Freshwater Colonization Research Articles

A key metabolic gene for recurrent freshwater colonization and radiation in fishes.

Colonization of new ecological niches has triggered large adaptive radiations. Although some lineages have made use of such opportunities, not all do so. The factors causing this variation among lineages are largely unknown. Here, we show that deficiency in docosahexaenoic acid (DHA), an essential ω-3 fatty acid, can constrain freshwater colonization by marine fishes. Our genomic analyses revealed multiple independent duplications of the fatty acid desaturase gene Fads2 in stickleback lineages that subsequently colonized and radiated in freshwater habitats, but not in close relatives that failed to colonize. Transgenic manipulation of Fads2 in marine stickleback increased their ability to synthesize DHA and survive on DHA-deficient diets. Multiple freshwater ray-finned fishes also show a convergent increase in Fads2 copies, indicating its key role in freshwater colonization.

Na+ /K+ -ATPase gene duplications in clitellate annelids are associated with freshwater colonization.

Major habitat transitions, such as those from marine to freshwater habitats or from aquatic to terrestrial habitats, have occurred infrequently in animal evolution and may represent a barrier to diversification. Identifying genomic events associated with these transitions can help us better understand mechanisms that allow animals to cross these barriers and diversify in new habitats. Study of the Capitella telata and Helobdella robusta genomes allows examination of one such habitat transition (marine to freshwater) in Annelida. Initial examination of these genomes indicated that the freshwater leech H.robusta contains many more copies (12) of the sodium-potassium pump alpha-subunit (Na+ /K+ -ATPase) gene than does the marine polychaete C.telata (2). The sodium-potassium pump plays a key role in maintenance of cellular ionic balance and osmoregulation, and Na+ /K+ -ATPase duplications may have helped annelids invade and diversify in freshwater habitats. To assess whether the timing of Na+ /K+ -ATPase duplications coincided with the marine-to-freshwater transition in Clitellata, we used transcriptomic data from 18 annelid taxa, along with the two genomes, to infer a species phylogeny and identified Na+ /K+ -ATPase gene transcripts in order to infer the timing of gene duplication events using tree-based methods. The inferred timing of Na+ /K+ -ATPase duplication events is consistent with the timing of the initial marine-to-freshwater transition early in the history of clitellate annelids, supporting the hypothesis that gene duplications may have played a role in the annelid diversification into freshwater habitats.

Gryposuchus (Crocodylia, Gavialoidea) from the early Miocene of Venezuela

Here, a fragment of a mandible recently discovered in the Cerro Zamuro site (Castillo Formation, Lara State, northwestern Venezuela) is assigned to the giant gavialoid Gryposuchus. This specimen, recovered from putative brackish environments of the early Miocene (~18 Ma) age, is unequivocally the earliest record of the genus in South America. Gryposuchus, together with the other gryposuchine previously recognized from the Castillo Formation, Siquisiquesuchus venezuelensis, increases the early Miocene taxonomic diversity of the group in the northern Neotropics. This new information from the Castillo Formation supports the conclusion that early gryposuchine evolutionary stages were in coastal, shallow marine or brackish environments, while the presence of some genera, such as Gryposuchus, in middle to late Miocene freshwater environments, is secondary habitat colonization late in the evolution of the clade. Freshwater colonization is probably the result of the gradual adaptation of early marine-adapted gryposuchines to the extensive estuarine-like environments of northern South America lowlands associated with marine transgressions that systematically occurred during the middle Eocene to early Oligocene. This new record is evidence of the wide chronological distribution of Gryposuchus in northern South America, highlighting the importance of this area as the center of origin and radiation of this successful Miocene gavialoid.

Salinity-induced phenotypic plasticity in threespine stickleback sperm activation.

