Genetic and developmental analysis of abdominal pigmentation differences across species in the Drosophila dunni subgroup

Abdominal pigmentation pattern varies dramatically among the species of the Drosophila dunni subgroup across the islands of the Caribbean. Previously, we developed a quantitative measure of abdominal pigmentation to assess phenotypic variation within and between species of this group. In this paper, we use this quantitative measure in an interspecific genetic analysis to decipher the underlying genetic basis of pigmentation differences between one of the lightest and the darkest species in the group. Our analysis shows that pigmentation expression in different areas of the abdomen is under separate genetic control. For these different abdominal regions, we detected a wide range of genetic effects, including X-linked, autosomal additive, near single-gene dominant, and sex-specific effects. Combining these genetic results with our earlier phenotypic and phylogenetic analyses, we present a simple conceptual model to explain how change in the control of expression of pigmentation has evolved throughout the D. dunni subgroup.     

Contrasting modes of natural selection acting on pigmentation genes in the Drosophila dunni subgroup

Genes that encode for divergent adaptive traits may have genealogies that contrast with those from loci that are not functionally involved in differentiation. Here, we examine DNA sequence variation among the species of the eastern Caribbean Drosophila dunni subgroup at two loci, yellow and dopa decaboxylase (Ddc), which both play integral roles in pigmentation patterning of adult Drosophila. Phylogenetic analyses of these loci produce gene genealogies with topologies that mirror those described for other nuclear genes: the six morphologically distinct species within the subgroup are divided into only three lineages, with one lineage containing four species that share extensive ancestral polymorphism. At the Ddc locus these major lineages are delineated only by silent site variation. We observe a significantly higher rate of synonymous site divergence than non-synonymous divergence, consistent with strong purifying selection acting on the locus. In contrast, the yellow locus exhibits patterns of amino acid divergence and nucleotide diversity that are consistent with recent diversifying selection acting in two different lineages. This selection appears to be targeting amino acid variants in the signal sequence of the Yellow protein, a region which is tightly constrained among members of the larger D. cardini radiation. This result highlights not only the potential importance of yellow in the evolution of divergent pigmentation patterns among members of the D. dunni subgroup, but also hints that variation in signal peptide sequences may play a role in phenotypic diversification.     

Recent radiation of endemic Caribbean Drosophila of the dunni subgroup inferred from multilocus DNA sequence variation

Studies of island endemism provide a unique opportunity to elucidate fundamental mechanisms of speciation. Here we examine intra- and interspecific DNA sequence variation at four unlinked genetic loci among populations of the Drosophila dunni subgroup to provide a detailed genealogical portrait of the process of speciation among these island endemic species. Our data indicate two major rounds of diversification that have shaped the D. dunni subgroup. The first occurred 1.6-2.6 million years ago and separated three major lineages, one in Puerto Rico and the Virgin Islands, a second in the northern Lesser Antilles and Barbados, and a third in St. Vincent and Grenada. A second round of diversification occurred in the last 96,000 years in the northern Lesser Antilles and Barbados. The four distinct species that resulted from this recent round of diversification maintain relatively high amounts of genetic variation, similar to that of a closely related mainland species, and share extensive ancestral polymorphism. These data suggest a minimal role for population bottlenecks linked to founder events in the history of the D. dunni subgroup. Further, the recent divergence of these island populations highlights the extremely rapid development of reproductive isolation and distinct patterns of abdominal pigmentation that has occurred in these species.     

Evolution of abdominal pigmentation differences across species in the Drosophila dunni subgroup

The Drosophila dunni subgroup displays a nearly perfect latitudinal cline in abdominal pigmentation that likely resulted from selective forces acting in the habitat of each species during speciation. Here we characterize the nature of this clinal variation by developing a quantitative measure to assess variation in abdominal pigmentation within and between the D. dunni subgroup species. Using discriminant analysis, we confirm the existence of a cline and find that our quantitative measure of pigmentation distinguishes each of the species with singular efficacy. We then combine our quantitative phenotypic analysis of pigmentation with the phylogeny of the D. dunni subgroup species and map the species relationships into the three-dimensional morphological space defined by our pigmentation measures. In this manner, we can visualize how the species have traversed the morphological pigmentation space during the course of speciation. Our analysis reveals that natural selection has caused overall intensity of pigmentation among the northernmost species of the cline to converge. Along with this convergence in phenotype has been a relaxation in expression of sexual dimorphism in these species, indicating a possible shift in the relative intensity of natural and sexual selection. Our analysis indicates an accelerated rate of change in pigmentation for the darkest species in addition to this species evolving a novel abdominal pigmentation trait.     

