Variation in the genetic composition of coral (Pocillopora damicornis and Acropora palifera) populations from different reef habitats

The genetic structure of populations of the corals Pocillopora damicornis and Acropora palifera was examined in three habitats at One Tree Island during March and April 1993, using electrophoretically detectable variation at six allozyme loci. There were significant genetic differences among populations of P. damicornis within each of the reef crest, lagoon and microatoll habitats. The level of differentiation among populations was similar in each of the habitats. Differences between populations of P. damicornis from lagoon and microatolls were no greater than that within habitats, but genetic differentiation of these from crest populations was much higher. There was no difference in the genetic composition of A. palifera populations within or between the lagoon and microatolls, the only habitats where this species was found. Both coral species had observed:expected (G _O:G_E) genotypic diversity rations >0.80, indicating predominantly sexual reproduction. These data, the high genotype diversity and general conformance of genotype frequencies to those expected under conditions of Hardy-Weinberg, suggested panmixis at each site. The high degree of sexual reproduction in the P. damicornis populations is unusual for a species where asexual reproduction has been the dominant mode of reproduction reported to date. Gene flow in both species was considerable between the lagoon and the closed microatolls. The genetic differences between populations of P. damicornis in these habitats and the reef crest may reflect the relative isolation of all populations within the closed One Tree Lagoon from those outside. However, local currents appear to offer effective means of dispersal between the habitats, suggesting that the genetic differences result from natural selection in the different environments within One Tree Lagoon and the reef crest.     

Differences between native and invasive<Emphasis Type="Italic"> Caulerpa taxifolia</Emphasis>: a link between asexual fragmentation and abundance in invasive populations

The tropical and subtropical marine green alga Caulerpa taxifolia has invaded several temperate regions throughout the world, including southeastern Australia. In this study, I examined how invasive C. taxifolia from temperate southeastern Australia differed from native C. taxifolia from subtropical Moreton Bay, Australia, in the traits proposed as being important to its invasion success: thallus size and density, levels of asexual reproduction (fragmentation) and total biomass. Against the prediction of a large size for invasive C. taxifolia, native populations from Moreton Bay had larger stolons and fronds than invasive populations. However, invasive populations consistently had much higher densities of stolons, fronds and fragmented fronds; and a greater biomass compared to native populations. Average densities at invasive sites exceeded 4,700 stolons and 9,000 fronds/m² and were as high as 27,000 stolons and 95,000 fronds/m², which are the highest reported for C. taxifolia anywhere. Average densities of fragmented fronds at invasive sites were as high as 6,000/m² and up to 45% of all stolons at invasive sites could be directly linked to asexual recruitment via fragmented fronds. Importantly, at invasive locations there was a strong association between asexual reproduction and abundance demonstrated by positive correlations between the density of fragmented fronds and total biomass. These findings are the first to describe quantitative differences between native and invasive C. taxifolia and to demonstrate a link between the high levels of asexual reproduction and high abundance in invasive populations. Although the causes and consequences of high levels of asexual reproduction remain to be explored, this study suggests that changes in demographic and life-history traits during the invasion by C. taxifolia into temperate habitats may contribute to its success there.Communicated by M. S. Johnson, Crawley     

Allozyme electrophoresis demonstrates that the scleractinian coral Montipora digitata is two species

Allozyme electrophoresis at five loci was used to clarify the species status of two morphs of the scleractinian coral Montipora digitata (Dana, 1846), using specimens collected from three locations off the Queensland coast between October 1991 and April 1993. The two morphs, occurring sympatrically, were distinguished by one fixed gene difference at Locus LT-2 and frequency differences at three other loci, indicating reproductive isolation between them. Populations of both morphs were generally in Hardy-Weinberg equilibrium, suggesting that sexual reproduction does occur. Despite sampling designed to reduce the chance of collecting clonemates, the genetic diversity ratio (G _O:G_E) indicated that asexual reproduction also occurs. This is in agreement with the known modes of reproduction for this species. There was significant genetic differentiation between populations of one of the morphs of M. digitata. This could have been influenced by greater degrees of asexual reproduction in this morph, selection within sites, or variation in larval survival and recruitment patterns. Relatively low F _st (differentiation among populations) values found for a broadcast spawning species during this study are in accordance with the hypothesis that brooded planulae are adapted for rapid settlement, whereas planulae produced by broadcast spawners are adapted for widespread dispersal.     

