Other Sleeper Sharks

Greenland sharks (Somniosus microcephalus), Pacific sleeper sharks (Somniosus pacificus), and southern sleeper sharks (Somniosus antarcticus) are members of the Somniosus genus, and part of the Somniosidae family. While there are numerous species within the Somniosus genus, S. pacificus, S. microcephalus, and S. antarcticus are of particular interest due to their potential abundance, feeding habits, long life spans, and morphological similarities.

Like Pacific sleeper sharks, southern sleeper and Greenland sharks are slow-moving, cryptic, ambush predators and scavengers that are found in polar and temperate waters. Greenland sharks are most frequently found in the Arctic and north Atlantic while reports of southern sleeper sharks originate from regions in the southern hemisphere. Despite their seeming commonality in polar waters, Somniosus spp. are also known in temperate and even subtropical waters. In 2001, a sleeper shark (likely S. microcephalus) was observed at 2647 m near a deep-water oil well in the Gulf of Mexico. Other Somniosus specimens have been recorded in the western North Atlantic and within the Caribbean (Benz et al. 2007).  

Diet

microcephalus and S. antarcticus are generalist feeders that feed on a variety of fishes, invertebrates, and marine mammals. Both Greenland sharks and southern sleeper sharks are known to actively feed on living pinnipeds (Lydersen et al. 2016; Van den Hoff & Morrice 2007). Isotope analysis of S. microcephalus in the Kongsfjorden region of Svalbard reveals that this population relies heavily on seal predation (McMeans et al. 2013). Other Greenland shark populations make use of offal from whaling operations (Leclerc et al. 2011). Southern sleeper sharks are known to feed on large cephalopods, including giant squids (Cherel & Duhamel 2004).

Reproduction

As with S. pacificus, both S. microcephalus and S. antarcticus are ovoviviparous, giving birth to live young. Pup size at birth appears to be variable, with reports of pup sizes from 40 to 100 cm; litter sizes are currently unknown, but it is possible that the largest reproductive females may produce litters of 300 (MacNeil et al. 2012).

Taxonomic debate

Overlapping ranges and morphological similarities make species identification dificult in the wild, and some researchers have put forth that such identifications cannot be made without the aid of genetic data (Benz et al. 2007).Genetic data points toward two groupings, based upon cytochrome c haplotypes: the Greenland shark group, and the Pacific and southern sleeper shark group (Murray et al. 2008). Despite this seeming separation of Greenland sharks from other Somniosus species, the Pacific sleeper shark cytochrome c haplotype has been found in sharks collected around Baffin Bay, Canada, giving evidence to continued gene flow between the two species (Hussey et al. 2015; Walter et al. 2017).