Intraspecific competition in Tridacna crocea,a burrowing bivalve

Summary Intraspecific competition for space and light occurred when Tridacna crocea burrowed into coralline substratum of boulders on leeward coral reefs in the central Great Barrier Reef near Townsville, Australia. Intensity of competition was linearly related to clam density. Above about 200 clams/m², all clams physically contacted one another and all shells sustained damage. Mortality in isolated populations due to intraspecific competition was estimated at 40%. Principles of intraspecific competition in plants were tested for applicability to T. crocea populations. Juvenile mortality due to competitive stress was density dependent. Aggregated distributions of one year old clams changed to random or regular distribution of adults. Normal size-frequency distribution for juveniles became skewed for older groups. A bimodal size-frequency distribution of the population was related to selective mortality in 1–3 year old clams. Adult mortality due to crowding was less severe but significant. Growth rates were inhibited by competition. Deformations in morphology resulted from crowding. Intraspecific competition for space and light by adults inhibited recruitment of young. Animal adaptations to reduce mortality under crowded conditions were also important. Larvae aggregated on settling and oriented with posterior ends pointed away from nearest neighbors. Positional alignment within the substratum was selectively advantageous. Burrowing posteriorly was preferential, but anterior and sideways burrowing as well as twisting within the burrow were also observed. Movement within substratum served to reduce local damage to the shell. Proteinaceous deposits secreted through perforations in the shell reduced subsequent damage. T. crocea populations exhibited many animal adaptations that reduced mortality during the first years of life, but as cohorts matured, plant-like patterns of competitive interaction became more significant.     

Patchy recruitment patterns in marine invertebrates: a spatial test of the density-dependent hypothesis in the bivalve Spisula ovalis

Density-dependent and density-independent processes have been shown to influence the population dynamics of marine invertebrates, especially recruitment. However, their relative importance has not been evaluated in natural populations. High adult densities have been suggested to inhibit recruitment, especially in suspension-feeders which may ingest incoming larvae. Age structure and juvenile abundance were investigated in the bivalve Spisula ovalis in order to evaluate the importance of density dependence in generating spatial patterns. Age structure is readily established in this species owing to annual shell lines. An extensive sample (from about 100 sites a few hundred meters apart over 4 consecutive years) was analyzed in the statistical framework of spatial analyses, avoiding spurious correlations due to non-independence between neighboring sites. The area studied supports about ten annual cohorts, though only a few occur at each site. The overall picture is a mosaic of kilometer-scale patches of contrasted age structures, as revealed by highly significant spatial autocorrelations. To our knowledge, such large-scale spatial patterns in age structure have not previously been described in benthic invertebrates. Strong patterns are detected even for juveniles, and are independent of the adult biomass present before settlement. Therefore, patchy patterns of age structure mainly reflect density-independent effects, such as spatial variations in larval supply, passive transport of juveniles, or predation on recruits. In the absence of detailed spatial analyses, such patterns have been misinterpreted previously as negative effects of adult density on settlement success. Received: 21 November 1996 / Accepted: 20 February 1997     

Reversed optimality and predictive ecology: burrowing depth forecasts population change in a bivalve

Optimality reasoning from behavioural ecology can be used as a tool to infer how animals perceive their environment. Using optimality principles in a 'reversed manner' may enable ecologists to predict changes in population size before such changes actually happen. Here we show that a behavioural anti-predation trait (burrowing depth) of the marine bivalve Macoma balthica can be used as an indicator of the change in population size over the year to come. The per capita population growth rate between years t and t+1 correlated strongly with the proportion of individuals living in the dangerous top 4 cm layer of the sediment in year t: the more individuals in the top layer, the steeper the population decline. This is consistent with the prediction based on optimal foraging theory that animals with poor prospects should accept greater risks of predation. This study is among the first to document fitness forecasting in animals.     

