Optimal growth and maximal survival temperatures of Atlantic Laminaria species (Phaeophyta) in culture

The effects of temperature on growth rate of rapidly-growing cultured macrosporophytes of 9 isolates of Atlantic Laminaria comprising 4 species have been investigated. No significant population variation was observed within species despite wide variations in temperature between the original collecting sites. L. saccharina showed a broad temperature optimum in the 10°–15°C range, whereas L. longicruris had a sharp optimum at 10°C. L. digitata and L. hyperborea grew more slowly, with only slightly sub-optimal growth over a wide temperature range, but with peaks at 10°C (L. digitata) and 15°C (L. hyperborea). The maximum survival temperatures of individual male and female vegetatively-growing gametophytes were ascertained for these species plus the Arctic L. solidungula, and were as follows: L. saccharina and L. longicruris, 23°C; L. digitata (male), 23°C; L. digitata (female), 22°C; L. hyperborea, 21°C; L. solidungula, 18°C. The lack of within-species differences demonstrates that the success of the genus in areas with different temperature regimes is brought about by phenotypic plasticity of individuals rather than the selection of temperature races or ecotypes.     

Influence of ultraviolet-radiation on chlorophyll fluorescence and growth in different life-history stages of three species of Laminaria (phaeophyta)

Zoospores, gametophytes, young sporophytes and discs cut from mature sporophytes of Laminaria digitata, L. hyperborea and L. saccharina were exposed in the laboratory to UV-radiation, with a spectral composition and irradiance similar to natural sunlight, for periods ranging from 15 min to 8 d, and were then returned to white light. Germination of zoospores and the growth of gametophytes were reduced after exposures to UV longer than 1 h, whereas UV had little effect on the growth of young or mature sporophytes unless exposure continued for more than 48 h. The variable fluorescence (F _v:F_m) of all stages was strongly reduced immediately after short exposures to UV, but recovered almost completely within 24 h. However, exposure of gametophytes to UV for >4 h resulted in little or no recovery of F _v:F_m, whereas >16 h of UV were required to produce this result in young sporophytes, and >48 h in mature sporophytes. Thus, sensitivity to UV-radiation decreased from gametophytes to sporophytes, and with increasing age of sporophytes, but, in gametophytes, growth appeared to be a more sensitive indicator of UV-damage than F _v:F_m after 24 h recovery. The responses to UV of the zoospores and gametophytes of all three species were similar, but both growth and fluorescence measurements suggested that the sporophytes of L. saccharina were more sensitive to UV than those of the other two species.     

Experimental interbreeding between geographically separated populations of the marine wood-boring isopod Limnoria tripunctata with preliminary indications of hybrid vigor

Populations of the marine wood-boring isopod Limnoria tripunctata Menzies were cultivated and interbred in the laboratory in order to determine whether geographically separated populations would interbreed. Two series of interbreeding studies were conducted. In the first, field populations from 11 Atlantic and Caribbean sites and 3 Pacific sites were interbred with a North Western Atlantic population from Beaufort, North Carolina (USA). Viable crossings, to at least F ₁, occurred in all but one case—that of a BeaufortxChatham (Massachusetts, USA) cross. In the second series of experiments, field populations from 9 Atlantic and Caribbean sites were crossed with populations from St. Teresa, Florida, Gulf of Mexico. In this case also, all crosses except one produced viable offspring, not only to the F ₁, but to the F ₂ generations as well. The St. Teresa x Chatham cross was not successful. Geographic distance was not a factor in the success of crossbreeding between disjunct populations. Chatham populations (mainly) are now believed to represent a valid but previously obscure species, Limnoria tuberculata Sowinski, and not a variant of L. tripunctata Menzies. Enhanced viality or vigor, as determined from rate of wood destruction, population growth rate and survival, was indicated in several crossings, but this was not necessarily correlated with distance between populations. Observations were made on the manner in which the species has achieved a worldwide distribution, and a hypothesis is put forward to explain the evolution of species related to L. tripunctata.Contribution from Zoology Department, Duke University, Durham, North Carolina, and Duke University Marine Laboratory, Beaufort, North Carolina, and the Department of Oceanography, No. 289, Florida State University, Tallahassee, Florida. Support in part from NSF Grant No. GB-6898.     