Phenotypic expression may be and often is influenced by an organism's developmental environment, referred to as phenotypic plasticity. The sperm cells of teleosts have been found to be inactive in the seminal plasma and are activated by osmotic shock for most fish species, through release in either hypertonic (for marine fish) or hypotonic (for freshwater fish) water. If this is the case, the regulatory system of sperm mobility should be reversed in salt- and freshwater fish. We tested this hypothesis by first activating sperm of salt- and freshwater populations of threespine stickleback in salt- and freshwater. The sperm from saltwater stickleback could be activated in either salinity, which matches the freshwater colonization history of the species, whereas the sperm from the freshwater population acted as predicted by the osmotic shock theory and was activated in freshwater only. As the freshwater population used here was calculated to be thousands of years old, we went on to test whether the trait(s) were plastic and sperm from freshwater males still could be activated in saltwater after individuals were exposed to saltwater. After raising freshwater stickleback in saltwater, we found the mature males to have active sperm in both saltwater and freshwater. Further, we also found the sperm of wild-caught freshwater stickleback to be active in saltwater after exposing those mature males to saltwater for only 2 days. This illustrates that the ability for stickleback sperm to be activated in a range of water qualities is an environmentally induced plastic trait.

Evolutionary bottlenecks in brackish water habitats drive the colonization of fresh water by stingrays.

Species richness in freshwater bony fishes depends on two main processes: the transition into and the diversification within freshwater habitats. In contrast to bony fishes, only few cartilaginous fishes, mostly stingrays (Myliobatoidei), were able to colonize fresh water. Respective transition processes have been mainly assessed from a physiological and morphological perspective, indicating that the freshwater lifestyle is strongly limited by the ability to perform osmoregulatory adaptations. However, the transition history and the effect of physiological constraints on the diversification in stingrays remain poorly understood. Herein, we estimated the geographic pathways of freshwater colonization and inferred the mode of habitat transitions. Further, we assessed habitat-related speciation rates in a time-calibrated phylogenetic framework to understand factors driving the transition of stingrays into and the diversification within fresh water. Using South American and Southeast Asian freshwater taxa as model organisms, we found one independent freshwater colonization event by stingrays in South America and at least three in Southeast Asia. We revealed that vicariant processes most likely caused freshwater transition during the time of major marine incursions. The habitat transition rates indicate that brackish water species switch preferably back into marine than forth into freshwater habitats. Moreover, our results showed significantly lower diversification rates in brackish water lineages, whereas freshwater and marine lineages exhibit similar rates. Thus, brackish water habitats may have functioned as evolutionary bottlenecks for the colonization of fresh water by stingrays, probably because of the higher variability of environmental conditions in brackish water.

Variation in Lateral Plate Quality in Threespine Stickleback from Fresh, Brackish and Marine Water: A Micro-Computed Tomography Study.

IntroductionIt is important to understand the drivers leading to adaptive phenotypic diversity within and among species. The threespine stickleback (Gasterosteus aculeatus) has become a model system for investigating the genetic and phenotypic responses during repeated colonization of fresh waters from the original marine habitat. During the freshwater colonization process there has been a recurrent and parallel reduction in the number of lateral bone plates, making it a suitable system for studying adaptability and parallel evolution.ObjectiveThe aim of this study was to investigate an alternative evolutionary path of lateral plate reduction, where lateral plates are reduced in size rather than number.Materials and MethodsA total of 72 threespine stickleback individuals from freshwater (n = 54), brackish water (n = 27) and marine water (n = 9) were analysed using microcomputed tomography (μCT) to determine variation in size, thickness and structure of the lateral plates. Furthermore, whole-body bone volume, and bone volume, bone surface and porosity of lateral plate number 4 were quantified in all specimens from each environment.ResultsThe results showed a significant difference in plate size (area and volume) among populations, where threespine stickleback from polymorphic freshwater and brackish water populations displayed lateral plates reduced in size (area and volume) compared to marine sticklebackConclusionsReduction of lateral plates in threespine stickleback in fresh and brackish water occurs by both plate loss and reduction in plate size (area and volume).

