Mitochondrial phylogeography of the killifish Aphanius fasciatus (Teleostei, Cyprinodontidae) reveals highly divergent Mediterranean populations

We studied the genetic structure of the Mediterranean killifish Aphanius fasciatus. Analysis of the sequence variation in a 372-bp portion of the mitochondrial control region in 623 fish from 27 sampling sites along the species’ distributional range (Tyrrhenian coast, Sardinia, Sicily, Adriatic coast, Malta, Tunisia, and Greece) yielded 120 distinct haplotypes. Most of the haplotypes are unique, and only 15 % are shared among different populations. The high F _ST value (=0.80) suggests a strong population genetic structuring. The phylogenetic analysis based on Bayesian inference, maximum likelihood and maximum parsimony, and the median-joining network show a sharp separation of the Southeastern Sicilian populations (belonging to the Hyblean region) and of the fluvial Tunisian population of Rio Melah from the others. The Adriatic, the Eastern Sicilian, and the Greek populations are well differentiated, while the group of populations from the Central-Western Mediterranean does not show a clear pattern of differentiation. Our findings indicate that the current genetic structuring of A. fasciatus reflects historical geographical patterns occurring within the Mediterranean basin from the Late Miocene to the Pleistocene. The presence of divergent evolutionary entities in the Hyblean region and the Tunisian Rio Melah supports their inclusion as target areas for the conservation of A. fasciatus.     

Phylogeographic study of the dwarf oyster, Ostreola stentina, from Morocco, Portugal and Tunisia: evidence of a geographic disjunction with the closely related taxa, Ostrea aupouria and Ostreola equestris

Despite the economic importance of oysters due to the high aquaculture production of several species, the current knowledge of oyster phylogeny and systematics is still fragmentary. In Europe, Ostrea edulis, the European flat oyster, and Ostreola stentina, the Provence oyster or dwarf oyster, are both present along the European and African, Atlantic and Mediterranean, coasts. In order to document the relationship not only between O. stentina and O. edulis, but also with the other Ostrea and Ostreola species, we performed a sequence analysis of the 16S mitochondrial fragment (16S rDNA: the large subunit rRNA-coding gene) and the COI fragment (COI: cytochrome oxidase subunit I). Oysters were sampled from populations in Portugal (two populations), Tunisia (two populations) and Morocco (one population), identified as O. stentina on the basis of shell morphological characters. Our data supported a high degree of differentiation between O. stentina and O. edulis and a close relationship between O. stentina and both Ostrea aupouria (from New Zealand) and Ostreola equestris (from Mexico Gulf/Atlantic). The status of this geographic disjunction between these closely related species is discussed. Furthermore, although identified in a separate genus Ostreola by Harry (Veliger 28:121–158, 1985), our molecular data on O. stentina, together with those available for the other two putative congeneric species, O. equestris and Ostreola conchaphila, would favour incorporation of Ostreola in Ostrea. Finally, a PCR-RFLP approach allowed the rapid identification of O. edulis and O. stentina.     

Phylogeography and genetic structure of the Mediterranean killifish <Emphasis Type="Italic">Aphanius fasciatus</Emphasis> (Cyprinodontidae)

The purpose of this study was to determine the phylogeographic structure of the brackish-hypersaline cyprinodont fish Aphanius fasciatus (Valenciennes, 1821), using sequencing and RFLP analysis of a 1,330 bp mitochondrial DNA segment containing part of the 16S rRNA gene as well as the genes for tRNA-Leu, NADH subunit 1 and tRNA-Ile. Individuals were collected from 13 different sites in Greece and Turkey, while seven published A. fasciatus sequences were also included to cover the area of distribution of the species. Pairwise sequence divergence values ranged from 0 to 4.51%. Congruent phylogenies were recovered with maximum likelihood, maximum parsimony and neighbour-joining methods. All analyses revealed two main groups. The first group consists of populations from almost all localities that drain into the Aegean Sea. The second group comprises the remaining population samples, which in some cases seem to consist of population-specific subgroups. Our results show that vicariant events have predominantly affected the evolution of A. fasciatus, with the Messinian salinity crisis having shaped the present genetic structure of its populations. Additionally, the life-history traits of the species, which determine a low potential for dispersal, coupled with the typical fragmentation of brackish-hypersaline water habitats have led to a high degree of isolation of A. fasciatus populations, even at restricted spatial scales. Analysis of the partitioning of the total amount of polymorphism with analyses of molecular variance (AMOVA) gave a value of F _ST = 84.6%. Potential conservation policies concerning A. fasciatus should also consider the low-genetic variability in the majority of its populations and the presence of fixed haplotypes in some of them.     

