Update: 12/28/11 – In reply to my request to identify the shark species involved, Seattle Aquarium shark biologist Jeff Christiansen sent the following :
“It is difficult to make a definitive ID from a partial view and with only one photo but I would hazard a guess the shark in the photo is a sevengill shark Notorynchus cepedianus or possibly even Heptranchias perlo. While I can’t rule out a sixgill the spotting pattern visible on the dorsal surface of the shark is characteristic of the sevengill. The photo is not sharp enough to tell if there are six or seven gill slits. Its definitely one of the Cowsharks as the six and sevengill sharks are called and finding them in groups is not unusual for that species… pack hunting and association is documented in sevengills. The Sevengill can tolerate much higher light levels than its deep water cousin the sixgill.”
I’ll update again when I hear back from a New Zealand orca researcher who may have more information on the orca pod that appeared to be hunting the sharks.
On Boxing Day (the day after Christmas) in New Zealand a pod of orcas managed to corral at least a half dozen sharks and apparently gave them a good thrashing in the shallows off the beach. Orcas are known to kill and eat sharks in many parts of their range – often eating the nutrient dense livers and discarding the rest – so although witnessing the interaction must have been amazing, the news that the whales went after the sharks was in and of itself not surprising.
But the fact that the orcas managed to get several sharks together in a group for the presumed mayhem is fairly astounding, and I was as curious about the sharks as I was the orcas as I read the article.
Copyright protection prohibits me from being able to use the photo of one of the sharks thought to have been involved in the altercation with the orcas, but it is similar to this one, and is one of the few species that is easily identified with just a head shot. If you look carefully you will see that the shark has six gills [note update above, this may have been a related species of cowshark, not a sixgill] instead of the usual five, and logically is called a six-gill shark (the sixth gill is thought to help extract oxygen from the deep water where the adult sharks are found).
The following video shows both the massive size this species can attain, and the depth to which it can be found. “This six-gill shark (Hexanchus) was filmed during a submersible dive off the northeast coast of Molokai at a depth of 1000m (3280ft). The 2 red laser dots are 6 inches apart, resulting in a length of about 18 ft for the shark.”
But if they were chasing this deep sea species of shark, how did the orcas manage to get several, albeit smaller, of these sharks rounded up in the shallows off the beach?
I found the answer through the Seattle Aquarium’s shark research program, and though I have not gotten a confirmation that this is the shark species involved in the New Zealand fracas, what I learned about six-gill sharks is fascinating and over-turns much of what we believe to be true about sharks in general.
The biologists have discovered that in this local population, sibling six-gill sharks stay together for many years, at least until they become large enough to move out of the protected environment of the Salish Sea.
The mother shark can give birth to over 100 baby sharks (called pups), and each litter of brothers and sisters remains together for years in “loosely associated groups”, possibly finding food together and avoiding predators.
The eyes of six-gill sharks are adapted to low light conditions and they shun the bright light of the surface during daylight hours, so the orcas might have managed to find a group of the sharks in deeper water, driven them to the surface where the sharks would have been blinded and disoriented causing them to swim up onto the beach (the news article reported that observers thought the sharks swam up to get out of the water, certainly possible but not likely.)
Although six-gill sharks are opportunistic feeders often scavenging about anything they can find, humans are not on the menu, and fortunately for them these sharks aren’t usually on our menu either – what a shame it would be to kill off the sharks of the world without ever knowing their secrets.
Brother and sister sharks growing up together, who knew?
For more on orcas and sharks, please see “Orcas and sharks, just who is hunting whom?” Also this report on New Zealand orca expert Ingrid Visser’s discovery of orcas hunting thresher sharks.
Source: SeattlePI.com
Source: Stuff.co.nz
