Plicopurpura pansa is a cultural resource with economic importance since ink can be extracted from it to dye clothes. This snail inhabits the rocky intertidal zone of Mexican Pacific, is found attached to rocks in crevices and holes. The objective was to analyze the P. pansa population structure in Costa Chica of the State of Guerrero based in density, sex ratio, size and weight, besides to realize observations on mating and recruitment, for which quarterly sampling was made during an annual cycle in three beaches. The density was 4.83 snails/m 2. Sex ratio ranged from 0.76:1.00 to 1.00: 0.96 (F:M). Average size was 20.91 mm. The best represented sizes in females were from 10.5 to 28.5 mm and in males 13.5 to 28.5 mm. Averaged weight was 1.99 g, females showed a coefficient of variation in weight of 96.53%. Recruitment occurred for the entire year. In December 2006 and September 2007 a larger frequency of recruits was shown. Sex ratio showed a 1:1 tendency. Snail population was characterized by being small sized and in high densities. Weight variations in females were found related to their reproductive cycle.

Extinctions are occurring at an unprecedented rate as a consequence of human activities. Vertebrates constitute the best-known group of animals, and thus the group for which there are more accurate estimates of extinctions. Among them, freshwater fishes are particularly threatened and many species are declining. Here we report the extinction of an endemic freshwater fish of Mexico, the Catarina pupfish Megupsilon aporus, the sole species of the genus Megupsilon. We present a synopsis of the discovery and description of the species, the threats to, and degradation of, its habitat, and the efforts to maintain the species in captivity before it became extinct in . The loss of the Catarina pupfish has evolutionary and ecological implications, and highlights the crisis of freshwater fish extinctions. It is a warning of the likely fate of more than  freshwater fish species threatened with extinction in Mexico. To save these species, the country urgently needs a national strategy to articulate a bold conservation effort, with better policies on ecosystem management and water use.

Resumen. Plicopurpura patula pansa es un caracol carnívoro característico de las costas rocosas del Pacífi co mexicano. Presenta una glándula hipobraquial que secreta un fl uido que adquiere una coloración púrpura en presencia de oxígeno y luz solar. Se determinó el arreglo espacial del caracol en playas que difi eren en cuanto al tipo de sustrato, exposición al oleaje y pendiente de playa. Se calculó entre 0.1522 y 0.5435. Las menores fl uctuaciones del I p se presentaron en 5 playas que tienen en común sustratos fi jos, gran cantidad de grietas y oquedades. Las mayores fl uctuaciones correspondieron a 4 playas con sustratos móviles, como los cantos rodados. Palabras clave: costa rocosa, Guerrero, patrón espacial, caracol púrpura.