Population fragmentation is a common symptom of the decline of species, including freshwater fishes. It occurs naturally, but has also proliferated in response to human interventions that increase the prevalence and intensity of isolating barriers and events. In regulated rivers, for example, fish are affected by the loss of connectivity between habitats that is associated with hydrological changes. The process has evolutionary consequences by limiting gene flow, reducing genetic diversity and rendering the isolates vulnerable to local environmental changes. Comparative studies of related species may help to elucidate the causes and consequences of fragmentation. For example, they may identify habitat features that influence the spatial separation of congeneric species. An opportunity for such a study arises with small fishes (Atherinidae) in the intensively-regulated River Murray, southeastern Australia. Whereas the unspecked hardyhead Craterocephalus stercusmuscarum fulvus is widespread and abundant, the Murray hardyhead C. fluviatilis has a patchy distribution and is listed as 'endangered‘ by the International Union for Conservation of Nature and 'vulnerable‘ under the Australian Environment Protection and Biodiversity Conservation Act 1999. These two species rarely cohabit, implying that they could be separated by particular habitat characteristics. In the past, several species of Craterocephalus, including C. fluviatilis and the Darling River hardyhead C. amniculus, have been regarded as C. eyresii sensu lato. The taxonomic separation of C. s. fulvus has been confirmed, but some doubt remains about the relationship of C. fluviatilis and C. amniculus. This issue needs resolution to ensure that appropriate targets are set for conservation. This study is a comparative investigation of the aforementioned species. It was designed (1) to identify the habitat characteristics that influence the distribution and abundance of C. fluviatilis and, given that salinity emerged as a key factor, (2) to explore the biological implications of salinity through a comparative study of osmoregulation in C. fluviatilis and C. s. fulvus, (3) to determine whether the osmoregulatory responses of population isolates of C. fluviatilis differ at varying salinities, and (4) to evaluate the genetic population structure of C. fluviatilis, confirm its taxonomic separation from C. amniculus and identify genetic 'management units‘ for conservation. Field sampling showed that C. fluviatilis is confined mainly to saline waters (0.4-20‰), whereas C. s. fulvus is absent from salinities >7‰. Comparisons were made of osmoregulation in these two taxa over a salinity range of 0.03-85‰, with additional reference to the small-mouth hardyhead Atherinosoma microstoma, a related estuarine species that tolerates salinities >94‰. The three species all are euryhaline, although the osmoregulatory ability of C. s. fulvus falters above about 35‰ salinity. C. fluviatilis is a better osmoregulator than A. microstoma at salinities <1‰, but both species tolerate hypersaline conditions (85‰). Osmoregulation was compared in C. fluviatilis from two isolated populations in different salinity regimes (Wyngate: 0.4-1.5‰, Disher Creek: c. 1.0-45‰) to determine whether they show related phenotypic differences. Fish from both populations remained healthy at salinities from 5-65‰. The Disher Creek population maintained a significantly lower blood osmotic concentration than the Wyngate population at salinities ≤1‰, suggesting that there is a physiological difference between them. The genetic population structure of C. fluviatilis and its taxonomic distinction from C. amniculus were investigated using complementary allozyme and mtDNA markers. This confirmed that C. fluviatilis is genetically distinct from its sister taxon, C. amniculus. It also identified several genetically-defined 'management units‘ as a framework for future conservation. Further, it revealed that C. fluviatilis in habitats downstream of Lock 1 on the Murray (274 km from the river mouth) displays a genetic signature indicating introgression with C. amniculus. Clearly, these findings have implications for the conservation of C. fluviatilis. For example, isolates can be prioritised for protection, and re-introduction programs can be modified accordingly. The findings may be applied to other freshwater fish, especially populations of closely-related species subject to salinisation or other stressors, and they may also contribute toward understanding of the factors and processes underlying rarity and fragmentation. It is clear that salinity can be a significant factor in population fragmentation, and that closelyrelated species with similar ranges may be segregated by differences in osmoregulatory ability. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1363300 / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2009
| Identifer | oai:union.ndltd.org:ADTP/264740 |
| Date | January 2009 |
| Creators | Wedderburn, Scotte Douglas |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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