Extreme mitochondrial DNA divergence within populations of the deep-sea gastropod Frigidoalvania brychia

The deep sea supports a diverse and highly endemic invertebrate fauna, the origin of which remains obscure. Little is known about geographic variation in deep-sea organisms or the evolutionary processes that promote population-level differentiation and eventual speciation. Sequence variation at the 16 S rDNA locus was examined in formalin-preserved specimens of the common upper bathyal rissoid Frigidoalvania brychia (Verrill, 1884) to examine its population genetic structure. The specimens came from trawl samples taken over 30 years ago at depths of 457-1,102 m at stations in the Northwest Atlantic south of Woods Hole, Massachusetts, USA. Near the upper boundary of its bathymetric range (500 m), extremely divergent haplotypes comprising three phylogenetically distinct clades (average uncorrected sequence divergence among clades ~23%, ~3% within clades) were found at stations separated by a maximum distance of ~80 km, suggesting the presence of high levels of intraspecific divergence or the possibility of morphologically cryptic species. Only one of these clades was found at two stations in the mid- to lower part of F. brychia's depth distribution (800-1,100 m), suggesting lower clade diversity with increasing depth, although among-sample divergence, with a single exception, was minimal. One station was genetically divergent from all others sampled, containing a unique suite of haplotypes including two found only at this site. Steep vertical selective gradients, major oceanographic changes during the late Cenozoic, and habitat fragmentation by submarine canyons might have contributed to an upper bathyal region that is highly conducive to evolutionary change.     

Genetic structure in native and non-native populations of the direct-developing gastropod Crepidula convexa

In many marine invertebrate species, larval development plays an important role in population connectivity and gene flow: species with direct benthic development generally show more genetic structure than those with planktonic development. We used nuclear markers (microsatellites) to determine population genetic structure of the direct-developing snail Crepidula convexa (Gastropoda: Calyptraeidae) in seven populations with 15–85 individuals each within its native range of the northwest Atlantic and compared it to Crepidula fornicata, a congener with planktonic development. Our results are consistent with general expectations and previous work in these species with other markers: C. convexa had greater population structure and even at a regional scale shows significant isolation-by-distance, in contrast to C. fornicata. We also genotyped a single population of C. convexa introduced to the northeastern Pacific to investigate the prediction of reduced genetic diversity following introduction (founder effect). We did not find a reduction in genetic diversity, suggesting that this non-native population may be characterized by multiple introductions. This pattern is consistent with many other introduced populations of marine invertebrates, including C. fornicata.     

Genetic differentiation between spatially-disjunct populations of the deep-sea,hydrothermal vent-endemic amphipod Ventiella sulfuris

Species endemic to deep-sea hydrothermal vent ecosystems have disjunct distributions imposed by the island-like arrangement of their specialized habitats. Using allozyme electrophoresis, we examined genetic population structure of the hydrothermal vent amphipod Ventiella sulfuris Barnard and Ingram, 1990. Samples from five sites along the East Pacific Rise (EPR) and two along the Galapagos Rift were collected in 1990 and 1988, respectively. Variability, based on 12 enzyme loci, was relatively high (proportion of polymorphic loci whose most common allele not greater than 0.95 in frequency) =41.6%; (mean heterozygosity) =0.158] compared with shallow-water marine and freshwater amphipods, and similar to the deep-sea lysianassid Eurythenes gryllus. Genetic divergence among populations spread along a cöntiguous rift axis (i.e., EPR) was low [Nei's genetic distance (D) ranged from <0.001 to 0.018]. Genetic tructure analysis suggests that along a contiguous ridge axis migration occurs in a stepping stone manner and is unconstrained by distances as great as 1200 km (migration rate, , ranged from 1.9 to 67.8 ind. generation^–1). However, genetic divergence between populations on disjunct ridge axes was extremely high (D ranged from 0.438 to 0.476). Most of the variance in gene frequencies was due to the differences between the major subpopulations inhabiting the two distinct ridge axes, EPR and Galapagos Rift. Apparently, very little migration and gene flow occur between these major subpopulations (M1). This level of genetic divergence may be sufficient to justity separation of EPR and Galapagos Rift populations at the species level. Futher analyses of morphological characters is required before taxonomic status can be assigned.     

