Association between the pen shell Atrina tuberculosa and the shrimp Pontonia margarita

This is the first report on the monogamous and gonochoric shrimp Pontonia margarita found in the mantle cavity of the pen shell Atrina tuberculosa. The mantle cavity of 25 pen shells A. tuberculosa, collected from a sand bank revealed the presence of the shrimp P. margarita which was found in all the A. tuberculosa collected (ranging from 210 to 266 mm in shell height). The shrimp size increased with the size of the host pen shell (r = 0.81for males, and r = 0.76 for females, P < 0.05). P. margarita measurements indicated that male shrimps were smaller than females. Thus, P. margarita is a species with reversed sexual dimorphism. The positive relationship between shrimp size and pen shell size, a prevalence of male–female pairs of shrimp (sex-ratio was 50 % males and 50 % females) and morphological measurements, suggest that a long-term symbiotic association exists between them, and that the mating system of the shrimp involves social monogamy.