Reference: Binl. Bull 188: 32-45. (February/March. 1995) Prespawning Behavior, Spawning, and Development of the Brooding Starfish Leptasterias polaris JEAN-FRANCOIS HAMEL AND ANNIE MERCIER Depart ement d'Oceanographie, Universite dii Quebec a Rimouski, Centre Oceanographique de Rimouski, 310 allee des Ursulines. Rimouski (Quebec), Canada G5L 3A1 Abstract. Our study focused on the precise reproductive behavior of the starfish Leptasterias polaris (Miiller 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 cor-responded to field observations in the Lower St. Lawrence Estuary. In experimental tanks provided with natural en-vironmental 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, be-cause aggregative and spawning behaviors were not ob-served under stable temperature conditions. Male spawn-ing is first initiated when the temperature falls to about 2C during minimum daylength (<9 h-d '). In seawater, the spermatozoa are negatively buoyant and tend to de-posit 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 con-tact with the oocytes, ensuring fertilization. In the labo-ratory, 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. Introduction Successful fertilization constitutes a critical stage in marine invertebrate reproduction, and many organisms Received 7 December 1993; accepted 4 November 1994. develop strategies to maximize this important step (Him-melman. 1981; Giese and Kanatani, 1987). Starfish show diversified reproductive behaviors. In many species, ga-metes are broadcasted by both sexes, with fertilization in the water being enhanced by synchronization of spawning (Hyman, 1955; Strathmann, 1987; Chia and Walker. 199 1 ). In other starfish, males broadcast spawn in the usual fashion, and females emit fewer gametes but brood their embryos to fully developed young starfish (McClary and Mladenov, 1990; Chia and Walker, 1991). Leptasterias polaris, which protects its embryos for 5 to 6 months, is among the few species that brood by overlaying the eggs deposited on the substrate (Emerson, 1977; Himmelman et ai, 1982; Boivin el a/.. 1986). Although brooding star-fish are generally small-sized, with lecithotrophic devel-opment (Chia and Walker, 1991). L. polaris can reach diameters up to 50 cm (Boivin et al., 1986) and are prob-ably among the largest brooders. Prespawning and spawning behaviors are very impor-tant to reproductive success in marine invertebrates. Breeding aggregations have been observed in a number of asteroids (Chia, 1968; Komatsu, 1983; Minchin, 1987; Young et a/., 1992; Slattery and Bosch, 1993). Many au-thors suggest that such aggregations could minimize sperm dilution and increase fertilization success (Ormond et al., 1973; Levitan, 1991; Levitan et al., 1992), as exemplified by the pairing strategies in Archaster typicu.i (Run et al.. 1988) and Neosmilaster georgianus (Slattery and Bosch, 1993). In those species, the male, after finding a female, mounts her before spawning (Ohshima and Ikeda, 1934; Komatsu, 1983; Run et al.. 1988; Slattery and Bosch, 1993). There is also evidence that the spatial distribution of broadcast spawners has a major influence on the prob-ability of fertilization due to gamete viability (Pennington, 1985; Yund, 1990; Levitan et al.. 1992; Young et al.. 1992). Young et al. (1992) suggested that aggregations 32
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