Characterization and isolation of novel microsatellites from the Drosophila dunni subgroup

We have isolated and characterized 77 novel microsatellites from two species, Drosophila dunni and Drosophila nigrodunni, which are closely related Caribbean-island endemics from the Drosophila cardini species group. These species are very distantly related to all other Drosophila from which microsatellites have previously been characterized. We find that the average length of microsatellites isolated in these species is quite small, with an overall mean length of 9.8 repeat units for dinucleotide microsatellites in the two study species. The nucleotide composition of dinucleotides differs between the two species: D. nigrodunni has a predominance of (AC/GT)n repeats, whereas D. dunni has equal numbers of (AC/GT)n and (AG/CT)n repeats. Tri- and tetranucleotide repeats are not abundant in either species. We assayed the variability of eight microsatellites in a closely related third species, Drosophila arawakana, using wild-caught individuals from the island of Guadeloupe. We found the microsatellites to be extremely variable in this population, with observed heterozygosities ranging from 0.541 to 0.889. DNA amplification trials suggest that these eight microsatellites are widely conserved across the D. cardini group, with five of the eight producing amplification products in every species tested. However, the loci are very poorly conserved over greater phylogenetic distances. DNA amplification of the microsatellite loci was unreliable in members of the closely related Drosophila quinaria, Drosophila calloptera, Drosophila guarani and Drosophila tripunctata species groups. Furthermore, these microsatellites could not be detected in the genome of Drosophila melanogaster, despite the conservation of microsatellite flanking regions at some loci. These data indicate that Drosophila microsatellite loci are quite short lived over evolutionary timescales relative to many other taxa.     

Ethanol Tolerance and Alcohol Dehydrogenase (ADH; EC1.1.1.1) Activity in Species of the cardini Group of Drosophila

Ethanol tolerance, alcohol dehydrogenase (ADH;EC1.1.1.1) activity, and tissue-specific expression wereexamined in species of the cardini group ofDrosophila using D. melanogaster as astandard of comparison. In contrast to most fruit-breeding species, allcardini species examined, two from the cardini subgroupand five from the dunni subgroup, were ethanol sensitive(LC₅₀ 2.05%) and the mean ADH activityof males ranges from only 8 to 16% that of D.melanogaster Adh^FF. Among all sevencardini species, there were small but significantdifferences in ethanol tolerance and ADH activity.Differences in enzyme mobility were in accordance with the proposedphylogeny for the dunni-subgroup species. ADH isexpressed in the fat body and midgut. Males of D.acutilabella and of D. belladunni havesignificantly less ethanol tolerance and express less ADH activitythan females in zymograms and histologicalpreparations.     

Postmating Reproductive isolation between strains of Drosophila willistoni

Speciation can occur through the presence of reproductive isolation barriers that impede mating, restrict cross-fertilization, or render inviable/sterile hybrid progeny. The D. willistoni subgroup is ideally suited for studies of speciation, with examples of both allopatry and sympatry, a range of isolation barriers, and the availability of one species complete genome sequence to facilitate genetic studies of divergence. D. w. willistoni has the largest geographic distribution among members of the Drosophila willistoni subgroup, spanning from Argentina to the southern United States, including the Caribbean islands. A subspecies of D. w. willistoni, D. w. quechua, is geographically separated by the Andes mountain range and has evolved unidirectional sterility, in that only male offspring of D. w. quechua females × D. w. willistoni males are sterile. Whether D. w. willistoni flies residing east of the Andes belong to one or more D. willistoni subspecies remains unresolved. Here we perform fecundity assays and show that F1 hybrid males produced from crosses between different strains found in Central America, North America, and northern Caribbean islands are reproductively isolated from South American and southern Caribbean island strains as a result of unidirectional hybrid male sterility. Our results show the existence of a reproductive isolation barrier between the northern and southern strains and suggest a subdivision of the previously identified D. willistoni willistoni species into 2 new subspecies.     

