Metamorphic Individuals Research Articles

Outbreeding reduces survival during metamorphosis in a headwater stream salamander.

Assessing direct fitness effects of individual genetic diversity is challenging due to the intensive and long-term data needed to quantify survival and reproduction in the wild. But resolving these effects is necessary to determine how inbreeding and outbreeding influence eco-evolutionary processes. We used 8 years of capture-recapture data and single nucleotide polymorphism genotypes for 1906 individuals to test for effects of individual heterozygosity on stage-specific survival probabilities in the salamander Gyrinophilus porphyriticus. The life cycle of G. porphyriticus includes an aquatic larval stage followed by metamorphosis into a semi-aquatic adult stage. In our study populations, the larval stage lasts 6-10 years, metamorphosis takes several months, and lifespan can reach 20 years. Previous studies showed that metamorphosis is a sensitive life stage, leading us to predict that fitness effects of individual heterozygosity would occur during metamorphosis. Consistent with this prediction, monthly probability of survival during metamorphosis declined with multi-locus heterozygosity (MLH), from 0.38 at the lowest MLH (0.10) to 0.06 at the highest MLH (0.38), a reduction of 84%. Body condition of larvae also declined significantly with increasing MLH. These relationships were consistent in the three study streams. With evidence of localised inbreeding within streams, these results suggest that outbreeding disrupts adaptations in pre-metamorphic and metamorphic individuals to environmental gradients along streams, adding to evidence that headwater streams are hotspots of microgeographic adaptation. Our results also underscore the importance of incorporating life history in analyses of the fitness effects of individual genetic diversity and suggest that metamorphosis and similar discrete life stage transitions may be critical periods of viability selection.

Tail regeneration at different ontogenetic stages of the tiger salamander Ambystoma tigrinum suggests possible changes in regeneration between larval and metamorphic individuals

AbstractUrodele amphibians possess remarkable regenerative abilities, allowing them to rebuild lost body parts. Contrary to lizards, salamanders can fully restore their tails, including the neural spine and components of the vertebral column. The axolotl (Ambystoma mexicanum) is the vertebrate model organism for regeneration research due to its ease of breeding in captivity. However, axolotls are paedomorphic, retaining larval somatic features throughout adulthood and do not naturally undergo metamorphosis, a transition phase from aquatic larvae to terrestrial adults with profound morphological and physiological changes. We investigate the influence of metamorphosis on salamander tail regeneration after conspecific biting in the metamorphosing sister taxon Ambystoma tigrinum using histological analysis to answer two key questions: (1) Does regeneration continue during metamorphosis, or is it halted? (2) How does regeneration differ histologically among larval, metamorphosing and postmetamorphic individuals? Our findings demonstrate that regeneration continues even during metamorphic climax, indicating the simultaneous coordination of metamorphosis and regeneration. Additionally, notable distinctions were observed between developmental stages concerning the speed of regeneration and structural differences in the formation of an apical epithelial cap (AEC). While the approach taken in this study necessarily restricts sample size, it offers valuable insights into regeneration in a metamorphosing species under natural conditions.

Progenesis as an intrinsic factor of ecological opportunity in a polyphenic amphibian

Abstract Paedomorphosis, a developmental heterochrony involving the retention of larval traits at the adult stage, is considered a major evolutionary process because it can generate phenotypic variation without requiring genetic modifications. Two main processes underlie paedomorphosis: neoteny, a slowdown of somatic development, and progenesis, a precocious maturation associated with body size reduction. Being essentially a truncation of ontogeny, progenesis has often been deemed an evolutionary dead end with advantages attributed to precocious reproduction or small body size required in specific environmental contexts (e.g. parasitism, interstitial life), but there is a lack of studies on the immediate ecological consequences of progenesis. Because body size is a key factor determining trophic ecology in animals, we hypothesized that progenesis might intrinsically promote ecological opportunity via body size reduction (i.e. ‘the trophic advantage of progenesis’ hypothesis). We tested this hypothesis in facultatively progenetic palmate newts Lissotriton helveticus using stable isotope niche modelling and diet reconstruction in conjunction with traditional stomach content analyses and body condition (BC) assessment. We show that not only did progenetic individuals occupy a different trophic niche than metamorphic individuals in all populations, but the smaller they were compared to metamorphs due to progenesis, the more different they were in terms of trophic ecology, with no negative effect on their BC. Altogether, the results suggest that via body size reduction, progenesis may generally act as an intrinsic factor of ecological opportunity, allowing the use of existing but previously unavailable resources, even in habitats where seemingly little opportunity exists. We argue that beyond the classically recognized fitness advantages of progenetic development, this process may also generally bring an immediate trophic advantage via body size reduction, which would have important implications to understand the evolution and adaptiveness of this process in many different taxa, from marine meiofauna to primates. A free Plain Language Summary can be found within the Supporting Information of this article.

