Predator induced life-history shifts in a freshwater cladoceran

Summary Life-history theory predicts that maturity and resource allocation patterns are highly sensitive to selective predation. Under reduced adult survival, selection will favour genotypes capable of reproducing earlier, at a smaller size and with a higher reproductive effort. When exposed to water that previously held fish, (size selective predators which prefer larger Daphnia), individuals of Daphnia hyalina reproduced earlier, at a smaller size and had a higher reproductive investment. Hence the prey was able to switch its life history pattern in order to become less susceptible to predation by a specific predator. The cue that evokes the prey response is a chemical released by the predator.     

Energy allocation rules inDaphnia magna: clonal and age differences in the effects of food limitation

Summary The allocation of energy to carapace formation, respiration, growth, and reproduction were examined in two parthenogenetic clones ofDaphnia magna (Cladocera) cultured at two levels of food (Chlorella) concentration. Clonal differences in energy allocation were more apparent at high ration (1.5 g C mL^-1) than at low ration (0.3 g C mL^-1). These differences included respiratory and molting costs, and the timing of energy allocation to growth and reproduction. A comparison of active vs. anesthetized animals revealed that the interclonal difference in respiration rate was the result of a difference in activity level. In both clones mass-specific rates of respiration, growth, and brood production all decreased at low vs. high ration levels, whereas mass-specific molt-loss rate increased. Lowered food concentration decreased the relative allocation of energy to growth and reproduction, but increased allocation to maintenance (respiration and carapace formation). These allocation responses to food limitation indicated that for both clones the highest energy priority was carapace formation. However, the relative priority of respiration, growth and reproduction varied with age and clone. In juveniles (instars 1–4) the priority ranking of growth was essentially equal to that of respiration, whereas respiration always had higher priority in adults (instars 5–9). All three possibilities for the relative ranking of growth and reproduction (i.e., growth>reproduction, growth=reproduction, and reproduction>growth), as specified by different models in the literature, were observed depending on age and clone. The energy allocation rules were also shown to vary between other daphniid species. Furthermore, metabolic responses to chronic food limitation may be different from responses to acute food deprivation. In this study, one clone showed a greater decrease in respiration rate as a result of lifetime food limitation than did the other, but the opposite was true when these clones were exposed to 48 h of starvation. These differences in allocation rules and in acute vs. chronic responses may have to be considered when using physiological data to modelDaphnia populations.     

Nocturnal stachyose metabolism in leaf tissues of Xerosicyos danguyi H. Humb.

Leaf discs obtained from mature leaves of Xerosicyos danguyi were found to contain appreciable levels of stachyose throughout an 8-h nocturnal period during which this plant performs Crassulacean acid metabolism (CAM). In contrast, in mesophyll tissues obtained from paradermal sections of these same leaf discs and which were devoid of vascular tissues, stachyose pools were rapidly depleted during the nocturnal phase. The pattern of this depletion followed closely the depletion pattern observed for starch, indicating that mesophyll stachyose was possibly involved in nocturnal CAM processes and was not necessarily being used for export. Pulse-labelling of intact X. danguyi leaves prior to excision of leaf discs and mesophyll samples also indicated that, while labelled stachyose had turned-over completely in the mesophyll tissues by the end of the nocturnal period, substantial levels of labelled stachyose were always recovered from the leaf discs from which these mesophyll samples were derived. The data indicate the existence of two separate pools of stachyose in the X. danguyi leaf, one a mesophyll pool which turns over rapidly at night and which may be involved to a small extent in nocturnal CAM processes, and the other a pool associated with and possibly synthesized by the vascular tissues and which presumably represents the phloem-transport pool.Abbreviation CAM Crassulacean acid metabolism This work was supported by National Science Foundation Grant DCB 8901785.     

Mating behavior in the black hamlet — gamete trading or egg trading?

Synopsis The black hamlet (Hypoplectrus nigricans, Serranidae) is a simultaneous hermaphrodite, like many other serranines. It has external fertilization and planktonic eggs and engages in a kind of reciprocal spawning consisting of three components: (1) The clutch is divided into sequentially spawned parcels. (2) Partners regularly alternate release of parcels in a spawning bout. (3) Courtship is associated with the female spawning role. Two hypotheses have been proposed to account for this pattern. The egg trading hypothesis states that the pattern results from competition for fertilizations and assumes that reproductive success (RS) as a male is limited by access to eggs and that female RS is not limited by access to sperm. The gamete trading hypothesis states that the pattern results from eggs being at substantial risk of not being fertilized — i.e. female RS is limited by access to sperm. An analysis was performed of data from the black hamlet and three other serranines to determine whether significant sperm limitation occurs. The evidence fails to support the hypothesis that access to sperm limits female RS. Unspawned eggs were not found in fish collected outside the spawning period, but fish without eggs always had milt (sperm). The percentage of eggs fertilized did not decrease over the course of a spawning bout. There was also a slight positive correlation between the number of eggs released in a spawning and the percentage that were fertilized, and serranines that divide the clutch into parcels do not tend to have higher fecundities than those that do not. The gamete trading hypothesis can therefore be rejected for H. nigricans.     

