Short‐term larval food stress and associated compensatory growth reduce adult immune function in a damselfly

Abstract 1. In animals with a complex life cycle, larval stressors may carry over to the adult stage. Carry‐over effects not mediated through age and size at metamorphosis have rarely been studied. The present study focuses on the poorly documented immune costs of short‐term food stress both in the larval stage and after metamorphosis in the adult stage. 2. The present study quantified immune function [number of haemocytes, activity of prophenoloxidase (proPO) and phenoloxidase (PO)] in an experiment where larvae of the damselfly Lestes viridis were exposed to a transient starvation period. 3. Directly after starvation, immune variables were reduced in starved larvae. Levels of proPO and PO remained low after starvation, even after metamorphosis. In contrast, haemocyte numbers were fully compensated by the end of the larval stage, yet were lower in previously starved animals after metamorphosis. This can be explained as a cost of the observed compensatory growth after starvation. Focusing only on potential costs of larval stressors within the larval stage may therefore be misleading. 4. The here‐identified immunological cost in the adult stage of larval short‐term food stress and associated compensatory growth strongly indicates that physiological costs may explain hidden carry‐over effects bridging metamorphosis. This adds to the increasing awareness that the larval and adult stages in animals with a complex life cycle should be jointly studied, as trade‐offs may span metamorphosis.     

Three‐dimensional fate mapping of larval tissues through metamorphosis in the glass sponge Oopsacas minuta

The tissue of glass sponges (Class Hexactinellida) is unique among metazoans in being largely syncytial, a state that arises during early embryogenesis when blastomeres fuse. In addition, hexactinellids are one of only two poriferan groups that already have clearly formed flagellated chambers as larvae. The fate of the larval chambers and of other tissues during metamorphosis is unknown. One species of hexactinellid, Oopsacas minuta, is found in submarine caves in the Mediterranean and is reproductive year round, which facilitates developmental studies; however, describing metamorphosis has been a challenge because the syncytial nature of the tissue makes it difficult to trace the fates using conventional cell tracking markers. We used three‐dimensional models to map the fate of larval tissues of O. minuta through metamorphosis and provide the first detailed account of larval tissue reorganization at metamorphosis of a glass sponge larva. Larvae settle on their anterior swimming pole or on one side. The multiciliated cells that formed a belt around the larva are discarded during the first stage of metamorphosis. We found that larval flagellated chambers are retained throughout metamorphosis and become the kernels of the first pumping chambers of the juvenile sponge. As larvae of O. minuta settle, larval chambers are enlarged by syncytial tissues containing yolk inclusions. Lipid inclusions at the basal attachment site gradually became smaller during the six weeks of our study. In O. minuta, the flagellated chambers that differentiate in the larva become the post‐metamorphic flagellated chambers, which corroborate the view that internalization of these chambers during embryogenesis is a process that resembles gastrulation processes in other animals.     

Common otolith microstructure related to key early life‐history events in flatfishes identified in the larvae and juveniles of cresthead flounder Pseudopleuronectes schrenki

Otolith microstructure of reared and wild cresthead flounder Pseudopleuronectes schrenki larvae and juveniles was used to investigate the daily periodicity of ring formation, morphological change and unique otolith structure related to important life events. By comparing microstructural features of P. schrenki with those reported for other flatfish species, it was shown that there may be microstructural features that are common to all flatfishes. In the sagittae and lapillus, a check (a distinct ring) was formed in the centre of otoliths at c. 6 days post hatching, and the daily formation of rings observed outside the check was confirmed. During metamorphosis, accessory primordia (AP) of otolith growth were formed on the outer edge of the sagittae, and the shape of the sagittae became more complex. No AP was formed on the lapilli, however, and otolith rings were concentrically formed throughout the larval and juvenile (≤51·6 mm standard length, L_S) stages. It is proposed, therefore, that lapilli are more appropriate than sagittae for analysis throughout the larval and juvenile (≤51·6 mm L_S) stages. During metamorphosis, unique rings that are relatively wide and show weak contrast are formed on lapilli (metamorphosing zone, MZ). Hence, the duration of metamorphosis, larval duration and the days of juvenile life can be estimated by the number of rings within the MZ, using rings from the check to outermost ring of the MZ, and that of rings formed outside MZ, respectively. The formation of AP on sagittae as well as the absence of AP, bilateral asymmetry and the formation of a unique structure during metamorphosis on lapilli have also been reported for other flatfishes.     