Chemically induced predator avoidance behaviour in the burrowing bivalve Macoma balthica

The responses of the burrowing bivalves Macoma balthica and Cerastoderma edule to chemical cues emitted by feeding shore crabs Carcinus maenas were investigated. M. balthica held in the laboratory and exposed to chemical signals in effluent water discharging from tanks containing C. maenas fed 20 M. balthica day^− 1 reacted by increasing their burial depths from approximately 30 mm to depths of > 60 mm, over a period of several days. When the signal was removed the bivalves gradually returned to their original depth over 5 days. C. edule similarly exposed to effluent from crabs feeding on conspecifics showed no response. In an attempt to identify the signal inducing this burrowing response, M. balthica were exposed to a variety of chemical signals. Crabs fed M. balthica elicited the strongest response, followed by crabs fed C. edule. There were also small responses to effluent from crabs fed on fish, crabs previously fed on M. balthica and to crab faeces, but no responses to starved crabs, crushed M. balthica, or controls. We conclude that increased burrowing depth of M. balthica is induced by some as yet unidentified chemical cue produced by feeding crabs and is strongest when the crabs were fed on M. balthica. Unexpectedly, neither the presence of crabs themselves, nor of damaged conspecifics, was effective in eliciting a burrowing response. The mortality rates of M. balthica and C. edule selected by crabs when burrowed at normal depths and after exposure to effluent from feeding crabs were different. Crabs selected 1.5 times more C. edule than M. balthica when both species were burrowed at their normal depths, but 15 times more after the tanks had been exposed to effluent from feeding crabs for 5 days. The burrowing response of M. balthica thus appears to reduce mortality significantly by displacing predation pressure on to the more accessible C. edule.     

Evolution of viviparity in warm-climate lizards: an experimental test of the maternal manipulation hypothesis

The maternal manipulation hypothesis for the evolution of reptilian viviparity has been claimed to apply to any situation where gravid females are able to maintain body temperatures different from those available in external nests, but empirical data that support this hypothesis are very limited. Here, we tested this hypothesis using gravid females of a warm-climate lizard, Mabuya multifasciata, by subjecting them to five thermal regimes for the whole gestation period. We found gravid females selected lower body temperatures and thermoregulated more precisely than did nongravid females. Offspring produced in different treatments differed in head size, limb length and sprint speed, but not in overall body size or mass. Variation in morphological traits of offspring was induced primarily by extreme temperatures. Sprint speed of offspring was more likely affected by the mean but not by the variance of gestation temperatures. Gravid females maintained more stable body temperatures than did nongravid females not because these temperatures resulted in the optimization of offspring phenotypes but because the range of temperatures optimal for embryonic development was relatively narrow. Our data conform to the main predictions from the maternal manipulation hypothesis that females should adjust thermoregulation during pregnancy to provide optimal thermal conditions for developing embryos and that phenotypic traits forged by maternal thermoregulation should enhance offspring fitness.     

Host manipulation as a parasite transmission strategy when manipulation is exploited by non-host predators

Trophically transmitted parasites often alter their intermediate host's phenotype, thereby predisposing hosts to increased predation. This is generally considered to be a parasite strategy evolved to enhance transmission to the next host. However, the adaptive value of host manipulation is not clear, as it may be associated with costs, such as increased susceptibility to predator species that are unsuitable next hosts for the parasites. Thus, it has been proposed that, to be adaptive, manipulation should be specific by predisposing hosts more strongly to predation by target hosts (next host in the life cycle) than to non-hosts. Here we formally evaluate this prediction, and show that manipulation does not have to be specific to be adaptive. However, when manipulation is nonspecific, it needs to effectively increase the overall predation risk of infected hosts if it is to increase the parasite transmission probability. Thus, when initial predation risk is low, even highly nonspecific manipulation strategies can be adaptive. However, when initial predation risk is high, manipulation needs to be more specific to increase parasite transmission success. Therefore, nonspecific host manipulation may evolve in nature, but the adaptive value of a certain manipulation strategy can vary among different parasite populations depending on the variation in initial predation risk.     

A canopy-scale test of the optimal water-use hypothesis

Common empirical models of stomatal conductivity often incorporate a sensitivity of stomata to the rate of leaf photosynthesis. Such a sensitivity has been predicted on theoretical terms by Cowan and Farquhar, who postulated that stomata should adjust dynamically to maximize photosynthesis for a given water loss.
In this study, we implemented the Cowan and Farquhar hypothesis of optimal stomatal conductivity into a canopy gas exchange model, and predicted the diurnal and daily variability of transpiration for a savanna site in the wet–dry tropics of northern Australia. The predicted transpiration dynamics were then compared with observations at the site using the eddy covariance technique. The observations were also used to evaluate two alternative approaches: constant conductivity and a tuned empirical model.
The model based on the optimal water-use hypothesis performed better than the one based on constant stomatal conductivity, and at least as well as the tuned empirical model. This suggests that the optimal water-use hypothesis is useful for modelling canopy gas exchange, and that it can reduce the need for model parameterization.     

