Author: Rafinesque, 1810
Field Marks:
Unmistakable compressed, rough-skinned small sharks with high, sail-like spined dorsal fins and no anal fin.
Diagnostic Features:
Trunk stout and compressed, high, with strong abdominal ridges. Head slightly depressed. Fifth (last) gill slits not abruptly expanded in width. Spiracles large and close behind eyes. Nostrils enlarged and close to each other, separated by a space much less than their width. Mouth transverse, with very long labial furrows that virtually encircle the mouth; lips expanded and papillose. Teeth strongly differentiated in upper and lower jaws, with a strong cusp and no cusplets; upper teeth small and narrow, not bladelike, lowers large, compressed and bladelike, and with erect, broad, triangular cusps. Two very large, high, sail-like, spined dorsal fins, both larger than the pelvic fins, with spines buried in the fin up to their tips; first dorsal with its base extending anteriorly to over the pectoral fin bases and gill openings. Caudal fin with a subterminal notch.
Habitat, Distribution and Biology:
Oxynotids are temperate to tropical, poorly known, deepwater bottom sharks of distinctive and bizarre appearance, that live on the upper continental and insular slopes and outer shelves at depths of 40 to 720 m. They currently have a disjunct distribution, off Venezuela in the western Atlantic, British Isles to South Africa in the eastern Atlantic, and Australia and New Zealand in the westernPacific. All the four species are small and harmless sharks, mostly smaller than 1 m but exceptionally to 1.5 m in one species.
These sharks, judging from their appearance are probably sluggish and rely on their expanded bocy cavities and large oily livers (16 to 23% of total weight in one species) to attain neutral buoyancy, so they can hover and slowly swim above the substrate without needing forward motion for lift. The prey of these sharks is little known, and includes polychaetes. Their mouths are very small and ringed with papillose lip5, and their teeth are small though strong, suggesting a diet of small bottom invertebrates and fishes. The very large nostrils and nasal organs and labial papillae may be especially important in locating prey. Development is ovoviviparous, without a placenta; litters 7 or 8.
Interest to Fisheries:
Very limited, as these sharks are relatively uncommon catches in bottom and pelagic trawl fisheries. They are mostly processed for fishmeal and oil, but also prepared smoked and dried salted for human consumption.
Remarks:
This single genus Oxynotus, is very close to the genera of the Family Squalidae, Subfamily Somniosidae (genera Centroscymnus, Scymnodon, Scymnodalatias, and Somniosus) of Compagno (1973c), and is often synonymized with the Squalidae. Pending further works on squaloid interrelationships I prefer to retain it as a family following Bigelow and Schroeder (1957) and Bass, d'Aubrey and Kistnasamy (1976).