Inbreeding and hermaphroditism in the sessile, brooding bryozoan Celleporella hyalina

This study examined the occurrence and effects of inbreeding in the sessile , colonial hermaphrodite Celleporella hyalina. The results are discussed wi th regard to theoretical explanations for the prevalence of hermaphroditism in sessile clonal organisms. C. hyalina exhibited inbreeding depression at all stages, including the pre-zygotic. Outcrossing, sib and half-sib matin gs produced offspring but selfing did not. There was inbreeding depression in embryo production and survival. Inbred colonies showed slower growth and later maturation, with fewer reproductive zooids. The relative numbers of male and female zooids were affected by inbreeding; increased production of males is a sign of stress in C. hjalina. When mated with an outbred non-re lative, inbred colonies had lower success both as males and as females in e mbryo production and offspring survivorship. The low survivorship of embryo s fathered by inbred colonies is a clear effect of inbreeding on the F2 gen eration. These results indicate that C. hyalina is unlikely to inbreed in t he wild, supporting the "space-limited" model of hermaphroditism for this s pecies. This study indicates that hermaphroditism, by avoiding much of the cost of sex, can confer optimal reproductive fitness for sessile brooding a nimals in a space-limited habitat even in the absence of inbreeding.