ICES mar. Sei. Symp., 199: 13-18. 1995 Potential depensatory mechanisms operating on reproductive output in gonochoristic molluscs, with particular reference to strombid gastropods Richard S. Appeldoorn Appeldoom, R. S. 1995. Potential depensatory mechanisms operating on repro­ ductive output in gonochoristic molluscs, with particular reference to strombid gastro­ pods. - ICES mar. Sei. Symp., 199: 13-18. Molluscs are typically sessile or slow moving, yet successful reproduction requires close proximity to potential mates. Three mechanisms are identified whereby repro­ ductive potential of a population can be limited under conditions of low density. The first is the reduction in numbers of spawners as abundance decreases. The second reflects the difficulty in finding mates, and takes the form of either (1) search time for slow but motile species, or (2) wasted spawning (or non-spawning) for sessile species, where gametes are not fertilized. The third mechanism is a breakdown of a positive feedback loop between contact with males (either direct or through chemical cues) and rate of gametogenesis and spawning in females, i.e., sexual facilitation. The second and third mechanisms are functions of local density, rather than overall abundance. Literature review and present studies on strombid gastropods indicate the potential for these mechanisms to occur. Many species are characterized by behaviours, such as aggregative settlement, that serve to overcome this problem. Intensive fishing prac­ tices may invoke these depensatory mechanisms as local density and abundance are reduced, thereby increasing the chance of recruitment failure. Richard S. Appeldoom: Department of Marine Sciences, University of Puerto Rico, Mayagiiez, Puerto Rico 00681-5000 [tel: (+809) 899 2048, fax: (+809) 899 5500], Introduction dation, cannibalism, competition, and starvation) occurring during early life stages. Only a few studies Successful reproduction is essential to stock mainten­ have emphasized mechanisms operating at other life ance. This salient point has been recognized both in stages, particularly those controlling reproductive nature and in management. Strong selective pressure potential (factors affecting fecundity, egg size, etc.) has, over evolutionary time, resulted in reproductive (Larkin et a i, 1964; Nikolskii, 1969; Bagenal, 1973; behaviours consisting of multiple mechanisms acting in Ware, 1980). The majority of these are concerned, as parallel to ensure successful reproduction (Audesirk above, with compensatory mechanisms. Larkin et al. and Audesirk, 1985). In fisheries, intense exploitation (1964) recognized the importance of possible depensa­ can adversely affect stock maintenance by reducing tory mechanisms, i.e., those mechanisms acting to dis­ spawning-stock size to a point were reproductive poten­ proportionately reduce recruitment when stock size is tial is insufficient to ensure adequate recruitment. Re­ reduced. However, their treatment of this was limited to cruitment overfishing can have serious consequences, the effects of predation. and its prevention has become a principal objective of This article reviews possible depensatory mechanisms management theory and practice (Gulland, 1983). affecting recruitment in molluscs, through control of Theoretical approaches to stock-recruitment re­ reproductive potential, at both the individual and popu­ lationships have largely been concerned with compensa­ lation levels. 1 do not intend to argue here that these tory mechanisms (e.g., Ricker, 1954). Such mechanisms mechanisms are of overriding importance and must be serve to maintain stock-size stability by increasing re­ considered in the practical management of molluscan cruitment rate at low spawning-stock levels and de­ resources, although there may be species where this is creasing recruitment at high stock levels. Additionally, so. My purpose is to explore the evidence for their emphasis has been placed on mechanisms (e.g., pre­ existence and possible significance in an attempt to 14 R. S. Appeldoom ICES mar. Sei. Symp., 199 (1995) understand reproductive processes and their population may result from an excess of eggs being produced, consequences more fully. whereupon recruitment is primarily independent of Throughout this article particular emphasis is given to stock size and dependent upon environmental varia­ conchs, large marine gastropods of the genus Strombus. bility. However, where environmental factors have been Conchs support a valuable commercial and recreational accounted for, underlying stock-dependent recruitment fishery throughout the Caribbean region, with the queen has been found (e.g., Crecco et al., 1986). The effect conch (S. gigas) being most important. Conchs repeat­ may also go unobserved