The symbiotic shrimp Ascidonia flavomaculata lives solitarily in the tunicate Ascidia mentula: implications for its mating system

Symbiotic crustaceans are expected to live solitarily with their hosts when members of their host species are small (relative to symbiont body size) and structurally simple. We tested the hypothesis of a solitary lifestyle in Ascidonia flavomaculata, a symbiotic shrimp that inhabits the branchial chamber of the relatively small and structurally simple tunicate Ascidia mentula in the subtidal zone of the Islas Baleares, Spain. We found that members of A. flavomaculata dwell as solitary individuals in the branchial chamber of ascidians at a higher frequency than expected by chance alone. Given this host use pattern of A. flavomaculata, we hypothesized that males actively move among host individuals in search of receptive females. We provide several lines of evidence consistent with that hypothesis. First, a positive correlation between shrimp and host body size was detected for females, but not for males, during one of the sampling seasons. If males as well as females spend long periods of time within their host individuals, a positive correlation between shrimp and host body size should have been found for both males and females. Second, the body sizes of individuals in the few male-female pairs observed during this study were poorly correlated. If males of A. flavomaculata shared their host individuals with females for long periods of time, a positive correlation between the sizes of males and females in a pair should have been found. Lastly, the body sizes of paired males were larger than those of solitary males during summer, when reproduction was more intense. This difference in body size between paired and solitary males additionally suggests competition among putatively roaming males for receptive females. Manipulative experiments and behavioral observations are necessary to reveal the details of the mating system of A. flavomaculata and other symbiotic crustaceans with a solitary lifestyle.