Abstract
Larvae of the caridean shrimp Hippolyte inermis persist in the plankton of the Mediterranean up to about one month. Since they need to reach appropriate coastal areas for their recruitment in seagrass meadows, we hypothesized that leaves of Posidonia oceanica or, alternatively, algae present in their epiphytic biofilms, might be physically recognised as target substrates and trigger larval metamorphosis and settlement. Chemical cues could improve the finding of suitable habitats for settlement. Thus, the effects of leaves of P. oceanica and biofilms of the diatom Cocconeis scutellum parva, seasonally abundant in the leaf epiphytic stratum, were investigated along with the effect of volatile organic compounds (VOCs) extracted from the epiphytic diatom. The physical induction with P. oceanica accelerated larval settlement, stimulating an earlier and faster metamorphosis of larvae. C. scutellum parva produced a weaker effect on settlement; however, diatom’s VOCs had evident influence and accelerated metamorphosis and settlement. We concluded that such chemical cues as the VOCs produced by epiphytic diatoms, reinforce the effect of physical cues for the identification of suitable settlement locations for this shrimp.
Highlights
Moulting and settlement are key processes in aquatic environments, and the identification of suitable locations for recruitment strongly influences the fitness of invertebrates and the connectivity of their natural populations[1,2]
A non-polar metabolite extracted from bacteria (Tetrabromopyrrole)[28] is responsible for partial or complete metamorphosis in coral planulae, which demonstrated a highly specific response to single infochemicals[29]
The role of various algae-produced cues was demonstrated for echinoderms[31], corals[1] and polychaetes[32]. Both physical and morphological cues influence the settlement of larvae, and the final choice depends on a multi-factorial strategy including surface topography[5] and the availability of trophic resources, along with chemical signals[4]
Summary
Moulting and settlement are key processes in aquatic environments, and the identification of suitable locations for recruitment strongly influences the fitness of invertebrates and the connectivity of their natural populations[1,2]. Chemical properties of substrates are important cues for habitat selection[33,34] For these reasons, rather than investigating “if ” a given species uses specific cues to improve the settlement process, we should strive to understand “which” cues influence the larval settlement, “how” they work and “why” they are employed, according to ecological and physiological needs of given invertebrates. A limited number of studies demonstrated the effects of physical and chemical cues on metamorphosis and settlement of marine crustacean decapods[35], mainly on brachyuran crabs With this perspective we hypothesized that the shrimp Hippolyte inermis, strictly associated with the seagrass Posidonia oceanica in the Mediterranean sea[36], might perceive and react to the presence of plant leaves. The main aims of this study were: (a) to test the effects of physical and chemical cues within the known time for post-larvae production[8]; (b) to test the effects of diatom films on the settlement of H. inermis and those of volatile organic compounds (VOCs) produced by C. scutellum parva[8,44]; and (c) to compare the proportion of larvae metamorphosing in presence of inducers and in controls, as suggested by Forward et al.[3] for larvae of brachyuran crabs
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