Broadnose sevengill shark

2/16/2016

This week, we are focusing on Broadnose sevengill sharks (Notorynchus cepedianus). They are in the Hexanchidae family, related to the bluntnose sixgill shark, sharpnose sevengill shark, and frilled sharks.

Does the name “Broadnose sevengill” not sound familiar? They go by a few names, actually: “cowshark,” is rather popular, as is “sevengill cowshark.” In Tasmania, it’s even referred to as the, “Tasmanian tiger shark.”

They can get rather large, up to 3m (10 ft), and a maximum weight of 107 kg (326 lbs). They can supposedly live up to, and maybe beyond, 50 years.

A few distinguishable characteristics are that these sharks only have one dorsal fin, unlike most sharks that have two. They also have a wide head and blunt nose (hence the common name, “broadnose”). And, as the common name also says, they do have seven gills (most sharks have five). They range in coloration from a copper brown color to a silvery color, its body speckled with black spots (like stars, almost). These spots can actually be used as an identification; in fact, Ocean Sanctuaries has a project dedicated to this.

Now, here’s something cool about their hunting style: they sometimes hunt in packs. Not to mention they also rush behind their intended prey and munch! Crunch! Gulp! After a satisfying meal (usually a sea mammal or other shark), they tend to not eat for a while, letting their reward digest in their tummy for a while. In the meantime, you can see them cruising over their preferred rocky bottom habitats, but also over sandy and muddy substrates, mostly near the bottom and rarely seeing the surface.

These sharks are viviparious with yolk-sac, giving birth in shallow bays during the spring/summer after a 12 month gestation period. Their litters can be large, numbers rising up to 82 pups! The young pups will stay in the safety of the shallows until they move offshore later on.

They can be aggressive, especially when provoked, and have been known to show this aggressiveness towards divers in aquariums. (I’ll cut the shark some slack, I’ve bitten people for tickling me and pinning me down… though I doubt anyone was tickling these guys…)

This shark does have a wide range, but that doesn’t mean it doesn’t face troubles. In fact, they are heavily fished by inshore fisheries. In the 1930’s and 1940’s, sevengills were sought for their liver oil and completely overfished in San Francisco Bay. This fishery collapsed, but that didn’t stop people from fishing them for sport and competitions, completely depleting the population. There is little to not fishery data for these animals elsewhere, however, which is why the IUCN lists them as Data Deficient (DD); it is listed as Near Threatened (NT) in the eastern Pacific Ocean, however.

have you ever gone diving with these sharks?

glossary

Dorsal fin: Fin located on the back.
IUCN: International Union for Conservation of Nature.
Pup: Give birth to baby sharks (pups).
Viviparous with yolk-sac: Also known as aplacental viviparous and previously known as ovoviviparous/ovoviviparity; it is the production of eggs that are fertilised and hatch inside the female shark but the embryos lack a placental connection to the oviduct or uterus and so do not feed off the mother.