Temperature tolerances of two southern African Ecklonia species (Alariaceae: Laminariales) and of hybrids between them

The temperature tolerances in culture of Ecklonia maxima (Osbeck) Papenf., the dominant kelp in the Benguela upwelling region on the west coast of southern Africa, and the smaller E. biruncinata (Bory) Papenf., growing on the warmer south coast, have been investigated. Vegetative growth and reproduction of gametophytes show very similar patterns in the two species, although temperature optima in E. biruncinata are consistently 2 ° to 3 °C higher than those of E. maxima. Reciprocal crosses between clonal female and male gametophytes of the two species produced healthy juvenile sporophytes, indistinguishable from the parent crosses. All parents and hybrids grew well over a wide range of temperatures (8 ° to 22°C), with E. biruncinata sporophytes again having slightly higher optima than E. maxima. Both reciprocal crosses exhibited optima intermediate between the parent crosses. The results are discussed with respect to the origins and evolution of Ecklonia species in the region.     

Standing crop and leaf area index of the sublittoral Laminaria species near Helgoland

Standing crop values of Laminaria species sampled between 0 to 2 m depth in the sublittoral of Helgoland, a small rocky island in the southern North Sea, are similar to those reported by other workers from Norwegian or Scottish coasts. Below 2 m water depth, however, standing crop values are found to be reduced to a far greater extent with increasing depth than in the Norwegian or Scottish areas studied. Maximum standing crop values are not found in Laminaria digitata and L. saccharina growing just below the low water mark of spring tides, but in the upper L. hyperborea forest at 2 m depth. Population density and leaf area index (LAI) decrease with increasing depth (but not the average weight of individuals constituting the L. hyperborea forest). It is suggested that the depth at which LAI becomes 1 represents an important ecological threshold, since growth rates of L. hyperborea specimens appear to decrease at greater depths but not to increase above the threshold. Self-shading might equalize, to a certain extent, the quantity of underwater irradiance available to an individual phylloid at all depths at which a close vegetation of Laminaria species occurs.     

Population differentiation in megrim (<Emphasis Type="Italic">Lepidorhombus whiffiagonis</Emphasis>) and four spotted megrim (<Emphasis Type="Italic">Lepidorhombus boscii</Emphasis>) across Atlantic and Mediterranean waters and implications for wild stock management

Megrim, Lepidorhombus whiffiagonis, and four spot megrim, Lepidorhombus boscii, are two marine fish species of high commercial interest. Despite their quite heavy exploitation little is known on the genetic structure of their populations. The present work aimed at characterizing the first seven microsatellites markers available for the two megrim species. These new markers were in a second step employed to describe the population structure of the two species among their almost entire habitat range (Atlantic and Mediterranean samples). Our study confirmed the existence of a strong genetic difference between Atlantic and Mediterranean megrim species already described in the literature for L. whiffiagonis on the basis of variations at ribosomal genes. Additionally our analysis gave the first evidences of a strong genetic differentiation among Atlantic populations in both megrim species (within Atlantic global F_ST in L. whiffiagonis and L. boscii were respectively 0.158 and 0.145). When describing megrim population structure, the comparison between allele-frequency-based tests (F_ST comparisons) and genotype-based inferences (Bayesian approach) gave evidences of a hierarchical structure of the populations. In conclusion, our work enlighten the existence of two different stocks within the Atlantic Ocean and one in the Mediterranean Sea that will clearly need to be managed separately. As the present results do not fully support the current megrim stock boundaries they will surely help to rethink megrim management policies in the future.     

Ecological energetics of the seaweed zone in a marine bay on the Atlantic coast of Canada. I. Zonation and biomass of seaweeds

An intertidal and underwater survey of the zonation of seaweed in St. Margaret's Bay, NovaScotia, Canada showed 8 major zones as one proceeded away from shore: (1) Fucus and Ascophyllum; (2) Chorda with filamentous browns; (3) Chondrus crispus; (4) Zostera marina; (5) Laminaria digitata with L. longicrusis; (6) Laminaria longicruris; (7) L. longicruris with Agarum cribrosum; (8) Agarum cribrosum with Ptilota serrata. Zostera occurred at the same level as O. crispus but replaced it in sheltered water. Ascophyllum was more abundant in sheltered water. L. digitata was confined to the more exposed, steeper shore. The average horizontal extent of the seaweed zone was 369 m, and the greatest depth of significant amounts of seaweed 20 to 30 m. Laminaria longicruris contributed 36% of the total biomass, and Laminaria spp. and Agarum together constituted 83%, while intertidal seaweeds contributed less than 10% of the biomass. The estimated average total biomass per m of shore line was 1,481 kg fresh weight, 326 kg dry weight, 98 kg carbon, or 980x10³ kcal. When averaged over the whole area of the bay, the corresponding figures were 1.38 kg/m² fresh weight, 0.30 kg/m² dry weight, 91 g/m² carbon or 912 kcal/m². Dry matter of Laminaria was 15 to 27% of fresh weight in blades, 10 to 12% in stipes. The dry matter content of blades was least in spring and highest in autumn, but carbon content and calorific value of dry matter showed little difference with species or season.Contribution to the International Biological Programme CCIBP 108.Bedford Institute Contribution BI 249.     

