The third way: spermcast mating in sessile marine invertebrates

Marine invertebrates belonging to a broad range of taxa disperseaquatic spermatozoa to fertilize eggs that are retained ratherthan spawned. We outline the occurrence of this mechanism, whichwe refer to as spermcast mating, and identify tentative generalizationsrelating to it. Contrasts are drawn where appropriate with broadcastspawning of both eggs and sperm for external fertilization,and with copulation or pseudocopulation. Spermcast mating mayinvolve the gradual accumulation of long-lived spermatozoa fromdilute suspension, probably during suspension feeding, and thesubsequent storage of spermatozoa by the recipient (acting female)prior to fertilization. This process may involve extensive contactbetween spermatozoa and recipient (maternal) tissue. Matingmay be influenced by compatibility systems, and receipt of compatibleallosperm may trigger female investment, giving apparent scopefor sexual conflict over levels of maternal investment. Externalfertilization of cohesive egg masses remaining close to theacting female may appear somewhat intermediate between spermcastmating and broadcast spawning but, while it may be possibleto envisage a continuum between the 2 modes, the end pointsare distinct, commonplace, and involve contrasting reproductivecharacteristics. Three variants of the typical pattern of spermcastmating are briefly discussed: the spawning of zygotes (ratherthan the more usual brooding of progeny), polyembryony, andthe dispersal of spermatophores rather than individual spermatozoa.