Alternative life histories in sex changing shrimp: a phenotype limited ESS

Summary In many populations of protandrous shrimp, two alternative life histories coexist. One way to interpret this fact is that one age (or size) group consists of a mixture of males and females. This is a nice example of a phenotype limited evolutionarily stable strategy (ESS), in the sense of Parker (1982, p. 187). This paper explores the ESS theory for the mixture.     

Temporal patterns of ovule allocation,fruit set,and seed predation inAnemonopsis macrophylla (Ranunculaceae)

Ovule deployment patterns were studied inAnemonopsis macrophylla Sieb. & Zucc. to develop hypotheses about major selective forces underlying different temporal strategies. Carpel number per flower decreased significantly with relative flowering date. The trend was present at individual, population, and among-population, levels. Both ovule number per carpel and ripe seed weight decreased with later flowering date. Such temporal allocation patterns may be adaptive in late-flowering species with extended flowering patterns, because successively later flowers run a higher risk of lower resource availability and premature death due to deteriorating climatic conditions.     

Comparative Ecology of Rubus Species (Rosaceae) I. Ecological Distribution and Life History Characteristics of Three Species, R. palmatus var. coptophyllus, R. microphyllus and R. crataegifolius

Abstract The ecological distribution and life history characteristics of three Rubus species, i.e., R. palmatus Thunb. var. coptophyllus (A. Gray) O. Kuntze, R. microphyllus L.f. and R. crataegifolius Bunge were investigated, with particular emphasis on the seasonal growth cycles, modes of the alternation of aerial organs, and patterns of dry matter allocation, and these features were critically compared with one another. It was found that seasonal changes of aboveground biomass are strongly related to the modes of cane alternation, to underground branching producing stolons, and to their dry matter allocation patterns. The modes of cane alternation, underground branching and dry matter allocation patterns were very similar in R. microphyllus and R. crataegifolius , taking about five months from May to September. But R. palmatus var. coptophyllus showed much more gradual replacement of aerial organs, extending over seven months from May to December. These three Rubus species are often predominant in the early successional stages of pioneer communities which develop after cutting of forests in central Japan, and differentiations of the above-mentioned life history traits found in Rubus species seem to be related to the constraints inherent in their somewhat labile, changing environments, e.g., at the forest margin or clearings produced temporarily.     

Female common lizards (Lacerta vivipara) do not adjust their sex-biased investment in relation to the adult sex ratio

Sex allocation theory predicts that facultative maternal investment in the rare sex should be favoured by natural selection when breeders experience predictable variation in adult sex ratios (ASRs). We found significant spatial and predictable interannual changes in local ASRs within a natural population of the common lizard where the mean ASR is female-biased, thus validating the key assumptions of adaptive sex ratio models. We tested for facultative maternal investment in the rare sex during and after an experimental perturbation of the ASR by creating populations with female-biased or male-biased ASR. Mothers did not adjust their clutch sex ratio during or after the ASR perturbation, but produced sons with a higher body condition in male-biased populations. However, this differential sex allocation did not result in growth or survival differences in offspring. Our results thus contradict the predictions of adaptive models and challenge the idea that facultative investment in the rare sex might be a mechanism regulating the population sex ratio.     

Sperm competition and male ejaculate investment in Nauphoeta cinerea: effects of social environment during development

Selective pressure arising from sperm competition has been predicted to influence evolutionary and behavioural adjustment of ejaculate investment, but also may influence developmental adjustment of ejaculate investment. Immature males able to target resources strategically based on the competitive environment they will experience when they become sexually mature should be at a selective advantage. In our study we investigated how the presence of potential competitors or mates affects ejaculate and testes investment during development in the cockroach Nauphoeta cinerea, a species where males control female remating via their ejaculate size (large spermatophores prevent females from remating and therefore function to avoid sperm competition for males) and females store sperm. Our aim was to determine whether the social environment influences developmental adjustment of ejaculate investment and the relative importance of ejaculate components with different functions; avoidance of or engagement in sperm competition. We conclude that the social environment can influence developmental and behavioural flexibility in specific ejaculate components that may function to avoid or engage in sperm competition.