Growth and respiration throughout the life-cycle of Nematospiroides dubius Baylis (1926) (Nematoda: Heligmosomidae): the parasitic stages

The development of spicules in Heligmosomoides bakeri (Nematoda, Heligmosomidae)

The spicules of male parasitic nematodes are key morphological features, which vary between species in shape and length and are used often for species identification. However, little is known about spicules and particularly if/how their length varies during growth. We first assessed the degree of variation in spicule length of male Heligmosomoides bakeri 21 days post infection (PI), and then in two follow-up experiments measured spicule lengths at half daily/daily intervals between days 6 and 14 PI. Mean spicule length in 21-day worms was 0.518 mm with a range of 94 μm, and variation between the two spicules of individual worms from 2 to 32 μm. Spicules were first detectable on day 6-6.5, after which their lengths increased until day 7 PI (mean = 0.61 and 0.59). This was followed by significant contraction, initially relatively quickly over the following 48 h and then more slowly over a longer period, stabilizing by days 10-14, with only minor further reduction in length. We conclude that the length of spicules varies significantly over the first few days after they have formed, and, consequently, the age of worms is an important factor for consideration when spicule lengths are measured for experimental/diagnostic or taxonomical purposes.

Beyond the '3/4-power law': variation in the intra- and interspecific scaling of metabolic rate in animals

In this review I show that the '3/4-power scaling law' of metabolic rate is not universal, either within or among animal species. Significant variation in the scaling of metabolic rate with body mass is described mainly for animals, but also for unicells and plants. Much of this variation, which can be related to taxonomic, physiological, and/or environmental differences, is not adequately explained by existing theoretical models, which are also reviewed. As a result, synthetic explanatory schemes based on multiple boundary constraints and on the scaling of multiple energy-using processes are advocated. It is also stressed that a complete understanding of metabolic scaling will require the identification of both proximate (functional) and ultimate (evolutionary) causes. Four major types of intraspecific metabolic scaling with body mass are recognized [based on the power function R=aMb, where R is respiration (metabolic) rate, a is a constant, M is body mass, and b is the scaling exponent]: Type I: linear, negatively allometric (b<1); Type II: linear, isometric (b=1); Type III: nonlinear, ontogenetic shift from isometric (b=1), or nearly isometric, to negatively allometric (b<1); and Type IV: nonlinear, ontogenetic shift from positively allometric (b>1) to one or two later phases of negative allometry (b<1). Ontogenetic changes in the metabolic intensity of four component processes (i.e. growth, reproduction, locomotion, and heat production) appear to be important in these different patterns of metabolic scaling. These changes may, in turn, be shaped by age (size)-specific patterns of mortality. In addition, major differences in interspecific metabolic scaling are described, especially with respect to mode of temperature regulation, body-size range, and activity level. A 'metabolic-level boundaries hypothesis' focusing on two major constraints (surface-area limits on resource/waste exchange processes and mass/volume limits on power production) can explain much, but not all of this variation. My analysis indicates that further empirical and theoretical work is needed to understand fully the physiological and ecological bases for the considerable variation in metabolic scaling that is observed both within and among species. Recommended approaches for doing this are discussed. I conclude that the scaling of metabolism is not the simple result of a physical law, but rather appears to be the more complex result of diverse adaptations evolved in the context of both physico-chemical and ecological constraints.

Helminth growth in vertebrate hosts: does host sex matter?

Helminth infections are usually more severe in male than in female vertebrate hosts. If parasite establishment is easier in male hosts, parasite growth may also be facilitated in males. This was tested with a meta-analysis of published between growth rates of worms in male and female vertebrate hosts. Two-thirds of the 48 comparisons found showed higher growth in male hosts than in females, but the average relative difference did not differ from zero. However, after controlling for sample size and for the variability in the original data, a small but significant effect of host sex was found. The meta-analysis suggests that male hosts harbour not only more helminths than females, but also slightly larger ones.

The distribution of Nematospiroides dubius within the small intestine of laboratory mice

Analyses of primary infections of Nematospiroides dubius in male and female laboratory mice show that on days 4--6 post-infection worms occupy up to 50% of the small intestine but as the age of the infection increases worm populations are highly aggregated in relation to the anterior sections of the small intestine. Individual worms are also shown to be aggregated with respect to each other. Between days 42 and 60 post-infection, although the pattern of distribution of N. dubius along the small intestine does not significantly change, survival of worms is influenced by both age and sex of ASH/CSI S.P.F. mice, reasons for which are discussed in the light of previous work on primary infections of N. dubius in other strains of laboratory mice.