Abstract
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 (103 ind. L -1 ) and low ( -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.
Highlights
While it is currently accepted that tintinnid taxonomy needs to be complemented with cytological and/or molecular analyses, up to date this kind of information is available only for fewer than 30 tintinnid species (Agatha and Strüder-Kypke, 2007)
A literature review later performed by Alder (1999) on South Atlantic tintinnids, led to the conclusion that both the constancy of the oral diameter and the continuity of the lorica length make it impossible to establish a cutting point between species previously reported as belonging to the genus, for which reason they should be considered as synonyms of H. subulata
Species belonging to the genus Helicostomella Jörgensen, 1924 were rearranged by Kofoid and Campbell (1929), who maintained the hierarchy of H. subulata (Ehrenberg, 1833) Jörgensen, 1924, while raising four variants up to the species level: H. edentata (Fauré-Fremiet, 1924), H. fusiformis (Meunier, 1910), H. kiliensis (Laackmann, 1906) and H. longa (Brandt, 1906)
Summary
While it is currently accepted that tintinnid taxonomy needs to be complemented with cytological and/or molecular analyses, up to date this kind of information is available only for fewer than 30 tintinnid species (Agatha and Strüder-Kypke, 2007) This represents approximately 5% of the total species included in the most widely used classification system (Kofoid and Campbell, 1929, 1939), which encompasses hundreds of taxa often distinguished on the basis of minute variations in the shape, size and details of the lorica. Regardless of such opposite systematic criteria, the major limitation for both cases is the fact that their conclusions emerge only from the revision of previous descriptions, as up to date there have been practically no field studies focusing on the relationships between the morphological variability of this species and environmental forces
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