Ezequiel Mabragaña

Bathyraja cousseauae sp.n.: A New Softnose Skate from the Southwestern Atlantic (Rajiformes, Rajidae)

Abstract A new species of softnose skate, Bathyraja cousseauae, is described from two adult and 19 juvenile specimens collected from different localities of the southwestern Atlantic. Bathyraja cousseauae is distinguished from all other southwestern Atlantic softnose skate species by its color pattern, squamation pattern of dorsal surface, lack of interspace between dorsal fins, and clasper morphology. The new species has a round pale area ocellus-like, margined with dark brown on posterior part of each pectoral base of dorsal surface, paler and dark spots over the disk, a continuous row of 21–27 median thorns from nuchal region to first dorsal fin and upper side of disc densely covered with dermal denticles. Males have rod-shaped claspers not expanded at proximal section of glans as it does in Bathyraja brachyurops. Besides the external morphological features, skeletal characteristics (neurocranium, scapulocoracoids, and claspers) are also described and illustrated here.

From coexistence to competitive exclusion: can overfishing change the outcome of competition in skates (Chondrichthyes, Rajidae)?

Competition for food could be a major force driving changes in the community structure of skates (Rajidae) subjected to fishing exploitation. Under this hypothesis, small skates are released from competition with larger skates after fishing has depleted the larger species. Here, we compare the abundance patterns of two sympatric skates with similar niches but different life histories, Bathyraja albomaculata (larger and slow-reproducing) and Bathyraja macloviana (smaller and faster-reproducing), before (1971, 1978) and after (1998-2004) a 108% increase in industrial bottom trawling on the southeastern South American shelf in order to test the prediction that B. macloviana should competitively exclude B. albomaculata after the increase in fishing mortality. In 1971 and 1978, there was no relationship between the abundance of both species, indicating that they coexisted over large scales. In 1998-2004, the relationship between the abundances of these skates was bell-shaped, indicating that both species increased in abundance at low densities until peaking, after which B. albomaculata decreased when B. macloviana became more abundant, consistent with resource competition. We tested whether food may be a potential limiting resource by comparing the diet of both species. The two species consumed mostly polychaetes, differing only in the consumption of polychaetes from the family Nephthyidae, which was much higher for B. macloviana. Bathyraja macloviana could replace B. albomaculata at high densities when food resources may become scarce. These results support the hypothesis that competition release is an important factor explaining the changes in skate communities in overexploited areas.

Dental abnormalities of the southern thorny skate Amblyraja doellojuradoi (Chondrichthyes, Rajidae)

Teeth morphology has been widely used in taxonomic, biological and fossil teeth studies of Chondrichthyes. Herein, dental abnormalities in Amblyraja doellojuradoi are described. Almost 10% of 182 specimens sampled presented any of these types of dental anomalies: (1) an additional incomplete tooth row between two complete rows, (2) an increasing tooth base size and division of its cusps until the complete splitting of teeth in the same row, and (3) an irregular tooth arrangement on the right side of the lower jaw. Possible causes producing these malformations such as mutation, damage by feeding or reproductive behavior are discussed.

Using otolith morphometry for the identification of three sympatric and morphologically similar species of Astyanax from the Atlantic Rain Forest (Argentina)

In several genera, the otolith shape is species-specific and the use of this structure provides a useful tool aiding in the species identification. In many studies regarding Neotropical fish fauna, species of the genus Astyanax are commonly identified at the genus level, mainly due to the phenotypic plasticity of the morphological characters traditionally used for species determination. In consequence, additional tools intended to better elucidate the taxonomic boundaries between species of Astyanax are certainly needed. In the last decade, the shape of otoliths has allowed to discriminate among closely related species. In this work, Fourier descriptors and shape indices of lapillus otolith were evaluated for the discrimination among three sympatric species of genus Astyanax inhabiting streams of the Atlantic Rain Forest (Argentina). Aspect ratio, roundness and ellipticity of otoliths were significantly different between the species (p < 0.05) while, no significant differences were found for circularity, rectangularity and form factor (p > 0.05). PERMANOVA analysis reveal significant differences between species using Fourier descriptors (F = 96.7, 0.0001 < p < 0.02) and the reclassification rates of quadratic discriminant analysis were high, averaging 86.3% (82.7 - 88.6%). Multivariate analyses of shape indices were not effective to discriminate between species. Instead, high classification percentages suggest that the otolith outline is a potential tool for the identification of sympatric morphologically similar species of Astyanax. Our results could contribute to future taxonomic and phylogenetic studies and may be an interesting input for both paleontological and trophic studies in sympatric species.

Molecular and taxonomic characterisation of introduced specimens of Poecilia reticulata in the lower Paraguay River basin (Cyprinodontiformes: Poeciliidae)

Fil: Rosso, Juan Jose. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina. Fundacion Bosques Nativos Argentinos para la Biodiversidad; Argentina