On staples and little boxes

Sea lettuce. Barnacles. And eelgrass. The staple foods for my aquarium critters. Barnacles for the leafy hornmouth snail, sea lettuce for the hermits, the crabs, and the bubble shells. And eelgrass, preferably decorated with hydroids, for the hermits and snails. Anything else is a special treat, but these three are essentials.

This week, the high tide brought in a truckload of fresh, bright green sea lettuce, which pleased the bubble shells; one ate so much of it that I could see the green in his stomach right through the shell and flesh.

But though I walked a long way, just at the edge of the incoming waves, I only found one small eelgrass plant. And it was a meagre, frayed one, mostly straggly stem and browning leaves. I would have left it there, except that it was the only one available. I and the hermits would have missed a treat.

Settling it into the tank, I noticed a small patch of bryozoans on one thin blade of grass, just below the water surface. A live patch, too; I could even see, with a lens, movement on its surface.

I rarely get to see these; out of the water, they shut down instantly. Underwater, the turbidity and the depth make them into a faint blur. Too much light, and they're asleep. And the individual animals are so very tiny; millimetre-high, transparent funnels.

Encrusting bryozoan colony, Membranipora membranacea, awake and feeding.

This is a small colony, about 8 to 10 animals from edge to edge. Each individual zooid lives inside a little box; seen from above, they look like walls, but there is a top, as well. The animal lies horizontally inside, and when the situation looks right, extends its feeding funnel up into the water. At the slightest disturbance, the funnels disappear and all that can be seen are the walls.

Hydroids and anemones have stinging tentacles, to subdue their prey; these bryozoans do not, but are filter feeders like the barnacles, relying on water currents to deliver their groceries. They will eat diatoms and bacteria, as well as other planktonic swimmers, like my newly-hatched crab zoea.

The little spines at the corners of their boxes (difficult to see here, but we really need a microscope for a better view, like this one) help to make the colony an uncomfortable base for a hungry Doridella nudibranch. I found several of these a few years back, on kelp, eating bryozoans, spines or no spines. There were none on this little eelgrass; not enough prey to keep them here.

...

That was the beginning. I kept finding more and more interesting things on that eelgrass. Unidentified "thingies", tomorrow. And a thingie mimic.