Allozyme variation in European populations of the oyster Ostrea edulis

Variations at 22 enzyme coding loci were surveyed in 11 populations of the oyster Ostrea edulis L., which were sampled between 1988 and 1990 along the Atlantic and Mediterranean coasts of Europe. Atlantic oyster beds suffered a steady decline during the last century, and restocking of beds with oysters of foreign origin has probably resulted in a high degree of interbreeding of natural oyster stocks from all Atlantic Europe. Our study confirms the low levels of genetic variability previously reported for the oyster populations from the Atlantic coasts, and extends it to the Mediterranean coasts. The locus arginine-kinase (ARK ^*) exhibited a high degree of interpopulation differentiation (F _ST=0.289), resulting from extensive variation in gene frequencies along a geographical cline. However, the overall genetic differentiation between populations was slight, and similar to that reported for other local populations of bivalves (mean genetic distance between populations is 0.010, mean F _ST=0.062). A general pattern of increasing differentiation along the coastline in an Atlantic-mediterranean direction emerged; but genetic differentiation among the Atlantic populations was not significantly lower than that observed among the Mediterranean populations. This and other results suggest that the effects of extensive transplantation of oysters among various areas in Europe are detectable only in some particular localities. The geographical distribution of low-frequency alleles suggests a restriction to gene flow outwards from the Mediterranean Sea, across the Straits of Gibraltar.     

Phylogeographic patterns of the mysid Mesopodopsis slabberi (Crustacea, Mysida) in Western Europe: evidence for high molecular diversity and cryptic speciation

The phylogeographic patterns among populations of Mesopodopsis slabberi (Crustacea, Mysida), an ecological important mysid species of marine and estuarine habitats, were analysed by means of DNA sequencing of a fragment of the mitochondrial cytochrome c oxidase subunit I (COI) and the 16S ribosomal RNA genes. Samples of M. slabberi collected from five Atlantic and two Western Mediterranean populations were investigated. Very high levels of within-population molecular diversity were observed in all samples (mean h=0.807 and π=0.0083), with exception of the Mediterranean Ebro population which contained only one haplotype. Differentiation among populations was high, and a clear phylogeographic break was observed between the Atlantic and Mediterranean populations. Moreover, a strong differentiation was detected between both populations in the Western Mediterranean Sea (Alicante and Ebro delta), while two divergent lineages occurred in sympatry within the Atlantic Mondego estuary. The high congruence between both the COI and 16S rRNA sequence data, the reciprocal monophyly of the different mitochondrial clades and the levels of nucleotide divergence between them suggest the presence of a complex of cryptic species within M. slabberi. Estimations of divergence time between the different mitochondrial lineages indicate that a split occurred during the late Miocene/early Pliocene. Such a divergence could be concordant with vicariant events during sea-level drops within the Mediterranean region at that time. However, within the Mediterranean Sea, the potential of divergence through ecological diversification cannot be ruled out.     

