First record and new combination of a rissoid gastropod from off south Brazil

After 35 years of its original description, Rissoa cruzi Castellanos & Fernández, 1974 is first recorded in southern Brazilian waters. An analysis of both shell and radular characteristics indicated that R. cruzi does not conform to its current generic assignment. Based on shell characters, R. cruzi is placed in the genus Alvania Risso, 1826. A comparison with other rissoids from the same region is also provided.


Introduction
Rissoa cruzi Castellanos & Fernández, 1974 was first described based on material collected in the intestinal tract of the asteoridean Astropecten brasiliensis Müller & Troschel, 1842 from Uruguay and from material collected by the vessel "Austral" in Bahia Blanca, Argentina.
After the original description there was only one record of the species from Uruguay (Scarabino, 2004).Ponder and Worsfold (1994) state that the species described by Castellanos and Fernandez (1974), although not available to them for study, could be recognized (based on its description and illustrations) as a taxon different than those dealt with in their report.
Specimens collected off Rio Grande by Wiggers and Veitenheimer-Mendes (2004) as Alvania sp. are herein recognized as Alvania cruzi comb.nov.and are compared with A. valeriae, A. faberi and other similar species.

Material and Methods
All material was composed of empty shells and was deposited in the collection of the Departamento de Zoologia of the Universidade Federal do Rio Grande do Sul, lot UFRS 1387.Identification was confirmed by comparison with material described by Castellanos and Fernández (1974) as "other localities" and deposited in the Museo de La Plata, La Plata, Argentina (MLP), not numbered.
Shell measurements were taken under a dissecting microscope equipped with an ocular micrometer.The number of whorls was estimated to the nearest quarter of a whorl.Spiral ornamentation on body whorl is defined as the spiral threads between the suture and the region touched by the aperture's parietal region.Sculptures located below the aperture's parietal region are considered to be on base.Photographs were taken using a Nikon Coolpix 995 digital camera adapted on a dissecting microscope and were digitally processed.
Institutional abbreviations used are: MLP, Museo de La Plata -Argentina; UFRS, Coleção do Departamento de Zoologia da Universidade Federal do Rio Grande do Sul -Brazil.
Description: Shell minute, 2.2 to 3mm long, white.Protoconch smooth, with 1.25-1.5 whorls.Teleoconch with 3 convex whorls and suture well impressed, not canaliculated.Sculpture reticulated with 6 (5-7) spiral threads on body whorl and usually 5 (4-6) spiral threads on the base.Axial sculpture is highly variable, from 28 to 41 axial ribs, but usually with 30 to 35, on body whorl, which does not form nodules at intersections.Spaces between axial ribs are usually homogeneous, but in some individuals it decreases as it reaches the aperture, which is elliptic, almost circular, sometimes thickened.Umbilicus small, clink-like, well defined.

Discussion
According to Ponder (1985) the axial sculpture in Rissoa is usually predominant and spiral cords are weak, low, usually do not cross axial ribs and the radula has one pair of basal denticles where the outer pair of basal denticles derived from the lateral margins ventral edge of each tooth with moderately developed U-shaped extension.
Neither the radular nor the shell characteristics of Rissoa cruzi Castellanos and Fernández, 1974 conform with the genus Rissoa Desmarest, 1814.Radular characters as described by Castellanos and Fernández (1974) show a single pair of basal denticles and would lead us to assign this taxon in the genus Onoba, as "Onoba have a generalized rissoid radula with only a single pair of basal denticles on the central teeth and no denticle arising from the lateral margin" (Ponder, 1985) or in the genus Alvania, as Ponder (1985) noted that "the radular characters of Alvania are similar to those of Rissoa".Unfortunately, no radula from our material was available for study and therefore Castellanos and Fernández' (1974) observations could not be confirmed.Furthermore, Castellanos and Fenrnández' (1974) illustration is based on optical microscopy and the origin of the basal denticles is not clear.It is therefore impossible to count the number of cusps on the marginal teeth.
The shell characters of A. cruzi are similar to those of a number of other species included in the genus Alvania, and consequently we place this species in that genus.Absalão (1993) described A. valeriae and compared it with the Brazilian species A. auberiana, A. deliciosa and A. xanthias and with the Caribbean species A. colombiana Romer & Moore, 1988, A. porcupinae (Jeffereys, 1884), A. zylensis (Gofas & Warén, 1982) and A. imperspicua Palary, 1920. Absalão (1993) stated that A. valeriae is unique among its southwestern Atlantic congeners for it has a large number of axial ribs that clearly dominate the 12-14 spiral threads, a slightly canaliculated suture, and 2.5 protoconch whorls.Although A. cruzi comb.nov.closely resembles A. valeriae in shell sculpture and shape, it differs from the latter in having only 5-7 spiral threads that dominate the 30-35 axial threads, the suture is not canaliculated and it has only 1.25-1.5 protoconch whorls.Geographical range is also very distinct, as A. valeriae is recorded from Espírito Santo to Rio de Janeiro -Brazil, and A. cruzi is recorded from Rio Grande do Sul -Brazil to Bahia Blanca -Argentina.Jong and Coomans (1988) describe A. deboeri, A. moolenbeeki, A. arubensis, A. caribaea and A. faberi.All but A. faberi can be readily distinguished from A. cruzi on shell characters.The protoconch of A. cruzi is F. Wiggers e I. Veitenheimer-Mendes similar to that of A. faberi as both have 1.25 protoconch whorls, but the shell of A. cruzi is much longer, as it reaches 2.2-3mm with 3 teleoconch whorls, while A. faberi reaches 1.3mm with 4.25 teleoconch whorls.The sculpture is also very distinct, as A. faberi has only 3 spiral threads crossed by 17 axial ribs.