In the ocean, squids occupy a sort of middle position in the food web. While they are voracious predators that attack anything that moves, they themselves are prey for a variety of animals like larger predatory fish and marine mammals. Even the mighty giant squid is food to whales and fish at various stages of their lives. This makes them the perfect vehicle for parasites looking to climb their way up the food chain, starting from infecting small plankton to end up in the bowels of a large fish or a whale.
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| Left: An Antarctic flying squid (Todarodes filippovae). Top right: Anisakis larvae encysted on squid stomach (indicated by red arrows), Bottom right: Anisakis physeteris larva from a squid. Photo of flying squid and infected squid stomach provided by Dr Chenhua Li, photo of larval Anisakis from this paper |
To investigate the role played by squid in parasite transmission, scientists from the University of Otago dissected a whole bunch of squids and what they found was a scientific treasure trove of parasites. They had on their hands a mixed platter of large squids, including the likes of arrow squid (Notodarus sloanii), flying squids (Todarodes spp.), glass squid (Teuthowenia pellucida), cock-eyed squid (Histoteuthis sp.), and many more - include a giant squid (Architeuthis dux)! In total, they were able to obtain 11 different species of squid, fit for a giant cephalopod banquet. But this would have been a banquet with parasite fillings, because they also found six types of roundworms, three types of tapeworms, and a mystery fluke among those cephalopods.
Of the six different roundworm species, three of them were in the Anisakis genus - a notorious public health hazard. Anisakis worms complete their life cycles in whales, but they can cause problems when they end up in humans and their occurrence in people is highly dependent on eating habits. For example in Japan, due to the popularity of raw seafood, Anisakis causes over 19000 cases of illness per year, whereas it is far less common in other regions with different seafood consumption preferences.
The scientists also found the larvae of Crassicauda, a genus of very long (>3 m) whale parasite that lives in all kinds of awkward nooks and crannies in the whale's body, such as under their skin, in their kidneys, in their genitalia, or even occupying multiple parts of the body simultaneously. The complete life cycles of those worms are unknown, though evidently squids are involved for at least a part of it. Similarly mysterious is Lappetascaris, a nematode which has previously been featured on this blog. The full life cycle of these worms are also shrouded in mystery, but based on what little we know, they would have been on their way to a swordfish or other large predatory bony fishes.
Meanwhile, the tapeworms are ridesharing these squid in order to reach a different destination - sharks. Two of the tapeworms found were trypanorhynchans, which also use various fish as vehicles to reach their final hosts. They are armed with four gnarly hook-lined tentacles that anchor them to their hosts' intestinal wall once they finally reach a shark's bowel. The other tapeworm species was a genetic match with a tapeworm which has previously been reported to dwell in the belly of porbeagle sharks, which is perhaps to be expected as these sharks have been documented to frequently dine on squid.
Amidst all the worms riding these squid to various different destinations was a fluke lurking in the mysterious googly-eyed glass squid. It was not readily identifiable to a known species but based on its DNA, it seems to be genetically similar to a species of fluke which lives in the throat of ocean sunfish, from the family Accacoeliidae. Very little is known about the life cycle of this family of flukes, and what little we know indicates they use various oceanic invertebrates such as pelagic snails and man o'war as intermediate hosts. Given the habits and diet of the glass squid, it might have inadvertently picked up these flukes while chowing down its usual prey.
So simply by looking under the hood of some squids have given us some remarkable insights into the ecological interactions in the ocean, along with parasitic worms are able to zip between hosts via such interactions. For those parasites, cephalopods are like well-connected travel hubs through which they can reach a myriad array of the ocean's inhabitants, all linked via a network of transmissions.
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