yellow markings are far more strongly developed than is usual for a British specimen - they are usually spots to the sides of the abdomen with some in the midline and partially joined to form weak bands on the rear segments as shown in many books including Baldock (2008). A clearly striped individual like this is more like a continental European specimen - indeed, an experienced hymenopterist from BWARS remarked that he had never seen an individual like this in Britain. So, is this just an aberrant specimen, or - and I shall speculate a little - might something else be happening?

In Britain, A. manicatum is only locally common in the south, especially SE England, and becomes scarcer as you look further north. Until 1993 it was scarce even in the SE, after which it began to expand its range (Edwards, 1997) and become common is some southern locations. This suggests that temperature is a key factor in its distribution and to me indicates that this is one of many invertebrate species expanding it range as a result of climate change. Its common name of 'wool-carder' referes to its habit of shaving the hairs from downy plant leaves (such as Stachys) in order to line its nest - possibly a behavioural adaptation to provide insulation due to a requirement for high temperatures.

Considering the colour pattern, this specimen (like those in continental Europe) clearly seem to display wasp-mimicry, while typical British specimens do not, or at least only weakly. This would be a clear defensive adaptation, so why do British specimens tend not to show it? One possibility is that the relatively dark colouration permits more rapid warming when basking  - certainly this individual was also vibrating its wings at intervals, a typical insect behaviour used to warm the flight muscles. If, further south in Europe, this aid to warming is outweighed by the benefits from defensive mimicry, then the clearer stripes might be an advantage. This leads me to consider three possibilities:

1. This is an aberrant individual with no further significance.
2. It is a vagrant continental individual.
3. It is a British individual and increased temperatures are selecting for stronger stripes.

Of these, #3 is the most interesting but also the most speculative and there is no way of knowing from a single sighting. However, I intend to watch closely - males are highly territorial (and may kill other bees) so might use the garden regularly - and would be interested to hear from anyone else who has seen a specimen like this in Britain.

Anthidium manicatum basking on common bird's-foot trefoil Lotus corniculatus

References

Baldock, D.W. (2008). Bees of Surrey. Surrey Wildlife Trust, Woking.
Edwards, R. (ed.) (1997). Provisional Atlas of the Aculeate Hymenoptera of Britain and Ireland. Part 1. BWARS/BRC, Huntingdon.

The spread of the trident

Most of us are used to finding spiders indoors, but harvestmen (which are arachnids but not spiders) tend not to enter buildings quite so readily. However, for the last few days, one has taken up residence in the toilet-roll stack in our bathroom, so (as is often the case when an invertebrate grabs my attention), I decided to look a little more closely.

Harvestman on a toilet roll; body length approx 8mm

As there are several similar species of harvestman in Britain and this wasn't one of the more obviously distinctive ones, I headed to my bookshelves to consult Hillyard (2005), the standard work on the subject. However, the start of the key involves looking closely at the pedipalps (shorter leg-like appendages at the front - they have mainly sensory, and partly feeding-related functions) but I didn't want to capture the specimen on this occasion. Instead I decided that a photo would have to do and that I would see if an identification could be made on the balance of features. This is what I noted (see below for photos illustrating some of these points):

• The central stripe or 'saddle' is clearly outlines in black and paler in the middle, and its rear margin is cut off in a straight line.
• The legs are relatively short and robust (for a harvestman) unlike the longer and more spindly legs of genera such as Leiobunum. The femora are more or less cylindrical, while the tibiae and patellae are more angular in cross-section.
• The ocularium (eye-bearing structure, indicated by a blue arrow) is relatively small, has small bumps or 'tubercles' on top and is about twice its own length from the from edge of the cephalothorax.
• The trident (red arrow) is titled slightly upwards with the prongs diverging slightly and not especially different in length (the central one is a little longer, but starts further forward so the actual lengths are very similar).
• There is a paler line widening forward from the ocularium to the trident (outlined by green dashes).
• The sides of the cephalothorax bear small, variable prongs and protuberances (green circles).

Along with the indoor/building-related location, these features combine to indicate that this is Odiellus spinosus. There are a number of superficially similar species, but these do not have the same combination of spines/tubercles and leg features etc.while Mitopus morio does not have a trident.

