Identification of Illex coindetii, I. illecebrosus and I. argentinus (Cephalopoda: Ommastrephidae) throughout the Atlantic Ocean; by body and beak characters

Shortfinned squid species of the genus Illex support commercial fisheries throughout the Atlantic Ocean and Mediterranean Sea. Previous identification of interspecific and intraspecific populations by morphological and size-at-maturity studies have not provided conclusive results. We analysed morphometric body and beak variables (24 characters) in three species of the genus (I. coindetii, I. illecebrosus and I. argentinus), using a geographic and seasonal series of 33 populations for 1,500 specimens of I. coindetii, I. illecebrosus and I. argentinus. Residuals of the regression between each morphometric body and beak variable and mantle length were used as input in a stepwise discriminant analysis. Species discrimination by body and hectocotylus characters required at least eight variables and resulted in high correct-classification percentages for I. coindetii and I. argentinus (75% and 90%, respectively), whereas the best identification resulted from beak characters (83% correctly classified). Size of the suckerless basal arm, sucker-bearing length and beak lateral wall discriminated best among I. coindetii from northern Iberia, northwest Iberia (year-1996) and Ireland in the Atlantic and western Mediterranean versus middle and eastern Mediterranean samples. Canadian shelf and American samples were discriminated from Canadian slope I. illecebrosus. Winter/shelf and winter/slope samples of I. argentinus seemed to form a single biological group separated from Falkland Island, 46°S/autumn spawners and 46°S/1996 specimens along the Patagonian Shelf. No significant sexual or maturity polymorphism was obtained. Discriminant analysis optimised population diagnosis on a morphometric basis of interest in fisheries strategies. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0796-7.     

Persistent near-bottom aggregations of mesopelagic animals along the North Carolina and Virginia continental slopes

Submersible observations during four missions over the North Carolina and Virginia continental slopes (184–900 m) documented the occurrence of large aggregations of mesopelagic fishes and macronektonic invertebrates near or on the bottom. Aggregated mesopelagics formed a layer up to tens of meters deep positioned from a few centimeters to 20 m, usually <10 m, above the substrate. Aggregations were numerically dominated by microvores, notably the myctophid fish Ceratoscopelus maderensis and the penaeid shrimp Sergestes arcticus. Consistently present but in relatively lower numbers, were mesopelagic predators, including the paralepidids Notolepis rissoi and Lestidium atlanticum, the eel Nemichthys scolopaceus, the stomiid fishes Chauliodus sloani and Stomias boa ferox, and squids Illex spp. Near-bottom aggregations do not appear to be an artifact due to attraction to the submersible. Based on submersible observations in three areas in 4 years spanning a decade, near-bottom aggregations of midwater organisms appear to be a geographically widespread and persistent phenomenon along the continental slope of the southeastern US Aggregations may exploit areas of enhanced food resources at the bottom.     

A critical habitat for Mediterranean fish resources: shelf-break areas with<Emphasis Type="Italic"> Leptometra phalangium</Emphasis> (Echinodermata: Crinoidea)

