Reference: Bin/ Bull 193: 20-29. (August. 1997) Effects of Spatial Distribution and Reproductive Biology on in situ Fertilization Rates of a Broadcast-Spawning Invertebrate RAFEL COMA 1 AND HOWARD R. LASKER 2 Department of Biological Sciences, Stale University of New York at Buffalo. P.O. Box 601 300. Buffalo. New York 'l 4 260-1300 Abstract. In situ fertilization was examined in the gor-gonian Psendoplexaiira porosa during 1994 and 1995 spawning events in the San Bias Islands. Panama, to as-sess spatial and temporal variation in fertilization suc-cess and to determine whether in situ fertilization was sperm limited. Fertilization rates did not differ signifi-cantly between years (60% vs. 55%), but monthly means were significantly different, ranging from 22% to 66%'. Fertilization rate varied among days, ranging from to 85%; 80% of this variability was explained by daily vari-ation in the number of colonies that spawned. A weighted average of in situ fertilization rates suggests that 67% or more of spawned eggs are fertilized in nature. Sperm limitation did not occur on the nights when most of the colonies synchronously spawned and when most of the eggs were released. Eggs collected downstream of the population often had higher fertilization rates than eggs collected either adjacent to their source colony or eggs collected in the middle of the population, which in-dicates that in dense populations, eggs may have multiple opportunities to be fertilized. Traits such as highly synchronous spawning, high fecundity, large egg size, large polyps, and large colonies directly and indi-rectly enhance P. porosa gamete production and fertil-ization. These life-history traits reduce the effects of ga-mete dilution during spawning events and thus decrease the importance of sperm limitation in the population dy-namics of P. porosa. Received 7 August 1996; accepted 2 June 1997. 1 Current address: Institut dc Ciencies del Mar-CSIC. Passeig Joan de Borbo s/n. 08039 Barcelona. Spam. 2 Author to whom correspondence should he addressed. Introduction Whether or not fertilization rates constrain the repro-ductive success of free-spawning organisms has become a subject of increasing interest in the last 10 years (see Levitan. 1995, for review). Despite a growing number of studies on fertilization rates, the questions still remain whether low fertilization rates are common, whether low fertilization rates are caused by sperm limitation, whether fertilization rates limit reproductive success, and whether morphologies and behaviors that enhance fertilization act as constraints on the life-history evolu-tion of broadcast-spawning taxa. We examined the fer-tilization rates of the Caribbean gorgonian Pseiuloplex-aura porosa in an effort to address these questions. The conclusion that fertilization success is an impor-tant factor controlling the overall reproductive success of broadcast-spawning species comes from three types of data: observations of low fertilization rates during natu-ral spawning events; experimental manipulations indi-cating that sperm density can limit fertilization success; and hydrodynamic models of gamete dilution. Low fer-tilization rates have been reported in taxa ranging from coelenterates to fishes (Petersen, 1991; Oliver and Bab-cock, 1992; Babcock and Mundy, 1992; Babcock el al.. 1992; Petersen el al.. 1992; Brazeau and Lasker. 1992; and Levitan, 1995). Moreover, fertilization rates are highly variable: even species with high average fertiliza-tion rates may exhibit cases of very low fertilization (e.g., Sewell and Levitan, 1992), and conversely, species with low fertilization rates sometimes demonstrate high rates when observed over many days (e.g., P. kuna; Lasker el al.. 1996). When they occur, low fertilization rates have generally
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