A flat, sandy beach may look the same in every direction, but when you explore it, you find that it is divided into many small habitats. (In the case of Crescent Beach, "small" is probably a misnomer.)
We varied our usual route this last visit to Crescent Beach; instead of aiming southwest from our parking spot, we walked down the railway and then angled north, towards the town and the river. And we found snails. Millions of snails, trillions. I think I must have crushed thousands under my feet, even though I was being careful.
The water was a few inches deep over most of the beach, and a fine eelgrass grew thickly everywhere. On top, the Eastern Mud Snail herd was grazing, like so many sheep:
I had seen a few of these snails on the south end of the beach, a minority among the Batillaria zonalis, the whelks, and the periwinkles. Here, a smattering of batillaria feeds with the mud snails; I saw no others.
This is not a pretty snail. The shells are almost shapeless, worn and "distressed" until they look more like dirty stones than snail shells. They range in colour from a muddy white to a dingy black. Algae and slime cover most shells. (A web page for students, describing how to examine one of these, starts off with, "Scrub its surface clean with a toothbrush.")
The body itself is attractive; the upper side of the foot comes in various patterns of black and white. The one in the photo above is speckled; others have streaks or marbling. The sole of the foot is white, maybe fringed with black. At the head are two antennae, white or black or mottled, and a long siphon, which works as a nose, sniffing out the scents of food and other snails, and bringing oxygen-laden water to the gills. And to eat, the snail extends a proboscis containing the mouth, radula (scraper) and the esophagus.
Ilyanassa obsoleta, the Eastern Mud Snail, is another invader; as the name says, he came from the East coast. He was first reported in San Francisco in 1907. By 1945, he'd moved up the coast to Washington, and in 1952 showed up on Crescent Beach. He's an omnivore, living mainly on microscopic diatoms and algae, but he doesn't turn up his siphon at a spot of dead fish or crab or a sea lettuce salad. He'll take live worms and eggs of other snails. He can handle varied levels of salt in the water, and temperatures from 10 to 22 degrees C. And he may live up to 40 years. Wide, flat, rich Crescent Beach suits him fine; he's taking over.
We had waded in the ankle-deep water through the herd for almost an hour when we stumbled on a party:
We watched for a while. It was hard to follow individual snails; they twisted and writhed in a sort of rythmless dance, often in pairs, sometimes in clutches of three or more. Feet extended, grabbed the next snail and swung it around, dropped it to cling to the next. Siphons waved madly.
A mating dance, an orgy.
It's not the first snail orgy on this beach; the eelgrass, everywhere, was spotted with snail eggs:
There's a downside to all this happy activity; the mud snail sometimes harbours a flatworm that causes swimmer's itch in humans.
The cycle goes from eggs that hatch in seawater, into a free swimming stage which invades mud snails, either through the mouth or skin. Inside, it turns into a reproductive organ, a sporocyst, and settles down to produce more sporocysts. These, then, produce a swimming form, the cercaria, which burrows through the snail and out into the water, where it goes looking for a bird (on Crescent Beach, probably a seagull). More burrowing; through the gull's skin, into the bloodstream, and close to the intestines. There they transmogrify once more, becoming a breeding adult. They mate, the females release the eggs into the intestine, and off they go to sea again.
The problem for humans is that sometimes the cercaria finds one of us instead of a bird. It settles on our skin until it dries out, then flees the dryness into the shelter of skin, leaving a tiny pinprick to show where it entered. Mistake; we are no home for them; they die without reaching the bloodstream. But we become sensitized after the first invasion, and react with a raised, itchy spot (photos), which fades in about a week. Sometimes the itching is minor, no worse than a mosquito bite, but a large infestation can become very uncomfortable.
There is a sign at our entrance to Crescent Beach, warning of this. Prevention is simple; the cercaria don't burrow into the skin while it is wet, just as it dries. So we rinse our feet and hands with clean water, and dry them well with a towel, which removes the cercaria and kills them.
Not everyone who wades or swims is attacked, of course. Hundreds of people come to the beach on summer weekends. A study done in 2004 (Leighton et al.) turned up 44 cases in 2002, a few the year before, none before that. All the infestations were in August of both years; whether because of the warmth or the number of kids in the shallow water is uncertain. What is clear is that, if sensible precautions are taken, swimmer's itch should not be a worry for beach lovers. Just take to heart Douglas Adams' advice; always carry a towel!
"A towel, it* says, is about the most massively useful thing an interstellar hitchhiker can have. Partly it has great practical value - you can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a mini raft down the slow heavy river Moth; wet it for use in hand-to- hand-combat; wrap it round your head to ward off noxious fumes or to avoid the gaze of the Ravenous Bugblatter Beast of Traal (a mindboggingly stupid animal, it assumes that if you can't see it, it can't see you - daft as a bush, but very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough."*The Hitchhiker's Guide to the Galaxy Tweet