Population structure and genetic variation in two tropical sea anemones (Cnidaria,Actinidae) with different reproductive strategies

Genetic variation and population structure of two tropical sea anemones, Bunodosoma caissarum Correa and Actinia bermudensis McMurrich were related to their different dispersal capabilities and reproductive modes. B. caissarum reproduces sexually and has a long-lived planktotrophic larva; A. bermudensis can reproduce both sexually and asexually, supposedly with short-range dispersal. Both species were sampled along 1150 km of Brazilian coastline between 1990 and 1991 and analyzed by horizontal starch gel electrophoresis for 16 enzyme loci in B. caissarum and 19 in A. bermudensis. B. caissarum had higher levels of heterozygosity (H=0.35) and lower levels of population structuring (F _ST=0.042) than A. bermudensis (H=0.17; F _ST=0.262). At one locality large genetic differences between two sympatric colour morphs of A. bermudensis provide evidence of possible cryptic speciation. Overall levels of genetic variation and heterozygosity in the two species are compatible with their known modes of reproduction.     

Dynamics of external brooding in the sea anemone Epictis Epiactis prolifera

The sexually produced young of the externally brooding actinian Epiactis prolifera Verrill, 1869 are attached to the parent's column just above the base. A transitory brood groove may be formed around the limbus when the parent contracts. In the population studied on the coast of Sonoma County, California, USA, from 27 to 49% of the adult anemones were brooding at any time, the proportion being inversely related to seawater temperature and directly related to size of the anemones. A brood usually consisted of young of various sizes. Overall, number of young being brooded was directly related to parent size and inversely related to size of the brooded juveniles. The growth rate of juvenile anemones was inferred to average about a millimeter in pedal disc diameter per month. Juveniles dislodged from the parent when smaller than 4 mm in basal diameter apparently cannot survive in the intertidal zone. Juveniles which successfully make the transition to adulthood have, therefore, been brooded for at least 3 months. Fewer than 50% of the smallest juveniles survived to a size at which they could live independently. The transition to adulthood was accompanied by a mortality of 80%. The survival rate, from egg to adult, was calculated to be about 1% per season.This work was done in partial fulfillment of the requirements for the Ph.D. degree in the Department of Zoology, University of California, Berkeley, and was supported by a U.S. National Science Foundation Traineeship.     

Effects of brooding and broadcasting reproductive modes on the population genetic structure of two Antarctic gastropod molluscs

Life-history characteristics exert a profound influence upon the population structure of many marine organisms. However, relatively few genetic studies have compared direct with indirect-developing species in the same ecosystem or geographical region, and none to our knowledge within an Antarctic setting. To address this issue, we have collected novel amplified fragment length polymorphism (AFLP) data from the direct-developing top shell Margarella antarctica to form a comparison with previously published data for the broadcast-spawning Antarctic limpet Nacella concinna. We scored 270 loci in 240 M. antarctica individuals sampled from five populations spanning the full length of the Antarctic Peninsula. Profound differences were identified in the strength and pattern of population structure between the two species, consistent with gene flow being highly restricted in M. antarctica relative to N. concinna.     

Reproduction in the externally brooding sea anemone Epiactis georgiana in the Antarctic Peninsula and the Weddell Sea

External parental care is uncommon among actiniarians but common in Epiactis species. Here, several aspects of reproduction are analyzed for of one of them, Epiactis georgiana. Samples were collected in December, January, February, March, and April in the Antarctic Peninsula and the eastern Weddell Sea, during 1998, 2000, 2002, and 2003. Most sexually mature individuals of E. georgiana are male or female, but some are hermaphrodites. This is the first report of hermaphroditism in E. georgiana, which is the third species of the genus with this sexual pattern. The results suggest that oogenesis starts in December and that at least two generations of oocytes overlap; a third generation is often brooded externally. Putative fertilization is likely internal, and larvae and/or embryos are externally brooded on the distal part of the adult column until an advanced developmental stage. Apparently E. georgiana reproduces seasonally, probably releasing the embryos/larvae in the last months of the austral spring (December). Inter-individual variability was observed in gametogenesis. In addition, specimens from the Antarctic Peninsula were larger than those from the Weddell Sea. This study represents the first step in understanding the reproductive mode of E. georgiana.     

