Molluscs of the Leschenault Inlet estuary: Their diversity, distribution, and population dynamics

Thirty-one species of mollusc were collected in Leschenault Inlet during 1982-1987. Seven species were common, with the remaining 24 species occurring sporadically, rarely or only once during the study. These seven most common species in order of general abundance were: Arthritica semen, Tellina deltoidalis, Nassarius burchardi, Spisula trigonella, Hydrococcus brazieri, Acteocina sp and Bedeva paivae. The molluscs of Leschenault Inlet can be classified as follows: (1) a stenohaline marine component: Bittium granarium, Mytilus edulis, Polinices conicus, Pholas australasiae, Nassarius nigellus, Solemya australis, Irus crenata, and Venerupis anomala; (2) a euryhaline marine component: Tellina deltoidalis, Tellina sp, Theora lubrica, Sanguinolaria biradiata, Philine angasi, Nassarius burchardi, Bedeva paivae, Spisula trigonella, Epicodakia sp, and Laternula creccina; and (3) a true estuarine component: Acteocina sp, Arthritica semen, Xenostrobus securis, Hydrococcus brazieri, Fluviolanatus subtorta, Assiminea sp, and Salinator sp. Across the inlet in general, molluscs inhabited tidal sand or tidal mud, shallow water platform sand or muddy sand, or deep water basin mud, within lower, middle, or upper estuarine salinity fields. In this context, seven broadly recurring assemblages or populations of mollusc could be discerned: (1) a mixed molluscan assemblage inhabiting the tidally flushed environments of the Preston River Delta and the tidal delta leeward of “The Cut”; (2) a Tellina (+ Spisula) assemblage inhabiting the deep water central muddy basin; (3) a Tellina-Nassarius (+ Bedeva) assemblage inhabiting seagrass-vegetated platforms; (4) Tellina populations inhabiting shallow subtidal mud flats, (5) Hydrococcus populations inhabiting tidal sandy beaches; (6) Acteocina populations inhabiting tidal mud flats; and (7) Arthritica populations inhabiting low tidal to shallow subtidal sand flats. Population structures were found to be different for the three common genera. Tellina populations appeared to be maintained by a relatively continuous low level of juvenile recruitment. Nassarius populations were dominated by a mature age cohort, with a low intermittent level of juvenile recruitment. Spisula populations were numerically dominated by one age cohort, and were not maintained by further recruitment. This study provides a five-year perspective of changes in mollusc populations in Leschenault Inlet, providing insight into the variability and longevity of the fauna. Some species are consistently present in the estuary although abundances varied seasonally, while others fluctuated markedly in their presence or absence. While the overall character in terms of diversity/abundance, and population structure of the mollusc assemblages may have remained similar, there were also changes in composition from year to year and from season to season. For many species there was a decrease in abundance, such that the relative abundance of species within an assemblage changed with time. Abundances of each species fluctuated largely independently of other species or a given habitat, and did not apparently occur in response to seasonal patterns in oxygen concentration, temperature, or salinity.