Differential Predation and Growth Rates of Bay Scallops Within a Seagrass Habitat

Journal of Experimental Marine Biology and Ecology, L 239 (1999) 299±314

Differential predation and growth rates of bay scallops within a seagrass habitat

Paul A.X. Bologna* , Kenneth L. Heck Jr. University of South Alabama, Department of Marine Sciences, Dauphin Island Sea Lab, P.O. Box 369-370, Dauphin Island, AL 36528, USA Received 9 October 1998; received in revised form 23 March 1999; accepted 30 March 1999

Abstract

The bay scallop, Argopecten irradians, is a common and commercially important bivalve species residing in shallow marine ecosystems dominated by seagrasses. However, unlike most bivalves, scallops have the ability to move considerable distances within and among habitats. Consequently, their adult distribution may not be set by larval settlement patterns. In St. Joseph Bay, FL, USA, scallops were signi®cantly more abundant at edges of turtle grass (Thalassia testudinum) beds (xÅÅ50.75 m22 ) than in their interior (x50.375 m22 ) or in nearby unvegetated sediments (xÅ 50.00). This difference in habitat use was shown by ®eld experiments to have two important consequences. First, scallops living along edges of T. testudinum beds experience signi®cantly higher predation potential (.20% loss to predation day21 ) than scallops living in the interior of grass beds or on open sediment (,5% predation loss day21 ). Second, scallops living along the edge of grass beds showed signi®cantly higher growth rates (0.031 mg dry wt. day21 ) than individuals living on open sediment (0.012) or in the interior of beds (0.019). Therefore, individual scallops appear to trade off higher predation risk for increased growth rates.  1999 Elsevier Science B.V. All rights reserved.

Keywords: Bay scallop; Seagrass; Edge effects; Predation; Growth; Argopecten irradians; Thalassia testudinum

*Corresponding author. Current address: Rutgers University Field Marine Station, 800 Great Bay Blvd. c/o 132 Great Bay Blvd., Tuckerton, NJ 08087-2004, USA. Tel.: 11-609-296-5260-x255; fax: 11-609-296- 1024. E-mail address: [email protected] (P.A.X. Bologna)

1. Introduction

Many marine organisms have planktonic larvae whose initial settlement patterns are controlled by physical transport processes (Butman, 1987; Gaines and Bertness, 1992). Although subsequent within habitat movement may occur (Walters, 1992), large scale physical oceanographic processes often control larval recruitment events (Cowen and Castro, 1994; Sale et al., 1994). Biogenic structures (e.g., kelp and seagrass) can dramatically affect these physical forces (Fonseca et al., 1982; Genin et al., 1986). Speci®cally, ¯ow-rate is greatly reduced within physically complex habitats (Fonseca and Fisher, 1986; Gambi et al., 1990) and this may affect faunal distributional patterns by producing settlement ``shadows'' (see Orth, 1992). This process occurs because relative larval settlement rates are increased near the edge of structural habitats due to passive depositional forces, and reduced in the interior due to larval depletion. Patterns expected based upon passive deposition may predict sessile adult distributions within habitats (Eckman, 1990). However, initial settlement patterns do not necessarily determine adult distributions for organisms with motile juveniles and adults (Kneib, 1987; Armonies and Hellwig-Armonies, 1992; O'Connor, 1993). Most bivalves are sessile, or nearly so, but adult scallops are atypical in possessing the ability to swim long distances during their life time (Morton, 1980; Winter and Hamilton, 1985; .0.5 km Bologna, pers. obs.) and escape attacks by predators (Pitcher and Butler, 1987). Hence, adult scallop distributions may not re¯ect initial larval settlement patterns because individuals may redistribute themselves within habitats, or migrate elsewhere (see Brand, 1991). Bay scallops (Argopecten irradians (Lamarck)) are locally abundant in shallow coastal waters. Three subspecies have been identi®ed, with distributions ranging from Nova Scotia, Canada to Laguna Madre, Texas (Clarke, 1965). Scallops are an important commercial and recreational ®shery in many Atlantic and Gulf of Mexico coastal states. They are intimately tied to seagrass beds, which they use as a primary settlement site (Gutsell, 1930; Eckman, 1987). Speci®cally, scallops settle and cling to blades via byssal threads until they are too large to remain suspended (Thayer and Stuart, 1974). During this life stage, the seagrass canopy provides protection from benthic predators (Pohle et al., 1991). Although growth rates may be reduced for juveniles climbing higher on blades, the reduction in predation creates a favorable trade off between growth and mortality (Ambrose and Irlandi, 1992). The goals of this study were to: (1) identify adult distributional patterns in a Gulf of Mexico bay scallop population; (2) determine whether these patterns re¯ect habitat selection; and, (3) examine the growth and survival of scallops occupying different habitats.