Phylogeny and geographic differentiation of Atlanto–Mediterranean species of the genus Xantho (Crustacea: Brachyura: Xanthidae) based on genetic and morphometric analyses

The crab genus Xantho Leach, 1814 is restricted to the northeastern Atlantic Ocean and the Mediterranean Sea. It consists of four species, Xantho hydrophilus (Herbst, 1790), X. poressa (Olivi, 1792), X. pilipes A. Milne-Edwards, 1867, and X. sexdentatus (Miers, 1881). X. hydrophilus has been divided into two geographic forms, of which one, X. h. granulicarpus (Forest, 1953), is postulated to be endemic to the Mediterranean Sea. In this study, we reconstruct phylogenetic relationships of the genus Xantho and related genera from the Atlantic Ocean or Mediterranean Sea and compare different geographic populations of Xantho hydrophilus and, to a lesser extent, of X. poressa by means of population genetic and morphometric analyses. The molecular phylogeny is based on two mitochondrial genes (large subunit rRNA and cytochrome oxidase I) and indicates that X. poressa, X. hydrophilus and X. sexdentatus form a monophyletic group, the latter two species sharing identical haplotypes. On the other hand, X. pilipes shows affinities to Xanthodius denticulatus. Population genetics based on the COI gene reveal genetic differentiation within X. hydrophilus. Morphometric results also give evidence for distinct geographic forms in X. hydrophilus with a clear discrimination. In comparison, morphometric discrimination between different geographic populations of X. poressa is less clear, but still significant. We therefore suggest a recent/ongoing morphological and genetic differentiation within Xantho hydrophilus, restricted gene flow between its Atlantic and Mediterranean populations (not allowing subspecific differentiation) and possible mtDNA introgression between the species X. hydrophilus and X. sexdentatus.     

When does bearing magnets affect the size of deflection in clock-shifted homing pigeons?

Pigeons whose internal clock is shifted by 6 h show deflections from the direction of untreated controls, yet these deflections are often smaller than predicted. Magnets temporarily disabling the magnetic compass increased these the deflections significantly (R. Wiltschko and Wiltschko 2001), indicating a compromise between sun compass and magnetic compass. – Recently, Ioalé et al. (2006) claim that they could not replicate our findings. The reason lies in a difference in the behavior of the clock-shifted pigeons without magnets: in the study of Ioalè et al. (2006), their deflections was already almost as large as that of our pigeons carrying magnets. This difference is probably caused by the limited experience of the pigeons of Ioalè et al. (2006): Their birds, in contrast to ours, had not used their sun’ compass during extended homing flights at various times of the year and, not having been faced with the necessity to compensate the saisonal changes of the sun’s arc, gave the sun compass more weight than our birds did.A comment to the paper by Ioalè, Odetti and Gagliardo (2006) Behav Ecol Sociobiol 60: 516–521.     

Genetic structure of populations of two species of Chthamalus (Crustacea: Cirripedia) in the north-east Atlantic and Mediterranean

Protein electrophoresis on starch gels was used to investigate population genetic structure of the barnacles Chthamalus montagui Southward and C. stellatus (Poli) over their north-east Atlantic and Mediterranean ranges. In each species, a single locus exhibited marked differentiation of allele frequencies between Atlantic and Mediterranean localities; in C. stellatus, genetic differentiation between the two basins had not previously been noted. In both species, mean heterozygosity per locus appeared higher in the Mediterranean samples than in the Atlantic, and Mediterranean populations had more alleles at the loci studied. Possible explanations for the differentiation between the Atlantic and the Mediterranean populations are discussed. Received: 30 May 1996 / Accepted: 17 September 1996     

Current and historic distribution and abundance of the inarticulated brachiopod, <Emphasis Type="Italic">Lingula reevii</Emphasis> Davidson (1880), in Kaneohe Bay,Oahu, Hawaii,USA

