The Basking Shark

First off, thanks to Markus Bühler of Bestiarium for inspiring this post by sending me this:

A juvenile basking shark 2.6 m in length from Izawa and Shibata (1993). I was completely unaware of this morphological feature - possible because only a handful of smaller specimens are known. Apparently the elongated snout functions in a similar manner as the cephalic fins of Manta rays.

As the above photo hopefully illustrates, many aspects of basking shark biology remain poorly known. I have a blatant bias towards covering large animals, but given the vulnerability and emerging literature on the animal, I think I can justify this post.

Tracking Basking Sharks

The most interesting aspect of basking shark biology which is now receiving long overdue attention is data on their range and movements. For several decades it was widely assumed (based on very circumstantial evidence) that basking sharks shed their gill rakers and "hibernated" in deep water for the winter! Basking sharks are observed year-round in Monterey Bay, although this population is apparently atypical (Francis and Duffy 2002). Francis and Duffy (2002) analyzed 203 basking shark records (consisting of sightings, captures, and strandings) from New Zealand and determined that basking sharks were swimming in midwater during the winter and did not lose their gillrakers. Sims et al. (2003) used "pop-up" satellite transmitters to gather tracking data for the first time in nearly two decades and directly demonstrated that hibernation does not occur (this study occured off the UK); the study also demonstrated that the sharks traveled extensively vertically and horizontally to productive areas. Skomal et al. (2004) demonstrated that the sharks do not hibernate off the West Atlantic and I think that old hibernation hypothesis can be regarded as completely dead.

Of course, these recent studies have done more than refute the bizarrely widespread hibernation hypothesis. Francis and Duffy (2002) provided unique records of basking sharks 4 km into the brackish Lake Ellesmere - made all the more impressive by (much more common) records of sharks off the continental shelf in water at least 904 m deep. Mancusi et al. (2005) provided the first data on basking sharks from the Eastern Mediterranean and Sandoval-Castillo et al. (2005) provided the first definitive records from Mexico (Pacific coast) - which leads up nicely to Skomal et al. (2009). Skomal et al. (2009) discovered, amazingly, that basking sharks do not have an antitropical distribution as widely assumed previously, but do live in the tropics after all! Sharks tagged off Cape Cod, Massachusetts and found tags in the Sargasso Sea, the Bahamas, Puerto Rican Trench, Caribbean Sea, Guyana, and Brazil - the latter record (a 6480 km trip) is the first transequatorial movement recorded via tagging from a "fish". It seems incredible that a species which was once heavily exploited completely avoided human detection over a considerable portion of its range - it goes to show that basic facts about megafauna are still being discerned. Mitochondrial DNA evidence from Hoelzel et al. (2006) demonstrates that basking sharks have low genetic diversity, a very low worldwide population (effective population ~10,000), and suggested a recent population bottleneck and worldwide panmixia; Skomal et al. (2009) suggested that if basking shark stocks are to recover there will have to be a worldwide effort.

Size & Growth

Since asymptotic size is used for growth calculations, this gives me an excuse to discuss basking shark size. The famous Stronsay carcass was described as a 16.8 m "beast" with all sorts of fanciful features - however its vertebrae were found to correspond exactly in morphology and size with a 9.3 m basking shark (then) recently described by Sir Everard Home (Wood 1982). Wood (1982) mentioned another case where a shark was lashed to the size of a boat and estimated via pacing at ~10.4 m - it was found to be 7.36 m when measured out of the water! Apparently other large figures have been obtained by measuring around the curve of the body. Such is the veracity of basking shark size claims - if a carcass hasn't been scientifically measured in recent decades the data should automatically be categorized as apocryphal.

