All divers interested in critters sooner or later will get interested in crustaceans. Some of the crustaceans are considered “must see” items, such as harlequin shrimp, tiger shrimp, several of the extremely small shrimp recently found here in Lembeh, emperor shrimp and a host of other shrimp. The keyword here is shrimp. Crabs on the other hand seldom elicit the same interest from divers. However, many crabs have at least as interesting life styles as shrimp, and many are also colorful and attractive, such as the amazing xeno crabs.
As those of you that have read my earlier posts might already understand, one of the aspects I find the most interesting when diving is the close symbiotic relationship between many marine animals, where one animal provides another animal with living quarters. Most crabs are free living and do not show symbiotic relationships to other animals. In contrast, here in Lembeh some really fascinating examples of symbiosis between crabs and other animals can be studied.
First of all, unknown to me until my last visit, there is a crab that quite violently excavates a hole into a soft coral and then lives there. It doesn´t seem to matter much what kind of a soft coral it is. This crab is extraordinarily hard to find, as it most of the time is totally buried into the coral. It comes in a couple of color varieties, which seems to some extent be related to which host they inhabit. This of course indicates that the crab sometimes will be outside its hole and walking around on the coral, using the color to blend in, but despite looking a lot for the crab I have never found it outside its lair.
The second crab on my list does a similar, but less violent, excavation into the rope sponges littering the sea floor in dive sites such as TK. This crab does not disappear into the sponge but at least during daytime it clings to the sponge, fitting into the excavation it has dug so tightly that ii is nearly indistinguishable from the sponge.
Third on my list, and a crab that I always look for but very rarely find, is the Xenia crab (not to be confused with the Xeno crab). This crab lives its life in between the polyps of some of the species of the pulsating Xenias soft corals.
Fourth are the swimming crabs living their whole life on a sea cucumber. These crabs are very flat, and have hooks on their legs. They use these hooks to attach their legs to the skin of the sea cucumber, and with the help of their claws they are able to pull the skin of the sea cucumber partly over them selves, thus being very hard to find for a predator. When they get tired of life on the outside of the sea cucumber, they actually hide themselves inside the ass of the host. Next time you see a sea cucumber, take a look up its butt. There just might be a crab looking out on you from the inside!
Fifth, and probably the most beautiful of the symbiotic crabs, are the soft coral crabs, or candy crabs as they are known as. These tiny crabs have small hooks on their exoskeleton where pieces of their host Dendronephtyid soft coral will be attached. The crab will also color match itself to the coral, so finding those crabs are best left to an experienced guide.
Finally, and maybe in many ways the weirdest and most special of the crabs here in Lembeh, are the Xeno crabs. These crabs live their whole life on a single wire coral, where they attach polyps from the wire coral to their exoskeleton, thus decreasing the risk that any fish will pick them of the wire coral.
Interestingly enough, very little is known about the details of any of the symbiotic relationships I have described here. Without doubt there is a career for a PhD-student interested in cool crab life styles waiting to be done. It is quite evident that some of the crabs incur a cost for their hosts, but it could also be that the hosts in some way has a benefit from its lodger. The quantification of costs and benefits for those relationships is probably quite hard to do, but wouldn’t it be the dream job of all!