In rivers worldwide, hydrological persistence and variability (i.e. environmental stability) typically parallel longitudinal changes in habitat. This interaction complicates determination of the hierarchy of mechanisms that structure fish communities along rivers. In this study, we examined fish species richness and presence–absence in pools of an intermittent river system containing underground water storages (Fortescue River, north-west Australia), a system that was predicted to uncouple this relationship. Stability, measured by pool persistence, was unrelated to a pool's maximum depth or its position in the catchment, indicating partial decoupling. However, pool stability remained correlated with habitat diversity and log-transformed surface area. Model selection indicated that species richness was better described by pool stability and the landscape factor stream order than by within-pool habitat descriptors. Permanent pools low in the catchment contained more species than unstable pools in headwater streams. We conclude that the distribution of fish in the Fortescue River is shaped predominantly by processes of extirpation and re-colonisation. Management efforts in this river and similar intermittent systems should focus on the preservation of refuge pools, and limit the construction of barriers that limit dispersal.
Here we report on the first quantitative survey of morphological variation in the sea urchin Heliocidaris erythrogramma within Western Australia and distinguish between two subspecies found to co-occur in this region. We surveyed urchins at multiple spatial scales along the Western Australian coastline to assess variation in dermis and spine color and, using landmark-based geometric morphometrics, spine morphology. Both color and morphology proved to be useful for separating subspecies within Western Australia. There were four major color morphs: red dermis/violet spines (56%), red/violet-green (23%), red/ green (7%), and white/green (10%). Members of the first two color morphs had bulbous spines with wide, flattened tips, a morphology that is unique to Western Australia and characteristic of H. e. armigera, and members of the latter two consistently exhibited the narrow, pointed spines typical of specimens of H. e. erythrogramma, which has a broader distribution. In Western Australia, H. e. armigera was relatively abundant both within and among sites, but H. e. erythrogramma was found only in a few localized patches. Shifts in the relative abundance of these two subspecies occurred at fine spatial scales (o5 km), although environmental correlates of these transitions were unclear. Contrary to expectations, neither dermis color nor spine morphology varied with relative wave exposure: individuals with a red dermis or thickened spine morphology occurred at most sites regardless of exposure, and while white dermis and thinner spines only occurred at high-exposure sites, these features were not common across the majority of exposed sites. Both color morph frequencies and spine morphology remained stable within sites over the 3-year duration of this study. While the ecological significance of this morphological variation remains unclear, the consistency of the association between color and spine morphology, occurring across fine spatial scales, suggests that strong environmental or genetic factors are involved in maintaining morphological differentiation between these two subspecies.Additional key words: geometric morphometrics, Heliocidaris erythrogramma armigera Invertebrate Biology 130(2): 161-174. r
The spatial scale over which genetic divergences occur between populations and the extent that they are paralleled by morphological differences can vary greatly among marine species. In the present study, we use a hierarchical spatial design to investigate genetic structure in Heliocidaris erythrogramma occurring on near shore limestone reefs in Western Australia. These reefs are inhabited by two distinct subspecies: the thick-spined Heliocidaris erythrogramma armigera and the thin-spined Heliocidaris erythrogramma erythrogramma, each of which also have distinct colour patterns. In addition to pronounced morphological variation, H. erythrogramma exhibits a relatively short (3-4 days) planktonic phase before settlement and metamorphosis, which limits their capacity for dispersal. We used microsatellite markers to determine whether patterns of genetic structure were influenced more by morphological or life history limitations to dispersal. Both individual and population-level analyses found significant genetic differentiation between subspecies, which was independent of geographical distance. Genetic diversity was considerably lower within H. e. erythrogramma than within H. e. armigera and genetic divergence was four-fold greater between subspecies than among populations within subspecies. This pattern was consistent even at fine spatial scales (< 5 km). We did detect some evidence of gene flow between the subspecies; however, it appears to be highly restricted. Within subspecies, genetic structure was more clearly driven by dispersal capacity, although weak patterns of isolation-by-distance suggest that there may be other factors limiting gene exchange between populations. Our results show that spatial patterns of genetic structure in Western Australian H. erythrogramma is influenced by a range of factors but is primarily correlated with the distribution of morphologically distinct subspecies. This suggests the presence of reproductive barriers to gene exchange between them and demonstrates that morphological variation can be a good predictor of genetic divergence.
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