Samuel M. Bashevkin, John H. Christy and Steven G. Morgan

Crabs have a larval life stage that occupies the plankton community until they grow large enough to enter the adult habitat on the seafloor. In the plankton, dark pigments protect larvae from damaging ultraviolet radiation (UVR), but these same pigments make plankton more visible to the multitudes of predatory fishes. Larval crabs have developed sharp spines to deter these fish predators. However, the extent of pigmentation and lengths of these spines varies widely among species of crabs, and even among individuals of the same species. We do not know why these traits are so variable or why all species do not express the strongest defenses against both threats.

We measured spine lengths, pigments, and body sizes in the larvae of 21 species of lab-hatched larvae and 23 unidentified species types of field collected larvae from Panama and California. Larvae with longer spines tended to have lighter pigments, and vice versa; larvae may be specializing in defending from either predators (long spines) or UVR (dark pigments). Furthermore, larger larvae had disproportionately longer spines, possibly because they are at higher risk of predation. Lastly, related species tend to have similar spine lengths and body sizes but not necessarily similar pigmentation levels. This may mean that the evolution of pigments is more flexibly and easily adapted to changing conditions than are spine and body sizes.

Our results improve understanding of how larval crabs defend against the perilous threats they face as members of the plankton community. We now know that they are not optimally defended from both visual predators and UVR and instead specialize. Their specialized defense may determine the habitat they can occupy since they will likely need to avoid threats from which they are poorly defended.

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