Prespawning Behavior, Spawning, and Development of the Brooding Starfish Leptasterias polaris.

Our study focused on the precise reproductive behavior of the starfish Leptasterias polaris (Muller and Troschel) before and during spawning--a subject of much speculation and evident ecological importance. Between the third week of December 1992 and mid-January 1993, we observed spawning in the laboratory that roughly corresponded to field observations in the Lower St. Lawrence Estuary. In experimental tanks provided with natural environmental conditions, the spawning was preceded by 7 to 8 weeks of complex aggregative interactions among the starfish. The individuals, which usually avoid each other, began to make discreet arm contact, which intensified with time and eventually led to the superposition of two or more starfish, independently of sex. The interactions seem to be associated with decreasing temperature, because aggregative and spawning behaviors were not observed under stable temperature conditions. Male spawning is first initiated when the temperature falls to about 2{deg}C during minimum daylength (<9 h.d-1). In seawater, the spermatozoa are negatively buoyant and tend to deposit as a sticky film on the substrate, where they enter a state of low activity. Stimulated by male spawning, females spawn on the layer of sperm, which is reactivated by contact with the oocytes, ensuring fertilization. In the laboratory, the fertilized eggs undergo first cleavage in 45 h, become brachiolaria in 40 days, and form fully developed young starfish within 5.5 to 6 months, synchronously with populations in the field. The embryos develop at the same rate even when not brooded, suggesting that the brooding behavior in L. polaris serves mainly to keep the eggs clean, healthy, and protected against predation.