Emergence patterns in male butterflies: A hypothesis and a test

A game theoretical model is advanced to explain the emergence time schedule of male butterflies under temporal “apostatic” selection, so that males emerging on different days enjoy equal fitness in evolutionary equilibrium. The model predicts not only the position of the peak date but also the shape of the male emergence curve for any given female emergence schedule. Where the female emergence curve is smooth with one peak, a flight season can be divided into an earlier phase, when some males emerge every day, and a later phase in which no male emerges. The male emergence curve is truncated at the boundary of the phases. The position of the truncation point is determined by the difference between pre- and postemergence mortality. Preemergence mortality also determines the rate coefficient of the decrease in sex ratio through the season. The model is applied to a well-studied population of the butterfly Euphydryas editha. The male presence curve fits well, but no clear truncation exists in male emergence, and some males emerge earlier than predicted. Reasons for deviations are discussed.     

A test of the air-seeding hypothesis using sphagnum hyalocysts

“Air-seeding” is a proposed mechanism for the initiation of water stress embolism in dead plant cells. During air-seeding, external air is drawn into the lumen of a dead plant cell through a pore or crack in the cell wall. The resulting bubble may expand to fill the lumen, thus embolizing the cell. The data presented confirm that Sphagnum hyalocysts can embolize by air-seeding when the pressure difference across the air-water meniscus is given by ΔP = 0.3/D (derived from the capillary equation), where ΔP is the pressure difference across the meniscus (megapascal), and D is the diameter (micrometer) of the pore through which the air bubble enters.     

The evolution of parasite manipulation of host dispersal

We investigate the evolution of manipulation of host dispersal behaviour by parasites using spatially explicit individual-based simulations. We find that when dispersal is local, parasites always gain from increasing their hosts' dispersal rate, although the evolutionary outcome is determined by the costs-to-benefits ratio. However, when dispersal can be non-local, we show that parasites investing in an intermediate dispersal distance of their hosts are favoured even when the manipulation is not costly, due to the intrinsic spatial dynamics of the host-parasite interaction. Our analysis highlights the crucial importance of ecological spatial dynamics in evolutionary processes and reveals the theoretical possibility that parasites could manipulate their hosts' dispersal.     

A test of the hypothesis of pheromone attraction in salmonid migration

Synopsis We tested the hypothesis that anadromous salmonids are guided on their homeward migration by population-specific pheromones. Our findings do not support the hypothesis. Wild migrant Arctic charr,Salvelinus alpinus, from Ikarut River, Labrador were transferred and held in a tributary previously uninhabited by anadromous fish. None of the charr migrating up Ikarut River entered the tributary after fish were transferred. Similarly, migrant charr, which were caught in Ikarut River and released in the tributary below the captive fish, did not remain in the tributary. We re-evaluated the data which have been used to uphold the concept of pheromone attraction in salmonid migration and concluded that support for the hypothesis is unsubstantiated.     

Evolution of tolerance: the genetic basis of a host's resistance against parasite manipulation

Despite considerable theoretical advances in the evolutionary biology of host–parasite systems, our knowledge of host–parasite coevolution in natural systems is often limited. Among the reasons for the lag of experimental insight behind theory is that the parasite's virulence is not a simple trait that is controlled by the parasite's genes. Rather, virulence can be expressed in several traits due to the subtle interactions between the host and the parasite. Furthermore, the host might evolve tolerance to the parasite if there is sufficient genetic variance to reduce the detrimental effect of the parasite on these traits. We studied the traits underlying virulence and the genetic potential to evolve tolerance to infection in the host–parasite system Aedes aegypti – Brachiola algerae . We reared the mosquitoes in a half-sib design, exposed half of the individuals in each full-sib family to the parasite and measured several life history traits – juvenile mortality, age at pupation and adult size – of infected and uninfected individuals. Virulence was due in large part to a delay of the mosquito's age at pupation by about 10%. Although this imposes strong selection pressure on the mosquito to resist the parasite, all of the mosquitoes were infected, implying a lack of resistance. Furthermore, although additive genetic variance was present for other traits, we found no indication of additive genetic variation for the age at pupation, nor for the delay of pupation due to infection, implying no potential for the evolution of tolerance. Overall, the results suggest that in this host–parasite system, the host has little evolutionary control over the expression of the parasite's virulence.     

