The prevalence and effects of the African pea crab Afropinnotheres monodi on the condition of the mussel Mytilus galloprovincialis and the cockle Cerastoderma edule

Abstract Some pea crabs are parasite of bivalves and may cause a significant effect on the host's condition. In this study, the prevalence of the pea crab Afropinnotheres monodi and its effect on host condition are assessed for two commercially important bivalves (the mussel Mytilus galloprovincialis and the cockle Cerastoderma edule ) from three natural beds of the Gulf of Cadiz (SW of Iberian Peninsula). The average prevalence of A. monodi ranged from 4.0% (Bay of Cadiz) to 51.4% (Carreras River) for cockles and from 30.5% (Bay of Cadiz) to 45.4% (Carreras River) for mussels. For both hosts, most bivalves were infested by a single pea crab (96.0% of cockles and 92.4% of mussels); among multiple infestations, one male and one female was the dominant combination (72.7% of cockles and 77.8% of mussels). Infested mussels showed a significantly decreased condition index (14.7%, on average), whereas crab infestation did not significantly affect the condition index of cockles (2.2%, on average). When the wet weight of the pea crab represented >5% of the wet weight of the host, an average condition loss of 12.7% and 15.9% were observed for cockles and mussels, respectively. As the relative condition loss of both mussels and cockles was negatively correlated with the relative size of the pea crab, the mentioned interspecific differences in the pea crab effects may be mainly due to the higher prevalence of soft females (larger size and obligatory parasitism) in mussels (70.6%, on average) and of hard females and males (smaller size and facultative parasitism) in cockles (81.1%, on average). This study highlights the importance of knowing the life cycle and host use of the pea crab A. monodi to ascertain the possible negative effect of this parasite on mussel farms and exploited cockle beds.