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----- ----------------------------------------------------------------------------------------- MARINE ECOLOGY PROGRESS SERIES Vol. 180: 223-232, 1999 Published May 3 Mar Ecol Prog Ser Resuspension of postlarval soft-shell clams Mya arenaria through disturbance by the mud snail Ilyanassa obsoleta Robert Dunn •, Lauren S. Mullineaux• *, Susan W. Mills Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA ABSTRACT: Transport and mortality of newly settled post larvae potentially have a large influence on the population dynamics and adult distributions of coastal benthic species, including the soft-shell clam Mya arenaria. Post-settlement transport typically occurs when boundary shear stresses are high enough to resuspend the surface sediments in which the small clams reside. The objective of the pre­ sent study was to examine the effect of disturbance by the mud snail Ilyanassa obsoleta on the hydro­ dynamic transport of recently settled NI. arenaria. Laboratory flume experiments showed that distur­ ·',, bance by activities of I. obsoleta caused suspension of small clams (1.8 and 2.3 mm) at boundary shear velocities (1.0 and 1.3 em s· 1) that were too slow to suspend undisturbed clams. In shear velocities high 1 enough to cause bulk sediment transport (1.4 and 2.0 ern s· ), more clams were suspended in the pres­ ence of snails than in their absence. Manipulative field experiments using cages to exclude snails demonstrated that abundances of juvenile lvl. arenaria (year-1 recruits) were lower in sediments where snails were present than where snails were absent. These results suggest that biological disturbance, such as that imposed by activities of mobile, benthic deposit feeders," may play an important role in postlarval transport and, eventually, in the adult distributions of infaunal bivalves. KEY WORDS: Postlarval transport · Mya arenaria · Soft-shell clam · Ilyanassa obsoleta · Disturbance · Boundary shear stress · Caging experiments INTRODUCTION rents, and benthic conditions while settling, and these factors have been used to predict settlement patterns Historically, most studies of species composition and (e.g. Eckman 1990). The location and density of juve­ population dynamics of benthic ecosystems have niles, however, may be influenced by post-settlement focused primarily on adult organisms. More recently, transport and mortality over weeks to months (Beu­ efforts to understand the variability in recruitment of kema & de VIas 1989, Commito et al. 1995, Armonies marine benthic organisms have included investigation 1996). As a result, when rates of transport of juveniles of processes influencing the patterns of settlement of are high, the larval settlement patterns may bear little their planktonic larval stages (e.g. reviews by Butman resemblance to adult distributions. 1987, Rodriguez et al. 1993). However, many sedi­ The soft-shell clam !vfya arenaria (L.) is subject to ment-dwelling infaunal organisms continue to move as post-settlement transport, as reported in both field juveniles (Palmer 1988, Emerson & Grant 1991). Lar­ (Emerson & Grant 1991) and laboratory (Roegner et al. vae are influenced by water-column conditions, cur- 1995) studies. This transport may contribute to the high temporal and spatial variation in recruitment observed in many populations (e.g. Moller & Rosenberg 1983). -Present address: Dept. of Ecology & Evolutionary Biology, Suspension of juveniles and their subsequent translo­ U-43 University of Connecticut, Storrs, Connecticut 06269- cation is due to boundary shear stress on the clams and 3042, USA --Addressee for correspondence. the surrounding sediments. Susceptibility of juvenile E-mail: [email protected] clams to resuspension can be influenced by their be- ©Inter-Research 1999 Resale of full article not permitted 224 Mar Ecol Prog Ser 180: 223-232, 1999 havior. Clams can facilitate suspension by extending Doppler Velocimeter (LDV) that provided simultane­ their byssal fibers (Sigurdsson eta!. 1976) or by migrat­ ous along-stream and vertical velocity measurements ing to the surface. They can make suspension Jess (described in Trowbridge eta!. 1989). The water height likely by attaching to the substrate with their byssal was set at 12 em during each trial run, and 4 min veloc­ fibers (Newell & Hidu 1986) or burrowing deeply (as ity averages were measured at 10 heights over the cen­ demonstrated for meiofauna by Palmer 1984, Fegley ter of the channel. Calculations of boundary shear 1987). The depth to which clams can burrow, however, velocity (u.) were made from the resulting velocity is limited by the length of their siphon, and therefore profile using the log profile technique (e.g. Jumars & their size (Peterson 1985, Zwarts & Wanink 1989). Nowell 1984). Specific flows were replicated by using Thus, small clams are restricted to the upper layer of the same pump speed and outlet weir setting. For all sediment, making them the most susceptible to resus­ treatments, a removable bottom panel of the flume was pension.