CAN SMALL MALE SHRIMPS ACHIEVE COPULATION IN THE PRESENCE OF LARGER ONES

The capacity for insemination and competitive ability of small males o f the partially protandrous alpheid shrimp Athanas kominatoensis Kubo which lives in symbiosis with the Japanese purple sea urchin Anthocida ris crassispina (A. Agassiz) were examined. Although male shrimps 1.5 mm in carapace length or larger produced sperm, it was not until they grew to 2.4 mm that they became functional. Males lost their inseminat ion ability when their endopodite (or the whole) of the 2nd pleopods w ere amputated. The amputation of the 5th pleopods did not affect the i nsemination ability. The males also lost insemination ability after be ing infected by 2 species of Isopod ectoparasites, probably because of the transformation of the 2nd pleopods. Small males were shown to be able to achieve copulation in cohabitation experiments with amputated or parasitized large males. The total parasite infection rate in the f ield was highest in December and lowest in May. From August to October , when the small and large males coexisted, on the average 15.9% of la rge males were infected and transformed by the parasites. The presence of parasites offers the small males an opportunity for reproduction a nd is thought to play an important role in the evolution of protandry in this shrimp.