This green, 14-legged pod critter came home on the eelgrass I picked by the border marker. I had seen one swimming about, but by the time I turned the camera that way, it had completely disappeared. This guy can manage that, even in a small, lighted tank. He lies on the eelgrass, lengthwise along the blade; he just fits, and his colour matches exactly. His feet are little hooks that anchor him there securely, even in a strong current.
In the aquarium, I've run my fingers through the eelgrass, looking; no isopod. I worry; did he get eaten already? (And I blame the crabs and hermits; greedy beasts!) I look a bit later, and there he is, parked on a blade of eelgrass, just a hint of vibrating paddles (the uropods) at the tail revealing his presence.
He's an eelgrass isopod, Idotea resecata. The species is confirmed by the concave bite out of his tail; no other local isopod has that. And I think he's a male because of the green stripe down his back, visible in the top photo, above the dark line of the gut inside.
He will get to about 1.6 inches, not including the antennae; he's just over an inch now. I'll have to keep him supplied with fresh eelgrass, for resting and feeding. Maybe I can find him a female, too. It's springtime, after all.