Comparative Histomorphology of Oviducts from Metamorphic and Paedomorphic Ambystoma talpoideum (Caudata: Ambystomatidae)

Oviducts of paedomorphic and metamorphic Ambystoma talpoideum were examined to determine if previously observed differences in egg mass characteristics between the two morphs originated from differences in oviductal histomorphology/histochemistry. We found no evidence to support this hypothesis. At least seven oviductal regions were easily identified in the oviducts of both paedomorphic and metamorphic individuals; these were as follows (from cranial to caudal): 1) pars recta (termed Region A), 2) pars convoluta (termed Regions B through F), and 3) the uterine region (termed Region G). While slight structure and staining differences were evident in Region D of the pars convoluta when paedomorphic A. talpoideum from South Carolina were compared with metamorphic A. talpoideum from Arkansas, there were no differences observed between paedomorphic and metamorphic individuals of A. talpoideum from Arkansas. Thus, any observed oviductal variation was most likely the result of geographic variation and not from differences involving metamorphosis between paedomorphic and metamorphic A. talpoideum. The number of oviductal regions of A. talpoideum was comparable with previous studies on plethodontids and salamandrids, and this study represents the first treatise of oviductal anatomy of an ambystomatid using a histochemical approach beyond general staining with hematoxylin and eosin.

Detritus Quality and Locality Determines Survival and Mass, but Not Export, of Wood Frogs at Metamorphosis.

Single-site experiments have demonstrated detritus quality in wetlands can have strongly negative, neutral, and even positive influences on wildlife. However, an examination of the influence of detritus quality across several regions is lacking and can provide information on whether impacts from variation in detritus quality are consistent across species with wide ranges. To address this gap in regional studies we examined effects of emergent and allochthonous detritus of different nutrient qualities on amphibians and assessed a mechanism that may contribute to potential impacts. We used aquatic mesocosms to raise wood frogs (Rana sylvatica) from two regions of the United States with whole plants from purple loosestrife (Lythrum salicaria), leaf litter from native hardwood trees, and a mixture of both. We examined several metrics of amphibian fitness and life history, including survival, number of days to metamorphosis, and size at metamorphosis. Further, we quantified whether the effects of detritus type could translate to variation in anuran biomass or standing stock of nitrogen or phosphorus export. Our results show detritus with high nutrient quality (purple loosestrife) negatively influenced survival of wood frogs, but increased size of metamorphic individuals in two different regions of the United States. Despite the decrease in survival, the increase in size of post-metamorphic anurans raised with high quality detritus resulted in anuran biomass and standing stock of N and P export being similar across treatments at both locations. These results further demonstrate the role of plant quality in shaping wetland ecosystem dynamics, and represent the first demonstration that effects are consistent within species across ecoregional boundaries.

Artificial Metamorphosis: Evolutionary Design of Transforming, Soft-Bodied Robots.

We show how the concept of metamorphosis, together with a biologically inspired model of multicellular development, can be used to evolve soft-bodied robots that are adapted to two very different tasks, such as being able to move in an aquatic and in a terrestrial environment. Each evolved solution defines two pairs of morphologies and controllers, together with a process of transforming one pair into the other. Animats develop from a single cell and grow through cellular divisions and deaths until they reach an initial larval form adapted to a first environment. To obtain the adult form adapted to a second environment, the larva undergoes metamorphosis, during which new cells are added or removed and its controller is modified. Importantly, our approach assumes nothing about what morphologies or methods of locomotion are preferred. Instead, it successfully searches the vast space of possible designs and comes up with complex, surprising, lifelike solutions that are reminiscent of amphibian metamorphosis. We analyze obtained solutions and investigate whether the morphological changes during metamorphosis are indeed adaptive. We then compare the effectiveness of three different types of selective pressures used to evolve metamorphic individuals. Finally, we investigate potential advantages of using metamorphosis to automatically produce soft-bodied designs by comparing the performance of metamorphic individuals with their specialized counterparts and designs that are robust to both environments.

