Why do some echinoderms gather together before spawning?

Many echinoderms aggregate before spawning primarily to increase the probability of successful fertilization of their eggs.12

Because these animals typically release their eggs and spermatozoa directly into the water column, gathering in close proximity ensures that gametes are more likely to encounter one another rather than being dispersed or lost in the environment. In addition to facilitating reproduction, aggregation in echinoderms may provide other ecological benefits, such as offering protection from predators and allowing individuals to better exploit food sources, such as plankton carried by strong currents.12