Population biology of the annual grass Triplasis purpurea in relation to distance from shore on Staten Island, New York

Four populations of the native annual grassTriplasis purpurea were surveyed on coastal beaches along the south shore of Staten Island, NY, to determine the potential of this species to colonize shoreline habitats mostly devoid of other vegetation. If the species can establish and maintain dense populations, it may have conservation value for urban beaches disturbed by human activities. For two populations, survivorship, growth, and reproduction were monitored at different distances from shore to determine the ability of this species to maintain viable populations. At three sites,T. purpurea occurred in >75% of all quadrats and the highest density was 1195 plants/m² at 74 m from shore in one recently disturbed site. Density generally increased with increasing distances from shore at low tide (from ca. 40–90 m). Plants showed the greatest growth and reproduction at close distances to shore (30–40 m); part of this effect was due to density in one population, but when density effects were removed statistically, there still remained a decline in growth and reproduction with increasing distance from shore. Improved vigor nearest to shore may be due to continual sand deposition. Survivorship showed a Type I pattern, with low mortality throughout the growing season. By colonizing newly-deposited and continually shifting sands,T. purpurea can contribute to the earliest stages of ecological succession along disturbed beaches in eastern North America and may be valuable to the development and management of urban coastal plant communities.     

Population biology of the annual grassTriplasis purpurea in relation to distance from shore on Staten Island, New York

Four populations of the native annual grassTriplasis purpurea were surveyed on coastal beaches along the south shore of Staten Island, NY, to determine the potential of this species to colonize shoreline habitats mostly devoid of other vegetation. If the species can establish and maintain dense populations, it may have conservation value for urban beaches disturbed by human activities. For two populations, survivorship, growth, and reproduction were monitored at different distances from shore to determine the ability of this species to maintain viable populations. At three sites,T. purpurea occurred in >75% of all quadrats and the highest density was 1195 plants/m² at 74 m from shore in one recently disturbed site. Density generally increased with increasing distances from shore at low tide (from ca. 40 – 90 m). Plants showed the greatest growth and reproduction at close distances to shore (30 – 40 m); part of this effect was due to density in one population, but when density effects were removed statistically, there still remained a decline in growth and reproduction with increasing distance from shore. Improved vigor nearest to shore may be due to continual sand deposition. Survivorship showed a Type I pattern, with low mortality throughout the growing season. By colonizing newly-deposited and continually shifting sands,T. purpurea can contribute to the earliest stages of ecological succession along disturbed beaches in eastern North America and may be valuable to the development and management of urban coastal plant communities.     

Sociogenetic organization of the red ant Myrmica rubra

Knowledge of the sociogenetic organization determining the kin structure of social insect colonies is the basis for understanding the evolution of insect sociality. Kin structure is determined by the number and relatedness of queens and males reproducing in the colonies, and partitioning of reproduction among them. This study shows extreme flexibility in these traits in the facultatively polygynous red ant Myrmica rubra. Relatedness among worker nestmates varied from 0 to 0.82. The most important reason for this variation was the extensive variation in the queen number among populations. Most populations were moderately or highly polygynous resulting in low relatedness among worker nestmates, but effectively monogynous populations were also found. Polygynous populations also often tend to be polydomous, which is another reason for low relatedness. Coexisting queens were positively related in two populations out of five and relatedness was usually similar among workers in the same colonies. Due to the polydomous colony organization and short life span of queens, it was not possible to conclusively determine the importance of unequal reproduction among coexisting queens, but it did not seem to be important in determining the relatedness among worker nestmates. The estimates of the mating frequency by queens remained ambiguous, which may be due to variation among populations. In some populations relatedness among worker nestmates was high, suggesting monogyny and single mating by queens, but in single-queen laboratory nests relatedness among the worker offspring was lower, suggesting that multiple mating was common. The data on males were sparse, but indicated sperm precedence and no relatedness among males breeding in the same colony. A comparison of social organizations and habitat requirements of M. rubra and closely related M. ruginodis suggested that habitat longevity and patchiness may be important ecological factors promoting polygyny in Myrmica. Received: 15 May 1995/Accepted after revision: 17 October 1995     

