One of the basic features of toxins in the environment is bioaccumulation. Many toxic chemicals don't get broken down or washed out of an animal's body, they're simply stored forever. As the animal ingests more and more chemicals, they accumulate in its tissues. That means toxins appear in higher and higher concentrations as you move up the food chain.
To illustrate this principle, imagine a whole crowd of plankton encountering a toxin in the water, let's say a polycyclic aromatic hydrocarbon, just for the sake of argument. Each plankter (that's the singular of plankton! awesome, right?) absorbs a little bit of toxin and stores it. Now a fish comes along and eats the whole crowd of plankton. It stores all those little bits of toxins, thereby concentrating, or bioaccumulating, them in its own body. Now a squid eats a whole bunch of these fish. Now a whale or dolphin eats a whole bunch of those squid. You see where this is going?
By the time you get up to a teuthophagous (squid-eating) cetacean (whale or dolphin), there's been a whole lot of bioaccumulation, and that poor beast can end up carrying around a significant toxic load. A recent study of sperm whales (perhaps the most teuthophagous of all cetaceans) across the Pacific Ocean has found the animals were all contaminated with aromatic hydrocarbons--nasty little molecules! The study was published in Environmental Health Perspectives by Godard-Codding et al. this month.
Now, one thing that's particularly interesting is that hydrocarbons are lipophilic (fat-loving). That means they are especially easy to store in fat tissues. Read: blubber. So most of the whales' toxic load is being carted around in their blubber, and whales have a lot of blubber, so it makes sense they could hold a lot of toxin.
What most interests me (you knew I had to get back to squid eventually) is this: what about the squid they were eating? Presumably that's where they got most of their toxins. But squid, in general, have very little fat. They're basically swimming tubes of protein, which might be what makes them such appealing snacks to every predator in the ocean. So where were the squid carrying their hydrocarbons, before they were messily devoured?
Here's one thought: it's true that most squid, and particularly the well-known market squid, don't have very much fat. But the Humboldt squid, a key component of the sperm whale diet, does have a rather large organ called the hepatopancreas that is just full of fat. No one really knows why. But it's large and greasy enough to often be the first organ than anyone notices during a squid dissection--"Hey, what's that?"
"It's the hepatopancreas," I usually respond, "or digestive gland. It's very fatty, and we don't know why." Then I move blithely on to point out how incredibly interesting the squid's three hearts are.
With a little bit more research, I wonder if I could add, "It's also a key element in the bioaccumulation of aromatic hydrocarbons. It basically poisons the sperm whales who eat these guys."
At the moment, though, that's pure conjecture!
Godard-Codding, C., Clark, R., Fossi, M., Marsili, L., Maltese, S., West, A., Valenzuela, L., Rowntree, V., Polyak, I., Cannon, J., Pinkerton, K., Rubio-Cisneros, N., Mesnick, S., Cox, S., Kerr, I., Payne, R.,&Stegeman, J. (2010). Pacific Ocean–wide Profile of CYP1A1 Expression, Stable Carbon and Nitrogen Isotope Ratios, and Organic Contaminant Burden in Sperm Whale Skin Biopsies Environmental Health Perspectives DOI: 10.1289/ehp.0901809 
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