|Antarctic toothfish. Alexander Remeslo (c) used w/permission|
|Alexander Remeslo (c)|
|Colossal squid hook/suckers taken from stomach|
of toothfish (c) Alexander Remeslo
(or possibly they lure prey with bioluminescence?) it does make sense that they would feed on toothfish in a weakened state caught on longline, according to the study depredation rates of about 1%.
|(not so) Colossal squid hooks from inside stomach's of toothfish (c) Remeslo|
Was this a predatory event or scavenged?
Now the authors - as well as the article in deep sea news about this paper - suggest that this parcel of colossal squid was either scavenged or taken off of an ailing individual. Possible predators the authors note that could have set the table for such a morsel ending up in the stomach of the toothfish include; sperm whales - which were witnessed at least once by the authors; and other colossal squid - the authors note that cannibalism is common in teuthids and most attacks concentrate on the mantle, leaving the tentacles behind which are then easily scavenged.
Another predator that consumes colossal squid is the southern sleeper sharks. Studies of the shark genus Somniosus suggest a capable - if very cryptic and sluggish - predator of large, active, and agile prey including large fish, cephalopods and marine mammals. Indeed dietary analysis of the southern sleeper shark suggests colossal squid is a mainstay of its diet and it takes larger squid on average than sperm whales. Now it is possible that southern sleeper sharks were simply scavenging these large squid but consider that we have evidence of southern sleeper sharks taking on the largest mammalian carnivoran of all time - the southern elephant seal.
Now whether or not attempted attacks on elephant seals by southern sleeper sharks are ever successful is debatable - but given colossal squids' adaptation as a very slow, drifter I think we should consider sleeper sharks as a predator of even large adult individuals. Regardless, data does not suggest southern sleeper sharks even range far enough south into the habit of Antarctic toothfish. And, although data is lacking, maybe toss in killer whales and various pinnipeds as potential predators of colossal squid - especially elephant seals, southern fur seals, and leopard seals.
But there is another possibility, one that both the authors of the study and the deep sea news out-right dismiss, that the Antarctic toothfish bit those arms off of a healthy colossal squid itself.
The reason why they assume that these remains do not evince a predation event is that the colossal squid is larger and that the teeth of a toothfish are ill-equipped to wreak such carnage i.e. "too needle like". Let's unpack this a bit.
|Colossal Squid (c) Alexander Remeslo|
|Patagonian Toothfish (c) Alexander Remeslo|
To prime your mind just a bit just watch this insane clip below to see what a group of (smaller) Chinaman Leatherjacket (Nelusetta ayraud) do to a larger octopus that they corner.
When I watch some of the plethora of online videos showing toothfish longlining hauls they all seem to be of the same size class... do we know if toothfish travel in schools? If so that would tip the scale a bit in favor of a fish that has to deal with several potential Antarctic predators and also maybe suggest a way to tackle large prey...
And finally let's keep in mind that colossal squid are not top predators but better characterized as mesopredators and are prey for sperm whales among others. It would make sense for such a slow moving animal to develop defensive strategies such as jettisoning off one or more tentacles when bit by a predator, like this squid species does. Anecdotally I see this a lot when looking at interactions between large cephalopods and predators - remember some of the first video evidence of Architeuthis showed individuals missing one or more tentacles - it could be a very common tactic used by cephalopods to distract predators...
|Giant squid missing two largest tentacles|
|Giant squid washed up in Japan. missing both large tentacles|
Should we be so surprised that a fish that is often compared to sharks does a shark like thing? That in the icy depths an exceptionally old, experienced toothfish had suffered enough from its cephalopod tyrants and took the fight to a colossal squid and decided that hey I am going to take some of your arms off today how do you like me now?
I have not yet mentioned that the name of the paper by Remeslo et al. is titled Alien Vs. Predator: Interactions between the colossal squid (Mesonychoteuthis hamiltoni) and the Antarctic toothfish (Dissostichus mawsoni) Journal of Natural History (2015). That is of course an awesome title and the allusion to color changing in the predator and cephalopods should be recognized. But in the cold Antarctic depths we best imagine this battle played out in super slow motion unlike the hyper speed assault of the fictionalized version. A gelatinous blob with a ridiculously slow metabolism begat by a much smaller fish that bleeds clear blood with only an incipiently faster pace of life. A slow motion clash of fin, tentacle, tooth and beak.
And an ecological association we are only beginning to get a handle on just as the cast of players in the Antarctic and sub-Antarctic oceans might be changing radically due to climate change.
|(c) Alexander Remeslo|
Special thanks to Alexander Remeslov for his kind correspondence and generous sharing of imagery.