I strongly suspect that this crab does not feed on pytoplankton. I'm not 100% certain on this, but the feeding appendages that it uses for filterfeeding almost certainly are designed for larger plankton (zooplankton, I would think)...I have not seen your crab, but other porcelain crabs I have seen seemed to be designed for larger food particles. I'd guess that most phytoplankton would pass right through (and even if it did not, our reef tanks normally don't have enough phytoplankton to keep phytoplankton feeders well-fed anyway).
I have kept a small brown species of porcelain crab from the Gulf of Mexico a number of times. These were hitchhikers on large tunicates and barnacles I had shipped in for teaching purposes. These crabs did pretty well, and lived at least a year in tanks that had lots of small critters (mysid shrimp, etc). Though they probably were catching some of these critters, I think they were mostly eating the food being fed to the tank. When we fed, they would come out and start beating their feeding appendages (I think these are probably modified mouthparts..I need to look this up sometime) and would catch particles of food drifting past. They would not go out and forage for such food using their claws, but they seemed to be perfectly happy to eat it if they caught it in their filter-feeding appendages as it drifted past. We tend to feed our tanks in the lab a lot, and there would be quite a few food partcles drifting about for a little while after feeding, which increased the chances of the crabs catching some of it.
The foods I would try include crumbled flake food, small-particled foods such as the small-pellet version of vibragrow, misc frozen foods that have been mashed up a bit, etc.
An ideal food would probably be live
baby brine shrimp. I currently have some barnacles that are doing really well on live baby brine shrimp. Barnacles feed in pretty much the same fashion as procelain crabs (though the body plans of these two different filter-feeding crustaceans are quite different of course). I have always had these barnacles (ivory barnacles from the Gulf of Mexico) starve to death fairly quickly (in a week or two) in the past (in the same tanks where the porcelain crabs did OK), but after about 1.5 - 2 months they are doing well and even growing on this baby brine shrimp diet. They are in the breeder box with a bunch of my baby bangaii cardinals, so they are surrounded by a pretty high concentration of live baby brine shrimp several times a day. I'd guess that a porcelain crab would be quite happy in that breeder box.
Cyclop-eze would also be good to try.
You should be able to see whether or not your crab is catching food and eating, and whether it likes any of the foods that you try.
Feeding filter feeders like this in a reef tank can get you into trouble though, especially if it is a new system and you don't have lots of worms and other critters established to mop up the majority of the food that misses its intended target. You can quickly turn a new reef tank into an algae farm this way (I have done so, trying to feed barnacles pulverized dry foods back when my first reef system was young!). You might be able to get enough food to the crab though by a few gentle puffs of food a day in its direction using a turkey baster. Make sure it is out and beating its feeding appendages when you do this though.
If you have a small refugium-type tank that you could hang in the inside of your main tank, with some holes to allow some water circulation from the main tank (like my breeder box), you could probably concentrate the food for this crab better, giving it more time to feed and a higher probablility of catching enough to keep it healthy without dumping near so much food into your system.
I think this crab is going to need a decent amount of food on a regular (ideally daily?) basis. Though it is a filter feeder, it is an active animal, and it expends a lot of energy trying to filter plankton from the water (and in our tanks, it probably gets little or nothing except for when we feed). If you see it catching and eating a few decent sized bits of food (or a lot of tiny particles) each day my guess is that it will be fine. Think to yourself how much you would expect an animal that size to eat each day (e.g. if it was a "normal"crab like an
emerald crab, or sllay lightfoot)...I'd guess that your procelain crab will probably need a similar amount of food (??).
Bill
-A procrastinator's work is never done.-