Boundary between the north and south Atlantic populations of the swordfish (<Emphasis Type="Italic">Xiphias gladius</Emphasis>) inferred by a single nucleotide polymorphism at calmodulin gene intron

Genetic differentiation of the Atlantic swordfish (Xiphias gladius) was investigated by a single nucleotide polymorphism (SNP) at the calmodulin gene (CAM) intron locus. Clearly distinct allele and genotype frequencies were observed between the north (20–41°N) and mid-south (10°N–33°S) Atlantic samples. Much lower frequency of A allele (37.5–57.1%) was observed in the north samples (n = 160 in total) than in the mid-south samples (83.3–92.6%; n=354), and homozygote BB was common in the north samples (23.4–31.3%) but very rare or absent (0–3.9%) in the mid-south samples. Very strong population subdivision was observed between the two groups (F _ST = 0.34, P < 0.001), while the allele and genotype frequencies within each ocean basin persisted over time (1990–2002 in the north, and 1994–2002 in the mid-south). Of two samples from the presumed boundary zone, one (n = 18) (14°N, 48°W) presented intermediate frequencies of the A allele (66.7%) and BB homozygote (11.1%), while the other (n = 23) (10–17°N, 28–37°W) shared similar frequencies of the A allele (89.1%) and BB homozygote (4.3%) with those of the mid-south Atlantic samples. These results indicate that the gene flow and individual migration between the north and mid-south Atlantic populations are considerably restricted and that the current management boundary between the north and south Atlantic swordfish stocks of 5°N should be reconsidered.     

Effects of ultraviolet radiation on different life cycle stages of the south Pacific kelps, Lessonia nigrescens and Lessonia trabeculata (Laminariales, Phaeophyceae)

The effects of exposure to ultraviolet radiation (UVR), 280–400 nm, in different life histories and development stages of the kelps, Lessonia nigrescens and L. trabeculata, collected in the south-east Pacific coast (30°S) were evaluated in the laboratory. Germination and viability (motile zoospores, settled spores), diameter of the primary cell of the gametophytes, percentage of female gametophytes, fertility and sporophytes production were measured after exposure to three radiation treatments (PAR; PAR + UVA; PAR + UVA + UVB). The effects of UVR in young sporophytes (diploid stage) were evaluated as changes in maximal quantum yield of chlorophyll fluorescence of photosystem II (PSII) (F _v/F _m). A significant decrease in all variables was observed for the treatment that included UVB (PAR + UVA + UVB) after 2 and 4 h of exposure, in relation to the control. The motile spores were more sensitive to UVR exposure compared to settled spores and gametophytes, suggesting that along with an increase in ontogenetic development; there is an increase in the tolerance to UVR. In addition, it was observed that early stages of the intertidal L. nigrescens were more tolerant to UVR compared to the subtidal L. trabeculata. These results allow initially to infer that UVR may be regarded as an important environmental factor influencing the upper limit of distribution of these species, mainly through its detrimental effects on the early stages of the life cycle.     

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

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

Molecular genetic variation in tarpon (<Emphasis Type="Italic">Megalops atlanticus</Emphasis> Valenciennes) in the northern Atlantic Ocean

