The fascinating subfamily Polydectinae

Hark, O readers of the Keeper Stories! I—Daniel!—have returned! And this time we’re going to talk about boxer crabs. Why boxer crabs? While we don’t have this species at Zoo Atlanta, I happen to be particularly especially very much interested in crabs, and thought this could be a nice deviation from the normal scaly or slimy post from our department (those are still awesome by the way!). Therefore, let’s talk boxer crabs!

Also known as pom-pom crabs, boxer crabs are in the subfamily Polydectinae which has only around a dozen species, all native to shallow waters around coral reefs in the Indo-Pacific. They’re small and incredibly cute, so they’re popular in the aquarium trade. Their most distinctive characteristic is the reason for their common names: they carry a pair of small anemones, one in each claw, and ‘box’ them at passersby, in a manner vaguely reminiscent of a cheerleader’s pom-poms.

Now, the important bit of this blog: if you look at any pop science article on this family, or watch a nature documentary that features them, they’ll tell you that the crabs carry the anemones for defensive purposes and swing them around to deter predators, which is a lovely example of symbiosis. And that MAY be true! But I have read every single scientific paper that has ever been published on these crabs (which is not very many), and I am here to tell you that, at the very least, that’s an oversimplification. This is an area of active research (or at least it will be if I am able to fund my graduate research), and there are a ton of unanswered questions about the crab-anemone relationship, but there’s a lot more going on than is obvious at first glance.

Symbiotic relationships between crabs and anemones are common and aside from boxer crabs, are well understood. The obvious parallels here are the decorator crabs (superfamily Majoidea) and hermit crabs (family Paguridae) that place anemones on their shells and walk around with them. Those are true mutualisms; both the crabs and the anemones live their lives largely as they would have individually, but the crab benefits from the anemone’s defenses and the anemone benefits from the crab’s mobility and access to food. However, there are several major differences when compared to boxer crabs.

The first is that the boxer crabs’ anemones are tiny compared to the size of the crab, to the degree that it’s not clear how useful they really are for defense. With other crabs, the anemones are large enough to provide an obvious deterrent (in the hermit crabs’ case, the cloak anemones they use often cover their entire shells), but with the boxer crabs it hasn’t yet been established if the anemones can give a noticeable sting to predators big enough to eat them. Functionally nothing is known about the anemones—many of the species that pair with boxer crabs haven’t even been identified, much less studied—so it’s hard to say much for sure about their defensive properties. There are a few videos out there of fish appearing to contact the anemones and moving backwards, so it’s possible that they sting enough to keep some fish away, but the crabs move so fast that it’s very hard to effectively interpret these interactions with the naked eye. The only formal study ever conducted on their defensive behavior found that in intraspecific combat (that is, boxer crab versus other boxer crab), they appear to swing the anemones at each other but carefully avoid ever making contact. My conclusion after poring through the literature was that boxer crabs probably use their anemones defensively, but that hasn’t been proven. They do wave the anemones at anything that approaches. That may be purely visual signaling rather than a threat of stings (arrow crabs at cleaning stations do the same thing, for example, without anemones). Ultimately, for the moment we don’t know anything about the behavior, which predators it might be aimed at (hermit crabs use their anemones to deter octopi, for example), how much the boxer crabs rely on it over other strategies, or anything else about it.

The second major difference between boxer crabs and other anemone-using crabs is that others just place the anemones on their backs and then go about their crabby business, whereas the boxer crabs carry them in their claws. In fact, they’ve completely given up the use of their claws for other things; they’re specially shaped for holding the anemones, and are too frail to be useful for other tasks even if the anemones are removed. That’s a huge tradeoff, for a crab; their claws are their main tools for nearly everything they do, so the fact that they’ve specialized so strongly towards carrying the anemones is striking, when the example of the other crabs would suggest that just sticking the anemones on their backs would have worked at least as well if all they wanted was a layer of defense.

Now, I hear you asking: “But, Daniel, why would boxer crabs carry anemones around if not for defense? What else could they be doing with them?” Well, that’s the million dollar question, isn’t it? (Actually, the $37,000 question if someone funds my grant proposal) I don’t know the answer, but a few hypotheses have been suggested.

First, they might use the anemones to passively collect food, waving them around to filter small particles out of the water and eating the particles off of them. Boxer crabs unquestionably do eat food particles that stick to their anemones, but we don’t have any data on whether those particles account for a significant part of their diet, or whether the nutrition they get from that is just a side benefit of keeping the anemones clean, or from taking the food away from the anemones so they don’t grow too large to carry. In fact, we don’t know anything about their natural diet; we know they’ll eat frozen brine shrimp in human care, but nobody’s ever studied it in the wild. If you take the anemones away from boxer crabs, they can clumsily pick up food particles with their walking legs, so we know they’re not completely reliant on the anemones to feed themselves. We don’t know!

Secondly, they might use the anemones for visual signaling, either with predators or with other boxer crabs. A number of crustaceans use brightly colored waving legs as visual signals, but the anemone thing would be a vastly higher investment than just having some yellow stripes on the legs. There’s absolutely no information whatsoever related to this, although it has been theorized by some authors. You’ll notice that’s been kind of a recurring pattern here.

