| Summary: | Crassostrea gigas was first introduced into the UK in 1965 for aquaculture purposes. Light natural spatfalls have been observed in estuaries in South-West England including an abundant population at the Yealm estuary, Devon. This study reports the first evidence of reproduction in the wild population of C. gigas at the Yealm estuary. Monthly sampling took place between January and August 2012 at two shore heights. Fully ripe and post-spawning gametogenic stages were observed in sampled C. gigas from both shore heights providing support for some individuals in the population successfully spawning. Environmental parameters such as the chlorophyll a concentration of seawater samples, sea surface temperature and sediment flux were monitored in order to examine possible interactions with the gametogenic cycle and population density of this wild population. A suppression of spawning was observed under reduced seawater temperatures in a thermal mesocosm experiment confirming a role for seawater temperature in the progression of normal gametogenesis and spawning. The ecological effects of C. gigas invasion on intertidal communities have been poorly studied particularly on rocky habitat. At the rocky mid shore Yealm estuary significantly different communities were supported by the natural rocky habitat and C. gigas shell in a field clearance and settlement experiment. Diversity indices revealed remarkably similar values across substrate giving comparable species diversity, richness and evenness of the associated macrofauna. A significantly lower juvenile Semibalanus balanoides survival rate post-settlement was found on C. gigas shell when compared to rock and this may have consequences for population success in this cold water barnacle species. This study provides support for a case by case outlook following a thorough examination of the consequences of invasion before the impacts of invasion are determined positive or negative.
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