· Suspension also may be facilitated through fitted with a sediment tray. The tray was a clear acrylic local disturbance by benthic organisms and fish, as sheet with a recessed 20 x 20 x 2 em deep central box well as by more-intensive anthropogenic disturbances to hold sediment. (i.e. Brown & Wilson 1997). Experiments were conducted with 2 different sizes of The mud snail Ilyanassa obsoleta (Say) occurs com­ clams: the smaller group had a mean length of 1.8 mm monly in high densities and may be an important dis­ (SD = 0.15 mm; n = 100), corresponding in size to juve­ turber of soft-shell clams. This snail feeds on surface nile Mya arenaria midway through their first summer sediments and carrion (Feller 1984) and is common in season in Barnstable Harbor (1 to 2 mm; as estimated intertidal flats and shallow subtidal areas on both from field data not shown) and the larger had a mean coasts of the continental USA. It has been shown to length of 2.3 mm (SD = 0.30 mm; n = 100). correspond­ reduce the abundance of the nematode Pseudotheris­ ing late-season juveniles in the field (2 to 3 mm in tid nematodes (Nichols & Robertson 1979). and length). The larger juveniles were acquired from Beals bivalves and other organisms (Hunt et a!. 1987), and Island Shellfish Hatchery, Beals, Maine, USA, where induce migration of the amphipod Microdeutopus they had been hatched on May 21, 1997. We con­ gryllotalpa (DeWitt & Levinton 1985) and the snail ducted experiments with these clams from July 22 to Hydrollia totteni (Levinton eta!. 1985). In the presence 23. A second group of smaller clams spawned on July 8 of I. obsoleta, the larger species such as M. gryllotalpa at Mook Sea Farm, Walpole, Maine, was used in and H. totteni emigrate actively, but the mechanism by experiments from August 13 to 21. All clams were which I. obsoleta excludes smaller infaunal species maintained in plastic containers covered with 64 Jlm remains unclear. mesh and supplied with running seawater. The clams Our objective in the present study was to examine were fed Isochrysis galbana algae at least once a day the role of the mud snail in the disturbance and subse­ and kept at approximately 21 oc. quent suspension of postlarval soft-shell clams. We Sediment for all trials was collected in Barnstable used laboratory flume experiments to determine Harbor, Massachusetts, USA; it was sieved through a whether snails affect the boundary shear stress at 2 mm sieve to remove the largest fraction and retained which clams are suspended and the proportion of on a 180 Jlm sieve to remove the smallest size fraction. clams suspended in a particular flow environment. The large size fraction was removed to eliminate large These experiments were intended to provide the abil­ pieces of shell and other material that might cause ity to predict the range of boundary shear stresses in small-scale variation in boundary shear stress. The which snails may have an effect, and to estimate the smallest fraction was removed to make the distribu­ probable magnitude of the effect. We then used tions of grain size more consistent from run to run manipulative field experiments to determine whether because there was spatial variability in the amount of the presence or absence of snails affected suspension very fine sediment and flocculent material within field and transport of postlarval clams in field flows. collection sites. The median grain size of the processed sediment was in the 150 to 180 )lm range. For each trial, the tray was filled with well-mixed sediment and MATERIALS AND METHODS evened off so that the sediment surface was flush with the surface of the tray. Adult snails (Ilyanassa obsoleta) Flume experiments. All flume experiments were roughly 1.2 to 2.0 em in length were collected from our conducted in a 17m long, 60 em wide, steady re-circu­ Thatch Island site (northwest region of Little Thatch lating flume located in the Rinehart Coastal Research Island) in Barnstable Harbor, and maintained for 2 to Center (RCRC) at Woods Hole Oceanographic Institu­ 4 d in plastic containers filled with a layer of sediment. tion (Butman & Chapman 1995). The flume was The container was kept in an unfiltered, aerated run­ equipped with an impeller pump and a 2-axis Laser ning seawater table. Dunn et al.: Resuspension of soft-shell clams by a mud snail 2')-_;, Clams were introduced into the sediment tray while however, was no greater than the variation among it was submerged in a running seawater table through replicate runs (conducted for a separate study) of a sin­ a clear acrylic cylinder ( 10 em diameter, 20 em high) gle clam size. Based on these measurements, a shear held approximately 1 em above the sediment. The velocity of 1.0 ems·' was selected as the minimum flow cylinder was used to distribute the clams evenly on the for flume experiments of the smaller clams. Bulk trans­ sediment, and to avoid disturbing the sediment sur­ port of the sediment did not occur at this shear velocity, face.

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