Occurrence, infestation rate, and spatiotemporal distribution of the African pea crab (Afropinnotheres monodi) inhabiting Mediterranean mussels (Mytilus galloprovincialis) from southern Portugal

The present study aimed to analyze the occurrence and prevalence, infestation rate and intensity, host–guest colonization pattern, morphometric relationships, and spatiotemporal distribution of the African pea crab (Afropinnotheres monodi) inhabiting the shell of Mediterranean mussels (Mytilus galloprovincialis) from the Algarve coast and Ria Formosa lagoon (southern Portugal). Sampling campaigns were performed with a trimestral periodicity in 2018–2019 and comprised 13 collecting sites distributed in the Ria Formosa, along the Algarve coast, and in an offshore mussel farm. Altogether, 10,109 individuals of M. galloprovincialis in a broad range of sizes (9.0–91.2 mm in shell length) were sampled, among which 571 host mussels (14.6–73.6 mm in shell length) contained a total of 598 individuals of A. monodi, corresponding to an infestation rate of 5.6% (prevalence rate of 5.9% due to multiple occurrences). The sample of pea crabs also covered an extensive range of sizes (1.7–14.5 mm in carapace width) and included eight juveniles, 163 males, and 427 females (82 hard females and 345 soft females, of which 128 were non-ovigerous and 217 ovigerous). Overall, hard (13.9%) and soft (58.5%) females prevailed over males (27.6%), corresponding to female-biased sex ratio of 1 male:2.6 females. In terms of infestation intensity, 95.4% of host mussels contained a single pea crab, with multiple infestations (two or three crabs inside a single mussel) being more frequent in the lagoon than along the coast. Morphometric relationships between host shell length and pea crab carapace width were highly correlated (r values ranged .218–.631), although there were no significant differences between sexes or female demographic categories and maturity stages. Overall, the present study provided further insights into the colonization of M. galloprovincialis by A. monodi.