A load of old Balanus

We're all familiar with barnacles, at least in broad terms - we see them on rocks, ships, whales etc, they turn up in A-level college biology projects, and if we slip on them, they are sharp and they hurt. However, how often do we really look at them?

A typical view of a barnacle-encrusted rocky shore (Start Point, south Devon, England)

They are of course crustaceans (of the class Cirripedia, order Thoracica) and come in three main forms, the suborders Lepadomorpha (the stalked 'goose' barnacles), Verruomorpha (like 'typical' barnacles but with only two opercular plates - I'll get on to that later), and Balanomorpha (the 'typical' barnacles'). I'm only going to focus on the latter, though I can't let the opportunity pass by without mentioning the existence of the order Rhizocephala. These are evolutionarily derived relatives of the barnacles and parasitise decapod crustaceans (such as crabs), their free-swimming juveniles settling on their hosts and developing into an 'interna' of root-like growths and an 'externa' which is a sac of reproductive parts. They lack as body as such, and their name means 'root head', and as with many internal parasites, they redirect their hosts' physiology and behaviour to their own ends. If I find any, I'll definitely write about them, but until then, here's a fine little 4-minute video introducing their life cycle through the medium of hand-drawn cut-out animation.

Anyhow, back to barnacles as we know them... The rocky shore above was well encrusted with barnacles of several species. There were many of the small limpet-shaped ones (such as Chthamalus stellatus in the family Chthamalidae), but what grabbed my attention were some larger, less flattened specimens typical of the the family Balanidae.

A barnacle of the family Balanidae.

As you can see, this barnacle is fairly tall (rather than being a low flat cone). Although the joins are rather obscured in this mature specimen, there are six plates forming the outer wall. It isn't clear from the photo, but the base forms a calcareous layer on the underlying rock (rather than a membranous layer) and hence this is in the genus Balanus. Identification past this point is a little tricky because the colours are so variable and corrode to a greyish mishmash in old specimens, but the obscure sutures, sharp 'beak' (tergum) and ribbed outer surface suggest either B. perforatus or B. balanus.

The scutum (seen here as a 'keel' beneath the sharply beaked tergum) is slightly saw-edged, and the smaller barnacles (which I assume are the same species) below the large one have yellowish rims with brown banding below/around them. Also, the opening in the large specimen isn't especially small. These features combine to suggest that this is B. balanus - a common species although south Devon is at the very SW extent of its range as it is absent from Cornwall, but is a place where this species has been regularly recorded. A closer look should also help clarify a couple of the features mentioned here.

The operculum, tergum and scutum of Balanus balanus

Aside from the smaller barnacles that are using it as a substrate, the beaked tergum is clearly visible as is the rough, serrated edge of the scutum. Both these structures are paired (you can see the join at the top of the tergum), forming four internal plates (collectively the 'operculum') that can move and open to allow the feeding structures (feathery 'cirri') to waft in the water and catch food particles.


So, a closer-than-usual look at a familiar organism - and a relatively rare (for me) foray into marine and littoral/intertidal habitats. Maybe I'll have to do more on this as it was the habitat that first grabbed my attention in terms of the invertebrate fauna...

Why Smurfs are like slipper limpets

Yes, I do mean Smurfs, those little blue Belgian cartoon characters... and slipper limpets are marine gastropods, Crepidula fornicata. So, why are they similar? Well, you probably know that, although there are lots of Smurfs (101 in fact), only one is female - Smurfette. Now, this could easily lead into pornographic territory (and undoubtedly has, somewhere on the Internet), but that's not what the Ecology Spot is about... instead I want to be a bit speculative and look at how this might affect Smurfs biologically if they were real...

One possibility would be that they became eusocial (like ants, bees and termites for example), with Smurfette as the only reproductive female (I assume Smurfs are viviparous, but maybe there are Smurf eggs - who knows?). However, Smurfette does not appear to be a large sedentary egg-layer (or large sedentary birther-of-live-young Smurflings), nor do there appear to be non-reproductive females rendered infertile by Smurfette pheromones. This is the case in, for example, the honey bee Apis mellifera, where the queen emits Queen Mandibular Pheromone (QMP), a pheromone set which, among other functions, inhibits ovary development in other females. So, the queen bee remains on the throne, and the princesses have to wait in line.With no other females present, and Smurfette running around actively, this seems unlikely. Instead, I think Smurfs might be an example of sequential hermaphroditism (SH).

