A molecular approach to the ecology of a mussel (Mytilus edulis – M. trossulus) hybrid zone on the east coast of Newfoundland, Canada

Mytilus trossulus Gould and M. edulis L. coexist in mixed populations in Atlantic Canadian waters. Although there is evidence that the two species hybridize in natural populations and that hybrids produce progeny through backcrosses, no study of the microgeographic distribution of the two forms and their hybrids has been made. Here we examine subtidal samples of mussels taken in July 1997 from two locations in eastern Newfoundland (Canada) and from wave-exposed and protected environments within each location. Shell lengths ranged from 15 to about 90 mm. Mussels were classified as pure forms or hybrids (F₁, F₂ and from backcrosses) based on four diagnostic markers, two allozyme loci (Mpi and Est-D) and two nuclear PCR-based DNA markers (ITS and Glu-5). In addition, a PCR-based mtDNA marker (COIII) was used to characterize the distribution of mtDNA mitotypes among pure and hybrid individuals. There were differences in the proportions of pure M. edulis and M. trossulus between sites and between environments at one location. M. trossulus was the predominant species at one of the two exposed sites. In all four samples, M. trossulus was also the predominant form among small individuals. The frequency of hybrids was 26% overall and did not differ among samples. Hybrids consisted mostly of backcrosses that were M. trossulus-biased among small mussels and M. edulis-biased among large ones. We conclude that both intrinsic genetic factors and extrinsic environmental factors influence the relative frequency of M. edulis, M. trossulus and their hybrids. Received: 29 June 1998 / Accepted: 4 November 1998     

Coexistence and reproductive isolation of the sympatric echinoids<Emphasis Type="Italic"> Diadema savignyi</Emphasis> Michelin and<Emphasis Type="Italic"> Diadema setosum</Emphasis> (Leske) on Kenyan coral reefs

The sympatric echinoids Diadema savignyi and D. setosum coexist in shallow reef lagoons throughout East Africa. The reproductive strategies of these echinoids were studied to investigate reproductive isolation as a possible mechanism for maintaining the coexistence of these closely related species. The annual reproductive cycle and lunar periodicity were determined by gonad index measurements, histological examination of gametogenesis, and induction of spawning with injections of KCl. The peak reproductive period of D. savignyi coincided with the north-east monsoon period (when light and temperatures are high) as gonad indices were high (>8%) beginning in February and peaked at 9.7% in May. Gonad indices subsequently rapidly decreased (by 26%) in June at the beginning of the cooler south-east monsoon period. However, the presence of sperm and ova in most months of the year indicates continuous gametogenesis with reduced reproductive effort during the cooler months. The annual cycle for D. setosum showed less of a seasonal trend as gonad indices remained above 7% throughout much of the year but tended to be highest when temperatures were lower. This is the first confirmation of continuous reproduction in these two species at the equator. The reproductive patterns of both species remained consistent over 2 years of sampling. Both species exhibited a synchronized lunar spawning periodicity during the 3 months sampled, with D. setosum spawning on lunar days 8–10 and D. savignyi spawning after the full moon (lunar days 17–18). Whereas spawning in D. savignyi was very tightly synchronized, 20% of D. setosum individuals still spawned after the peak spawning period. The coexistence of these closely related species appears to be maintained by temporal reproductive isolation during the lunar spawning period reinforced by seasonal differences in reproductive effort.Communicated by O. Kinne, Oldendorf/Luhe     

Effect of hybridization with genome exclusion on extinction risk

Human‐induced habitat changes may lead to the breakdown of reproductive barriers between distantly related species. This phenomenon may result in fertile first‐generation hybrids (F₁) that exclude the genome of one parental species during gametogenesis, thus disabling introgression. The species extinction risk associated with hybridization with genome exclusion is largely underappreciated because the phenomenon produces only F₁ hybrid phenotype, leading to the misconception that hybrids are sterile and potentially of minor conservation concern. We used a simulation model that integrates the main genetic, demographic, and ecological processes to examine the dynamics of hybridization with genome exclusion. We showed that this mode of hybridization may lead to extremely rapid extinction when the process of genome exclusion is unbalanced between the interbreeding species and when the hybridization rate is not negligible. The coexistence of parental species was possible in some cases of asymmetrical genome exclusion, but show this equilibrium was highly vulnerable to environmental variation. Expanding the exclusive habitat of the species at risk allowed its persistence. Our results highlight the extent of possible extinction risk due to hybridization with genome exclusion and suggest habitat management as a promising conservation strategy. In anticipation of serious threats to biodiversity due to hybridization with genome exclusion, we recommend a detailed assessment of the reproductive status of hybrids in conservation programs. We suggest such assessments include the inspection of genetic content in hybrid gametes.     