The inarticulated brachiopod, Lingula reevii Davidson (1880) is a filter-feeding invertebrate that burrows vertically in sandy or mixed sediments. Its only recorded occurrence is from Kaneohe Bay, Oahu, Hawaii, southern Japan, and Ambon, Indonesia. Past surveys of Kaneohe Bay populations suggested a distinct decrease in abundance following the diversion of sewage effluent from the bay in 1978/1979. In the summer of 2004 and 2007, visual surveys were conducted in areas of historical L. reevii abundance as well as in areas appearing to have suitable habitat. In 2004, approximately 2,950 m² at 20 sites within the bay were surveyed using quantitative belt transecting methods. A maximum density of 4 Lingula/m² was observed, a decrease from previous maximum estimates of 500 individuals/m² (Worcester, Dissertation, Zoology Department, University of Hawai′i, pp 49, 1969) and 100 individuals/m² (Emig, J Exp Mar Biol Ecol 52:47–61, 1981). When these 20 sites were revisited in 2007, many had fewer or no L. reevii; therefore, broader scale presence/absence surveys were conducted at 16 additional sites in the bay (also surveyed in 2004). The highest density of L. reevii found in 2007 was 0.94 individuals/m². The continued decline in abundance of L. reevii in Kaneohe Bay may be due, in addition to decreased organic enrichment from diversion of sewage discharge almost 30 years ago, to the more recent reduction of suitable habitat by the invasion of mat-forming alien algae species.     

Population structure of the common sole (<Emphasis Type="Italic">Solea solea</Emphasis>) in the Northeastern Atlantic and the Mediterranean Sea: revisiting the divide with EPIC markers

Spatial and temporal population genetic structures of the common sole, Solea solea, were studied in Northeastern Atlantic and Mediterranean Sea populations, using three polymorphic exon-primed intron-crossing (EPIC) markers. Results demonstrated significant multilocus differentiation among Eastern Mediterranean and a group composed by Western Mediterranean and Atlantic populations (θ = 0.150, P < 0.001), but also suggested unrecorded genetic differentiation of the Adriatic Sea population. No pattern of isolation-by-distance was recorded across the range covered by sampling, from the Kattegat to the Aegean Sea. Conversely to genetically structured Mediterranean populations, Atlantic populations ranging from Denmark to Portugal could be considered as representative of the same panmictic unit (θ = 0.009, not significant). Results further demonstrated stability of multilocus genetic structure among temporarily replicated cohort samples [0+, 1+, subadults] from several coastal and estuarine locations from Bay of Biscay, excepted for the amylase locus Am2B3-2 at one location (Pertuis d’Antioche). Despite coherence of such observed patterns of multilocus differentiation with previous allozymic surveys in sole, and with patterns generally obtained for other marine fish species, single-locus results from EPICs indicated divergent coalescence schemes supporting a complex response to ecology and history of sole’s populations. Results stress the use of nuclear genes such as EPIC markers to investigate population structure, but also historical, demographic, and possibly selective processes in marine fishes.     

Biology of Serranicotyle labracis,ectoparasite of Dicentrarchus labrax (Teleostei): contribution to the study of its populations

Serranicotyle labracis (Van Beneden and Hesse, 1863) Maillard, Euzet and Silan, 1988 (synonym: Microcotyle labracis Van Beneden and Hesse, 1863) is a monogenean Polyopisthocotylea and a gill parasite of the bass Dicentrarchus labrax (Linné, 1758). Analysis of a western Mediterranean population of this parasite (Gulf of Lions, Languedoc, France) clarified several aspects of its biology. Its location in the gill biotope (Silan et al. 1987) was not fundamentally different from that found by Oliver (1977) and Winch (1983), but its distribution in host populations differed from that reported in the earlier studies; it displayed a negative binomial distribution in the study zone. At a finer level of observation, the behaviour of the hosts, as a function of degree of maturity and sex, appeared to determine more than any other factor the extent of infestation by the parasite. Previous suggestions in the literature to account for these differences in population dynamics are discussed.     