A survival-analysis-based simulation model for Russian wheat aphid population dynamics

A simulation model for Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), populations is built by integrating survival-analysis-based development and survivor functions and the same-shape reproduction distribution model in the framework of Leslie [Leslie, P.H., 1945. On the use of matrices in certain population mathematics. Biometrika 33, 183–212] matrix structure. Survival analysis is utilized to model both the development and survival of RWA populations, and the Cox (1972) proportional hazards model is fitted with the data sets from our laboratory observation of 1800 RWA individuals under 25 factorial combinations of five temperature regimes and five barley plant-growth stages. Rather than using simple age-specific survivor rates as in the traditional Leslie matrix, the survivor functions based on survival analysis describe age-specific, temperature and plant stage-dependent RWA survival probabilities. Similarly, a probability model from survival analysis to estimate the probability that an individual will reach mature adult stage is utilized to describe the development process; this makes the transition from nymphal stage to mature adult stage dependent on RWA age as well as temperature and plant-growth stage.Inspired by the same-shape distribution and rate-summation approach for modeling insect development, a similar approach for modeling insect reproduction under variable temperature is developed. This new same-shape reproduction distribution model incorporates individual variation in reproduction capability, as well as the effects of RWA age, temperature and plant-growth stage. Consequently, the same-shape reproduction distribution model replaces the simple age-specific fecundities in Leslie matrix model. To the best of our knowledge, this work is the first to introduce survival analysis to simulation modeling in entomology and ecology and also the first to integrate our newly developed same-shape reproduction distribution model into application.     

Population structure in the coral Pavona cactus: clonal genotypes show little phenotypic plasticity

Electrophoretic data were used to examine the relationship between genotype and growth form, and to assess the contribution of asexual reproduction to recruitment within six local populations of the agaricid coral Pavona cactus from the central and northern Great Barrier Reef. The data revealed the presence of highly replicated clonal genotypes in the five densest populations. In three cases, samples of 50 to 60 colonies collected from 25 m² areas within the Eclipse Island and Pandora Reef populations each consisted of colonies with only two distinct 4-locus genotypes. More intensive sampling of the population at Eclipse Island showed that colonies with the same 4-locus genotypes were separated by distances of up to 93 m. In contrast, the population at Watson's Bay (Lizard Island) consisted of a few widely scattered and genetically distinct colonies. The samples collected from each population contained a range of growth forms including, in some cases, the entire morphological range described for this species. A strong association of genotype and growth form was detected in samples from all populations, with the exception of Watson's Bay where no such comparison was possible. Nevertheless, some genotypes were represented by more than one growth form and this could reflect the effects of limited phenotypic plasticity. The effects of asexual reproduction reduced the value of these data as a test of the genetic connectedness of the six populations studied. The genetic distance between samples was not simply correlated with geographic distance. This may reflect either the true structure of the breeding population(s) or the effects of asexual reproduction on estimates of allelic frequencies.Contribution No. 47 from the Ecology and Genetics Group of the University of Wollongong     