Effects of Pesticide Mixtures on Host-Pathogen Dynamics of the Amphibian Chytrid Fungus.

Anthropogenic and natural stressors often interact to affect organisms. Amphibian populations are undergoing unprecedented declines and extinctions with pesticides and emerging infectious diseases implicated as causal factors. Although these factors often co-occur, their effects on amphibians are usually examined in isolation. We hypothesized that exposure of larval and metamorphic amphibians to ecologically relevant concentrations of pesticide mixtures would increase their post-metamorphic susceptibility to the fungus Batrachochytrium dendrobatidis (Bd), a pathogen that has contributed to amphibian population declines worldwide. We exposed five anuran species (Pacific treefrog, Pseudacris regilla; spring peeper, Pseudacris crucifer; Cascades frog, Rana cascadae; northern leopard frog, Lithobates pipiens; and western toad, Anaxyrus boreas) from three families to mixtures of four common insecticides (chlorpyrifos, carbaryl, permethrin, and endosulfan) or herbicides (glyphosate, acetochlor, atrazine, and 2,4-D) or a control treatment, either as tadpoles or as newly metamorphic individuals (metamorphs). Subsequently, we exposed animals to Bd or a control inoculate after metamorphosis and compared survival and Bd load. Bd exposure significantly increased mortality in Pacific treefrogs, spring peepers, and western toads, but not in Cascades frogs or northern leopard frogs. However, the effects of pesticide exposure on mortality were negligible, regardless of the timing of exposure. Bd load varied considerably across species; Pacific treefrogs, spring peepers, and western toads had the highest loads, whereas Cascades frogs and northern leopard frogs had the lowest loads. The influence of pesticide exposure on Bd load depended on the amphibian species, timing of pesticide exposure, and the particular pesticide treatment. Our results suggest that exposure to realistic pesticide concentrations has minimal effects on Bd-induced mortality, but can alter Bd load. This result could have broad implications for risk assessment of amphibians; the outcome of exposure to multiple stressors may be unpredictable and can differ between species and life stages.

First record of facultative paedomorphism in the Kosswig’s newt Lissotriton (vulgaris) kosswigi (Freytag, 1955) (Urodela; Salamandridae), endemic to northwestern Turkey

Facultative paedomorphism is regularly observed in salamanders, including newts of the Lissotriton vulgaris group. We document the first record of paedomorphism in L. (v.) kosswigi from a locality near Karasu, in the vicinity of Sakarya, Turkey. We compare measurements of 5 paedomorphic individuals with those of 10 metamorphic individuals from the same population and with measurements taken from the literature. Paedomorphs are on average bigger than metamorphs; intriguingly, the opposite pattern has been observed in the Turkish L. (v.) schmidtlerorum.

Experimentally induced metamorphosis in axolotls reduces regenerative rate and fidelity.

While most tetrapods are unable to regenerate severed body parts, amphibians display a remarkable ability to regenerate an array of structures. Frogs can regenerate appendages as larva, but they lose this ability around metamorphosis. In contrast, salamanders regenerate appendages as larva, juveniles, and adults. However, the extent to which fundamental traits (e.g., metamorphosis, body size, aging, etc.) restrict regenerative ability remains contentious. Here we utilize the ability of normally paedomorphic adult axolotls (Ambystoma mexicanum) to undergo induced metamorphosis by thyroxine exposure to test how metamorphosis and body size affects regeneration in age‐matched paedomorphic and metamorphic individuals. We show that body size does not affect regeneration in adult axolotls, but metamorphosis causes a twofold reduction in regeneration rate, and lead to carpal and digit malformations. Furthermore, we find evidence that metamorphic blastemal cells may take longer to traverse the cell cycle and display a lower proliferative rate. This study identifies the axolotl as a powerful system to study how metamorphosis restricts regeneration independently of developmental stage, body size, and age; and more broadly how metamorphosis affects tissue‐specific changes.

Why are the prevalence and diversity of helminths in the endemic Pyrenean brook newt Calotriton asper (Amphibia, Salamandridae) so low?

