2004. Antipredator behaviour mediated by chemical cues: the role of conspecific alarm signalling and predator labelling in the avoidance response of a marine gastropod. -Oikos 104: 43 -50.Gastropods represent a challenge in the understanding of alarm signalling. We studied predator avoidance (climbing behaviour) of the marine snail Tegula funebralis in laboratory experiments. Snails were exposed to crude extract of conspecifics, and to water conditioned by actively feeding or non-feeding predatory crabs. Crabs had previously been maintained on different diets, and were accordingly labelled by chemical cues of various origins. Tegula-extract alone released climbing behaviour in May, but not in June. However, during both these months, snails responded to chemical cues from crabs that were actively feeding on Tegula. Crabs labelled by Tegula-diet, and actively feeding on Tegula, also caused more climbing responses compared to crabs labelled by other diets. Chemical cues derived from crabs actively feeding on another snail species, or from non-feeding crabs, did not induce snail climbing no matter the previous feeding history of the predators. When snails received Tegula-extract combined with water conditioned with a non-feeding, Tegulalabelled crab, no climbing occurred. However, when the non-feeding, Tegula-labelled crab was present in the solution of Tegula-extract, moderate climbing responses were obtained. The results imply that climbing responses of T. funebralis are in general caused by the action of a two-component system. This system seems to be a mixture of chemical cues leaking from the tissue of conspecifics when being eaten, and latent conspecific chemicals that are modified in crabs and presumably released with the urine of chemically labelled predators. The modified chemical labels appear to be fully released by crabs when feeding, and moderately released when detecting food. The responses obtained in May with crude extract alone may result from a seasonal change in alarm signalling properties, or a change in behavioural responsiveness of snails exposed to a variable predator regime.

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High resolution images of zooplankton organisms were recorded in situ using a digital autonomous video plankton recorder (VPR) in a semi-enclosed bay in northern Norway. Data from vertical VPR profiles was used to estimate the depth distribution and abundance of zooplankton along a bay-wide grid. Images were recorded simultaneously with hydrographical data, and the spatial distribution of different organisms was related to several water column characteristics, such as chlorophyll a, density, and buoyancy frequencies. In June, unusually high abundances of hydromedusae (Rathkea octopunctata and Obelia sp.) were located in a fine-scale layer of a few metres thickness. This layer could be associated with relatively high buoyancy frequencies, reflecting a density gradient present throughout the entire sampling area. Aggregation of hydromedusae around density discontinuities may have enabled retention of medusae in the bay. Ctenophores and appendicularians were distributed above the medusae layer, while peaks of copepod abundance were found outside the layer boundaries. We suggest that the bloom of R. octopunctata was initially caused by liberation of new medusae from successful benthic hydroid colonies, and that proliferation of this species was sustained by asexual, manubrial budding in the medusae stage. KEY WORDS: Zooplankton · Hydromedusae · Video plankton recorder · Vertical distributionResale or republication not permitted without written consent of the publisher