Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river

1. Algal growth in lotic systems is controlled either from the bottom‐up (e.g. nutrients and light, which determine growth rates) or from the top‐down (e.g. grazing pressure, which reduces accumulated biomass). Nutrient‐enriched streams that support large and diverse grazing macroinvertebrate populations and those with shaded riparian corridors rarely suffer from excessive algal growth. 2. In this study, the density of benthic algivorous macroinvertebrates was experimentally manipulated in shaded and open nutrient‐enriched stream habitats of the Owennagearagh River, south‐west Ireland. The ability of macroinvertebrate grazers and riparian shade to control benthic algal growth [particularly the nuisance alga, Cladophora glomerata (L. Kütz)] was investigated. Three sites with markedly different concentrations of plant nutrients (one site upstream and two sites downstream of the sewage outfall) were selected. The density of grazing invertebrates colonising ceramic tiles was reduced using high‐voltage localised electric pulses. Replicates of treatment (grazer‐excluded) and control (grazed) tiles were deployed in open and shaded (<25 and >80% canopy cover, respectively) patches of stream bed, in each site. 3. After 2‐week Cladophora cover, periphytic chlorophyll a and biofilm ash‐free dry mass (AFDM) were quantified for all experimental tiles. Values for all three parameters were highest on grazer‐excluded tiles from open patches. Grazed tiles from open patches accrued little Cladophora and had significantly lower levels of chlorophyll a and AFDM. Nutrient inputs were found to have an impact on the density of grazing invertebrates, with higher densities of Baetis nymphs at the most nutrient‐enriched site. 4. Our results demonstrate that in eutrophic, high‐light streams, filamentous algae can quickly accumulate to nuisance levels in the absence of invertebrate grazers. In future, greater attention should be paid to the role of grazing invertebrates in controlling nuisance algae in streams, in addition to algal–nutrient relationships.     

The shredding activity of gammarids facilitates the processing of organic matter by the subterranean amphipod Niphargus rhenorhodanensis

1. The functional feeding group approach has been widely used to describe the community structure of benthic invertebrates in relation to organic matter resources. Based on this functional framework, positive interactions between feeding groups (especially shredders and collector‐gatherers) were postulated in the River Continuum Concept. However, relationships with organic matter have been poorly documented for invertebrates living in the hyporheic zone. 2. We hypothesised that the common subterranean amphipod Niphargus rhenorhodanensis would feed on fine particulate organic matter (FPOM), which is more abundant than coarse particulate organic matter (CPOM) in hyporheic habitats, and should be favoured by the occurrence of shredders that produce FPOM from CPOM. 3. We used laboratory experiments to quantify leaf litter processing by N. rhenorhodanensis and a common shredder, the surface amphipod Gammarus roeselii. We estimated rates of feeding and assimilation (using nitrogen stable isotopes) of the two species separately and together to reveal any potential shredder–collector facilitation between them. 4. Measured leaf litter mass loss showed that N. rhenorhodanensis did not act as a shredder, unlike G. roeselii. Organic matter dynamics and ¹⁵N/¹⁴N ratios in tissues of niphargids indicated that N. rhenorhodanensis was a collector‐gatherer feeding preferentially on FPOM. We also found a positive influence of the gammarid shredders on the assimilation rate of N. rhenorhodanensis, which fed on FPOM produced by the shredders, supporting the hypothesis of a positive interaction between surface shredders and hyporheic collector‐gatherers.     

Chemical mediation of reciprocal mother–offspring recognition in the Southern Water Skink (Eulamprus heatwolei)

Abstract Kin recognition has been demonstrated to play an important role in the social structure of a wide range of animals. Most studies to date have examined parent–offspring recognition only in species that provide offspring with direct parental care, however, there are several advantages to parent–offspring recognition even in the absence of direct parental care. In this study we investigated reciprocal mother–offspring recognition in the Australian scincid lizard Eulamprus heatwolei, a species that does not show direct parental care. We examined whether neonates could discriminate between their mothers and unrelated females, and whether females could discriminate between their offspring and unrelated neonates, via chemical cues, using retreat site selection experiments. We conducted trials when neonates were 1 and 4 weeks old to investigate whether responses are maintained as neonates age. We found that both neonates and mothers could discriminate between related and unrelated individuals when neonates were 1 week old. Mothers were more likely to take refuge under tiles treated with the odours of their own offspring, while neonates spent less time in areas treated with the odours of unrelated females. At 4 weeks of age, mothers no longer exhibited discriminatory behaviour between their offspring and unrelated neonates, while neonates were more likely to associate with the odour of any female over the odourless control. We hypothesize that reciprocal mother–offspring recognition in E. heatwolei reduces interference competition between mothers and their offspring and also may be important in habitat selection and territory establishment.     

Density‐dependent prophylaxis in primary anti‐parasite barriers in the velvetbean caterpillar

1. Organisms rely on a set of primary barriers to prevent invasion by parasites, and secondary defences to fight parasites that breach the primary barriers. However, maintaining these defences to be active and effective is costly. Thus, hosts increase investment in anti‐parasite defences under situations of high risk of infection and reduce defences when the risk is reduced (the ‘Density‐Dependent Prophylaxis’ hypothesis). 2. In the present study, it was tested whether the midgut primary defences of the velvetbean caterpillar Anticarsia gemmatalis Hübner present density‐dependent plasticity, and also whether these defences could be induced by a viral pathogenic challenge. The aim was to examine whether morphometry and the structure of the midgut and peritrophic matrix (PM) change in accordance with colour transition in caterpillars, and whether such changes may provide the caterpillars a more protective barrier against invasion by Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV). 3. It was found that PM and the midgut epithelium of the velvetbean caterpillar change plastically according to phenotype, itself a response to changes in population density. Caterpillars reared at high densities (black phenotype) had a considerably thicker midgut epithelia and peritrophic matrices than those reared individually (green phenotype), and there was also more chitin in the PM of the former. 4. This was interpreted as the first demonstration of increased investment in primary, barrier, defences against parasites, in response to increased conspecific density and an increased risk of infection. The possibility that this arises as a positive result of pleiotropy is discussed further, wherein the biochemical pathways responsible for the up‐regulation of the immune system are also involved in midgut properties.     

