Morphometric and electrophoretic study of Aphidius species (Hymenoptera: Aphidiidae) reared from a variety of aphid hosts

Abstract. Morphological and electrophoretic variation is evaluated in 272 female and 424 male specimens of Aphidius reared from known hosts representing seventeen aphid-host species. Much of this material was reared under experimental conditions in order to examine the effects of environmental conditions such as temperature and different host species, and to induce as much variation as is possible in known single populations. Multivariate analysis and electrophoresis are used to demonstrate the degree to which entities defined by morphological characters and electrophoretic bands correspond to collection from a common host.
Many characters commonly used in the identification of this group, e.g. sculpturing on the propodeum, are shown to be very variable. Despite this, several morphologically and electrophoretically definable groups of Aphidius are demonstrated, many of them corresponding to individual aphid-hosts, while others, especially those parasitizing pea and cereal aphids were found to occur on more than one host. The possible nominal species associated with each defined group are discussed and morphological keys given to these complexes, and to the species of of Aphidius known to parasitize pea and cereal aphids.     

A COMPARISON OF TEMPERATURE-INDUCED POLYPHENISM IN AFRICAN BICYCLUS BUTTERFLIES FROM A SEASONAL SAVANNAH-RAINFOREST ECOTONE

Temperature-induced variation and norms of reaction have been analyzed for wing pattern elements of six species belonging to the African butterfly genus Bicyclus (Lepidoptera, Nymphalidae, Satyrinae). Five of these species are sympatric in Malawi and exhibit seasonal polyphenism in the savannah-rainforest ecotone. The sixth species originated from Cameroonian equatorial rainforest. The organisms were laboratory reared under four different temperature conditions ranging from 17–28°C. The variation in response to temperature is described by principal component analysis (PCA). Discrimination on the basis of plastic wing pattern characters was performed by discriminant function analysis (DFA) and unweighed pair-group method algorithm (UPGMA) clustering. A phylogenetic reconstruction based on adaptive plastic wing characters was compared with a cladogram built on “nonadaptive” characters. Results demonstrate that: (1) Phenotypic plasticity of wing pattern characters in response to temperature in laboratory-reared organisms is reminiscent of variation induced by seasonal change in the field. (2) Different wing pattern characters are under different control: “exposed” characters of butterflies at rest position are highly sensitive to temperature variation, whereas “hidden” characters, only visible during active behavior, are dominated by species differences. In general the sensitivity of the former can be attributed to their proposed function in deflecting predators. (3) The sexes differ especially in the size of those eyespots that are displayed during active behavior. (4) Species from seasonal and aseasonal environments react in a broadly similar manner to temperature variation. However, savannah species and species of aseasonal rainforest exhibit relatively shallow reaction norms, whereas reaction norms are steeper in species from the savannah-rainforest ecotone. Such a strong response was also apparent in so-called correlation networks between principal components for these species. (5) Phylogenetic distances are to some extent reflected in ordination in both PCA-space and DFA-space: closely related species of the safitza group remain close in both ordinations. The more distantly related species differ in ordination from a pattern as suggested by a phylogenetic reconstruction. It is argued that the wing pattern variation of these species reflects both adaptive processes and historical relationships.     

Phenotypic plasticity of pigmentation and morphometric traits in Pnigalio soemius (Hymenoptera: Eulophidae)

Species of the genus Pnigalio Schrank are ectoparasitoids on several pest insects. Most species are polyphagous parasitoids of lepidopteran and dipteran leafminers. Despite their potential economic importance, information on intraspecific phenotypic variability is insufficient. Pnigalio soemius (Walker) was reared at five different temperatures (10, 15, 20, 25, 30 degrees C) on mature larvae of one of its natural hosts, Cosmopterix pulchrimella Chambers (Lepidoptera: Cosmopterigidae), to investigate the influence of temperature on size, colour and other morphological traits, and to measure the range of variation of several characters. Thermal developmental reaction norms, which represent the effect of temperature during growth and development on the value of some adult traits, were produced. The results confirmed the influence of temperature on numerous characters and that these characters had a larger range of variation than realized previously in the construction of taxonomic keys to species. In particular, the number and position of the costulae on the propodeum and colour of the gaster were affected by rearing temperature.     

