Abstract. The majority of the published data about the reproductive biology of the decapod shrimp, Hippolyte inermis support the idea that this species is a protandric hermaphrodite, as is reported to be the case for certain other caridean species. However, our studies, based on the relative growth of the male reproductive appendage and histological examinations of the ovary, testes, oviducts, and deferentia vasa, indicate that there is no evidence supporting the occurrence of protandry. The first report of anomalies in the process of sexual inversion is credited to Reverberi (1950); however, we have obtained no evidence to support this phenomenon in H. inermis. We have not found either ovotestes or testes transforming into ovotestes. Therefore, we propose that H. inermis is a gonochoric species.

The study of a decapod community in a Cymodocea nodosa meadow from Southeastern Spain (Western Mediterranean Sea) showed a stable structure, in which the families Hippolytidae, Processidae, Majidae and Portunidae were the most abundant and the species Hippolyte niezabitowskii dominated. The animal community was more numerous and diverse during the night, showing the existence of nychthemeral movements, which are essentially related to the trophic behaviour and shelter. In this way, many species increased their abundance as a result of an increasing activity and, also, of an influx of other species and specimens from adjacent sandy bottoms, such as Processa spp. (mainly P. modica) Sicyonia carinata, Liocarcinus spp. (mainly juveniles) and several species of hermit crabs, which were rare or absent during the day. All these changes produced modifications in the dominance curves and in the values of all ecological indices (richness, diversity and evenness). Monthly samples were grouped and ordered (MDS) by the factor ''day-night'', which showed slight qualitative and quantitative differences (SIMPER, dissimilarity average of the factor day-night=61.67). On the other hand, no global seasonal differences have been found (one way ANOSIM), but there was a significant level of similarity between winter and spring, while the summer samples were the most different. The differentiation of the summer 1999 can be attributed to a decrease in species abundance and richness, probably due to the dynamics of the decapod populations and the balance with predators (fishes), while that of the summer 2000, to an anomalous event: the massive proliferation of filamentous algae, mainly Ectocarpus s.l., which modified the environmental conditions.

The present work completes a series of studies on the biology of the shrimp Hippolyte inermis Leach 1815, where we suggested the species to be gonochoristic. The morphology of the male reproductive system (testes, vasa deferentia, gonopores) and the different stages of male germ cell development are described for the first time in the genus Hippolyte, using TEM, SEM, and histological methods. All males from 1.70 to 3.42 mm in carapace length had active testes and well-developed vasa deferentia. No case of sex reversal could be found.

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