Genetic variation among populations of Mytilus spp. in eastern Newfoundland

Small-scale population densities of tidal creek eastern mudsnails, Ilyanassa obsoleta Say (studied in 1986 and in 1992 at West Meadow Creek, Stony Brook, New York) corresponded more to variation in water flow velocity than to surface sediment chlorophyll a. Higher densities were found at low flow sites. Short-term behavioral responses are likely to be responsible for density variation. Experiments using laboratory flumes and field observations both demonstrated that the snails responded to strong flow by burrowing into the substratum. Burrowing may prevent dislodgment from the sediment surface, but it also appears to be disadvantageous since burrowed mudsnails have smaller amounts of food in their guts. Snails released in sites of periodic high flow conditions moved greater distances and were soon found near the quiet-water periphery of the creek, whereas snails released at the quiet-water periphery moved far less. It is not clear whether movement from the high flow site was through crawling or through hydrodynamic transport. Laboratory flume experiments demonstrated an active crawling movement towards areas of lower current velocity. This evidence suggests that strong bottom flow in the creek center results in a combined response of burial to avoid dislodgment and a net movement towards quiet water, which reduces exposure to the high velocity conditions of the creek center.     

Seasonal breeding aggregations in low-density populations of the bathyal echinoid Stylocidaris lineata

Reproduction of bathyal cidaroid sea urchins was studied between 1985 and 1991 with the aid of a manned submersible in the northern Bahamas. During May 1988 and February 1990, discrete, single-species aggregations of Stylocidaris lineata containing 2 to 6 individuals were observed between 510 and 640 m depths. Reproductive conditions of isolated and aggregated individuals were determined histologically. In February, all individuals, regardless of their spatial distribution, contained developing gametes. During the May spawning season, most isolated individuals contained spent gonads with only relict gametes, whereas individuals in discrete aggregations mostly contained ripe gonads with mature or nearly-mature gametes. Aggregations were not observed during the autumn months, when gonads were spent or immature. These data suggest that bathyal cidaroids aggregate for reproduction. An in situ survey during May indicated that pairs of S. lineata are more common than larger clumps, but no more so than predicted by chance. Homosexual and heterosexual pairs occur at the relative frequencies predicted on the basis of sex ratio. We evaluated the need for reproductive aggregation by applying Denny's (1988) model that predicts downstream sperm concentration in a turbulent boundary layer. Flow parameters were measured with dye injected from the submersible, sperm-release rate was estimated in the laboratory, and the relationship between fertilization success and sperm concentration was determined in a laboratory dilution-experiment. The model predicted that, on smooth sandy bottoms at bathyal depths, sperm concentration should remain high enough to fertilize at least some eggs several meters directly downstream from a spawning male. Aggregation may facilitate spawning synchrony, increase gamete encounter probabilities, or cause gametes to be retained at high concentrations near the adults long enough for fertilization to occur.     

Temporal patterns in diversity and species composition of deep-sea benthic copepods in bathyal Sagami Bay,central Japan

Little is known about temporal changes in the diversity and species composition of deep-sea metazoan meiofauna and their relationships with changes in the food supply. Those changes were studied for benthic copepod assemblages based on 2-year time-series data at a bathyal site in Sagami Bay (1430 m depth), central Japan, where annual fluctuation in the abundance of benthic foraminiferans was previously observed. Species diversity of benthic copepods at the site was as high as, or slightly higher than, that observed at other deep-sea sites, but did not fluctuate temporally through the study period. Multivariate analyses did not reveal any clear seasonal or directional change occurring over the longer term in their species composition, although there was some consistent pattern. These results indicate a lack of, or only weak, seasonality in the diversity and species structure of the deep-sea benthic copepod assemblages, even though the fresh organic food supply fluctuates seasonally. They also suggest that there are differences between copepods and foraminiferans in the response to changes in environmental factors, and that spatial differences in the composition of copepod communities are greater than temporal ones at this deep-sea site.Communicated by T. Ikeda, Hakodate     

Genetic comparison of two populations of the deep-sea vent shrimp Rimicaris exoculata (Decapoda: Bresiliidae) from the Mid-Atlantic Ridge