Otolith chemistry of prey fish consumed by a fish predator: does digestion hinder Russian doll techniques?

The effect of digestion by a predatory fish (largemouth bass Micropterus salmoides) on stable isotopic (δ¹³C and δ¹⁸O) and trace elemental (Sr:Ca and Ba:Ca) compositions of prey fish (bluegill Lepomis macrochirus) otoliths was investigated in a laboratory experiment. Trace element and stable‐isotopic signatures of L. macrochirus otoliths were not significantly altered for up to 16 h after L. macrochirus were consumed by M. salmoides. Prey fish otoliths recovered from predator digesta can retain environmental stable isotopic and trace elemental signatures, suggesting that determination of environmental history for prey fishes by stable‐isotope and trace‐element analysis of otoliths recovered from stomachs of piscivorous fishes will be feasible.     

Variations in larval growth and metabolism of an estuarine shrimp Palaemonetes pugio during toxicosis by an insect growth regulator

1. Exposure of the estuarine shrimp, Palaemonetes pugio, to a juvenile hormone analogue (⩾8 μg methoprene 1⁻¹) throughout larval development inhibited successful completion of metamorphosis.2. Methoprene exposure retarded growth in early larval stages and postlarvae, but enhanced growth in premetamorphic larvae.3. Respiration rates of early larvae were elevated by methoprene exposure, but not so older larvae or post larvae.4. Lower net growth efficiency (K₂ values) in methoprene-exposed early larvae suggests that increased metabolic demands reduced assimilated energy available for growth.5. Modifications in O:N ratios of premetamorphic larvae and postlarvae suggest that methoprene altered substrate utilization patterns during metamorphosis.     

Sequential predator effects across three life stages of the African tree frog, <Emphasis Type="Italic">Hyperolius spinigularis</Emphasis>

While theoretical studies of the timing of key switch points in complex life cycles such as hatching and metamorphosis have stressed the importance of considering multiple stages, most empirical work has focused on a single life stage. However, the relationship between the fitness components of different life stages may be complex. Ontogenetic switch points such as hatching and metamorphosis do not represent new beginnings—carryover effects across stages can arise when environmental effects on the density and/or traits of early ontogenetic stages subsequently alter mortality or growth in later stages. In this study, I examine the effects of egg- and larval-stage predators on larval performance, size at metamorphosis, and post-metamorphic predation in the African tree frog Hyperolius spinigularis. I monitored the density and survival of arboreal H. spinigularis clutches in the field to estimate how much egg-stage predation reduced the input of tadpoles into the pond. I then conducted experiments to determine: (1) how reductions in initial larval density due to egg predators affect larval survival and mass and age at metamorphosis in the presence and absence of aquatic larval predators, dragonfly larvae, and (2) how differences in mass or age at metamorphosis arising from predation in the embryonic and larval environments affect encounters with post-metamorphic predators, fishing spiders. Reduction in larval densities due to egg predation tended to increase per capita larval survival, decrease larval duration and increase mass at metamorphosis. Larval predators decreased larval survival and had density-dependent effects on larval duration and mass at metamorphosis. The combined effects of embryonic and larval-stage predators increased mass at metamorphosis of survivors by 91%. Larger mass at metamorphosis may have immediate fitness benefits, as larger metamorphs had higher survival in encounters with fishing spiders. Thus, the effects of predators early in ontogeny can alter predation risk even two life stages later.     

GABA is an inhibitory neurotransmitter in the neural circuit regulating metamorphosis in a marine snail

The marine mud snail, Tritia (=Ilyanassa) obsoleta, displays a biphasic life cycle. During the initial phase of early development, embryos hatch from benthic egg capsules to become weakly swimming veliger larvae. In the second phase, adult T. obsoleta are facultative carnivores and major agents of community disturbance. Metamorphosis is the irreversible developmental event that links these two life history stages. When physiologically competent, larvae can respond to appropriate environmental cues by settling onto their mudflat habitat and transforming themselves into miniature adult snails. Two neurotransmitters—serotonin and nitric oxide—have opposing effects on the metamorphic process in this species. In multiple other species of gastropod and bivalve molluscs, a third neurotransmitter, the classically inhibitory compound γ‐aminobutyric acid (GABA), can induce settlement or metamorphosis upon external application to competent larvae. In this situation, GABA is presumed to mimic the action of ligands from the juvenile environment that bind to larval chemosensory receptors and activate the metamorphic pathway. Results of our experiments contradict this commonly reported action of GABA on molluscan larvae. External application of GABA to competent larvae of T. obsoleta elicited no response, but instead attenuated the action of serotonin (5‐HT), a metamorphic inducer. Our investigations into the responses of larval T. obsoleta to multiple GABAergic reagents support our hypothesis that GABA functions internally as a neurotransmitter in the pathway that controls the initiation of metamorphosis. Our results also suggest that GABA acts directly on or downstream from serotonergic neurons to regulate the metamorphosis‐inducing effects of this neurotransmitter. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 736–753, 2018     

