M. Wolff et E. Alarcon, STRUCTURE OF A SCALLOP ARGOPECTEN-PURPURATUS (LAMARCK, 1819) DOMINATED SUBTIDAL MACROINVERTEBRATE ASSEMBLAGE IN NORTHERN CHILE, Journal of shellfish research, 12(2), 1993, pp. 295-304
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
.