Temperature and population density: interactional effects of environmental factors on phenotypic plasticity,immune defenses,and disease resistance in an insect pest

Temperature and crowding are key environmental factors mediating the transmission and epizooty of infectious disease in ectotherm animals. The host physiology may be altered in a temperature‐dependent manner and thus affects the pathogen development and course of diseases within an individual and host population, or the transmission rates (or infectivity) of pathogens shift linearly with the host population density. To our understanding, the knowledge of interactive and synergistic effects of temperature and population density on the host–pathogen system is limited. Here, we tested the interactional effects of these environmental factors on phenotypic plasticity, immune defenses, and disease resistance in the velvetbean caterpillar Anticarsia gemmatalis. Upon egg hatching, caterpillars were reared in thermostat‐controlled chambers in a 2 × 4 factorial design: density (1 or 8 caterpillars/pot) and temperature (20, 24, 28, or 32°C). Of the immune defenses assessed, encapsulation response was directly affected by none of the environmental factors; capsule melanization increased with temperature in both lone‐ and group‐reared caterpillars, although the lone‐reared ones presented the most evident response, and hemocyte numbers decreased with temperature regardless of the population density. Temperature, but not population density, affected considerably the time from inoculation to death of velvetbean caterpillar. Thus, velvetbean caterpillars succumbed to Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV) more quickly at higher temperatures than at lower temperatures. As hypothesized, temperature likely affected caterpillars' movement rates, and thus the contact between conspecifics, which in turn affected the phenotypic expression of group‐reared caterpillars. Our results suggest that environmental factors, mainly temperature, strongly affect both the course of disease in velvetbean caterpillar population and its defenses against pathogens. As a soybean pest, velvetbean caterpillar may increase its damage on soybean fields under a scenario of global warming as caterpillars may reach the developmental resistance faster, and thus decrease their susceptibility to biological control by AgMNPV.     

Physiological bases of genetically determined variation in growth of marine invertebrate larvae: A study of growth heterosis in the bivalve Crassostrea gigas

Many species of marine animals have larval stages whose rates of growth in the plankton are regulated by complex combinations of biological and environmental factors. In this study, we focus on the physiological bases that underlie endogenous variation in growth potential of larvae. Our approach was based on experimental crosses of gravid adults from pedigreed families of the Pacific oyster, Crassostrea gigas. This produced large numbers of larvae with different growth rates when reared under similar environmental conditions of food and temperature. A total of 35 larval families were reared to test hypotheses regarding the physiological bases of growth variation. Growth rate of these larval families varied over a five-fold range, from 3.4 (± 0.5, S.E.) to 17.6 (± 0.6) μm day^− 1. The suite of integrated measurements applied to study growth variation included size, biochemical compositions, rates of particulate and dissolved nutrient acquisition, absorption efficiencies, respiration rates and enzyme activities. We show that a complex set of physiological processes regulated differences in genetically determined growth rates of larvae. One-half of the energy required for faster growth came from an enhanced, size-specific feeding ability. Differences in absorption rates were not significant for slow- and fast-growing larvae, nor were differences in size-specific respiration rates. Metabolic processes accounted for the additional 50% of the energy “savings” required to explain enhanced growth rates. We propose that different protein depositional efficiencies could account for this energy saving. Quantitative analyses of the endogenous physiological factors that cause variation in growth rate will allow for a more sophisticated understanding of growth, survival and recruitment potential of larvae.     

Correlated evolution of colour pattern and body size in polymorphic pygmy grasshoppers, Tetrix undulata

Theory posits that selection on functionally interrelated characters will promote physical and genetic integration resulting in evolution of favourable trait-value combinations. The pygmy grasshopper Tetrix undulata (Orthoptera: Tetrigidae) displays a genetically encoded polymorphism for colour pattern. Colour morphs differ in several traits, including behaviours, thermal biology and body size. To examine if these size differences may reflect phenotypic plasticity of growth and development in response to temperature we used a split brood-design and reared hatchlings from mothers belonging to different morphs in different thermal environments (warm or cold) until maturity. We found that time to maturity was longer in the cold compared with the warm treatment. In the warm (but not in the cold) treatment time to maturity also varied among individuals born to mothers belonging to different colour morphs. Although low temperature and long development time are normally accompanied by increased body size in ectotherms, our results revealed no difference in size at maturity between individuals reared in the two temperature treatments. There was also an increase (not a decrease) in adult body size with shortened time to maturity across families within each treatment. Taken together, this suggests that body size is canalized against environmental perturbations, and that early maturation does not necessarily trade off against a size-mediated decrease in fecundity. Heritability of body size was moderate in magnitude. Moreover, body size at maturity varied among individuals belonging to different morphs and was influenced also by maternal colour morph, suggesting that a genetic correlation exists between colour pattern and body size. These findings suggest that different characters have evolved in concert and that the various colour morphs represent different evolutionary strategies, i.e., alternative peaks in a multi-modal adaptive landscape.     

