Evidence of a reproductive barrier between two forms of the marine periwinkle Littorina fabalis (Gastropoda)

Studies of allozyme variation may reveal unexpected patterns of genetic variation which challenge earlier conclusions of species delimitations based on morphological data. However, allozyme variation alone may not be sufficient to resolve this kind of problem. For example, populations of the marine intertidal snail Littorina fabalis (=Littorina mariae) from wave exposed parts end from protected parts of the same shores are distinguished by different alleles of arginine kinase (Ark) while indifferent, or very nearly so, in another 29 loci. Intermediate populations have large deficiencies of exposed/sheltered heterozygote classes of Ark and we have earlier suggested habitat-related selection in this locus as the explanation. In this study we estimated growth rate of individual snails of different Ark-genotypes in three different habitats (exposed, sheltered and intermediate). In all habitats the snails homozygous for alleles of ‘exposed’ type grew faster and matured at a larger size than did snails homozygous for alleles of ‘sheltered’ types. This relationship was indirectly confirmed in three additional sites of intermediate exposure where exposed AA-genotypes dominated among large (>8 mm) snails while the sheltered genotypes dominated among small (<5 mm) snails of truly sympatric samples. We furthermore found small differences in allele frequencies of two other loci (Pgi and Pgm-2) and in shell colour frequencies, comparing sympatric snails of exposed and sheltered Ark-homozygotes. Although we found no signs of habitat-related selection among snails of different Ark-genotype, or selection against heterozygotes, we cannot reject selection in Ark, as our experiments only covered one island, one season and grown-up snails. The coupling between allozyme and phenotypic characters in strictly sympatric samples of snails suggests the presence of two gene pools. Perhaps the large and small forms of L. fabalis represent very closely related cryptic taxa. However, introgression between them seems a possible explanation for the striking similarities in the vast majority of morphological and allozyme characters.     

Ecological and morphological patterns in communities of land snails of the genus Mandarina from the Bonin Islands

The land snail genus Mandarina has undergone extensive radiation within the Bonin Islands in the west Pacific. The preferred above-ground vegetation heights of sympatric species were clearly different. They separated into arboreal, semi-arboreal, exposed ground and sheltered ground ecotypes. Shells of species with different ecotypes differ markedly, but shells of species with the same ecotype are very similar to each other. Shell morphologies of some phylogenetically distantly related species with the same ecotype were indistinguishable. Character evolution estimated parsimoniously using a phylogenetic tree suggests that the speciation among sympatric species is accompanied by ecological and morphological diversification. In addition, species coexistence of Mandarina is related to niche differentiation. The above findings suggest that ecological interactions among species contribute to the ecological and morphological diversification and radiation of these land snails in this depauperate environment.     

Plastic and Heritable Variation in Shell Thickness of the Intertidal Gastropod Nucella lapillus Associated with Risks of Crab Predation and Wave Action,and Sexual Maturation

The intertidal snail Nucella lapillus generally has thicker shells at sites sheltered from wave action, where crabs are abundant and pose a high risk of predation, than at exposed sites where crabs are rare. We studied two populations showing the opposite trend. We reciprocally transplanted snails between field sites and measured shell length, width and lip thickness of those recaptured 12 months later. Snails transplanted to the sheltered site grew larger than sheltered-site residents, which in turn grew larger than transplants to the exposed site. Relative shell-lip thickness was greater in residents at the exposed site than at the sheltered site. Transplants from shelter to exposure developed relatively thicker shells than their controls and relatively thinner shells from exposure to shelter. Progeny of the two populations were reared for 12 months in a common garden experiment presenting effluent from crabs feeding on broken conspecifics as the treatment and fresh sea-water as the control. The crab-effluent treatment decreased foraging activity, concomitantly reducing cumulative somatic growth and reproductive output. Juveniles receiving crab-effluent grew slower in shell length while developing relatively thicker shell lips than controls, the level of response being similar between lineages. F₂ progeny of the exposed-site lineage showed similar trends to the F₁s; sheltered-site F₂s were too few for statistical analysis. At sexual maturity, shell-lip thickness was greater in snails receiving crab-effluent than in controls, indicating plasticity, but was also greater in the exposed-site than in the sheltered-site lineage, indicating heritable variation, probably in degree of sexual thickening of the shell lip. Results corroborate hypotheses that ‘defensive’ shell thickening is a passive consequence of starvation and that heritable and plastic control of defensive shell morphology act synergistically. Shell thickening of juveniles was similar between lineages, contrary to hypotheses predicting differential strengths of plasticity in populations from low- or high-risk habitats.     