This by all means doesn't mean that basking sharks aren't huge animals. Francis and Duffy (2002) looked at clasper length in assorted male basking sharks from New Zealand and determined that they reached sexual maturity at ~7.5 m in length; if this is accurate (and it may be somewhat conservative) it means that males in the East Coast region (<>

Anyways, Pauly (2002) used 10 m and ~7.5 tonnes* as an asymptotic size for a von Bertalanffy equation and determined (contra previous studies) that basking sharks do in fact grow very slowly, so slow in fact that they may have a gestation period of 2.6 years. The slow growth also implies considerable longevity (~50 years) and presumably a considerable age at maturity (well over 10 years judging by Figure 4) (Pauly 2002). Estimates of the mortality rate due to the fishery indicates it was impossible for any fish species to withstand for long, particularly one as extremely vulnerable as the basking shark (Pauly 2002). Compagno (2002) notes that in some regions basking sharks numbers have shown no signs of recovery even after decades. It's such a shame that some people are just catching on to this whole "responsibility" thing - hopefully those continuing to hunt the fish in East Asia will figure this out before yet another collapse occurs.

* Wood (1982) cites a 2.99 tonne/7 m individual which scales up to ~8.7 tonnes for 10 m. His 4.65 tonne/7.9 average figure scales up to ~9.4 tonnes for 10 m. I'm guessing that the weight estimation in Pauly (2002) is probably too conservative, but it would be nice to have more data on the subject.

That's about all I want to say about basking sharks. It certainly is remarkable that the biology of a species so heavily exploited could still be a largely emerging picture. Things aren't entirely bleak for the species and fortunately it is getting some good publicity.

References:

Compagno, Leonard J. V. 2002. Sharks of the World. Available (in part)

Francis, M. P. and Duffy, C. 2002. Distribution, seasonal abundance and bycatch of basking sharks (Cetorhinus maximus) in New Zealand, with observations on their winter habitat. Marine Biology 140, 831-842

Hoelzel, A. R. et al. 2006. Low worldwide genetic diversity in the basking shark (Cetorhinus maximus). Biol. Lett. 2, 639–642. Available

Izawa, Kunihiko and Shibata, Terukazu. 1993. A Young Basking Shark, Cetorhinus maximus, from Japan. Japan J. Ichthyol. 40, 237-245. Available

Mancusi, Cecilia et al. 2005. On the presence of basking shark (Cetorhinus maximus) in the Mediterranean Sea. Cybium 29, 399-405.

Pauly, D. 2002. Growth and mortality of the Basking Shark Cetorhinus maximus and their Implications for Management of Whale Sharks Rhincodon typus. In: Elasmobranch biodiversity: Conservation and Management. Available

Sandoval-Castillo, J. et al. 2005. First record of basking shark (Cetorhinus maximus) in Mexico? JMBA2 - Biodiversity Records

Sims, David W. et al. 2003. Seasonal movements and behaviour of basking sharks from archival tagging: no evidence of winter hibernation. Mar. Ecol. Prog. Ser. 248, 187-196. Available

Skomal, Gregory B. et al. 2004. Archival tagging of a basking shark, Cetorhinus maximus in the western North Atlantic. J. Mar. Biol. Ass. U.K. 84, 1-6.

Skomal, Gregory B. et al. 2009. Transequatorial Migrations by Basking Sharks in the Western Atlantic Ocean. Current Biology 19, 1–4.

Wood, Gerald. 1982. The Guinness Book of Animal Facts and Feats.

Captain Hanna's Mystery Fish

In the spirit of this recent post, I'll attempt to critically re-analyze an alleged "sea serpent' carcass reported from New Harbor, Bristol, Maine. I believe that a plausible candidate has been overlooked by prior analyses, but of course I'll have to critically examine the possibilities lest I fall victim to Phylogenetic Roulette!