This paper considers the potential role of the crinoid Leptometra phalangium as an indicator of highly productive areas along the shelf break that can sustain large biomasses of benthopelagic fish and recruits. The structure of fish assemblages in the central Mediterranean Sea (central-western coast of Italy), analysed on the basis of surveys carried out in summer and autumn from 1997 to 2001, revealed the presence of a well-defined group of species on the shelf break. This area, occurring at a depth of between 120 and 170 m, is characterised by detritic organogenic sediments colonised by the crinoid L. phalangium, a suspension-feeding macro-epibenthic species confined in the Mediterranean to the shelf-break area. Its abundance in the studied area can reach 12–15 ind. m^–2. A total of 121 species belonging to 66 families of demersal organisms (crustacean decapods and stomatopods, cephalopods, selaceens and teleosteens) were caught at shelf-break stations from September to October. The species which typified the assemblage were the fishes Trisopterus minutus capelanus, Merluccius merluccius, Glossanodon leioglossus, Argentina sphyraena, Capros aper, Macroramphos scolopax and Lepidotrigla cavillone, the crustacean decapod Parapenaeus longirostris and the cephalopods Illex coindetii and Todaropsis eblanae. Detritic shelf-break stations showed a higher abundance of demersal organisms than stations distributed on muddy bottoms in the same depth range (100–200 m). Such differences appeared to be significant in September–October, when a clear increase in benthopelagic zooplanktivorus species, such as Glossanodon leioglossus, Trachurus trachurus, Trachurus picturatus, was found. The length structure of species occurring on the shelf break showed that for some of them the selection of this area is related to specific phases of their life cycle. Significant highest abundance of recruits and juveniles was observed for Merluccius merluccius, Helicolenus dactylopterus, Phycis blennoides, Parapenaeus longirostris and Capros aper in at least one of the two seasons. Similarly, an increased abundance of spawners of red mullet Mullus barbatus and four-spotted megrim Lepidorhombus boscii was observed on the shelf break. Results of this study may have important consequences for management of fish stocks and assemblages in the central Mediterranean. The co-occurrence of high densities of L. phalangium and benthopelagic fish, occurring mainly with juveniles and spawners, strongly indicates a potential role of L. phalangium as an indicator of highly productive areas around the shelf break. Such areas appear to play a major role in the production of some of the most abundant and commercially important fish species, such as the Mediterranean hake and red mullet.Communicated by R. Cattaneo-Vietti, Genova     

Chemical and structural characteristics of statoliths from the short-finned squid Illex illecebrosus

Statoliths of the short-finned squid Illex illecebrosus were chemically analyzed to define their chemical composition and surveyed by scanning electron microscope to differentiate internal structural patterns. X-ray diffraction data demonstrated that I. Illecebrosus statoliths were composed principally of CaCO₃ in the aragonite crystal form. The crystals occurred in a protein matrix to form incremental patterns which radiated from the nucleus to the edge of the statoliths. The protein matrix comprised approximately 5% of the statolith by weight. The protein was principally composed of acidic amino acids. A high abundance of aspartic acid in the protein matrix indicated that the matrix would function as a template in the initiation and acceleration of the crystal growth of CaCO₃. The rhythmic microstructural patterns, constructed of aragonite crystals in the protein matrix, were suggested to be daily in formation and subsequent growth estimations were in agreement with known life history information. The stable isotopic composition of the carbonate of I. illecebrosus statoliths suggested that oxygen may be deposited in isotopic equilibrium with the surrounding environment while carbon appeared to be related to biological processes. The information recorded in the statoliths as incremental growth and stable isotopic composition could provide valuable insights into the ecological history of squid.     

<Emphasis Type="Italic">Patella aspera</Emphasis> and<Emphasis Type="Italic"> P. ulyssiponensis</Emphasis>: genetic evidence of speciation in the North-east Atlantic