Why get big in the cold? Size–fecundity relationships explain the temperature‐size rule in a pulmonate snail (Physa)

Most ectotherms follow a pattern of size plasticity known as the temperature‐size rule where individuals reared in cold environments are larger at maturation than those reared in warm environments. This pattern seems maladaptive because growth is slower in the cold so it takes longer to reach a large size. However, it may be adaptive if reaching a large size has a greater benefit in a cold than in a warm environment such as when size‐dependent mortality or size‐dependent fecundity depends on temperature. I present a theoretical model showing how a correlation between temperature and the size–fecundity relationship affects optimal size at maturation. I parameterize the model using data from a freshwater pulmonate snail from the genus Physa. Nine families were reared from hatching in one of three temperature regimes (daytime temperature of 22, 25 or 28 °C, night‐time temperature of 22 °C, under a 12L : 12D light cycle). Eight of the nine families followed the temperature‐size rule indicating genetic variation for this plasticity. As predicted, the size–fecundity relationship depended upon temperature; fecundity increases steeply with size in the coldest treatment, less steeply in the intermediate treatment, and shows no relationship with size in the warmest treatment. Thus, following the temperature‐size rule is adaptive for this species. Although rarely measured under multiple conditions, size–fecundity relationships seem to be sensitive to a number of environmental conditions in addition to temperature including local productivity, competition and predation. If this form of plasticity is as widespread as it appears to be, this model shows that such plasticity has the potential to greatly modify current life‐history theory.     

Cooler butterflies lay larger eggs: developmental plasticity versus acclimation

We use a full factorial design to investigate the effects of maternal and paternal developmental temperature, as well as female oviposition temperature, on egg size in the butterfly Bicyclus anynana. Butterflies were raised at two different temperatures and mated in four possible sex-by-parental-temperature crosses. The mated females were randomly divided between high and low oviposition temperatures. On the first day after assigning the females to different temperatures, only female developmental temperature affected egg size. Females reared at the lower temperature laid larger eggs than those reared at a higher temperature. When eggs were measured again after an acclimation period of 10 days, egg size was principally determined by the prevailing temperature during oviposition, with females ovipositing at a lower temperature laying larger eggs. In contrast to widely used assumptions, the effects of developmental temperature were largely reversible. Male developmental temperature did not affect egg size in either of the measurements. Overall, developmental plasticity and acclimation in the adult stage resulted in very similar patterns of egg size plasticity. Consequently, we argue that the most important question when testing the significance of acclamatory changes is not at which stage a given plasticity is induced, but rather whether plastic responses to environmental change are adaptive or merely physiological constraints.     

Morphological variation and mating compatibility within the mealybugs Pseudococcus calceolariae and P. similans (Hemiptera: Pseudococcidae), and a new synonymy

A laboratory study of isolines of Pseudococcus calceolariae (Maskell) and P. similans (Lidgett) collected from Hawke's Bay and Auckland, two widely separated regions in the North Island of New Zealand, threw doubt on the validity of the defining characters of these species. For P. similans, the number and position of oral rim tubular ducts varied widely and sometimes fell outside the defined limits for the species, and characteristic ‘stout abdominal setae’ were lost in the F₁ generation. The morphological characters that separate one species from the other were manipulated by changing the temperature at which the mealybugs developed, such that cohorts of F₁ sisters reared in the laboratory contained phenotypes of both P. calceolariae and P. similans. No impediments were found to breeding among populations of P. calceolariae and P. similans from Hawke's Bay and Auckland. All combinations of crosses between virgin females and males produced viable progeny. Those reared at 16°C laid more eggs than those reared at 23°C. The data did not suggest the existence of cryptic or sibling species, and contrasted with experiments elsewhere which quite clearly showed species incompatibility of closely related mealybugs. Examination of 160 ‘wild’ specimens of P. calceolariae from New Zealand, Australia and California (U.S.A.) and P. similans from New Zealand and Australia showed a morphological continuum from one species to the other. It is concluded that P. calceolariae and P. similans merely represent the phenotypic extremes of one widely polymorphic species, with the morphological characters of individual adults determined by the microenvironment in which they developed. Pseudococcus calceolariae is thus the senior synonym of P. similans.     

