ABSTRACT
Our study focused on describing the precise reproductive behavior of the starfish Leptasterias polaris (Müller and Troschel), before and during spawning, as it has been subject to many speculations and is of evident ecological importance. Spawning in laboratory was observed between the last week of December 1992 and mid-January 1993 and roughly corresponded to field observations in the St. Lawrence Estuary. In experimental tanks, provided with natural environmental conditions, this event was preceded by 7 to 8 weeks of complex prespawning interactions between the starfish. The individuals, which usually avoid each other, began to make discreet arm contacts, intensifying with time and eventually leading to massive superposition of two or more starfish, independently of sex. The interactions seem to be associated with decreasing temperature, since prespawning and spawning behavior cannot be observed under stable temperature conditions. Male spawning is first initiated when the temperature reaches 2 °C during minimum day length (<9 h-d−1). In sea water, the spermatozoa show negative buoyancy and have a tendency to deposit as a sticky film on the substrate where they enter a resting state. Stimulated by male spawning, females spawn on deposited sperm, which is reactivated by contact with the oocytes, ensuring egg fertilization to be followed by a long protective brooding period. In the laboratory, the fertilized eggs undergo first cleavage in 45 h, reach brachiolaria in 40 d and fully developed young in 5.5–6 mo, synchronously with populations in the field. The embryos develop at the same rhythm even when not brooded, suggesting that the brooding behavior in L. polaris serves mainly in keeping the eggs clean, healthy and protected against predation. Also, this work presents the first description of embryo development allowing a determination of developmental kinetic in asteroids under natural environmental conditions.