Fissiparity and population genetics of Coscinasterias calamaria

The effect of asexual reproduction on the population genetics of the fissiparous seastar Coscinasterias calamaria was examined at Rottnest Island and the adjacent mainland, Western Australia, in 1985. Field samples and laboratory observations on growth rate and regeneration showed that fission is common at all 14 sites examined. Electrophoretic analysis of six polymorphic enzymes revealed low genotypic diversity and strong genetic disequilibrium at each site, confirming the highly clonal structure of local populations. Striking variation in clonal composition over distances as short as 50 m emphasizes the very localized subdivision of these populations. In the combined sample from all sites, however, the expected range of multilocus genotypes was found in the proportions expected from random mixing of sexually produced larvae, confirming that clonal diversity results from sexual reproduction. Local genotypic diversity was not correlated with fission, emphasizing the difficulty of determining the short-term roles of asexual and larval recruitment in the maintenance of populations. Subtidal populations, however, appear to have both lower incidence of fission and lower larval recruitment than do intertidal populations.     

Population dynamics of natural colonies of <Emphasis Type="Italic">Aurelia</Emphasis> sp. scyphistomae in Tasmania,Australia

The aim of this study was to identify potential environmental controls of the asexual phases of reproduction by measuring the rates of asexual reproduction (budding and strobilation) and mortality in naturally occurring populations of Aurelia sp. scyphistomae at different spatial and temporal scales. The percentage cover, density of colonies of Aurelia sp. scyphistomae, and density of the population of two naturally occurring colonies of Aurelia sp. scyphistomae were examined over 2 years in southern Tasmania. Artificial substrates were also deployed to investigate colony dynamics when density dependent effects were reduced. Clear spatial and temporal differences in the population dynamics of the colonies were observed. Density dependent effects controlled budding and recruitment of new scyphistomae to the substrate when populations were dense and space limiting. In contrast, environmental controls of budding and strobilation were more apparent in a colony with significantly greater area of bare substrate and hence room for expansion. Water temperature and rainfall (as a proxy for salinity) were linked to changes in population size. Annual and seasonal differences in population dynamics were not observed in a colony limited by space but were apparent in a colony where space was not limited. When space was removed as a limiting factor by deploying artificial substrates, a seasonal environmental effect on the rate of growth of the colony was observed. These studies suggest that the growth, survival and reproduction of the sessile colonial phase of Aurelia sp. is regulated by a combination of density dependent factors and environmental conditions, which are consequently important to the formation of jellyfish blooms.     

Introduction of the northern Pacific asteroid Asterias amurensis to Tasmania: reproduction and current distribution

The northern Pacific asteroid Asterias amurensis (Lütken) has been recently introduced to Tasmania and is now well-established in east and southeast Tasmania. This sea star is conspicuous throughout the Derwent River estuary and is particularly abundant in the Hobart port area. Reproduction of two populations of A. amurensis at Sullivans Cove and at Sandy Bay was investigated from August 1993 to October 1994 by gonadal histology and measurement of the gonad index (GI). An intense period of vitellogenic and spermatogenic growth started in April, with the breeding condition reached by June. In 1994, peak GI was recorded at Sullivans Cove in July and at Sandy Bay in August. Although these data indicated that the Sullivans Cove population spawned before the Sandy Bay population, histological examination revealed that major spawning activity occurred in both populations from July onwards. In the early part of the breeding season, gametogenesis proceeded in parallel with spawning, with released gametes being replaced by continual gametogenesis. Maintenance of a higher GI during the early part of the breeding season at Sandy Bay was due to prolonged gametogenic replacement at this site. Spawning continued to October 1994, resulting in a sharp decline in the GI. The cyclic expansion and regression of the genital haemal sinus coincident with germinal proliferation and growth, respectively, support the contention that haemal fluid provides nutrients for gametogenesis. In Tasmania, A. amurensis experiences temperature and photoperiod regimes similar to those experienced by endemic populations of this species in the north Pacific. Comparison of reproduction of A. amurensis in Japan and Tasmania at similar latitudes shows that gametogenesis of the northern and southern populations is 6 mo out of phase. This phase shift provides evidence for photoperiodic regulation of gametogenesis in A. amurensis. The similar trend in sea-temperature regimes associated with the stages of gametogenesis in northern and southern populations suggests that temperature also plays a modulatory role. Based on the periodicity of reproduction and the ontogeny of A. amurensis, the planktotrophic larvae of this species would be expected to be in the plankton for several months from the end of winter through summer. Considering the highly fecund, dispersive life-history of A. amurensis, the southeast Tasmanian populations have considerable potential to serve as a seed source for establishment of new populations of this asteroid elsewhere in Tasmania and in mainland Australia. Received: 18 October 1996 / Accepted: 22 October 1996     

