Differential Predation and Growth Rates of Bay Scallops Within a Seagrass Habitat
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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) 0022-0981/99/$ ± see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0022-0981(99)00039-8 300 P.A.X. Bologna, K.L. Heck / J. Exp. Mar. Biol. Ecol. 239 (1999) 299 ±314 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 dis- tributions 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 (În 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).