Search

Fragmented populations of freshwater fish may develop genotypic and phenotypic differences as adaptations to local habitat conditions. These differences contribute significantly to biological diversity and may lead to speciation. In the Murray–Darling Basin, Australia, the Murray hardyhead Craterocephalus fluviatilis, listed as ‘endangered’ by the World Conservation Union, has a wide but fragmented distribution that is apparently related to salinity. To determine whether this pattern has a physiological basis, we compared osmoregulation in fish from two isolated populations in different salinity regimes (Wyndgate: 0.4–1.5‰; Disher Creek: c. 1.0–45‰). In laboratory trials, fish from both populations remained healthy at high salinities (5–65‰). The Disher Creek population maintained a significantly lower blood osmotic concentration than the Wyndgate population at salinities ≤1‰, suggesting that there is a physiological difference between them. The findings have implications for the conservation of C. fluviatilis and other fish populations whose distributions are fragmented by salinity. [ABSTRACT FROM AUTHOR]

Comparative studies of related species may reveal the determinants of distribution and abundance, and elucidate the causes of fragmentation. The intensively regulated River Murray in south-eastern Australia contains fragmented populations of several small fish species that have more common, widespread congeners. The Murray hardyhead Craterocephalus fluviatilis (McCulloch) (Atherinidae) has a wide but patchy distribution, whereas the unspecked hardyhead Craterocephalus stercusmuscarum fulvus (Ivantsoff, Crowley & Allen) has a similar range but is generally abundant. The two species rarely cohabit. Salinity and other aspects of ‘habitat complexity’ appear to be key factors separating the species and associated fish assemblages, although this requires confirmation. The findings are discussed in the context of other studies of closely related species. [ABSTRACT FROM AUTHOR]