The group of subspecies of Patella ulyssiponensis, described by Christiaens, was widely known as Patella aspera until recently. The group extends throughout the Mediterranean, on all Macaronesian islands, along the North African coast, and in Europe, as far north as southern Norway. Throughout its range it displays great variation in shell sculpture and colour. The aim of this work was to re-examine the various subspecies proposed by Christiaens and, genetically, to test the hypothesis that European continental populations belong to a different species than the one composed by Macaronesian (north-west African) populations. In the present work, this group was studied by allozyme electrophoresis of 21 loci and by six morphological variables. The monomorphic locus of malate dehydrogenase (Mdh-1) was found to be diagnostic for distinguishing European continental populations from those of north-west African archipelagos, confirming the specific status of both groups. The allele observed at this locus in the Macaronesian populations was novel, while European continental populations showed a plesiomorphic one, shared with all other north-west Atlantic patellids with the exception of Patina pellucida, suggesting a more recent origin of the Macaronesian species from the continental forms. Both species showed a genetic identity of 0.730±0.061, which allowed rough estimations of 6.5–3 Mya since speciation. The subspecies Patella ulyssiponensis deserta described by Christiaens was not confirmed by our genetic data. Other loci (glyceraldehyde-3-phosphate dehydrogenase and the second locus of lactate dehydrogenase) were partially diagnostic, in which both species showed different most common alleles. Morphologically, both species are easily recognised by shell characters and the results agree with previous findings, that continental populations are more homogeneous in shell morphology and radula characters than populations from the Macaronesia. This work supports retention of the earliest valid name, Patella ulyssiponensis Gmelin, with Lisbon, Portugal as type locality, for the European continental species, and Patella aspera Röding for the Macaronesian populations. Population subdivision within species was measured by theta, the estimator of F_st, showing in both P. aspera and P. ulyssiponensis a high degree of genetic structuring (=0.226 and 0.182, respectively) mostly explained by the large distances separating the populations within species.Communicated by J.P.Thorpe, Port Erin     

Geographical patterns of variability at allozyme loci in the European oyster Ostrea edulis

The variability of 14 enzyme-coding genes has been analysed in samples from 19 populations of the oyster Ostrea edulis L., collected along the Atlantic and Mediterranean coasts of Europe. We found an abundance of clines, which appeared at 8 loci, including the most polymorphic (AP-2 ^*, ARK ^*, EST-4 ^*, MDH-2 ^*, ME-1 ^*, 6PGH ^*, PGI ^* and PGM ^*). Another 6 loci (ALDH ^*, EST-3 ^*, EST-5 ^*, IDH-2 ^*, MDH-1 ^*, ME-2 ^*) exhibited V-shaped patterns of gene-frequency variation, with clines at one or both sides of the Straits of Gibraltar. The observation of coincident clines at many loci can be explained by a model of secondary intergradation. The geographical location of the midpoints of the clines and V-shaped patterns suggests the existence of two ancient Atlantic and Mediterranean oyster stocks which became differentiated in allopatry and subsequently merged. Clines observed along Atlantic and/or Mediterranean coasts at the loci with V-shaped patterns must have arisen independently. The large heterogeneity observed in the levels of gene differentiation (G _ST ) across loci (G _ST ranged from 0.008 to 0.290) and important differences in estimates of gene flow obtained by different methods suggest that the populations of O. edulis are not in genetic equilibrium. Lack of population equilibrium can be due to natural selection and/or restrictions to gene flow. The average among-population variability was higher than in other oyster species that do not show incubatory habits, and represented 8.8% of the total heterozygosity. Levels of intrapopulation variability were lowest in populations from the North Atlantic, suggesting low population sizes in that area.     

Genetic evidence for the cryptic species pair, <Emphasis Type="Italic">Lottia digitalis </Emphasis>and <Emphasis Type="Italic">Lottia austrodigitalis</Emphasis> and microhabitat partitioning in sympatry

It has been proposed that the common West Coast limpet, Lottia digitalis, is actually the northern counterpart of a cryptic species duo including, Lottia austrodigitalis. Allele frequency differences between southern and northern populations at two polymorphic enzyme loci provided the basis for this claim. Due to lack of further evidence, L. austrodigitalis is still largely unrecognized in the literature. Seven additional enzyme loci were examined from populations in proposed zones of allopatry and sympatry to determine the existence of L. austrodigitalis as a sibling species to L. digitalis. Significant allele frequency differences were found at five enzyme loci between populations in Laguna Beach, southern California, and Bodega Bay, northern California; strongly supporting the existence of separate species. Both species exhibit two microhabitat morphotypes, a gooseneck barnacle morph in the mid-intertidal zone and a rock morph in the high-intertidal zone. In sympatry, L. austrodigitalis was more abundant higher in the intertidal on rocks, whereas L. digitalis was more abundant lower in the intertidal on barnacles. This finding supports earlier claims of microhabitat partitioning in this sibling species pair. In addition to this finding, the transition zone between the species was found to have shifted substantially northward in only two decades, from Monterey Peninsula, CA to near Pigeon Point, CA, where L. digitalis previously dominated.     