That was a quick and unscientific summary of one (one!) unanswered question about the boxer crab-anemone relationship. There are many (many many) more, at least as complicated as that one. For example, if a boxer crab loses one of its anemones, it responds by tearing the remaining one in half to make a replacement, effectively causing it to reproduce asexually. In some of the anemone species involved here, that appears to be the primary way the anemones reproduce; nearly all of them are clones of ones held by other crabs. That suggests that the crabs are exercising an enormous amount of control over their anemones’ reproduction, to the extent that it might even be fair to call them “domesticated.” What does that imply? We have no idea. Probably a lot, though. Also, this really looks like not just tool use, but tool production, which would be extraordinarily unusual in an invertebrate, but nobody’s ever studied the system from that angle.

I had several more pages of material here, but I have to wrap this up at some point, and none of the rest of it was any more satisfying than that last paragraph was; it was basically more lists of things we don’t know. I will leave you with one last incredibly weird fact: There is at least one species of boxer crab that, instead of carrying anemones, instead carries a pair of small nudibranchs. Now, if you’re not big on marine invertebrates, it may not be immediately obvious just how weird that is. Nudibranchs are sea slugs! They’re active, mobile animals! And how do they even catch nudibranchs, or convince them to stay in their claws? How could that have possibly started? What does any of this mean? Somebody should work on that!

If you found this distraction from the usual reptile and amphibian-related content you’re used to interesting and want to read more papers about how we don’t know anything about boxer crabs, feel free to consult the references below! Thanks for indulging me!

(Lybia tessellate photo: Wikimedia Commons)

References:

Antoniadou, C., Vafeiadou, A.M., & Chintiroglou, C. (2013). Symbiosis of sea anemones and hermit crabs in temperate seas. In Camisão, A. F. & Pedroso, C.C. (eds), Symbiosis: Evolution, Biology and Ecological Effects. NOVA Science, New York: 95–118.

Baba, K., & Noda, H. (1993). A rare collection of a small species of Gymnodoris (Nudibranchia: Polyceridae) held alive by the chelipeds of the crabs, Lybia hatagumoana (Brachyura: Xanthidae), from the bottom off Kanayama Bay, Kii, Japan. Venus 52: 283–289.

Borradaile, LA. (1902). Marine crustaceans III: The Xanthidae and some other crab. In Gardiner, JS. (ed), The fauna and geography of the Maldives and Laccadive archipelagos, vol 1, part 3. Cambridge: University Press. 237-271.

Duerden, JE. (1905). On the habits and reactions of crabs bearing actinians in their claws. Proceedings of the Zoological Society of London 2: 494-511.

Guinot, D. (1976). Constitution de quelques groupes naturels chez les crustaces decapodes brachyoures l La superfamille des Bellioidea et trois sous-familles de Xanthidae (Polydectinae Dana Trichiinae de haan Actaeinae Alcock). Mémoires du Muséum national d’Histoire naturelle (A) 97: 1-308

Guinot, D., Doumenc, D., & Chintiroglou, C.C. (1995). A review of the carrying behavior in brachyuran crabs with additional information on the symbioses with sea anemones. Raffles Bulletin of Zoology 43: 377-416.

 

Karplus, I., Fiedler, G.C., & Ramcharan, P. (1998). The intraspecific fighting behavior of the Hawaiian boxer crab Lybia edmondsoni—fighting with dangerous weapons? Symbiosis 24: 287-301.

 

Mendoza, J.C.E., & Ng, P.K.L. (2011). The Polydectinae Dana, 1851, of the Philippines, with description of a new genus for Lybia hatagumoana Sakai, 1961 (Crustacea: Decapoda: Brachyura: Xanthidae). Zootaxa 3052: 51–61.

 

Mori, M., Abelló, P., Mura, M. & De Ranieri, S. (1995). Population characteristics of the crab Monodaeus couchii (Crustacea, Brachyura, Xanthidae) in the Western Mediterranean. Misc. Zool. 18: 77-88.

Ross, D.M. (1974). Evolutionary aspects of associations between crabs and sea anemones. In Vernberg, W.B. (ed) Symbiosis in the Sea. University of South Carolina Press, Columbia.

Ross, D.M. (1979). Stealing of the symbiotic sea anemone Calliactis parasitica in intraspecific and interspecific encounters of 3 species of Mediterranean pagurids. Canadian Journal of Zoology 57(6): 1181-1189.

Sakai, T. (1961). New Species of Japanese Crabs from the Collection of His Majesty the Emperor of Japan. Crustaceana, 3(2): 131-150. Retrieved from http://www.jstor.org/stable/20102431

Schnytzer, Y., Giman, Y., Karplus, I., & Achituv, Y. (2013). Bonsai sea anemones: growth suppression of sea anemones by their associated kleptoparasitic boxer crab. Journal of Experimental Marine Biology and Ecology 448: 265-270.

Schnytzer, Y., Giman, Y., Karplus, I., & Achituv, Y. (2017). Boxer crabs induce asexual reproduction of their associated sea anemones by splitting and intraspecific theft. Peer J 5:e2954

Schmitt, W. L. (1965). Crustaceans. Michigan, Ann Arbor Science Paperbacks, 204 pp.

 

Daniel E.
Seasonal Keeper, Herpetology