Early Miocene amber inclusions from Mexico reveal antiquity of mangrove-associated copepods.

Copepods are aquatic microcrustaceans and represent the most abundant metazoans on Earth, outnumbering insects and nematode worms. Their position of numerical world predominance can be attributed to three principal radiation events, i.e. their major habitat shift into the marine plankton, the colonization of freshwater and semiterrestrial environments, and the evolution of parasitism. Their variety of life strategies has generated an incredible morphological plasticity and disparity in body form and shape that are arguably unrivalled among the Crustacea. Although their chitinous exoskeleton is largely resistant to chemical degradation copepods are exceedingly scarce in the geological record with limited body fossil evidence being available for only three of the eight currently recognized orders. The preservation of aquatic arthropods in amber is unusual but offers a unique insight into ancient subtropical and tropical ecosystems. Here we report the first discovery of amber-preserved harpacticoid copepods, represented by ten putative species belonging to five families, based on Early Miocene (22.8 million years ago) samples from Chiapas, southeast Mexico. Their close resemblance to Recent mangrove-associated copepods highlights the antiquity of the specialized harpacticoid fauna living in this habitat. With the taxa reported herein, the Mexican amber holds the greatest diversity of fossil copepods worldwide.

Responses to simulated winter conditions differ between threespine stickleback ecotypes

Abiotic factors can act as barriers to colonization and drive local adaptation. During colonization, organisms may cope with changes in abiotic factors using existing phenotypic plasticity, but the role of phenotypic plasticity in assisting or hindering the process of local adaptation remains unclear. To address these questions, we explore the role of winter conditions in driving divergence during freshwater colonization and the effects of plasticity on local adaptation in ancestral marine and derived freshwater ecotypes of threespine stickleback (Gasterosteus aculeatus). We found that freshwater-resident stickleback had greater tolerance of acute exposure to low temperatures than marine stickleback, but these differences were abolished after acclimation to simulated winter conditions (9L:15D photoperiod at 4 °C). Plasma chloride levels differed between the ecotypes, but showed a similar degree of plasticity between ecotypes. Gene expression of the epithelial calcium channel (ECaC) differed between ecotypes, with the freshwater ecotype demonstrating substantially greater expression than the marine ecotype, but there was no plasticity in this trait under these conditions in either ecotype. In contrast, growth (assessed as final mass) and the expression of an isoform of the electroneutral Na(+)/H(+) exchanger (NHE3) exhibited substantial change with temperature in the marine ecotype that was not observed in the freshwater ecotype under the conditions tested here, which is consistent with evolution of these traits by a process such as genetic assimilation. These data demonstrate substantial divergence in many of these traits between freshwater and marine stickleback, but also illustrate the complexity of possible relationships between plasticity and local adaptation.

Mid-Carboniferous diversification of continental ecosystems inferred from trace fossil suites in the Tynemouth Creek Formation of New Brunswick, Canada

We report Skolithos, Scoyenia and Mermia Ichnofacies from sub-humid tropical fluvial megafan deposits in the Lower Pennsylvanian Tynemouth Creek Formation of New Brunswick, Canada, and discuss their evolutionary and palaeoecological implications, especially regarding the colonization of continental freshwater/terrestrial environments. The Skolithos Ichnofacies comprises annelid/arthropod spreite in the upper storey of a fluvial channel. The Scoyenia Ichnofacies comprises tetrapod tracks, arthropleurid trackways, and shallow annelid/arthropod burrows in active/abandoned fluvial channels and rapidly aggrading levees/splays in proximal interfluve areas. The Mermia Ichnofacies comprises abundant xiphosuran trackways, along with diverse traces of other arthropods, annelids, mollusks, and fish, in shallow freshwater lakes, ponds, and coastal bays in slowly aggrading distal interfluve areas. Transitional Mermia/Scoyenia Ichnofacies comprise tetrapod, mollusk and annelid/arthropod traces in coastal bay deposits on the distal edge of the megafan. These trace fossil suites (1) provide the clearest documentation yet of the mid-Carboniferous diversification event, when tropical continental environments became more densely populated; (2) suggest that euryhaline visitors (xiphosurans, microconchids, and other taxa) from open marine settings played a key role in this episode of freshwater colonization; and (3) provide empirical support for the “Déjà vu Effect”, the evolutionary concept that new or empty ecospace, recurrent in spatially and temporally variable environments, is colonized by simple ichnocoenoses.