Hybridization and introgression between Indo-Pacific species of Diadema

We present the first case of hybridization between echinoid species evaluated through genetic markers and morphology. We examined the three tropical Indo-Pacific species of the genus Diadema: D. paucispinum A. Agassiz, D. savignyi (Audouin) Michelin and D. setosum (Leske). Specimens morphologically intermediate between two of these species, D. savignyi and D. setosumhave previously been noted. Fertile hybrids have also been produced in the laboratory. To determine extent of hybridization, we first assayed the allozyme products of 22 loci in individuals which, on the basis of morphology and collection locality, could be unambiguously assigned to one of the three species. We found four loci that were either diagnostic or semi-diagnostic between D. setosum and the other two species, and one locus semi-diagnostic between D. savignyi and D. paucispinum.We then assayed individuals of intermediate morphology to find out whether they had hybrid genotypes. In the Ryukyu Islands, where D. setosum and D. savignyi coexist, we found one specimen which on the basis of all four diagnostic loci was an F₁ hybrid, and several individuals that could be either F₂ (or later-generation) hybrids or progeny of backcrosses.We also found one individual that on both genetic and morphological grounds appeared to belong to D. paucispinum (even though this spcies has only been reported from Hawaii and Kiribati) and three other individuals that carried alleles characteristic of D. paucispinum. Thus, previous reports of hybridization between D. setosum and D. savignyi were correct; it is also possible that larvae of D. paucispinum occasionally arrive at localities outside Hawaii, reach sexual maturity, and hybridize with the other two species. Counts of pure and hybrid morphotypes in other populations across the western tropical Pacific revealed a low but widespread incidence of apparent F₁ hybrids and backcrosses of D.savignyi and D. setosum. However, the existence of diagnostic or semi-diagnostic loci, low interspecific gene-flow estimates based on F _st statistics, and the lack of Hardy-Weinberg or linkage disequilibria among individuals of pure morphology all suggest that gene introgression between the three species is limited.     

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     

Separation of the two Atlantic species of Pomatoceros,P. lamarckii and P. triqueter (Annelida: Serpulidae) by means of biochemical genetics

In order to confirm the status of two proposed species of the serpulid Pomatoceros, the electrophoretic mobilities of 13 enzymes encoded at 19 loci in P. lamarckii (Quatrefages) and P. triqueter (L.) were compared using horizontal starch gel electrophoresis. The serpulids were sampled from Menai Bridge (North Wales), Plymouth (England) and the Isle of Man in the winter of 1981/1982. Six loci were monomorphic or nearly so and showed no useful or significant difference between species. At a further 7 loci, some of the allozymes common to both species were present, though usually at significantly different frquencies, while others were species-specific. The remaining 6 loci presented species-specific allozymes. In P. lamarckii, 61% of the loci were polymorphic compared with 50% in P. triqueter. The mean observed heterozygosity was 0.169 for P. lamarckii and 0.208 for P. triqueter. Calculation of genetic distance and identity gave values of 0.821 and 0.440, respectively. This study, therefore, confirms the existence of two separate species of Pomatoceros as has been previously proposed on the basis of opercular structure and supported by ecological separation where these species occur sympatrically.     

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.     

