Sex change and spatial distribution pattern in an intertidal holothurian Polycheira rufescens in the reproductive season

The patterns of sex change and spatial distribution in an intertidal holoth urian, Polycheira rufescens (Chiridotidae; Echinodermata), were investigate d on a stony beach in Amakusa, western Kyushu. Field caging experiments rev ealed that some individuals of P. rufescens underwent a sequential sex chan ge from male to female via hermaphrodite stages and back to male again with in a single reproductive season. The sex ratio of the population gradually changed from male dominance at first to equal proportions of males and fema les as the reproductive season progressed. Toward the end of the reproducti ve season, immature or spent individuals increased in proportion. Stone siz e appeared to be an important factor affecting the occurrence of P. rufesce ns individuals on a stony beach. Analysis of spatial distribution by means of Morisita's index of dispersion and nearest neighbor distances indicated that (i) males showed a slightly stronger tendency to aggregate than female s, while females had a tendency toward uniform distribution; (ii) females t ended to attract males, as shown by relatively short female-to-male distanc es; and (iii) there was a substantial variation in male-to-female distances , such that some males were positioned close to females while others were n ot. It may be suggested that spatial distribution of P. rufescens individua ls during the reproductive period is partly dictated by the differential ne eds of individuals of different sexual states.