Allozyme data are presented for populations of the bresiliid shrimp Rimicaris exoculata from two hydrothermal vent fields, Trans-Atlantic Geotraverse (TAG) and Broken Spur, located along the Mid-Atlantic Ridge. These indicate that all morphotypes of R. exoculata examined, including those previously interpreted as representing separate species, are conspecific. Conversely, genetic identity between a single specimen of Chorocaris sp. and R. exoculata was high for intergeneric comparisons. Genetic variation in the populations of R. exoculata (H _o =0.034 to 0.056) was in the lower range of that estimated for other vent organisms, but similar to values obtained for other species of caridean shrimps in previous genetic studies. F-statistics were used to examine the population structure of R. exoculata. Estimates of variance of allele frequencies among populations (F _ST ) between TAG and Broken Spur were very low (mean F _ST =0.001), indicating no significant genetic differentiation between these populations although they were separated by 370 km. The number of migrants per generation was estimated from F _ST and by a private-alleles method, and indicates that migration between the two fields exceeds 100 individuals per generation. This may be because of efficient larval or adult migration or a combination of both. Estimates of the correlation between homologous alleles between individuals within local populations (F _IS ) of R. exoculata were high at two enzyme loci and indicate a heterozygote deficiency which caused a significant deviation from genotype frequencies expected under Hardy-Weinberg conditions. This deficiency was caused by the occurrence of rare homozygous genotypes in small individuals. In large individuals, rare alleles decreased in frequency or disappeared completely. This is discussed in relation to previous genetic investigations on other vent and nonvent organisms.     

Genetic structure of local populations and divergence between growth forms in a clonal invertebrate,the Caribbean octocoral Briareum asbestinum

Although the genetic structure of many populations of marine organisms show little deviation from panmixia, in those marine species with limited larval dispersal, patterns of microgeographic genetic differentiation may be common. The octocoral Briareum asbestinum should show local population differentiation because colonies reproduce asexually by fragmentation, most matings occur between colonies in very close proximity, and the sexually produced larvae and sperm appear to disperse only short distances. Variability in secondary chemistry of individual B. asbestinum colonies from different populations in close proximity also suggests local population differentiation. We determined the genetic composition of local populations by surveying allozyme variation of three shallow and two deep populations within a 300 m² area at San Salvador Island, Bahamas and at a site 161 km away on Little San Salvador, Bahamas in July 1990. As B. asbestinum occurs as either an erect branching form or an encrusting mat often at the same sites, we sampled both morphs to examine the extent of genetic exchange between them. Five of 21 loci were polymorphic and most populations showed a deficit of heterozygotes. Allele frequencies differed significantly between morphs at each site where they occurred together. The mean genetic distance (D=0.065) between morphs is consistent with the interpretation that the two morphs are genetically isolated. Despite the close spatial proximity of the San Salvador populations, both the branching and encrusting morphs showed significant genetic heterogeneity among neighboring populations. Similarly, pooled allelic frequencies for samples collected from the islands of San Salvador and Little San Salvador differed significantly at 1 locus for the branching morph and at 3 out of 5 loci for the encrusting morph.     

Population characteristics of the deep-sea lobsters Polycheles typhlops and Stereomastis sculpta (Decapoda: Polychelidae) in a bathyal mud community of the Mediterranean Sea

Distribution and abundance as a function of depth, and population parameters such as sex and size structure of the population and aspects of reproductive biology have been studied in the deep-sea polychelid lobsters Polycheles typhlops and Stereomastis sculpta. Samples were taken by otter trawl in the Balearic Sea, a deepsea basin in the northwestern Mediterranean Sea from 1985 to 1989. In both species the largest size classes were dominated by females. In S. sculpta, male and female sizes were very similar. In P. typhlops, ovigerous females and those with external spermatophores were 23 and 25 mm in carapace length (CL), respectively; males with external spermatophores were 17 mm in CL. In S. sculpta, ovigerous females and those with external spermatophores were 24 and 19 mm CL, respectively, and males with external spermatophores 19 mm CL. Highest densities of P. typhlops occurred along the middle slope at depths between 500 and 1000 m. Only small-sized individuals were found at the deepest depths sampled; some recruitment must therefore occur in waters much deeper than those usually inhabited by the adult population. The sex-ratio was 1:1 in most samples, but in some of the shallowest samples females predominated. The depth distribution range of S. sculpta was 981 to 2253 m: densities clearly increased with increasing depth. There were no apparent variations in size distribution as a function of depth. Since very few adult males and females were captured, the population in the survey area would seem to be comprised mainly of juveniles. As a function of depth, females were numerically dominant only in some of the shallowest samples taken in the distribution range of this species. There is strict habitat partitioning between the two species. In both species, the variation in the sex-ratio as a function of depth suggests differential migration between the sexes, probably related to egg incubation and hatching.     

