Hexanchids

Hexanchids are a family of sharks belonging to the order Hexanchiformes. This order is notable for containing the living representatives of some of the most ancient shark lineages. The family name, Hexanchidae, refers to the six gill slits present on each side of the head, a characteristic distinguishing them from most other sharks which have five gill slits. Some fossil species within this order are known to have had up to seven gill slits.

There are currently four recognized species within the Hexanchidae family, divided into two genera: Heptranchias and Hexanchus. The bluntnose sixgill shark, Hexanchus griseus, and the sharpnose sevengill shark, Heptranchias perlo, are the most well-known members. These sharks are generally found in deep waters across temperate and tropical oceans worldwide, inhabiting continental slopes and abyssal plains.

Hexanchids are characterized by their elongated bodies, large heads, and large, vertically slit pupils. Their dentition is also distinctive, with multicusped teeth adapted for their diet, which typically consists of fish, cephalopods, and carrion. They are slow-moving and benthic or bathydemersal, meaning they live near the seabed or in the lower part of the water column. Reproduction in hexanchids is ovoviviparous, with eggs developing inside the mother before hatching. They are considered to be relatively primitive sharks due to their anatomical features, which are thought to resemble those of early shark ancestors.