Genetic structure of the flounders Platichthys flesus and P. stellatus at different geographic scales

The genetic structure of the flounders Platichthys flesus L. and P. stellatus Pallas was investigated on different spatial scales through analysis of allozyme variation at 7 to 24 polymorphic loci in samples collected from different regions (Baltic Sea, North Sea, Brittany, Portugal, western Mediterranean, Adriatic Sea, Aegean Sea and Japan) in 1984 to 1987. No geographic variation was evident within a region. Some pattern of differentiation by distance was inferred within the Atlantic, while the Mediterranean comprised three geographically isolated populations and was itself geographically isolated from the Atlantic (fixed allele differences at up to three loci were found among P. flesus populations from the Atlantic, the western Mediterranean, the Adriatic Sea, the Aegean Sea and also P. stellatus from the coast of Japan). Sea temperature during the reproductive period probably acts as a barrier to gene flow between populations. Genetic distances among European flounder populations (P. flesus) were higher than, or of the same magnitude as, the genetic distance between Pacific (P. stellatus) and European flounder populations, suggesting that P. flesus is paraphyletic and/or there is no phylogenetic basis to recognising P. stellatus as a different species. The divergence between P. flesus and P. stellatus was thus inferred to be more recent than the divergence between the present P. flesus populations from the NE Atlantic and eastern Mediterranean. The eastern Mediterranean populations are thought to originate from the colonisation of the Mediterranean by a proto-P. flesus/P. stellatus ancestor, whereas the present western Mediterranean population has undergone a more recent colonisation event by P. flesus. Patterns of mitochondrial DNA variation, established on a smaller array of P. flesus samples, were in accordance with the geographic patterns inferred from the allozyme survey. In addition, they supported the hypothesis of a two-step colonisation of the western Mediterranean. These results contribute to our understanding of the biogeography of the Mediterranean marine fauna, especially the group of boreal remnants to which P. flesus belongs. Received: 7 February 1997 / Accepted: 26 March 1997     

Phylogeographic differentiation of storm petrels (<Emphasis Type="Italic">Hydrobates pelagicus</Emphasis>) based on cytochrome<Emphasis Type="Italic"> b</Emphasis> mitochondrial DNA variation

We evaluated mitochondrial DNA (mtDNA) sequence variation in a 910 bp region of the cytochrome b gene of the storm petrel, Hydrobates pelagicus. Samples from birds collected from five populations in the North Atlantic Ocean and the Mediterranean Sea were investigated. Genetic differentiation within the Mediterranean basin was low but high in the Atlantic. Strong differences were noted between the Atlantic and the Mediterranean populations, confirming the distinction of the subspecies H. p. pelagicus and H. p. melitensis for the Atlantic and Mediterranean seabirds, respectively. Divergence between the two subspecies probably resulted from paleogeographic changes in the Strait of Gibraltar, which was likely the route used by H. pelagicus to invade the Mediterranean Sea. Current and past demography and ecology of the storm petrel is regarded as an explanation for the level of differentiation observed within each oceanic basin. We compare the phylogeographic pattern of the storm petrel to other seabirds that breed in the same regions.Communicated by S. A. Poulet, RoscoffC.C. and L.B. contributed equally to this paper.     

Population structure of Brachidontes pharaonis (P. Fisher, 1870) (Bivalvia, Mytilidae) in the Mediterranean Sea, and evolution of a novel mtDNA polymorphism

Brachidontes pharaonis (Fisher P, 1870) is an Indo-Pacific mussel that has colonized the Mediterranean Sea via the Suez Canal. Mussels may have migrated by natural dispersal of pelagic larvae, or they may have been transported on the hulls of ships, or in ballast water, or by some combination of these. Mitochondrial COI sequences (618 bp) from 101 mussels from six localities in the central and eastern Mediterranean Sea and from one site in the Red Sea were used to describe population structure. Analysis of molecular variance indicated that frequencies differed among populations, and that 92% of the variation resided within populations. The majority of haplotypes were private alleles. No simple pattern of longitudinal variation was detected for haplotype frequencies, haplotype diversity, or nucleotide diversity. A variety of tree-building algorithms (NJ, MP, ME) gave consistent results, showing two sister groups. One clade had a leucine (L-form) and the other one a methionine (M-form) at the 30th amino acid. These sympatric forms were detected within all localities and they had similar frequencies in males and females. The M- and L-form were separated by a 7.3% sequence divergence, indicating that this is an ancient polymorphism. Closely related species of mussels had exclusively the L-form, so we infer that the M-form evolved from the L-form lineage of B. pharaonis, probably in the northern portion of the Red Sea. Neutrality tests detected an excess of rare mutations, a pattern of variation expected in expanding populations. Patterns of variation detected by Tajima’s D suggest that some form of balancing selection has maintained this polymorphism.     