Adult sex ratio affects divorce rate in the monogamous endoparasite Schistosoma mansoni

“Divorce” (mate switching) rate is known to vary largely both between and within socially monogamous species. Although the adult sex ratio can have an important influence on mating patterns, very few studies have investigated the influence of sex ratio on divorce rate in monogamous species, and even less so from an experimental point of view. In addition, most studies on the causes and consequences of divorce have been performed on vertebrate species, whereas data for invertebrate monogamous species remain scarce. Schistosoma mansoni is a monogamous endoparasite with a complex life cycle characterized by asexual reproduction in the intermediate host and sexual reproduction in the definitive host. In the wild, populations of S. mansoni inside their definitive hosts are characterized by a male-biased sex ratio. We studied the influence of experimentally varying the adult sex ratio on divorce rate in S mansoni, using controlled infections of hosts with clonal populations. The more male-biased the sex ratio was, the more the divorce rate increased, whereas no such effect was observed under a female-biased sex ratio. In this study and for the first time, we showed, by handling the sex ratio, that the divorce rate increases in adult male-biased sex ratio conditions in a monogamous species.     

Genetical structure within populations of the coral Pocillopora damicornis

Genetic variation of allozymes within populations of Pocillopora damicornis from southwestern Australia was consistent with a primary role of local asexual proliferation of clones in population maintenance. Populations were composed typically of two to four multilocus genotypes accounting for 40 to 80% of individuals, with the remainder assigned to genotypes occasionally in twos or threes but more commonly singly. In the three populations where recruitment was examined genetically, 84% of all first-year recruits was assigned to clones represented in the population's resident adults. The majority of these recruits came from the most highly-replicated of the adult clones. The observed genotypic diversity was, on average, about half that calculated to occur for the same allelic frequencies in a sexually-reproducing population with free recombination. Despite the prevalence of asexual reproduction, both through planulae and fragments, the existence of a sexual mode of reproduction was inferred from the high level of variation produced by pooling populations, the existence of novel genotypes and the concordance of clonal gene frequencies at many sites with the predictions of Hardy-Weinberg equilibria.     

Evidence for broadcast spawning as well as brooding in the scleractinian coral Pocillopora damicornis

The reproductive activity of 88 colonies of Pocillopora damicornis at Rottnest Island, Western Australia, was examined over 12 mo (20 August 1988 to 20 August 1989). Larvae were found close to the three new moons of February, March and April, 1989, but in large numbers only during March; 67% of all corals produced larvae. There was no obvious connection between gametogenesis and the production of brooded larvae; however, those corals which did not produce, larvae also did not produce ova but did produce, sperm, and must be considered male colonies. All colonies which produced ova went on to produce planulae. Spermatogenesis occurred throughout the year, but oogenesis occurred between September 1988 and April 1989. Dramatic declines in the percentage of polyps containing eggs and sperm occurred in early February and early April 1989. On these dates the amount of lipid in the tissue of the corals decreased, which suggests that the decline in the percentage of polyps with sperm and eggs was not due to the gametes being resorbed by the polyps. Therefore, P. damicornis at Rottnest Island appears to be both a brooder and a broadcaster with asexual reproduction through brooded larvae and sexual reproduction through spawning of gametes. This accounts for earlier evidence in the literature that some input from sexual reproduction had been occurring in the populations of P. damicornis at Rottnest Island. There is no direct evidence for spawning, as broadcasting of gametes has not been observed, but the indirect evidence provides a strong argument for the existence of this dual reproduction.     