Retention time as a primary determinant of colour and light attenuation in two tropical Australian reservoirs

1. The light climates of Darwin River Reservoir (DRR) and Manton River Reservoir (MRR), in northern Australia, are compared for an 8-year period. The reservoirs are subject to the same wet/dry tropical climate and have similar catchment characteristics, but differ in their basin morphology, retention time and trophic status.
2. Median euphotic depths in DRR and MRR were 9.7 and 4.4 m, respectively. Seasonal variation in each reservoir's euphotic depth was best explained by colour, based on a stepwise linear regression. Turbidity was excluded from the two regressions, while chlorophyll a concentration was significant only for the MRR regression.
3. Both reservoirs showed the same seasonal pattern for colour. Wet season inflow increased reservoir colour, and was followed by a reduction in colour due to photodegradation and microbial decomposition of humic material, reaching a minimum before the next wet seasons inflow.
4. Although the colour of catchment run-off into both reservoirs was similar, MRR colour was two to three times greater than that measured in DRR. The higher colour and greater light attenuation of MRR is attributed primarily to its shorter retention time, and therefore shorter time for colour removal. Annual retention time accounted for 97% of the variability of average annual colour (measured as absorption at 440 nm), based on a linear regression of log₁₀ transformed data for both reservoirs.
5. Long retention times favour reduced colour, increasing water transparency, particularly in water bodies of low trophic state and inorganic turbidity.     

The functional morphology of mating in the Silurian eurypterid, Baltoeurypterus tetragonophthalmus (Fischer, 1839)

The functional morphology of the genital appendages, horn organs and modified prosomal limb III of the Silurian eurypterid, Baltoeurypterus tetragonophthalmus (Fischer, 1839) (Chelieerata: Eurypterida) is investigated. The longer type A genital appendage, interpreted as a female structure, is associated with a pair of horn organs, regarded as spermathecae. The shorter type B appendage, interpreted as male, is associated with scimitar lobes on prosomal limb III, regarded as clasping structures. A mechanism is proposed by which the type A appendage was lowered through flexure of the genital operculum acting along sutures in the cuticle. A model of mating is suggested, involving the male depositing an unstalked spermatophorc on the substrate and the female retrieving it for storage in the spermathecae. This model is more arachnid-like than mating in Limutus (an extant aquatic chclicerate), and suggests that sperm transfer using spcrmatophores and storage of sperm in spermathecae, allowed the timing of egg production in curyptcrids to be controlled.     

Respiration Rate and Mitochondrial Oxidase Activity in Arum Spadix

1. Mitochondria prepared from young Arum spadix oxidise succinicand -ketoglutaric acids at about 300µl. O₂/hr./g. wetwt. but as the spadix ages the activity of the mitochondriaincreases more than tenfold. The increase results from a smallrise in the quantity of mitochondrial nitrogen brought downin the centrifuge and a larger rise in oxidase activity permg. mitochondrial nitrogen. 2. The rate of respiration of spadix slices has been measuredunder conditions in which it is not limited by slow diffusionof oxygen. Slices of young spadix respire at about 1,000–2,000µl. O₂/hr/g. but as the inflorescence opens the rate soarsto 20,000µl. O₂/hr/g. and the spadix becomes warm. 3. Cytochrome oxidase was assayed in mitochondria treated withdigitonin. There is at all times enough cytochrome oxidase toaccount for the rates of oxidation of succinate and -ketoglutarate;and enough to account for the relatively slow respiration ofthe young spadix slices but not for the fastest respirationof the mature spadix. 4. The respiration of spadix slices is found, in contradictionto an earlier report, to be sensitive to malonate.     

Influence of adult breeding experience on growth and provisioning of Wandering Albatross Diomedea exulans chicks at South Georgia

For most seabirds, reproductive performance improves with age; in albatrosses this is thought not to be so (experience being acquired before starting breeding) but only one study (of chick growth in a single season at one site) has specifically addressed this. We compared the provisioning performance and growth rates of chicks of Wandering Albatrosses Diomedea exulans breeding for the first (IN), second and third (LE) and fourth or more times (EE) on Bird Island, South Georgia in the austral winters of 1996 and 1997. Eggs from EE adults were significantly heavier than the other two categories and these chicks had a greater mass and longer wings up to 160 days of age and longer culmen and tarsus up to 115 days old. However chicks from all categories fledged at the same average mass, size and age. No significant differences between categories in feeding frequency or meal size were detected but experienced adults made shorter long foraging trips and spent more time at the nest than less experienced birds. Adults that remained at the nest gave chicks smaller meals than those that left immediately after feeding the chick. Although provision of smaller but more frequent meals by experienced adults promotes more rapid chick growth, the resulting differences do not persist into the late chick-rearing period. Our results were very similar to those from Iles Crozet in the Indian Ocean, supporting the hypothesis that when Wandering Albatrosses start to breed they are fully competent foragers but that it takes a while, during early chick-rearing, for birds breeding for the first time to adapt to the additional demands of provisioning a chick.