Thyroid hormone-responsive genes mediate otolith growth and development during flatfish metamorphosis

Flatfish begin life as up-right swimming, bilaterally symmetrical larvae that metamorphose into asymmetrically shaped juveniles that swim with a highly lateralized posture. We have previously shown that TH induces abrupt growth and mineralization of one component of the vestibular system, the otoliths, during early larval development and metamorphosis. Here we report that four of five vestibular-specific genes that we tested (alpha-tectorin, otogelin, otolith matrix protein, and otopetrins 1 and 2 that are known to be associated with otolith development in other vertebrates are up-regulated 1.5- to 7-fold in larval flatfish during spontaneous metamorphosis and/or following 72 h of TH treatment. These findings suggest that otolith growth and development are mediated by diverse TH-responsive genes during flatfish metamorphosis.     

The effects of nurse eggs and sibling interactions on the larval development of the poecilogonous annelid Boccardia proboscidea (Spionidae)

In poecilogony, different types of larvae are produced within the same species. Previous studies have suggested maternal control of the production of larval types; however, it is not clear which factors or mechanisms generate contrasting developmental patterns among siblings. The spionid polychaete Boccardia proboscidea produces within the same capsule adelphophagic larvae that eat nurse eggs and siblings and complete all or most of their development inside the capsule (Type A larvae), and larvae with little growth until they hatch as planktotrophic larvae (Type B larvae). In this study, we manipulated capsule content to explore the factors determining larval type in B. proboscidea and the role of extra‐embryonic maternal nutrition and sib–sib interaction in the developmental fate of offspring. When early larval stages were grown individually in vitro, with nurse eggs as the only food source, some of them remained small, while others continue developing into larger pre‐competent larvae by feeding on nurse eggs. This suggests that larval types in B. proboscidea are determined very early in development and are not solely the product of sib–sib interaction inside the capsule. However, our data also suggest that hatching size variability within larval types of a clutch depends on nurse egg availability. Type B larvae grew normally to metamorphosis when phytoplankton was available, but suffered high rates of cannibalism by Type A larvae. These results are consistent with the hypothesis that individual larval fates are determined very early in development and that once their fate is determined, hatching size and intracapsular survival are affected by maternal food provisioning and sibling interaction.     

Growth of larval and juvenile Diaphus theta (Pisces: Myctophidae) in the transitional waters of the western North Pacific

Diaphus theta is one of the most common myctophid fish species in the subarctic and transitional waters of the North Pacific. The growth of larval and juvenile D. theta was investigated using sagittal otolith increment analysis of specimens caught in transitional waters of the western North Pacific. Samples taken over a 24-h period demonstrated that otoliths exhibited daily growth cycles, allowing accurate determination of age. Calcification of the incremental zone of otoliths took place only at night, suggesting that the formation cycle of the increment of juvenile D. theta was different from that of shallow-water fishes and would be related to their diel vertical migration. The relationships between standard length (SL) and daily growth increment (D) were expressed as linear equations: SL = 2.65 + 0.141D (r ² = 0.942) for larvae of 5.1–9.6 mm SL and SL = 3.54 + 0.129D (r ² = 0.933) for juveniles of 13.7–27.6 mm SL. The growth rates were 0.14 mm d⁻¹ in larvae and 0.13 mm d⁻¹ in juveniles; this is slow compared with tropical or subtropical mycto-phid species, in which growth occurs at about twice these rates. The larval period, including the metamorphic stage, was long compared with species at lower latitudes and was estimated to be 71 days. The slow growth rate and long period of larval stage of D. theta would be the life history pattern of high-latitudinal species adapted to a low-temperature habitat. Received: March 23, 2001 / Revised: July 5, 2001 / Accepted: July 19, 2001     