As detailed in the Bulletin of the U.S. Fish Commission, at some point in August 1880 one Captain S. W. Hanna caught a dead "young sea-serpent", badly tearing up his net in the process. Hanna thought he would have lost a considerable amount of money (~ $480 in 2008 dollars) by towing it to shore and let the carcass be. Fortunately he did make a sketch of the 24-25 foot (~7.5 m) fish later in the next month:


What immediately strikes me about the illustration is the placement of the dorsal fin directly behind the head and the subsequent superficial resemblance to elongated members of the actinopterygian order Lampriformes such as Lophotidae, Radiicephalidae, and Regalecidae. Of all these species, only Regalecus glesne can reach 7.6 meters; citations give a maximum of an incredible 11 meters and there is strong documentation of them at least reaching 9 meters. I'd highly recommend clicking on the last hyperlink - wow.

Wood (1982) mentions that circa 1885, a 25 foot/7.6 m oarfish weighing 600 lbs/272 kg was caught by fishermen off Pemaquid Point, Maine. There is no such town as "Pemaquid Point" in Maine - but that landmark is a portion of New Harbor which itself is a village within Bristol. It is very frustrating that Wood (1982) does not provide a direct citation for this account - the "circa 1885" certainly needs to be explained as does the possibility that people other than Hanna may have examined the carcass. The "circa 1885" implies that this is not an interpretation of the case as outlined by Hanna (1883) and the fact that it wasn't mentioned by Hanna could imply that there was a coincidental, second case involved. Such a coincidence seems rather improbable and it could be possible that Hanna was unaware of either the case or the synonymy with his own case.

To make matters even more confusing, Kendall (1914) states Chlamydoselachus anguineus or frilled sharks have occured in New Harbor and Pemaquid [sic] Maine, citing Hanna (1883) and Goode and Beane (1896)! It seems exceedingly odd that they didn't realize the locale synonymy. Anyways, Goode and Beane (1896) state that Hanna's description "might correspond very closely" (?) with a frilled shark and sea serpent stories; despite including an extensive discussion of frilled shark morphology there is absolutely no discussion for their reasoning. While the authors use confusing tenses ("At this time it was suggested") they only cite Hanna (1883) and appear to be the originators of the strongly suggested and poorly supported identification.

Heuvelmans does not cite Goode and Beane (1896), Kendall (1914), or any accounts other than Hanna (1883) yet also suggests that the Captain Hanna/New Harbor/Pemaquid "sea serpent" is either a frilled shark or a relative of some sort. This has proven to be the most popular identification and it has been echoed by Bright (1968) and by Shuker (cited by Roesch 1997). The popularity of this identification is baffling - Roesch (1997) points out that Hanna's description is hardly diagnostic of a shark and the only argument put forth by Heuvelmans and Shuker for the shark identification are the apparent multiple "gill slits". Roesch (1997) considers the possibility of artistic license, but Hanna (1883) (which he didn't appear to have access to) specifically states that the "three strokes" on the "neck" are "corresponding with those of the shark". Of course there are no known sharks with only three gill slits; none have less than five and frilled sharks have six - and they're distinctive enough to inspire the common name. Roesch (1997) suggests that Hanna could have been mistaken about operculum morphology; members of the lampriform family Lophotidae have operculums which can create the impression of three "gill slits" and I'd suggest that this could be an interpretation for the family Regalecidae as well.

Roesch (1997) puts forth several lines of evidence which convincingly suggest that the Hanna (1883) account describes a bony fish and not a shark; given the lack of availability for his paper (and anything published in The Cryptozoological Review) his points are worth summarizing here. The illustration and account clearly refer to pectoral fins with readily visible rays ("not stiff pointed fins like the shark... more like the side-fins of the cod or sun-fish") - a diagnostic trait for a member of Actinopterygii. The pectoral fin placement at the side of the body and not sloping down is also more characteristic of a ray-finned fish than a shark. The comparison of the dorsal fin to that of the cod also supports the notion that lepidotrichia are present and an anguilliform-like caudal region is also drawn with fin rays. Roesch notes that no sharks have such an area of continuous fins "like those of an eel" and this should be readily observed even on something like the frilled shark.