Comparative analysis of Japanese three-spined stickleback clades reveals the Pacific Ocean lineage has adapted to freshwater environments while the Japan Sea has not.

Divergent selection and adaptive divergence can increase phenotypic diversification amongst populations and lineages. Yet adaptive divergence between different environments, habitats or niches does not occur in all lineages. For example, the colonization of freshwater environments by ancestral marine species has triggered adaptive radiation and phenotypic diversification in some taxa but not in others. Studying closely related lineages differing in their ability to diversify is an excellent means of understanding the factors promoting and constraining adaptive evolution. A well-known example of the evolution of increased phenotypic diversification following freshwater colonization is the three-spined stickleback. Two closely related stickleback lineages, the Pacific Ocean and the Japan Sea occur in Japan. However, Japanese freshwater stickleback populations are derived from the Pacific Ocean lineage only, suggesting the Japan Sea lineage is unable to colonize freshwater. Using stable isotope data and trophic morphology, we first show higher rates of phenotypic and ecological diversification between marine and freshwater populations within the Pacific Ocean lineage, confirming adaptive divergence has occurred between the two lineages and within the Pacific Ocean lineage but not in the Japan Sea lineage. We further identified consistent divergence in diet and foraging behaviour between marine forms from each lineage, confirming Pacific Ocean marine sticklebacks, from which all Japanese freshwater populations are derived, are better adapted to freshwater environments than Japan Sea sticklebacks. We suggest adaptive divergence between ancestral marine populations may have played a role in constraining phenotypic diversification and adaptive evolution in Japanese sticklebacks.

Timing marine–freshwater transitions in the diatom order Thalassiosirales

With species found throughout both marine and fresh waters, the diatom order Thalassiosirales is one of the most phylogenetically and ecologically diverse lineages of planktonic diatoms. A clear understanding of the timescale of Thalassiosirales evolution would provide novel insights into the rates and patterns of species diversification associated with major habitat shifts, as well as provide valuable context for understanding the age and evolutionary history of the model species, Cyclotella nana (= Thalassiosira pseudonana). The freshwater fossil record for Thalassiosirales is extensive, well characterized, and generally supportive of a Miocene origin for the major freshwater lineages. The marine record is, by comparison, more sparse and in many cases, unverified. The discovery of freshwater thalassiosiroids in Eocene sediments pushed the freshwater fossil record considerably further back in time, highlighting an apparent gap of some 30 million years. An alternative interpretation is that the Miocene and Eocene reports represent competing hypotheses. In the absence of additional independent and decisive fossil data, I explored the relative plausibility of these two scenarios with Bayesian relaxed molecular clock methods under a range of fossil calibration schemes. Although I found no support for the Eocene fossil dates, the two major freshwater colonization events probably occurred much earlier than previously thought—as early as the Paleocene for Cyclotella, followed by an Eocene origin for the cyclostephanoid lineage. Much of the extant freshwater diversity in both lineages traces back to the Miocene, however, giving the impression of a single Miocene origin. Efforts to infer the timescale of Thalassiosirales evolution more accurately would benefit from a systematic reevaluation of the marine fossil record and formal integration of fossil species into existing phylogenetic hypotheses.

