Informing the conservation and restoration of a keystone species : the larval behaviour of the European oyster Ostrea edulis

Abstract

The European oyster Ostrea edulis is a keystone species that is internationally recognised as ‘threatened and declining’ in the North-East Atlantic and several nations have adopted strategies for its conservation and restoration. The overall goal of the present work was to inform conservation and restoration efforts. The purpose of this thesis, therefore, was to study the larval behaviour and ecology of O. edulis in as much as is relevant to the dispersal of this species. Specifically, the larvae’s vertical distribution, swimming speeds, settlement preferences and pelagic duration were studied in laboratory experiments. Most larvae concentrated at the bottom of the aquarium, independently of the developmental stage, light, food or temperature. In addition, larvae behaved actively in ~50% of all bottom observations, indicating a behavioural function other than resting. Advection close to the seabed is known to be slower than in any other part of the water column. The observed demersal behaviour would therefore most likely reduce dispersal from natal populations and enhance self-recruitment. At the surface, larvae frequently formed aggregations. In the water column, larvae swam with high vertical directionality and their distribution was homogenous. Swimming speeds ranged from 0.001 mm/s to 9.07 mm/s. O. edulis larvae settled preferentially among conspecifics (100% in < 24h), and if conspecifics were absent, larvae also settled in response to habitat-associated biofilms (81% of settlement after a 45h delay). Sterile shells and terrestrial stones did not induce more settlement than control treatments (0-14% settlement). Pelagic duration was strongly dependent on temperature, food and a suitable settlement cue. In the absence of an appropriate settlement cue, 80% of larvae delayed metamorphosis for up to 14 days, when the experiment was terminated. In contrast, 95-100% of larvae delaying their metamorphosis settled when presented with a conspecific. Such a delay in metamorphosis enhances the risk of predation and, ultimately, of losing most larvae to mortality if target habitats are absent. The results of this thesis provide strong evidence that O. edulis larvae are targeting their own beds, and that the behaviour of larvae plays a crucial role in their dispersal and successful recruitment. Restoring European oyster beds at a scale that is large and dense enough to promote the retention of larvae may be crucial to the success of restoration efforts.