Are meiofaunal species cosmopolitan? Morphological and molecular analysis of Xenotrichula intermedia (Gastrotricha: Chaetonotida)

Many meiofaunal species are reported to be cosmopolitan, but due to uncertainties of identification, the affiliation of specimens from geographically distant areas to the same species-taxon is problematic. In this study, we examined morphological and molecular variation in samples of Xenotrichula intermedia Remane (Gastrotricha: Chaetonotida) from the Mediterranean Sea, the northwestern Atlantic and the northern Gulf of Mexico. Univariate analysis of 16 morphological traits was unable to detect differences among populations, except for the length of the pharynx, which was significantly shorter in the Gulf of Mexico specimens. Canonical discriminant analysis separated the Gulf of Mexico specimens from the other two populations, with pharynx length contributing about half of the total discrimination. Molecular analysis based on restriction-fragment length polymorphisms (RFLPs) in a 710-base pair polymerase chain-reaction (PCR) produet representing roughly half of the mitochondrial cytochrome oxidase I (COI) gene detected four haplotypes: one each from the Mediterranean and the Gulf of Mexico populations and two coexisting within the Atlantic population. The estimated nucleotide-sequence divergence calculated for each pairwise combination of haplotypes (based on the proportion of shared fragments) ranged from 5.3 to 11.5%. The high genetic divergence and the inability to clearly separate populations based on morphology suggest that individuals characterized by different haplotypes are genetically isolated sibling species.     

Molecular population structure of the kuruma shrimp <Emphasis Type="Italic">Penaeus</Emphasis><Emphasis Type="Italic"> japonicus</Emphasis> species complex in western Pacific

In a previous study on the kuruma shrimp Penaeus japonicus from the South China Sea, we detected high genetic divergence between two morphologically similar varieties (I and II) with distinct color banding patterns on the carapace, indicating the occurrence of cryptic species. In the present study, we clarify the geographical distribution of the two varieties in the western Pacific by investigating the genetic differentiation of the shrimp from ten localities. Two Mediterranean populations are also included for comparison. Based on the mitochondrial DNA sequence data, the shrimps are separated into two distinct clades representing the two varieties. Variety I comprises populations from Japan and China (including Taiwan), while variety II consists of populations from Southeast Asia (Vietnam, Singapore and the Philippines), Australia and the Mediterranean. Population differentiation is evident in variety II, as supported by restriction profiles of two mitochondrial markers and analysis of two microsatellite loci. The Australian population is genetically diverged from the others, whereas the Southeast Asian and Mediterranean populations show a close genetic relationship. Variety I does not occur in these three localities, while a small proportion of variety II is found along the northern coast of the South China Sea and Taiwan, which constitute the sympatric zone of the two varieties. The present study reveals high genetic diversity of P. japonicus. Further studies on the genetic structure of this species complex, particularly the populations in the Indian Ocean and Mediterranean, are needed not only to understand the evolutionary history of the shrimp, but also to improve the knowledge-based fishery management and aquaculture development programs of this important biological resource.     