Genetic variation between geographic populations of the amphipods Gammarus zaddachi and G. salinus

Spatial and temporal patterns of gene-enzyme variation were estimated in the sibling species Gammarus zaddachi Sexton and G. salinus Spooner by starch gel electrophoresis. Twenty-one G. zaddachi and 18 G. salinus populations from coastal and estuarine areas in the Baltic Sea, North Sea and other localities of north-western Europe were surveyed. Both amphipods display similar electrophoresis patterns of the enzyme systems studied. Considerable interspecific and interpopulational differences were detected in allele frequencies at three highly polymorphic loci, phosphoglucose isomerase (PGI), glutamate oxalacetate transaminase (GOT) and arginine phosphokinase (APK). G. zaddachi exhibits a pronounced genetic heterogeneity in most areas of the sampled range. Populations from northern French and western English coasts differ significantly from the other samples in allele frequencies at the PGI or APK locus, respectively. Baltic populations are widely uniform in their genetic composition but can be distinguished from samples taken at North Sea sites in allele frequencies at the APK locus. The latter reveal a clinal variation, ranging from the Danish to the French coast. In contrast to G. zaddachi, a low degree of genetic differentiation was observed among the G. salinus populations examined. This indicates that migration and interregional mixing may be more important in maintaining the genetic structure than in G. zaddachi which, compared to G. salinus, prefers habitats of lower salinity levels. Evidently, less extensive dispersal capabilities owing to the confinement of G. zaddachi to brackish waters of dilute salt concentrations may account for a diminished gene flow and considerable genetic separation of local populations. This assumption is supported by the genetic homogeneity documented in Baltic G. zaddachi populations. In view of the low and constant salinities in wide areas of this brackish-water sea such barriers do not exist. Survey studies performed with selected populations over a 3-yr period demonstrated a general pattern of temporal constancy in the allozyme variation observed.     

Genetic mechanisms of pollution resistance in a marine invertebrate.

Pollution is a common stress in the marine environment and one of today's most powerful agents of selection, yet we have little understanding of how anthropogenic toxicants influence mechanisms of adaptation in marine populations. Due to their life history strategies, marine invertebrates are unable to avoid stress and must adapt to variable environments. We examined the genetic basis of pollution resistance across multiple environments using the marine invertebrate, Styela plicata. Gametes were crossed in a quantitative genetic breeding design to enable partitioning of additive genetic variance across a concentration gradient of a common marine pollutant, copper. Hatching success was scored as a measure of stress resistance in copper concentrations of 0, 75, 150, and 350 microg/L. There was a significant genotype x environment interaction in hatching success across copper concentrations. Further analysis using factor analytic modeling confirmed a significant dimension of across-environment genetic variation where the genetic basis of resistance to stress in the first three environments differed from that in the environment of highest copper concentration. A second genetic dimension further differentiated between the genetic basis of resistance to low and high stress environments. These results suggest that marine organisms use different genetic mechanisms to adapt to different levels of pollution and that the level of genetic variation to adapt to intense pollution stresses may be limited.     

Genetic variation in the eel: a critique

A number of papers have dealt with protein variation in samples of eels of the genus Anguilla from throughout the North Atlantic Ocean. Examination of these published data from the standpoint of observed electrophoretic variations as well as statistical comparisons with genetic models, suggests that most genetic inferences concerning eels have been unjustified.     