Genetics of a difference in pigmentation between Drosophila yakuba and Drosophila santomea

Abstract.— Drosophila yakuba is a species widespread in Africa, whereas D. santomea, its newly discovered sister species, is endemic to the volcanic island of São Tomé in the Gulf of Guinea. Drosophila santomea probably formed after colonization of the island by its common ancestor with D. yakuba. The two species differ strikingly in pigmentation: D. santomea, unlike the other eight species in the D. melanogaster subgroup, almost completely lacks dark abdominal pigmentation. D. yakuba shows the sexually dimorphic pigmentation typical of the group: both sexes have melanic patterns on the abdomen, but males are much darker than females. A genetic analysis of this species difference using morphological markers shows that the X chromosome accounts for nearly 90% of the species difference in the area of abdomen that is pigmented and that at least three genes (one on each major chromosome) are involved in each sex. The order of chromosome effects on pigmentation area are the same in males and females, suggesting that loss of pigmentation in D. santomea may have involved the same genes in both sexes. Further genetic analysis of the interspecific difference between males in pigmentation area and intensity using molecular markers shows that at least five genes are responsible, with no single locus having an overwhelming effect on the trait. The species difference is thus oligogenic or polygenic. Different chromosomal regions from each of the two species influenced pigmentation in the same direction, suggesting that the species difference (at least in males) is due to natural or sexual selection and not genetic drift. Measurements of sexual isolation between the species in both light and dark conditions show no difference, suggesting that the pigmentation difference is not an important cue for interspecific mate discrimination. Using DNA sequence differences in nine noncoding regions, we estimate that D. santomea and D. yakuba diverged about 400,000 years ago, a time similar to the divergences between two other well‐studied pair of species in the subgroup, both of which also involved island colonization.     

Bat metacommunity structure on Caribbean islands and the role of endemics

Aim We evaluate characteristics of species ranges (i.e. coherence, species turnover and range boundary clumping) to determine the structure of bat metacommunities and metaensembles from Caribbean islands. We evaluate the effects of endemic species on that structure, and quantify associations between island characteristics and latent environmental gradients that structure these metacommunities and metaensembles. Location Sixty‐five Caribbean islands throughout the Bahamas, Greater Antilles and Lesser Antilles. Methods Metacommunity structure is an emergent property of a set of ecological communities at different sites defined by species distributions across geographic or environmental gradients. We analysed elements of metacommunity structure (coherence, range turnover and range boundary clumping) to determine the best‐fit pattern for metacommunities from all Caribbean islands, as well as from the Bahamas, the Greater Antilles and the Lesser Antilles separately. For each island group, analyses were conducted for all bats and for each of two broadly defined guilds (i.e. carnivores and herbivores). In addition, analyses were conducted for all species and for a subset in which endemic species were removed from the fauna. Spearman rank correlations identified island characteristics (area, elevation, latitude, longitude) that were associated significantly with island scores for ordination axes based on reciprocal averaging. Results Metacommunity structure for all bats and for carnivores was similar for each island group, with Clementsian distributions (i.e. discrete communities with groups of species replacing other groups of species along the gradient) for all islands, the Bahamas and the Lesser Antilles, but with nested distributions for the Greater Antilles. Herbivore distributions were random for the Bahamas, but were Clementsian for all other island groups. Removal of endemic species affected the best‐fit model of metacommunity structure in only 3 of 12 cases. In general, ordination scores for islands were correlated with longitude or latitude, but not with island area or elevation. Main conclusions Characteristics of bat species ranges and associated metacommunity structure were primarily dependent on the number and geographic arrangement of primary sources of colonization, and not on interspecific interactions, species‐specific levels of environmental tolerance, or the physical characteristics of islands. Endemic species did not greatly affect metacommunity structure in Caribbean bats.     

A molecular phylogeny for the Drosophila melanogaster subgroup and the problem of polymorphism data

Drosophila melanogaster belongs to a closely related group of eight species collectively known as the melanogaster subgroup; all are native to sub-Saharan Africa and islands off the east coast of Africa. The phylogenetic relationships of most species in this subgroup have been well documented; however, the three most closely related species, D. simulans, D. sechellia, and D. mauritiana, have remained problematic from a phylogenetic standpoint as no data set has unambiguously resolved them. We present new DNA sequence data on the nullo and Serendipity-alpha genes and combine them with all available nuclear DNA sequence data; the total data encompass 12 genes and the ITS of rDNA. A methodological problem arose because nine of the genes had information on intraspecific polymorphisms in at least one species. We explored the effect of inclusion/exclusion of polymorphic sites and found that it had very little effect on phylogenetic inferences, due largely to the fact that 82% of polymorphisms are autapomorphies (unique to one species). We have also reanalyzed our previous DNA-DNA hybridization data with a bootstrap procedure. The combined sequence data set and the DNA-DNA hybridization data strongly support the sister status of the two island species, D. sechellia and D. mauritiana. This at least partially resolves what had been a paradox of parallel evolution in these two species.      

