Effects of Salinity on Distribution of Alien Invasive and Indigenous Byssally-attached Bivalves in the Songkhla Lake Basin

Abstract

Mytilopsis sallei (Récluz, 1849) and Mytella strigata (Hanley, 1843) are alien invasive byssally-attached bivalves that invaded Songkhla Lake which is a part of the largest lagoon system in Thailand. Another bivalve, a native Perna viridis (Linnaeus, 1758) was commonly found in the lake before, but it has been very rare in recent years, instead it occurs seasonally on the coast near the mouth of the lake. This work investigated distribution pattern and recruitment variability of these dominant bivalves in the Songkhla Lake with a focus on the effects of salinity which fluctuates greatly in this estuarine system. Growth rate, condition index and survival of juvenile bivalves rearing at different salinities (0, 20, 35) were also monitored to examine mechanisms influencing distribution of these bivalves. Spatial pattern of bivalve recruitment and adult distributions were congruent. Recruits of M. sallei and adults were found at salinities ranging from freshwater to mid-range salinity. M. strigata recruited at salinities from brackish to saline condition, where the adults were present. Recruits and adults of P. viridis were found at the mouth of the lake and on the shore of the Gulf of Thailand where salinity was high. Observations around the lake also showed that the distribution of the adult bivalves changed due to the seasonal variations of salinity in the lake. M. strigata and P. viridis seemed to migrate to the inner part of the lake when salinity became higher. From the experiment, M. sallei survived at all salinities and having the highest condition index at salinity 35. M. strigata and P. viridis survived only at salinity 20 and 35. The study suggests that salinity is the key factor influencing recruitment process that regulates distribution of the dominant byssallyattached bivalves in this estuarine system