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