Blowin' in the wind… 100 Ma old multi-staged dinoflagellate with sexual fusion trapped in amber: Marine–freshwater transition

Here we report the unexpected discovery of multi-staged dinoflagellates with organic cellular contents from a paralic habitat trapped in 100millionyear (100Ma) old amber. Amber formed from resin, a fluid medium, that trapped, then remarkably embedded and preserved soft parts of organisms usually destroyed by fossilization processes. We assume that the marine-costal dinoflagellates reached the sticky resin carried away by the wind, inside spray droplets. We answer to a fundamental question dealing with the Peridiniaceae: the paratabulation of Cretaceous cysts reflects the tabulation of Cretaceous thecae. We provide the first life cycle of the fossil record with evidence of cellulosic thecae, sexual fusion and zygote. We highlight an ancestral behaviour for the sexual phase: naked gametes complete fusion outside of gametic thecae, a process known in rare extant Peridinium species. The new taxon, Succiniperidinium inopinatum gen. et sp. nov., belongs to the Peridiniaceae and shares characters with extant marine-brackish Scrippsiella and two freshwater Peridinium clades identified by morphological characters (tabulation, cingular plates, ecdysis, plasmogamy) and molecular phylogenies. Understanding the processes of marine–freshwater transition in microbial lineages is a central goal in evolutionary ecology. Marine dinoflagellates passed through the osmotic barrier and the studies of marine-costal Cretaceous species help to understand the adaptation and the diversification of these species. We discuss the migration according to biological, paleontological and molecular phylogenies data and suggest that the freshwater colonization was the result of Cretaceous species adapted to costal habitats; their lineage isolated in ponds following the Cenozoic global sea-level fall passed through the osmotic barrier. A Scrippsiella-like group (i.e. Subtilisphaera terrula, Palaeoperidinium cretaceum and Succiniperidinium inopinatum gen. et sp. nov.) is suggested as intermediate species in the Peridinium freshwater colonization line.

Do habitat shifts drive diversification in teleost fishes? An example from the pufferfishes (Tetraodontidae)

Habitat shifts are implicated as the cause of many vertebrate radiations, yet relatively few empirical studies quantify patterns of diversification following colonization of new habitats in fishes. The pufferfishes (family Tetraodon-tidae) occur in several habitats, including coral reefs and freshwater, which are thought to provide ecological opportunity for adaptive radiation, and thus provide a unique system for testing the hypothesis that shifts to new habitats alter diversification rates. To test this hypothesis, we sequenced eight genes for 96 species of pufferfishes and closely related porcupine fishes, and added 19 species from sequences available in GenBank. We time-calibrated the molecular phylogeny using three fossils, and performed several comparative analyses to test whether colonization of novel habitats led to shifts in the rate of speciation and body size evolution, central predictions of clades experiencing ecological adaptive radiation. Colonization of freshwater is associated with lower rates of cladogenesis in pufferfishes, although these lineages also exhibit accelerated rates of body size evolution. Increased rates of cladogenesis are associated with transitions to coral reefs, but reef lineages surprisingly exhibit significantly lower rates of body size evolution. These results suggest that ecological opportunity afforded by novel habitats may be limited for pufferfishes due to competition with other species, constraints relating to pufferfish life history and trophic ecology, and other factors.

No short-term effect of salinity on oxygen consumption in threespine stickleback (Gasterosteus aculeatus) from fresh, brackish, and salt water