Population structure and energetics of the shallow-water antarctic sea star Odontaster validus in contrasting habitats

Individuals and populations of Odontaster validus Koehler differed markedly among different habitats, as revealed in a study from October 1984 through January 1986 in McMurdo Sound, Antarctica. At McMurdo Station, individual sizes (wet weight) and population biomass (g wet wt m^-2 and kJ m^-2) decreased significantly with increasing depth. Individuals from shallow (10 to 20 m) habitats were in superior nutritional condition to those from deeper water (30 and 165 m), as shown by higher gonad and pyloric cecum indexes, and by higher lipid and energetic levels in the pyloric ceca. Moreover, gonadal output (reproductive output) was higher in shallow-water individuals. Higher levels of chlorophyll in the pyloric ceca and richer yellow to red coloration of the body wall in the shallow-water individuals indicate that they utilize the higher levels of primary production at shallow depths. At East Cape Armitage, where nearly permanent, thick, snow-covered ice most of the year resulted in very low levels of benthic primary production, the lowdensity sea stars were all very small and nutritionally similar to the deep-water individuals at McMurdo Station. At Cape Evans, where the generally snow-free sea-ice that broke up in mid-summer resulted in a luxurient benthic cover of diatoms and macroalgae, the sea stars were smaller than at McMurdo Station at comparable depths, but population densities were higher, resulting in 4 to 9 times greater biomass. Growth rates of sea stars fed in the laboratory were very low, especially compared to laboratory-reared temperate and tropical species; well-fed individuals need about 9 yr to reach 30 g wet weight, near the mean size of shallowwater individuals at McMurdo Station. No growth was detected in individuals caged at McMurdo Station for one year, suggesting even lower growth rates in the field. The stable size-frequency distributions at the different sites and depths throughout the year-long study suggest highly stable populations with low temporal variability in recruitment, migration and mortality. These data indicate that individuals and populations of O. validus quantitatively and qualitatively reflect the general level of productivity of a habitat. Differences noted in size, coloration, nutrition, and reproductive effort may be the result of long-term integration of local levels of primary production. These ubiquitous sea stars may serve as a biotic indicator of productivity in localized habitats around the continental shelf of Antarctica.     

Comparative population genetic structure of marine gastropods (Littorina spp.) with and without pelagic larval dispersal

 Population genetic theory predicts that marine animal species with planktonic larvae will have less genetic structure than those with direct development. We compared the genetic structure of four species of littorinid snails – two with planktonic egg capsules that hatch as planktonic larvae and two with benthic egg masses that hatch as crawl-away juveniles. We used DNA sequencing and single stranded conformational polymorphism (SSCP) to assess sequence variation in a 480 bp fragment of the mitochondrial cytochrome b gene and then used an analysis of molecular variance (AMOVA) to estimate Φ_st for populations from the northeastern Pacific coast. One of the two direct-developing species, Littorina subrotundata, had a moderate amount of population structure (Φ_st=0.209) as expected but the other direct-developing species, L. sitkana, was nearly fixed for a single haplotype that made it impossible to precisely estimate Φ_st. One of the two planktonic-developing species, L. scutulata, did not show any significant population structure (Φ_st=0.004). In contrast to our expectations, the other planktonic-developing species, L. plena, showed some weak but statistically significant population structure (Φ_st=0.052). We discuss how differences in population genetic structure between species with the same type of development may reflect differences in their historical demography. Received: 22 December 1999 / Accepted: 24 July 2000     

Fine-scale genetic structure in the surface brooding Caribbean octocoral,Antillogorgia elisabethae

Larval dispersal is a critical component of marine species’ life histories because it controls their population dynamics. Dispersal distance can be inferred by the presence and scale of spatial genetic structure (SGS). The larvae of Antillogorgia elisabethae, a surface brooding Caribbean octocoral, have been observed to settle within meters of maternal colonies, although many disperse over greater distances. Using a spatially hierarchical sampling design, A. elisabethae recruits were collected from a site off the coast of Great Abaco, The Bahamas, in June of 2009–2011. Seven microsatellite loci were used to determine whether genetic structure was present among recruits on scales of <1 m to 10s of meters. Multilocus genotypes were autocorrelated within spatial distance classes using Moran’s I coefficient as an estimate of genetic similarity, and Rousset’s genetic distance (â). Genetic structure was present on a scale of <10 m in 2010, but not in the other 2 years, suggesting that larval dispersal was more localized in 2010. Moran’s I coefficients were positive and significant for 0–1, 1–2 and 2–5 m classes in 2010, and the average genetic distance (â) among recruits in the 0–1 m class was significantly lower than the other classes. Autocorrelograms suggested that genetic patch sizes were 7.5 m in 2010, and 20 m in 2009 and 2011. Differences in the SGS of recruits between years might reflect temporal variation in temperature, wind and/or current speeds affecting larval dispersal, or variation in reproduction at the site.     