Genetic differentiation and reproductive incompatibility among Baja California populations of the copepod Tigriopus californicus

Populations of the harpacticoid copepod Tigriopus californicus were sampled from five sites from San Diego, California, to Playa Altamira, Baja California, Mexico. Allozyme analyses revealed that all the study populations are sharply differentiated genetically. At the extreme, two populations, Punta Baja and Playa Altamira, have no alleles in common at the seven allozyme loci studied. All pairwise interpopulation crosses successfully produced F₂ hybrids except those involving the Playa Altamira population. All crosses using Playa Altamira females failed to produce F₁ hybrids, while Playa Altamira males successfully produced F₁ progeny with females from all other sites. These F₁ offspring, however, were completely sterile (with San Diego and Punta Banda females) or only occasionally produced F₂ offspring (with Punta Morro and Punta Baja females). These results suggest that allopatric differentiation among Baja populations has resulted in exceptionally high levels of genetic divergence and nearly complete reproductive isolation of the Playa Altamira population, which should now be recognized at the semispecies (or perhaps sibling species) level.     

Phenotypic assortative mating and genetic population structure in the crab Trapezia digitalis

Colonies of Pocillopora spp. containing the crab Trapezia digitalis were collected from Hawaii in summer of 1979 and of 1980. Individuals of T. digitalis in Hawaii vary extensively in coloration of the carapace: crabs were separated into phenotypic classes by their degree of carapace reticulation. When presented with two females of different phenotype, males preferentially paired with the female which was similar to them in reticulation. The genetic population structure of T. digitalis was unusual by comparison with other species of Trapezia. Previously reported homozygote excesses at esterase-4, hexokinase-1, phosphoglucoseisomerase, and general protein-2 cannot be explained by the single-locus Wahlund effect, since there was not significant genetic differentiation among phenotypic classes, as measured by Wright's F _ST statistic. There was significant variation among classes in allele frequencies at a fifth locus, hexokinase-2. Two pairs of loci were in significant gametic disequilibrium. Gametic disequilibrium was not detected in seven other species of Trapezia. It is suggested that multi-locus genetic analysis and an increased geographic sampling range are needed to determine the genetic population structure of T. digitalis in relation to observed variation in coloration.     

Genetic evidence for hybridisation between Haliotis rubra and H. laevigata

Seventeen abalone whose appearance was intermediate between the two commonly fished species in south-eastern Australia, blacklip (Haliotis rubra) and greenlip (H. laevigata), were collected between 1986 and 1989. Protein electrophoresis revealed that rare, morphologically intermediate abalone are predominantly F₁ hybrids; others are backcross individuals. There is also evidence of possible local introgression of H. rubra genes into H. laevigata and vice versa in sympatric populations. These two species exhibit marked morphological, behavioural and genetic differences, and yet hybridisation, although rare, occurs in at least several regions of sympatry. The commonly invoked explanations for the breakdown of species barriers, environmental disturbance and rarity of one of the parent species, do not appear applicable, unless microhabitat separation creates local rarity of one species. The frequency of hybridisation is discussed in relation to regional biogeography.     

Reproductive isolation between populations from Northern and Central Europe of the leaf beetle Chrysomela lapponica L.