Shell shape and habitat use in the North‐west Pacific land snail Mandarina polita from Hahajima,Ogasawara: current adaptation or ghost of species past?

The endemic land snail genus Mandarina of the Ogasawara Islands provides an excellent model system to investigate adaptive radiation. Previously, it has been shown that coexisting species of the islands segregate by microhabitat, so that they are either predominantly found on the ground in relatively wet and sheltered sites, dry and exposed sites, or else are arboreal. Moreover, shell morphology correlates with microhabitat, so that species in wet and sheltered sites tend to have high-spired shells with a high aperture, and those in dry and exposed sites tend to have relatively low-spired shells with a wide aperture. We have now found that on Hahajima, Mandarina polita have variable shell morphology, and there is a correlation between morphology and the depth of leaf litter, as well as the presence/absence of other terrestrial species. Specifically, when high-spired terrestrial Mandarina ponderosa is present, M. polita tend to be low-spired and have a large aperture, indicative of character displacement. When M. ponderosa is absent, the shell shape of M. polita is much more variable, the overall spire is higher, individuals are found in deeper litter, and there is a strong correlation between litter depth and spire height. We argue that these patterns are due to local adaptation, but it remains possible that they are an artefact due to the 'ghost of species past'.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 149–159.     

Population and microhabitat effects of interspecific interactions on the endangered Andalusian toothcarp (A<Emphasis Type="Italic">phanius baeticus</Emphasis>)

The Andalusian toothcarp, Aphanius baeticus, is a critically endangered cyprinodontid species, with only nine known extant populations. Although not yet studied in the field, the distribution and abundance of Andalusian toothcarp are thought to be strongly influenced by interspecific interactions. We analysed the abundance and microhabitat use of Andalusian toothcarp in two water courses, one in which several other fish species occurred (sympatric site) and one hypersaline stream in which toothcarp was the only species present (allopatric site). Fish were sampled using plastic minnow traps and results were analysed separately for three size categories. Toothcarps were clearly more abundant in the allopatric population than in the sympatric one, though the difference was less apparent in the smallest size category. In coexistence with other species, toothcarp occupied shallower microhabitats, but in both sites in the absence of shelter fish selected deeper positions than in its absence. While in the sympatric site sheltered microhabitats were used predominately by small individuals, in the allopatric ones they were used by larger ones. Observed patterns strongly suggest that predation is the main mechanism involved in the differences in abundance and microhabitat use between sites. Our results confirm that the presence or absence of coexisting species is an important habitat feature for Andalusian toothcarp populations.     

SYMPATRIC CONVERGENCE AND ENVIRONMENTAL CORRELATION BETWEEN TWO LAND-SNAIL SPECIES

In the southern Appalachian region of North America, the phylogenetically convergent shells of the polygyrid snails Triodopsinae Neohelix major (Binney) and Polygyrinae Mesodon normalis (Pilsbry) are even more convergent in size and shape in sympatry (7 sites) than in allopatry (23 and 10 sites). Environmental correlations account for 34% and 30% of size and shape variations in N. major (larger, taller, and more loosely coiled at northern, high-altitude, sheltered sites), but for only 14% and 9% in M. normalis (larger, flatter, and more loosely coiled at south-facing, exposed sites). The statistical significance of the sympatric convergence dropped out when these correlations were removed. This phenomenon helps account for the many cases in eastern North America of nearly identical land-snail shells in sympatry and questions the importance of competitive character displacement in the evolution of land-snail shell morphology. This apparently nonmimetic case of sympatric convergence provides an unusually precise and well-delimited, naturally replicated experiment in evolutionary morphology, which is analyzed for controlling factors in a follow-up paper.     

