Does Salinity Gradient Affect Distributions of Byssally-Attached Bivalves in Songkhla Lake, the Largest Estuary in Thailand?

Abstract

Bivalves are typically considered as ecosystem engineers, so knowledge of their distributions is important to understanding the structure of the benthic community in a lagoon system. This study aims to characterize the spatial distribution and examine mechanisms regulating the distribution of Mytilopsis sallei, Mytella strigata and Perna viridis in relation to salinity gradient, which is a key hydrographic feature, in Songkhla Lake, southern Thailand. Surveys showed that M. sallei was abundant in areas upstream with low salinity, while My. strigata were largely found in mid-range salinity. P. viridis was very rare and observed only at the mouth of the lagoon where it connects to the sea. Most M. sallei and My. strigata recruited at salinity conditions similar to where their adults were found. However, only a few recruits and adults of P. viridis was observed in the system, so their spatial relationship could not be evaluated. Results from an experiment testing the effect of different salinity levels on survival, growth, and condition of the bivalves showed that juveniles of M. sallei survived at all salinities (0, 20, 35 psu), while My. strigata and P. viridis survived only at 20 and 35 psu. Our study highlights the role of salinity as a key factor determining distribution of the byssally-attached bivalves in this estuarine system and demonstrates that the degree of influence recruitment and post-recruitment mechanisms have on distributions varies by species. The information obtained is crucial for understanding the dispersal mechanism of alien invasive species that is useful for species management.