One of the best-known examples of SH is C. fornicata. Though native to the eastern coast of North America, it has been widely introduced into the coastal waters of Europe, Japan and the NW Pacific, where it is invasive (having no predators away from its original range), competing with native filter-feeders for food. For more on its British history see here.

A stack of C. fornicata (plus a small chiton on the left) - photo by F. Lamiot, and used here under the Creative Commons Attribution-Share Alike 1.0 Generic license.

They can often be found in stacks and chains, their SH reproductive strategy meaning that the largest, oldest individuals, found at the base of  the stack are female, while the younger, smaller ones at the top are male, and some in between are 'transient'. If the female(s) die, the largest male becomes a new female.

Proestou (2005) showed that C. fornicata tended towards a 1:1 sex ratio, and that as a male's distance from a female increased, his reproductive success decreased i.e. that the males closest to the female have a competitive advantage. From this, it follows that if these males suffer a reduction in reproductive success (e.g. from competition with other males) that is greater than that due toswitching sex at a small size, then they should change. Only the lowest male in a stack can change sex, a process that takes around 60 days, during which the penis regresses and the pouches and glands of the female duct develop. If a juvenile settles on an existing stack, it develops as a male and may stay like this for up to 6 years due to pheromones released by females at the base of the stack (Fretter & Graham, 1981). Presumably the death of a female means this pheromone ceases to be produced and thus the male can change sex - another process must prevent others from changing, possibly pheromones from the new female-to-be? As there are 'transients' which complicate the picture, a pheromone gradient seems plausible.

So, although the sex ratio is different in Smurfs (100:1 rather than 1:1), an SH strategy fits well. If Smurfette dies, then as the oldest male, Papa Smurf should become Mama Smurf, with some of the others (who after all, could be 'transient' and we wouldn't know by looking at them) waiting in line.

Next post - normal service will resume!



References

Fretter, V. & Graham, A. (1981). The Prosobranch Molluscs of Britain and Denmark. Part 6. Journal of Molluscan Studies Supplement 9: 309-313.
Proestou, D.A. (2005). Sex change in Crepidula fornicata: Influence of environmental factors on reproductive success and the timing of sex change. Dissertation, University of Rhode Island.

Some are squat and some are squatters: more of the intertidal

About a year ago when my blog was just getting going, I posted photos of some specimens from a Southampton Natural History Society (SNHS) visit to Calshot Beach in southern England, a site with areas of stones, gravels, sands and shingle as well as nearby saltmarsh (the latter not covered here). That visit produced some excellent views of spectacular species such as the Dahlia Anemone Urticina felina, and we were keen to see what could be found this year. This is an annual SNHS event and the findings go towards building up a late summer/early autumn species list for the site - this year, although we picked low tide, it wasn't as low as last year's so we expected a somewhat different range of species.

Some species, or their signs, were familar from last year, such as the shells of Haminoea sea slugs, the Snakelocks Anemone Anemonia viridis, and the introduced North American bivalve, the Quahog Mercenaria mercenaria. Many others were different however - some are presented here and I hope you enjoy them. The first group I want to cover are the molluscs, starting with some primitive armoured species (the chitons) and then a larger (and edible) introduction.

Lepidochitona cinerea (family Ischnochitonidae). Probably the commonest North European chiton, this species is often found on the underside of stones, though this one was on a large bivalve shell. Though it looks smooth, the valves (sections) are rough to the touch, like fine sandpaper. This specimen is about 15mm long - the maximum is about 24mm.

The fairly common Acanthochitona crinitus (family Acanthochitonidae). The valves are less smoothly arranged than in L. cinerea and there are 18 tufts of coarse bristles. This specimen is about 20mm long - the maximum is about 34mm.

