Assessing the effect of environmental factors on the spawning activity of the sea urchin Strongylocentrotus intermedius through video recording observations

We used video recordings of spawning behaviour of the sea urchin Strongylocentrotus intermedius conducted in parallel with an analysis of gonadal state and measurement of environmental variables (tidal level, phytoplankton, temperature, salinity and oxygen concentration) in situ to study the role of environmental factors in triggering sea urchin spawning. The observations were performed during 3 reproductive seasons, each of 2 mo duration, in 2 bays with different levels of phytoplankton (measured as chlorophyll a [chl a]). The only significant positive correlation found was between sea urchin spawning frequency and chl a concentration. Despite a high level of gonad maturity, sea urchin spawning was rarely recorded in the bay with low levels of chl a whereas mass spawning occurred in the bay with higher chl a concentration, with the peak of spawning activity centred around the full moon. Spawning was predominantly nocturnal. Immediately before and during spawning, males actively moved. Females began to spawn only when there were several spawning males nearby. In both sexes, each spawning event included several episodes of gamete release of different duration and with different intervals between them. The same male likely can participate in several spawning events. The following hierarchy of drivers of S. intermedius spawning activity was revealed: (1) increasing the phytoplankton concentration induced active motion and spawning in males, (2) both an elevated phytoplankton level and the presence of sperm stimulated spawning in females and (3) the night time and the new and full lunar phases increased the probability of spawning.