Genetic variability and differentiation in the temperature niche component of the diatom Thalassiosira pseudonana

The acclimated reproduction rates of 14 clones of the diatom Thalassiosira pseudonana were measured at 12°, 16°, 20°, and 24°C. Reproduction rate increased monotonically with an increase in temperature in all 14 clones. Significant genetic variability in reproduction rates and electrophoretic mobility of isozymes were observed among clones within a population from a single water bottle collected in a warm core eddy overlying slope water. The major genetic differentiation was between neritic and oceanic populations, with neritic clones having higher reproduction rates at all 4 temperatures. Surprisingly small genetic differences were observed among clones from neritic waters around the world. The slopes of the temperature-reproduction rate plots of all 14 clones were quite similar.Woods Hole Oceanographic Institution Contribution No. 4679     

Genetic variability and differentiation of sporophytes and gametophytes in populations of Gelidium arbuscula (Gelidiaceae: Rhodophyta) determined by isozyme electrophoresis

Genetic differentiation and genetic variability of sporophytic and gametophytic populations of Gelidium arbuscula (Bory) from three localities sampled in 1989 and 1990 in the Canary Islands (Spain) were examined by isozyme electrophoresis. Twenty-three to 29 putative alleles corresponding to 22 gene loci, were compared. High deviations in Hardy-Weinberg equilibrium, and significant differences between allelic frequencies of sporophytic and gametophytic subpopulations at the same locality were found, suggesting a predominant asexual reproduction of G. arbuscula. The genetic variability (percentage of polymorphic loci, mean number of alleles per locus and average gene diversity) of haploid subpopulations was lower than that of diploid subpopulations at all three localities, being the lowest described for seaweeds. No correlation between genetic and geographical distance was found. The high genetic differentiation coefficient between all subpopulations suggests a very reduced genetic flow between subpopulations of the same and of different localities. These results suggest that the genetic structure of the populations of G. arbuscula from the Canary Islands is due to a founder-effect combined with a predominance of asexual reproduction. This is the first report comparing allelic frequencies between sporophytic and gametophytic subpopulations of seaweeds.     

Are direct developers more locally adapted than planktonic developers?

The hypothesis that populations of direct developers exhibit greater geographic differentiation in life history features than populations of planktonic developers, was tested with two species of grazing snails of the genus Littorina from 1986 to 1987. Littorina sitkana (direct developer) and L. scutulata (planktonic developer) coexist on sun- and wave-sheltered beaches from Alaska to Oregon, USA. Seasonal patterns in growth, survival and reproduction were monitored for samples from four geographically separated populations of each species grown in population cages at a common site, Friday Harbor, Washington, USA. The environmental and population effects on growth in the two species were determined in a four-way reciprocal transplant experiment with the same populations. Both the direct and planktonic developers exhibited geographic differentiation in life history features. Differentiation in the direct developer occurred over distances shorter than 30 km, while differentiation in the planktonic developer occurred over the 500 km distance examined (greater than their larvae would likely travel).     

Genetic structure of dictyoceratid sponge populations on the western Coral Sea reefs

Allozyme variation at six polymorphic loci was examined in foliose dictyoceratid sponges from isolated reefs in the western Coral Sea. Four major genetic groups corresponding to the species Phyllospongia lamellosa, P. alcicornis, Carterospongia flabellifera and Collospongia auris were examined. A further two rare morphotypes from individual reefs formed genetic outliers to the P. lamellosa group, and may represent further taxa related to P. lamellosa. Gene frequencies in individual reef populations were largely in Hardy-Weinberg equilibrium, suggesting that random mating occurred in local populations of all four common species. Genetic variability was high and observed heterozygosities within populations ranged from 0.13 to 0.40. All four taxa showed significant genetic differentiation among populations (F _ST=0.05 to 0.36). Genetic distances (Nei's D) among populations within species ranged from 0 to 0.723 and increased with increasing geographical separation. There was evidence that genetic differentiation between populations to the north and to the south of the southern limit of the South Equatorial Current (SEC) divergence was greater than expected on the basis of their geographical separation. The SEC divergence may form a partial barrier to gene flow among populations of these ecologically important sponges on the submerged Queensland Plateau. Levels of migration among populations of three of the species was less than those required to prevent divergence of the populations through genetic drift (Nm<1). Restricted migration among populations may provide a mechanism to explain the occurrence of highly divergent populations of dictyoceratid sponges whose specific identity is not clear, and may allow them additionally to develop partial reproduction isolation from other populations.     

