Effect of fish predation on intertidal benthic fauna is modified by crab bioturbation

Abstract

The burrowing crab Chasmagnathus granulatus is an important bioturbator in SW Atlantic estuaries where they generate dense and extended intertidal beds. Its bioturbation leads to profound changes in the structure, quality and dynamics of sediments with concomitant impacts on the entire benthic community. In this study, we evaluate whether the presence of this crab affects the predator–prey interaction between juvenile fishes and their benthic prey. Gut content and benthic prey selection by juvenile fishes inside and outside crab beds were evaluated, and predation effect was experimentally contrasted between areas using fish exclosures. The results show that in crab beds the percentage of fish with empty guts was lower and the number of polychaetes consumed by fish higher than outside crab beds. The silverside Odontesthes argentinensis and the catfish Pimelodella laticeps fed on larger polychaetes outside than inside crab bed areas, while the white mouth croaker Micropogonias furnieri preyed upon larger polychaetes inside crab beds. In addition, field experiments shows that fish predation decreases polychaete abundances only in crab beds. These results suggest that crab bioturbation facilitate fish predation on benthic prey.

Introduction

Predation has important effects on benthic community structure (Lenihan and Micheli, 2001). In intertidal soft bottom habitats, infaunal organisms are prey of a variety of predators, including crabs and fishes during high tide (e.g., Virnstein, 1977), and shorebirds (e.g., Quamenn, 1984, Botto et al., 1998, Palomo et al., 2003b), ants (e.g., Palomo et al., 2003a), and terrestrial mammals (e.g., Sheldon, 1991, Navarrete and Castilla, 1993) during low tide. Experimental evaluations of the effect of predation on intertidal soft bottom habitats (Wilson, 1991) have shown that predation impact depends on prey abundance and behavior (Seitz et al., 2001), habitat structure (e.g., Micheli, 1997, Corona et al., 2000) and sediment type (Quamenn, 1982, Quamenn, 1984, Micheli, 1996, Seitz et al., 2001). The potential role of bioturbators in modifying the outcome of predator–prey interactions has rarely been explored (but see Palomo et al., 2003b).

There are a variety of mechanisms by which bioturbators could affect interactions between predators and infaunal prey. They can direct or indirectly modify abundance (Posey, 1986, Dobbs and Guckert, 1988), vertical distribution in the sediment (Escapa et al., 2004), body condition and surficial activities of benthic organisms (Palomo et al., 2004), and could consequently change their availability to predators. Also, changes in turbidity promoted by bioturbators (Murray et al., 2002) can affect prey detection (Blaber and Blaber, 1980, Macia et al., 2003), modify granulometry (Botto and Iribarne, 2000) and change prey capture efficiency (Quamenn, 1982, Seitz et al., 2001). Dense beds of large burrows can also alter predator foraging pattern (Iribarne et al., 2004). Despite all these possibilities, the role of bioturbators as modifiers of predator–prey interactions is poorly known.

In Southwestern Atlantic estuaries, mudflats are dominated by the burrowing crab Chasmagnathus granulatus Dana (Boschi, 1964, Spivak et al., 1994, Iribarne et al., 1997, Bortolus and Iribarne, 1999). This bioturbator excavates large (up to 30 cm diameter) semi-permanent burrows and reworks large amount of sediments (up to 5 kg m⁻² day⁻¹). It occurs in extensive beds (Botto and Iribarne, 2000, Iribarne et al., 2003) with densities of up to 40 burrows m⁻² and covering up to 80% of the mudflat surface (Escapa et al., 2004). Given their high densities and burrowing activity, this crab has a profound influence on sediment characteristics (Botto and Iribarne, 2000), and benthic community structure (Botto and Iribarne, 1999, Palomo et al., 2004). The presence of C. granulatus is known to affect predator–prey interactions between shorebirds and polychaetes (Palomo et al., 2003b, Escapa et al., 2004), and ants and polychaetes (Palomo et al., 2003a). In both cases, the effect of the predator is significantly greater in areas not dominated by crab burrows than in similar areas dominated by crab burrows, indicating that the presence of burrowing crabs interferes in some ability of the predator (e.g., movement throughout the habitat, prey detection, prey capture efficiency). It is unknown whether this effect also applies to fish predation. Fishes are important predators in intertidal mudflats and infaunal preys are important in their diets (Norton and Cook, 1999). Some estuarine fishes feed exclusively on benthic infauna (Little, 2000). Any effect on the availability or abundance of benthic infauna will also affect predator–prey interaction between fishes foraging in these areas and their benthic prey.

In this study, we evaluate whether the presence of the burrowing crab C. granulatus affects the predator–prey interaction between juvenile fishes and their benthic prey. With this aim, gut content and benthic prey selection by juvenile fishes in areas with and without crab burrows were evaluated. In addition, we performed an experiment to compare the effect of predation by fishes on benthic fauna between areas with and without crab burrows.