Eocene origin of anemone-carrying behaviour in polydectine crabs (Brachyura: Xanthidae)

Abstract

The crabs in the subfamily Polydectinae (family Xanthidae) have adapted a defence behaviour in which living invertebrates are used as protection from predators. The polydectine crabs carry a living invertebrate, a sea anemone or nudibranch, in each claw, which is positioned in front of the body and waved to scare off attackers. In an attempt to trace the origin of this behaviour, we sequenced the complete mitochondrial genome of 19 crabs (encompassing 12 genera) from museum samples. The complete mitochondrial genomes were included in a larger dataset with previously published sequences and analysed using maximum likelihood and Bayesian inference. The divergence times of the polydectine crab radiation were estimated using the mitochondrial dataset and fossil calibration points. Our results show that the anemone-carrying polydectine crabs consist of at least three deep evolutionary lineages that evolved in the Late Eocene. All three lineages share the unique behaviour suggesting that it emerged in the ancestor to Polydectinae nearly 40 million years ago and has persisted in all living species.