Possessing either 6 or 7 gill-slits, these sharks are readily separable from other species in the region, all of which have only 5. Indeed, the only non-hexanchid shark to have more than five gills, the sixgill sawshark Pliotrema warreni, does not occur in the region. Similarly, only a single seven-gilled shark - the sharpnose sevengill Heptranchias perlo - frequents the North Atlantic. A larger species, the broadnose sevengill Notorynchus cepedianus, occurs in the southern Eastern Atlantic, but ranges only as far north as Namibia on the southwest African coast.
These sharks are primarily deepwater inhabitants, usually near to the bottom on outer continental or insular shelves and upper slopes but will occur sporadically near the surface, particularly in the case of bluntnose sixgills Hexanchus griseus, which make nocturnal excursions to the surface. It is not uncommon to encounter both this species and the sharpnose sevengill closer inshore, especially in areas where deepwater lies only a short distance offshore or where small islands are encircled by abruptly deep water (for example, many of the Mediterranean islands). All of these sharks are fish-eaters and the largest, Hexanchus griseus, will readily ingest mammalian remains through scavenging and possibly active predation. The deepwater habits of these sharks present few chances of encounters with divers or bathers and only the bluntnose sixgill - a powerful, omnivorous species - can be considered as potentially hazardous. Nevertheless, despite its comparative abundance in the Mediterranean, it has been identified in only one injurious encounter with a diver - which, instructively, was a provoked interaction.
| Field Identification Key | |
|---|---|
| Ordinal Identification | |
| Six or seven gill slits. One dorsal fin, sited posteriorly, its origin over or just behind the pelvic fin insertions. Anal fin present. Caudal fin strongly asymmetric, with lower lobe short or absent. Body essentially cylindrical in cross-section; varying from stout and fusiform to slender and eel-like. | |
| Family Identification | |
| Slender, eel-like body; head reptilian in appearance with mouth almost terminal; no subterminal notch in caudal fin; teeth very numerous, trident-like with three prominent cusps. First gill-slits continuous under the throat | Chlamydoselachidae Chlamydoselachus anguineus, frill shark |
| Body stout or moderately slender, not eel-like; mouth underslung and containing distinctive comb-like teeth in the lower jaws. First gill-slit not continuous under the throat | Hexanchidae Cowsharks |
| Species Identification, Hexanchidae | |
| 7 gill-slits slender body; pointed snout | Heptranchias perlo Sharpnose seven-gill shark |
| 6 gill slits | |
| -Bluntly-rounded snout; large size; caudal peduncle short and thick; 6 rows of comb-like lower teeth | Hexanchus griseus Bluntnose six-gill shark |
| -Narrow head, bluntly-pointed snout; slender body; size up to 180cm; caudal peduncle long and slender; 5 rows of comb-like lower teeth | Hexanchus nakamurai Bigeyed sixgill shark |