There are two species of horshoe crabs. One which grows to an enourmous size (2 feet +) which can be seen of the eastern coast of the United states, and like the one pictured above with seafood and such. Then there is a smaller version, like the ones sold on SWF.com, these do NOT grown huge like the other ones. In fact they will satay small all their lives. The eat crap off the bottom of your tank which is a good thing, although they will also eat a little of your micro fauna possibly, but they are slow and never seem to cover the entire area of the sand bed, and in a large tank they will do just fine!
SWF.com suggests that they will do better in a FO tank rather than a reef tank. If you look at their description you will also get the information about them being a smaller species than those huge ones. The ones I have had would learn to accept flake food as well as frozen brine shrimp, although they are not too good at catching the stuff, it has to be sitting there on the bottom. I have had one that lasted about 9 months, but most of them die sooner. I would only reccomend them in a well established tank with LS and a lot of micro fauna and no triggers or puffers that might pick at them.
If you want something that you can see, then forget this little guy, cause he will be buried more than he will be out, and they are, for the most part, nocturnal. after spending 5 minutes doing a search online I have learned that there are in fact 4 spiecies of horseshoe crabs... the north American one being the most common and largest. The ones sold for marine aquariums will never reach 20 inches!! Also if you have LR a lot of oyur micro fauna will live in and under them in places your horseshoe crab cannot reach. It is impposible for them to consume your entire population of well- established micro fauna and then die a few days later. If this happens it is simply a water quality problem, a too stressful acclimation into your tank, or they were taken from the ocean and never got used to aquarium life. If your micro fauna all died, then you have a bigger problem then a horseshoe crab could cause. They will eat some good things in your reef tank that they can get to. but they are poor climbers and poorer swimmers, and under the right circumstances make an interesting part of your aquarium family. The main drawback in my opinion is their track record of longevity which doesn't seem to be good. If anyone wants to list their longevity reports with these guys that would be cool, maybe even here?
