Morphological variability, spatial distribution and abundance of <em>Helicostomella</em> species (Ciliophora: Tintinnina) in relation to environmental factors (Argentine shelf; 40-55°S)

For the first time a taxonomic and ecological study of species belonging to Helicostomella was performed over a mesoscale spatial distribution by examining ca.3000 loricae collected in argentine shelf waters during the austral autumn.Microscopic and statistical analysis revealed that the general shape and the oral diameter remained practically constant in the whole area surveyed, despite a continuous length fluctuation of ca.300 µm, which includes the entire range present in eight previously reported species.consequently, we consider that the genus may be represented only by H. subulata, whose strong fluctuations in length (mostly attributed to an increase in the collar length), density and biomass seem to respond to temperature, food availability and front-related processes.in Buenos aires coastal waters associated with a quasi permanent estuarine front (40-40.5°S),mixed conditions would favour moderate abundances of long loricae, whereas in northern (42.5-45.5°S)and southern (46-54.5°S)Patagonian waters, high (10 3 ind.L -1 ) and low (<5 ind.L -1 ) densities of short loricae seem to be a consequence of stratification and encystment, respectively.the non-occurrence of the species at 41-42°S, together with a three-fold reduction in length-related parameters between specimens from Buenos aires and Patagonian waters, suggests a disjunct distribution.