Population genetics of the Antarctic heteronemertean Parbolasia corrugatus from the South Orkney Islands

An allozyme survey, using starch-gel electrophoresis, was carried out on eight populations of the Antarctic nemertean worm Parborlasia corrugatus (McIntosh, 1876) collected from locations around the South Orkney Islands, Antarctica. These populations were separated by distances in the order of tens of kilometres. Genetic variation was estimated over 22 enzyme loci for all populations examined, giving an observed heterozygosity of 0.142. This was much lower than the expected heterozygosity (H _e  = 0.201), and it was found that there was a significant deficiency of het‐erozygotes across four enzyme loci ( p ≤ 0.01). A more detailed examination of this deficiency of heterozygotes was undertaken for the six populations and six variable enzyme loci for which the most complete data sets existed. A significant deficiency of heterozygotes was found at the enzyme locus Odh-1 for four of the six populations examined ( p ≤ 0.01). Mean F _is (0.240) indicated a significant ( p ≤ 0.01) within-population component of the heterozygote deficiency estimated for the six populations sampled, and this was mainly due to the␣Ap-1, Odh-1 and Pgm-1 loci. The mean F _st value (0.036) was also significant ( p ≤ 0.01), indicating a degree of genetic differentiation between populations. The observed levels of genetic differentiation between populations of P. corrugatus and the significant heterozygote deficiencies were unexpected, because this species has been reported to have a long-lived planktotrophic larva. It is hypothesised that recruitment of P. corrugatus in the South Orkney Islands originates from genetically distinct populations located in the Weddell Sea and to the west of the Antarctic Peninsula. Shifts in the relative position of the Weddell Sea Front, Weddell–Scotia Confluence and Scotia Front, relative to the South Orkney Islands, provide a mechanism for variation in the origin of recruits over time. Received: 24 July 1997 / Accepted: 31 October 1997     

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     

Selective grazing of the isopod Idotea baltica between Fucus evanescens and F. vesiculosus from Kiel Fjord (western Baltic)

Grazing rates of the isopod Idotea baltica on Fucus evanescens and F. vesiculosus (Phaeophyta) were quantified in laboratory feeding preference experiments. Fucus species were offered alone (no-choice) or simultaneously (choice). In three of four no-choice experiments and in all four choice experiments, I. baltica significantly preferred F. vesiculosus to F. evanescens. F. evanescens recently immigrated into Kiel Fjord and has increased in abundance since 1990. One possible reason for the competitive success of this species may be that, compared to F. vesiculosus, it is less preferred by I. baltica, the most abundant mesograzer at the study site.     

Effect of hyposalinity on the photophysiology of <Emphasis Type="Italic">Siderastrea radians</Emphasis>

Tolerance to hyposalinity of the scleractinian coral S. radians was examined in a mesocosm study. Colonies of S. radians were collected from five basins in Florida Bay, USA, which occur along a northeast-to-southwest salinity gradient. Salinity treatments were based on historical salinity records for these basins. Photophysiology of the endosymbiont Symbiodinium spp. (maximum quantum yield; F _v/F _m) was measured as an indicator of holobiont stress to hyposalinity. Colonies from each basin were assigned four salinity treatments [The Practical Salinity Scale (PSS) was used to determine salinity. Units are not assigned to salinity values because it is a ratio and has no unit as defined by UNESCO (UNESCO Technical papers no. 45, IAPSO Pub. Sci. No. 32, Paris, France, 1985)] (30, 20, 15, and 10) and salinities were reduced 2 per day from ambient (30) to simulate a natural salinity decrease. Colonies treated with salinities of 20 and 15 showed no decrease in F _v/F _m versus controls (i.e. 30), up to 5 days after reaching their target salinity. This indicates a greater ability to withstand reduced salinity for relatively extended periods of time in S. radians compared to other reef species. Within 1 day after salinity of 10 was reached, there was a significant reduction in F _v/F _m, indicating a critical threshold for hyposaline tolerance. At the lowest treatment salinity (10), F _v/F _m for the more estuarine, northeast-basin colonies were significantly higher than the most marine southwest-basin colonies (Twin Key Basin). Our results suggest that historical salinity ranges within basins determine coral population salinity tolerances.     

