Abstract. The taxonomical and trophic structures of the vagile fauna communities of the leaf stratum in a Posidonia oceanica meadow at Ischia (Gulf of Naples, Italy) were investigated at five stations along a depth gradient (1 to 25 m). Sampling was performed in July, November, February, and May. The analyzed groups ‐ polychaetes, molluscs, tanaids, isopods, amphipods, and decapods ‐ exhibited similar distributional trends in all seasons, with coenotic discontinuities occurring at well‐defined depths. The same zonation pattern was produced by feeding‐guild analysis. Eleven trophic groups were identified. The most abundant groups were: Herbivores, which were found mainly at the shallow stations; Herbivores‐deposit feeders, which were widely distributed along the transect; Deposit feeders‐carnivores, found mainly at the deep stations. This study suggests that in the Posidonia leaf stratum, herbivores and herbivores‐deposit feeders, as consumers of epiphytic micro‐ and macroflora and deposited particulate organic matter, play an important role in the energy transfer from producers to higher trophic levels of the system.

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Abstract. Studies on leaf growth and production were performed in two stands, at depths of 5 m and 22m, in the Posidonia oceanica (L.) Delile bed off Lacco Ameno, Ischia (Gulf of Naples). Experiments were carried out in situ from May 1988 to August 1989 at monthly intervals. Leaf growth and production profiles differed in the two stands investigated. Growth rates were higher at 5 m, with a peak in autumn (5.8 mg · shoot‐1· d‐1) and another in March (3.8 mg · shoot‐1· d‐1). At 22 m the maximum rate occurred in May (3.9 mg · shoot‐1· d‐1). These differential growth rates resulted in a delayed maximum leaf surface and biomass at the deep stand, where lower irradiance values and different temperature patterns were also found. The biomass of epiphytic algae showed trends similar to those of leaves; however, there were remarkable differences in the values between the two stations and for the two investigated years. The production data of the present study are compared with those of other reports, and it is shown that growth processes are greatly influenced by physical factors.

The effect of diatoms of the genus Cocconeis on the sex reversal of the shrimp Hippolyte inermis Leach was examined in the laboratory. Randomised experiments were carried out to investigate the functional response of shrimps to various diets. The benthic diatom Cocconeis neothumensis was offered, alternatively, during the larval phase and during the postlarval phase, and the results obtained with shrimps produced by individual females were compared. Results demonstrated that diets based on green alga Enteromorpha sp. or dry commercial food did not influence the normal protandric development, as most shrimps at sexual maturation were males. Conversely, diets containing C. neothumensis did influence the protandric development, as most shrimps at sexual maturation were females and the sex ratio was significantly different from that obtained with diets not containing C. neothumensis. These results provide an explanation of the patterns observed in the field, and are in accordance with the seasonal abundances of diatoms in the leaf stratum of Posidonia oceanica.