Features associated with the cepahlothorax of Odiellus spinosus

Leg segments of Odiellus spinosus. (1) femur, (2) patella, (3) tibia, (4) metatarsus, (5) tarsus

Although known from Britain since the middle of the 19th century (it is found from northern Spain to the Balkans in continental Europe), this is a species that has recently been spreading northwards through Britain, having reached at least as far as Cumbria. It is probably that this is another of the many invertebrates whose range is expanding with the warming due to climate change - as it avoids high ground in its continental range, it is likely that temperature is a key controlling factor. So, if you do see one of these, and you are sure what it is, why not help map its changing distribution by sending the details to your local Biological Records Centre.

References

For a cheaper option than Hillyard's 2005 work, Richard's 2010 fold-out guide is also a very useful starting point for a few pounds.

Hillyard, P.D. (2005). Harvestmen (3rd ed.). FSC, Shrewsbury.
Richards, P. (2010). Guide to Harvestmen of the British Isles. FSC, Shrewsbury. [laminated fold-out card]

Making the summer

You may have heard the old adage, 'one swallow doesn't make a summer', but what about 300 or more? Given the exceptionally wet weather recently (following a dry winter and an unusually hot early spring leading to drought), the feeling that summer might be coming is certainly a pleasant one, even though objectively the rain is welcome in farms and gardens. So, while spending the weekend at Downton for the Cuckoo Fair, although I'd been seeing small numbers for a few weeks, it was good to see a more sizeable flock of swallows (Hirundo rustica) that had just arrived...

There were about 300 in total and as you can see, when this video was taken (at about 11:00) they were mostly feeding low over the River Avon with some skimming low over the fields - traditional grazed pastures favoured by swallows. By the afternoon and evening, their feeding behaviour had changed - they were flying higher and were not following the river; instead they were moving more widely over the fields, occasionally shifting en masse as a loose flock, presumably following the patterns of movement of their insect prey.

Filming these birds with their rapid, darting flight isn't too difficult (as long as it's just a broad view that you want), but photography is another matter - group shots aren't too tricky, but focusing on individuals is another matter, hence the blurriness, though it is possible to pick out the blue head and the white flashes on the underside of the tail.

Swallows feeding over the River Avon, Downton, Wiltshire

Swallows performing close aerial passes...

Within this flock of swallows, there were a few other birds that looked different - one (black rather than blue, with a white rump-patch) was the house martin (Delichon urbicum) several of which could be found away from the main swallow flock in a nearby field. Another was smaller, fairly uniform on top, with a pale underside and dark throat-collar - the sand martin (Riparia riparia) which is also associated with this low-over-water feeding behaviour.

Among the chaos of the swallow flock, one of the three sand martins seen - and the best photo I could manage!

As well as heralding summer in the UK, migratory species such as these can be excelllent (if sometimes worrying) indicators of environmental conditions. Wintering in southern Africa, Britain's swallows face a long and hazardous journey, with perils including starvation, exhaustion and death in storms. The precise routes have not always been clear, but tracking technology has provided useful data and we now know that some cross the Sahara while others follow the west coast or fly up the Nile Valley. Rather than gaining much weight prior to migration, swallows find food en route (they fly by day and at low altitude), progressing around 200 miles (320km) each day.

It's long been known that annual weather fluctuations affect swallow populations - a cold, wet breeding season reduces insect numbers and hence chicks starve - but there are longer-term trends with populations having fallen across Europe since around 1970. Although the precise causes are uncertain, there are a number of likely reasons for this:

• The effects of climate change on swallows' African wintering grounds and migration routes. Certainly,  swallows are returning to their breeding grounds in poorer condition and laying fewer eggs than was previously the case. One factor seems likely to be the expansion of the Sahara desert,  making this already major barrier increasingly difficult to cross.
• The effects of climate change in Europe. Cold springs (including late frosts) reduce insect numbers. Similarly, very hot, dry summers cause pools to dry out, also reducing insect numbers, and as well as the risk of starvation, chicks die from heat exhaustion and dehydration.
• Land use changes across Europe may be reducing the numbers of nest sites and flying insects. Swallows tend to forage over grazed pastures (as seen at Downton), and the loss of cattle grazing has impacted on swallows in some areas.

So, there are clear concerns about swallow populations (as there are about many other migratory species) and solid evidence for population declines even if some of the causes are not fully understood. It is also a good example of the difference between 'weather' and 'climate' - in very simple terms, climate can be seen as 'average weather'. However, if you have outbuildings (garage, barn, workshop etc), there are some things you can do to encourage swallows to nest - they like to nest on dark ledges and in nooks and crannies, as these stay warm in cold weather and cool in hot weather. They can enter buildings via surprisinly small holes and need very little light (dark areas are at less risk from predators) - my favourite swallow nesting site is at Monkey World Ape Rescue Centre in Dorset where swallows nest high up inside one of the Chimpanzee viewing galleries, darting between visitors as they fly to and from the nest. To encourage swallows to nest in an outbuilding, the following should help:

• Make a small opening (minimum 50 mm high & 70 mm wide), under the eaves or simply leave a window or door open if security is not an issue
• Attach a nest platform where you would like them to be - high up, out of the reach of cats. Use flat pieces of wood to make an open-fronted box (the front should be tilted slightly upwards or have a low lip to stop the nest falling out - robin nest-box designs are sometimes used), or if you are feeling more creative, attach a sawdust-and-cement or papier-mache cup to a wooden backing plate. Block off places where you don't want the birds to nest.
• Put a plastic bag below the nest to catch droppings 
• If the weather is very hot, put an old carpet or blanket on the outside of the roof above the nest and soak it regularly with water. A couple of buckets of bathwater on such material takes several hours to dry and helps keep the temperature down inside the outbuilding.

Now, I think I've spotted a suitable nesting spot, so I'm off to rummage through my timber store to make a box. If it works, watch this space for nest photos! Until then...

Even swallows have to rest sometimes..

Banking on bees

Much has been written about the importance of sunny banks for invertebrates - they are often included (sometimes as 'beetle banks') in habitat management plans and as sometimes appear as recommended features related to mitigation of various types of site development. There is much about this and other aspects of habitat management for invertebrates in e.g. the excellent Kirby (2001), but I want to focus on a local example of a feature that has just this function, albeit unintentionally - a south-facing bank alongside the footpath leading to the front of a nearby church in Hampshire, southern England.

The south-facing bank in a nearby churchyard

This is not a huge feature - it's around 10 metres long and no more the around a metre high at most (as the path rises, the bank gets lower until it merges with the rest of the churchyard). However, there are nearby flowers as pollen sources, plus small bare patches of light soil, just right for burrowing. It is also south-facing and hence sunny and warm - this is key as the north-facing slope on the other side of the path does not support the range of pollinators described below. A couple of days ago, these were found and photographed during less than an an hour on a sunny day in what is currently proving to be an unusually warm spring (I'll leave out discussion of climate change on this occasion). So, what did I find?

The bee-fly Bombylius major

The commonest bee-fly in the UK, B. major can be identified by its broad, dark wing-bands (not to mention the long proboscis characteristc of this gorup) and parasitises a number of solitary bee species - so, as well as being a herald of spring, it's also an indicator that there are bees around somewhere. However, in this case, a indicator of bee activity wasn't needed - I just needed to look at the grassy bank to see a flurry of activity.

Andrena cineraria - an attractive black and white ground-nesting species

The most obvious bee species was A. cineraria - I spotted around 15 at one time (it's difficult to be more precise as they rarely stop flying around), including some using entrance holes to burrows, plus brief 'tussles' between pairs of individuals. In some places, this species nests in huge aggregations of thousands of females and attendant males (Baldock, 2008) and I wonder if these 'tussles' are aggressive encounters related to access to nest-burrows, or are failed mating attempts - they are too fast to be certain about the sex of the participants. Of course, where one species is found, its parasites often follow - in this case the cleptoparasite Nomada lathburiana which can be very numerous in the large aggregations mentioned above. Cleptoparasites are also known as 'cuckoo bees' as they lay their eggs in nests of other bee species, their  larvae feeding on the food provided for the host larvae. Needless to say, I was on the lookout for N. lathburiana and it wasn't long before I saw one investigating the entrances to A. cineraria nests.

Nomada lathburiana showing the yellow spots and bands seen in several species of this genus.

However, although this was an interesting find, there were other bee species, including a small (aroung 6mm long) fairly dark bee that was extremely difficult to photograph due to its rapid darting flight and tendency to re-launch whenever I got near. However, I did manage to get a shot and it turned out to be another cleptoparasite, N. flavoguttata.

Nomada flavoguttata - note the reddish-brown bands on the abdomen, including a pair of tiny yellowish dots (the species name means 'yellow spots').

