The effects of salinity on the physiological ecology of Choromytilus chorus (Molina, 1782) (Bivalvia : Mytilidae)

Abstract

Choromytilus chorus frequently occurs in the estuaries of southern Chile, an environment that subjects it to changing ambient salinities. The effect of different salinities on the physiological processes of C. chorus was measured at 15, 18, 24 and 30‰ and at 12 °C. Scope for growth and net growth efficiency were used as physiological stress indices. Filtration rate was greater and similar at the two higher salinities (24 and 30‰), decreased at 18‰ and was considerably reduced at 15‰. Assimilation efficiency was high (≈ 80 %) and independent of both body weight and salinity. Oxygen uptake was also reduced by low salinity. There were no significant differences between weight exponents or between elevations when standard metabolism was compared at the three higher salinities. However, when the lowest salinity was included in the analysis, significant differences were observed. The energy lost due to excretion was very low at all salinities. The lowest value obtained at 15‰ was associated with the pronounced valve closure shown at this salinity, resulting in a build up of ammonia in the mantle cavity and body tissue and an inhibition in the production of NH 4. Scope for growth in C. chorus of given size was similar between 24 and 30‰ salinity and always positive in this range. Low feeding activity and a relatively high metabolic rate at the lower salinities resulted in a negative scope for growth for the larger animals at 18‰ and for all the size classes at 15‰ salinity. The stress imposed on C. chorus by low salinity was also reflected in a decreased net growth efficiency ( K 2). The data suggest that the growth rate as well as the degree to which this species can penetrate estuaries is partially determined by the amount of time which the mussels are able to spend in positive scope for growth under the estuarine conditions of reduced and fluctuating salinities.