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Volume 42 - Number 1 - 2007

The origins of symbiosis as a lifestyle in marine crabs (genus Petrolisthes) from the eastern Pacific: Does interspecific competition play a role?

J. Antonio Baeza1


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




The adoption of a symbiotic lifestyle is one of the main environmental adaptations in marine crustaceans. Interspecific competition might be considered important in explaining the evolution of symbiosis in this and other groups of marine invertebrates. Here, the historical origin of symbiosis as a lifestyle in marine crabs (genus Petrolisthes) from the eastern Pacific was examined, and the evolutionary role of intra-guild interspecific competition in explaining this ecological trait was explored. A previously published phylogenetic hypothesis for 29 species of Petrolisthes generated by Minimum Evolution and Maximum Parsimony methods of inference (using sequences of 16s rRNA mitochondrial DNA as a genetic marker) was used as framework for this analysis. When the lifestyle of each species was mapped onto the phylogenetic tree for Petrolisthes, a free–living habit was inferred to be the ancestral character state and symbiosis appeared to evolve independently at least twice in this group. Crabs with symbiotic associations occur in temperate waters in the southern hemisphere; Allopetrolisthes spinifrons inhabiting sea anemones, and Liopetrolisthes mitra inhabiting sea urchins. A comparative analysis by independent contrasts suggested that interspecific competition is not important in explaining either the evolution of symbiosis or the occurrence of less frequent partnerships between Petrolisthes crabs and other macroinvertebrates. On the other hand, the adoption of a symbiotic lifestyle has important consequences for the social life of crabs. Symbiotic crabs are proposed as model system to examine the role of the environment in shaping the social behavior of marine invertebrates


Key words: Symbiosis evolution, competition, phylogeny, comparative method, Petrolisthes



1Laboratorio de Zooplancton, Instituto de Ciencias del Mar y Limnología,Universidad Nacional Autónoma de México. A. P. 70-305, 04510 México, D. F. México
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