Although possibly not that familiar, this is a fairly common species which parasitises several small Andrena species. This is important because at this point I hadn't seen any small Andrena, only A. cineraria, so I continued looking. Then, among the flurry of black & white bee bodies, I noticed a likely candidate - a small bee crawling around under the leaf litter. Again, photography was tricky as, when on the ground, it crawled underneath leaves, and flew rapidly away as soon as it emerged - definitely camera-shy! However, I did get the following picture, and (from other views, including while feeding on dandelion Taraxacum flowers) enough detail to identify it as A. minutula, the commonest of several similar species and a known host of N. flavoguttata.

Andrena minutula crawling underneath leaves.

Two host-parasite pairs was good for an hour's work, but there was more bee-biodiversity to be found as I saw another Andrena species exiting a burrow.

Andrena flavipes flying away from its burrow

A. flavipes is identified by the pale abdominal hair-bands and orange-yellow pollen-hairs in the hind tibiae, and like A. cineraria, can be found in large nesting aggregations. It also has its own cleptoparasite, N. fucata although I could not find this although it is present at almost all of its host's nest sites - more hunting will be required.

So, I hope these observations have reinforced the importance of sunny banks as invertebrate habitat - including for our often-declining, yet essential, populations of pollinators. If you have a little spare space in your garden or another piece of land, why not add a little bank - even a small one will attract invertebrates.

An insect's-eye view of the bank with A. cineraria flying overhead.

References

Baldock, D.W. (2008). Bees of Surrey. Surrey Wildlife Trust, Pirbright.
Kirby, P. (2001). Habitat Management for Invertebrates: a Practical Handbook (revised reprint). RSPB, Sandy.

OMG in the OMZ: massive marine microbes

Today, I'm drawing inspiration from the Census of Marine Life, a decade-long project which has produced a huge inventory of marine life - a baseline catalogue to be used for further research and to inform the management and conservation of marine life. The Census looked at all scales from microbes to whales, at all latitudes and at all depths. The Census has produced a range of books, both popular and technical - one of the most straightforward and non-specialist, 'Citizens of the Sea' (Knowlton 2010) provided a couple of snippets that induced me to delve into the detail rather more...

First up, megabacteria - not the disease of budgies (which is actually a yeast), but very large true bacteria discovered off the coast of Chile and Peru in the 1960s. Placed in the genus Thioploca, the bacteria are filamentous and 2 to 7 cm (yes, cm) long. Secreting mucus, they form vast mats (the largest covering 130,000 km-sq) in/under the 'oxygen minimum zone' (OMZ), an area at 40-280 m with very little dissolved oxygen; instead they have to rely on hydrogen sulphide in the sediments. They oxidise this using nitrates (from sea water) which they can concentrate up to 500 mM in the liquid vacuole that occupies over 80% of their cell volume, even though the concentration of nitrates in sediment is only around 25 μM. Mucus-sheathed transport filaments send this nitrate 5–10 cm down into the sediment and reduce it, thus oxidising the hydrogen sulphide and creating a coupling of the nitrogen and sulphur cycles in the sediment (Fossing et al. 1994), producing pyrite and elemental sulphur as a result (Ferdelman et al. 1997). Thus, organic matter (in the form of anaerobic dissolved organic carbon) can be oxidised at low oxygen concentrations. The mats also provide food and shelter for a range of animals including squat lobsters (Pleuroncodes monodon), amphipods, and ophiuroids (Grupe 2011). As the OMZ shares features with conditions during the Proterozoic period (2.5 bya to 650 mya), and similar microfossils have been found, such bacteria may provide an insight into ancient life forms and ecology as well as performing a still little-known but key function in nutrient cycling. Research is ongoing with one recent example investigating Thioploca found in Danish waters where (in the species T. ingrica) nitrate accumulation was lower at around 3 mM, with bicarbonates and acetates used as carbon sources, and no mat being formed (Høgslund et al. 2010). 

A core from a Thioploca bacterial mat. The core is about 8 cm across and the mat about 1 cm thick. The mat is made up of many bacterial filaments with individual cells visible to the naked eye as white threads. Huge for bacteria! Photo courtesy of NOAA/Lisa Levin.