Phenotypic homogeneity of two intertidal snails across a wave exposure gradient in South Australia

Dislodgement by the large drag forces imparted by breaking waves is an important cause of mortality for intertidal snails. The risk of drag-induced dislodgement can be reduced with: (1) a smaller shell of lower maximum projected surface area (MPSA); (2) a streamlined shell shape characterized by a squatter shell; and/or (3) greater adhesive strength attained through a larger foot area or increased foot tenacity. Snails on exposed coasts tend to express traits that increase dislodgement resistance. Such habitat-specific differences could result from direct selection against poorly adapted phenotypes on exposed shores but may reflect gastropod adaptation to high wave action achieved through phenotypic plasticity or genetic polymorphism. With this in mind, we examined the size, shape and adhesive strength of populations of two gastropod species, Austrocochlea constricta (Lamarck) and Nerita atramentosa (Reeve), from two adjacent shores representing extremes in wave exposure. Over a 5 day period, maximum wave forces were more than 10 times greater on the exposed than sheltered shore. Size-frequency distributions indicate that a predator consuming snails within the 1.3-1.8 cm length range regulates sheltered shore populations of both snail species. Although morphological scaling considerations suggest that drag forces should not place physical limits on the size of these gastropods, exposed shore populations of both snails were small relative to the maximum size documented for these species. Therefore, selective forces at the exposed site might favour smaller individuals with increased access to microhabitat refuges. Unexpectedly, however, neither snail species exhibited between-shore differences in shape, foot area or foot tenacity, which are likely to have adaptive explanations. Hence, it is possible that these snails are incapable of adaptive developmental responses to high wave action. Instead, the homogeneous and wave-exposed nature of Australia's southern coastline may have favoured the evolution of generalist strategies in these species.     

Male-male competition and nuptial-colour displacement as a diversifying force in Lake Victoria cichlid fishes

We propose a new mechanism for diversification of male nuptial-colour patterns in the rapidly speciating cichlid fishes of Lake Victoria. Sympatric closely related species often display nuptial colours at opposite ends of the spectrum with males either blue or yellow to red. Colour polymorphisms within single populations are common too. We propose that competition between males for breeding sites promotes such colour diversification, and thereby speciation. We hypothesize that male aggression is primarily directed towards males of the common colour, and that rare colour morphs enjoy a negatively frequency-dependent fitness advantage. We test our hypothesis with a large dataset on the distributions and nuptial colorations of 52 species on 47 habitat islands in Lake Victoria, and with a smaller dataset on the within-spawning-site distributions of males with different coloration. We report that territories of males of the same colour are negatively associated on the spawning site, and that the distribution of closely related species over habitat islands is determined by nuptial coloration in the fashion predicted by our hypothesis. Whereas among unrelated species those with similar nuptial colour are positively associated, among closely related species those with similar colour are negatively associated and those with different colour are positively associated. This implies that negatively frequency-dependent selection on nuptial coloration among closely related species is a sufficiently strong force to override other effects on species distributions. We suggest that male-male competition is an important and previously neglected agent of diversification among haplochromine cichlid fishes.     

Micro- and macrogeographic allozyme variation in Littorina fabalis; do sheltered and exposed forms hybridize?

