Morphological variability of Littoridina australis (d'Orbigny, 1835) (Hydrobiidae) in the Bonaerensian marine Holocene (Argentina)

Fossil and living shells of Littoridina australis (d'Orbigny, 1835) (Mesogasti-opoda, Hydrobiidae) from the marine Holocene in the Bonaerensian coastal area (Argentina) and the modern SW Atlantic littoral zone (Southern Brazil, Uruguay, Argentina) document its wide intraspecific morphological variability and offer implications for the interpretation of paleoenvironmental changes that have occurred since about 7.6 ka BP. Shell morphs previously described as different taxa, i.e. L. australis, Littoridina isabelleana (d'Orb.), Littoridina australis crassa (Gaillard), Littoridina conexa (Gaillard), Littoridina parchappii (d'Orb.), can be linked together in a chain of transitional forms that reflect responses or adaptations to different salinity conditions. A biometrical study of L. australis allows independent testing of the hypothesis of a polymorphic species ranging from medium-sized chubby shells ( = L. conexa) (oligomesohaline environments, 3-18parts per thousand) to a smaller elongate morph L. isahelleana) (polyeuhalille habitats, 18-35parts per thousand) with the predominant bigger morph ( = L. australis lectotype; L. crassa) (genuine brackish species in typically mesohaline waters, 8-18parts per thousand) in the middle of the morphological gradient. Results of the covariance analysis applied to 10 fossil and eight modern sample groups indicate: (1) no significant discrepancies between L. australis vs. L. crassa, L. australis vs. L. conexa or L. australis vs. L. isabelleana; (2) significant differences between L. conexa vs. L. isabelleana; (3) highly significant differences between L. australis and L. parchappii; (4) slight differences between L. conexa and L. parchappii. Together with paleoecological data and geographical distribution, these results suggest that: (1) L. parchappii represents a separate species; (2) 'conexa', 'isabelleana' and 'crassa' are intraspecific variations (ecomorphs) of L. australis; (3) 'conexa' is intermediate between both species; (4) during the time span of the Holocene transgression (ca. 7.6-1.4 ka BP) only two species lived in the area: L. parchappii in oligohaline waters and L. australis with its optimum in brackish (mixohaline) conditions and ecomorphic variations within the estuarine environment; (5) the modern restriction of L. australis is a consequence of salinity changes linked to sea-level fluctuations along the Bonaerensian littoral. (C) 2002 Elsevier Science B.V. All rights reserved.