The recent evolutionary origin of the phenotypically novel amphipod Hyalella montezuma offers an ecological explanation for morphological stasis in a closely allied species complex

Numerous molecular studies have identified morphologically cryptic, freshwater invertebrate species, but have not suggested possible mechanisms for their phenotypic stasis. The amphipod crustacean genus Hyalella contains numerous morphologically cryptic species in the H. azteca complex, as well as a small number of morphologically very divergent, narrowly endemic taxa. One such taxon, Hyalella montezuma, is the sole planktonic filter-feeder within the North American amphipod fauna, and is known only from Montezuma Well, a fishless travertine spring mound in Arizona, USA. In this study, we conduct a phylogenetic analysis of mtDNA sequence data using likelihood, Bayesian and cladistic approaches to determine both the relationship of H. montezuma to the H. 'azteca' species complex, and to ascertain if its morphological and ecological differentiation have been comparatively recent. The results show that H. montezuma has a very close phylogenetic affiliation with one lineage in the H. azteca complex, indicating that its origin has been recent. We present evidence suggesting that fish predation is an important ecological factor, which constrains morphological and ecological diversification within the genus Hyalella, and that Montezuma Well has provided a relaxation on this constraint.     

Diatom and micro-invertebrate communities and environmental determinants in the western Australian wheatbelt: a response to salinization

Diatom and micro-invertebrate communities were analyzed in 56 lentic wetlands from the wheatbelt of Western Australia. Sixteen water quality parameters were measured at each habitat and were tested as determinants of diatom and micro-invertebrate distribution. Nearly all waters were dominated by Na¹⁺ and Cl^1– and over half had low buffering capacity (<2 meq/L as TCO). Salinities ranged from <1 to > 240 mS/cm; some hypersaline habitats had pH values as low as 2.1, but most sites were alkaline.Diatoms (165 taxa) were present in all habitats, with indications that assemblages are correlated with land-use practices, i.e., historical clearing of native vegetation for dryland agriculture with resultant hydrological problems of salinization and water-logging. A non-metric multidimensional ordination showed pH, salinity, TCO, and total phosphorus to be significantly correlated with diatom community structure. Principal components analysis showed similar patterns. A salinity index was also developed for numerically important diatom taxa. Principal components analysis showed similar patterns. Certain tolerant Bacillariales, including Hantzschia amphioxys, Nitzschia hybrida, Tryblionella hungarica, and N.recta, were numerically important taxa in wetlands with visible signs of salinization.Micro-invertebrates (143 taxa) were present in habitats with salinities up to 200 mS/cm. Multidimensional ordinations showed salinity to be significantly correlated with micro-invertebrate community structure. The cladoceran Daphniopsis queenslandensis, ostracods Australocypris insularis, Mytilocypris tasmanica chapmani, and Platycypris baueri, and copepods Apocyclops dengizicus and Mesochra nr. flava were numerically important micro-invertebrates in moderate to high salinity environments, including those with signs of salinization. The diatoms, Navicula minuscula var. muralis and Pinnularia divergentissa var.subrostrata, and the copepod, Calamoecia trilobata, were common inhabitants of the extreme hypersaline (75 mS/cm), acidic (<4) habitats. Diatom communities were similar to those previously reported for other regions in Australia subjected to salinization; there were some differences in micro-invertebrate communities.     

An energetic evaluation of a predator–prey (leech–amphipod) couple in Montezuma Well, Arizona, USA

1. The highly stable conditions in Montezuma Well, Arizona, have contributed to a very close predator–prey relationship between the endemic leech, Erpobdella montezuma , and its endemic amphipod prey, Hyalella montezuma , which forms nearly 90% of the leech diet.
2. Using a simple bioenergetic model, it was determined why a benthic opportunistic foraging strategy, typical of most other erpobdellids, is inferior to the specialized foraging behaviour utilized by E . montezuma and why reproduction by the leech is restricted during certain times of the year.
3. Our model demonstrated that to attain a biomass in excess of 2000 mg (maximum field size) during periods of low amphipod densities in the winter E . montezuma must feed on alternative prey types and have a foraging activity < 4 h per day.
4. Leech reproduction 4–6 months prior to the onset of winter food shortages of prey increases juvenile and adult survivorship with highest reproductive success for E . montezuma occurring in June–July.
5. Rather than being an obligate forager on H . montezuma, E . montezuma exploits a periodically abundant amphipod during most of the year in addition to alternate prey types during the winter thereby achieving an overall higher rate of energy gain than could be achievable by other closely related species of erpobdellids utilizing an opportunistic foraging approach.     

The diatom flora of Lake Eyre South: a large episodically filled salt lake in South Australia

Thirty-three diatom taxa were collected from the large terminal salt lake, Lake Eyre South, located in south-central Australia. The diatom assemblage included taxa that were widely distributed in marine and saline athalassic habitats in both northern and southern hemispheres, with the diatom flora of Lake Eyre South most similar to that of southern Africa. This study represents the first report on the diatom flora of Lake Eyre South and supports the findings that episodically filled lakes do not appear to be good evolutionary loci.