Chitons (class Polyplacophora) have a mantle skirt which forms a toughened 'girdle' around the whole edge of the animal and this is where the fringing spines etc are found. The head is small and covered by the girdle and the dorsal surface generally has armoured 'valve plates' as seen here. They graze plant and algal material from hard substrates and, like limpets, are able to withstand wave shocks without being dislodged.

The introduced Mediterranean/Biscay species Crassostrea gigas, the Portuguese Oyster. The shell has several large wrinkles and smaller concentric lines. It can grow to 180mm in length and is attached to the substrate at the hinge end - here it is attached to a dislodged stone.

Sticking with shelled species, but adding legs, a number of crustaceans were also found. As well as the common shore crab Carcinus maenas, some possibly less familiar species were worthy of a photo or two...

OK, this probably is quite familiar - it's the common hermit crab Pagurus bernhardus, using the shell of a Netted Dog-whelk Hinia reticulata. Gotta love hermit crabs! Note the larger right claw which is covered in small knobs or 'tubercles'.

One of my favourites, the Hairy Crab Pilumnus hirtellus. It is covered in hairs which are broader at the tip than the base and help camouflage it among the sediment and detritus of the intertidal zone. The shell is up to 20mm across, which is about the size of this specimen, doing its best to hide in a white tray with a few scraps of seaweed.

Another favourite, and less commonly seen, the squat lobster Galathea squamifera. The rostrum is triangular and pointed with 4 spines on each side, the rear ones also being the smallest. These spines are usually red-tipped (as here) as are those on the outer edge of the claws. The claws are also covered in flat scale-like tubercles. Despite the claws, it filter-feeds on suspended detritus.

The shrimp Palaemon elegans with a straight (rather than clearly up-curved) rostrum and dainty little claws. Note the telson (the flap at the end of the tail) doesn't have any lateral spines. These features separate it from similar species such as P. serratus.

Lastly, I'd like to introduce a couple of soft-bodied species - not the large showy ones, but a couple that are often likely to be overlooked.

The sea anemone Sagartia troglodytes. Similar to S. ornata (some specimens may be extremely difficult to separate to species), but this one clearly shows the pale brown column darkening at the top with speckles. Note the numerous tentacles; there may be up to 200 arranged in a roughly hexagonal pattern (here it is clear that the arrangement is not circular). The attachment to a buried stone is typical.

The Leathery Sea-squirt Styela clava. This is an introduced Pacific species and can be found attached to stones (as here) and pilings around the south-west coasts of Britain.

As with last years post, this is only a snapshot of what can be found on a diverse section of intertidal habitat, but hopefully an interesting one. However, it does illustrate how important volunteers are for recording wildlife, especially less 'popular' groups like many in marine and intertidal habitats - and with knowledge comes at least the potential for conservation.


References

Crothers, J. & Crothers, M. (1988). A key to the crabs and crab-like animals of British inshore waters. Field Studies 5(5): 753-802 (revised reprint).
Gibson, R., Hextall, B. & Rogers, A. (2001). Photographic Guide to the Sea & Shore Life of Britain & North-west Europe. OUP. Oxford.
Hayward, P.J. & Ryland, J.S. (eds) (2000). Handbook of the Marine Fauna of North-West Europe (2000 revision). OUP, Oxford.

Further reading

Crothers, J.H. (1997). A key to the major groups of British marine invertebrates. Field Studies 9(1): 1-177. Very useful if you are new to marine invertebrates.
Hayward, P.J. (1988). Animals on Seaweed. Richmond, Slough. For those interested in generally small intertidal species found attached to seaweeds.
Hayward, P.J. (1994). Animals of Sandy Shores. Richmond, Slough. Not used here although Calshot Beach is sandy in places and supports some of the species in this book.
Hiscock, S. (1979). A field key to the British brown seaweeds (Phaeophyta). Field Studies 5(1): 1-44. Useful alongside Hayward (1988).

Spines, sockets and a sculptured mystery...

We're off to the beach today - not literally, but it is time to go back to a wet and windy visit to Cley Marshes (Norfolk, England) on Dec 4th 2009. Why? Well, when I was there, I picked up some sea urchins which were neatly cleaned out and had been washed up on the beach following rough weather. They've been on my curio shelf ever since, and earlier today I decided to have a closer look in order to (a) identify the species, and (b) look at the structure in more detail. What I didn't expect was (c) - investigate a mystery item...

