Right: Sony F-717 digital
Sunpak MZ4400AF flash off-camera
11mm (Macro) at f8
1/500 at ISO 100
Middle: Canon Elan IIe
380EX flash off-camera
Sigma 105 Macro w/ 36mm extension tube
Fuji Superia 400
1/125 second, f8, Macro Tank
Bottom: Sony F-717 digital
Sunpak MZ4400AF flash off-camera
9.7mm (Macro) at f8
1/500 second, ISO 100

Feeding Habits
green porcelain crab Petrolisthes armatus or spotted porcelain crab Porcellana sayana

The lagoon plays home to a truly stunning number of these little guys, which are porcelain crabs. Near as I can determine, I've had two subspecies in my tank, and may even be showing both in these photos – the green porcelain crab (Petrolisthes armatus) and the spotted porcelain crab (Porcellana sayana.) It's very difficult to tell, so one of these days I'll have to pester a marine biologist.

They're relatively benign little critters, charismatic as far as I'm concerned, who feed by straining microorganisms from the water with two seinelike appendages at their mouth, which can be seen at top – these are specially adapted versions of the little manipulative limbs, called millipeds, that most crustaceans have to handle food. They use a rapid alternating motion, 1 to 3 sweeps per second, which makes it hard to time a photo to catch them in a decent position.

But they also have huge flat chelipeds (pincers,) seemingly way out of proportion for their little oval bodies, and I'm not sure of the full functionality of these yet. Part of me suspects that they can press flat into the rocks and crevices, presenting a hard target for predators to pry loose. But I've also observed one, only once, jetting around the tank in midwater by using their belly 'scales,' much like shrimp do, and there's a vague suspicion that the pincers then come in handy for steering. Since I've only ever seen this once, I'm not putting a lot of faith in the idea yet.

green porcelain crab Petrolisthes armatus or spotted porcelain crab Porcellana sayana
green porcelain crab Petrolisthes armatus or spotted porcelain crab Porcellana sayana

If you were sharp-eyed (don't blame yourself – I missed it for a long time), you'll notice they only have three pairs of walking legs, which doesn't seem proper for crustaceans – normally there's four pairs. But with porcelain crabs, the fourth pair is there, much smaller and tucked up tight against the sides of their carapace. One can vaguely be seen here, folded alongside the shell at the base of the legs. I also haven't determined the purpose behind these yet, but suspect they're useful for keeping the carapace and gills clean. I've seen them used to dig under the carapace as if searching out an itch.

Like most crabs they vary in size, but typically get no bigger than 2.5 cm (1 inch) elbow-to-elbow. Fairly timid and non-aggressive towards intrusions, even when feeding, and they're considered beneficial to saltwater tanks. So they were welcome to stay for the entire time I maintained the tank, especially since I enjoyed watching them.