2. Materials and methods

2.1. Study site

Work was conducted during 1992 and 1993 in St. Joseph Bay, FL, USA (298 N, 85.58 W, Fig. 1), a shallow semienclosed lagoon with little fresh water input. Salinities in St. P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 301

Fig. 1. St. Joseph Bay, FL, USA 298 N, 85.58 W.

Joseph Bay range from 22½ to 35½ and temperatures from 8.58Cto328C (Bologna, 1998b). Extensive seagrass meadows occupy the shallows (,2 m) and cover approxi- mately 2300±2400 hectares (Savastano et al., 1984; Iverson and Bittaker, 1986). The meadows are comprised of Thalassia testudinum, Halodule wrightii, and Syringodium ®liforme with T. testudinum being the dominant species. Research was conducted in an extensive, shallow sand-T. testudinum habitat mosaic (depth ,1.2 m mean low water). For experimental investigations, T. testudinum edge was de®ned as vegetated bottom within 1 m of the sand±seagrass interface, and interior was de®ned as vegetated bottom greater than 10 m from the sand±seagrass interface. These delineations were established because changes in the physical regime associated with seagrass habitat edges decline rapidly as one moves from the interface to the bed interior (see Fonseca et al., 1982; Orth, 1992). Therefore, a 1 m distance from the sand±grass interface was chosen as ``edge'' to represent a transition from unvegetated to vegetated habitat. Average T. testudinum shoot density was signi®cantly greater at interior experimental sites (961.7664.7 m222 (mean6SE; n524, 0.01824 m cores)) than at edges (765.3664.3 22 m,n524, t4652.25, P,0.03).

2.2. Scallop abundance

In August 1992, mean scallop density was estimated in a T. testudinum-sediment habitat mosaic using a strati®ed quadrat (1 m2 ) sampling design (n5139). Quadrats 302 P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 landing within continuous T. testudinum were classi®ed as ``grass bed'' (n548), those falling in vegetation, but within 1 m of the T. testudinum-unvegetated sediment interface were classi®ed as ``edge'' (n548), and those landing in nonvegetated areas were classi®ed ``sediment'' (n543). Quadrats were visually surveyed and adolescent and adult scallops (.30 mm shell height) were removed and counted to determine mean ]] density in the three subhabitats. Scallop density was square-root transformed (Œn 1 0.5) and analyzed using a one-way analysis of variance (ANOVA). Signi®cant differences among means in different habitats was determined using Fisher's least-signi®cant- difference test (a 50.05). During June 1993, scallops used for habitat selection were marked by cleaning, drying, and gluing a numbered tag to the right (ventral) valve. Thirty one marked individuals (31.4±45.1 mm shell height) were released on June 11 in an unvegetated region of the T. testudinum habitat mosaic (|4 m from sand/grass interface). On June 15, 48 additional marked individuals (33.4±42.75 mm shell height) were released into an interior portion of a T. testudinum grass bed (|6 m from sand/grass interface). A circular area of 10.5 m radius from the point of each release was searched to relocate marked scallops. At periods of 24, 48, and 72 h, scallops were relocated from the open sand release and their position (distance from release) and habitat location were recorded. Unfortunately, due to inclement weather and logistical constraints, scallops released in T. testudinum did not have their position and habitat location recorded until 216 h had elapsed (June 24).