Fungal disease resistance of Caribbean sea fan corals (Gorgonia spp.)

An on-going, Caribbean-wide epizootic affecting sea fan corals (Gorgonia spp.) is caused by the fungus Aspergillus sydowii (Thom et Church). We examined the role of crude extracts in resistance of two species of sea fans, Gorgonia ventalina (L.) and G. flabellum (L.), against A. sydowii and a bacterial pathogen of fish, Listonella anguillarum (MacDonell et Colwell). Sea fans were collected in January 1997 from San Salvador, Bahamas, and in June 1997 and January 1998 from Alligator Reef, Florida Keys, USA. Crude extracts from both species were tested to determine concentrations inhibiting germination of A. sydowii spores. Crude extracts from both species inhibited spore germination at concentrations as low as 1.5 mg ml⁻¹; most samples were active at 5 to 10 mg ml⁻¹. These concentrations are within the range estimated in living tissue and were higher in healthy colonies suggesting their role in mediating disease susceptibility. We also detected within-colony gradients in antifungal activity, which varied with the disease state of the colony. In healthy sea fans, resistance was highest at colony edges and lowest in medial and central regions of the colony. Among sea fans with lesions in the colony center, resistance in tissue from proximal and medial regions was as high as tissue from the colony edge (i.e. distal region). The increase in antifungal activity suggests an inducible response by the coral host to the fungal pathogen. This response is most evident among sea fans with lesions in the colony center and not among colonies with lesions at the edge. Antibacterial activity of crude extracts against L. anguillarum was highest at the colony edge but did not vary with disease state or tissue location. Received: 10 March 1999 / Accepted: 10 November 1999     

Lights and shadows: growth patterns in three sympatric and congeneric sponges (Ircinia spp.) with contrasting abundances of photosymbionts

The life-history traits of long-lived benthic littoral invertebrates remain poorly understood. In this study, we analysed patterns of growth in three abundant sublittoral sponges from the western Mediterranean Sea, chosen for their close phylogenetic relatedness, sympatric distribution, and contrasting amounts of photosymbionts: high in Ircinia fasciculata, lower in I. variabilis, and absent in I. oros. Sponge area, perimeter, number of oscula, and epibiont abundance were quantified from in situ digital images taken monthly for 1.5 years and volumetric growth rates were calculated from empirical area–volume relationships. Volumetric growth rates were different among species and coherent with the photosymbiont abundance: high in I. fasciculata (40.03 ± 4.81 % year^?1, mean ± SE), low in I. variabilis (5.65 ± 6.11 % year^?1), and almost nil in I. oros (?0.04 ± 3.02 % year^?1). Furthermore, a marked seasonality was observed in the first two species, with greater growth during the warm season. The high growth rates of I. fasciculata were likely fuelled by symbiont-derived photosynthates and required to compete in the well-lit, algal-dominated habitats this species prefers. In contrast, I. variabilis and I. oros tended to dwell in shaded habitats, where competition from slow-growing invertebrates is intense, and featured lower growth rates. The flattened morphology and lower circularity of I. variabilis indicates a capacity for adaptation to any space that is freed, while I. oros had less oscula and was more massive and circular, suggesting a strategy of passive occupation and minimisation of biological interactions. The results show that even congeneric species living sympatrically can achieve important biomass using different growth and substrate occupation strategies.     

