Population dynamics of the infaunal bivalve, Soletellina alba

The population dynamics of the infaunal bivalve Soletellina alba were investigated at three sites situated in close proximity to the mouth of the Hopkins River estuary from 1997 to 1999. The distribution and abundance of juvenile and adult S.alba was very variable across all dates, sites and channel elevations (i.e. water depths). An experimental test comparing the recruitment of juveniles at different channel elevations and in sediments of varying particle size was conducted during 1999. The results of these tests showed that recruitment was greatest at the shallowest channel elevation and there was little evidence that sediment particle size influenced recruitment. In contrast to 1999, recruitment during 1997 or 1998 was very low.

Growth rates were monitored using tagged individuals held in caged and uncaged plots, which revealed that growth was highly variable among individuals, but not between sites. These tests also revealed that growth was negligible during the colder, winter months, and that the fastest growing individuals were capable of growing 0.2 mm/day.

Salinity tolerance experiments showed that bivalves exposed to low salinities ( 14 ppt), while death of bivalves exposed to salinities < 1 ppt occurred after 8 days of exposure. These tests provide evidence that low salinities are probably the principal cause of mass mortalities that are observed during winter flooding, although the interaction between salinity, temperature and turbidity also deserve consideration in the future.

It is hypothesised that the survival of very young juveniles (between 0.5mm and 1mm shell length) and rapid growth rates are important features of the life history of S.alba that explain its successful persistence within the Hopkins River estuary. It is highly likely that this species is capable of completing its entire life cycle within the estuary. The absence of other nearby populations, and periods of mouth closure, are likely to greatly limit the potential contribution made by larvae entering from the surrounding marine environment. This study has added to our knowledge of how an infaunal bivalve copes with life in the intermittently closing estuaries that typify semi-arid coastlines in the Southern Hemisphere.