2.3. Biomass estimation

To determine differences in initial size and natural growth rates of individuals in the three habitats, the relationship between scallop size and biomass was determined. Scallops (n5161, shell heights 27±64 mm) were collected and transferred to the laboratory. Scallop shell height and width were measured before all body tissue was removed and dried for 48 h at 808C. A regression equation using both shell height and width as independent variables was calculated to predict scallop biomass. This equation (ln(dry wt. (g))529.77910.79093ln(shell height)12.21243ln(shell width); r 2 50.92, P<0.0001) was then used to estimate initial and ®nal dry weights of living specimens.

2.4. Predation

Two series of tethering experiments were undertaken during June and October 1993 to assess the relative predation rates of scallops living within the three habitats. Previous studies identi®ed several groups of scallop predators, including gulls, gastropods, and decapods (Peterson et al., 1989; Prescott, 1990). For predation experiments carried out in June, 77 scallops with shell heights between 30 and 48 mm and shell widths between 14 and 22.75 mm were used. An initial comparison of ln transformed data showed no signi®cant difference in scallop shell height (one-way ANOVA; F2,7451.95, P.0.15), shell width (F52.26, P.0.11), or dry weight for individuals tethered among habitats (F52.2, P.0.12). For predation experiments conducted in October, 18 scallops with shell heights between 33 and 69 mm and shell widths between 17.5 and 33 mm were P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 303 used. Initial size analysis showed no signi®cant differences in shell height (one-way

ANOVA, F2,1550.22, P.0.8), shell width (F50.098, P.0.9) or biomass (F50.37, P.0.69) among habitats. Scallops were tethered in the following manner: right (ventral) valves were cleaned and allowed to dry, shell height and width were measured for each scallop and then a piece of mono®lament was SupergluedE to the right valve, ventrally adjacent to the umbo. This technique ensured that the valves were not accidentally glued together. Mono®lament tethers were at least 50 cm to ensure that the scallop could easily move about within the habitat. The scallop tethers were then attached to 12-gauge wire and marked for later identi®cation. In June and October, scallops were transferred to the ®eld and placed in one of three habitats: interior T. testudinum (June n524; October n56, respectively), T. testudinum edge (n531, n56), or sand (n522, n56). Individuals remained in the ®eld for 24 h, after which they were relocated and classi®ed as either live, eaten, or missing. The results were analyzed using a G-Test of Independence (Sokal and Rohlf, 1981). Six tethered scallops were also placed in an aquarium to determine the potential effects of handling and the tethering process. All six survived for over 2 weeks at which point they were released into the ®eld. This result suggests that tethering procedures had no adverse effects on scallop survival.

2.5. Growth rate

Scallops were measured and tethered in the same manner described above and placed in one of the three habitats during June 1993: interior T. testudinum (n524), T. testudinum edge (n531), or sand (n522). Initial size of tethered individuals did not differ signi®cantly among treatments (see comparisons for predation experiment). Scallops remained in the ®eld and were monitored from 4 to 14 days depending upon weather and logistical constraints. They were then remeasured and their initial and ®nal biomass estimated using the regression equation shown above. Scallop growth rate (mg dry wt. day21 ) was then calculated and compared for scallops living in each of the three habitats using one-way ANOVA. Fisher's least-signi®cant-difference multiple comparison F-test was used to compare means for growth rates among habitats with a 50.05.

2.6. Mortality to foraging rates

To determine whether scallop distributions re¯ect trade offs between growth rates and mortality rates due to predation, m/f ratios were calculated following Gilliam and Fraser (1987). Changes in this ratio indicate differences in population mortality and/or growth. Minimizing this ratio is preferable and by comparing these ratios among habitats it is possible to assess the relative value of each habitat for the population. m/f ratios were calculated for scallops tethered among the interior, edge and sand habitats using the following equation: 304 P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314

N 2 N ]]]t50 t5]1 FGN m 5 ]]]]t50 ] t and 1 n w 2 w f 5 ] S SD]]]it51 it]50 nti51 where,

m 5calculated instantaneous predation mortality rate f5calculated average increase in scallop biomass N5total scallops living at a given time n5number of scallops surviving to assess growth t5time w5calculated dry weight of an individual scallop.