A cornerstone in parasitology is why some species or populations are more parasitized than others. Here we examine the influence of host characteristics and habitat on parasite prevalence. We studied the helminths parasitizing the Pyrenean brook newt Calotriton asper (n=167), paying special attention to the relationship between parasites and ecological factors such as habitat, sex, ontogeny, body size and age of the host. We detected two species of parasites, Megalobatrachonema terdentatum (Nematoda: Kathlaniidae) and Brachycoelium salamandrae (Trematoda: Brachycoeliidae), with a prevalence of 5.99% and 1.2%, respectively. Marginally significant differences were found in the prevalence between sexes, with females being more parasitized than males. The present results show significant differences in the body length of paedomorphic and metamorphic individuals, the former being smaller. Nevertheless, no significant correlations between parasite prevalence and either newt body length, ontogenetic stage or age were found. In comparison with other Salamandridae living in ponds, prevalence and diversity values were low. This may be due to a long hibernation period, the species' lotic habitat and its reophilous lifestyle, which probably do not allow for a high parasite load.

Downstream migration and hematophagous feeding of newly metamorphosed sea lampreys (Petromyzon marinus Linnaeus, 1758)

The metamorphosis of sea lamprey (Petr- omyzon marinus Linnaeus, 1758) allows young post- metamorphic individuals to migrate to the sea and start the hematophagous feeding. However, the information about this phase is very limited, especially for European populations. Herein, we provide for the first time a comprehensive study on the phenology of downstream migration, the timing and location of first feeding and the prey species in the River Ulla and its estuary (NW Spain). Results show that downstream migration occurs between October and May with a peak in March. At least for a part of the postmeta- morphic lampreys this migration stops for several months when they reach the estuary, where lampreys find shelter and abundant food, before moving to coastal waters. Hematophagous feeding in the estuary allows postmetamorphics to increase their total length and weight exponentially. Our results also suggest that part of the postmetamorphics (10-30%) start the hematophagous feeding in the river, with a special affinity for anadromous species, probably because of their larger size.

Plasticidade morfológica e de desenvolvimento em larvas de Physalaemus santafecinus (Anura: Leiuperidae) em resposta a estímulos químicos de diferentes predadores

Many antipredator responses are mediated or induced by the ability of the prey to identify chemical cues of predators. The presence of chemicals produced by predators may alert tadpoles to the presence of the predators, and a heightened response to alarm cues or predator presence may increase the possibility of prey survival. We examined changes in morphology, and growth and development rates of Physalaemus santafecinus tadpoles reared in the presence of chemical cues of water beetles (Hydrophilidae) and a fish(Characidae). We recorded the time to metamorphosis, as well as weights of metamorphic individuals to determine if the larval stage is accelerated. The experiments were performed under microcosm conditions, with three treatments—chemical cues from fish, water beetles, and a control group. Each treatment was replicated 30 times. To obtain independent data from different variables, treatments were conducted on individual larvae in separate containers. The principal results were, as follow. (1) Larval morphology was significantly affected by the presence of a predator. (2) Control tadpoles were significantly larger than those subjected to the other two treatments (cues of water beetles and fish). (3) Growth and development rates did not differ significantly among any treatments. (4) Neither time to metamorphosis nor weights of metamorphs varied significantly among treatments. Our results suggest that tadpoles are able to perceive predators by chemical cues released in the water, and P. santafecinus tadpoles alter their morphology to chemical cues that indicate predation. However, chemical cues of predators had no detectable effect on growth rate and developmental rates of these tadpoles.

Life history and habitat use of the speckled worm eel, Myrophis punctatus, along the east coast of the United States

Many species of fishes along the east coast of the United States have complex life histories, especially those that move over hundreds of kilometers across ocean and estuarine habitats. To further unravel the life history of one of these, the speckled worm eel, Myrophis punctatus we examined samples from extensive time series and discrete samples collected in the ocean and estuaries between Florida and Massachusetts. We now surmise spawning occurs between fall and early winter in the ocean south of Cape Hatteras, North Carolina and in the vicinity of the Bahamas. The pelagic leptocephalus larvae (10–80 mm Total Length [TL]) are transported north in the Gulf Stream and across the continental shelf to arrive at estuarine inlets at ages of 53 – 110 days. Their estuarine immigration and abundance varies along the east coast, with higher levels occurring at inlets in South Carolina (North Inlet), and North Carolina (Beaufort Inlet), during the winter and early spring. Much lower abundances occur in New Jersey (Little Egg Inlet) in winter and spring and again in the summer. These ingressing individuals were euryodontic leptocephali and metamorphic stages and were shrinking to lengths of 76–52 mm TL as these stages progressed. Metamorphic individuals and glass eels subsequently settle and burrow in estuarine sediments, as do all subsequent stages, and thereby become relatively unavailable to many sampling gears. In estuaries they attained sizes up to 440 mm TL. Later, they presumably enter the ocean to spawn because that is where the smallest larvae are found.

