Bivalve contribution to shallow sandy bottom food web off mar del plata : inference from stomach contents and stable isotope analysis

ABSTRACT sum of two units infaunal species, the purple clam Amiantis purpurata and the razor clam Solen tehuelchus, are for the use of all species in the 15-20 m sandy bottom sediments between southern Brazil and central Argentina. the one and the other species are food sources for the coastal cheer web, but the extent of their contribution to this nutrition web is still unknown. Based upon stomach content analysis and [[delta].sup.15]N and [[delta].sup.15]C stable isotope signatures we explored the position of these clams in the nourishment web. Stomach content shows that the ray Sympterigia bonapartei and the white croaker Micropogoniasfurnieri loot on entire A. purpurata and s tehuelchus. The stable isotopic analysis confirms this proceed The elephant fish Callorynchus callorynchus and the Brazilian codling Urophycis brasiliensis also present to view values of C and N consistent with those awaited from species that are preying upon A. purpurata. Based on the prediction from the isotopic analysis the clam A. purpurata is a subsistence source for the shrimps Artemesia longinaris, Pleoticus muelleri, the anemone Antholoba achates and the gastropod Buccinanops monilifer. The gastropods Adelomelon brasiliana, Olivancillaria deshayesiana, Olivancillaria urceus and Zidona dufresnei also present to view evidence of consuming A. purpurata on the contrary with contribution from other species with heavier C satisfied According to complementary results the razor clam Solen tehuelchus may be this other species. The stable isotopic analysis displays that both clam species are at the base of the consumer in the sustenance web. A. pupurata showed lighter C mark than s tehuelchus, but the N isotopic mark showed higher value on the contrary still being within the same trophic level

lock opener WORDS: food web, bivalves, fish, crustaceans, gastropods, SW Atlantic

INTRODUCTION

sum of two units infaunal bivalves, the purple clam Amiantis purpurata (Lamarck, 1818) and the razor clam Solen tehuelchus (D'Orbigny 1843) are for the use of all species in the coastal ([les than or equal to]20-m depth) sandy bottom of the SW Atlantic. The purple clam is distributed from Espirito Santo, Brazil (20[degrees]S) to the Golfo San Matias, Argentina (42[degrees]S) (Carcelles 1944 de Castellanos 1967 Scarabino 1977) occurring in high-density patches (maximum abundance: 632 clams [msup-2]) at deepnesss ranging from 10-18 m dominated through 1- or 2-year-classes (Morsan 2003) The razor clam s tehuelchus ([less than or equal to]61-mm length) exhibits similar distribution, from Rio de Janeiro, Brazil (22[degrees]54'S) to Bahfa Blanca, Argentina (38[degrees]44'S) (Rios 1994 Capitoli 1997) However, the abundance of this species is not in fact reflected in the literature, probably because of their down-reaching burrowing habits. There is evidence that the one and the other species are important food sources for a number of species in shallow coastal areas on the other hand the extent of their contribution to the commons web is still unknown.

Several predators have been described for these species based upon gut content analysis (Olivier et al. 1968 Radonic 1997) For example, siphon tips of A. purpurata are the main pabulum item of the electric ray Discopyge tschudii, Heckel 1846 (Arrighetti et al. 2005) which is the regionally greatest in quantity abundant torpediniform species. Also, entire clams are pillageed by the fish Pogonias cromis (Linnaeus, 1766) (Garcia & Gianuca 1997) However, intestine content analysis is limited and may be biased toward larger species that can gripe [i]or[/i] grip the bivalves for longer time (see Sutela & Huusko 2000 Hyslop 1980) and toward regimen items recently consumed; moreover, this manner cannot provide information on the rate of ingestion and regarding long-term assimilation (Creach et al. 1997)

As an alternative, measurement of stable isotope ratios has become widely used to define relationships between consumer and their commons sources (Peterson & Fry 1987 Michener & Schell 1994 Cabana & Rasmussen 1994) and they have been applied to research foraging, migration and other life history phenomena (Hesslein et al. 1991 Alisauskas & Hobson 1993 Walker et al. 1999 Best & Schell 1996 Hansson et al. 1997 Kline et al. 1998 Griffin & Valiela 2001 McGinnis & Emslie 2001) Stable isotopes are relatively consistently fractionated through biological and physical processes as they pass [i]or[/i] part of to the other food webs becoming heavier by dint of 3%0 to 4%0 for N and by dint of 1%o to 2%0 for C with each trophic transfer (Peterson & cook in boiling fat 1987, Cabana & Rasmussen 1994) This fractionation from potential provisions sources to tissues in consumer has been used to identify trophic relationships. Trophic relationships between motile consumer like fishes and their nutriment sources may depend on size of consumer and foraging patterns. Larger animals are able to fe higher in the regimen web and often acquire heavier isotopic signatures in their tissues (France et al. 1998 Harvey et al. 2002) Therefore, combining isotope analysis with intestine contents can provide a better understanding of trophic pathways, given that more [i]or[/i] less items could be ingested on the contrary not assimilated. Also, isotope analysis can provide information upon those organisms in which it is not possible to evaluate intestine content.