3. Results

3.1. Natural density

Adult scallops showed a signi®cant difference in the density of individuals among habitats (F2, 136513.5, P,0.0001), with greatest densities at the grass bed edge (xÅ 50.75 m22 ) followed by grass bed interior (x] 50.375 m22 , (Fig. 2)). Although

Fig. 2. Scallop density (number m222 ) estimated from 1 m quadrats collected in sand (n543), Thalassia testudinum bed interior (n548), and T. testudinum bed edges (n548). Values represent mean and standard error of the data. Letters above bars indicate signi®cant density differences (a 50.05). P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 305 scallops were observed in open sand habitats during the study period, their density was below the detection limit of quadrat sampling.

3.2. Habitat selection

The data on scallop movement and habitat choice are presented in Table 1. Although the time between relocation of scallops for the sand and T. testudinum releases is different, it is apparent that scallops released on the open sand rapidly moved into T. testudinum grass beds. Additionally, the average distance moved for sand released scallops was greater than that for scallops released in T. testudinum. This suggests that scallops actively select against open sand habitats, thus providing a mechanism to explain adult distributional patterns.

3.3. Predation

Individual scallop predation was scored as one (1) for live and zero (0) for eaten. Missing individuals were not included in the analysis as their fate was not known (i.e., they may have escaped, they may have been ``harvested'' by humans, or they may have been carried off by predators). Because only a few individuals were missing from the June experiments (sand51, grass bed51, edge50) and none from the October experiment, it is doubtful that the results were affected by missing individuals. Scallop predation rates, presented as total proportion eaten per habitat (Fig. 3), were signi®cantly greater along edges of T. testudinum beds than on sand or interior T. testudinum beds for 22 both June (x 22515.6, P,0.001) and October (x 511.38, P,0.005). A Kolmogorov± Smirnov two sample test was conducted posthoc to determine whether size of scallop eaten differed from size of scallop offered. Results from the analysis indicate that there was no apparent predator size selectivity (Dmax50.19, P.0.2), as all sizes of individuals were taken (e.g., 30±69 mm shell height). Therefore, there appears to be no size refuge from predation for scallops less than 69 mm in shell height in St. Joseph Bay. The principal predators identi®ed in this study (Bologna, pers. obs.) include large gastropods (e.g. whelks (Busycon spp.), horse conchs (Pleuroploca gigantea), and tulip shells (Fasciolaria spp.)), which accounted for 79% of successful predation attacks. Stone crabs (Menippe mercenaria), accounted for an additional 5%, and 16% were from unidenti®ed sources (shells present and intact, but not attached at the hinge).

Table 1 Comparison of scallop movements for an open sand release and an interior Thalassia testudinum release; distance expressed in cm6SD

Release habitat Time No Distance moved % Sand % Grass bed % Edge % Not recovered Sand 24 hrs. 31 287.76108.8 35.5 0 41.9 22.6a Sand 48 hrs. 24 375.86190.7 12.5 33.3 54.2 0 Sand 72 hrs. 24 396.96138.7 0 25.0 70.8 4.2 Grass Bed 216 hrs. 48 226.76214.1 0 52.1 2.1 45.8 a One individual eaten. 306 P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314

Fig. 3. Relative predation rates for tethered scallops among sand, Thalassia testudinum bed interior, and T. testudinum bed edge from June and October predation experiments. Values are presented as proportion of scallops eaten during a 24 h predation experiment executed during either June or October. Differing letters above bars indicate signi®cant differences in potential predation in June by a G-Test of Independence, P,0.001. Letters with apostrophes indicate signi®cant differences in potential predation for October by a G-Test of Independence, P,0.005.