Roesch (1997) concludes that the Hanna fish is more likely a bony fish than a shark and concludes that it "quite likely represents a new species". I'm not so convinced and it seems odd to me that he didn't mention the only bony fish which appears to reach or exceed the reported size with some frequency. Could the Hanna fish have been Regalecus glesne? There are a number of statements in Hanna (1883) which make this identification problematic: the body was described as round or nearly so (and 8-10" thick) instead of laterally flattened, none of the dramatic color was observed (it was slate gray dorsally to gray and white ventrally), the pelvic fins were not observed, an anguilliform anal fin was observed (oarfish lack anal fins), and "fine, briery teeth" were located at the "extreme end" of the head (oarfish lack teeth). On the plus size the mouth was described as small and like that of a sucker (which is an OK comparison to an oarfish) and the reference to very fine skin is consistent with the scaleless oarfish. It should be noted that since this fish was viewed when still in the water and possibly partially submerged (with the head most elevated?), some of the morphology and coloration was simply not observed - note this live specimen and the difficulty of observing the dorsal fin and the apparent dull coloration. Alternately and/or additionally some of the misinterpretations/misidentifications could be due to post-mortem damage to the specimen - possibly inflicted by the nets used to capture it. The teeth are still rather problematic, although I'll have to check to see if the oarfish has gill rakers which can potentially be confused for teeth - this could be why they were described as "briery". The presence of an anal fin would have to be the results of assumption on the part of Captain Hanna - it could have been an optical illusion of some sort.


The notion of an oarfish-sized species remaining undescribed is not totally outside the realm of possibility, although the Hanna carcass does not demonstrate this convincingly. The reference to an identically-sized oarfish from the same location and roughly the same time is incredibly suspicious, as is the roughly similar morphology reported. Although the oarfish has become something of a cliche for "sea serpent" candidates, I think the evidence does lean towards such an identification regardless of some of the weird morphology described by Hanna. Further investigation into the case described by Wood could theoretically prove this hypothesis.


I've got to concentrate on little fish for the time being, but I hope to write more extensive posts about oarfish and frilled sharks at some point in the near future.



References:

Bright, Michel. 1989. There are Giants in the Sea. Robson, London.

Hanna, S. W. 1883. Description of an Eel-like Creature Taken in a Net at New Harbor, Maine, in 1880. Bull. U.S. Fish. Commn. 3, 407.

Heuvelmans, Bernard. 1968. In the Wake of the Sea-Serpents. Hill and Wang, New York.

Goode, George Brown and Bean, Tarleton H. 1895. Oceanic Ichthyology, a treatise on the deep-sea and pelagic fishes of the world. Available (Caution ~50 mb!)

Kendall, William Converse. 1914. Proceedings of the Portland Society of Natural History III, 1-198. Available

Roesch, Ben S. 1997. A Review of Alleged Sea Serpent Carcasses Worldwide (Part One --- 1648-1880) The Cryptozoological Review 2, 6-27

Wood, Gerald.1982. The Guinness Book of Animal Facts and Feats. Guinness Superlatives, Middlesex.

The Surprisingly Predatory Sleeper Sharks

Sleeper sharks (Somniosus spp.) belong to the family Somniosidae in the order Squaliformes, so they're relatives of dogfish (Squalidae). The taxonomy of this genus has recently been given an overhaul by Yano et al. 2004, creating two sub-genera and re-establishing two other species. The sub-genus Rhinoscymnus is composed of two species: S. rostratus from the Mediterranean and North Atlantic and S. longus from the Pacific. They were formerly grouped into one species and are both under 1.4 m (4'7"), a calcified vertebral column, leaf-shaped denticles, and semi-oblique cusps in the lower jaw. For this blog post at least I'll be focusing on Somniosus (Somniosus) spp., the huge sleeper and Greenland sharks.