Earlier studies of Swedish populations of the marine snail Littorina fabalis show that snails from different microhabitats (with a greater and lesser exposure to wave action) have almost diagnostic differences in one allozyme locus (arginine kinase, Ark ), and differ in adult size. Snails with 'sheltered' and 'exposed' Ark genotypes occur in sympatry in intermediary exposed sites and here adult sizes remain distinct. Approaching the microgeographic differentiation we studied the parts of two populations where the frequency of Ark changes dramatically over zones 50–120 m wide. The aim was to test if the transitional zones are best described as areas of mixing of two genetically separate populations, or if hybridization between the exposed and sheltered groups occurs. Heterozygotes were in deficiency along both clines but were still roughly twice as common as expected from a pure mixing of 'sheltered' and 'exposed' groups suggesting hybridization. Hybridization was also supported by the observation that snails homozygous for sheltered and exposed alleles mated at random with each other in both populations. On the macrogeographic scale, we found populations from exposed and sheltered sites in France and Wales being fixed for the same exposed and sheltered Ark alleles as found in Sweden. However, variation in three other highly polymorphic loci indicated geographic affinity rather than habitat similarity being the main factor of genetic coherence. These observations support a hypothesis of gene flow between exposed and sheltered populations of L. fabalis . Two Spanish populations were remarkably different with unique alleles at high frequencies in three of four strongly polymorphic loci.     

Temperature-related diversity of shell colour in the intertidal gastropod Batillaria

The intertidal snail Batillaria exhibits remarkable variationin the shell colour within and among populations. Field studywas conducted to determine the factors in maintaining observedshell colour polymorphism. Geographical variations in shellcolour polymorphisms in B. attramentaria were significantlycorrelated with the temperature of the locality of the population.Darker morphs were predominant in colder regions, whereas lightermorphs increase their proportion in warmer regions. A consistentassociation was also found in B. multiformis that co-existedwith B. attramentaria. Strong predatory pressure imposed bydigenean trematode parasites was observed in B. attramentaria.However, it is unlikely to affect the colour variations, becauseno correlation exists between colour morphs and trematode parasitism.Although visual selection may also contribute to colour variationin Batillaria, no evidence is found for the existence of visualpredators that affect colour patterns of these snails. The deficitof variation in cold regions is possibly due to selection againstbrighter morphs, because bright colours reflect heat. Althoughdark shells absorb sunlight and may therefore be exposed tothe risks of overheating and drying up in a hot habitat, thedarkest morph was frequently observed in the warmer regions,suggesting that physical selection on the colour morphs canbe relaxed in the warmer environment. Our results suggest thatclimatic selection is one of the significant factors maintainingshell colour polymorphism in these intertidal snails. (Received 13 December 2006; accepted 14 April 2007)     

Refuge function of marine algae complicates selection in an intertidal snail

Species with restricted gene flow often show trait-shifts from one type of environment to another. In those rock-dwelling marine gastropods that lack larval dispersal, size generally decreases in wave-exposed habitats reducing risk of dislodgement, while increases in less exposed habitats to resist crab-crushing. In Littorina fabalis, however, snails of moderately exposed shores are generally much larger (11–14 mm) than snails of sheltered shores (5–8 mm). Observations from the White Sea (where crabs are not present) indicate that in the absence of crabs snails are small (6–7 mm) in both habitats. We assumed that the optimal size for L. fabalis in the absence of crabs is less than 8 mm, and thus that increased size in moderately exposed habitats in areas with crabs might be a response to crab predation. In a crab-rich area (Sweden) we showed that crab predation is an important mortality factor for this snail species in both sheltered and moderately exposed habitats. In sheltered habitats, snails were relatively more protected from crab-predation when dwelling on their habitual substrate, fucoid algae, than if experimentally tethered to rocks below the algae. This showed that algae function as snail refuges. Snail dislodgement increased, however, with wave exposure but tethering snails in moderately exposed habitats showed that large snails survived equally well on rocks under the algae as in the canopy of the algae. Thus in sheltered habitats a small snail size is favored, probably due to life-history reasons, while increased risk of being dislodged from the algae refuges promotes a large size in moderately exposed habitats. This study shows an example of selection of a trait depends on complex interactions of different factors (life-history optimization, crab predation, wave induced dislodgement and algal refuges).     

