BY ALPER ÖZKAN (MSN/PhD)
d_ozkan@ug.bilkent.edu.tr
I come bearing good news. Some time ago, I inherited inherited a small fish tank from a fellow student in my dorm, and for the past few months I’ve been keeping an apple snail in it. I am now proud to announce that she has produced her first clutch of eggs. I very much look forward to their hatching, and to mark the occasion, I’ll break routine today and devote this column to snails and slugs of every shape and description (well, except the sea angels and such, which I wrote about in a previous column).
Although they are excellent carriers of many parasites, and some snails can embed themselves into the skin of echinoderms to drain body fluids (while others drill into their digestive systems to steal food directly from their mouths), gastropods in general do not have many parasitic forms. But predators do occur often among their number, each with a unique way of capturing its quarry. Olive snails, for example, may rapidly engulf their prey (which, as one paper puts it, is anything that moves—including pencils and swimmers’ toes, though with the notable exception of sea urchins) into a sac-like compartment on their foot, sometimes suffocating it to death prior to consumption, while cone snails are able to “harpoon” fish with a long, thin, highly venomous dart that is stabbed into the prey—their radula is adapted for this very purpose, and their ammunition is in fact their highly modified teeth. Free-floating gastropods, such as the violet snail Janthina (which is unique in that it lives upside down on a bubble “raft” it produces) and some pelagic nudibranchs, often target the highly toxic Portuguese man o’ war, while snails that feed on armored prey are able to drill through shells by a combination of acidic secretions from their foot glands and mechanical abrasion by the radula—some can even take down crabs, in addition to bivalves and other snails. While predatory limpets are impressive, plant-eaters also have a few tricks up their sleeves: it is well-known that many sea slugs can maintain chloroplasts from the algae they graze on, while some carnivorous nudibranchs supplement their diet by maintaining symbiotic algal colonies in their bodies (they may even acquire their photosynthetic machinery by eating jellyfish or other sea slugs containing a population of plastids or zooxanthellae). Some grazing limpets also form symbiotic relationships with their own crops, maintaining “gardens” that are dominated by a single type of alga—any foreign species is rapidly trimmed down by the gastropod, preventing the native symbiont from being ousted from its niche, while the limpet gardener gets a steady source of food in return.
Despite the range of their offensive tactics, however, the forte of snails is defense, and the toughest nut among them is the scaly-foot gastropod. While snails normally form their shells by depositing calcium into an organic matrix, this deep-sea snail is able to enhance its defenses with a third layer of iron compounds, derived from the hydrothermal vents it frequents. While its three-layered shell is impressive enough by itself, the snail’s body is also covered with a thick, scaly armor that, due to its iron sulfide-based composition, is attracted by magnets (this is an example of life imitating art, since the existence of magnetic sea life was of course predicted by the iconic song “Lobster Magnet”). Snail defenses are also co-opted by other animals, with hermit crabs, certain bristleworms and small cichlids all using them as makeshift homes. But the greatest shell-jackers of all are the larvae of snail-eating beetles—these animals require a quiet, well-defended place to complete their molt, and may acquire one by devouring a suitably-sized snail and hiding in its shell (this phenomenon is referenced in the Pokemon Shelmet and Karrablast, based respectively on a snail and a snail-eating beetle—they only evolve when traded with each other, and their evolutions are a shell-less ninja snail and an armored beetle knight).
Oh yes, one last note: It is now autumn, and it has been raining frequently, which means that our local snail population is often out and about. They’re especially common on the ramp going to the dorms; I often see them crushed along that path, and that is a shame—I would really be glad if you paid a bit more attention to them, and the earthworms, during the morning hours.