The tarpon (Megalops atlanticus) is a highly valued game fish and occasional food fish in the eastern and western Atlantic Ocean. Tarpon have a high capacity for dispersal, but some regional biological differences have been reported. In this study we used two molecular genetic techniques—protein electrophoresis of nuclear DNA loci, and restriction fragment length polymorphism analysis of the mitochondrial DNA (mtDNA)—to assess this species population genetic structure in the eastern (coastal waters off Gabon and Sierra Leone, Africa) and western (coastal waters off Florida, Caribbean Sea) Atlantic Ocean north of the equator. Genetic differentiation was observed between tarpon from Africa and tarpon from the western Atlantic Ocean. A unique allele and haplotype, significant differences in allozyme allele and mtDNA haplotype frequencies between the African and western Atlantic samples, and significant F_ST analyses suggest that levels of gene flow between tarpon from these two regions is low. Among the western Atlantic Ocean collections, genetic diversity values and allele and haplotype frequencies were similar. AMOVA analyses also showed a degree of genetic relatedness among most of the western Atlantic Ocean collections: however, some significant population structuring was detected in the allozyme data. A regional jackknifed F_ST analysis indicated the distinction of the Costa Rica population from the other western Atlantic populations and, in pairwise analyses, F_ST values tended to be higher (i.e., genetic relatedness was lower) when the Costa Rican sample was paired with any of the other western Atlantic samples. These data suggest that Costa Rican tarpon could be partially isolated from other western Atlantic tarpon populations. Ultimately, international cooperation will be essential in the management of this species in both the eastern and western Atlantic Ocean.Communicated by P.W. Sammarco, Chauvin     

Geographic range limitation of the intertidal gastropods Littorina sitkana and L. planaxis

Geographic ranges of marine intertidal organisms are not entirely governed by discontinuities in temperature gradients. This is especially true in the eastern north Pacific Ocean, where temperature gradation is less steep than in the western north Atlantic Ocean. The southern limit of distribution of Littorina sitkana (Philippi, 1845) and the northern limit of L. planaxis (Philippi, 1847) occur at Charleston, Oregon (Latitude 43.4°N). No discontinuity in air or water temperature can be correlated with these distributional boundaries. A third species, L. scutulata (Gould, 1849), has the widest range, overlapping with L. planaxis in the south and L. sitkana in the north (Latitude 27.5° to 58°N). L. planaxis transplanted to northern Washington survived for 4 years, grew and produced planktonic larvae. Either larval mortality or south-flowing currents during planktonic life prevent this species from dispersing northward. L. sitkana has direct development and occurs only in habitats where adults are sheltered from waves and egg masses from desiccation. Lack of suitable habitat, coupled with the presence of the predatory crab Pachygrapsus crassipes (Randall, 1839) prevent the southward establishment of L. sitkana.     

Regulation of growth in Laminaria digitata: use of in-vivo nitrate reductase activities as an indicator of nitrogen limitation in field populations of Laminaria spp.

The seasonal variation in growth rate of a population of Laminaria digitata (Huds.) Lamour growing at Arbroath, Scotland was studied between August 1981 and September 1982, and was found to follow the biphasic annual cycle typical of this genus. Growth rates were maximum (0.3 cm cm^-1 mo^-1) in early June and minimum (0.05 cm cm^-1 mo^-1) between September and January. An analysis of the relationship between the seasonal changes in environmental factors (inorganic nitrogen concentrations, irradiance and temperature) with those of growth rate and the accumulation or mobilisation of cellular reserves of carbohydrates and nitrate, indicated that growth was nitrogen-limited between June and October and light-limited (with a possible co-involvement of temperature) for the remainder of the year. These conclusions were supported by the seasonal changes in the ratio of actual: potential in-vivo nitrate reductase activities in L. digitata, thus confirming the suitability of this technique for monitoring the occurrence of nitrogen limitation in Laminaria spp. The seasonal changes in blade nitrate reductase activities closely followed those of growth rate, with maximum activities [0.3 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] being present in late May and minimum levels [0.01 mol NO ₃ ^- reduced g^-1 (wet wt) h^-1] occurring between November and March. The correlation observed between nitrate reductase activities and growth rate is consistent with the ability of Laminaria spp. to store excess inorganic nitrogen, available during winter and early spring, as NO ₃ ^- , and with the requirement to conserve enzyme protein during the summer period of nitrogen limitation.     

