Clearance rates achieved by an intertidal oyster, Isognomon ephippium, under different tidal emersion and temperature conditions

Abstract

Restoring ecosystems in coastal estuaries impacted by increasing modification and habitat loss requires species-specific data to increase success. Oyster reefs, for example, have been extensively lost around the world, and while restoration efforts are underway, in Australia for example, species data is limited which means that restoration efforts are supported by knowledge obtained from overseas programs. What is shared among oyster species is that their position in the intertidal zone is a function of their tolerance to air exposure but also ability to access sufficient food for energy which occurs when covered by tidal waters. Here we collected Isognomon ephippium, a species at risk of loss across its range in Australia, from a tropical estuary (Townsville; −19.25°S, 146.82°E) in northern Australia to examine how the feeding rate (clearance rate; CR) changes under different tidal emersion (0, 2.5 and 5 h s) and seasonal air and water temperatures (determined from long term air and water temperature records – scenarios were based on winter, annual average, summer and extreme conditions). Results revealed a negative trend in CRs as exposure time to air and water temperature conditions increased from 0 to 5 h.. An exception to this was for oysters exposed to the extreme air and water temperatures as CRs were low or negligible, suggesting our experiment had reached potential thermal limit for oysters. The results outline that the natural shift in tides from season to season in Townsville (winter months the low tide occur during day, while summer months the high tide occurs during the day) provides important thermal refugia strategies for this intertidal species, whereby it may not frequently encounter extreme ambient conditions during summer months when temperatures are expected to exceed their upper thermal limit because the high tide coincides during these critical times of the day. We advocate for managers to investigate local species-specific ecological requirements in order to increase the likely success of shellfish restoration.