Journal of the Marine Biological Association of the United Kingdom

Research Article

Molecular phylogeny of enigmatic Caribbean spider crabs from the Mithrax–Mithraculus species complex (Brachyura: Majidae: Mithracinae): ecological diversity and a formal test of genera monophyly

J. Antonio Baezaa1a2 c1, Juan A. Bolañosa3, Soledad Fuentesa4, Jesús E. Hernandeza3, Carlos Liraa3 and Régulo Lópeza3

a1 Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Ancón, Republic of Panama

a2 Smithsonian Marine Station at Fort Pierce, 701 Seaway Drive, Fort Pierce, FL 34949, USA

a3 Grupo de Investigación en Carcinología, Escuela de Ciencias Aplicadas del Mar, Núcleo Nueva Esparta, Universidad de Oriente, Isla Margarita, Venezuela

a4 NOAA Fisheries, Milford Laboratory, 212 Rogers Avenue, Milford CT 06460, USA


Crabs from the Mithrax–Mithraculus species complex are known for their diversity of lifestyles, habitats, and coloration. This group includes small, colourful, symbiotic species and much larger, reef-dwelling crabs targeted by fishermen. The evolutionary relationships between the species within this complex are not well-defined. Previous studies based upon morphological characters have proposed the separation of this complex into two genera (Mithrax and Mithraculus), but cladistic analyses based upon larval characters do not support this division. A molecular phylogeny of the group may help to resolve this long-standing taxonomic question and shed light on the ecological conditions driving the diversity of these crabs. Using a 550-bp alignment of the 16S rRNA mitochondrial DNA segment we examined the phylogenetic relationships between 8 species within the Mithrax–Mithraculus complex native to the Caribbean. The resulting phylogeny indicates that this complex is paraphyletic, as it includes the genus Microphrys. The analyses revealed a well-supported, monophyletic group containing four species of Mithraculus (M. cinctimanus, M. coryphe, M. sculptus and M. forceps) and supported one pair of sister species from the genus Mithrax (M. caribbaeus and M. spinosissimus). No complete segregation of species, according to genera, was evident, however, from tree topologies. Bayesian-factor analyses revealed strong support for the unconstrained tree instead of alternative trees in which monophyly of the two genera was forced. Thus, the present molecular phylogeny does not support the separation of the species within this complex into the genera Mithrax and Mithraculus. A review of the literature demonstrated considerable phenotypic variation within monophyletic clades in this group.

(Received May 19 2009)

(Accepted August 02 2009)

(Online publication November 23 2009)


c1 Correspondence should be addressed to: J.A. Baeza, Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Ancón, Republic of Panama email: