PEMBERTON, AJ*; BISHOP, JDD; Marine Biological Association of the UK; Marine Biological Association of the UK and University of Plymouth: Plant-like mating in sessile animals by the release and uptake of aquatic sperm

In contrast to copulatory mating or broadcast spawning for external fertilization, a diverse range of marine animals release dispersing aquatic sperm which fertilise eggs that have been retained by the female; the resulting progeny are often brooded. This may be termed spermcast mating, and has apparent parallels with reproductive processes in some algae and with angiosperm mating by pollen dispersal. Studies on laboratory cultures of two sessile, modular, marine invertebrates with spermcast mating have revealed extensive similarities between the two species, despite their belonging to different major clades of the Metazoa. Both are self-sterile hermaphrodites. Sperm are long-lived following release, and may be utilised even after considerable dilution in the water column. Largely freed from the constraints of sperm limitation that threaten the success of broadcast spawning organisms, but without the massive insemination of sperm seen in copulating taxa, such species represent an intriguing middle ground for studies of sexual selection. Fertilization is truly internal, and follows extensive contact between sperm and maternal tissue. Mating is governed by compatibility systems. Compatible allosperm may undergo prolonged storage before syngamy occurs, allowing the gradual accumulation of male gametes by the female, potentially from different sources. In both species, the receipt of compatible allosperm triggers vitellogenic egg growth, which is otherwise deferred. In one species, the budding of entire female modules is also triggered by uptake of sperm. In the other, the size to which eggs grow depends on the identity of the sperm source.