World Congress of Malacology Antwerp ... - Unitas Malacologica

Diversity of sub-antarctic and antarctic Scissurellidae and Anatomidae
(Gastropoda: Vetigastropoda): Consequences for the region’s biogeography
Zelaya, Diego G. 1 ; Geiger, Daniel L. 2
1. División Zoología Invertebrados, Museo de La Plata, Paseo del Bosque s/n, 1900, La Plata,
Argentina,
Email: dzelaya@fcnym.unlp.edu.ar
2. Santa Barbara Museum of Natural History, 2559 Puesta del Sol Road, Santa Barbara, CA 93105,
USA,
Email: geiger@vetigastropoda.com
The southern tip of South America, the so called Magellan Region, and the adjacent Antarctic areas
were linked in the past, as part of the Gondwana supercontinent, and are physically and
geographically separated at present. Under this framework, the study of the molluscan fauna can be
used to address particularly interesting questions pertaining to evolution and biogeography, among
them, those that refer to the effects of the Polar Front as a biogeographic barrier, and the West Wind
Drift as a possible disperser.
In the present study we performed a systematic revision of the species of Scissurellidae and
Anatomidae occurring in the Sub-Antarctic and Antarctic Southern Ocean and, based on the
geographic distribution exhibited by these species, we re-evaluate the faunistic affinities between the
Magellan and Antarctic Regions. This study was based on the material collected during several field
trips, and the examination of museum collection.
Out of 15 nominal species of scissurellids and anatomids previously reported from Sub-Antarctic and
Antarctic waters, twelve proved to be valid species, and the remaining three species were synonyms.
The generic allocation of all these species was confirmed through SEM. In addition, three new
species were found. The examination of the radula morphology and gross anatomy provided
additional characters for generic definitions.
The studied species showed varied distributional patterns, ranging from species with rather restricted
geographic distribution to widely distributed species. However, the Polar Front is regarded as a
strong biogeographic barrier for scissurellids and anatomids, separating two different realms: a group
of Sub-Antarctic species at the north, and a second group of Antarctic species at the south; only one
scissurelid species occurred at both areas. As part of this study it was possible to confirm the
circumantarctic distributions of three of the species, a pattern most probably facilitated by the West
Wind Drift.
Eye structure and vision among gastropods
Zieger, Marina
2348 Stone Road Ann Arbor, MI 48105 USA,
Email: marinazieger@yahoo.com
Species, living in different light environments, are compared with each other in relation to retinal
designs, optics, cellular composition of the retina and visually-guided behaviour. The eyes of some
aquatic pulmonates do differ from those of terrestrial pulmonates; the latter, in species such as
Cepaea nemoralis, Trichia hispida, Arion rufus and Deroceras agreste possess conventional, cupshaped
retinas, but the aquatic species Lymnaea stagnalis, Radix peregra, Physa fontinalis, and
Planorbarius corneus have retinas that are partitioned into dorsal and ventral depressions. The eyes
of L. stagnalis and R. peregra are well adapted to function in both water and in air, but the eyes in P.
fontinalis, Pl. corneus are less modified from those of their terrestrial ancestors. Terrestrial snails and
slugs have relatively poor vision. Good resolving power may be of greater importance in aquatic
pulmonates than in the terrestrial species. Theoretical calculations of the eye’s performance are
supported by behavioural tests. The dominant cellular components of the retinae are pigmented
(supportive) cells and microvillar photoreceptors, the latter occurring in two morphologically distinct
types that likely represent dim- and bright-light receptors. V. viviparus has only one type of
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