Remarkable Consistency of Larval Release in the Spermcast-Mating Demosponge Amphimedon queenslandica (Hooper and van Soest)

ABSTRACT

Many marine invertebrates, including many sponge species, reproduce by spermcast spawning, in which sperm released externally disperse in the water column to fertilize eggs retained internally by the maternal adult. The population consequences of a sexual reproduction mode that depends upon uptake of free spermatozoa from dilute suspension in the water column are not yet well understood. In the spermcast-spawning tropical demosponge Amphimedon queenslandica, we observed continuous fertilization and development in healthy maternal brood chambers. This results in a constant release of larvae into the water column. On average in our study population on Heron Island reef, a hermaphroditic adult will have 45 potential sperm donors available within a 4 m radius to fertilize the eggs retained within its brood chambers. A single adult may brood more than 300 embryos at one time, and all stages of development are always represented. These data can be explained by adult sponges releasing a steady trickle supply of sperm into the water column, perhaps in combination with the existence of a mechanism for sperm storage or post-fertilization developmental stasis in this species.