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Los cladóceros son la segunda comunidad zooplanctónica más abundante en el Sistema de Maracaibo y todas las especies reportadas hasta el momento han sido dulceacuícolas (1). Son pocos los estudios realizados en este sistema y más específicamente en su zona norte, en la boca San Carlos - Zapara, la cual es la principal entrada de agua salada proveniente del Golfo de Venezuela y con ella especies marinas. El presente estudio tiene como objetivo reportar por primera vez la presencia de los cladóceros marinos, Penilia avirostris(Dana, 1849) y Pseudevadne tergestina(Claus, 1877) en el Sistema de Maracaibo. Las muestras se obtuvieron durante cuatro muestreos; dos en época de lluvia y dos en época de sequía, a dos profundidades y durante marea entrante y saliente, en seis (6) estaciones establecidas en la boca San Carlos ”“ Zapara. Los resultados obtenidos muestran una mayor frecuencia en la captura de P. avirostris, la cual mostró preferencia por aguas superficiales durante marea entrante y en mayor densidad en época de sequía. P. tergestina por el contrario, se observó solo en la estación lluviosa, desplazándose principalmente por el fondo, presentando mayor densidad durante la marea entrante, al igual que P. avirostris. La aparición de estos organismos, tal vez se deba a los fenómenos de surgencia que ocurren al noreste del Golfo de Venezuela, favoreciendo la aparición ocasional de estos organismos en la boca San Carlos ”“ Zapara. Se recomienda extender el área de estudio de estas especies para una mayor comprensión de las comunidades de cladóceros en el Sistema de Maracaibo.

We studied the horizontal and vertical distributions of marine cladocerans across the Catalan Sea shelf (NW Mediterranean) in July and September 2003, and in June and July 2004. At the seasonal scale, Penilia avirostris appears first in June in the southern region, where temperatures are warmer, and its populations develop northward during the summer. Evadne-Pseudevadne did not show a clear pattern, likely because several species were pooled. In 2003 successive heat waves affecting southwestern Europe resulted in surface seawater temperatures about 2 °C higher than usual across the whole study region. These high temperatures were associated with much lower abundance of P. avirostris. Overall, the mesoscale distributions of cladocerans were associated with the presence of low salinity, productive and stratified waters of continental origin, and negatively linked to the intrusion of offshore waters. On the vertical scale P. avirostris was located within or above the thermocline, whereas Evadne-Pseudevadne was much shallower; no evidence of diel migration was detected in either group. Our study provides new insights regarding the environmental limits for marine cladocerans in the NW Mediterranean; in the particular case of P. avirostris that knowledge can define the likely boundaries of its new distributions as it expands poleward under climate change

Penilia avirostris is a cosmopolitan marine cladoceran that inhabits coastal and shelf waters of most low and mid latitude ecosystems. In this study we describe the life history and population dynamics of P. avirostris at a fixed coastal station in the NW Mediterranean Sea. This marine cladoceran was very seasonal, having population maxima in summer (2500-3000 ind m–3), and being practically absent from the water column for the rest of the year. The population typically collapsed in late summer – early autumn, and this decline was accompanied by a shift to gametogenetic reproduction, allowing the production of resting eggs to ensure the continuity of the species in the following season. Estimated birth rates of P. avirostris in the Catalan Sea ranged between 0.097 and 0.46 d–1 and seemed to be decoupled from changes in P. avirostris standing stocks. Birth rates were mainly dependent on the changes in abundance of embryo-carrying females and their brood size, because during the period of population plateau the temperature-dependent embryonic developmental time was rather similar (2.3 to 2.7 days). Neonates are like miniature adults and need a very short period of growth and moulting to reach the reproductive stage. This life-history trait can explain the fast blooming and dominance of plankton communities by P. avirostris under suitable conditions. Finally, the causes for the seasonal decline of P. avirostris populations are also discussed