Variable consumer responses to invasion by Microstegium vimineum: The role of indirect effects

Abstract Background/Question/Methods As both basal resources and autogenic ecosystem engineers, plants play an important role in structuring consumer communities. Plant invasion can alter important ecosystem properties such as habitat complexity, nutrient availability, and abiotic properties, and the hyper-productive, high-nutrient nature of many invaders may make the influence of these plants especially pronounced. While direct influences of these processes on competitors and predators may be relatively foreseeable, the consequences of indirect effects on consumers living within these invaded habitats are more difficult to predict. In order to determine the response of various forest floor consumers to invasion by the exotic grass _Microstegium vimineum_ we stocked large enclosures spanning eight independent invasion fronts with metamorphic individuals of three amphibian species; Anaxyrus [Bufo] americanus (American toads), Lithobates [Rana] sphenocephalus (southern leopard frogs), and Notophthalmus viridescens (eastern newts). Using robust capture periods we tracked the survival and growth of these individuals. We also investigated the mechanisms behind these responses by tracking invertebrate abundance and abiotic factors on either side of each of these invasion fronts and performing additional manipulations, such as predator manipulations and analyses of foraging success, designed to investigate the effects of both bottom-up and top-down influences on these consumers. Results/Conclusions We found that invasion by M. vimineum could have significant effects on consumer performance (for example, the survival of metamorphic A. americanus was significantly lower in invaded areas) but that the nature and strength of these influences varied between species as well as ontogenetically, as L. sphenocephalus was not similarly affected and the influences on A. americanus dissipated over time. It appears that the negative influences of invasion on metamorphic toad survival were largely driven by top-down processes associated with the accumulation of invertebrate predators in invaded areas, while abiotic and bottom-up influences were more important factors for individuals large or fast enough to escape capture by invertebrate predators. We conclude that the influences of plant invasion on consumer communities can be varied, complicated, and far-reaching.

Measurement of the attachment strength of brachiolaria larvae and metamorphic individuals of the sea star Asterina gibbosa by a centrifugation method

Two methods are generally used to measure the adhesive strength of invertebrate larvae: direct measurement with a force transducer connected to the organisms and indirect measurement with a water flow used to dislodge the organisms. Each of these methods, however, has its drawbacks. The present study aimed to design a simple and straightforward method to measure the adhesion strength of marine invertebrate larvae based on centrifugation. This centrifuge technique works in immersed conditions and applies forces acting at 45° to the substratum, therefore mimicking natural conditions. It was tested with three different substrata on two developmental stages of the sea star Asterina gibbosa: the brachiolaria larvae, which use temporary adhesion, and the metamorphic individuals which use permanent adhesion. Measurements were completed by SEM and TEM observations of the larval adhesive organs. The critical detachment force (force required to detach 50% of the larvae) of brachiolaria larvae attached to glass (36 ±9 µN) and rough PMMA (43 ± 16 µN) were equivalent and both significantly higher than the critical detachment force measured on smooth PMMA (11 ± 8 µN). Most metamorphic individuals, on the other hand, resisted to the highest centrifugation speed used, corresponding to a force of 2.13 mN. For the hydrodynamics of larval settlement and metamorphosis, force is the ecologically relevant factor, and adhesion forces obtained by centrifugation are strikingly similar to forces measured for other marine invertebrate larvae with other methods. This indicates the usefulness of the centrifugation technique to compare adhesion of larvae between different species or development stages, or between different treatments.

Soil pH influences embryonic survival in Pseudophryne bibronii (Anura: Myobatrachidae)

Abstract: There has been limited research examining the role that terrestrial habitat characteristics play in influencing persistence of amphibian populations. In this study we investigated the influence of a terrestrial habitat attribute on the distribution of the terrestrial egg‐laying toadlet, Pseudophryne bibronii. Eggs of this species are deposited in depressions, or under leaf litter, and develop to a stage where they can hatch to free swimming tadpoles when water covers breeding sites or embryos are washed into water. Because rainfall can be intermittent, eggs may sit for extended periods in terrestrial nests before sufficient rainfall initiates hatching. Appropriate egg‐laying sites must be chosen by both males and females to ensure embryos survive these periods. A study of 20 sites found that soil pH at sites where P. bibronii were recorded as present was significantly lower than pH at sites where P. bibronii were recorded as absent. To determine whether soil pH influenced the hatching success in P. bibronii, experiments were conducted in which batches of eggs were translocated to local areas with different soil pH. Survival rate of eggs was significantly influenced by relatively small changes in pH, with hatching success higher at lower pH. In a laboratory trial survival of eggs raised on sterilized soil was not affected by soil pH. Fungal infection of eggs was observed in the field and laboratory suggesting that hatching success might be affected via a complex interaction between soil pH, fungi and other soil biota. Choice trials using P. bibronii metamorphs indicated that individuals were capable of distinguishing between small differences in pH. Therefore, distribution of P. bibronii may be influenced by either differential mortality of embryos, or habitat choice by post metamorphic individuals, and further experiments are required to determine the relative importance of these factors.