3.4. Growth rates

Of the 77 scallops tethered for growth rate comparisons, only 46 were relocated to assess growth rate (n59 sand, n518 edge, n519 interior). A posthoc analysis of scallop size showed no difference in initial size among scallops tethered in each habitat relocated for growth comparisons (F2,4351.3, P.0.27). Field experiments demonstrated that increases in scallop biomass were signi®cantly greater along the edges of T. testudinum beds (xÅ 50.031 mg dry wt. day21 ) than from interior portions of the bed

(xÅÅ50.019) or open sand (x50.012; F2,4354.46, P,0.02; Fig. 4).

3.5. Mortality to foraging rates

Results from calculated m/f ratios indicate that scallops tethered along the edge of T. testudinum beds had the greatest m/f ratio (7.28), while scallops from the interior of the bed had the lowest ratio (2.29) and tethered scallops from the open sand had an intermediate ratio (3.97; Fig. 5). Minimized m/f ratios indicate a positive bene®t to the population through reduced mortality rates and/or increased growth rates. Results from these calculated ratios indicated that interior T. testudinum provides the optimum habitat for the scallop population and this was followed by open sand, while the edges of T. testudinum beds provided the least favorable habitat for the scallop population. P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 307

Fig. 4. Estimated growth in biomass (mg dry wt. day21 ) for scallops tethered in sand, Thalassia testudinum bed interior, and T. testudinum bed edge. Different letters above treatments indicate signi®cant growth rate differences, P,0.05.

Fig. 5. Calculated m/f ratios for scallops tethered in Thalassia testudinum bed interior, edge and from unvegetated regions. Smaller values represent preferable habitat for a population due to reduced mortality rates and/or increased growth rates. 308 P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314

