"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

May 18, 2021

Anisakis physeteris

Being on top of the food chain sounds like it'd be pretty awesome - all the other animals in the ecosystem are potentially your food and nothing else hunts you. In reality, it also means that there are many parasites out there that see you as prime real estate, a nice place to settle down and start a family. And there's no way for you to avoid them since many of those parasites would be climbing their way up the food chain via the prey animals you have been eating. Nowhere is that more obvious than in the ocean.

The oceans are filled with parasites - not that you'd necessarily know since the vast majority of them are hidden out of plain sight within the body of their hosts. Many of them are parasitic worms that treat the oceanic food web like a transit system, using predator-prey interactions to get from one host to another. This post is about a study on two nematodes that cross path inside some oceanic squids

Left (a, c): Lappetascaris larva (top) embedded in squid mantle muscle, (bottom) viewed under the microscope.
Right (b, d): Anisakis physeteris larva (top) in squid testis, (bottom) viewed under the microscope
Photos from Figure 1 of the paper

A group of researchers from Italy looked at parasitic roundworms that are found in the umbrella squid and the reverse jewel squid. Both of them belong to a group of squid called the "cock-eyed squids", which are commonly found in the mesopelagic zone. The squid that the researchers examined were caught as by-catch from commercial trawling vessels that were operating off the coast of Italy and Naples, and every squid that they looked at were infected with some kind of nematode larvae. 

Most of the nematodes were of a genus called Lappetascaris, along with another species which was identified as Anisakis physeteris. While both of those parasites look superficially similar and sometimes co-infect the same squid, there are some key life history and life cycle differences between them. 

For parasites, a host is not a single homogenous entity, but a collection of different microhabitats, and each parasite species has their own taste when it comes to fine-scale real estate. In this case, the researchers found that A. physeteris mostly settled in the squid's testis whereas Lappetascaris preferred embedding itself in the firm mantle musculature (the part of squid which are sold on the market as "squid tubes").

But these worms don't just differ in the part of the squid they prefer, but also which species of squid they infect. While Lappetascaris was found in both the umbrella squid and the reverse jewel squid, A. physeteris was choosier, and was only found in the umbrella squid. Finally, the two worms complete their life cycles in totally different animals. Lappetascaris reaches maturity in the gut of large teleost fishes such as swordfish and billfish, whereas A. physeteris needs to get into the stomach of a sperm whale - as denoted by its species name (the genus name for sperm whale is Physeter).

This may explain why A. physeteris was only found in the umbrella squid. Compared with the reverse jewel squid, umbrella squid venture into much deeper water which overlaps with the sperm whale's usual hangouts. And this exposes them to infective stages that are being released from sperm whales which have hundreds and thousands of adult Anisakis worms in their gut.

While the popular perception of the sperm whale often depict them as duking it out with the giant squid, the majority of their diet is composed of more modestly sized cephalopods, and the umbrella squid seems to form a major part of their diet. That's not to say umbrella squid is not on the menu of other large oceanic predators like swordfish and billfish too (hence it is also infected by Lappetascaris larvae), but if you are a parasite that is looking for the ideal ride to get you into the belly of a sperm whale, you can't do much better than the umbrella squid. 

What about the Lappetascaris which are sharing that squid with A. physeteris? Well they better hope a swordfish would come along and snatched it up before it ends up in the belly of a marine mammal - an environment that it is ill-equipped to live in.

So while these two worms may sometimes meet in the same squid, they eventually have to go their separate ways - and reaching their respective final hosts would unfortunately spell doom for the other worm in the shared squid. As for the sperm whales, a belly full of yummy squid must inevitably lead to a stomach full of wriggly worms.