Now, ocean acidification due to carbon emission from fossil fuels may affect marine microbes - with microbial ecosystems responsible for between 50 and 90% of all marine biomass and over 95% of marine respiration, they maintain Earth's habitability though their influences on climate (they can sequester atmospheric carbon dioxide), nutrient cycling and the decomposition of pollutants (Leahy 2012). So, this could be very serious indeed and current research is looking at the  sensitivity of marine microbes to acidification. If I find links to results from this research, I'll post an update, plus I have some more bacterial and marine posts (among others) in the pipeline.

References

Ferdelman, T.G., Lee, C., Pantoja, S., Harder, J., Bebout, B.M. & Fossing, H. (1997). Sulfate reduction and methanogenesis in a Thioploca-dominated sediment off the coast of Chile. Geochimica et Cosmochimica Acta 61(5): 3065-3079. Fossing, H, Gallardo, V.A., Jørgensen, B.B., Hüttel, M., Nielsen, L.P., Schulz, H., Canfield, D.E., Forster, S., Glud, R.N., Gundersen, J.K., Küver, J., Ramsing, N.B., Teske, A., Thamdrup, B. & Ulloa, O. (1994). Concentration and transport of nitrate by the mat-forming sulphur bacterium Thioploca. Nature 374: 713-715.
Grupe, B. (2011). Sea Floor Habitats of the Chile Margin. NOAA Ocean Explorer [accessed 15/02/2012].
Høgslund, S., Nielsen, J.L. & Nielsen, L.P. (2010). Distribution, ecology and molecular identification of Thioploca from Danish brackish water sediments. FEMS Microbiology Ecology 73(1): 110-120.
Knowlton, N. (2010). Citizens of the Sea: Wondrous Creatures from the Census of Marine Life. National Geographic, Washington DC.
Leahy, S. (2012). Giant Bacteria Colonize the Oceans. Tierramérica. [accessed 15/02/2012].

Antarctic sea spiders: polar or abyssal gigantism?

It is well known that in certain situations, some species evolve to be unusually large members of their taxonomic groups - the phenomenon of gigantism. Two such situations are polar and abyssal (deep-sea) gigantism, but why do large species evolve in polar and/or deep sea waters? As we will see, the answers are not always straighforward and are not necessarily the same for both situations. To illustrate this, I want to look at sea-spiders - not actually spiders but marine arthropods in the class Pycnogonida. A brief but informative introduction to this group can be found here, but in summary they are mostly free-living and are found at all latitudes and ocean depths. Superficially resembling spiders (though their taxonomic link to other groups is unclear), they have a cephalon (head) and a 4- (sometimes 5- or 6-) segmented body, each segment with a pair of walking legs (the rear segment bears a small abdomen), while the cephalon has various feeding appendages and palps, plus in males a pair of ovigers (leg-like appendages primarily used for carrying eggs & caring for young, but also for cleaning and courtship) which are found only in the Pycnogonida. Males also have 'cement glands' which they use to form eggs into round masses that are carried on the ovigers (Barnes 1980). Their biology is poorly studied - see Arnaud & Bamber (1987) for a useful review - but they reproduce by hatching as larvae or post-larvae with some being dispersed by medusae (jellyfish) and appear to feed on sessile animal prey or algae. Having such small bodies, their guts extend into their legs and in females, eggs are carried inside the femora. Around Britain, one common sea-spider is Pycnogonum littorale, a temperate shallow-water species (distribution given here) with a body around 5mm long and hence a leg-span of around 20mm.

A display of Pycnogonum littorale at the OUMNH

In contrast, polar and/or deep-water species may have leg-spans up to 750mm, especially in the family Colossendeidae.

Colossendeis wilsoni, A large Antarctic sea-spider also in the OUMNH collection. For scale, the label font is the same size as in the photo of P. littorale above.

So, why this gigantism in polar and abyssal marine environments? As noted in the excellent Deep Sea News, it is unclear whether the cause is the same in both cases, and the two tend to be confused in scientific reporting by the media. Also, although giant sea-spiders are familiar examples of Antarctic gigantism, many are found in deep water and therefore a species may be subject to the processes of adaptation to both polar and abyssal conditions, making it difficult to separate the two effects. For example, specimens of C. wilsoni (photo above) in the Smithsonian Museum were found at depths between 36m and 801m, while the most common Antarctic species in this genus, C. megalonyx, has been found between 3m and 4,900m (Wu & Mastro 2004)! Hence, it is not clear whether gigantism in this species is polar or polar-and-abyssal.

