The association between the coquina clam Donax fossor say and its epibiotic hydroid Lovenella gracilis clarke

Journal of Shellfisheries Research, Jan, 2005 by J.R. Dougherty, Michael P. Russell

ABSTRACT The northern coquina clam, Donax fossor, is often found in association with the hydroid Lovenella gracilis in the intertidal zone of exposed sandy beaches of southern New Jersey. Although previous workers have suggested that the Donax-Lovenella relationship is one of commensalism, this assumption rarely has been quantified or experimentally investigated. Our objectives are: (1) to eliminate the possibility that the association is a recent occurrence, (2) to quantify whether the hydroid has any affect on predation rates of the clam, (3) to quantify the effect of the hydroid on the ability of the clam to swash-ride, and (4) to document seasonal variation in the association. Monthly field samples collected at two sites over a 2-y period revealed that D. fossor and L. gracilis are seasonal in occurrence. The proportion of the D. fossor population with an epibiotic hydroid varied among season, clam size, and field site. Hydroid abundance also varied between years, among seasons, and between field sites. Overall, larger clams were more likely to support a hydroid colony than smaller clams. Predation by the moonsnail Neverita duplicata on D. fossor was affected by the presence of L. gracilis. Clams without a hydroid colony were drilled more frequently than clams with a hydroid colony, suggesting that D. fossor benefits from the hydroid by way of predator deterrence. The results of this study indicate that the Donax-Lovenella association is quite common during the summer months although it has been overlooked for decades. The evidence to date implies that the Donax-Lovenella relationship is a candidate for a case of mutualism because of the reduced predation rates by moonsnails on clams with a hydroid colony.

KEY WORDS: Donax fossor, Lovenella gracilis, epibiosis

INTRODUCTION

Along the Atlantic coast of North America, the hydroid Lovenella gracilis is a facultative epibiont on intertidal bivalves in the genus Donax (Ruppert & Fox 1988). Although epibiosis is common in rocky intertidal and subtidal habitats, it rarely has been documented in exposed sandy beaches (but see Manning & Lindquist 2003) or muddy habitats (Stachowitsch 1977, Gili et al. 1993, Jarms & Tiemann 1996, Patil & Anil 2000). Few infaunal or epifaunal macroinvertebrates occur in wave-swept sandy beaches due to the dynamic nature of the substrate. Thus, this habitat is largely devoid of macrofauna to serve as basibionts. However, the valves of living Donax spp. often serve as substrate for a variety of epibionts in this habitat (Ansell 1983). In addition, most hydroid species are subtidal and rarely occur in the intertidal (Jackson 1977, Gili & Hughes 1995). Consequently, the range of habitats for L. gracilis is broadened by the presence of Donax spp. in the swash zone. Although D. fossor is a common and highly abundant species and its association with L. gracilis is highly visible, few studies focus on this association, and no one has quantified the temporal variation of the distribution of L. gracilis on D. fossor.

Clearly L. gracilis benefits from its association with Donax spp. by using the clam as a substrate. Large colonies of L. gracilis are never found on the plethora of abiotic hard substrata (e.g., broken shells, pebbles, etc.) in the swash zone (personal observation). Rather, L. gracilis colonizes the posterior margins of the valves of living Donax spp. (Fig. 1A). The clam provides an anchor in the swash zone and acts as a stabilizing structure maintaining the hydrorhizae at the surface-water interface (Fig. 1B). This predictable orientation of the clam shell allows the hydroid colony to receive the full impact of the swash. Furthermore, Donax spp. actively maintain their position in the swash zone by migrating with the tide, a behavior known as "swash-riding" (Ellers 1995). Lovenella gracilis benefits from the tidal migratory behavior of the clam thus avoiding desiccation and overheating while constantly being exposed to food and dissolved gases in the swash zone.

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Although Donax spp. are highly abundant on sandy beaches on the northwest Atlantic, its association with L. gracilis has been seldom documented and rarely quantified. Only Manning and Lindquist (2003) addressed the question of costs and benefits of the epibiotic hydroid on Donax spp. In a series of elegant experiments they showed that L. gracilis enhanced predation rates from some predators and decreased rates for another. They also quantified the role of the hydroid on the clam's abilities to orient in the swash, burrow, and to migrate in the backwash.

In this study, we examined whether the Donax-Lovenella association is a recent phenomenon by examining museum specimens of D. fossor. We also examined the role of the hydroid on clam predation by a gastropod. The moon snail, Neverita duplicata (formerly Polinices), which preys upon D. variabilis in Texas (Loesch 1957) and Florida (Mikkelsen 1978), appears to be a significant predator of D. fossor along beaches in southern New Jersey. We also examined the ability of the clam to swash-fide both with and without a hydroid colony. Finally, we quantified the frequency, abundance, and seasonal occurrence of L. gracilis on D. fossor in southern New Jersey.