The relationship between habitat permanence and larval development in California spadefoot toads: field and laboratory comparisons of developmental plasticity

We evaluated differences in larval habitats and life history of three species of spadefoot toads, then compared their life histories in a common garden study. Our field work defined the selective regime encountered by each species. Our Great Basin spadefoot (Spea intermontana) bred asynchronously in permanent streams and springs where there was no risk of larval mortality due to drying. The water chemistry remained fairly stable throughout the larval period. The western spadefoot toad, Sp. hammondii, bred fairly synchronously following heavy spring rains in temporary pools that remained filled an average of 81 d. Fifteen % of the breeding pools dried completely on or before the day the first larvae metamorphosed. The desert spadefoot toad, Scaphiopus couchii, bred synchronously after heavy summer showers in very short duration pools; 62% of the breeding pools dried completely on or before the day the first larvae metamorphosed. The concentration of ammonium nitrogen and CaCO₃ increased markedly as the Sp. hammondii and S. couchii pools dried. S. couchii attained metamorphosis at a much earlier age and smaller size than the other two species. S. couchii also showed little variation in the age at metamorphosis but considerable variation in the size at metamorphosis, while the other two species varied in both age and size. The results identify some variables that could serve as cues of pool drying and demonstrate an association between breeding pool duration, breeding synchrony, development rate, and larval development. Our laboratory study yields information about the genetic basis of the differences in development and controlled comparisons of phenotypic plasticity. We manipulated food supply to study the plastic response of age and size at metamorphosis and hence construct the reaction norm for these variables as a function of growth rate. The growth rates ranged from below to above those observed in natural populations. As in the field, in the lab S. couchii attained metamorphosis at an earlier age and smaller size than the other two species. All three species had a similarly shaped reaction norm for size(y‐axis) and age (x‐axis) at metamorphosis, which was a concave upward curve. A consequence of this shape is that age at metamorphosis changes more readily at low levels of food availability and size at metamorphosis changes more readily at high levels of food availability. If we restrict our observations to just those growth rates that are seen in nature, then S. couchii has almost no variation in the age at metamorphosis but considerable variation in size at metamorphosis, while the other two species vary in both age and size at metamorphosis. All three species increased in size at metamorphosis with increased food levels. Our comparative reaction norm approach thus demonstrates that S. couchii has adapted to ephemeral environments by shifting its growth rate reaction norm so that age at metamorphosis is uniformly fast and is not associated with growth rate. The realized variation is concentrated in size rather than age at metamorphosis.     

Poor Nutritional Conditions During the Early Larval Stage Reduce Risk‐Taking Activities of Fire Salamander Larvae (Salamandra salamandra)

Environmental conditions experienced early in life have been shown to significantly affect growth trajectories at later stages in many vertebrate species. Amphibians typically have a biphasic life history, with an aquatic larval phase during early development and a subsequent terrestrial adult phase after completed metamorphosis. Thus, the early conditions have an especially strong impact on the future survival and fitness of amphibians. We studied whether early nutritional conditions affect the behavioural reaction of fire salamander larvae (Salamandra salamandra) before completion of metamorphosis. Fire salamander larvae reared under rich nutritional conditions were heavier and larger, displayed better body condition overall throughout the first three month of life and metamorphosed earlier compared with larvae raised under poor nutritional conditions. Specifically, we tested whether larvae reared under these different conditions differed with respect to their risk‐taking behaviour and activity. We found no differences in the activity of larvae with respect to their experienced early food conditions. However, larvae reared under poor nutritional conditions hid significantly more often in a risk‐taking test than larvae reared under rich food conditions. This increase in shelter‐seeking behaviour might be an adaptation to reduce the risk of larval drift or an adaptation to compensate for physiological deficits in part by appropriate behavioural reactions. Our results indicate that environmental conditions, such as food availability, may lead to different behavioural strategies.     

