Species-specific fertilisation kinetics in two structurally distinct Desmophyllum cold-water corals

Abstract Cold-water corals have been increasingly studied over recent decades; however, quantitative information on their fertilisation kinetics is lacking. Here, we quantified and compared sperm motility, egg-mediated sperm activation, gamete contact time, and fertilisation assays in two cold-water coral species from the same genus: the colonial reef-forming coral Desmophyllum pertusum from Norway, and the solitary species D. dianthus from the deep-water emerged populations in Chile. Using a Bayesian modelling framework, we identified two distinct fertilisation strategies between species. The strategy displayed by D. dianthus was characterised by efficient fertilisation at low sperm concentrations and short contact times, whereas the strategy displayed by D. pertusum required sperm concentrations two orders of magnitude higher and at least twice the gamete contact time to achieve comparable fertilisation levels. In both species, the presence of eggs increased the proportion of progressively motile sperm, while swimming speed showed no biologically meaningful changes. Fertilisation success varied substantially among D. pertusum crosses but was consistently high across D. dianthus crosses. These patterns suggest contrasting reproductive strategies, with fertilisation in D. pertusum potentially influenced by cross-specific compatibility and synchronised spawning within dense reef structures, while D. dianthus , which typically occurs as scattered individuals in the deep sea, appears to rely on a highly efficient fertilisation mechanism. Together, these results establish a baseline for fertilisation kinetics in cold-water corals and enable comparison with fertilisation dynamics described for tropical broadcast-spawning corals. Such information is critical for informing conservation and management of vulnerable cold-water coral ecosystems.