Marine threespine stickleback ( Gasterosteus aculatus L., 1758) have repeatedly colonized Holarctic freshwater environments after the retreat of the Pleistocene glaciers, and based on their ability to move rapidly between salinities have apparently retained a robust osmoregulatory apparatus that can cope with both short- and long-term exposure to non-native salinity environments. Standard metabolic rate (SMR), measured as oxygen consumption at rest, can be used as an indicator of the cost of osmoregulation when fish are exposed to new environmental conditions. Following freshwater colonization, reduction in the number of lateral plates, an antipredator defence structure, is common. Completely plated fish dominate in the sea, low-plated fish dominate in fresh water, and partially plated fish often dominate in brackish water environments. In a laboratory experiment, we estimated SMR in locally adapted populations from salt, brackish, and freshwater environments at three different salinities (0, 15, and 30 practical salinity units (PSU)). In addition, we tested for correlations between SMR and lateral plate number and lateral plate genotype at the Ectodysplasin locus for stickleback originating from the brackish water population. Contrary to our expectations, no differences were found in SMR between any of the experimental groups in our experiment. Apparently, the threespine stickleback is able to move among salinity environments without large short-term metabolic costs, irrespective of their environment of origin.

Invasion of freshwater and variable marginal marine habitats by microconchid tubeworms – an evolutionary perspective

Microconchids are an extinct group of Spirorbis-like tentaculitoid tubeworms that dwelled in a variety of aquatic environments, ranging from normal marine, through brackish and hypersaline, to freshwater. An analysis of published microconchid occurrences focusing on their ecology and palaeoenvironmental distribution through geological time is conducted in order to establish the timing of microconchid colonization of freshwater and marginal marine habitats. Microconchids originated during the Late Ordovician in shallow shelf, normal marine environments where they thrived until their extinction at the end of the Middle Jurassic (latest Bathonian). Microconchid colonization of marginal marine brackish habitats seems to have started already by the Early Silurian (Wenlock). The freshwater habitats were invaded by microconchids in the Early Devonian, nearly simultaneously in several regions (Germany, Spitsbergen, USA). Since shallow marginal marine and freshwater habitats are more unstable, especially in terms of temperature and salinity fluctuations, as well as prone to desiccation, than normal marine, shelf environments, the drivers of the colonization of these habitats by microconchids are currently incompletely understood. We hypothesize that by colonizing such environments, microconchids gained access to abundant food resources in the form of suspended organic matter delivered from the land by rivers and streams. These, combined with their biology, enabled microconchids to reproduce fast and in large numbers. Microconchids are considered to have gone extinct by the end of the Middle Jurassic (Late Bathonian). Their youngest occurrence in freshwater environments is known from the Late Triassic and it is currently not known whether microconchids continued to occupy such habitats later on in the Jurassic. All the Middle Jurassic records of microconchids come from marine settings. Thus, more focused research on Jurassic brackish and freshwater deposits worldwide is needed to check whether they may have thrived in such environments at some locations, until their hypothesized extinction.

Abrupt changes in defence and trophic morphology of the giant threespine stickleback (Gasterosteussp.) following colonization of a vacant habitat

(8%: 0.5%), lateral plate 2 frequency (46%: 17%), lateral plate 3 height (17%: 18%), pelvic spine length (12%: 6%), dorsal spine length (21%: 16%), gill raker number (5%: 1%), and gill raker length (37%: 43%), and an increase in gill raker spacing (+5%: 0%), jaw length (+5%: +4%), and eye diameter (+8%: +7%). These changes within eight generations represent one-third of the morphological differences observed between naturally established large lake and pond populations in these archipelago and are all in the direction predicted from the change in habitat. Field samples indicate strong selection in the colonists although lab-reared individuals implicate an additional role of plasticity for dorsal spine and gill raker lengths, which may contribute to the rapid adaptation into novel and highly divergent selective landscapes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107, 494‐509. ADDITIONAL KEYWORDS: benthic adaptation ‐ freshwater colonization ‐ Gasterosteidae ‐ gigantism ‐ Haida Gwaii ‐ Haldanes ‐ macroinvertebrate ‐ plasticity ‐ rapid evolution ‐ selective landscape.

