Questioning success: Fertilisation outcomes in non-standard experiments in the sea-cake Dendraster excentricus

Abstract

Fertilisation in a laboratory environment is often not representative of ecological field conditions in which fertilisation naturally occurs, especially for broadcast-spawning species that release eggs and sperm into the water. Some challenges of broadcast-spawning include dispersal of gametes in the surrounding water, ability of eggs and sperm to interact, and competition from neighbouring male individuals. Here, we study the sea-cake or sand dollar Dendraster excentricus, a model study system, to understand how fertilisation success is affected by volume of water, varying sperm concentration, and mixed sperm from multiple males. Furthermore, we report sperm motility for D. excentricus. We found that with equal sperm concentration, fertilisation success was significantly lower in a larger volume of water compared to a standard 6-well laboratory plate. Finally, fertilisation was reduced by sperm from multiple males compared to sperm from one male at similar total concentrations. Our results elucidate how fertilisation success is related to conditions under which eggs and sperm meet and interact, and further highlight the challenges and remaining issues of gamete dispersal and gamete competition in external fertilisers, as is the case for many other echinoderms. Standard methods could potentially overestimate fertilisation success or increase the likelihood of polyspermia, and new techniques that more realistically reflect natural conditions could open new avenues for studying reproductive strategies and life histories beyond model organisms such as D. excentricus.