Tuesday, January 24, 2017

You are what you eat

Northern winters are tough on the little beasties that live at the intersection of sea and shore. Temperatures change, often drastically, in the course of a few hours. Storms toss whole colonies up on the beach, bury them in mounds of slimy, rotten eelgrass, drop logs on top of them. Cozy shelters in the seaweed are stripped bare; on the scoured stones, there is nothing left to eat, nothing to hide a scuttling crab from the myriads of hungry birds following the tide line.

My tank critters are spoiled. The temperature stays put; so does the water. Food drops from on high every day. There are no birds, and the cat doesn't relish getting wet.

Grainy-hand hermit on the remains of a kelp holdfast.

But even here, winter brings its losses. I comb the beaches year-round, searching for salad fixin's and gym equipment for the hermit crabs. In summer, there are bright sheets of sea lettuce, tall stalks of eelgrass, some loaded with delicious hydroids and diatom fuzz, knobbly Turkish towels and washcloths, long, green hair, stubby rockweeds, and more. But in winter? Bare stones, washed-out shreds of unidentifiable weeds, stripped of anything edible. And tea-leaf black, dried eelgrass.

In earlier years, the tank in winter gradually lost all its greenery, until the hermits wandered about disconsolately on bare sand and stone. I supplemented their diet with human food, sheets of green algae meant for sushi wrappings. They liked that, but it fouled the water.

The last couple of winters have been different. A red sheet algae established itself in the tank, and now grows as enthusiastically as the land plague, Himalayan blackberry. I have to keep cutting it back with every weekly water change. But at least there's something fresh to eat, and somewhere to climb.

Red algae is even trying to grow on the hermit's shell.

The change in diet has influenced the colours of some of the tank's residents. Red are redder, greens are browner. The colour of the shrimp's innards changes day to day, depending on his last meal; red today, transparent tomorrow.

And even the normally transparent copepods are striped with red and yellow. Look closely at the hermit above; there are a dozen copepods visible on his shell.

Three females with eggs, one male; the rest, I can't tell.

Most copepods have one red eye, a streamlined body, and a forked tail. The females carry their eggs around behind them; usually one pouch (Order Calanoida), sometimes two (Cyclopoida), one to each side. In the dark one at the lower left, the pattern is easy to see; red eye above, green bag beneath. The small, pale copepod at upper left may be a male of the Cyclopoida; his long, forked tail is just visible.

Drawing adapted from one by Jesse Cladgett.


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