Sex dimorphism of life-history traits and their response to environmental factors in spider mites

Sex dimorphism is ubiquitous in the animal kingdom and can be influenced by environmental factors. However, relatively little is known about how the degree and direction of sex difference vary with environmental factors, including food quality and temperature. With the spider mites from the family Tetranychidae as subjects, the sex difference of life-history traits in responses to host plant and temperature were determined in this meta-analytic review. Across the 42 studies on 26 spider mite species (N?=?8057 and 3922 for female and male mites, respectively), female spider mites showed longer developmental duration than the males in all except two species. The direction of sex difference in development was consistent regardless of temperature and host plant. The 16 spider mite species in 33 studies generally showed female-biased longevity, with an overall effect size of 0.6043 [95%CI = 0.4054–0.8031]. Host plant significantly influenced the sex difference in longevity, where the males lived longer than females below 22.5 ℃, but the reverse was true at higher and fluctuating temperature. Host plant also influenced the magnitude of sex difference in longevity, with females living longer than males when reared on herbs but not on trees. This study indicated that life-history traits are highly variable between sexes under temperature and host plant influence, highlighting that environmental conditions can significantly shape the direction and magnitude of sexual dimorphism of life-history traits.

     

Many ways to be small: different environmental regulators of size generate distinct scaling relationships in Drosophila melanogaster

Static allometries, the scaling relationship between body and trait size, describe the shape of animals in a population or species, and are generated in response to variation in genetic or environmental regulators of size. In principle, allometries may vary with the different size regulators that generate them, which can be problematic since allometric differences are also used to infer patterns of selection on morphology. We test this hypothesis by examining the patterns of scaling in Drosophila melanogaster subjected to variation in three environmental regulators of size: nutrition, temperature and rearing density. Our data indicate that different environmental regulators of size do indeed generate different patterns of scaling. Consequently, flies that are ostensibly the same size may have very different body proportions. These data indicate that trait size is not simply a read-out of body size, but that different environmental factors may regulate body and trait size, and the relationship between the two, through different developmental mechanisms. It may therefore be difficult to infer selective pressures that shape scaling relationships in a wild population without first elucidating the environmental and genetic factors that generate size variation among members of the population.     

An age–size reaction norm yields insight into environmental interactions affecting life‐history traits: a factorial study of larval development in the malaria mosquito Anopheles gambiae sensu stricto

Environmental factors frequently act nonindependently to determine growth and development of insects. Because age and size at maturity strongly influence population dynamics, interaction effects among environmental variables complicate the task of predicting dynamics of insect populations under novel conditions. We reared larvae of the African malaria mosquito Anopheles gambiae sensu stricto (s.s.) under three factors relevant to changes in climate and land use: food level, water depth, and temperature. Each factor was held at two levels in a fully crossed design, for eight experimental treatments. Larval survival, larval development time, and adult size (wing length) were measured to indicate the importance of interaction effects upon population‐level processes. For age and size at emergence, but not survival, significant interaction effects were detected for all three factors, in addition to sex. Some of these interaction effects can be understood as consequences of how the different factors influence energy usage in the context of a nonindependent relationship between age and size. Experimentally assessing interaction effects for all potential future sets of conditions is intractable. However, considering how different factors affect energy usage within the context of an insect's evolved developmental program can provide insight into the causes of complex environmental effects on populations.     

Effects of Temperature on the Early Development, Growth, and Survival of Shortnose Sturgeon, Acipenser brevirostrum, and Atlantic Sturgeon, Acipenser oxyrhynchus, Yolk-Sac Larvae

We reared shortnose and Atlantic sturgeons at different temperatures after hatch and measured yolk utilization rate and efficiency (YUE), maximum standard length, survival and development of escape response. Newly hatched Atlantic sturgeon, were smaller in size, more efficient at utilizing yolk (incorporating yolk to body tissue) and reached developmental stages sooner than shortnose sturgeon reared at the same temperatures (13–15°C). Within each species, decreasing temperature delayed yolk absorption, escape initiation, time to reach maximum size, and time to 100% mortality. However, YUEs and the size of the larvae at these 'stages' were independent of rearing temperature for both species. These results suggest that even as temperature drives metabolic processes to speed up development, these two species are still extremely efficient at transferring yolk energy to body tissues. The lower efficiencies experienced by larval shortnose may reflect difference in yolk quality between the two species and/or the Atlantic sturgeon's higher conversion efficiency. The ability of these two sturgeon species to develop successfully and efficiently under a wide range in temperatures may provide a competitive advantage over more stenothermic species and explain their persistence through evolutionary time.     