A global comparison of plant invasions on oceanic islands

Oceanic islands have long been considered to be particularly vulnerable to biotic invasions, and much research has focused on invasive plants on oceanic islands. However, findings from individual islands have rarely been compared between islands within or between biogeographic regions. We present in this study the most comprehensive, standardized dataset to date on the global distribution of invasive plant species in natural areas of oceanic islands. We compiled lists of moderate (5–25% cover) and dominant (>25% cover) invasive plant species for 30 island groups from four oceanic regions (Atlantic, Caribbean, Pacific, and Western Indian Ocean). To assess consistency of plant behaviour across island groups, we also recorded present but not invasive species in each island group.We tested the importance of different factors discussed in the literature in predicting the number of invasive plant species per island group, including island area and isolation, habitat diversity, native species diversity, and human development. Further we investigated whether particular invasive species are consistently and predictably invasive across island archipelagos or whether island-specific factors are more important than species traits in explaining the invasion success of particular species.We found in total 383 non-native spermatophyte plants that were invasive in natural areas on at least one of the 30 studied island groups, with between 3 and 74 invaders per island group. Of these invaders about 50% (181 species) were dominants or co-dominants of a habitat in at least one island group. An extrapolation from species accumulation curves across the 30 island groups indicates that the total current flora of invasive plants on oceanic islands at latitudes between c. 35°N and 35°S may eventually consist of 500–800 spermatophyte species, with 250–350 of these being dominant invaders in at least one island group. The number of invaders per island group was well predicted by a combination of human development (measured by the gross domestic product (GDP) per capita), habitat diversity (number of habitat types), island age, and oceanic region (87% of variation explained). Island area, latitude, isolation from continents, number of present, non-native species with a known invasion history, and native species richness were not retained as significant factors in the multivariate models.Among 259 invaders present in at least five island groups, only 9 species were dominant invaders in at least 50% of island groups where they were present. Most species were invasive only in one to a few island groups although they were typically present in many more island groups. Consequently, similarity between island groups was low for invader floras but considerably higher for introduced (but not necessarily invasive) species – especially in pairs of island groups that are spatially close or similar in latitude. Hence, for invasive plants of natural areas, biotic homogenization among oceanic islands may be driven by the recurrent deliberate human introduction of the same species to different islands, while post-introduction processes during establishment and spread in natural areas tend to reduce similarity in invader composition between oceanic islands. We discuss a number of possible mechanisms, including time lags, propagule pressure, local biotic and abiotic factors, invader community assembly history, and genotypic differences that may explain the inconsistent performance of particular invasive species in different island groups.     

Out of Cuba: overwater dispersal and speciation among lizards in the Anolis carolinensis subgroup

Overwater dispersal and subsequent allopatric speciation contribute importantly to the species diversity of West Indian Anolis lizards and many other island radiations. Here we use molecular phylogenetic analyses to assess the contribution of overwater dispersal to diversification of the Anolis carolinensis subgroup, a clade comprising nine canopy-dwelling species distributed across the northern Caribbean. Although this clade includes some of the most successful dispersers and colonists in the anole radiation, the taxonomic status and origin of many endemic populations have been ambiguous. New mitochondrial and nuclear DNA sequences from four species occurring on small islands or island banks (Anolis brunneus, Anolis longiceps, Anolis maynardi, Anolis smaragdinus) and one species from the continental United States (A. carolinensis) are presented and analysed with homologous sequences sampled from related species on Cuba (Anolis allisoni and Anolis porcatus). Our analyses confirm that all five non-Cuban species included in our study represent distinct, independently evolving lineages that warrant continued species recognition. Moreover, our results support Ernest Williams's hypothesis that all of these species originated by overseas colonization from Cuban source populations. However, contrary to Williams's hypothesis of Pleistocene dispersal, most colonization events leading to speciation apparently occurred earlier, in the late Miocene-Pliocene. These patterns suggest that overwater dispersal among geologically distinct islands and island banks is relatively infrequent in anoles and has contributed to allopatric speciation. Finally, our results suggest that large Greater Antillean islands serve as centres of origin for regional species diversity.     