Effect of water velocity and temperature on energy use,behaviour and mortality of pallid sturgeon Scaphirhynchus albus larvae

Natural reproduction of pallid sturgeon Scaphirhynchus albus has been limited for decades and a recruitment bottleneck is hypothesized to occur during the larval stage of development. In this study, we evaluated the effects of water velocity and temperature on the swimming activity, energy use, settling behaviour and mortality of endogenously feeding larvae. The swimming activity of drifting sturgeon larvae (i.e., fish exhibiting negative rheotaxis) increased at low water velocity. In subsequent experiments, we observed greater energy depletion and resultant mortality of larvae in no-flow environments (0 cm s⁻¹) compared to tanks with water velocity ranging from 3.5 to 8.3 cm s⁻¹. The growth rate of drifting larvae was positively related to water temperature (18.7–23.3°C), but reduced growth rate at low water temperature (18.7°C) resulted in protracted development that extended average drift duration by ~4 days compared to larvae reared at 23.3°C. This study provides evidence that cooler summer water temperatures, characteristic of present-day conditions in the upper Missouri River, can reduce larval development and extend both the drift duration and distance requirements of S. albus. Moreover, if dispersed into low velocity environments, such as in reservoir headwaters, larvae may experience increased mortality owing to a mismatch between early life stage drift requirements and habitat conditions in the river. Manipulation of water releases to increase seasonal water temperature below dams may aid survival of S. albus larvae by shortening the time and distance spent drifting.     

HABITAT DURATION,LENGTH OF LARVAL PERIOD,AND THE EVOLUTION OF A COMPLEX LIFE CYCLE OF A SALAMANDER,AMBYSTOMA TEXANUM

In many organisms, genotypic selection may be a less effective means of adapting to unpredictable environments than is selection for phenotypic plasticity. To determine whether genotypic selection is important in the evolution of complex life cycles of amphibians that breed in seasonally ephemeral habitats, we examined whether mortality risk from habitat drying in natural populations of small-mouthed salamanders (Ambystoma texanum) corresponded to length of larval period when larvae from the same populations were grown in a common laboratory environment. Comparisons were made at two levels of organization within the species: 1) among geographic races that are under strongly divergent selection regimes associated with the use of pond and stream habitats and 2) among populations within races that use the same types of breeding habitats. Morphological evidence indicates that stream-breeding A. texanum evolved from pond-breeding populations that recently colonized streams. Larvae in streams incur heavy mortality from stream drying, so the upper bound on length of larval period is currently set by the seasonal duration of breeding sites. We hypothesized that selection would reduce length of larval period of pond-breeders that colonize streams if their larval periods are inherently longer than those of stream-breeders. The results of laboratory experiments support this hypothesis. When grown individually in a common environment, larvae from stream populations had significantly shorter larval periods than larvae from pond populations. Within races, however, length of larval period did not correlate significantly with seasonal duration of breeding sites. When males of both races were crossed to a single pond female, offspring of stream males had significantly shorter larval periods than offspring of pond males. Collectively, these data suggest that differences in complex life cycles among pond and stream-breeders are due to genotypic selection related to mortality from habitat drying. Stream larvae in the common-environment experiment were significantly smaller at metamorphosis than pond larvae. Yet, the evolution of metamorphic size cannot be explained readily by direct selection: there are no intuitively obvious advantages of being relatively small at metamorphosis in streams. A positive phenotypic correlation was observed between size at metamorphosis and length of larval period in most laboratory populations. A positive additive genetic correlation between these traits was demonstrated recently in another amphibian. Thus, we suspect that metamorphic size of stream-breeders evolved indirectly as a consequence of selection to shorten length of larval period.     

Plasticity in the timing of a major life‐history transition and resulting changes in the age structure of populations of the salamander Hynobius retardatus

Variation in age and size at life‐history transitions is a reflection of the diversifying influence of biotic or abiotic environmental change. Examples abound, but it is not well understood how such environmental changes influence the age structure of a population. I experimentally investigated the effects of water temperature and food type on age and body size at metamorphosis in larvae of the salamander Hynobius retardatus. In individuals grown at a cold temperature (15 °C) or given Chironomidae as prey, the time to metamorphosis was significantly prolonged, and body size at metamorphosis was significantly enlarged, compared with individuals grown at a warmer temperature (20 °C) or fed larvae. I also examined whether larval density (a possible indicator of cannibalism in natural habitats) generated variation in the age structure of natural populations in Hokkaido, Japan, where the climate is subarctic. Natural ponds in Hokkaido may contain larvae that have overwintered for 1 or 2 years, as well as larvae of the current year, and I found that the number of age classes was related to larval density. Although cool water temperatures prolong the larval period and induce later metamorphosis, in natural ponds diet‐based enhancement of development translated into a shorter larval duration and earlier metamorphosis. Geographic variation in the frequency of cannibalism resulted in population differences in metamorphic timing in H. retardatus larvae. It is important to understand how environmental effects are ultimately transduced through individual organisms into population‐level phenomena, with the population response arising as the summation of individual responses. Without a thorough comprehension of the mechanisms through which population and individual responses to environmental conditions are mediated, we cannot interpret the relationship between population‐level and individual‐level phenomena. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 100–114.     

