Further polyphyly of pinnotheroid crabs: the molecular phylogenetic position of the polychaete-associated Aphanodactylidae

Abstract

Pea crabs of the superfamily Pinnotheroidea De Haan, 1833 are known for their obligate commensal relationships with other marine invertebrates. The concomitant specialisations and adaptations of pinnotheroids have resulted in superficially similar body forms that include a high degree of structural reduction. This has confounded interpretation of their phylogenetic position in the Brachyura and interrelationships within Pinnotheroidea, though all were nevertheless believed to be united by a monophyletic origin of obligate commensalism. The family Aphanodactylidae Ahyong & Ng, 2009 was proposed for a group of genera associated with tube-dwelling polychaetes formerly classified in Pinnotheridae, and provisionally retained in the Pinnotheroidea. We investigated the phylogenetic position of Aphanodactylidae using molecular data from three markers (mitochondrial 12S and 16S rRNAs, and nuclear histone H3) covering five of the 12 described aphanodactylid species and a total of 15 thoracotreme families. We found Aphanodactylidae to be monophyletic, but widely distant from Pinnotheridae and instead most closely related to Macrophthalmidae (Ocypodoidea) and Varunidae (Grapsoidea). Therefore, the family Aphanodactylidae is corroborated, but its placement in Pinnotheroidea is rejected. Instead, the phylogenetic position of Aphanodactylidae, as clearly distant from other pinnotheroids, demonstrates that obligate commensalism has evolved independently multiple times within Thoracotremata.