DISTRIBUTION OF EARLY SETTLEMENT OF MESODESMA DONACIUM (LAMARCK, 1818) (MOLLUSCA: BIVALVIA: MESODESMATIDAE) IN THREE BAYS OF THE FOURTH REGION, CHILE: PHYSICAL AND CHEMICAL CHARACTERIZATION Universidad Católica

Abstract

The objective oí the present study was to investígate the location oí early settlement oí the bivalve Mesodesma donacium, an important fishery resource in Chile. Studies focused on the beaches oí Coquimbo, Guanaqueros and Tongoy bays (IV Región, Chile) during two successive reproductivo seasons (1993-1994). Mid-littoral and subtidal transects (down to3 m depth) were established as follows: 138 transects in Coquimbo Bay, 86 in Guanaqueros Bay and 112 in Tongoy Bay. Transects were separated by a distance oí 130 m and cores were taken along each transect at depths of mid-littoral, 1,2 and 3 m. A standard volume of sand (2.4 dm3) was obtained from each core and sieved using a 0.5 mm mesh. Subtidal samples were taken at a depth between 5 and 10 m using a drag net (0.5 x 0.5 m) towed parallel to the coast. In Coquimbo, Guanaqueros and Tongoy Bays, respectively, tows were collected from 16,12 and 14 locations. Each bay was characterized with respect to presence and magnitudeof rivera, streamsand others freshwater sources, beach morphodynamicstate, sediment characteristics (sizegrain and organic content), and salinity. In all threebaysstudied juveniles oí Mesodesma doiiactutn were found alongall parts oí the beaches, with exception oí areas with predominantely fine sediment, high organic content, areas containing seaweeds and seagrasses in suspensión or cast ashore, and areas adjacent to river streams, or other sources of freshwater.