So, starting at the start, the ID was straightforward enough. The specimens were all 25-40mm in diameter, somewhat flattened on top and greenish with spines (of those still bearing spines) fairly short, mostly of about the same length, and green with purplish tips. This, along with investigation of the plates and pores, makes them specimens of Psammechinus miliaris, a common species around the coasts of Britain (Hayward & Ryland, 2000).

40mm specimen with spines

25mm specimen without spines

Looking more closely, the structures become more and more fascinating. Firstly, the spines - these look like simple structures, but a closer view reveals a world of detail. The spines themselves are grooved and at the base, fit into pores like a ball-and-socket joint in a human skeleton - there are even traces of the muscle fibres attaching them and allowing the joint to be flexible, and the microstructure of the urchin's plates can be seen in the foreground..

Basal joint/attachment of spines.

Greek columns? No, a garden of spines

There are other structures too. The basal joint has pale 'drumsticks' or cotton-buds' littered about and these are numerous all over the specimen. As far as I can tell, these are the remains of 'pedicellariae' - small articulated structures (they are often toxic) which can be used to defend against predators (such as sea stars) and may have other functions (some bear tiny 'jaws' with three 'teeth'). The spines even move aside to allow poisonous pedicelliarae easier access to the threatening predator - small attackers may be paralysed and larger ones driven away. There are also some small secondary spines, but these don't have the differentiated tip shapes.

Looking at the top, the periproct (anal area) and surrounding plates can clearly be seen. There is a large plate (the 'madreporite') with numerous small pores and one larger gonopore, plus four other genital plates, each also with a gonopore - as the names suggest these are involved in reproduction.

The periproct, surrounded by four genital plates and the larger madreporite (upper right).

Now, back to the spines, and towards the mystery... Looking at the spinal bases and surface structures, I kept noticing tiny (around 0.5mm) roughly oval objects attached to the spines, mostly pink, a few more-or-less colourless. With all the other small structures and debris, I initially didn't think much of them until I realised that they were at different points on the spines (and so probably separate from them and not, for example, bits of broken-off spine tip), and then a bit of lucky lighting showed up what looked like a suture or coarse sculpturing. So, are they some other part of the urchin, or something entirely separate?

Above and below, mysterious pink ovals

 

Though tiny, they look almost like little shells, but for now, they are known as UPOs (unidentified pink objects). So, dissecting pin in hand, I managed to extract one (it was quite well attached) and put it under the microscope. This is what I saw...

Top image x40, middle pair x100, bottom image x400

First of all, I must admit I had no real idea what the UPO might be. It looks like a tiny bivalve or maybe brachiopod, but I've no clue about the size or form of, say, early post-larval bivalves, even of common species. It appears to have an attachment structure and threads at what I'm currently calling the 'hinge' or 'base'. A join/suture separates two 'valves'; this is straight basally, and sinuous apically. There is coarse surface sculpturing, although this is fine on the paired bulbous structures.

*** UPDATE - MYSTERY SOLVED ***

I posted images on iSpot and sent some to the Marine Biological Association (MBA), in the hope of more information appearing soon - and iSpot did its job after just a day. I had wondered if it was something unusual, or something very familar to marine invertebrate specialists, but otherwise not so widely known about. It was even suggested that it might be a seed blown onto the stranded urchin as there are plant seeds with sculpturing not unlike this. However, an iSpot user came up with a much more straightforward suggestion which turns out to be correct - it's the tip (or 'business end') of a pedicellaria, and so part of the urchin after all. I had been completely thrown by the unfamiliar structure without a stalk, but of course in hindsight it's obvious - the stalks seen all over the surface have in many cases got flat ends because the tips have become detached. There are few good images around showing this structure, but once I knew what I was looking for, I came across this site (the only one so far) with excellent photos, including the whole structure - the text is in Japanese, but the images do their job.