because some compensatory edly copulate and spawn over a protracted spawning mechanism is operating, such as increased survival season, with maximum rates of spawning occurring among eggs and larvae. However, as stock size declines during summer months (Randall, 1964; Davis et al., a point is reached beyond which the loss of egg produc­ 1984; Weil and Laughlin, 1984). Females lay demersal tion cannot be compensated. egg masses and can store viable sperm from a single This first mechanism is obvious. In conjunction with copulation for several weeks (D’Asaro, 1965; Weil and age-specific fecundity rates, it is usually incorporated Laughlin, 1984). Spawning occurs at discrete sites. Esti­ into models attempting to quantify the effects of harvest mates of spawning rate are few. For queen conch, Weil strategy on reproductive potential (e.g., Botsford and and Laughlin (1984) reported an average of 8.7 days Hobbs, 1986; Prager et al., 1987). between spawnings in isolated females during the height of the spawning season. Davis and Hesse (1983) and Davis et al. (1984) reported a maximum rate of 8.3 days Mechanism 2: Number of eggs fertilized (via between spawnings. Averaged over the entire 6-month copulation) or fertilization rate (broadcast season, the time between egg masses per female was spawning) are a positive function of density 24.4 days. Maximum observed spawning rates for S. In molluscs, because they are sedentary or slow moving, pugilis are somewhat higher (Reed, 1992). Taking an stock density is also important. In conchs, males must average of 400000 eggs per egg mass (Thorson, in find and copulate with females. As long as individuals Robertson, 1959; Randall, 1964; Weil and Laughlin, are aggregated this should not be a problem. However, 1984), with an egg mass spawned every 24.4 days over a in heavily fished areas aggregations are destroyed when 6-month spawning season, a female S. gigas would be harvested, with only scattered individuals remaining. expected to spawn 3000000 eggs/year. This estimate is When this happens there is added a “search time” cost to similar in magnitude to those for other large molluscs reproduction, both in terms of energy and time. Conchs (see, e.g., Galtsoff (1964) for oysters; Hahn (1989) for unable to find mates at all or at a rate insufficient in abalone). Conch eggs hatch in a few days, producing comparison to their rate of gametogenesis will not repro­ planktotrophic veligers with an estimated larval life of duce to their full capacity. This density effect is additive two to five weeks (see review in Mitton et al., 1989). to the stock abundance effect given above. This mechanism ought to operate similarly in sessile species with broadcast spawning. Here the loss at low Depensatory mechanisms density takes the form of wastage of gametes not ferti­ lized. Low densities of eggs and sperm will reduce the Presented below are three potential mechanisms relat­ probability of their contact (demonstrated by Levitan ing stock to reproduction and recruitment, one depen­ (1991) for sea urchins). dent upon total abundance and two dependent upon This effect of density on reproduction, although density. The former is characteristic of all species; occur­ recognized (Thorson, 1950; Audesirk, 1977), has not rence of the latter two will depend upon species-specific been studied specifically for any mollusc. Yet, its im­ characteristics. After each mechanism is defined, evi­ portance is manifested by the number of mechanisms dence for their existence is reviewed. evolved to counteract the effects of low density, and the use of more than one mechanism by many species. These are briefly reviewed below. Mechanism 1: Total reproductive output is a Although interest here is with gonochores, it is worth positive function of the number of spawners mentioning an evolved strategy that specifically acts to The influence of total abundance follows along the lines offset low density: hermaphroditism. Theoretical of typical stock-recruitment relationships, as observed studies have generally shown hermaphroditism to be or implied in other species. As abundance of spawners increasingly advantageous as density decreases or declines there is a concomitant reduction in egg produc­ motility is reduced (Tomlinson, 1966; Ghiselin, 1969). tion for the population. At high stock levels this relation­ Hermaphroditism is common among many bivalves and ship may often go unnoticed (for examples in finfishes, gastropods (Coe, 1942; Audesirk and Audesirk, 1985). see Hennemuth (1979), Hennemuth et al. (1980)). This Facultative sex determination is another obvious i c e s mar. Sri. Symp., 199 (1995) Depensatory mechanisms and reproductive output in gonochoristic molluscs 1 5 mechanism, often found in sequentially hermaphroditic