Small-scale genetic structure in the sea palm Postelsia palmaeformis Ruprecht (Phaeophyceae)

Documenting the scale of movement among populations is an important challenge for marine ecology. Using nine microsatellite markers, evidence of genetic structure in a marine kelp, the sea palm Postelsia palmaeformis Ruprecht, was examined in the vicinity of Cape Flattery, Washington state, USA (48° 24′ N, 124°44′ W). Genetic clustering analysis implemented without reference to geographic structure strongly suggested that a number of distinct genetic clusters existed among the 245 plants sampled in August in the years 1997–2001. Subsequent analysis showed that clustering was associated with geographically defined populations both among (km scale) and within (m scale) sampling sites. F _st analysis of geographically defined populations revealed significant genetic differentiation among populations of plants as little as 5 m apart, evidence of genetic structuring at even smaller scales, and a sharp increase in F _st across populations separated by up to 23 m. F _st values were also high and approximately unchanging (F _st=0.470) for populations separated by greater distances (up to 11 km), consistent with a scenario of rare dispersal by detached, floating plants carried by variable currents. The results corroborate natural history observations suggesting that P. palmaeformis has extremely short (1–3 m) spore dispersal distances, and indicate that the dynamics of sea palm populations are more affected by local processes than recruitment from distant populations.     

Inter- and intraspecific patterns of reproductive effort in four species of cone shells (Conus spp.)

Patterns of reproductive effort were examined both within and between 4 species of the tropical marine gastropod genus Conus from Hawaii in 1979. Caloric content was measured for the somatic tissues of female cone shells and for their egg capsule masses. These data were used to calculate (1) current reproductive effort, which was the ratio of annual energy produced as spawn to the sum of the annual energy produced as spawn plus the annual energy produced as somatic tissue; and (2) weight-specific reproductive effort, which was the ratio of annual energy produced as spawn to the total energy content of the female body just before spawning. For C. pennaceus, during the approximately 10 yr of life of a female, current reproductive effort rose from about 0.35 at 2 yr to about 0.85 at 10 yr. In contrast, weight-specific reproductive effort remained relatively constant throughout life for C. pennaceus (about 0.2 to 0.5), C. abbreviatus (about 0.2 to 0.3), C. flavidus (about 0.2 to 0.3) and C. quercinus (about 0.1 to 0.2). It is suggested that weight-specific reproductive effort may be useful as an index of the selective importance of the survival cost of present reproduction, while current reproductive effort more closely reflects the selective importance of energy allocation tradeoffs. Data on the 4 species of Conus examined support the hypothesis that reproductive effort should be positively correlated with extrinsic adult mortality and negatively correlated with the variability of juvenile survivorship. The data also suggest a possible relationship between reproductive effort and the cost of parental care.     

Population structure in the sexually reproducing sea anemone Oulactis muscosa

The intertidal sea anemone Oulactis muscosa (Drayton) is dioecious and most individuals are sexually mature throughout the year. Biochemical genetic evidence was used to determine the genetic structure of populations and to infer the relative contributions of sexual and asexual reproduction to recruitment. Data were collected for six enzyme-encoding loci from local populations spread along 735 km of the south east coast of Australia. The genetic structure of each of the nine local populations studied was consistent with recruitment by sexually produced individuals. In almost all cases, the observed single-locus genotypic frequencies closely matched those expected for hardy-Weinberg equilibria, however, consistent deficits of heterozygotes were detected for all loci. No apparent subdivision of the population was detected within the sampling area. Low levels of genetic differentiation were found between local populations and standardised variance (F _ST ) values were similar to those for other species with widespread planktonic dispersal of larvae.Contribution No. 60 from the Ecology and Genetics Group of the University of Wollongong     

Population genetic structure of the prosobranch Nassarius reticulatus (L.) in a ria seascape (NW Iberian Peninsula) as revealed by RAPD analysis

Randomly amplified polymorphic DNA (RAPD) banding patterns were compared between samples of the netted dogwhelk Nassarius reticulatus from 11 locations along the NW Iberian Peninsula coast. To detect if rias (estuaries formed by drowned river valleys) might promote genetic differentiation, five sampling sites were located within a ria (ria of Muros) and the remaining six were scattered along open-coast areas at increasing distances from the ria mouth. Population differentiation statistics (Φ-values) were estimated using a hierarchical analysis of molecular variance (AMOVA) with samples sorted into two groups: open-coast and ria populations. Despite a high potential to disperse, AMOVA demonstrated a modest, statistically significant genetic heterogeneity among N. reticulatus populations. Most of the genetic structure resided in differences among open-coast populations; ria populations were genetically homogeneous. No obvious geographical pattern was detected for the pairwise genetic distances (non-metric multidimensional scaling; UPGMA tree; Mantel test). Unlike previous studies with other species at a variety of estuarine systems other than rias, there was no evidence that the ria of Muros may enhance the genetic divergence of N. reticulatus populations. This discrepancy is discussed in relation to the biological features of the species (high dispersal potential and a preference for mid-low estuarine habitat) and the strong hydrographic connectivity between ria and neighbouring off-shore waters.     