So, after all this detective work, I remain fascinated - I generally work on terrestrial and freshwater species and had no idea that these structures existed (it might have been mentioned in Zoology 101 way back, but that was a while ago...) - small mobile jaws and poison darts: wonderful! It still makes sense for a small organism to hide among urchin spines - fossil brachiopods have even been found on fossil urchins in the US (Schneider 2003). This is the sort of location where it is quite possible no-one ever looks, so even something common may well be under-recorded and such places (like the holdfasts of seaweeds) remain a potential source of interesting finds. It's also a reminder to assume nothing!

References

Hayward, P.J. & Ryland, J.S. (eds.) (2000). Handbook of the Marine Fauna of North-West Europe. OUP, Oxford. [a great book if you can get it].

Schneider, C.L. (2003). Hitchhiking on Pennsylvanian Echinoids: Epibionts on Archaeocidaris. Palaios 18(4-5): 435-444.

Iridescence in the common limpet (Patella vulgata)

Limpets are familiar seashore animals and school biology (well, mine anyway) tells us about the conical shells that allow them to remain attached and store water in the challengingly energetic and sometimes inconveniently dry conditions of the intertidal zone. However, looking at an empty limpet shell on the Isle of Wight a couple of days ago, I couldn’t help but notice the iridescent colours inside its peak. The white colour is, if I remember rightly, one why of identifying this limpet species, but the iridesence is something generally associated with other molluscs such as ormers, abalones, top shells and of course oysters and their pearls.

Inside an empty limpet shell.

Now, I know that I’ve seen iridescence in all sorts of mollusc species, and am aware that the colours are due to the effect of small-scale structures of a size similar to the wavelengths of light and the resulting interference patterns. What piqued my curiosity was what these structures actually are and why they exist – given that the colours are often hidden away inside shells, it seems that they must be an aesthetically pleasing by-product of something with a function other than the decorative; is it simply the crud-capture associated with pearl formation?

Close-up of limpet iridescence

Nacre or ‘mother-of-pearl’ is a composite material (organic and inorganic) made of thin layers of hexagonal platelets of aragonite (a type of calcium carbonate) separated by sheets of elastic biopolymers such as chitin and silk-like materials. This structure is strong and crack-resistant (more precisely it resists crack propagation) and smooths the inside of the shell. It also helps defend the animal against parasites by encysting them in layers of nacre. So, it is both structural and a capturer of crud, which incidentally  happens to be iridescent due to the size of aragonite platelets. Limpets may not be the most well-known iridescent mollusc, but they deserve their place here (there are other types of layering and shine forms in the molluscs as well); for an electron-microscope close-up, have a look at images from the University of Tokyo’s Kogure Lab here (scroll down to number 6 for limpets).

Gratuitous intertidal images

A couple of months ago, during a helpfully low tide, I went to Calshot Beach (Hampshire, southern England) with a group from the Southampton Natural History Society to see what intertidal and saltmarsh beasts we could discover. With few if any of us being marine/littoral specialists, there was a good chance that we would expand our joyous wildlife experience with some new species (well, new to us anyway).

Although many of our findings were invertebrates, one early specimen was a plant, the Perennial Glasswort Salicornia perennis.

On the saltmarsh, Perennial Glasswort showing tiny scale-like leaves.

From there, interest moved onto the invertebrates - some annotated examples as follows - enjoy!

Tapes decussatus, the Cross-cut Carpet Shell from the strandline

Above and below, two colour forms of the Dahlia Anemone Urticina felina. The upper specimen was around 10-12cm across, the lower one around 8cm. Splendid creatures.

 

The splendid Snakelocks Anemone Anemonia viridis, around 8-10cm across.

Not the usual type of seashell, but the shell of a Haminoea sea-slug. Around 1.5cm, several found on the strandline.

 

A N.American introduction, the Quahog Mercenaria mercenaria, carrying another one from across the pond, a Slipper Limpet Crepidula fornicata as well as some tiny Spirorbis worms.

Though not in edible condition, an Edible Crab Cancer pagurus with various hitchhikers...

To finish, above and below views of the Turban Top Shell Gibbula magus - I particularly like the sculptural form of this shell.