Population regulation and demography in a harvested freshwater crayfish from Madagascar

Despite advances in statistical techniques for investigating population dynamics based on mark–recapture data, the majority of our understanding about demography and regulation comes from relatively few taxa. Most proposed generalisations about the association between demography and variation in population size are based on data from vertebrates, there are few sufficiently detailed invertebrate studies to examine whether these generalisations are widely supported. The population biology of freshwater invertebrates is especially poorly known. We present a large-scale mark–recapture study of an endemic freshwater crayfish from Madagascar ( Astacoides granulimanus ). Variation in density, caused by difference in fishing pressure due to local taboos, allowed us to investigate density-dependent regulation. We found evidence of density dependence in fecundity operating through the proportion of reproductive females by size but no significant evidence of density dependence in growth. Using a prospective analysis based on the elasticities from a size-structured matrix model, we found that both recruitment rates and survival rates of large individuals were strongly associated with deterministic population growth – a result that differs from generalisations drawn from vertebrate studies. A central assumption in mark–recapture studies is that handling does not affect mortality. By treating the number of times an individual was captured as an individual covariate, easily done using the freeware program MARK, we were able to test for, and take account of, handling-induced mortality. Our results show interesting similarities, and important differences, to generalisations based on vertebrate studies and emphasise the importance of population studies on poorly known taxa.     

A comparative study of the morphology and viability of hyphae of Penicillium expansum and Phytophthora nicotianae during freezing and thawing

The changes in morphology of Penicillium expansum Link and Phytophthora nicotianae Van Breda de Haan during freezing and thawing in a growth medium with and without the cryoprotective additive glycerol were examined with a light microscope fitted with a temperature-controlled stage. Viability of 0.5-1.0 mm diameter colonies of both fungi was determined after equivalent rates of cooling to -196 degrees C in the presence or absence of glycerol. In P. expansum shrinkage occurred in all hyphae at rates of cooling of less than 15 degrees C min-1; at faster rates intracellular ice nucleation occurred. The addition of glycerol increased the rate of cooling at which 50% of the hyphae formed intracellular ice from 18 degrees C min-1 to 55 degrees C min-1. This species was particularly resistant to freezing injury and recovery was greater than 60% at all rates of cooling examined. At rapid rates of cooling recovery occurred in hyphae in which intracellular ice had nucleated. In contrast, during the cooling of Ph. nicotianae in the growth medium, shrinkage occurred and no samples survived on thawing from -196 degrees C. However, on the addition of glycerol, shrinkage during freezing decreased and viable hyphae were recovered upon thawing; at rates of cooling over 10 degrees C min-1 the loss of viability was related to glycerol-induced osmotic shrinkage during cooling rather than to the nucleation of intracellular ice.     

Temperature and the tropical tortoise Kinixys spekii: tests of thermoregulation

The mean body temperature (T_b) of active Kinixys spekii did not vary with sex or type of activity, or between hot days (maximum ambient temperature T_a > 29°C) and cool days. On hot days, T_b increased rapidly in the morning, and was constant during the late afternoon after midday inactivity. On cool days, T_b increased more slowly in the morning, and decreased during the afternoon. The slopes of T_b on T_a overall, and on hot days, were close to one, suggesting that tortoises were thermoconformers; the slope was greater than one on cool days. In a second test of thermoregulation, T_bs were compared with temperatures of null models (T_m). Tortoises were clearly thermoregulators compared to 'active-all-day' models. 'Activity-time' models had T_m more similar to T_b. Nevertheless, detailed comparison showed that tortoises were thermoregulating in the late morning, and that this was by choice of microenvironment, rather than ceasing activity when T_b reached a high level. These results are discussed in relation to E, a measure of the effectiveness of thermoregulation based on comparison of T_b and T_m with the set point range (T_set) selected in a thermal gradient. A set of three indices, which separate the variability of T_b, the difference between T_b and T_m, and between T_b and T_set, is suggested as a more generally applicable summary of thermoregulation in ectotherms.     

Lesions of the Basal Forebrain Cholinergic System in Mice Disrupt Idiothetic Navigation

Loss of integrity of the basal forebrain cholinergic neurons is a consistent feature of Alzheimer’s disease, and measurement of basal forebrain degeneration by magnetic resonance imaging is emerging as a sensitive diagnostic marker for prodromal disease. It is also known that Alzheimer’s disease patients perform poorly on both real space and computerized cued (allothetic) or uncued (idiothetic) recall navigation tasks. Although the hippocampus is required for allothetic navigation, lesions of this region only mildly affect idiothetic navigation. Here we tested the hypothesis that the cholinergic medial septo-hippocampal circuit is important for idiothetic navigation. Basal forebrain cholinergic neurons were selectively lesioned in mice using the toxin saporin conjugated to a basal forebrain cholinergic neuronal marker, the p75 neurotrophin receptor. Control animals were able to learn and remember spatial information when tested on a modified version of the passive place avoidance test where all extramaze cues were removed, and animals had to rely on idiothetic signals. However, the exploratory behaviour of mice with cholinergic basal forebrain lesions was highly disorganized during this test. By contrast, the lesioned animals performed no differently from controls in tasks involving contextual fear conditioning and spatial working memory (Y maze), and displayed no deficits in potentially confounding behaviours such as motor performance, anxiety, or disturbed sleep/wake cycles. These data suggest that the basal forebrain cholinergic system plays a specific role in idiothetic navigation, a modality that is impaired early in Alzheimer’s disease.