4. Discussion

Bay scallops, unlike other bivalves, have the ability to travel relatively long distances as adults (.0.5 km over 3 months, pers. obs.). This ability allows them to make choices about the habitat in which they reside as both post-settlement juveniles and adults, and suggests that habitat choice may be linked to their survival, growth, and reproduction. Surveys of the bay scallop population in St. Joseph Bay show a distinct pattern of very low abundances in the open sand and high, clumped concentrations along the edges of T. testudinum beds. Other studies have shown similar results (Winter and Hamilton, 1985), but proposed no mechanism for these patterns (but see Hamilton and Koch, 1996). Results from the tag and release experiment clearly show that scallops released on sand rapidly moved into grass beds. Given that seagrass habitats are mosaics of vegetated and unvegetated patches (Irlandi et al., 1995), the active rejection of sand indicates that scallops prefer to be in vegetated regions (Hamilton and Koch, 1996; Table 1). Although the timing of relocation for scallops released within T. testudinum beds makes direct comparisons dif®cult between sand and T. testudinum releases, the results show that over 50% of T. testudinum individuals remained near their point of release (Table 1). Other individuals may have dispersed beyond the 315 m2 survey area, given the arti®cially in¯ated density at the release site. Although several studies have documented the effectiveness of seagrasses as a predation deterrent (see Orth et al., 1984; Heck and Crowder, 1991), this was not the case for bay scallops of the size tested here. In fact, the relative predation potential was signi®cantly higher for edge tethered scallops than for those in unvegetated habitats. The disparity in potential predation rates among predatory species may relate to the dominant predators of tethered scallops: large gastropods. They often rest in unvegetated regions, move into the grass beds to feed, and subsequently return to sandy areas (pers. obs.). This suggests that scallops living along edges of seagrass beds could potentially encounter a gastropod predator twice as often as those living in the interior of the bed or on sand. Additionally, there appears to be no size refuge for scallops in St. Joseph Bay, as even the largest scallops tethered were successfully attacked. Tethered scallops placed along the edge, however, experienced higher growth rates than scallops in interior beds or open sand (Fig. 4). This suggests that scallops living along the edges of grass beds have an advantage in obtaining food resources. It also suggests that scallops, by avoiding unvegetated regions, may be actively selecting habitats which maximize their growth rate. Several studies have shown that the arrangement and density of seagrass shoots in¯uence both survival and growth of bivalves (Irlandi and Peterson, 1991; Irlandi, 1994; Irlandi et al., 1995). Results from this study suggest that scallops living on the edge are confronted by a trade off between increased growth rates and increased predation mortality. Many organisms show distributions that apparently re¯ect trade offs between the risk of predation and their foraging success rates (Kneib, 1987). Suspension feeding organisms living along habitat edges may experience higher food ¯uxes relative to those in interior portions (Muschenheim, 1987a,b; Eckman, 1987; but see Judge et al., 1993). If individual scallops respond to food resources, then they should follow an ideal free distribution (Fretwell and Lucas, 1970). Under these circumstances, scallop density P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 309 would be highest where food (i.e., growth rate) is maximal and lower in habitats where growth rate is reduced. This may partially explain the difference in scallop density between interiors and edges of T. testudinum beds, but not their near absence from the sand, where they also showed positive growth rates (Fig. 4). A risk sensitive foraging trade off (Holbrook and Schmitt, 1988), whereby scallops assess their relative risk in different habitats and distribute themselves accordingly, also does not fully describe scallop distributions. Scallop densities were signi®cantly greater where predation rates were the highest. Consequently, either scallops do not exhibit risk sensitive foraging or they do not ascertain risk. Although scallops respond to predatory attacks and have a highly developed escape mechanism (Peterson et al., 1982; Pitcher and Butler, 1987), little information exists on their ability to ascertain predation risk (see Brand, 1991). Gilliam and Fraser (1987) discuss a model where individuals in a population should attempt to minimize mortality (m), while maximizing foraging rate ( f ). Their ``minimize m/f '' rule suggests that any individual in the population must survive and grow, but more importantly, the population must have a favorable m/f ratio. Calculating these ratios for scallops tethered within the interior, edge and sand habitats, showed that slower growing scallops in interior grass beds should be more successful in surviving and reaching reproductive sizes than individuals living elsewhere (Fig. 5). Although this examination of the data may be useful in assessing the relative costs and bene®ts for a population living in various habitats, it failed to explain why scallop density is signi®cantly greater at the edge than in the interior, nor can it explain their absence from the sand (Fig. 2). Consequently, other factors must explain scallop habitat preferences. It is possible that the escape response of scallops was altered by tethering so that predation mortality was overestimated, thus biasing predation rates and m/f ratios. Recently, Barbeau and Scheibling (1994a,b) reported that tethering affected the interaction between predators and juvenile Placopecten magellanicus. They found that tethering increased vulnerability to predation by sea stars, but not crabs, in laboratory studies. However, Barbeau and Scheibling (1994b) only reported signi®cant tethering artifacts in experiments with sea star predators and small scallops (|10.3 mm shell height). Lower vulnerabilities of larger juvenile P. magellanicus determined in their ®eld research (Barbeau et al., 1994) suggests that