The effect of host species on growth and variability of Echinorhynchus salmonis Müller, 1784 (Acanthocephala: Echinorhynchidae), with special reference to the status of the genus

Summary The sex and age (as measured by length) of Echinorhynchus salmonis Müller, 1784 and the host species of this acanthocephalan considerably affected worm body form and size, as well as size of proboscis, proboscis hooks, proboscis receptacle, lemnisci, testes and cement glands. Linear regression analysis indicated that curves describing the growth pattern of these characters by worm length were significantly different as a function of host species. The larger worms recovered from bloater (Coregonus hoyi: Salmonidae) almost invariably showed higher regression coefficient compared to those from smelt (Osmerus mordax: Osmeridae) in all characters. Taxonomic implications of these findings are discussed. Abnormalities in body wall, proboscis hook orientation, lemnisci and cement gland ducts as well as variations in proboscis hook and cement gland numbers are reported, some for the first time. Findings from studies of cement gland pattern invalidate Petrochenko's (1956) splitting up of the genus into three: Echinorhynchus, Metechinorhynchus and Pseudoechinorhynchus. It is proposed that the designation of the last two genera as junior synynoms of the first be accepted. ac]19791205     

Effect of larval rearing temperature on adult female morphology of Anopheles (Nyssorhynchus) albimanus (Diptera: Culicidae)

Summary

Time to pupation, percent survival to pupation, and percent adult emergence of Anopheles albimanus Wiedemann decreased at higher larval rearing temperature. Mosquitoes reared at 30°C experienced higher mortality during the pupal stage than did mosquitoes reared at 22°C. Analysis of variance revealed that wing length and costal wing spot patterns of adult female A. albimanus were affected by larval rearing temperature. Female A. albimanus reared at 22°C had longer wings, and larger basal pale + prehumeral pale, prehumeral dark, and humeral pale costal wing spots than did female siblings reared at 30°C. Female A. albimanus reared at 30°C had larger subcostal pale spots than did female siblings reared at 22°C. Analyses of 2x2 contingency tables indicated that sex ratio was independent of larval rearing temperature, whereas survival to the adult stage and coalescence of wing spots were not independent of rearing temperature. The need to examine stability of morphological characters under differing environmental conditions is discussed.     

PHENOTYPIC PLASTICITY IN THE SAILFIN MOLLY,POECILIA LATIPINNA (PISCES: POECILIIDAE). II. LABORATORY EXPERIMENT

Field studies indicate that the influence of environmental factors on growth rate and size and age at maturity in sailfin mollies (Poecilia latipinna) is inconsistent over time and suggest that the marked interdemic variation in male body size in this species is the result of genetic variation. However, the role of specific environmental factors in generating phenotypic variation must be studied under controlled conditions unattainable in nature. We raised newborn sailfin mollies from four populations in laboratory aquaria under all possible combinations of two temperatures, three salinities, and two food levels to examine explicitly the influence of these environmental factors. Males were much less susceptible than females to temperature variation and were generally less plastic than females in terms of all three traits. Members of both sexes matured at larger sizes and at later ages in less saline and in cooler environments. Food levels were not sufficiently different to affect the traits we studied. The effects of temperature and salinity were not synergistic. Males from different populations exhibited different average ages and sizes at maturity, but females did not. The magnitudes of the effects we found were not substantial enough to account for the consistent interdemic differences in male and female body size that have been observed previously. Our results also indicate that no single environmental factor is solely responsible for the environmental effects observed in field experiments on growth and development. These studies, together with other work, indicate that the strongest sources of interdemic variation are genetic differences in males and differences in postmaturation growth and survivorship in females.     

