This article originally appeared in issue 3 of Bang! Science magazine, November 2009.
Hail to the Snail
Jessica Law gets under the shell of the garden gastropod.
There are strange beings in the garden. Thousands of them. They have tentacles, eyes on stalks, can walk upside down. They’re out there, watching you through tiny pinhole – camera eyes. And, as in any good Hammer Horror movie, they are all extremely slimy.
Slugs and snails belong to the class Gastropoda, meaning “belly footed”. Their muscular underside acts rather like a conveyor belt, with waves of muscle contraction moving from tail to head, propelling the snail forward like the tracks of a tank (but at a measly 0.03 miles per hour).
What snails lack in speed they make up for with teeth: snails have twenty-five thousand of them. Each. Their gnashers form a mechanism called a radula, something like a cross between a cat’s tongue and a JCB digger. The snail scrapes the radula over the food and the rows of jagged teeth gouge out chunks, which are then swallowed. The sound they make while eating is surprisingly loud: about 30dB, similar to a stage whisper.
Snails have two pairs of tentacles. The eyes on the top pair are rather basic, like a pinhole camera; they can detect light, dark, movement and shapes. The lower pair is for tasting and smelling. Both sets retract by turning inside out, like pushing in the finger of a washing up glove. To retract the whole body into the shell, however, the first gastropods had to undergo a series of bizarre adaptations.
To make things a little easier, most of the snail’s vital organs are already inside the shell, including the heart, liver and a lung. However, a snail’s orifices look as if they’ve been positioned by a blindfolded child in a party game. In order for the whole body to retract into the shell in one go, the mouth and anus both need to be at the front. So “torsion” evolved – while the snail is in its larval stages, its gut twists back on itself during development so that the anus opens above the head. This also enables it to excrete even when inside its shell.
Snails’ mating organs are also pinned on in a bizarre location. Most land snails are hermaphrodites, with both male and female reproductive organs positioned in the side of their neck. Prior to mating, they often wind around each other and join mouthparts as though kissing. The mating ritual really gets going when one snail, cupid-esque, fires a calcareous love-dart from the side of its neck into the neck of its prospective partner, binding the snails together and stimulating the exchange of sperm.
If the snail’s elaborate courtship ritual makes you think they’re compensating for something, that’s because they are: their parenting. After sex, each snail goes away to lay tens or hundreds of eggs, which it will bury and abandon. After a few days, land snail babies hatch as miniature adults. However, marine snail larvae often differ greatly from the adult. Whelk larvae have a wing-like “vellum”, which they flap to swim through the ocean. The pioneering offspring find fresh pastures to settle down and change into a ponderous adult.
However, life for a snail isn’t all love and cucumbers. Some live in fear of infection by the hideous parasitic flatworm, Leucochloridium paradoxum. Bird droppings are full of flatworm eggs, which the amber snail unwittingly eats. The eggs enter the snail’s digestive system and hatch. There they produce long, swollen tubes containing several hundred nematode larvae each. These grow and move up into the eyestalks, which causes them to stretch and pulsate in changing colours.
The larvae also affect the snail’s brain, making it climb, zombie-like, to the highest, most exposed point it can find. There it sits and waits for birds, which mistake the distended eyestalks for caterpillars. The bird picks off the eyestalks and, miraculously, the snail usually survives – but more parasites are waiting to move in when the eyestalks regenerate. Meanwhile, inside the digestive tract of the bird, the larvae mature, reproduce and lay eggs that are excreted in droppings, ready to infect a new generation of victims.
So there you have it. Replete with lettuce, love and danger, life is far from slow for the most mundane of molluscs.