This PhD study takes a multidisciplinary approach to the investigation of anemonefish host-imprinting to explain the mechanism and place it in an ecological context. To do this I have collected information on three different types of investigations;

1) An ethological study experimentally examined how the juvenile anemonefish A. melanopus recognized its host, and whether recognition and selection are mediated by an imprinting phenomenon. Specifically, I experimentally examined the host-selection made by A. melanopus that had been reared under constant conditions, but whose embryos had received one of 3 treatments: (a) in contact with a known natural host sea anemone, Entacmaea quadricolor (Am-Eq); (b) in contact with the sea anemone Heteractis malu that is not a host for A. melanopus in nature, but is a host for anemonefish of other species (Am-Hmalu); (c) without a sea anemone (or chemical cues released from sea anemones) at any life stage (Am -). My study showed that olfaction, not vision, was used by juvenile A. melanopus to recognize host anemones. Furthermore the choice of a settlement site for juvenile A. melanopus was strongly influenced by events that occur early in development, prior to the dispersal of larvae from their natal site. I suggest juvenile A. melanopus possess an innate preference for E. quadricolor; a preference that is enhanced by imprinting. It was not possible to imprint A. melanopus larvae to the non-host sea anemone Heteractis malu, which suggests that anemonefish host-imprinting may be rather restricted.

2) A field investigation was conducted, which examined whether anemonefishes exhibited any specific spawning site preferences in the field, and whether this was influenced by possible imprinting. All species studied (Amphiprion akindynos, A. bicinctus, A. melanopus & A. perideraion), showed the same pattern of spawning site preference. (1) Egg clutches were always placed in contact with the host anemones column or pedal disc, with the tentacles hiding the egg clutch. (2) The steeper the substrate slope, where the host anemone was attached, the more likely it was for the parental fish to choose a spawning site exactly above the host anemone. I infer the findings of this study show that, with regard to the spawning site preference, anemonefish host-imprinting, together with predator protection, outweighs: (1) the danger of loosing newly hatched embryos, killed by the stinging tentacles; (2) the importance for the embryos to easily find the direction towards the surface, right after hatching by phototaxis. The findings are support for the anemonefish host-imprinting hypothesis.

3) Lastly, the development of the olfactory system in anemonefish embryos of A. melanopus was examined from day six postfertilization, through to hatching (day nine). For anemonefish host-imprinting to function, a pre-requisite is a requirement for the olfactory system to be fully functional prior to the larvae hatching in order that imprinting to the chemical cue from the parental anemone can occur. A functional olfactory placode with receptor cells lining the olfactory epithelium and nerve axons from the olfactory placode into the olfactory bulb were observed on newly hatched embryos. Two different secondary bilateral solitary receptor systems were found. I infer that these findings: (1) support the anemonefish host imprinting hypothesis; (2) Indicate that the ontogenetic timing of this imprinting mechanism occurs towards the end of embryonic development; (3) Support the hypothesis that chemotaxis plays an important role in the mechanisms of larval settlement of some reef fish species.

The findings of this study have brought new knowledge of the dynamic processes of tropical fish ecology, in terms of an enhanced understanding of tropical fish settlement. This will help identify factors that can influence the abundance patterns and recruitment and survival of newly settled reef fish. The findings of this study have shown that anemonefish host-imprinting is an advanced form of chemotaxis, helping the settling juveniles recognize a suitable host, i.e. microhabitat on the coral reef.