Predominance of obligate outbreeding in the simultaneous hermaphrodite Celleporella hyalina sensu lato

Simultaneous hermaphroditism affords the potential for self-fertilization, which could be selectively advantageous under restricted opportunity for outbreeding. Such restriction may apply to free- spawning sessile invertebrates, such as bryozoans, when the vagaries of settlement place colonies at a distance from potential partners. Contrary to this scenario, however, most populations studied within the Celleporella hyalina species complex showed little or no evidence of self-fertilization. Most colonies kept in reproductive isolation failed to produce embryos or produced few, abortive embryos, whereas controls kept together in culture vessels copiously produced viable embryos. These results, together with previous work demonstrating severe inbreeding depression and tiie capacity of juvenile colonies to store allosperm, suggest that Celleporella hyalina is typically, although not exclusively, an obligate outbreeder.