Summary. Allopatric populations of the leaf beetle Chrysomela lapponica are known to feed upon either willow (Salicaceae) or birch (Betulaceae). This study aimed to elucidate the differentiation process of these allopatric populations. We investigated whether these allopatric populations specialized on different host plants are still able to produce fertile offspring when interbreeding. Individuals from a population in Finland (willow specialists) and one in the Czech Republic (birch specialists) were crossed in laboratory. Hybrid formation succeeded only between females from the Czech, birch specialized population and males from the Finnish, willow specialized population, while no eggs were produced by females of the willow specialists having mated males of the birch specialists. Behavioral, morphological, physiological, and chemical features of the F₁ hybrids were studied. The chemical composition of larval defensive secretion and feeding preferences of the resulting F₁ hybrids mainly showed similarities with the paternal phenotype, while the area of black coloring on the offspring’s elytra was intermediate between those of the parental elytra. F₁ hybrids did not accept the host plant (birch) of their mothers for feeding and only survived on willow. Thus, since mothers only lay eggs on birch, we found evidence for a postzygotic isolation mechanism between the individuals of the two investigated populations: when having been mated with willow specialized Finnish males, the birch specialized Czech mothers place the hybrid eggs on a plant species (birch), on which the hatching larvae cannot survive.     

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.     

Population genetics of the nurse shark (Ginglymostoma cirratum) in the western Atlantic

The nurse shark, Ginglymostoma cirratum, inhabits shallow, tropical, and subtropical waters in the Atlantic and the eastern Pacific. Unlike many other species of sharks, nurse sharks are remarkably sedentary. We assayed the mitochondrial control region and eight microsatellite loci from individuals collected primarily in the western Atlantic to estimate the degree of population subdivision. Two individuals from the eastern Atlantic and one from the Pacific coast of Panama also were genotyped. Overall, the mtDNA haplotype (h = 48 ± 5%) and nucleotide (π = 0.08 ± 0.06%) diversities were low. The microsatellite data mirror the mitochondrial results with the average number of alleles ([`(N)]<sub>A</sub> \bar{N}_{A}  = 9) and observed heterozygosity ([`(H)]_O \bar{H}_{O}  = 0.58) both low. The low levels of diversity seen in both the mtDNA and the microsatellite may be due to historical sea level fluctuations and concomitant loss of shallow water habitat. Eight of the 10 pair-wise western Atlantic F _ST estimates for mtDNA indicated significant genetic subdivision. Pair-wise F _ST values for the microsatellite loci indicated a similar pattern as the mtDNA. The western Atlantic population of nurse sharks is genetically subdivided with the strongest separation seen between the offshore islands and mainland Brazil, likely due to deep water acting as a barrier to dispersal. The eastern and western Atlantic populations were closely related. The eastern Pacific individual is quite different from Atlantic individuals and may be a cryptic, sister species.     

Genetic structure and allozyme variation of sea bass (Dicentrarchus labrax and D. punctatus) in the Mediterranean Sea

This paper reports data on 28 allozyme loci in wild and artificially reared sea bass (Dicentrarchus labrax) samples, originating from either coastal lagoon or marine sites in the Mediterranean Sea. F _ST analysis (θ estimator) indicated strong genetic structuring among populations; around 34% of the overall genetic variation is due to interpopulation variation. Pairwise θ estimates showed that, on average, the degree of genetic structuring was much higher between marine populations than between samples from lagoons. Six polymorphic loci showed differences in allele frequencies between marine and lagoon samples. Multivariate analyses of individual allozymic profiles and of allele frequencies suggested that different arrays of genotypes prevail in lagoons compared to marine samples, particularly at those loci that, on the basis of previous acclimation experiments, had been implicated in adaptation to freshwater. On the other hand, variation at “neutral” allozyme loci reflects to a greater extent the geographic location of populations. Allozyme differentiation was also studied in a D. labrax population from the Portuguese coast. Average genetic distance between this population and the Mediterranean populations was quite high (Nei's D = 0.236) and calls into question the taxonomic status of the Portuguese population. Finally, genetic relationships between D. labrax and D. punctatus were evaluated. Average Nei's D was 0.648, revealing high genetic differentiation between the two species, even for two sympatric populations of these species in Egypt; thus gene flow was not indicated between species. Received: 24 October 1996 / Accepted: 27 November 1996     

Genetic differentiation in Hediste diversicolor (Polychaeta: Nereididae) for the North Sea and the Baltic Sea