Systematics, biogeography, and evolution of the Neotropical peacock basses Cichla (Perciformes: Cichlidae)

To investigate forces influencing diversification in Neotropical fishes, the phylogenetic relationships among species and populations of the cichlid genus Cichla were examined. Mitochondrial DNA was sequenced for 454 individuals of the 5 nominal Cichla species and several putative undescribed species. Phylogenetic analyses support the distinction of two major clades of Cichla. Clade A includes C. temensis and two undescribed species from the lower Amazonas and Xingu Rivers. Clade B includes C. orinocensis, C. monoculus, C. ocellaris. C, intermedia, and an undescribed species from the upper Madeira River. Species boundaries were relatively well-circumscribed for clade B, while incomplete lineage sorting was inferred for clade A. Three probable instances of introgression were observed, including a regional population of C. orinocensis from the Negro River that shows a history of introgression. Biogeographic patterns from Cichla are partially congruent with those seen in several other Neotropical fish clades, and the diversification of Cichla species is inferred to result from both vicariance and sympatric divergence.     

Visual selection of shell colour in two littoral prosobranchs

The rough periwinkle Littorina “saxatilis” exhibits a wide range of shell colours. In current literature it is claimed that these colours are not related to the environment, that they arise randomly as genetic accidents and that they have little positive survival value. In a previous paper it has been shown that, in Wales, “saxatilis” consists of four separate, fully sympatric species. This paper reports an investigation as to whether the colour of two of these species, nigrolineata and rudis, is related to the colour of the background. Because of difficulties in quantitatively describing the colour of the background it was decided to concentrate mainly upon one aspect: whether or not the frequency of red shells is larger upon shores consisting of red sandstone than elsewhere in Wales. In both species the association between red shells and red sandstone is highly significant. All nigrolineata samples in which red was found at a frequency above 15% are from red sandstone. On red sandstone, red shells of this species were found mainly upon sheltered shores, their frequency decreasing and that of white shells increasing with exposure. This may be because barnacles, which occupy the same vertical zone as nigrolineata, are more abundant upon exposed shores. Partly covering the red rock, barnacles create a white background upon which white shells, rather than red, are cryptic. Yellow shells are found mainly on sheltered shores, where the brown alga Fucus is abundant. It was observed that when the tide is in and the algae spread out, a yellow shell situated beneath them is well concealed. Yellow is also found upon barnacles which because of fungal and lichen infections, are dirty yellow. It is suggested that a striped (“nigrolineated”) pattern breaks up the shape of the shell. It also resembles the colour of the dark rock and the dark sutures between barnacle plates. In rudis 80% of the samples containing over 25% red are from red sandstone. Contrast to nigrolineata, in this species the relative frequency of red decreases on sheltered shores. This could be because the brown alga Pelvetia, which occupies the same vertical zone as rudis, is more abundant in sheltered conditions and its colour partly covers over that of the rock beneath. The fact that in both species red shells are more frequent upon red sandstone than elsewhere in the study area, suggests that visual selection is restricting their distribution to the background that they match most. Rock pipits and shore-crabs prey upon winkles. They have colour perception and could be partly responsible for this selection.     

