Take a look at the three pictures to the right (Figs. 1-3). First you have the delicious snow crab, named for its sweet, delicate, snow-white meat. Next you have the famous and gigantic red king crab from the Discovery Channel’s “Deadliest Catch” series. Finally, you have a tiny and cute hermit crab like the ones that you have surely tormented during one of your beach vacations. Based on those pictures, what crabs do you think are more closely related to each other?
Unless you are an accomplished invertebrate zoologist working on crabs, your most likely answer will be that snow crabs and king crabs look quite similar and therefore are more related. At first glance it is a reasonable answer; however, appearances can be deceiving. In fact king crabs and hermit crabs are the most closely related. The snow crabs belong to brachyurans, or true crabs, which typically have a crab-like form with a very short body hidden under the thorax and ten visible legs. In contrast, king crabs and hermit crabs belong to the anomurans, or false crabs, which have a wide variety of body forms and only eight visible legs - the last pair being reduced and hidden.
King crabs are among the largest crustaceans worldwide, hard-bodied and free-living, whereas hermit crabs are small, soft-bodied and depend on a shell for protection. Although you would not think these two are related because their striking differences, an evolutionary connection between both groups has been recognized since the 19th century. When one examines their anatomy in detail, some features clearly reveal their relatedness. Many hermit crabs have an elongated and asymmetrical coiled body concealed in an empty shell, and traces of this asymmetry can also be observed in the shortened and folded body of the king crabs. Moreover, both groups have asymmetrical claws, with the right one being bigger than the left. These morphological commonalities have recently received further support from DNA data, and mounting evidence depicts an evolutionary scenario where king crabs were derived from a hermit crab-like ancestor.
The process whereby a crustacean evolves into a crab-like form from a non-crab like ancestor is known as carcinization, and it generally involves the acquisition of a broad, calcified carapace along with reduction of the body, which is fully or partially folded beneath the carapace. This is believed to have happened multiple times in different groups of false crabs porcelain, hairy stone, mole and king crabs with the latter being the most remarkable example (Figs. 4-7 to the left). No other crustacean more closely approximates the crab-like shape of true crabs than king crabs do.
As different as the hermit and king crabs may seem, scientific evidence helps us to imagine how evolution surprisingly shaped the insignificant and inedible hermit crabs into the mighty and tasty king crabs. Understanding the morphological transformations that took place in the hermit to king transition remains one of the biggest mysteries of crustacean systematics, and unraveling it will be my challenge. The Department of Invertebrate Zoology at the National Museum of Natural History USNM holds what is arguably the world’s largest collection of hermit and king crabs, along with cutting edge molecular lab facilities. Thus, this will provide me with a unique opportunity to make use of traditional and modern techniques to address this enigmatic evolutionary question.
References
Bracken-Grissom, H. D., Ettinger, M., Cabezas, P., Feldmann, R. M., Schweitzer, C. E., Ahyong, S. T., Felder, D. L., Lemaitre, R. & Crandall, K. A. (2013) A comprehensive and integrative reconstruction of evolutionary history for Anomura (Crustacea: Decapoda). BMC Evolutionary Biology, 13, 128.
Cunningham, C. W., Blackstone, N. W. & Buss, L. W. (1992) Evolution of king crabs from hermit crab ancestors. Nature, 355, 539-542.
McLaughlin, P. A. & Lemaitre, R. (1997) Carcinization in the Anomura-fact or fiction? I. Evidence from adult morphology. Contributions to Zoology, 67, 79-123.
McLaughlin, P. A., Lemaitre, R. & Tudge, C. C. (2004) Carcinization in the Anomura-fact or fiction? II. Evidence from larval, megalopal and early juvenile morphology. Contributions to Zoology, 73, 165-206.
Tsang, L. M., Chan, T. Y., Ahyong, S. T. & Chu, K. H. (2011) Hermit to King, or Hermit to All: Multiple transitions to crab-like forms from hermit crab ancestors. Systematic Biology, 60, 616-629.
by Patricia Cabezas, IZ Postdoctoral Fellow