Genetic differentiation between North Sea and Baltic Sea Hediste diversicolor (O.F. Müller, 1776) (Polychaeta: Nereididae) populations was studied by allozyme electrophoresis on starch gel. Thirteen loci were analyzed in eight populations. The level of genetic variation was very low (mean H _o = 0.000 to 0.015). Differentiation between H. diversicolor populations is quite high (F _ST = 0.892) and reflected by three enzyme loci (MDH-I*, MDH-II*, IDH-I*). The reduced gene flow (N _m<1) may be explained by the limited dispersal capacity of the species. Regardless of whether found in the North Sea or Baltic Sea, there appear to be two different genetic types which are parapatric or sympatric in some places. The two types hybridize at three localities, but no signs of hybridization have been found at one (Tallinn). Received: 26 June 1997 / Accepted: 10 September 1997     

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 genetics and systematics of theLeptasterias hexactis (Echinodermata: Asteroidea) species complex

Morphological characters and 14 enzyme loci were examined for 1 040 sea stars, currently recognized as forms ofLeptasterias hexactis, from Lynn Canal, Alaska, and Puget Sound, Washington, USA, between March 1988 and April 1989. Three morphologically and two genetically distinctLeptasterias forms were identified. The most common form found at both localities wasL. epichlora (Brandt)sensu Verrill.L. hexactis (Stimpson)sensu Verrill co-occurred withL. epichlora at all study sites and apparently hybridizes extensively withL. epichlora in the Puget Sound region, but rarely, if at all, in Alaska. The presumptive product of this hybridization morphologically resembledL. aequalis (Stimpson)sensu Fisher, and was conspicuously absent from Alaskan samples. Considerable genetic distance existed betweenL. epichlora andL. hexactis (Nei'sD=0.19±0.01) and moderate genetic differentiation occurred between populations of each species from Alaska and Washington (Weir and Cockerham'sF _RT =0.29±0.04 forL. epichlora and 0.21±0.15 forL. hexactis). A significant (p<0.05) deficiency in the proportion of heterozygous individuals was found compared to Hardy-Weinberg expectations (Wright's fixation index,F _ID=0.12±0.04 and 0.31±0.08 forL. epichlora andL. hexactis, respectively). However, mean observed heterozygosity for each species (0.09±0.03, 0.14±0.04 and 0.14±0.04 forL. epichlora, L. hexactis andL. aequalis, respectively) fell within the range of reported values for other asteroid species (ca. 0.04 to 0.37). The results of this study indicate that considerable genetic integrity is maintained betweenL. epichlora andL. hexactis, which warrants their recognition as distinct species despite their apparent hybridization in the Puget Sound region.     

Electrophoretic patterns of serum proteins in the blood of four species of puff-fish,genus Fugu,and their F1 hybrids

In a series of hematological investigations to analyze species characteristics and ecological factors controlling the distribution of marine fish, comparisons of the electrophoretic patterns of the serum proteins in the blood of four species of puff-fish (Fugu rubripes, F. xanthopterus, F. poecilonotus and F. pardalis) and two artificial F₁ hybrids (male F. rubripesxfemale F. xanthopterus; female F. poecilonotusxmale F. pardalis) were made by using the cellulose acetate membrane technique. The number of components, as well as the position and ratio of each component, differed from species to species and with sex. The serum protein pattern of the F₁ hybrid was much the same as that derived from the arithmetic mean of the parental patterns. The opening made in a portion of the abdomen to sample the blood directly from the auricle could be sutured successfully by biotissue glue Biobond.Contribution No. 220 from the Seikai Regional Fisheries Research Laboratory, Nagasaki.     

Asymmetric mate choice,hybridization, and hybrid fitness in two sympatric grasshopper species

Hybridization between animal species has long been regarded as unusual, but is meanwhile accepted as a widespread phenomenon. Typically, sexual interactions among species are studied in secondary contact zones of closely related species (hybrid zones) or between invasive and native species, whereas hybridization between sympatric congeners has received little attention. Here, we present a study on the hybridization potential of two grasshopper species, Chorthippus parallelus and Chorthippus montanus, which occur sympatric in large parts of Eurasia. We performed a mate choice experiment with specimens from an area of local parapatry in northwestern Germany. Most copulations were conspecific, but males of both species and females of C. parallelus were indiscriminate in their mate choice, while females of C. montanus clearly preferred conspecific males. Although these results suggest asymmetric introgression, a no-choice hybridization experiment revealed that hybridization is possible in both directions. The hatching success of the hybrids was intermediate between the parental species. Female hybrids showed no clear mate preferences, indicating that back crossing in both directions is possible in principle, but the fertility of the hybrids and the fitness of the F2 generation remain unknown. Our study suggests that hybridization between sympatric species might occur more often than assumed. It might affect the ecology and local distribution of animals in a similar way as competition does.     