Epistasis, phenotypic disequilibrium and contrasting associations with climate in the land snail Theba pisana

Hotter conditions favour effectively unbanded (EUB) shells in the snail Theba pisana. T. pisana is also polymorphic for colour of the shell's apex, determined by a pair of alleles at a locus linked to the banding locus. Apex colour is epistatic to shell banding, such that banded snails with a dark apex have darker bands. Annual censuses over 22 years across an ecotone between a sheltered Acacia thicket and open dune vegetation showed a persistent association of both EUB shells and pale apex with the Open habitat. The parallel variation was due partly to strong phenotypic disequilibrium, as the combination of EUB with dark apex was rare. Nevertheless, in fully banded shells the frequency of pale apex was also higher in the Open habitat, confirming independent, parallel associations of the two contributors to paleness. Within the Acacia habitat, temporal variation of the frequencies of banding morphs was much greater than for apex colour, and EUB shells were associated with hotter summers. Consistent with its primary effect only on the very small snails, apex colour did not vary with summer conditions, but instead, higher frequencies of pale apices were associated with sunnier winters. The intensity of selection was lower on apex colour than shell banding, due partly to the constraint of phenotypic disequilibrium. The shell traits in T. pisana are an example of complex responses to climatic variation, in which phenotypic disequilibrium constrains evolution of apex colour, but separate mechanisms of selection are evident.     

The interplay between multiple predators and prey colour divergence

Evolutionary divergence in the coloration of toxic prey is expected when geographic variation in predator composition and behavior favours shifts in prey conspicuousness. A fundamental prediction of predator‐driven colour divergence is that the local coloration should experience lower predation risk than novel prey phenotypes. The dorsal coloration of the granular poison frog varies gradually from populations of conspicuous bright red frogs to populations of dull green and relatively cryptic frogs. We conducted experiments with clay models in four populations to examine the geographic patterns of taxon‐specific predation. Birds avoided the local phenotype while lizards consistently selected for decreased conspicuousness and crab predation did not depend on frog coloration. Importantly, birds and lizards favoured low conspicuousness in populations where relatively cryptic green morphs have evolved. This study provides evidence for the interplay among distinct selective pressures, from multiple‐predator taxa, acting on the divergence in protective coloration of prey species. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 580–589.     

Colour plasticity and background matching in a threespine stickleback species pair

Examining differences in colour plasticity between closely‐related species in relation to the heterogeneity of background colours found in their respective habitats may offer important insight into how cryptic colour change evolves in natural populations. In the present study, we examined whether nonbreeding dorsal body coloration has diverged between sympatric species of stickleback along with changes in habitat‐specific background colours. The small, limnetic species primarily occupies the pelagic zone and the large, benthic species inhabits the littoral zone. We placed benthic and limnetic sticklebacks against extremes of habitat background colours and measured their degree of background matching and colour plasticity. Benthics matched the littoral background colour more closely than did the limnetics, although there was no difference between species in their resemblance to the pelagic background colour. Benthics were able to resemble both background colours by exhibiting greater directional colour plasticity in their dorsal body coloration than limnetics, which may be an adaptive response to the greater spectral heterogeneity of the littoral zone. The present study highlights how habitat‐specific spectral characteristics may shape cryptic coloration differences between stickleback species. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 902–914.     

Interspecific variation in plumage colour among birds: species recognition or light environment?

Abstract The traditional explanation for interspecific plumage colour variation in birds is that colour differences between species are adaptations to minimize the risk of hybridization. Under this explanation, colour differences between closely related species of birds represent reproductive character displacement. An alternative explanation is that interspecific variation in plumage colour is an adaptive response to variation in light environments across habitats. Under this explanation, differences in colour between closely related species are a product of selection on signal efficiency. We use a comparative approach to examine these two hypotheses, testing the effects of sympatry and habitat use, respectively, on divergence in male plumage colour. Contrary to the prediction of the Species Isolation Hypothesis, we find no evidence that sympatric pairs of species are consistently more divergent in coloration than are allopatric pairs of species. However, in agreement with the Light Environment Hypothesis, we find significant associations between plumage coloration and habitat use. All of these results remain qualitatively unchanged irrespective of the statistical methodology used to compare reflectance spectra, the body regions used in the analyses, or the exclusion of areas of plumage not used in sexual displays. Our results suggest that, in general, interspecific variation in plumage colour among birds is more strongly influenced by the signalling environment than by the risk of hybridization.     