A widely cited paper by Chapelle & Peck (1999), found that the maximum size of amphipods (shrimp-like crustaceans) was related to dissolved oxygen rather than temperature or salinity, with polar waters being high in dissolved oxygen, because water can hold more oxygen at low temperatures. Similar effects in bivalve molluscs have also been found (e.g. Pörtner et al. 2006). The reasoning behind this 'oxygen hypothesis' is that as the size of an organism increases, its surface area:volume ratio reduces. This means that larger animals have more tissue volume requiring oxygen, but relatively less surface area with which to sequester it from the surrounding water. In warmer waters, not only is their less dissolved oxygen, but the oxygen needs of animal tissues is higher. Thus polar gigantism occurs due to cold water temperatures and high levels of dissolved oxygen. However, more recent research involving the self-righting abilities of 12 different-sized species of sea-spider (Woods et al. 2008) did not fully support the oxygen hypothesis, although it did agree that oxygen availability was likely to be one important factor, just not the only one. A possible explanation is that, being an apparently early branch of sea-spider evolution (Arango & Wheeler 2007), Colossendeis species have been adapting to cold, well-oxygenated waters for a longer period that other genera and have oxygen delivery systems which are more finely tuned to such conditions. If this is the case, then climate change is a potentially serious threat to a specialist groups of species functioning with narrow oxygen safety margins i.e. warmer waters leading to higher oxygen demand and lower availability could push Colossendeis beyond these margins more quickly than it can adapt.

So, although it seems that polar gigantism is a result of oxygen availability plus other factors, abyssal gigantism is in some ways more mysterious. Firstly, as noted by Deep Sea News, much work has looked at deep sea dwarfism rather than gigantism because so many taxa show this reduction in size, suggesting that the deep ocean is primarily a small-organism habitat (McClain et al. 2005, Kaariainen & Bett 2010). Thus, the incidence of abyssal gigantism (seen particularly in crustaceans, but also a range of other taxa) contrasts strongly with what appears to be the 'normal' situation in the deep ocean.

Abyssal dwarfism has generally been attributed to low food availability, with most animal communities (away from seeps and vents) relying on the 'marine snow' of detritus sinking from the surface, with occasional larger localised inputs such as dead whales. Thus little food arrives, especially away from productive shallow coastal waters. However, several possible explanations for the rarer gigantism have been proposed e.g.:

• Higher oxygen availability (Chapelle & Peck 2001) as the amount of available oxygen determines the amount of sustainable tissue, with cell size and number both increasing with higher oxygen concentration in Drosophila fruit flies (Frazier et al. 2001) and freshwater amphipods (Peck & Chapelle 2003). In gastropods, a link between larger size and more oxygenated deep-sea sites has been noted (McClain & Rex 2001), but giant isopods Bathynomus sp. are known from low-oxygen regions in the Gulf of Mexico.
• Longer lifespans due to reduced predation (few predators) and slower growth rates in cold water with larger cell size in crustaceans (Timofeev 2001) with a similar process suggested for other taxa (e.g. Van Voorhies 1996).

However, although key effects such as the link between oxygen levels and cell size/number have been described, these are the result of work on unrelated taxa and it remains unclear precisely why Colossendeis sea-spiders (let alone giant isopods) should exhibit gigantism while others do not - and so it is tie for a little (hopefully not too idle) speculation:

• Through development of fat reserves, larger size may allow longer gaps in feeding when food is scarce (although sea-spiders do not appear to have much space for such storage) or larger foraging areas. 
• It may be that gigantism is linked to the species' evolutionary past as island biotas also show a mixture of dwarfism and gigantism related to the size of their mainland ancestors (e.g. Lomolino 2005). Could Colossendeis (or Bathynomus) be descendents of larger ancestors from warmer and/or shallower waters and thus display gigantism rather than dwarfism when adapted to polar/abyssal conditions? 
• Is their large size actually adaptive or is it simply a random evolutionary trait which happens to serve them as well as dwarfism might? 
• With many abyssal species tending towards dwarfism, might it provide a form of niche-separation and thus reduce competition?
• Does large size itself reduce predation?
• Might large size (through the ability to exploit a large food patch or larger food items) reduce the need to move and thus expend energy? Would this be a successful trade-off against the need for more energy/food to maintain a larger body size?
• With the smaller surface area: volume ratio, larger bodies can mean easier temperature regulation, but would this be sufficiently adaptive and if so, why in only a few species?
• With some hydrothermal vent and seep species such as vestimentiferan tubeworms showing great longevity (e.g. Fisher et al. 1997), and gigantism being at least partly associated with slow growth over a long period in a stable, if food-scarce environment, might gigantism be linked to an adaptive function of increased individual longevity in areas away from vents and seeps?

