Predators, Prey, and Parasites
No man, mollusc, or microbe is an island; we all exist in a world populated with our fellow creatures. The study of interactions between organisms and their environments is called ecology, and in the ecology of the open ocean, Humboldt squid stand front and center. They are hunted and consumed throughout their lives by a wide variety of predators. The squid themselves are also voracious generalist predators, meaning that they eat a wide variety of prey, and lots of it. And, like all animals, Humboldt squid play the ecological role of host to numerous parasites.
A single female Humboldt squid can lay millions of eggs, of which only a few will survive to adulthood. The rest provide a high-protein buffet for any marine life that can catch them, their vast numbers creating a significant source of food for small predators. Once they grow up, Humboldt squid are too large to be eaten by the predators that stalked them when they were young, but there are plenty of larger predators out there. Tuna, sharks, dolphins, and especially sperm whales love to chow down on Humboldt squid. In fact, sperm whales in the Gulf of California live almost entirely on Humboldt squid. And of course, humans catch and eat hundreds of thousands of tons of Humboldt squid every year.
But Humboldt squid are hardly ecological victims—they know how to do some damage themselves. Hatchlings have an internal yolk sac that keeps them going for a few days, but they must quickly learn to find their own food. We don’t actually know what these tiniest of squid eat, but we suspect it’s some kind of plankton (tiny drifting plants and animals). As they grow larger and more skilled at hunting, they will devour fish, crustaceans, and their fellow squid.
Humboldt squid have a fairly small mouth relative to their body size, and their esophagus actually runs through the middle of their brain, so their food must be either very small, or torn into small pieces with their parrot-like beak. They can extend their “gape” to a certain extent by using their arms to grab and hold prey items. Anything that can be held in place can be eaten one small bite at a time. But creatures like whales and humans are far too big for this trick to work. Adult Humboldt squid tend to eat creatures less than half their own body size.
Some of our knowledge about what Humboldt squid eat comes from witnessing them in the wild, but a great deal of predation happens far below the surface. More thorough information comes from looking at the stomach contents of captured squid. These have contained some strange anomalies, like algae, bird feathers, and aluminum foil.
During your squid dissection, after separating the stomach from the rest of the internal organs, try carefully cutting it open. There are two layers to cut through: the external stomach wall, and an internal stomach lining that looks like a corrugated membrane. Squid digest their food very quickly, so it’s unlikely that you’ll find a whole recognizable animal, but you can look for hard parts: fish bones and scales, squid beaks and eyes, and crustacean limbs. It’s difficult to identify a fish just from bones and scales, but a fish’s distinctive otoliths, or ear-stones, can usually tell researchers both the species and size of fish that was eaten.
Humboldt squid eat different things depending on where they live. In Mexico, their diet is dominated by myctophids—small open-ocean fish, sometimes called lanternfish, that glow in the dark. They also eat swimming red crabs and small squid. In California, they also eat mostly myctophids, small squid and krill, but they sometimes add larger prey to their diet, such as flatfish, rockfish, hake and even salmon. This worries fishermen and fishery managers, because humans also like to catch and eat flatfish, rockfish, hake and salmon. If Humboldt squid compete with us for these resources, will the fisheries be in trouble? Scientists aren’t sure yet about the answer to this question.
While you’re looking at the squid’s stomach, you may see some worms—flabby white things as long as a couple of knuckles on your finger. They are tapeworms, related to the parasites of the same name that you may have heard about in pigs or cows. However, they are not the same species. They are actually shark tapeworms, which cannot mature and reproduce until the squid is eaten by a shark. You might also see nematodes, or roundworms, curled up in tight spirals on the stomach wall.
Because they have so many interactions with other species, as predators, prey, and competitors, Humboldt squid usually play a key role in the food webs where they occur. As their populations change in size and move around in the ocean, they are undoubtedly affecting local ecologies, but the exact impacts are difficult to predict. Some scientists think that these squid may become more abundant, and occupy a larger range, as a result of global climate change, which could change food webs in many complex ways. In ecology, even small changes often cascade further than we might expect.
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