Ancient DNA Suggests Single Colonization and Within-Archipelago Diversification of Caribbean Caviomorph Rodents

Reconstructing the evolutionary history of island biotas is complicated by unusual morphological evolution in insular environments. However, past human-caused extinctions limit the use of molecular analyses to determine origins and affinities of enigmatic island taxa. The Caribbean formerly contained a morphologically diverse assemblage of caviomorph rodents (33 species in 19 genera), ranging from ∼0.1 to 200 kg and traditionally classified into three higher-order taxa (Capromyidae/Capromyinae, Heteropsomyinae, and Heptaxodontidae). Few species survive today, and the evolutionary affinities of living and extinct Caribbean caviomorphs to each other and to mainland taxa are unclear: Are they monophyletic, polyphyletic, or paraphyletic? We use ancient DNA techniques to present the first genetic data for extinct heteropsomyines and heptaxodontids, as well as for several extinct capromyids, and demonstrate through analysis of mitogenomic and nuclear data sets that all sampled Caribbean caviomorphs represent a well-supported monophyletic group. The remarkable morphological and ecological variation observed across living and extinct caviomorphs from Cuba, Hispaniola, Jamaica, Puerto Rico, and other islands was generated through within-archipelago evolutionary radiation following a single Early Miocene overwater colonization. This evolutionary pattern contrasts with the origination of diversity in many other Caribbean groups. All living and extinct Caribbean caviomorphs comprise a single biologically remarkable subfamily (Capromyinae) within the morphologically conservative living Neotropical family Echimyidae. Caribbean caviomorphs represent an important new example of insular mammalian adaptive radiation, where taxa retaining “ancestral-type” characteristics coexisted alongside taxa occupying novel island niches. Diversification was associated with the greatest insular body mass increase recorded in rodents and possibly the greatest for any mammal lineage.     

Composition and structure of Caribbean bat (Chiroptera) assemblages: effects of inter‐island distance,area, elevation and hurricane‐induced disturbance

Aim Although bats of the Caribbean have been studied extensively, previous work is largely restricted to zoogeography, phylogeography or the effects of island characteristics on species richness. Variation among islands in species composition that is related to geographical or environmental variation remains poorly understood for much of the Caribbean. Location Caribbean islands, including the Bahamas, Greater Antilles and Lesser Antilles. Methods Using presence–absence data, we assessed the extent to which island area, maximum island elevation, inter‐island distance and hurricane‐induced disturbance affected patterns of composition and nestedness for bats in the Bahamas, Greater Antilles and Lesser Antilles. Analyses were conducted for all species, as well as for two broadly defined guilds: carnivores and herbivores. Results For the Bahamas, only inter‐island distance accounted for variation in species composition between islands. For the Greater and Lesser Antilles, differences in island area and inter‐island distance accounted for differences in species composition between islands. Variation in species composition was not related significantly to differences in elevation or hurricane‐related disturbance. In general, results of analyses restricted to a particular broad guild (i.e. carnivores or herbivores) mirrored those for all bats. Bat species composition was nested significantly in each island group. Nestedness was stronger in the Greater Antilles and in the Lesser Antilles than in the Bahamas. Carnivore assemblages were nested significantly in the Greater and in the Lesser Antilles, but not in the Bahamas. In contrast, herbivore assemblages were nested significantly in each island group. Main conclusions Inter‐island distance had a greater effect on compositional similarity of Caribbean bat assemblages than did island area, elevation or disturbance related to hurricanes. Differential immigration and hierarchical habitat distributions associated with elevational relief are likely to be primary causes for nestedness of Caribbean bat assemblages.     