These sharks are identified by teeth with strongly oblique cusps in the lower jaw, hooked denticles, an uncalcified spine with an expanded notochord and much larger size. (Yano et al. 2004). It appears that sharks over 4 m (13 feet) are not uncommon, and the three species are widely cited as reaching 5-7 meters (16-23 feet) in total length (Yano et al. 2007). Using Table 1 from Yano et al. 2007, it can be calculated that a 7 meter Somniosus would weigh between 3500 and 3800 kg (~4 tons) and presumably rival the largest Carcharodon carcharias in size (my assumption, not theirs). These sharks can be differentiated from all other species by their range, S. microcephalus and S. pacificus are the only two species of shark known to live above the Arctic circle.

Yano et al. 2004 define three species defined by range and morphology: S. microcephalus from the Arctic and North Atlantic, S. pacificus from the North Pacific and S. antarcticus from the South Indo-Pacific and South Atlantic. A paper by Benz et al. which I unfortunately can't access, describes the first record of the subgenus from the Gulf of Mexico (note the distribution) and noted that there was no taxonomic character that could clearly identify each member of the group. The abstract didn't cast doubt on the existence of species, just some identification records. Oh, and apparently this video is the observation mentioned. Murray et al. 2008 note the rather similar morphology and supported the notion of S. microcephalus as a separate species. S. pacificus and S. antarcticus and a possibly distinct population from Taiwan did not appear to be separate species, but it is noted that more mtDNA analysis is needed to determine exactly what the status of them is. Curiously, Parin and Kotlyar 2007 note the capture of a small (1.4 m) shark showing characteristics (hook shaped denticles, number of turns in spiral valve) of the Somniosus subgenus from the southeastern Pacific that was not identifiable to any of the known species. Since sleeper sharks seem to be much more widespread in tropical (and very deep) waters than what was previously thought there's still probably a lot of work left to be done.

One of the most famous traits of the sleeper sharks is a parasitic infection frequently found in their eyes. Ommatokoita elongata is a copepod which has been found attached to the eyes of northern hemisphere sleeper/Greenland sharks (Benz et al. 2002) although I'm not sure about the southern hemisphere population. The cornea is smooth and apparently the ideal location for attachment (it doesn't have denticles), and there is typically one female specimen per eye (sometimes with larvae present) (Benz et al. 2002). It compromises the ability of the eye to form images, but not to detect light and it is believed that the sharks have no debilitations* (Benz et al. 2002.). Older articles and Wikipedia claim that the parasite is bioluminescent and acts as a fishing lure (making it mutualism, not parasitism) BUT Benz et al. 2002 continuously refer to them as parasites and it has been specifically stated that they are not bioluminescent.

*In another abstract I can't access, it is noted Greenland sharks in the St. Lawrence River do have a different pattern of behavior involving different intra- and interspecific aggression and predatory behavior.

The null idea of a fishing lure was also used to explain how the supposedly sluggish sleeper and Greenland sharks were able to catch the prey that they did. The paper that was the genesis of this post was Hoff and Morrice 2008 which documented bite wounds on elephant seals from sleeper sharks (S. antarcticus) . On account of the shark's blindness and apparent sluggishness, this seems to be quite remarkable. The bites* were not only found on juveniles and females, but on adult male elephant seals as well which were around the same length as the shark (judging from the size of the bite) (Hoff and Morrice 2008). This suggests that sleepers could be capable of successful predation and raises the possibility that other organisms found in the stomach of sleepers weren't just there as a result of scavenging. It is suggested that the sharks are capable of sneaking up on them using cryptic coloration (and possibly by gliding) and have a large buccal cavity which allows for suction feeding (Sigler et al. 2006). Even our speedy friend Lissodelphis wound up in the stomach of a sleeper shark, apparently from a living specimen. Since sleeper sharks travel towards the surface at night (Hulbert et al. 2006) and have been observed on the surface (Stokesbury 2005) I can't help but wonder if it sneaks up on sleeping cetaceans. Elephant seals may sleep while diving.

* There were also unidentified "conical" shark bites (not from sleepers or great whites) on male seals. Do the bites of the bluntnose sixgill (Hexanchus griseus) meet this description?

