Pickle bug

I was pleased, last week, to get a recognizable photo of one of my rockweed isopods, on the far side of the tank. They are usually so tightly clasped onto the seaweeds that they become almost invisible, so that I can lift a rockweed out of the water, rinse it off, place it in a bowl, clean the tank, and replant the rockweed without noticing the isopod I've been handling until it decides to swim to a different clump of weed.

Last week, I was surprised to discover that there were two of these in the tank. Yesterday, cleaning the tank again, I found a third; no telling how long the three have been there.

And this time, one decided to explore the orangey algae collar of the big anemone, right up against the glass, where I could get a good look at him.

He's about an inch and a half long, not counting the antennae.

The tail end is rounded outward; the similar eelgrass isopod has a bite taken out of the tail.

These isopods have 7 legs on each side, each ending in a sharp, incurved claw, which they use to hold tightly to seaweeds waving in the current. They prefer rockweed or kelp, but I have found them on mud under rocks, on sea lettuce, and now on the delicate red algae around the neck of the anemone. They eat algae and other detritus.

The colour varies, according to the weeds they've been eating; the smallest of the isopods in my tank is a dark brown; this one is the brightest green.

Males are usually larger, and have fatter legs. The female broods her young; I'm going to be watching the smaller two closely, to see if I can catch one in berry.

Head shot, showing the kidney-shaped eyes, and one of the flat, hairy palps.

These are also known as kelp isopods, Wosnesenski's (or Vosnesenskys) isopod, olive-green isopod. And a name I hadn't seen before; pickle bug, because of the shape and colour of the greenest ones. The scientific name is Idotea wosnesenskii.