Prey location, recognition. and ingestion by the phagotrophic marine dinoflagellate Oxyrrhis marina

The initial ingestion rates of Isochrysis galbana and Dunaliella primolecta by Oxyrrhis populations precultured separately on these phytoplanktonic prey were quantified and related to the chemosensory responses elicited in Oxyrrhis by the filtrate from live and heat killed prey cells. Despite evidence to suggest that Oxyrrhis shows specific distaste towards Isochrysis (but not Dunaliella) such that consumption of N-deplete Isochrysis halted in grazing experiments, positive chemotaxis was observed towards the cell-free filtrate from both species. These results suggest that while tactile cues encountered upon contact with Isochrysis and Dunaliella may enable Oxyrrhis to recognise differences between the two species, the chemosensory responses observed towards dissolved chemical cues derived from potential prey items are non-specific. That chemosensory and ingestion behaviours do not appear to be tightly coupled raises important questions concerning the ecological implications of chemotaxis in Oxyrrhis. Chemotaxis may enhance the overall efficiency of prey detection; however, when confronted with a variety of chemical stimuli (i.e. from a mixed-prey assemblage) Oxyrrhis may be unable to discern the difference between cues that originate from high quality, poor quality (or even toxic) prey items. The positive chemosensory responses observed towards a range of synthetic amino acid, amino sugar and ammonium solutions suggest that chemotaxis could facilitate the detection of solute gradients in prey deplete environments for direct exploitation via osmotrophy. Furthermore, the positive chemotaxis elicited by regenerated ammonium and compounds derived from heat killed conspecifics suggests that Oxyrrhis may release chemical cues which induce cannibalistic behaviour as a 'life boat mechanism' when no other suitable (non-self prey items are available. Further work is required to explore the nature of the chemosensory apparatus and signal transduction pathways that mediate responses to dissolved chemical stimuli in Oxyrrhis and to investigate other sensory mechanisms that enable cells to recognise and differentiate between potential prey items. (c) 2006 Elsevier B.V. All rights reserved.