STRUCTURE OF A SCALLOP ARGOPECTEN-PURPURATUS (LAMARCK, 1819) DOMINATED SUBTIDAL MACROINVERTEBRATE ASSEMBLAGE IN NORTHERN CHILE

The structure and biomass of the subtidal, macro-invertebrate assembla ge of Tongoy Bay was analyzed from 255 samples taken by divers during the winter and summer periods of 1990 and 1998. The main purpose of th e study was to assess the relative importance (in numbers and biomass) of the scallops within the assemblage and to look for functional rela tionships between scallops. and associated species. Of 52 taxa found, the scallop Argopecten purpuratus was the dominant species (30% of tot al biomass) followed by the crab Cancer polyodon, the sea stars Meyena ster gelatinosus and Luidia magellanicus and the predatory snails Xant hochorus sp. and Priene rude. As shown by a cluster analysis, these 6 species (which present 70% of the biomass) are closely associated, sug gesting a functional unit with the scallop as prey and the others as p redators. This is confirmed by literature reports on the feeding behav ior of the above predators. As the species abundance data conformed to a straight line the log-series model was applied and die diversity in dex alpha was calculated based on the numbers of species ( = 7.5). For comparison with published data from Independence Bay (Peru), located about 2000 km to the north of the study area, the Shannon-Wiener diver sity index H' (= 3.6) and the index of species evenness J' (=0.64) wer e also calculated. Species richness (58), H' (4.4) and J' (0.76) were higher for the macro-invertebrate assemblage of the Peruvian Bay, whil e th dominant species and their rank order seemed similar, indicating important functional similarities between the two bays. The biomass fo und in Tongoy Bay (26.4 g m-2 wet wt, macrophytes excluded) is low whe n compared to reports from temperate zones and is also somewhat lower than that reported for the coast of Volta and Congo and West Africa. T his low biomass in Tongoy Bay is explained by a heavy clandestine scal lop fishery over the past years causing a two- to threefold decrease i n scallop biomass and a concomitant biomass decrease of associated spe cies. It is postulated that Argopecten purpuratus occupies a central r ole in the assemblage as a filter feeder that converts planktonic food into available prey biomass, and that is not fully replaceable by oth er species of the system. Scallops and associated species were found o n gravel, sand and soft sand bottoms, but scallops, the sea star M. ge latinosus and the snail P. rude were more frequent on gravel, and the crab C. polyodon and the sea star L. magellanicus on soft sand grounds .