Trophic assessment of three sympatric batoid species in the Southern Gulf of California.

The competitive exclusion principle establishes that the coexistence of closely related species requires a certain degree of resource partitioning. However, populations have individuals with different morphological or behavioral traits (e.g., maturity stages, sexes, temporal or spatial segregation). This interaction often results in a multi-level differentiation in food preferences and habits. We explored such resource partitioning between and within three batoid species: Hypanus dipterurus, Narcine entemedor, and Rhinoptera steindachneri in the southern Gulf of California, Mexico, using a combination of stomach content (excluding R. steindachneri) and stable isotope analyses. We found a clear differentiation between H. dipterurus and N. entemedor, where the latter exhibited more benthic habitats, supported by a greater association to infaunal prey and higher δ13C values. Though the degree and patterns of intra-specific segregation varied among species, there was a notable differentiation in both sex and stage of maturity, corresponding to changes in specialization (i.e., isotopic niche breadth) or trophic spectrum (varying prey importance and isotopic values per group). This work is a promising step towards understanding the dietary niche dynamics of these species in a potentially important feeding area within the southern Gulf of California, as well as the biological and ecological mechanisms that facilitate their coexistence.

Mechanical properties of the California sea lion (Zalophus californianus) and northern elephant seal (Mirounga angustirostris) lower jaws explain trophic plasticity.

The fossil record of pinnipeds documents a suite of morphological changes that facilitate their ecological transition from a terrestrial to an aquatic lifestyle. Among these is the loss of the tribosphenic molar and the behavior typically associated with it in mammals: mastication. Instead, modern pinnipeds exhibit a broad range of feeding strategies that facilitate their distinct aquatic ecologies. Here, we examine the feeding morphology of two species of pinnipeds with disparate feeding ecologies: Zalophus californianus, a specialized raptorial biter, and Mirounga angustirostris, a suction specialist. Specifically, we test whether the morphology of the lower jaws facilitates trophic plasticity in feeding for either of these species. We used finite element analysis (FEA) to simulate the stresses during the opening and closing of the lower jaws in these species to explore the mechanical limits of their feeding ecology. Our simulations demonstrate that both jaws are highly resistant to the tensile stresses experienced during feeding. The lower jaws of Z. californianus experienced the maximum stress at the articular condyle and the base of the coronoid process. The lower jaws of M. angustirostris experienced the maximum stress at the angular process and were more evenly distributed throughout the body of the mandible. Surprisingly, the lower jaws of M. angustirostris were even more resistant to the stresses experienced during feeding than those of Z. californianus. Thus, we conclude that the superlative trophic plasticity of Z. californianus is driven by other factors unrelated to the mandible's tensile resistance to stress during feeding.

Length at Maturity, Sex Ratio, and Proportions of Maturity of the Giant Electric Ray, Narcine entemedor, in Its Septentrional Distribution.

The size at which a certain fraction of a fish population reaches sexual maturity is an important parameter of life history. The estimation of this parameter based on logistic or sigmoid models could provide different ogives and values of length at maturity, which must be analyzed and considered as a basic feature of biological reproduction for the species. A total of 305 individuals of Narcine entemedor (N. entemedor) were obtained from artisanal fisheries in the Bahía de La Paz, Mexico. For the organisms sampled, sexes were determined and total length (TL) in cm was measured from October 2013 to December 2015. The results indicated that the females were larger, ranging from 48.5 cm to 84 cm TL, while males varied from 41.5 cm to 58.5 cm TL. The sex ratio was dominated by males ranging from 45-55 cm TL, while females were more abundant from 60 to 85 cm TL. Mature females were present all year long, exhibiting a continuous annual reproductive cycle. The length at maturity data were described by the Gompertz model with value of 55.87 cm TL. The comparison between models, and the model selection between them, showed that the Gompertz model had maximum likelihood and smaller Akaike information criterion, indicating that this model was a better fit to the maturity proportion data of N. entemedor.

Biochemical characterization and in vitro biological activities of the epithelial cell extracts from Hypanus dipterurus spine.

Ocean organisms live in competitive environments that demand the production of poisons and toxins. In some cases, these substances have been used in the pharmaceutical industry for human disease treatments. Most fish poisons generally have potent cytolytic activity, probably through cardiovascular and neuromuscular effects. In the case of marine stingrays, the injuries made by their tail venom apparatus are caused by the mechanical penetration of their sting and a subsequent venom release. This study focused on the evaluation of substances with cytotoxic activity in the epithelium that covers the venom apparatus from the marine stingray Hypanus dipterurus. To demonstrate the above, the hemolytic, proteolytic and cytotoxic capacities of H. dipterurus epithelium substances were determined. Discs impregnated with epithelial extract were used on blood agar plates. The proteolytic activity was analyzed using casein as substrate and for gelatin the liquefaction activity test. To determine the cytotoxicity degree of the extracts, the proliferation and cell viability MTT bioassay was implemented on human cervical carcinoma cells (HeLa). The results showed that no hemolytic or proteolytic activity existed against casein associated with the epithelial extract, but gelatin hydrolysis and cytotoxic activity against the HeLa cell line were observed. This study concludes that the substances found in the epithelium covering the H. dipterurus stingray venom apparatus are a mixture of various proteins, among which, glycosylated anionic proteins represent a potential source of molecules with cytotoxic and hydrolytic activity.

Putative extinction of two sawfish species in Mexico and the United States

All species of sawfish are listed by the International Union for the Conservation of Nature (IUCN) as endangered or critically endangered. In fact, the smalltooth sawfish Pristis pectinata, and the largetooth sawfish Pristis pristis, have been declared to be regionally and locally extinct from the US Atlantic coast and the Gulf of California, Mexico, respectively, likely due to overfishing. However, here we dispute these claims by illustrating how lack of existence of a given species within a region can be misconstrued as evidence for extinction.(AU)

Todas as espécies de peixe-serra são qualificadas pela União Internacional para a Conservação da Natureza como ameaçadas ou criticamente ameaçadas de extinção. De fato, o peixe-serra-de-dentes-pequenos, Pristis pectinata, bem como o peixe-serrade-dentes-grandes, Pristis pristis, têm sido declarados como regionalmente e localmente extintos da costa atlântica dos EUA e do Golfo da Califórnia e México, respectivamente, provavelmente devido ao excesso da pesca. No entanto, aqui pretendemos questionar estas extinções ilustrando como a falta de evidências da existência de uma espécie em uma região pode ser mal interpretada como evidência de extinção.(AU)