If humans call you a “prickly dogfish” and think you’re uglier than goblins, hammerheads and some of your other strange-looking cousins, you’d probably grow up with a chip on your dorsal fin. Now humans have another reason to pick on the prickly dogfish shark – researchers have discovered that this rare shark lives solely on the eggs of another of its cousins … the ghost shark. How big is that chip?
Not much is known about the prickly dogfish shark (Oxynotus bruniensis). Rarely seen, it lives off the coasts of Australia and New Zealand and is called “pepeke” by the Maori people of New Zealand. Unlike most sleek sharks, the brownish-grey prickly dogfish is more triangular than linear with a small flat head, a lateral keel on either side of its body and two bog, floppy dorsal fins. On the small side for sharks, males reach 60 cm (2 ft) and females 72 cm (28.3 in). Then there’s the rough scales known as dermal denticles that give the shark its distinctive name.
That’s about all that was known of the prickly dogfish - listed on the International Union for Conservation of Nature red list of threatened species as "Data Deficient" - until recently when Brit Finucci of New Zealand's Victoria University led a study on them. Published in the Journal of Fish Biology, it reveals that they caught and cut open 50 prickly dogfish (Didn’t someone say they were endangered?) and found nothing but the eggs of ghost sharks.
The chimaeras or ghost sharks are cartilaginous fish like other sharks but branched off from sharks 400 million years ago and are considered to be a prehistoric subclass known as Holocephali. Even less is known about these strange-looking fish, although they at least got a better nickname.
One thing that is known about the ghost shark is that females lay their eggs in leathery cases and that’s what were found in the stomachs of the prickly dogfishes. While they solved one mystery, Finucci found no evidence why they’re so finicky.
One area where we collected prickly dogfish, Chatham Rise, is known for its very productive waters and supports a very diverse collection of taxa, so prickly dogfish have no need to be picky eaters.
One theory is that the prickly dogfish is as slow and out-of-shape as it looks and the ghost shark eggs are the easiest things for it to catch and eat. This odd preference seems to run in the family. Oxynotus centrina, a European cousin, feed mostly on skate eggs but have been known to try other fish.
Prickly dogfish are considered to be by-catch by the fishing industry and over-fishing could contribute to their extinction. That might be good news for the ghost sharks which are disappearing rapidly because their eggs are disappearing rapidly.
Could all of this be solved by giving the prickly dogfish a nicer nickname? Any suggestions? Sandpaper of the Sea?