The Portuguese man-of-war is not to be messed with. The electric blue, elaborately frilled creature sails through the ocean with venom-laced tentacles trailing behind, and fish and other organisms unfortunate enough to get in its way die a horrible death. Even humans who encounter them have painful welts to show for it. There are exceptions to the rule, however, including the tiny, transparent larvae of lobsters. For nearly 50 years, these little slips of creatures have been observed in the open ocean, exuberantly munching on men-of-war and jellyfish. Slender limbs waving, they shimmy up the tentacles, perch on top, and proceed to eat, sometimes over a period of days, organisms capable killing things many times their size.
How can these larvae consume something so venomous with no apparent ill effects? This was the question on the minds of biologist Kaori Wakabayashi of Hiroshima University and her colleagues at the Tokyo University of Marine Science and Technology when they began a study published recently in Plankton and Benthos Research. They thought of three possibilities: The lobsters might be immune to the venom, which is packaged in little balls with something like a spring-loaded needle that extends when triggered. They might encase the balls, called nematocysts, in a barrier so the venom can’t damage the digestive tract. Or they might somehow keep the balls from going off, avoiding what the researchers delicately term “internal envenomation.”
To extract venom from jellyfish tentacles for experiments, the researchers soaked them in salt, filtered the resulting slurry through a nylon stocking, and then spun it a centrifuge to get the nematocysts. After draining venom from the balls, they injected it into 10 smooth-fan lobster larvae and gave another 10 larvae injections of saltwater, as controls. The saltwater group suffered no ill effects, but over the next 24 hours, 9 out of 10 of the venom group died. If the larvae have some way of neutralizing the venom, the researchers concluded, it has to be inside the gut, because it certainly isn't in the rest of the body.
Turning a microscope on the lobsters’ feces to address their other hypotheses, the researchers realized that each poop from their captive, jellyfish-fed population was covered in a thin membrane. This membrane, a kind of biological Saran wrap, has been seen before in other crustaceans and insects, where it's thought to protect their guts from rough or sharp objects they’ve ingested. Lobster guts are armored at the beginning and end of their length, but their midguts, where nutrients are absorbed, are tender. It could be that this wrapper keeps the nematocysts from injuring the midgut, says Wakabayashi. Within the poop were nematocysts that had been triggered, although whether that happened before or after they were consumed wasn’t clear. At any rate, they had not pierced the membrane.
In some ways, the work is preliminary; the researchers used different kinds of jellyfish to feed the lobster larvae than they used for the venom-injection experiments, as a matter of convenience. Both are eaten by larvae in the wild, but to really eliminate the possibility of immunity, more experiments that consistently use one kind of jellyfish would be required.
Nevertheless, this finding—that a wrapper around feces might protect lobster from the effects of consuming jellyfish—is a remarkable reminder that if there is a benefit to a behavior, like eating venomous creatures in open ocean where other food is scarce, evolution will find a way. It also adds to a body of knowledge that Wakabayashi hopes will aid in farming the creatures. Lobster farming operations rely on clam and mussel meat to sustain them, she says. It is possible that jellyfish, the larval lobsters’ natural prey, can also provide a source of nourishment.