Survival of three species of anuran metamorphs exposed to UV-B radiation and the pathogenic fungus Batrachochytrium dendrobatidis

When exploring the possible factors contributing to population declines, it is necessary to consider multiple, interacting environmental stressors. Here, we investigate the impact of 2 factors, ultraviolet radiation and disease, on the survival of anuran amphibians. Exposure to ultraviolet-B (UV-B) radiation increases mortality and results in various sub-lethal effects for many amphibian species. Infectious diseases can also negatively impact amphibian populations. In this study, we exposed metamorphic individuals (metamorphs) to both UV-B and Batrachochytrium dendrobatidis (BD), a fungal pathogen and cause of the disease chytridiomycosis, and monitored survival for 3 wk. We tested for possible interactions between UV-B and BD in 3 species: the Cascades frog Rana cascadae; the Western toad Bufo boreas; and the Pacific treefrog Hyla regilla. We found strong interspecific differences in susceptibility to BD. For example, R. cascadae suffered a large increase in mortality when exposed to BD; B. boreas also experienced mortality, but this effect was small relative to the R. cascadae response. H. regilla did not show any decrease in survival when exposed to either factor. No synergistic interactions between UV-B and BD were found for any of the test species. A previous study investigating the impact of BD on larval amphibians showed different species responses (Blaustein et al. 2005a). Our results highlight the importance of studying multiple life history stages when determining the impact of environmental stressors. The contrast between these 2 studies emphasizes how vulnerability to a pathogen can vary between life history stages within a single species.

Temporal response of the tiger salamander (Ambystoma tigrinum) to 3,000 years of climatic variation

BackgroundAmphibians are sensitive indicators of environmental conditions and show measurable responses, such as changes in phenology, abundance and range limits to local changes in precipitation and temperature regimes. Amphibians offer unique opportunities to study the important ecological and evolutionary implications of responses in life history characteristics to climatic change. We analyzed a late-Holocene fossil record of the Tiger Salamander (Ambystoma tigrinum) for evidence of population-level changes in body size and paedomorphosis to climatic change over the last 3000 years.ResultsWe found a significant difference in body size index between paedomorphic and metamorphic individuals during the time interval dominated by the Medieval Warm Period. There is a consistent ratio of paedomorphic to metamorphic specimens through the entire 3000 years, demonstrating that not all life history characteristics of the population were significantly altered by changes in climate on this timescale.ConclusionThe fossil record of Ambystoma tigrinum we used spans an ecologically relevant timescale appropriate for understanding population and community response to projected climatic change. The population-level responses we documented are concordant with expectations based on modern environmental studies, and yield insight into population-level patterns across hundreds of generations, especially the independence of different life history characteristics. These conclusions lead us to offer general predictions about the future response of this species based on likely scenarios of climatic warming in the Rocky Mountain region.

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea, Echinodermata) during the perimetamorphic period

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.

Neoteny and progenesis as two heterochronic processes involved in paedomorphosis in Triturus alpestris (Amphibia: Caudata).

Current theories on the evolution of paedomorphosis suppose that several ontogenetic pathways have appeared according to different selective pressures. The aim of this study was to find out whether two distinct processes can lead to paedomorphosis in the Alpine newt, Triturus alpestris. In this respect, we compared age structures of paedomorphic and metamorphic individuals in two newt populations where the two forms lived syntopically. Whereas paedomorphosis resulted in a slower rate of somatic development in one population, it resulted in an acceleration of sexual maturation in the other population. These processes correspond to neoteny and progenesis, respectively. These results suggest that phenotypic plasticity can result from contrasted ontogenetic pathways between two populations of the same species. They give support to models that consider gonadic development as the target of selection under different environmental pressures.