BackgroundStomach contents of 131 specimens of five elasmobranch species (Mustelus lunulatus, Dasyatis longa, Rhinobatos leucorhynchus, Raja velezi and Zapteryx xyster) caught in the central fishing zone in the Pacific Ocean of Colombia were counted and weighed to describe feeding habits and dietary overlaps.ResultsTwenty-one prey items belonging to four major groups (stomatopods, decapods, mollusks and fish) were identified. Decapod crustaceans were the most abundant prey found in stomachs. The mantis shrimp Squilla panamensis was the main prey item in the diet of M. lunulatus; tiger shrimp Trachypenaeus sp. was the main prey item in the diet of Rhinobatos leucorhynchus and Raja velezi, and Penaeidae shrimp were the main prey items in the diet of Z. xyster. Furthermore, fish were important in the diet of Raja velezi, Z. xyster and D. longa. The greatest diet breadth corresponded to Z. xyster whereas M. lunulatus was the most specialized predator. Finally, four significant diet overlaps between the five species were found, attributable mainly to Squillidae, Penaeidae and Fish.ConclusionShrimps (Penaeidae and stomatopods) and benthic fishes were the most important food types in the diet of the elasmobranch species studied. Diet breadth and overlap were relatively low. Determination of food resource partitioning among the batoid species studied was not possible. However, we identified partitions in other niche axes (time of feeding activity and habitat utilization). It is possible to assume that diffuse competition could be exceeding the biunivocal competition among the studied species. Therefore, this assemblage would have a strong tendency to trophic guild formation.

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Reproductive aspects of the round ray Urotrygon rogersi were studied based on 2005 specimens obtained in the artisanal shrimp fishery operating on the Colombian Pacific coast. Females reached greater maximum total length (L(T) ), disc width (W(D) ) and mass (M) (38·0 cm, 19·9 cm and 348 g) than males (32·5 cm, 17·0 cm and 165 g). Sex ratio of juveniles and adults was 1:1. Clasper length increased rapidly between 10·0 and 12·5 cm W(D) . The smallest mature male measured 10·5 cm W(D) and the largest immature individual 13·7 cm W(D) . Male first maturity was reached at 61·8% of maximum W(D) , and estimated W(D50) was between 11·5 and 11·8 cm. The smallest mature female measured 10·5 cm W(D) ; the size at first maturity was 52·8% of maximum W(D) , and estimated W(D50) was between 11·8 and 12·3 cm. Embryos were found in females ≥ 10·5 cm W(D) and maximum fecundity was three embryos per female (mode = 1) and varied with maternal size. Embryos were found in all months, but three birthing peaks per year were identified and a gestation period of 4-5 months estimated. Based on ovulation time, embryonic growth and parturition dates, a triannual reproductive cycle was inferred for this species, with overlapping ovarian and uterine cycles. These results suggest that U. rogersi has a reproductive strategy based on low fecundity, a rapid reproductive cycle (short ovulation and gestation time), three birth peaks per year and large embryos. This strategy probably allows U. rogersi to withstand the fishing pressure they are subject to on the Colombian Pacific coast. The results also suggest that the study area is an important nursery and reproductive area for this species.

The aims of the present study were to identify the trophic roles of shark and batoid species in a tropical marine food web and to examine how ontogenetic dietary changes affect these roles. Elasmobranch species at different stages of maturity (juveniles and adults) are distributed at medium and high trophic levels (3.2–4.2), preying on numerous fish and invertebrates. Nine trophic groups comprising species at different stages of maturity were identified. Considering the maturity stages, elasmobranchs were found in five trophic groups and 37.5% of the regular equivalence nodes in the web. These species had roles as both predator and prey in four trophic levels of the web, participating in most of the roles identified, and are highly redundant in their functions as prey and mesopredators, but not in their role as top predators. The results of the present study suggest that elasmobranchs can be fundamental to the structure and function of marine food webs and highlight the need to include the effect of ontogenetic changes in the diet of these predators in future assessments of their ecological relevance.

RESUMEN.La raya guitarra Rhinobatos leucorhynchus es comúnmente capturada como fauna acompañante en la pesca del camarón de aguas someras del Pacífico colombiano, tanto a nivel industrial como artesanal. A partir de 286 ejemplares capturados incidentalmente entre 2001 y 2007, se estudiaron diferentes aspectos biológicos de esta especie. Las hembras fueron proporcionalmente más grandes que los machos, aunque el crecimiento fue similar en ambos sexos (alométrico). Los embriones no presentaron diferencias sexuales significativas ni en talla ni en peso. La proporción sexual en adultos fue 2,4:1 (hembras-machos), mientras que en embriones esta proporción fue de 1:1. El 51,5% de machos presentó claspers desarrollados y calcificados y el 56,5% de las hembras estaban grávidas, presentando fecundidades entre 1 y 6 embriones, con tallas entre 3 y 19,5 cm, sugiriendo una talla de nacimiento entre 19 y 19,5 cm LT. La talla mediana de madurez de las hembras fue menor que la de machos (48,48 cm y 51,18 cm LT, respectivamente), y las hembras presentaron crías a partir de 44 cm LT. Los hábitos alimentarios de esta especie mostraron 30 ítems presa, con camarones de la familia Penaeidae, y especialmente Trachypenaeus sp. como alimento principal. La raya guitarra presentó diferencias significativas en la dieta de machos y hembras, y la amplitud de nicho indicó que es especialista en su dieta. Los resultados de este estudio aportan información útil para plantear medidas de manejo de R. leucorhynchus, especie que requiere atención, dado sus características biológicas y su importancia comercial y de consumo. Palabras clave: batoideos, reproducción, crecimiento, dieta, ecología alimentaria. Biology of the guitar ray Rhinobatos leucorhynchus (Günther, 1867) (Rajiformes: Rhinobatidae) in the Colombian PacificABSTRACT. The guitar ray Rhinobatos leucorhynchus is commonly caught as by-catch of industrial and artisanal shrimp trawling in the shallow waters of the Colombian Pacific. The biological aspects of this species were studied using 286 animals caught as by-catch between 2001 and 2007. The females were proportionally larger than males, although growth was similar (allometric) for both sexes. The embryos showed no significant differences by sex in either size or weight. The adult sex ratio was 2.4:1 (females-males), whereas in embryos this ratio was 1:1. 51.5% of males, 51.5% had developed, calcified claspers, and 56.5% of females were pregnant, carrying between 1 and 6 embryos of 3 to 19.5 cm, suggesting a size at birth of 19 to 19.5 cm TL. The median size at maturity for females was lower than that of males (48.48 cm and 51.18 cm TL, respectively), and the females had embryos starting at 44 cm TL. The feeding habits of this species showed 30 prey items, with shrimp from the Penaeidae family and especially Trachypenaeus sp. as main food. The diets of male and female guitar rays differed significantly, and the niche breadth indicated that this species has a specialist diet. The results of this study provide useful information for propos...