The Galápagos Islands: biogeographic patterns and geology

In the traditional biogeographic model, the Galápagos Islands appeared a few million years ago in a sea where no other islands existed and were colonized from areas outside the region. However, recent work has shown that the Galápagos hotspot is 139 million years old (Early Cretaceous), and so groups are likely to have survived at the hotspot by dispersal of populations onto new islands from older ones. This process of metapopulation dynamics means that species can persist indefinitely in an oceanic region, as long as new islands are being produced. Metapopulations can also undergo vicariance into two metapopulations, for example at active island arcs that are rifted by transform faults. We reviewed the geographic relationships of Galápagos groups and found 10 biogeographic patterns that are shared by at least two groups. Each of the patterns coincides spatially with a major tectonic structure; these structures include: the East Pacific Rise; west Pacific and American subduction zones; large igneous plateaus in the Pacific; Alisitos terrane (Baja California), Guerrero terrane (western Mexico); rifting of North and South America; formation of the Caribbean Plateau by the Galápagos hotspot, and its eastward movement; accretion of Galápagos hotspot tracks; Andean uplift; and displacement on the Romeral fault system. All these geological features were active in the Cretaceous, suggesting that geological change at that time caused vicariance in widespread ancestors. The present distributions are explicable if ancestors survived as metapopulations occupying both the Galápagos hotspot and other regions before differentiating, more or less in situ.     

A comparison of populations of island and adjacent mainland species of Caribbean Selenops (Araneae: Selenopidae) spiders

The role of the landscape in structuring populations has been the focus of numerous studies, in particular, the extent to which islands provide opportunities for isolation, and the consistency of such an effect across lineages. The current study examines this phenomenon using a series of relatively widespread taxa, all within a single genus of spiders, Selenops. We focus on the Caribbean Islands and adjacent Mesoamerican mainland to examine how the islands per se dictate structure across lineages. We use molecular genetic data from mitochondrial and nuclear genes to examine the population structure of seven species of Selenops. Comparisons are made between species found in the Greater Antilles, Lesser Antilles, and adjacent mainland. Results indicate that geography has little effect on the population structure of mainland species. In contrast, population structure appears to be partitioned by island in the insular Caribbean. Within islands, the amount of population structure for each species is variable and may be dictated more by ecological or demographic parameters, rather than geographic location. The overall conclusion is that the extent to which a given lineage is structured is highly variable across species, with this variability overwhelming any general signal of geographical isolation.     

Genomic tests of the species‐pump hypothesis: Recent island connectivity cycles drive population divergence but not speciation in Caribbean crickets across the Virgin Islands

Harnessing the power of genomic scans, we test the debated “species pump” hypothesis that implicates repeated cycles of island connectivity and isolation as drivers of divergence. This question has gone understudied given the limited resolution of past molecular markers for studying such dynamic phenomena. With an average of 32,000 SNPs from the genome of 136 individuals from 10 populations of a Caribbean flightless ground cricket species (Amphiacusta sanctaecrucis) and a complementary set of statistical approaches, we infer a stepping‐stone colonization model and high levels of genetic differentiation across the Virgin Islands, which have been periodically interconnected until 8 ka. Estimates of divergence times from models based on the site frequency spectrum coincide with a period of repeated connection and fragmentation of the islands at 75–130 ka. These results are consistent with a role of island connectivity cycles in promoting genomic divergence and indicate that the genetic distinctiveness of island populations has persisted despite subsequent and extended interisland connections identified from bathymetric data. We discuss these findings in the broader context of Caribbean biogeography, and more specifically why high levels of genomic divergence across the Virgin Islands associated with repeated connectivity cycles do not actually translate into species diversification.     

Invertébrés Sans Frontières: Large Scales of Connectivity of Selected Freshwater Species among Caribbean Islands

The freshwater fauna (crustaceans, molluscs, fish) of many tropical islands in the Caribbean and Pacific share an amphidromous life‐cycle, meaning their larvae need to develop in saline conditions before returning to freshwater as juveniles. This community dominates the freshwaters of much of the tropics, but is poorly known and at risk from development, in particular dam construction. Amphidromy can theoretically lead to dispersal between different freshwater areas, even to distant oceanic islands, via the sea. The extent and scale of this presumed dispersal, however, is largely unknown in the Caribbean. Recent genetic work in Puerto Rico has shown that many freshwater species have little or no population structure among different river catchments, implying high levels of connectivity within an island, whereas between‐island structure is unknown. We used genetic techniques to infer the geographic scales of population structure of amphidromous invertebrates (a gastropod and a number of crustacean species) between distant parts of the Caribbean, in particular Puerto Rico, Panama and Trinidad. We found virtually no geographic population structure across over 2000 km of open sea for these freshwater species. This implies that they are indeed moving between islands in sea currents as larvae, meaning that continued recruitment requires a continuum of healthy habitat from the freshwater to marine environment. We further discuss the role of amphidromy and suggest its ecological and biogeographic role may be more important than previously presumed.