Does Manila clam cultivation affect habitats of the engineer species Lanice conchilega (Pallas, 1766)?

Abstract

The major French site of Manila clam Ruditapes philippinarum (Adams and Reeves, 1850) cultivation is located in the Chausey Archipelago where the associated practices are highly mechanized: every steps of production are made with tractor-driven machinery. The Manila clam concessions are concentrated on Lanice conchilega (Pallas, 1766) bioherms, which are known to increase alpha-diversity and to locally modify sediment dynamics. This study focus on the impacts of Manila clam cultivation on (i) the natural populations of L. conchilega and on (ii) the structure of the associated benthic assemblages during the different steps of the farming production cycle. We found that the L. conchilega populations are significantly affected within the concessions where their total abundances drastically decrease, their spatial patterns are modified and the associated benthic assemblages are significantly altered. Our results are discussed in a context of a sustainable management of the Manila clam cultivation in coastal areas.

Introduction

Although the environmental hazards of shellfish farming are now well known (e.g. Crawford, 2003a, Crawford, 2004), the effects of Manila clam (Ruditapes philippinarum, Adams & Reeves, 1850) cultivation on the benthic compartment are little documented. The few studies available have shown mostly an organic enrichment (Bendell-Young, 2006, Spencer, 1996, Spencer et al., 1996) and an increase of sedimentation rates (Goulletquer et al., 1998, Spencer, 1996, Spencer et al., 1996, Spencer et al., 1997). Both on-growing and harvesting processes have the potential to alter benthic communities (Spencer et al., 1996, Spencer et al., 1997), but only short-term effects have been observed (Kaiser et al., 1996). In the Chausey archipelago (Normand-Breton Gulf, English Channel, France), Manila clam cultivation is highly mechanized throughout the production cycle, including tractor-towed tolls which allow large-scale operations and increase productivity. The effects of such mechanical processes have already been assessed for cockle cultivation (Ferns et al., 2000, Hall and Harding, 1997), but not for Manila clam. In 1989, the SATMAR (Société Atlantique de Mariculture) company started cultivation of this exotic species in Chausey and nowadays it annually produces 120 tons of marketable clams (first national production). One of the major features of this shellfish farming concerns the setting up of clam concessions on the sand-mason worm Lanice conchilega (Pallas) habitats.

L. conchilega is one of the most common tube-building polychaetes in the Northern European sandflats. This amphiboreal species (absent from Artic) ranges from intertidal to subtidal (up to 100 m depth) fine to coarse sediments and its populations are mainly concentrated below the low water neap tide level (Carey, 1987). This Terebellid worm has a mean life of about 3 years (Féral, 1988) and is a deposit/filter feeder changing its feeding strategy in response to density-dependant processes (Buhr and Winter, 1976). This tube-dweller has an aggregative distribution (Nicolaidou, 2003) with patches reaching densities of several thousands of individuals per square meter (extreme cases up to 20,000 ind. m⁻²; Buhr and Winter, 1976). The Lanice tube (diameter = 0.5 cm, total length up to 65 cm; Ziegelmeier, 1952) is prolonged by sand-fringes which protrude from the sediment and modify the small-scale benthic boundary layer patterns and consequently the local hydrosedimentary environment (Eckman et al., 1981, Luckenbach, 1986). Above a threshold density, current velocities decrease within clusters, deposition of fine sediment particles is facilitated and mounds are created (Friedrichs et al., 2000). Sand-mason worms stimulate oxygen exchanges at the water-sediment interface (‘piston-pumping mechanism: Forster and Graf, 1995, Zühlke et al., 1998) and promote the nutrient cycling of key elements (Bendell-Young, 2006). Moreover, L. conchilega clusters generate a more beneficial environment for other benthic species such as a refuge from predation, a source of food (Callaway, 2006, Zühlke, 2001, Zühlke et al., 1998) and a suitable surface for larval settlement (Armonies and Hellwig-Armonies, 1992, Callaway, 2003b, Gallagher et al., 1983). In fact, L. conchilega may be considered as an auto- and allogenic ecosystem engineer (Jones et al., 1994) with high positive effects on the diversity features of the associated benthic macrofauna, and consequently on secondary consumers (Petersen and Exo, 1999, Amara et al., 2001, Godet et al., in press).

The objective of this study was to assess the effects of this cultivation on the high value habitats of L. conchilega (Godet et al., in press) by testing the following hypotheses:

• The spatial distribution and the structure of L. conchilega populations are modified in the shellfish farming concessions when compared to a control site and they develop along the production cycle.

• The total abundance and the structure of the benthic assemblages associated to the L. conchilega habitats are altered within the Manila clam concessions.