BY ALPER ÖZKAN (MSN/PhD)
d_ozkan@ug.bilkent.edu.tr
…What music they make!
There is an unspoken link between niche specialization and animal weirdness. A generalist predator, for example, is usually something that has sharp claws or a nasty bite, something that either chases down its quarry or lies in wait for an ambush, and while such predators’ exact morphology may differ, you generally know what to expect from them. Specialist predators, though, run the gamut with their diversity. They include such things as a dung beetle that uses its knife-like head to decapitate millipedes, and a hyena that uses its sticky tongue to scoop up termites by the thousands. There’s a katydid that lures cicadas by mimicking the mating behavior of their females; there are assassin bugs and lacewing larvae that wear the corpses of their previous prey to disguise them from the next. Specialization, be it dietary or environmental, is often something of a license that justifies bizarre adaptations.
Little wonder, then, that endoparasites come in all kinds of shapes and display all sorts of behaviors, since most are by necessity specialized for living in just a single type of animal (I wonder if helminths, wasps, mites and the like would be half as diverse as they are, in an alternate universe where they never discovered parasitism), and further encouraging parasite diversity is the fact that parasites must constantly race with the defensive adaptations of their host in order to survive—the Red Queen’s world runs both ways. The insides of a larger animal aren’t the only mini-deathworld available to the would-be specialist, however: there are hypersaline salt marshes, boiling volcanic lakes and of course deserts, but there’s one biome in particular that stands out because it just may be the cradle of life as we know it—the deep sea.
The abyss is a harsh mistress—it is cold and lightless, there’s crushing pressure all around (that’s one reason that many deep sea animals seem so blobby after they’ve been fished up—they decompress), and there’s no food except for your fellow abyss-dwellers and the marine snow (a romantic word for the corpses and waste products of plants and animals from the upper reaches of the sea) coming down from above. While there are ways to avoid the other issues (get rid of the swimbladder to ease up the pressure problem, grab some symbiotic bacteria and grow photophores for light, avoid wasting energy in general), nutrition remains a particularly pressing concern in the depths.
Fish tend to solve this problem by eating anything that they come across, and I do mean anything. Most deep-sea fish have gigantic maws full of nasty-looking but ultimately weak and spindly teeth that serve as a cage for any prey item seeking escape—one, the needlebeard seadevil, has teeth outside its mouth, and another, the fangtooth, has teeth so long that they must be contained in scabbard-grooves on the sides of its brain. In order not to miss out on a meal at times when that potential meal is larger than themselves, these fish also have distensible stomachs—which can, in the case of the black swallower (note how most deep-sea fish are named after their eating habits), accommodate prey up to twice the size of the fish. It is also worth noting that swallowers sometimes die of surfeit, when they eat something too large to be digested in time and the prey item rots in their stomach, with the gaseous products of decomposition forcing them to the surface.
The other side of the coin, of course, is not eating at all, which is what the adult males of anglerfish and whalefish do—some anglerfish males fuse into the skin of the female, obtaining nutrients from her bloodstream and doing little else other than fertilizing her eggs, while the whalefish male subsists on an enormously enlarged liver and eats nothing during his adult life. This is by necessity, not by choice—his upper jaw bones now support his nasal organ, which is essential for finding a mate, and are no longer functional for eating, and in any case he has no gullet and no stomach.
Also mouthless, but far from unfed, are the beardworms, a group that until recently had its own phylum, but is now grouped with the annelids. A common sight near hydrothermal vents, beardworms are host to billions of bacteria contained in a specialized organ called the trophosome, which may take up to 35 percent of the worm’s body. Lacking a digestive tract, the beardworm relies solely on its legion of chemosymbiotic bacteria for nutrition, protecting them and providing them with oxygen in return. Other bacteria-farmers, too, inhabit the depths: the Pompeii worm Alvinella uses them for insulation, being able to tolerate up to 80°C thanks to the bacterial blanket covering its back, while the yeti crab Kiwa hirsuta is suspected of using them to purify the toxins common in the sulfur-rich hydrothermal vents.
Small animals are still rather common in the deep sea, and many abyssal horrors also subsist on them—even placid filter-feeders like sponges and tunicates grow ravenous in the depths, using their spicules to latch onto unlucky crustaceans or developing gigantic maws to capture small worms. (It is interesting that carnivorous tunicates retain the nerve cord that their filter-feeding relatives digest during metamorphosis—perhaps their predatory habit requires better coordination during rapid movements?) The prey animals, however, have one last trick up their sleeves: bioluminescent prey may alert a bigger, badder predator to the presence of their foe, allowing the doomed shrimp or amphipod to get postmortem revenge (there’s even a shrimp that spews luminescent liquid to confuse predators, like a deep-sea octopus—and speaking of cephalopods, the vampire squid also has the same trick, although it lacks true ink sacs).
This is the reason that even transparent deep-sea animals still have a blotch of red or black in their stomachs, so that the light of the prey items doesn’t reach the outside world—for a harsh and unforgiving world it is.