Genetic diversity increases population productivity in a sessile marine invertebrate

Reductions in genetic diversity can have widespread ecological consequences: populations with higher genetic diversity are more stable, productive and resistant to disturbance or disease than populations with lower genetic diversity. These ecological effects of genetic diversity differ from the more familiar evolutionary consequences of depleting genetic diversity, because ecological effects manifest within a single generation. If common, genetic diversity effects have the potential to change the way we view and manage populations, but our understanding of these effects is far from complete, and the role of genetic diversity in sexually reproducing animals remains unclear. Here, we examined the effects of genetic diversity in a sexually reproducing marine invertebrate in the field. We manipulated the genetic diversity of experimental populations and then measured individual survival, growth, and fecundity, as well as the size of offspring produced by individuals in high and low genetic diversity populations. Overall, we found greater genetic diversity increased performance across all metrics, and that complementarity effects drove the increased productivity of our high-diversity populations. Our results show that differences in genetic diversity among populations can have pervasive effects on population productivity within remarkably short periods of time.     

Geographical variation in metacercarial infection levels in marine invertebrate hosts: parasite species character versus local factors

Despite the important roles played by parasites in local population dynamics and community structure of marine ecosystems, there is a lack of information on the geographical variation in infection levels displayed by particular host–parasite species combinations. This study examines geographical variation in infection levels by the metacercarial stages of trematode parasites in crustacean and bivalve second intermediate hosts. Analyses were based on a dataset compiled from the literature, consisting of 164 local samples representing 49 host–parasite species pairs for crustaceans, and 338 entries representing 36 host–parasite species pairs for bivalves. The analyses indicate that for all measures of infection levels [prevalence (percentage of individuals infected), intensity (mean no. of metacercariae per infected individual), abundance (mean no. of metacercariae across all individuals in a sample)], there was statistically significant repeatability of infection values within host–parasite species pairs. However, it is only for values of intensity and abundance of infection in crustacean hosts that the repeatability was strong; this suggests that infection levels are specific properties of crustacean–trematode species pairs, showing significant consistency across localities despite spatial variation in abiotic and biotic conditions. Although the magnitude of variation in infection levels within parasite species pairs (measured as coefficients of variation) was independent of scale in crustacean hosts, infection levels in bivalves increased in variability at large (>100 km) spatial scales. These results suggest that there is a considerable geographical consistency in parasite load, especially in crustacean hosts, which should lead to consistent ecological and ecosystem effects of marine trematodes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Bathymetric patterns of genetic variation in a deep-sea protobranch bivalve, Deminucula atacellana

The origin of the deep-sea benthic fauna is poorly understood and represents an enormous gap in our understanding of basic evolutionary phenomena. One obstacle to studying evolutionary patterns in the deep sea has been the technical difficulty of measuring genetic variation in species that are typically minute, rare, and must be recovered from extreme depths. We used molecular genetic techniques to quantify variation in the 16S rRNA mitochondrial gene within and among populations of the common protobranch bivalve Deminucula atacellana (Schenck, 1939). We analyzed 89 individuals from nine samples collected in the 1960s along a depth gradient from 1100 to 3800 m in the western North Atlantic. Genetic variability within populations is much lower than between populations, and peak haplotype numbers occur near the center of its depth distribution. Continental slope (<2500 m) and rise (>2500 m) populations were genetically distinct despite the lack of any obvious topographic or oceanographic features that would impede gene flow. These findings indicate that the deep-sea macrofauna can have strong population structure over small (134 km) spatial scales, similar to that observed in shallow-water and terrestrial organisms. This surprisingly high biodiversity at the genetic level affords the potential for adaptation and evolutionary diversification, the ultimate historical causes of high species diversity in the deep-sea benthos. Received: 24 July 1997 / Accepted: 26 January 1998     

Genetic variance in Ophiomusium lymani (Ophiuroidea: Echinodermata) from lower bathyal depths in the Rocklall Trough (northeast Atlantic)

Specimens of the deep-sea brittle-star Ophiomusium lymani were collected from six sites in the Rockall Trough (northeast Atlantic_. Four monomorphic and four polymorphic loci were detected, with up to 75 individuals screened at any one locus. The results showed little difference in allele frequencies between sites and, consequently, estimates of genetic identity indicated no significant genetic differentiation between the populations sampled. Deficits of heterozygotes were observed at all polymorphic loci, but the deficiencies were only significant at the phosphoglucose isomerase locus from two locations. The deficit of heterozygotes was not the same across loci, suggesting that inbreeding is not the cause of the excess homozygosity. No relationship between heterozygosity and depth was observed.     