The use of nondestructive methods to assess a physiological status and conservation perspectives of <Emphasis Type="Italic">Eryngium maritimum</Emphasis> L.

Eryngium maritimum is a perennial species growing exclusively in a coastal littoral zone both on sand dunes and shingle beach and indicated as declining in Northern Europe. The objective of the present study was to prove the use of nondestructive physiological measurement methods to access physiological status of endangered plants, using E. maritimum as a model species. Plants from two Latvian sites were studied in comparison with other populations in Northern Europe (Estonia, Lithuania, Poland, United Kingdom) to find out if local differences in environmental factors affect vitality of E. maritimum individuals. Noninvasive chlorophyll analysis and chlorophyll a fluorescence measurements were used as indicators of plant physiological status through characterization of various aspects of photochemistry of photosystem II activity. Dynamics and morphology of individuals were investigated for evaluation of clonal growth potential of E. maritimum in natural conditions. Highly fluctuating trend of dynamics of individuals (within 40 groups at two Latvian sites) was established for E. maritimum in natural conditions over a five-year period. Disturbance of individuals lead to formation of new shoots from nodal root meristems. An exponential regression between fluorescence parameters F_V/F_M and Performance Index and summary monthly precipitation was found indicating that E. maritimum plants had significant tolerance to water shortage together with susceptibility to increased precipitation. Negative correlation between leaf chlorophyll content and more northern localization of individuals was found (r = 0.95 and r = 0.94, for generative and vegetative shoots, respectively). Northern populations of E. maritimum are endangered by high precipitation in conditions of low air temperature, leading to decrease of photosynthetic productivity and overall physiological status. High developmental plasticity at the root level leads to clonal growth and an efficient survival and relatively long life span of E. maritimum individuals. Chlorophyll a fluorescence is a useful method to search for the effect of suboptimal conditions on physiological status of endangered plant species without elimination and disturbance of individuals.     

Ecological aspects of ophiuroids from the phytal of S. W. Atlantic Ocean warm waters

Nine species of ophiuroids — Amphipholis januarii, Axiognathus squamatus, Hemipholis clongata, Ophiocnida scabriuscula, Ophiactis lymani, Ophiactis savignyi, Ophionereis reticulata, Ophioderma appressum and Ophiothrix angulata —were collected in various habitats from the mid-littoral and upper infralittoral on the northern coast of the State of São Paulo, Brazil. The abundance of 6 species occurring in 28 species of algae was investigated. A total of 220 samples was collected at monthly intervals from April, 1969 to May, 1970. The distributional pattern of the populations, as determined by Mobisita's index, shows aggregation of the ophiuroids. No ophiuroid species was limited to one algal species. Each species prefers certain algal species and/or a different level on the shore. The most abundant species on algae was Ophiactis lymani, with a maximum density of 1,510 individuals/100 ml of algae. Four species of ophiuroids occurred inside sponges. The most abundant species in sponges was O. savignyi, with a maximum density of 1,892 individuals/100 g of dry sponge. Two species of ophiuroids occurred inside the bryozoan Schizoporella unicornis. Feeding observations have shown Axiognathus squamatus to capture its food by mucous nets. Decapod crustaceans fed on ophiuroids in aquaria. The copepod Thespesiopsyllus sp. was found in the body cavity of the 2 Ophiactis species. Simultaneous sexual and asexual reproduction were abserved in O. savignyi.     