tethering artifacts may only be severe for small juveniles with poorly developed escape mechanisms (Barbeau and Scheibling, 1994a; R. Scheibling pers. comm.). Juvenile P. magellanicus are also fundamentally different from A. irradians, because juvenile A. irradians (,20 mm shell height) often climb seagrass blades to escape benthic predators (Pohle et al., 1991). This does not appear to be the case for P. magellanicus. However, even if tethering in¯ated our measurements of predation potential for scallops, tethering artifacts on the relative mortality rate (m) for all treatments would likely be similar. Thus, more favorable m/f ratios would still result for scallops residing in interior portions of T. testudinum beds than on the edge (as m →0, m/f →0). If tethering artifacts exist, and are habitat speci®c, then individuals tethered in habitats where entanglement may decrease tether length should show disproportionate increases in relative mortality to those tethered in less complex habitats (see Heck and Wilson, 1987; Aronson and Heck, 1995). In this case, interior T. testudinum beds with signi®cantly higher shoot densities should in¯ate 310 P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 predation potential relative to edges and m/f ratios would be even more favorable for scallops residing in interior grass beds. Consequently, we believe that tethering artifacts have little effect on the interpretation of our results. Other mechanisms that may explain the observed distributional pattern include migration of scallops from the interior to the edge of a grass bed or immigration from other seagrass patches. These two hypotheses, however, require a constant supply of new individuals from other locations within the system (i.e., a meta-population scenario) or from outside the system (i.e., larval recruitment). If scallops are migrating within the system, then the differences in predation rates would ultimately reduce the number of individuals in a local population and depletion would result. If a constant supply of settling recruits is occurring, then the potential exists of having a replenished scallop population in a patch and these mechanisms could be tested. Unfortunately, little is known about the reproductive cycles of scallops in the northern Gulf of Mexico. Previous research on scallop reproduction has suggested one major and possibly one minor peak in yearly spawning (Crenshaw et al., 1991; Arnold et al., 1998; Bologna, 1998a). Given the limited life span of A. irradians larvae (10±19 days; Sastry, 1965; Castagna and Duggan, 1971), this is probably inadequate to sustain the continuous recruitment hypothesis. If, however, individuals within the population choose to maximize their growth in order to maximize reproductive success, then increases in the potential predation risk would not inhibit an individual from living on the edge. Numerous studies have indicated that fertilization in broadcast spawners is often limited by sperm dilution (Pennington, 1985; Oliver and Babcock, 1992; Babcock et al., 1994; see Levitan and Petersen, 1995). Consequently, reproductive success may be a function of environmental factors (e.g., current ¯ow, hydrodynamics; see Denny, 1988), species traits (e.g., individual size (Levitan, 1991), reproductive allocation (Babcock et al., 1994), spawning synchrony (Lasker et al., 1996), and aggregation behavior (Pennington, 1985)). Bay scallops exhibit mass spawning (Bricelj et al., 1987) and aggregated distributions (Thayer and Stuart, 1974; this study) which may increase reproductive success. Their decision to avoid sediments and live on the edge may have two important ®tness components: increased growth rate with greater reproductive output (Barber and Blake, 1991) and increased fertilization success (see Mead and Denny, 1995). Consequently, their habitat preference may re¯ect an effective reproductive strategy. If scallops were to spawn in open sedimentary habitats, hydrodynamic forces could dilute the gametes and reduce fertilization success (Pennington, 1985; Denny, 1988; Yund, 1990; Babcock et al., 1994). Spawning in areas where hydrodynamic forces are greatly reduced (i.e., interiors of grass beds; Fonseca et al., 1982) would decrease gamete dilution and potentially increase fertilization success, but could also increase the potential for self fertilization in these hermaphrodites (Wilbur and Gaffney, 1991). Therefore, scallops living on the edge could bene®t from a moderate reduction in gamete dilution and low shear stresses which may increase gamete mixing (see Mead and Denny, 1995), while minimizing the potential for self fertilization. Consequently, scallops may trade off signi®cant increases in predation potential for increases in potential reproductive success. In summary, the results presented show that within habitat differences exist in potential predation and growth rates for bay scallops. It is unusual, however, that P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 311 predation rates were higher at the edge of seagrass habitats compared to unvegetated regions, since seagrasses have often been demonstrated to be a predation deterrent. Additionally, the differences in growth rates for scallops tethered in different habitats provides a mechanism which may be responsible for scallop distributional patterns. What is unusual is that had this study merely assessed growth and predation rates between vegetated and unvegetated habitats, no signi®cant difference would have been detected. But by including ``edge'' as a separate habitat, dramatic differences in these rates were found and suggest that potential trophic interactions at habitat edges should be addressed more closely.

Acknowledgements

We would like to thank John Valentine, Robert Orth, James Cowan, Robert Steneck, and two anonymous reviewers for critical evaluation of this manuscript. We would also like to thank T. Murdoch for assistance with the creation of Fig. 1. The Dauphin Island Sea Lab provided logistical support for this research. This work was supported by grants from the Mississippi±Alabama SeaGrant Consortium (NA16RG0155) and a Lerner± Gray Fellowship from the American Museum of Natural History. This is contribution [308 to the Dauphin Island Sea Lab.

References

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