title: Genetic structure and life history variation in a cryptic fish species complex, Australian smelt (Retropinna semoni) across south-east Queensland, Australia
Creators: name: Islam, R.
Publisher: name: Griffith University
Year: 2018
Subjects: subject: Genetic structure
subject: Cryptic fish species complex
subject: Retropinna semoni
subject: Australian smelt
subject: Fish populations

description: Generally freshwater fish exhibit higher levels of genetic structuring between spatially distinct populations than marine species due to the presence of natural and artificial barriers to dispersal in freshwater ecosystems. In addition, freshwater species are not able to move between populations that are separated by either terrestrial or marine habitat. Diadromy constrains the development of genetic structuring, even among geographically isolated populations due to potential connectivity via movement through the sea. As a result, higher levels of gene flow and lower population structuring tend to be observed in diadromous fishes than freshwater species. The Australian smelt (Retropinna semoni) is a native fish species complex widely distributed across coastal and inland drainages of south-eastern Australia. Recently, a complex of five or more cryptic species of Australian smelt has been recognized throughout their geographic range based on genetic studies. Variation in life history strategies has been observed in many cryptic fish species and multiple life history patterns were also found in southern smelt lineages (Retropinna sp.) where mainland Australian populations contain diadromous and wholly freshwater individuals and Tasmanian populations contain estuarine individuals. Despite the populations of southern smelt containing diadromous individuals, strong genetic structuring and low levels of connectivity were reported in at least some populations, which were suggested to result from apparently diadromous individuals being retained in the estuaries. It is possible that the different cryptic species may differ in life history. In the present study, I examine the genetic structure and life history variation of the South-east Queensland (SEQ) lineage of Australian smelt which was further subdivided into northern and southern lineages (SEQ-N, SEQ-S). I used both molecular and ecological approaches to understand the pattern of genetic structure and life history variation in this species to compare with other members of the species complex. Prior to this task, twenty one polymorphic microsatellite primers were developed (Chapter 2), which were then used for species delimitation and population structure analysis. In chapter 3, I used one mitochondrial gene (cyt b) and ten microsatellite loci to investigate patterns of genetic structuring in Australian smelt (R. semoni) and describe the genetic differences between these two cryptic lineages (SEQ-N and SEQ-S). These two lineages formed monophyletic clades in the mtDNA gene tree and among river phylogeographic structure was also evident within each clade. There was clear genetic divergence between the two lineages, suggesting that they have been separated historically by a hard barrier. Strong genetic structuring was observed from microsatellite analysis in both lineages (SEQ-S FST = 0.13; SEQ-N FST = 0.23) suggesting limited dispersal among rivers. Slightly lower levels of genetic structuring were observed in the SEQ-S lineage than the SEQ-N lineage. This might be the result of different microhabitat preferences between these two cryptic lineages (SEQ-N and SEQ-S), for example intolerance to water quality parameters. Another plausible explanation is that SEQ-S catchments may have been connected more recently and /or more often than those in the SEQ-N group during flood events. Contemporary movement of individuals only occurred between nearby sites within a river, but not between rivers, suggesting that if local extinctions occur in one or more of these rivers, then recolonization from elsewhere is unlikely to occur rapidly. Similarly, extinctions within a site are only likely to be recolonised from nearby sites in the same river. In chapter 4, I used otolith micro-chemistry analysis to examine the life history patterns of smelt at the northern extent of their range and to identify any differences in migratory behaviour of the two lineages (SEQ-N and SEQ-S) in this region. Based on otolith core-to-edge transects of Sr:Ca and Ba:Ca, there was no evidence of marine residence for either lineage suggesting that both are non-diadromous. This contrasts with the two southern smelt species, in which both exhibit evidence of diadromous movement of individuals within some populations. Significant differences in multi-elemental otolith chemistry signatures were observed among rivers and between paired sites within some rivers, suggesting no exchange of individuals among catchments and limited dispersal of individuals over large spatial scales within a river. This is almost certainly the result of the marine habitat conditions along the coastal drainages which isolate rivers and restrict movement of fish between them, combined with some limited dispersal within a river. In addition, movement of fish may also be precluded due to anthropogenic migration barriers within river catchments. This study has presented a holistic view of population structure using ecological and genetic markers and revealed that R. semoni is highly structured across south-east Queensland from the Mary River to Currumbin Creek. Genetic analysis delivers the general framework for applying ecological methods and substantial information regarding exchange between populations. Sensitive ecological methods such as otolith chemistry provide resolution of the fine-scale spatial separation within and between sample collection locations because movements within an individual lifetime can be inferred. The results of the present study emphasise the advantages of using complementary methods to evaluate the connectivity of fish populations. The combination of otolith chemistry and molecular markers provided insights into the role of migration in structuring smelt populations over a range of temporal and spatial scales. Overall, the current study furnished new insights into the population genetic structure and life history patterns of Australian smelt, which has significant implications for the sustainable management and conservation of this ecologically variable species along coastal drainages in south-east Queensland.