Progressive background in moths, and a quantitative measure of crypsis

A method is presented for quantitative estimation of the degree of crypsis of species seen by visual predators against known backgrounds. It is based upon a comparison between transects taken across animal and background colour patterns. The method was applied to day-resting moths in deciduous forest in New Jersey. Each species is found for two to four weeks at characteristic dates, and there is a constant turnover of species. In both moths and backgrounds there is a regular change in the colour pattern parameters from winter through spring to early summer. Moths are on average more cryptic at their normal dates than they would be if present earlier or later in the year. Species with known resting sites are on average more cryptic on their resting sites than other background habitats. Species that rest on more than one background habitat are less cryptic on their preferred habitats than are specialists. Species that rest under leaves and are not visible from above are not very cryptic. Specific v. general resemblance, disruptive coloration, and factors affecting 'aspect diversity' are discussed. The new method of estimating crypsis is useful for studies of crypsis as well as in sexual selection. It is necessary to know much about the resting sites and behaviour of moths, as well as other functions of colour patterns, to understand colour pattern evolution.     

Effects of habitat on growth and shape of contrasting phenotypes of Bembicium vittatum Philippi in the Houtman Abrolhos Islands,Western Australia

Translocation experiments were used to test the effect of habitat on growth and shape of three contrasting phenotypes of Bembicium vittatum: dwarf, highly domed snails from an usually dry tidal pond; large, moderately domed snails from a sheltered, regularly inundated pond; and relatively flat snails from a vertical, exposed shore. Snails from both ponds grew nearly twice as fast in the wet pond as in the dry pond, indicating a high degree of plasticity of growth. Associated with these changes in growth rates was convergence of shape. Under conditions of rapid growth, the dwarf snails became relatively flatter, and hence more similar to the native snails at that site. These results indicate that the dwarf phenotype is largely a plastic stunting in response to conditions of little submersion time. The snails from the exposed shore also grew faster in the sheltered, wet pond than at their native site. However, they not only retained their flat shape, but actually became flatter (and hence divergent from the pond snails) when grown in the pond. Thus, variation in shell shape was due to interactions between source population and a common plastic association of flatter growth profile with more rapid growth. Previous experiments had demonstrated high heritability of the flat phenotype, while the present results show that the expression of the genetically different types is affected substantially by the conditions of growth, and that phenotypic differences among populations may either overestimate or underestimate the underlying genetic differences. This unpredictability of the relationship between variation in shell form and its underlying genetic basis complicates interpretations of geographical variation or palaeontological sequences based on shell form.     

Habitat selection in amphidromous Gobiidae of Reunion Island: Sicyopterus lagocephalus (Pallas, 1770) and Cotylopus acutipinnis (Guichenot, 1863)

Freshwater populations of the Indo-pacific region are characterized by a large proportion of amphidromous species. In this paper, we analyse habitat selection by two amphidromous sympatric Gobiidae: Sicyopterus lagocephalus and Cotylopus acutipinnis in Reunion Island. A sampling method using Point Abundance Sampling (PAS) was conducted in 12 rivers. We used mixed logistic models in order to examine the presence probability of species according to location, downstream-upstream gradient, microhabitat variables (depth, velocity and predominant substrata) and presence of conspecifics and sympatric species. Presence probabilities varied between the sampled rivers. We observed a positive attraction between identical developmental stages of both species, which suggests that social interactions or similar preferences for environmental cues influenced their distribution. The presence probabilities of both species’ juveniles decreased from downstream to upstream. We showed that traditional microhabitat variables weakly explained the spatial distribution of both S. lagocephalus and C. acutipinnis, in Reunion Island. We suggest that weak habitat selection for these species is consistent with the amphidromous life style because of the unpredictability of juvenile settlement and the extreme hydrological variations in tropical rivers.