Chlorophyll fluorescence measures of seagrasses <Emphasis Type="Italic">Halophila ovalis</Emphasis> and <Emphasis Type="Italic">Zostera capricorni</Emphasis> reveal differences in response to experimental shading

In coastal waters and estuaries, seagrass meadows are often subject to light deprivation over short time scales (days to weeks) in response to increased turbidity from anthropogenic disturbances. Seagrasses may exhibit negative physiological responses to light deprivation and suffer stress, or tolerate such stresses through photo-adaptation of physiological processes allowing more efficient use of low light. Pulse Amplitude Modulated (PAM) fluorometery has been used to rapidly assess changes in photosynthetic responses along in situ gradients in light. In this study, however, light is experimentally manipulated in the field to examine the photosynthesis of Halophila ovalis and Zostera capricorni. We aimed to evaluate the tolerance of these seagrasses to short-term light reductions. The seagrasses were subject to four light treatments, 0, 5, 60, and 90% shading, for a period of 14 days. In both species, as shading increased the photosynthetic variables significantly (P < 0.05) decreased by up to 40% for maximum electron transport rates (ETR_max) and 70% for saturating irradiances (E_k). Photosynthetic efficiencies (α) and effective quantum yields (ΔF/Fm′) increased significantly (P < 0.05), in both species, for 90% shaded plants compared with 0% shaded plants. H. ovalis was more sensitive to 90% shading than Z. capricorni, showing greater reductions in ETR_max, indicative of a reduced photosynthetic capacity. An increase in E_k, Fm′ and ΔF/Fm′ for H. ovalis and Z. capricorni under 90% shading suggested an increase in photochemical efficiency and a more efficient use of low-photon flux, consistent with photo-acclimation to shading. Similar responses were found along a depth gradient from 0 to10 m, where depth related changes in ETR_max and E_k in H. ovalis implied a strong difference of irradiance history between depths of 0 and 5–10 m. The results suggest that H. ovalis is more vulnerable to light deprivation than Z. capricorni and that H. ovalis, at depths of 5–10 m, would be more vulnerable to light deprivation than intertidal populations. Both species showed a strong degree of photo-adaptation to light manipulation that may enable them to tolerate and adapt to short-term reductions in light. These consistent responses to changes in light suggest that photosynthetic variables can be used to rapidly assess the status of seagrasses when subjected to sudden and prolonged periods of reduced light.     

The invasion and subsequent die-off of <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> in Langebaan Lagoon,South Africa: effects on natural communities

The alien mussel Mytilus galloprovincialis invaded sand banks in Langebaan Lagoon on the west coast of South Africa in the mid-1990s. However, by 2001 these beds had completely died off, with only empty shells and anoxic sand remaining. In an effort to prevent the re-settlement of this aggressive invader, all dead mussel shells were then cleared. This study considered the impacts of the invasion and subsequent die-off on natural benthic communities. Community composition differed significantly between non-invaded and invaded areas (ANOSIM, R = 0.685 and P < 0.01) as the physical presence of mussel beds created a new habitat that promoted invasion by indigenous rocky-shore species. This dramatically increased faunal biomass from 1,132.9 g m⁻² ± 3,454.7 SD to 53,262.4 g m⁻² ± 23,052.6 SD and species richness from 38 to 49 species. Following the die-off of the mussel beds, communities remained significantly different between non-invaded areas and those in which mussel shells remained (ANOSIM, R = 0.663 and P < 0.01). Species richness was significantly greater in non-invaded areas (18 species) than in uncleared areas with remnant shells (four species) (Kruskal–Wallis ANOVA H _2,36 = 10.8964 and P = 0.032), as the previously dominant rocky-shore species became smothered by sediment and the compacted shells formed an impermeable layer excluding sandy-shore burrowing organisms. After the shells were cleared, 50% of the sandy-shore species associated with non-invaded areas returned within 5 months, but community structure still remained significantly different to non-invaded areas (ANOSIM, R = 0.235 and P > 0.05). Invasion thus dramatically altered natural communities and although the subsequent removal of the dead mussel shells appears to have aided recovery, community composition remained different from the pre-invasion state after 5 months.     