Larval carry‐over effects from ocean acidification persist in the natural environment

An extensive body of work suggests that altered marine carbonate chemistry can negatively influence marine invertebrates, but few studies have examined how effects are moderated and persist in the natural environment. A particularly important question is whether impacts initiated in early life might be exacerbated or attenuated over time in the presence or absence of other stressors in the field. We reared Olympia oyster (Ostrea lurida) larvae in laboratory cultures under control and elevated seawater pCO₂ concentrations, quantified settlement success and size at metamorphosis, then outplanted juveniles to Tomales Bay, California, in the mid intertidal zone where emersion and temperature stress were higher, and in the low intertidal zone where conditions were more benign. We tracked survival and growth of outplanted juveniles for 4 months, halfway to reproductive age. Survival to metamorphosis in the laboratory was strongly affected by larval exposure to elevated pCO₂ conditions. Survival of juvenile outplants was reduced dramatically at mid shore compared to low shore levels regardless of the pCO₂ level that oysters experienced as larvae. However, juveniles that were exposed to elevated pCO₂ as larvae grew less than control individuals, representing a larval carry‐over effect. Although juveniles grew less at mid shore than low shore levels, there was no evidence of an interaction between the larval carry‐over effect and shore level, suggesting little modulation of acidification impacts by emersion or temperature stress. Importantly, the carry‐over effects of larval exposure to ocean acidification remained unabated 4 months later with no evidence of compensatory growth, even under benign conditions. This latter result points to the potential for extended consequences of brief exposures to altered seawater chemistry with potential consequences for population dynamics.     

Influence of density and salinity on larval development of salt‐adapted and salt‐naïve frog populations

Environmental change and habitat fragmentation will affect population densities for many species. For those species that have locally adapted to persist in changed or stressful habitats, it is uncertain how density dependence will affect adaptive responses. Anurans (frogs and toads) are typically freshwater organisms, but some coastal populations of green treefrogs (Hyla cinerea) have adapted to brackish, coastal wetlands. Tadpoles from coastal populations metamorphose sooner and demonstrate faster growth rates than inland populations when reared solitarily. Although saltwater exposure has adaptively reduced the duration of the larval period for coastal populations, increases in densities during larval development typically increase time to metamorphosis and reduce rates of growth and survival. We test how combined stressors of density and salinity affect larval development between salt‐adapted (“coastal”) and nonsalt‐adapted (“inland”) populations by measuring various developmental and metamorphic phenotypes. We found that increased tadpole density strongly affected coastal and inland tadpole populations similarly. In high‐density treatments, both coastal and inland populations had reduced growth rates, greater exponential decay of growth, a smaller size at metamorphosis, took longer to reach metamorphosis, and had lower survivorship at metamorphosis. Salinity only exaggerated the effects of density on the time to reach metamorphosis and exponential decay of growth. Location of origin affected length at metamorphosis, with coastal tadpoles metamorphosing slightly longer than inland tadpoles across densities and salinities. These findings confirm that density has a strong and central influence on larval development even across divergent populations and habitat types and may mitigate the expression (and therefore detection) of locally adapted phenotypes.     

Seasonal changes in the stable isotope values of lake-dwelling chironomid larvae in relation to feeding and life cycle variability