MULTIPLE EVOLUTIONARY PATHWAYS TO DECREASED LATERAL PLATE COVERAGE IN FRESHWATER THREESPINE STICKLEBACKS

Adaptation to novel environments can be based either on standing genetic variation or variation attributable to new mutations. When standing genetic variation for a functional adaptation is lacking, and variation due to new mutations is not yet available, adaptation is possible only through alternative functional solutions. Reduction in the number of bony lateral plates as an adaptation to freshwater colonization by marine threespine sticklebacks (Gasterosteus aculeatus) has occurred in numerous independent cases through allelic substitution in the ectodysplasin-a (Eda) gene. Studying the phenotypic and genetic variation in plate number and size in five marine and six freshwater threespine stickleback populations, we found that when variation in Eda was limiting (i.e., alleles associated with the low-plate morph were missing or in extremely low frequency), plate number reduction did not take place in freshwater populations, but reduced lateral plate coverage was achieved by a reduction in the size of lateral plates. Our results suggest that this phenotypically and genetically discrete "small-plated" threespine stickleback-which is the dominant form in three northern European freshwater populations-may be functionally equivalent to the low-plated morph and hence, serve as an example of convergent evolution toward functional similarity in the face of genetic constraints.

Towards deciphering dynamic changes and evolutionary mechanisms involved in the adaptation to low salinities in Ectocarpus (brown algae).

Colonizations of freshwater by marine species are rare events, and little information is known about the underlying mechanisms. Brown algae are an independent lineage of photosynthetic and multicellular organisms from which few species inhabit freshwater. As a marine alga that is also found in freshwater, Ectocarpus is of particular interest for studying the transition between these habitats. To gain insights into mechanisms of the transition, we examined salinity tolerance and adaptations to low salinities in a freshwater strain of Ectocarpus on physiological and molecular levels. We show that this isolate belongs to a widely distributed and highly stress-resistant clade, and differed from the genome-sequenced marine strain in its tolerance of low salinities. It also exhibited profound, but reversible, morphological, physiological, and transcriptomic changes when transferred to seawater. Although gene expression profiles were similar in both strains under identical conditions, metabolite and ion profiles differed strongly, the freshwater strain exhibiting e.g. higher cellular contents of amino acids and nitrate, higher contents of n-3 fatty acids, and lower intracellular mannitol and sodium concentrations. Moreover, several stress markers were noted in the freshwater isolate in seawater. This finding suggests that, while high stress tolerance and plasticity may be prerequisites for the colonization of freshwater, genomic alterations have occurred that produced permanent changes in the metabolite profiles to stabilize the transition.

PREDATION-IMPOSED SELECTION ON THREESPINE STICKLEBACK (GASTEROSTEUS ACULEATUS) MORPHOLOGY: A TEST OF THE REFUGE USE HYPOTHESIS

The transition from marine to freshwater life in the threespine stickleback (Gasterosteus aculeatus) is accompanied by complex morphological changes-including reduction in bony armor and change in body shape-but experimental evidence for the selective agents behind these evolutionary transitions is sparse. We investigated whether selection by predatory fish affects threespine stickleback morphology differentially when refuge is absent (pelagic lifestyle-ancestral condition) or present (benthic lifestyle-derived condition). Our results show that selection favors low numbers of lateral plates in habitats with refuge, whereas fully plated individuals have a selective advantage in habitats without refuge. We also found that a decrease in the length of the caudal peduncle increased survival probability, irrespective of habitat. The effect of spine lengths on survival was evident only in a multivariate analysis of selection, implying that it is essential to account for phenotypic and genetic correlations between traits before drawing conclusions about the effects of selection on single traits. Apart from uncovering targets and patterns of predator-induced selection on threespine stickleback morphology, our results provide direct evidence to support the hypothesis that differences in antipredator strategies in pelagic versus benthic sticklebacks could play a role in the repeated, independent cases of plate number reduction following freshwater colonization in this species.

Life: Marine animals moved upriver early: Fossil worm burrows suggest rapid colonization of freshwater

Life: Marine animals moved upriver early: Fossil worm burrows suggest rapid colonization of freshwater