I suspect I could go on, but that is enough speculation for now. As always, comments and suggestions are most welcome -  this is an area of ongoing research where the processes involved are, in part, poorly understood, so this may require an update at some point in the not-too-distant future. And the answer to the original question - polar or abyssal gigantism? Well, it seems likely that both are involved and linked to some extent by the influence of oxygen availability, but the relative 'weight' of each type of gigantism can not currently be determined for certain. However, my feeling is that, for Colossendeis at least, the fact that a single species can be found anywhere from the sea surface to depths of thousands of metres suggests that it is the polar aspect that is constant and having a greater effect. Could be wrong though!


Further reading

For a key to coastal British species: King, P.E. (1986). Sea Spiders. A revised key to the adults of littoral Pycnogonida in the British Isles. Field Studies 6(3): 493-516.

References

Arango, C.P. & Wheeler, W.C. (2007). Phylogeny of the sea spiders (Arthropoda, Pycnogonida) based on direct optimization of six loci and morphology. Cladistics 23: 255–293. Arnaud, F. & Bamber, R.N. (1987). The Biology of Pycnogonida. Advances in Marine Biology 24: 1-96.
Bamber, R.N. & El Nagar, A. (eds.) (2012). Pycnobase: World Pycnogonida Database. [accessed 09/02/2012]
Barnes, R.D. (1980). Invertebrate Zoology (4th ed.). Holt-Saunders, Philadelphia.
Chapelle, G., & Peck L.S. (1999). Polar gigantism dictated by oxygen availability. Nature 399: 114-115.
Fisher, C.R., Urcuyo, I.A., Simpkins, M.A. & Nix, E. (1997). Life in the slow lane: growth and longevity of cold-seep vestimentiferans. Marine Ecology 18(1): 83-94.Frazier, M. R., Woods, H. A. & Harrison, J. F. (2001). Interactive effects of rearing temperature and oxygen on the development of Drosophila melanogaster. Physiological and Biochemical Zoology 74: 641-650.
Kaariainen, J. & Bett, B. (2010). Evidence for benthic body size miniaturization in the deep sea. Journal of the Marine Biological Association of the UK. 86(6): 1339-1345.
Lomolino, M.V. (2005). Body size evolution in insular vertebrates: generality of the island rule. Journal of Biogeography 32(10): 1683-1699.
McClain, C.R & Rex, M.A. (2001). The relationship between dissolved oxygen concentration and maximum size in deep-sea turrid gastropods: an application of quantile regression. Marine Biology 139: 681-685.
McClain, C.R., Rex, M.A. & Jabbour, R. (2005). Deconstructing bathymetric body size patterns in deep-sea gastropods. Marine Ecology Progress Series 297: 181-187.
Peck, L.S. & Chapelle, G. (2003). Reduced oxygen at high altitude limits maximum size. Proceedings of the Royal Society of London B (Suppl.) 270: S166-167.
Pörtner, H.O., Peck, L.S. & Hirse, T. (2006). Hyperoxia alleviates thermal stress in the Antarctic bivalve, Laternula elliptica: evidence for oxygen limited thermal tolerance. Polar Biology 29: 688-693.

Timofeev, S.F. (2001). Bergmann's Principle and deep-water gigantism in marine crustaceans. Biology Bulletin 28(6): 646-650.

Van Voorhies, W.A. (1996). Bergmann size clines: a simple explanation for their occurrence in ectotherms. Evolution 50: 1259-1264.

Woods, H. A., Moran, A. L., Arango, C. P., Mullen, L. & Shields, C. (2008). Oxygen hypothesis of polar
gigantism not supported by performance of Antarctic pycnogonids in hypoxia. Proceedings of the Royal Society B 276: 1069-1075.
Wu, N. & Mastro, J. (2004). Under Antarctic Ice. University of California, Berkeley CA.