Historical processes and landscape context influence genetic structure in peripheral populations of the collared lizard (<Emphasis Type="Italic">Crotaphytus collaris</Emphasis>)

Populations at the periphery of a species’ range often show reduced genetic variability within populations and increased genetic divergence among populations compared to those at the core, but the mechanisms that give rise to this core-periphery pattern in genetic structure can be multifaceted. Peripheral population characteristics may be a product of historical processes, such as founder effects or population expansion, or due to the contemporary influence of landscape context on gene flow. We sampled collared lizards (Crotaphytus collaris) at four locations within the northern Flint Hills of Kansas, which is at the northern periphery of their range, to determine the genetic variability and extent of genetic divergence among populations for ten microsatellite loci (n = 229). We found low genetic variability (average allelic richness = 3.37 ± 0.23 SE; average heterozygosity = 0.54 ± 0.05 SE) and moderate population divergence (average F_ST = 0.08 ± 0.01 SE) among our sample sites relative to estimates reported in the literature at the core of the species’ range in Texas. We also identified differences in dispersal rates among sampling locations. Gene flow within the Flint Hills was thus greater than for other peripheral populations of collared lizards, such as the Missouri glade system where most of the mesic grasslands have been converted to forest since the last glacial retreat, which appears to have greatly impeded gene flow among populations. Our findings signify the importance of considering landscape context when evaluating core-peripheral trends in genetic diversity and population structure.     

Population genetic structure and landscape connectivity of the Eastern Yellowbelly Racer (<Emphasis Type="Italic">Coluber constrictor flaviventris</Emphasis>) in the contiguous tallgrass prairie of northeastern Kansas,USA

The tallgrass prairie of North America has undergone widespread habitat loss and fragmentation (<4% remains). The Flint Hills region of Kansas and Oklahoma is the largest tallgrass prairie remaining and therefore provides an opportunity to study the population genetic structure of grassland species in a relatively contiguous landscape and set a baseline for evaluating changes when the habitat is fragmented. We adopted a landscape genetics approach to identify how landscape structure affected dispersal, population genetic structure, and landscape connectivity of the Eastern Yellowbelly Racer (Coluber constrictor flaviventris) across a 13,500-km² landscape in northeastern Kansas, USA. The racer population had high allelic diversity, high heterozygosity, and was maintaining migration-drift equilibrium. Autocorrelation between genetic and geographic distance revealed that racers exhibited restricted dispersal within 3 km, and isolation-by-distance. Significant isolation-by-distance occurred at broad regional scales (>100 km), but because of sufficient gene flow between locations, we were unable to define discrete subpopulations using Bayesian clustering analyses. Resistance distance, which considers the permeability of habitats, did not explain significant variation in genetic distance beyond Euclidean distance alone, suggesting that racers are not currently influenced by landscape composition. In northeastern Kansas, racers appear to be an abundant and continuously distributed snake that perceives the landscape as well connected with no cover type currently impeding snake dispersal or gene flow.     

Experimental deepwater culture of the Sydney rock oyster (Crassostrea commercialis=Saccostrea cucullata): I. Growth of vertical clumps of oysters (‘ren’)

Japanese style ren, consisting of scallop shells encrusted with small Sydney rock oysters and strung on vertical wires, were grown at 23 estuarine stations along the New South Wales coast for up to 21 months. Weight increments of ren were used to evaluate growth rates of oysters in non-cumulative (3–4 months immersion) and cumulative (6–21 months immersion) series. Oyster weight increased at a rate of approximately 10% per month, which was considered suitable for commercial production, but estimated labour costs to make up the ren were high. Mortality rate was 6.3% per 3–4 months immersion. Gaping oysters, through natural mortality, winter mortality, the haplosporidian parasite Marteilia sydneyi, and other causes, accounted for 4.5%. Broken oysters, resulting from fish and crab predation, accounted for 1.8%. Mortality caused by the polychaete mudworm Polydora websteri was negligible; only 1.7% of the 4.5% gapers showed the characteristic blisters.     

Experimental deepwater culture of the Sydney rock oyster (Saccostrea commercialis). VI. Commercial oyster cage systems

A growout cage that retains oysters, excludes predators and reduces maintenance is described; it is cut from 200 mm (8 in) steel reinforcing mesh as kitset units and then hot dip galvanised. Each cage is 2.2 m long, 0.8 m wide and 1.0 m deep (88 × 22 × 40 in) and contains five floors which are 203 mm (8 in) apart. The floors and the walls are covered with 19 mm ($\text{3}\text{4}$ in) galvanised wire mesh, or high density polyethylene plastic mesh. These cages may be slung from rafts, fitted with individual flotation or lowered to the bottom with a marker buoy; they weigh approximately 70 kg (154 lb) empty and about 220 kg (484 lb) when harvested. Each cage crop consists of approximately 2.5 bags totalling about 150 kg (330 lb), and in good growing areas three to four crops have been harvested from each cage per year. In two such crops culled spat (22–29 g whole wet weight) were converted to plate (first grade) oysters (40–67 g); in the other two, seconds (29–40 g) were converted to plate oysters. Growth rates, stocking densities and mortality data are given for 11 crops.     

Experimental deepwater culture of the Sydney rock oyster (Crassostrea commercialis = Saccostrea cucullata): II. Pontoon tray cultivation

Pontoons made from plastic pipe were tested as an alternative to racks for deepwater culture of the Sydney rock oyster. The growth and mortality of oysters permanently suspended in water on trays beneath floating pontoons were compared with oysters on trays in an intertidal zone. For both culled spat (30–31 g whole oyster weight) and seconds oysters (37–39 g) beneath pontoons the growth rate, measured by weight increases, was three times that of oysters on intertidal racks over a 5-month period. Mortality (from unknown causes) was higher beneath the pontoons. The mean mortality of spat oysters was 40% compared with 24% on the intertidal trays, and for seconds oysters was 51% compared with 34%.     

Experimental feeding of Sydney rock oysters (Crassostrea commercialis = Saccostrea cucullata): I. Optimum particle sizes and concentrations

Oysters removed cornflour particles of 0–4 μm diameter at a rate significantly different (5% level) from the controls. For 4–6 μm particles the difference was significant at the 10 but not the 5% level. There were no significant differences in removal rates for 6–10 μm particles. Particles less than 5 μm in diameter seemed optimum for ingestion.The optimum concentration for riceflour particles less than 5 μm in diameter was approximately 2 mg/l; at this concentration there was maximum faecal production and minimum pseudofaecal production. The changeover point (50% producing pseudofaeces) was approximately 18 mg/l; above this concentration pseudodefaecation steadily increased until 35 mg/l, when pseudofaeces (as well as faeces) were being produced by 100% of the oysters. Similar results were obtained for riceflour, starch gluten effluent and active dried yeast, but the ranges were different.     

Experimental feeding of Sydney rock oysters (Saccostrea commercialis). III. Food concentration and fattening procedures

Sydney rock oysters (Saccostrea commercialis) were fed an artificial diet at 0, 3, 6, 9, 12 and 15 mg dry food/l. At 9 mg/l oyster glycogen content, condition indices and meat weight gains were maximised without increasing mortality rates. In a comparison of fattening procedures, oysters held in a fertilised pond and those fed a second artificial diet fattened in 4 and 6 weeks respectively, whereas those held in an estuary did not fatten at all. The artificial diet produced higher oyster meat glycogen contents than the fertilised pond and estuary treatments.