Of great interest to me is the fact that some very large cephalopods wound up inside sleeper sharks. In examinations of 36 sleeper shark stomachs in the Southern Ocean, all of them contained cephalopods - one of which was a large unknown cirrate. Cherel and Duhamel noted that sleeper sharks had a sperm whale-like diet; four of the gigantic squids I discussed earlier (Kondakovia, Taningia, Architeuthis and Mesonychoteuthis) were found in the stomachs of sleeper sharks. That's right, both the giant and colossal squid were eaten by this species. Incredibly, the average size of cephalopod prey eaten by the shark was slightly larger than that of the sperm whale. The authors do state that it is problematic exactly how sleeper sharks could prey on the giant cephalopods, and state that it is unknown if the giant and colossal squids were eaten while alive or scavenged. It is troubling for the ~4 meter animals in the study, but some of the largest sleepers presumably weighing around 4 tons would considerably outweigh the giant cephalopods.


I'm not trying to suggest that sleeper and Greenland sharks are unstoppable killing machines, certainly a lot of what they eat has been scavenged. It still seems that the predatory abilities of this species is much greater than what is expected, as demonstrated by attacks on similarly sized (or more massive?) elephant seals. Evidence for sharks 7 meters long is based on video evidence as far as I know, but it appears to be fairly strong. Such specimens comparable to the largest white sharks would be impressive predators, even if they are blind and sneak up on prey.

References:

Benz, George W. et al. 2002. Ocular lesions associated with attachment of the copepod Ommatokoita elongata (Lernaeopodidae: Siphonostomatoida) to corneas of Pacific sleeper sharks Somniosus pacificus captured off Alaska in Prince William Sound. J. Parasitol., 88(3), pp. 474–481

Benz, George W. et al. 2004. A second species of Arctic shark: Pacific sleeper shark Somniosus pacificus from Point Hope, Alaska. Polar Biol 27: 250–25

Cherel, Yves and Duhamel, Guy. 2004. Antarctic jaws: cephalopod prey of sharks in Kerguelen waters. Deep-Sea Research I 51 17–31

Hoff, John van den and Morrice, Margaret G. 2008. Sleeper shark (Somniosus antarcticus) and other bite wounds observed on southern elephant seals (Mirounga leonina) at Macquarie Island. Marine Mammal Science 24(1): 239–247

Hulbert, L. B. et al. 2006. Depth and movement behaviour of the Pacific sleeper shark in the north-east Pacific Ocean. Journal of Fish Biology 6, 406-425.

Murray, Brent William et al. 2008. Mitochondrial cytochrome b variation in sleeper sharks
(Squaliformes: Somniosidae). Mar Biol 153:1015–1022

Parin, N. V. and Kotlyar, A. N. 2007. On Finding of Shark of the Genus Somniosus (Squalidae) at the Submarine Ridge of Nazca (Southeastern Pacific). Journal of Ichthyology, Vol. 47, No. 8, pp. 669–672.

Sigler, M. F. et al. 2006. Diet of Pacific sleeper shark, a potential Steller sea lion predator, in the north-east Pacific Ocean. Journal of Fish Biology 69, 392–405

Stokesbury, Michael J. W. et al. 2005. Movement and environmental preferences of Greenland sharks (Somniosus microcephalus) electronically tagged in the St. Lawrence Estuary, Canada. Marine Biology 148 (1): 159-165.

Yano, Kazunari et al. 2004. A review of the systematics of the sleeper shark genus Somniosus with redescriptions of Somniosus (Somniosus) antarcticus and Somniosus (Rhinoscymnus) longus (Squaliformes: Somniosidae). Ichthyological Research. 51: 360-373.

Yano, Kazunari et al. 2007. Distribution, reproduction and feeding of the Greenland shark Somniosus (Somniosus) microcephalus, with notes on two other sleeper sharks, Somniosus (Somniosus) pacificus and Somniosus (Somniosus) antarcticus. Journal of Fish Biology 70, 374–390