CONICET | Buscador de Institutos y Recursos Humanos

The Southwestem Atlantic Ocean is recognized as one of the most productivo áreas in the world and supports a very rich físhery. In order to study the biodiversity and seasonal dynamics of phytoplankton in Argentinean continental shelf and slope waters, a cooperative research project has been established (GEFPatagonia, Argentina). As part of this project, both qualitative and quantitative samples collected during spring 2005, were examined using light and scanning electrón microscopy. An extensive bloom of a small Thalassiosira bioculata var. rariprora was observed along shelf waters from 39-48°S, reaching concentrations up to 4.5 x 10^ cells per liter and representing more than 90% of total phytoplankton abundance. Thalassiosira bioculata var. rariprora is a taxon initially analyzed by Gaarder and Hasle, but never validly published (Hasle personal communication). In this work its ultrastracture is analyzed and described for the fírst time and this variety is recognized as valid for science. The observed specimens are small (8.5-12.0 um), solitary, rectangular in girdle view, and with numerous discoid chloroplasts. The valve face shows a single strutted process slightly subcentral, with an additional ring of strutted processes located between the valve face and mantle (3-4 per valve). The strutted processes are externally surrounded at their base by areolae with robust walls, and internally have four satellite pores and long tubular extensions. One labiate process is situated adjacent to the subcentral strutted process. The areolae pattem shows a tendency toward fasciculation (20-23 in 10 um), sometimes with radial ribs. The cingulum is composed of numerous copulae with vertical rows of pores and a valvocopula with a similar omamentation. The morphological features of these specimens are compared with related taxa, including an evaluation of the main morphological characteristics useful to differentiate them.

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