Population genetics of eight species of Trapezia (Brachyura: Xanthidae), symbionts of corals

Starch-gel electrophoresis was used to study gene-enzyme variation in thirteen populations of eight species of the genus Trapezia from Hawaii, Panamá, and Enewetak Atoll (Marshall Islands). Between 20 and 30 (mean = 27.8) gene-enzyme systems were resolved in each population, with 20 systems in common among all populations. The distribution of electrophenotypes was in agreement with Hardy-Weinberg-Castle expectations, except for T. digitalis, which consistently showed heterozygote deficiencies. Diagnostic loci among color forms support the hypothesis that color forms are distinct species. Low values of genetic distance among species suggest a recent radiation, perhaps during the Pleistocene. Genetic distance between the Hawaiian and Panamanian populations of T. ferruginea did not significantly differ from zero, indicating that the Eastern Pacific population of T. ferruginea has recently immigrated from the central Pacific, and/or that there is gene flow between the two areas. There were diagnostic loci between T. ferruginea and T. formosa from Enewetak and the populations of these species from Hawaii (T. ferruginea only) and Panamá (both species). Therefore, these geographic populations may represent separate species. The level and pattern of genetic variability in Trapezia spp. are in agreement with those observed in most other organisms.     

Genetic consequences of long larval life in the starfish Linckia laevigata (Echinodermata: Asteroidea) on the Great Barrier Reef

Gene flow between populations of the asteroid Linckia laevigata (Linnaeus) was investigated by examining over 1000 individuals collected from ten reefs throughout the Great Barrier Reef (GBR), Australia, for genetic variation at seven polymorphic enzyme loci. Despite geographic separations in excess of 1000 km, Nei's unbiased genetic distance (0 to 0.003) and standardised genetic variation between populations (F _ST) values (mean 0.0011) were small and not significant. Genetic homogeneity among L. laevigata populations is consistent with the long-distance dispersal capability of its 28 d planktonic larval phase, and is greater than that observed for other asteroid species, including another high-dispersal species, Acanthaster planci, which has a 14 d larval phase. Variation within populations was also higher than previously recorded for asteroids (mean heterozygosity=0.384; number of alleles per locus ranged from 5.1 to 6.0 in each population). Among asteroids, dispersal ability is positively correlated with gene flow and levels of variation, and negatively correlated with levels of differentiation.     

Genetic structure of populations of two closely related brittle stars with contrasting sexual and asexual life histories,with observations on the genetic structure of a second asexual species

Ophiocoma pumila Lütken andOphiocomella ophiactoides (H. L. Clark) are morphologically similar brittle stars with contrasting life histories, the former obligately sexual, the latter fissiparous (capable of both sexual reproduction and asexual proliferation by binary fission). Electrophoretic analysis of five polymorphic enzymes was used to assess the genetic consequences of these differing life histories and provide a genetic perspective on the taxonomic relationship between the two species. Genotypic diversity ofOphiocoma pumila collected at Discovery Bay, Jamaica, in 1985 conformed to expectations for a sexually reproducing population. In contrast, genotypic diversity ofOphiocomella ophiactoides at this site was significantly lower than expected for a sexually reproducing population, due largely to the predominance of clonal proliferation over larval recruitment. Large variation in clonal composition over a short (50 m) distance emphasized the very localized scale of clonal mixing in this species. Allozymic data are indicative of a close sibling species relationship betweenOphiocoma pumila andOphiocomella ophiactoides which suggests that the present generic separation of the two species should be re-examined. Electrophoretic analysis was also used to examine the genetic structure of sponge- and alga-dwelling populations of a second fissiparous brittle star,Ophiactis savignyi (Müller & Troschel), which was also collected at Jamaica in 1985. Striking differences in the allelic composition of sponge- and alga-dwellingO. savignyi were observed. Genotypic diversity ofO. savignyi in sponges was very low, each sponge being dominated by a single genotype. Genotypic diversity ofO. savignyi in algae was higher, although still significantly lower than expectations for a sexually reproducing population. In the light of the highly clonal composition of fissiparous brittle-star populations, the adaptive significance of clonal growth may be related to an increase in the overall fitness of dispersed clones (genets), compared to individuals of strictly sexual counterparts, through greater genotypespecific biomass and, hence, fecundity.Contribution No. 476 of the Discovery Bay Marine Laboratory of the University of the West Indies