Genetic subdivision in the subtidal,clonal sea anemone Anthothoe albocincta

Anthothoe albocincta, a common subtidal anemone along south-eastern Australia, reproduces both sexually through broadcast spawning and clonally through fission. Clones may be distinguished both by their electrophoretic genotypes and the colour of their tentacles and oral discs. Local populations typically consist of many, dense clonal aggregations. However, some clones appear to have locally extensive distributions, forming a series of separate aggregations. The capacity of clones to disperse among local populations is unknown. In this study we used an electrophoretic survey at six allozyme loci to quantify levels of variation among samples from each of 13 local populations and four geographic regions sampled between April 1992 and June 1993. These data revealed that populations of A. albocincta, separated by up to 930 km, were at least moderately subdivided. Levels of geographic variation were high and the average F _ST value (standardised genetic variation) was 0.27 based on genotypes of all individual polyps. This value reflects substantial variation both within (F _PR =0.13) and among (F _RT =0.16) regions. Estimates of gene flow among both neighbouring populations and regions are therefore low (N _e m=1.7 and 1.3, respectively). UPGMA (unweighted pair-group method using arithmetic averages) dendrograms suggest that a genetic discontinuity occurs at the very south-east corner of Australia, paralleling reports for two other south-eastern Australian marine invertebrates. In addition, our analyses and theoretical predictions imply that localised proliferation of clonal genotypes may have caused us to underestimate the potential importance of gene flow via larval dispersal. Moreover, the abundance and vast geographic range of this species suggests that widespread dispersal does occur. Collections from three populations covering the peroid December 1992 to June 1993 were examined by crude dissection along with histological sectioning, and showed A. albocincta to be dioecious, with unisexual clones. Eggs within the ovaries of six females sampled over a 3 mo period were small (96±4 m) and similar to those of related species that produce planktotrophic larvae. In contrast, we found no evidence that clones were dispersed (shared) among neighbouring local populations. An average of only 6% of six-locus genotypes were common to pairs of local populations separated by up to 125 km, this being equal to the percentage expected through sexual reproduction alone. In addition, the percentage of shared genotypes did not decline with increasing geographic separation. These data imply that although asexual reproduction may be used to maintain local populations, the sexual production of genotypically diverse larvae is the primary source of widely dispersed colonists and hence of new clones.     

Spatial autocorrelation analysis of small-scale genetic structure in a clonal soft coral with limited larval dispersal

The philopatric larval dispesal and small effective population sizes characteristic of many clonal species should promote the development of significant small-scale genetic structure within populations as a result of isolation-by-distance. We used spatial autocorrelation statistics to detect genetic structure, arising from both clonal reproduction and philopatric dispersal of sexual propagules, for five allozyme loci within populations of the soft coral Alcyonium sp. In a population on Tatoosh Island, Washington, USA, sampled in 1991/1992, we found significant positive spatial autocorrelation at all loci among individuals separated by <40 cm, reflecting the presence of significant smallscale genetic structure due to associations among clonemates. For 4 of 5 loci, however, we detected no significant spatial autocorrelation among the different clones within this population over distances of 1 to 40 m. Analysis of soft-coral populations from six additional, topographically diverse sites in the north-east Pacific also did not reveal significant spatial autocorrelation among clones at any loci. This general lack of spatial autocorrelation of genotypes among clones suggests that significant small-scale genetic structure has not arisen in populations of Alcyonium sp. as a consequence of isolation-by-distance.