Distribution and life history of the mysid Rhopalophthalmus mediterraneus in the Guadalquivir Estuary (SW Spain)

The Rhopalophthalmus mediterraneas population of the Guadalquivir Estuary ( SW Spain) was sampled monthly from January 1998 to December 1999. Density, population structure, brood size, and standing stock were recorded during b oth years. The estuarine population of R. mediterraneus was concentrated in the outer 20 km of the estuary. There were considerable seasonal changes i n the mysid abundance, with minima observed in winter and maxima in spring and summer. Such spatio-temporal distribution seemed to be related to estua rine environmental conditions: salinity was primarily responsible for the h orizontal distribution of the species within the estuary, while its seasona l trend was determined primarily by water temperature. The population was c haracterised by a clear dominance of adults on all sampling dates and sites , with females (66.8%, on average) significantly more abundant than males ( 23.8%, on average), and the proportion of adult males and females changed s ignificantly among sampling sites, being hi.-her at the outer area (ANOVA; SNK test, P < 0.01). Gravid females were collected throughout the year, wit h maximal reproductive activity occurring in spring and summer. The populat ion has a complex life cycle, being probably multivoltine and iteroparous. The brood size was significantly dependent on female size and season. The m ean length of adults of both sexes varied negatively with temperature: smal ler sizes occurred in summer.