Genetic population structure of a species' complex of blue mussels (Mytilus spp.)

Blue mussels representing two nominal species (Mytilus trossulus Gould, 1850 and Mytilus galloprovincialis Lamarck, 1819 were collected from 28 intertidal locations along the Pacific coast of the USA in 1990–1991 (total N=1255) and examined for variation at 15 allozyme loci. Twelve samples, mostly from a region of suspected hybridization, were analyzed for variation in seven shell characters. Principal-components analysis of allozyme data revealed three groups based on first principal-component scores, which were identified as M. trossulus, M. galloprovincialis, and hybrids. Canonical discriminant analysis of shell characters was less successful in separating mussels into discrete groups. Each location was characterized for four environmental variables: (1) temperature, (2) salinity, (3) tidal height and (4) degree of exposure to wave action, which were then used as independent variables in a series of multiple-regression analyses, with the proportions of the two species as dependent variables. Temperature and salinity had significant (P<0.05) effects on the macrogeographic distribution of the two species, whereas the effects of height in the tidal zone and degree of wave exposure were not statistically significant. Salinity was found to have a greater influence than temperature on the microgeographic distribution of the two species. M. trossulus was more abundant at locations with lower temperatures and greater salinity variation than M. galloprovincialis. The two species appear to be ecologically distinct, and their genetic integrity is at least partly the result of environmental heterogeneity.     

Population structure of the wreckfish Polyprion americanus determined with microsatellite genetic markers

 We examined population structure in the wreckfish, Polyprion americanus, by assaying six microsatellite loci in 422 individuals collected throughout the geographic range. Eighteen hapuku, P. oxygeneios, were assayed at the same loci for use as an outgroup. Expected heterozygosities ranged from 0.49 to 0.88 and averaged 0.66. Allele-frequency distributions at those loci indicated that samples from the eastern North Atlantic, western North Atlantic and the Mediterranean were genetically similar, confirming the pattern seen in a previous analysis of mitochondrial DNA (mtDNA) restriction fragment length polymorphisms (RFLPs). Both mtDNA and microsatellite studies differentiated northern and southern wreckfish stocks. However, in contrast to the mtDNA studies, allelic variation at microsatellite loci clearly differentiated wreckfish from two Southern Hemisphere locations, Brazil and the South Pacific. Far more genetic variation was observed at microsatellite loci than with mtDNA RFLPs (haplotype diversity averaged 0.01), and we saw more evidence of population structure with the microsatellite loci. The differentiation between southern and northern wreckfish supports the distribution records, which indicate that wreckfish do not occur in the tropics. Temperature profiles and current patterns throughout the southern oceans apparently also prevent significant gene flow between the South Pacific and Brazilian samples. Received: 29 January 2000 / Accepted: 27 June 2000     

Genetic divergence between east and west Atlantic populations of<Emphasis Type="Italic"> Actinia</Emphasis> spp. sea anemones (Cnidaria: Actiniidae)

The sea anemone Actinia equina was considered a highly variable species with a wide geographical distribution, but molecular systematic studies have shown that this wide distribution may be the result of the lumping of cryptic species. In this work enzyme electrophoresis was used to analyse the genetic variability of A. equina from the Atlantic coasts of Europe and Africa, as well as the relationships between those populations and other species of the genus. Samples of A. equina from the United Kingdom and France were compared with supposedly conspecific populations from South Africa and a recently described species from Madeira, Actinia nigropunctata. The South African and Madeiran populations were genetically very divergent from each other (genetic identity, I=0.15), as well as from the A. equina population from the United Kingdom and from the other species studied (I between 0.23 and 0.78), indicating that the African population of A. equina belongs to a different species. High genetic similarities were found between west Atlantic A. bermudensis and east Atlantic A. sali, in spite of the great geographical distance between them, and between the European A. equina and A. prasina. The results confirm the presence of the new species A. nigropunctata from Madeira and suggest a new species within the A. equina species group, from South Africa.Communicated by O. Kinne, Oldendorf/Luhe     

