Detecting and quantifying male sea urchin spawning with time-integrated fertilization assays

There has been much modeling and experimentation on factors affecting fertilization success in free-spawning marine invertebrates, but for many species spawning has rarely been observed in nature, if at all, and then mostly qualitatively. Understanding the dynamics of spawning and fertilization is especially important for species, such as the green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller), that have been widely depleted by harvesting. Time-integrated fertilization assays provide measures of sperm-availability adjacent to spawning populations for extended periods and with unprecedented temporal resolution. In such assays unfertilized eggs are held in baskets permeable to sperm and suspended in the water column as long as the eggs remain viable. In a consecutive series of 24- to 72-h assays the time course and spatial extent of sperm availability were measured near an aggregation of 250 sea urchins suspended in a cage at a field site in West Boothbay Harbor, Maine, USA. The assays were conducted in March and April, 2000, the spawning season of the green sea urchin. The distribution of embryonic stages within the assays revealed fine-scale temporal information on the timing of fertilization. The time series of multiple assays indicated that over a substantial portion of the spawning season there was an almost continuous low level of male spawning, a pattern consistent with a slow decline in gonad index observed in both male and female urchins. Fertilization rates averaged only 17% for the eggs in the basket nearest the spawning urchins (20 cm away), and the rates declined sharply over a distance of <1 m. Thus, despite the potential benefits of long egg life in integrating sperm supply over several days, the spatial effects on fertilization success were marked. Owing to the urchins' captivity and the lack of site replication, the generality of the observed spawning pattern must be confirmed. Nonetheless, the method lends itself to wider application among natural populations.