Allozyme heterozygosity and escape response performance of the scallops, <Emphasis Type="Italic">Argopecten purpuratus</Emphasis> and <Emphasis Type="Italic">Placopecten magellanicus</Emphasis>

Multilocus allozyme heterozygosity (MLH) has been positively correlated with growth in some marine bivalves and was suggested to facilitate swimming activity in pectinids. Using two highly mobile scallops, Placopecten magellanicus and Argopecten purpuratus, we examined escape response performance and morphometric characteristics as a function of allelic variability at metabolic loci. Ten allozyme systems were used for A. purpuratus and 7 for P. magellanicus. In each species, the morphometric characteristics and escape response parameters were analyzed separately using principal components analysis (PCA) and the scores of the major principal components were related to allozyme heterozygosity. In both P. magellanicus and A. purpuratus, positive correlations were found between MLH and morphometric parameters, but escape response parameters were only positively linked to MLH in P. magellanicus, and then weakly. The hypothesis that MLH improves fitness of pectinids by increasing the capacity to escape predators is not supported.     

Negligible evidence for regional genetic population structure for two shark species <Emphasis Type="Italic">Rhizoprionodon acutus</Emphasis> (Rüppell, 1837) and <Emphasis Type="Italic">Sphyrna lewini</Emphasis> (Griffith &; Smith, 1834) with contrasting biology

Biodiversity of sharks in the tropical Indo-Pacific is high, but species-specific information to assist sustainable resource exploitation is scarce. The null hypothesis of population genetic homogeneity was tested for scalloped hammerhead shark (Sphyrna lewini, n = 237) and the milk shark (Rhizoprionodon acutus, n = 207) from northern and eastern Australia, using nuclear (S. lewini, eight microsatellite loci; R. acutus, six loci) and mitochondrial gene markers (873 base pairs of NADH dehydrogenase subunit 4). We were unable to reject genetic homogeneity for S. lewini, which was as expected based on previous studies of this species. Less expected were similar results for R. acutus, which is more benthic and less vagile than S. lewini. These features are probably driving the genetic break found between Australian and central Indonesian R. acutus (F-statistics; mtDNA, 0.751–0.903, respectively; microsatellite loci, 0.038–0.047 respectively). Our results support the spatially homogeneous monitoring and management plan for shark species in Queensland, Australia.     

DNA microsatellite variability and genetic differentiation among natural populations of the Cuban white shrimp<Emphasis Type="Italic"> Litopenaeus schmitti</Emphasis>

Genetic variability within the Cuban population of the white shrimp Litopenaeus schmitti (Burkenroad, 1936) was assessed using five microsatellite loci and samples collected from four sites (Batabanó, Cienfuegos, Tunas de Zaza, and Manzanillo) from November 1999 to March 2000. All loci were polymorphic, and a total of 80 alleles were found, 13 of them private alleles occurring only in a single population and always in low frequencies. The Cienfuegos sample had the highest observed heterozygosity (H_o=0.653), and the Tunas de Zaza sample had the lowest values (H_o=0.605), but there were no significant differences among samples in heterozygosity or in the mean numbers of alleles per locus (ranging from 11.0 to 11.6). Significant differentiation among populations was detected (F_ST=0.012, P<0.001). Low but significant F_ST values were revealed in pairwise comparisons between populations. Assignation tests correctly assigned high percentages of individuals to their original populations (74.5%) using a Bayesian approach. The significant differentiation among populations could be due to the restriction of gene flow among populations of L. schmitti and is concordant with previous allozyme studies on Cuban populations.Communicated by J.P. Grassle, New Brunswick