Seasonal variation in reproduction and population structure of the bivalve Limopsis tajimae in the upper bathyal zone of Suruga Bay, Japan

Seasonal variation in reproduction and population size structure was investigated for the suspension-feeding bivalve Limopsis tajimae Sowerby inhabiting the upper bathyal zone (300 m deep) of Suruga Bay, central Japan. The bivalve was collected at 1- to 4-month intervals for a period of 22 months, and bottom environment was monitored concurrently to detect factors affecting seasonality in the bivalve. Bottom water temperature, organic carbon and nitrogen contents in the sediments did not exhibit seasonal variation. Size-adjusted soft-tissue weight varied slightly, but statistically significantly between stations and months. However, its seasonal pattern was not obvious, and the pattern of temporal variation was totally different between stations. The sex ratio did not deviate from 1:1, and there was no significant difference between shell lengths of females and males. Females possess both immature small oocytes and large developed oocytes in their ovaries throughout the year, suggesting that they can potentially undergo year-round continuous reproduction. The proportion of developed oocytes in each female varied greatly from month to month, although no seasonal cycle was obvious. Population size structure of L. tajimae was polymodal. A mode of the smallest size class occurred in most months, suggesting long periods of bivalve recruitment. These findings indicate that seasonal variation in reproduction of the bivalve was negligible, probably reflecting constant physical and nutritive conditions of the bottom environment. Received: 6 January 1998 / Accepted: 19 May 1998     

Seasonal variation in the white muscle biochemical composition of deep-sea macrourids in the North-east Atlantic

The peculiar limpet-like morphology of the genus Colobocentrotus is unique among the regular echinoids. This shape has been interpreted as an adaptation to life in areas of extreme wave exposure. In this study the attachment strength of C. atratus is compared with that of three sympatric species, Echinometra mathaei, Heterocentrotus trigonarius and Stomopneustes variolaris, which have more typical echinoid morphology and live in different microhabitats. For each species, the adhesion of individual sea urchins was measured as well as the tenacity of single tube foot and the mechanical properties of the tube foot stems. Colobocentrotus always presented the highest measured values, although not always significantly different from those of the other species. Of the mechanical properties of the stem measured, the stem extensibility was the only property that was significantly different among species. In general the stems of all the species studied became more extensible and more difficult to break with increasing strain rate, providing an adaptative advantage to the sea urchin when subjected to rapid loads such as waves. In terms of single tube foot tenacity, C. atratus tube feet attached with a tenacity (0.54 MPa) two times higher than the one of E. mathaei, H. trigonarius and S. variolaris (0.21–0.25 MPa). Individual sea urchins of the four species, however, attached with a similar strength (0.2–0.26 MPa). The calculation of safety factors showed that it is the very high number of adoral tube feet of C. atratus and not the overall shape of the animal that allows this species to withstand very high water velocities. However, C. atratus streamlined morphology may be a functional adaptation to reduce the impact of other hydrodynamic forces (such as wave impingement forces) or to cope with other selective environmental stresses (such as dessication), and thus to inhabit extremely exposed areas of the intertidal.     

Genetic variation in the mangrove periwinkle Littorina angulifera

Twenty populatios of Littorina angulifera, inhabiting islands composed of the mangrove tree Rhizophora mangle, were assayed at an esterase locus to determine whether genetic differentiation was associated with distance between populations. It was predicted, on the basis of larval dispersal in this species, that genetic differentiation between populations on islands separated by long distances should be greater than those on islands located near each other. A chi0square test of homogeneity revealed significant differences in esterase gene frequencies among the 20 island populations. However, there was no association of distance between islands and genetic heterogeneity. In addition, a cline in gene frequency was found to be assiciated with latitude. Factors responsible for the observed pattern of heterogeneity at the esterase locus are discussed.