Effects of salinity on the survival of the Brackwater mussel, <i>Brachidontes virgiliae</i>, in the St Lucia estuarine system, South Africa

Abstract

During drought periods, the razor clam Solen cylindraceus is the dominant bivalve in the St Lucia estuarine system, although restricted to its South Lake region. However, with the recent onset of a wet phase, the mussel Brachidontes virgiliae has become widespread and overwhelmingly dominant throughout the system. The salinity tolerance of B. virgiliae is here determined using both rapid and gradual changes in salinity. Mussels were collected at Esengeni in the Narrows (salinity ≈ 0) and Lister's Point in False Bay (salinity ≈ 20). Mortalities were recorded for animals exposed to a sudden change in salinity using 8 different treatments, ranging from 0 to 70. Additionally, animals were exposed to a gradual change in salinity, using treatments that exceeded the minimum and maximum salinities mussels were previously able to tolerate. In all four experiments, animals were able to tolerate salinity levels up to 20. However, a wider salinity tolerance, up to 50, was shown by animals collected from Lister's Point and those gradually acclimated to test conditions. With an increase in flood events predicted for this region, it is imperative to understand how key species may be affected. During wet phases B. virgiliae becomes ubiquitious throughout Lake St Lucia and it is unlikely that the species will disappear from the system, even if floods escalate in the future, as it has an ability to withstand near-freshwater conditions. During dry periods, however, the mussel will be concentrated in the Narrows (oligohaline to limnetic conditions), especially if an inverse salinity gradient with hypersaline conditions prevails within the system.

Short-Term Exposure of Mytilus coruscus to Decreased pH and Salinity Change Impacts Immune Parameters of Their Haemocytes

“…In comparison, salinity stress had a weaker effect on phagocytosis as compared with decreased pH. This suggests a higher tolerance of these mussels to salinity changes (Nel et al, 2015 ). A reduction of phagocytosis under low salinity condition was found in the mussel P. viridis (Wang et al, 2012b ), the mussel M. edulis (Bussell et al, 2008 ), the oyster C. gigas (Gagnaire et al, 2006a ), the abalone H. diversicolor (Cheng et al, 2004b ) and the saltwater clam C. gallina (Matozzo et al, 2007 ).…”

Biodiversity census of Lake St Lucia, iSimangaliso Wetland Park (South Africa): Gastropod molluscs

“…Since the end of 2011, the system has entered a wet phase, with above average rainfall leading to occasional flooding and the prevalence of oligo- to polyhaline conditions throughout the extent of the system. This was compounded by the excavation of a beach spillway in July 2012, which has since contributed substantial freshwater inflow from the Mfolozi River into the St Lucia system and also partial exchange of water with the open ocean ( Nel 2014 , van Elden et al in press ). These changes have led to the appearance of a number of new gastropod species of brackish to freshwater origin, including Bulinus natalensis , Cassidula labrella , Melampus parvulus , Melampus semiaratus and Melanoides tuberculata (Figure 2 ).…”

Population irruption of the clam Meretrix morphina in Lake St Lucia, South Africa

“…6 These shifts appear to be related to flood and drought states of the system, more specifically, its associated salinity regime. 21 St Lucia was previously dominated by B. virgiliae during wet phases and by S. cylindraceus during the preceding drought phase 22 ( Figure 2). However, during 2015, instead of the expected irruption of S. cylindraceus, M. morphina dominated (Figure 2, Table 1).…”