Wing morphology and fluctuating asymmetry depend on the host plant in cactophilic Drosophila

As in most insect groups, host plant shifts in cactophilic Drosophila represent environmental challenges as flies must adjust their developmental programme to the presence of different chemical compounds and/or to a microflora that may differ in the diversity and abundance of yeasts and bacteria. In this context, wing morphology provides an excellent opportunity to investigate the factors that may induce changes during development. In this work, we investigated phenotypic plasticity and developmental instability of wing morphology in flies on the cactophilic Drosophila buzzatii and Drosophila koepferae raised on alternative breeding substrates. We detected significant differences in wing size between and within species, and between flies reared on different cactus hosts. However, differences in wing shape between flies emerged from different cactus hosts were not significant either in D. buzzatii or in D. koepferae. Our results also showed that morphological responses involved the entire organ, as variation in size and shape correlated between different portions of the wing. Finally, we studied the effect of the rearing cactus host on developmental instability as measured by the degree of fluctuating asymmetry (FA). Levels of FA in wing size were significantly greater in flies of both species reared in non-preferred when compared with those reared in preferred host cacti. Our results are discussed in the framework of an integrative view aimed at investigating the relevance of host plant shifts in the evolution of the guild of cactophilic Drosophila species that diversified in South America.     

Sex differences in phenotypic plasticity of a mechanism that controls body size: implications for sexual size dimorphism

The degree and/or direction of sexual size dimorphism (SSD) varies considerably among species and among populations within species. Although this variation is in part genetically based, much of it is probably due to the sexes exhibiting differences in body size plasticity. Here, we use the hawkmoth, Manduca sexta, to test the hypothesis that moths reared on different diet qualities and at different temperatures will exhibit sex-specific body size plasticity. In addition, we explore the proximate mechanisms that potentially create sex-specific plasticity by examining three physiological variables known to regulate body size in this insect: the growth rate, the critical weight (which measures the cessation of juvenile hormone secretion from the corpora allata) and the interval to cessation of growth (ICG; which measures the time interval between the critical weight and the secretion of the ecdysteroids that regulate pupation and metamorphosis). We found that peak larval mass of males and females did not exhibit sex-specific plasticity in response to diet or temperature. However, the sexes did exhibit sex-specific plasticity in the mechanism that controls size; males and females exhibited sex-specific plasticity in the growth rate and the critical weight in response to both diet and temperature, whereas the ICG only exhibited sex-specific plasticity in response to diet. Our results suggest it is important for the sexes to maintain the same degree of SSD across environments and that this is accomplished by the sexes exhibiting differential sensitivity of the physiological factors that determine body size to environmental variation.     

Is the temperature-size rule mediated by oxygen in aquatic ectotherms?

Temperature is an important environmental factor that influences key traits like body size, growth rate and maturity. Ectotherms reared under high temperatures usually show faster growth, but reach a smaller final size, a phenomenon known as the temperature-size rule (TSR). Oxygen may become a limiting resource at high temperatures, when demand for oxygen is high, especially in water as oxygen uptake is far more challenging under water than in air. Therefore, in aquatic ectotherms, the TSR might very well be mediated by temperature effects on oxygen availability and oxygen demand. To distinguish between the direct effects of temperature and oxygen mediated effects, growth rate and final size were measured in the aquatic ectotherm Asellus aquaticus (Linnaeus, 1758) reared under different temperature and oxygen conditions in a factorial design. Growth could be best described by a modified Von Bertalanffy growth function. Both temperature and oxygen affected age at maturity and growth. Growth responses to temperature were dependent on oxygen conditions (interactive effect of temperature and oxygen). Only under hypoxic conditions, when oxygen was most limiting, did we find a classic TSR. Moreover, when comparing treatments differing in temperature, but where the balance between oxygen demand and supply was similar, high temperature increased both growth rate and final size. Thus effects of oxygen may resolve the life-history puzzle of the TSR in aquatic ectotherms.     

Temperature and oxygen tension influence the development of muscle cellularity in embryonic rainbow trout

Muscle cellularity at a developmental stage around the time of hatching was examined in rainbow trout which had been reared from the eyed stage at three different temperature regimes (5, 10 and 15° C) and different O₂ tensions [70% of air saturation value (ASV) at 5° C, 100% of ASV at all temperatures, and 150% of ASV at 10 and 15° C]. It was found that, as has been shown for other species, there was a difference in muscle fibre numbers and fibre cross-sectional areas between some of the regimes. There was a decrease in fibre number at the intermediate and higher temperature, and a decrease in fibre size at the high temperature. The temperature effects observed were modified by the applied changes in O₂ tension. An increased O₂ tension at 10° C led to an increase in fibre size whereas a decrease in O₂ tension at the low temperature resulted in a decrease in fibre number. The largest total white muscle cross-sectional area was achieved at 10° C under high O₂ conditions. Temperature and O₂ tension therefore had a clear effect on muscle cellularity and there was a significant interaction between the two parameters.