The successful settlement of the African pea crab Afropinnotheres monodi in the Bay of Cadiz was analysed to get new insights into the evolution of pea crab parasite life history traits. The pea crab lives symbiotically and with high prevalence in the bivalves Cerastoderma glaucum and Mytilus galloprovincialis, and with low prevalence in Scrobicularia plana. A remarkable monopolisation of host (one crab/bivalve) occurred irrespective of host species and crab demographic categories (males, hard females, soft females), probably as an optimisation of resources in smallsize hosts. However, there was a clear asymmetry in host use by the different crab 2 categories. A 1:1 sex ratio was found in crabs harboured by C. glaucum and S. plana, with most of the female crabs being small and hard specimens, whereas crabs inhabiting M. galloprovincialis were primarily large reproductive females. Ovigerous females were found throughout the year in M. galloprovincialis and there was a strong correlation between female size and fecundity, suggesting that the females harboured by this host were the major contributors to the reproductive effort of the studied population. Conversely, most of the new crabs recruiting to the population were harboured by the remaining two host species. We hypothesise that such a generalist but asymmetrical usage of bivalve hosts by pea crabs may have clear benefits for species in geographical expansion as A. monodi, facilitating its dispersal to new locations and its successful settlement in sheltered systems, as the Bay of Cadiz.

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