1. We studied seasonal changes in the carbon and nitrogen stable isotope ratios of larval Chironomus anthracinus and C. plumosus from the profundal sediments of four contrasting lakes. 2. Pronounced seasonal changes in both δ¹³C and δ¹⁵N values were evident in chironomid larvae of both species from two summer‐stratified, eutrophic lakes: Esthwaite Water and Wyresdale Park. Changes were most marked in the larvae of C. plumosus and in larvae from greater depths. In contrast, neither C. anthracinus in summer‐stratified but mesotrophic Schöhsee, nor C. plumosus in polymictic Großer Binnensee, showed marked seasonality in larval stable isotope ratios. 3. The particularly strong ¹³C‐depletion of larvae from the stratified, eutrophic lakes is attributed to a significant contribution of methane‐derived carbon to their diets. Feeding by larvae on isotopically light methanotrophic bacteria appears to occur mainly when autumn overturn of the water column restores oxygenated conditions to the sediment surface. At this time both δ¹³C and δ¹⁵N values of larvae decreased sharply. 4. Changes in the mean stable isotope ratio of the larval populations can also occur when larger, more isotopically light, larvae pupate and emigrate from the population to hatch as imagos. This effect can induce seasonal changes in larval isotope values even in lakes in which there is no evidence of a significant involvement of methane‐derived carbon in their diets. Variations in emergence patterns between species and between lakes may generate differences in the seasonal pattern of change in stable isotope ratios in larval populations. 5. Our results emphasise the importance of adequate seasonal sampling if stable isotope ratios are to be used as biomarkers to study the role of key groups, such as chironomid larvae, in the trophic structure of lakes.     

Models of metamorphic timing: an experimental evaluation with the pond-dwelling salamander Hemidactylium scutatum (Caudata: Plethodontidae)

Amphibian larvae vary tremendously in size at metamorphosis and length of larval period. We raised pond-dwelling four-toed salamander (Hemidactylium scutatum) larvae to test two models that predict a larva’s age and size at metamorphosis. The Wilbur-Collins model proposes that the developmental rate of a larva responds to changes in growth rate in an adaptive manner throughout the larval period, and that metamorphosis can be initiated after a minimum size has been reached. The Leips-Travis or fixed-rate model states that developmental rate is set early in the larval period, perhaps by early growth rate or food availability and their positive correlation with developmental rate, and that changes in growth rate during the larval period affect size at metamorphosis, but have no effect on the age of an individual at metamorphosis. A modified version of the Wilbur-Collins model suggests that a larva’s developmental rate becomes fixed about two-thirds of the way through the larval period, with changes in growth rate after that point only affecting size at metamorphosis. Larvae were raised on eight different feeding regimes which created two constant and six variable growth histories. Growth history did significantly affect size at metamorphosis. However, an a posteriori statistical test revealed a group of seven and an overlapping group of six treatments with indistinguishable lengths of larval period, indicating a general picture of a fixed developmental rate regardless of growth history. This result is unique among similar studies on invertebrates, fish, and frogs. There was no association between early growth or food level and development rates. Neither the Wilbur-Collins nor the Leips-Travis fixed-rate models were supported. The invariable developmental rate of Hemidactylium and recent osteological evidence from the literature suggest that larvae begin the process of metamorphosis as soon as they hatch, probably a trait selected for by strong predation pressure in the aquatic environment. A variety of different approaches (ecological, developmental, phylogenetic) are necessary to fully evaluate the adaptive nature of the timing of transitions between life cycle stages. Received: 3 June 1999 / Accepted: 18 March 2000     

Larval growth of two species of lanternfish at nearshore waters from an upwelling zone based on otolith microstructure analyses

Larval growth and hatching days of lanternfishes Diogenichthys laternatus and Myctophum nitidulum (Myctophidae) collected in September 2012 in nearshore waters (<1 km offshore) at Mejillones Bay, northern Chile, were estimated on the basis of microstructure analyses of sagitta otoliths, to establish potential differences in early traits of both species from the productive coastal waters of the Humboldt Ecosystem. Growth increments were well defined, and no accessory primordia were observed in the analyses of the largest individuals in either species (8.61 and 9.17 mm BL, respectively). Both larval species displayed slow and similar growth rates: 0.057 ± 0.016 mm day^?1 for D. laternatus, and 0.061 ± 0.005 mm day^?1 for M. nitidulum. A large variability in the size‐at‐age in larvae of both species was detected. However, a recent otolith growth index showed all M. nitidulum in similar condition 5 days before capture, but with three D. laternatus in better condition and only one in a poorer condition than the other D. laternatus individuals. Growth trajectories estimated by the microincrement width of sagitta otoliths, indicated the presence of fast‐ and slow‐growing larvae for both species. Also, the back‐calculated ‘birth’ days suggest a large hatching pulse for D. laternatus near the third‐quarter moon. The small sampling size of M. nitidulum precluded a robust conclusion on hatching patterns, although most individuals were hatched between the third quarter and the new moon. It is suggested that the slow growth rates estimated for both larval species might be caused by cold waters from upwelling events and/or allometric growth during early development of these lanternfish.