Morphological and genetic variation in Japanese populations of the anemonefish Amphiprion clarkii

Geographic populations of the anemonefish Amphiprion clarkii (Bennett) from 6 widely separated locations off the coast of southern Japan are morphologically different, exhibiting (1) latitudinal clinal patterns in color pattern and meristics and (2) degrees of genetic differentiation. Electrophoretic examination of 7 polymorphic loci (95% level) among 6 populations collected between July 1979 and January 1980 revealed an average genetic distance value of 0.008. Significance was demonstrated for 39 out of 105 (37%) tests of heterogeneity. Averaged over the 6 populations, the percentage of polymorphic loci ( _0.95) was 24.6% and the proportion of heterozygous loci per individual ( ) was 0.0613. The results indicate that genetic clines are absent and suggest that the morphological variation may not be genetic. Relative isolation of genetic populations may be maintained by (1) localized larval dispersal resulting from a relatively short larval stage, and (2) current gyres tending to trap larvae, increasing the return of juveniles to their adult coastal habitat. Morphological clines may be due to clines in ecological parameters related to latitude.Contribution No. 39 of the Tatsuo Tanaka Memorial Biological Station     

Genetic and taxonomic relationships among Northeastern Atlantic and Mediterranean populations of the soft coral Alcyonium coralloides

Species boundaries among taxa of colonial marine organisms are often obscured by intraspecific morphological and ecological variation; genetic comparisons of recognized “ecotypes” frequently reveal them to be reproductively isolated species. Based on morphological similarities, it has been proposed that the Mediterranean soft coral Alcyonium (=Parerythropodium) coralloides Pallas, 1766 and its Atlantic congener A. hibernicum belong to one highly variable and geographically widespread species, A. coralloides. I collected A. coralloides from ten Atlantic and three Mediterranean locations in 1990 and 1994, and used differences in colony form, substrate use and color to separate them into five distinct morphotypes. Two occur sympatrically in the Mediterranean (M1, M2) and three have overlapping distributions in the Atlantic (A1, A2, A3). I used allozyme electrophoresis to compare morphotypes genetically at 14 enzyme loci. Where two morphotypes occurred sympatrically, fixed allelic differences at 4 to 6 loci indicated reproductive isolation. In all but one pairwise comparison (M1 and A2), morphotypes whose ranges did not overlap were also separated by large genetic distances. From these results I suggest that the five morphotypes represent four distinct species. A. coralloides comprises two morphotypes (M1, A2) with relatively high genetic identity. A. hibernicum (=A1) is reproductively isolated from A. coralloides and should be retained as a valid species; levels of genetic diversity and heterozygosity within populations support the absence of outcrossing in this reportedly asexual species. Morphotypes M2 and A3 are also reproductively isolated from A. coralloides; they are taxonomically distinct from but belong to the same phylogenetic clade as A. hibernicum. Although preliminary observations suggest that differences in reproductive timing maintain species boundaries in sympatry, wider geographic sampling will be required to elucidate the events leading to speciation within this species complex. Received: 8 May 1998 / Accepted: 8 October 1998     

Genetic relationships between marine and marginal-marine populations of Cerithium species from the Mediterranean Sea

Results of isozyme electrophoresis were used to explore the genetic relationships between several Mediterranean morphs of Cerithium (Gastropoda: Prosobranchia), for which taxonomy is currently uncertain because of high intraspecific variability and low interspecific differentiation. The large species, classically known as C. vulgatum Bruguière, 1789 was identified at four sites (two in the French Mediterranean and two in southern Spain). Two different larval types were found in the French sites, but poecilogony could not be demonstrated. Individuals collected from harbours were not genetically distinct from open-sea populations of classic C. vulgatum. However, a population in the Embiez lagoon (French Mediterranean) which morphologically resembles C. vulgatum did display distinct genetic traits, supporting its status as a separate species. Of the small Cerithium species usually known as C. rupestre, two sympatric species (C.“rupestre” Risso, 1826 and C. lividulum Risso, 1826) were distinguished. Genotype frequencies within the analysed populations revealed much heterozygote deficiency. F _ST values (fixation index measuring the effects of population subdivision) suggest a higher genetic differentiation for C. lividulum populations than for C. vulgatum populations. We assume that a high larval dispersal capability (via planktotrophy) allows a high gene flow between populations of C. vulgatum. Received: 24 November 1998 / Accepted: 24 September 1999