A test of the heterozygote-advantage hypothesis in cystic fibrosis carriers.

We report a test of the hypothesis that the high frequency of cystic fibrosis (CF) in Caucasian populations is due to a fertility advantage in CF carriers. One hundred forty-three grandparent couples of Utah CF cases were compared with 20 replicate sets of matched control couples drawn from the Utah Genealogical Database. Ascertainment correction, which has not been applied in previous studies of CF carrier fertility, was applied to these data. Before ascertainment correction was applied, CF carriers appeared to manifest a significant fertility advantage over controls. After the correction formula was applied, this difference disappeared. Carriers and controls were also compared in terms of the length of intervals between births. Again, no significant differences were found. It was concluded that fertility differences are unlikely to account for the observed Caucasian CF gene frequency. Other mechanisms, particularly a past selective event or random genetic drift, are more likely to be responsible.     

A test of the hydraulic limitation hypothesis in fast-growing Eucalyptus saligna

The hydraulic limitation hypothesis proposes that (1) reduced growth in taller trees is caused by decreased photosynthesis resulting from a decrease in hydraulic conductance promoted by a longer root‐to‐leaf flow path, and (2) this mechanism reduces stand productivity after canopy closure. This hypothesis was tested by comparing the physiology of 7 m (1 year) and 26 m (5 year) Eucalyptus saligna plantations where above‐ground productivity for the 26 m trees was approximately 69% of that for the 7 m trees, and water and nutrients were not limiting. The study compared whole tree physiology [water flux (Q_l), average crown conductance (G_T), crown hydraulic conductance per unit leaf area (K_L), carbon isotope discrimination (δ¹³C)] and leaf physiology under light saturation (leaf water potential at the canopy top (Ψ_LEAF), photosynthetic capacity (A_max), and photosynthesis (A) and stomatal conductance (g_s). K_L was 50% lower in the taller trees, but whole tree Q_l and G_T were the same for the 7 m and 26 m trees. Photosynthetic capacity was the same for leaves at the canopy top, but δ¹³C was ?1.8‰ lower for the 26 m trees. A and g_s were either lower in the taller trees or equal, depending on sampling date. The taller trees maintained 0.8 MPa lower Ψ_LEAF during the day and had 2.6‐times higher sapwood area per unit leaf area; these factors compensated for the effects of increased height and gravitational potential in the taller trees to maintain higher G_T. The hydraulic limitation hypothesis (as originally stated) failed to explain the sharp decline in net primary productivity after canopy closure in this study. The effects of increased height appear to be a universal hydraulic problem for trees, but compensation mitigated these effects and maintained Q_l and G_T in the present study. Compensation may induce other problems (such as lower Ψ_LEAF or higher respiratory costs) that could reduce carbon gain or shift carbon allocation, and future studies of hydraulic limitation should consider compensation and associated carbon costs. In this study, the combination of similar G_T and lower δ¹³C for the 26 m trees suggests that total crown photosynthesis was lower for the 26 m trees, perhaps a result of the lower Ψ_LEAF.     

A test of the production line hypothesis of mammalian oogenesis

Summary Germ cells in female mammals become committed to meiosis and enter its prophase sequentially in fetal life and, according to the Production Line Hypothesis, the oocytes thus generated are released after puberty as mature ova in the same sequence as that of meiotic entry in fetu. This hypothesis in its original and complete form has a subordinate proposition that concerns chiasma (Xma) frequency; it postulates that Xma number would decrease with fetal age. Consequently, univalents would increase, leading to errors of chromosome disjunction at the first meiotic division (MI), and thus to maternal age-dependent numerical chromosome anomalies. By using an in vitro/in vivo approach, we radioactively labelled the DNA of germ cells at premeiotic synthesis as they sequentially entered meiosis, while the fetal ovaries were in culture. At the end of this in vitro phase, pachytene/diplotene (P/D) stages were studied to determine their labelled fraction. The ovaries were then transplanted to spayed females and, after the in vivo phase, mature ova were harvested and the proportion of labelled first and second meiotic metaphases (MI/MII) determined. By marking the germ cells with label while in vitro during periods equivalent to early and late gestation, and by comparing the observed proportions of labelled MI/MII with those of oocytes labelled at P/D, we concluded that, in the mouse, ova do not mature at random for release, but are formed according to a production line system in which the time of release after puberty is related to the time of entry into meiosis in fetu.