Factors influencing differential mortality within a marine mussel (Mytilus spp.) hybrid population in southwestern England: reproductive effort and parasitism

Two species of marine mussel, Mytilus edulis and M. galloprovincialis hybridize on the coasts of western Europe. Studies of hybrid mussel populations have shown that natural selection favors M. galloprovincialis-like genotypes within this hybrid zone. Many hypotheses have been proposed to explain differential mortality in these populations. This study tests two hypotheses addressing factors of mortality in a population, and describes yearly energy storage and reproductive cycles of these two species and their hybrids. No evidence was found that the two taxa have different overall levels of reproductive effort or parasite infestation. They do, however, have asynchronous spawning periods and divergent energy storage strategies. In the year of this study, 1993, the M. edulis genotypic class spawned as a group in June and July. After spawning, they built up a high level of mantle energy-storage tissues that are probably used for gametogenesis in the following winter and spring. The M. galloprovincialis genotypic group, however, spawned asynchronously, beginning in June and finishing by August, and did not build up high levels of energy-storage tissues in summer. These results add a temporal component to the interpretation of selective forces acting to shape this hybrid zone. Vulnerability of each species to mortality factors may differ because of their divergent reproductive and energy-storage cycles. Received: 15 January 1999 / Accepted: 26 July 2000     

Short-term stability of genetic structure in populations of the sea anemone Metridium senile

Populations of the sea anemone Metridium senile (L.) were sampled from several locations in eastern North America in two series, one collected from 1977–1979 and the other from 1981–1985. Fourteen populations were sampled twice at one- to six-year intervals. Samples were analyzed for temporal differences in genetic composition at both the single locus and multiple locus levels. Overall patterns of geographic variation in allele frequency did not change between series. Regressions describing clines did not differ significantly, and loci not showing clinal variation in the first series remained similar in the second. Analysis of populations sampled twice produced no systematic evidence of change in allele frequency with time for any of four polymorphic loci. Comparisons with computer simulations of repeated sampling of multiple locus genotypes from panmictic populations with free recombination also revealed little temporal change at this level. One population showed possible evidence of recruitment from a different gene pool. Other significant departures from expectations reflected more reproducibility of genotype distributions between samples than expected for sexual populations. This excess stability likely results jointly from clonal reproduction and little sexual recruitment. Despite these indications of genetic stability in adult populations, newly settled juveniles were genetically different from resistant adults in one population, demonstrating the potential for genetic change by immigration. Successful sexual recruitment seems to be rare, even though larvae regularly settle from the plankton. Although interpretation of these results is somewhat limited by lack of knowledge of longevities and generation times, to the extent that they reflect longer-term trends, they suggest that at least some of the observed patterns of geographic variation in allele frequency probably result from natural selection.     

Oceanic variability and coastal topography shape genetic structure in a long-dispersing sea urchin

Understanding the scale of marine population connectivity is critical for the conservation and sustainable management of marine resources. For many marine species adults are benthic and relatively immobile, so patterns of larval dispersal and recruitment provide the key to understanding marine population connectivity. Contrary to previous expectations, recent studies have often detected unexpectedly low dispersal and fine-scale population structure in the sea, leading to a paradigm shift in how marine systems are viewed. Nonetheless, the link between fine-scale marine population structure and the underlying physical and biological processes has not been made. Here we show that patterns of genetic structure and population connectivity in the broadcast-spawning and long-distance dispersing sea urchin Centrostephanus rodgersii are influenced by physical oceanographic and geographic variables. Despite weak genetic differentiation and no isolation-by-distance over thousands of kilometers among samples from eastern Australia and northern New Zealand, fine-scale genetic structure was associated with sea surface temperature (SST) variability and geography along the southeastern Australian coast. The zone of high SST variability is characterized by periodic shedding of eddies from the East Australian Current, and we suggest that ocean current circulation may, through its influence on larval transport and recruitment, interact with the genetic consequences of large variance in individual reproductive success to generate patterns of fine-scale patchy genetic structure. If proven consistent across species, our findings suggest that the optimal scale for fisheries management and reserve design should vary among localities in relation to regional oceanographic variability and coastal geography.