Distribution patterns of meiofauna associated with a sublittoral<Emphasis Type="Italic"> Laminaria</Emphasis> bed in the Cantabrian Sea (north-eastern Atlantic)

This study deals with meiofauna associated with a sublittoral population of the kelp Laminaria ochroleuca located on the northern coast of Spain. By sampling once a year over a 4-year period, we examined patterns of faunal distribution as a function of some environmental factors at the meso-scale level (depth, and exposure to waves and surge). We also examined the relationship between L. ochroleuca abundance (as dry weight biomass and number of plants per sampling quadrat) and abundance and diversity of meiofauna. Finally, we investigated patterns of within-plant distribution (algal frond vs. algal holdfast), using also the meiofauna of the adjacent bottom as a referent to estimate the level of "phytal dependence" of the meiofauna collected on L. ochroleuca. We found that the bulk of permanent meiofauna consisted of nematodes, copepods, mites, polychaetes, tanaids and ostracods, with copepods being predominant on the fronds of the alga and nematodes in the holdfasts. The temporary meiofauna consisted of juvenile amphipods, bivalves and gastropods, together with barnacle nauplii and cyprids. Abundance and major composition of meiofaunal taxa were unrelated to both depth and hydrodynamic exposure of the sampling quadrats. However, we detected significant qualitative and quantitative faunal differences as a function of microhabitat. All meiofaunal groups were more abundant in holdfast samples than in frond and bottom samples. The gross taxonomic composition of meiofauna in bottom samples was similar to that in holdfast samples, but substantially different from that of meiofauna associated with the fronds. The L. ochroleuca holdfasts, in which dense aggregations of meiofauna can occur, appear to function as ecotone between phytal and rocky-bottom microhabitats. All together, our results suggest that the distribution of meiofauna within the Laminaria bed is mostly affected by factors operating at the microhabitat level rather than the meso-scale level.Communicated by L. Hagerman, Helsingør     

Cryptic species and population structuring of the Atlantic and Pacific seabob shrimp species, Xiphopenaeus kroyeri and Xiphopenaeus riveti

Seabob shrimps of the genus Xiphopenaeus are important fishery resources along the Atlantic and Pacific coasts of Central and South America. The genus was considered to comprise two species: the Atlantic Xiphopenaeus kroyeri (Heller, Sitzungsber Math Naturwiss cl kaiserliche Akad Wiss Wien 45:389–426, 1862), and the Pacific Xiphopenaeus riveti (Bouvier, Bull Mus Hist Nat Paris 13:113–116, 1907). In a recent review, Xiphopenaeus was regarded as a monotypic genus, on the basis that no clear morphological differences could be found between Pacific and Atlantic specimens (Pérez Farfante and Kensley, Mem Mus Nat Hist Nat Paris 175:1–79, 1997). In the present work, nuclear (allozymes), and mitochondrial (Cytochrome Oxidase I) genes were used to demonstrate the validity of X. riveti and reveal the presence of two cryptic species of Xiphopenaeus within X. kroyeri in the Atlantic Ocean. The high levels of molecular divergence among these species contrast with their high morphological resemblance. Interspecific sequence divergences (Kimura 2-parameter distance) varied from 0.106 to 0.151, whereas intraspecific distances ranged from 0 to 0.008 in Xiphopenaeus sp. 1, from 0 to 0.003 in Xiphopenaeus sp. 2, and from 0.002 to 0.005 in X. riveti. In addition, five diagnostic allozyme loci were found between sympatric samples of Xiphopenaeus sp. 1 and 2 along the Brazilian coast. The results suggest that Xiphopenaeus sp. 2 from the Atlantic is more closely related to the Pacific X. riveti than to the Atlantic Xiphopenaeus sp. 1. Furthermore, a high level of genetic structuring (Xiphopenaeus sp. 1: F _ST =0.026; P<0.05; Xiphopenaeus sp. 2: F _ST =0.055; P<0.01) was found in the Brazilian Xiphopenaeus populations, indicating the presence of different genetic stocks in both Atlantic species. These findings have important commercial implications as they show that the fisheries of the two Atlantic species must be managed separately, and that each one is comprised of different populations.Communicated by O. Kinne, Oldendorf/Luhe     

The role of behavioral responses to predators in modifying urchins' (Strongylocentrotus droebachiensis) destructive grazing and seasonal foraging patterns

We documented spatial and temporal patterns of urchins (Strongylocentrotus droebachiensis) and periwinkles (Littorina littorea) in three habitats: a persistent Laminaria longicruris and L. digitata bed; an urchin dominated barrens, and the edge of the kelp bed that formed a boundary between the two. Urchins were rare in the kelp and, when present, always large and well hidden, a pattern we interpret as a response to crab and lobster predation. Urchins were abundant in the barrens, and, in the summer when predaceous fish were active during the day, foraged only at nigh. We observed the formation of a dense urchin feeding front along the kelp bed edge, and these urchins remained exposed and feeding even during the summer. Laboratory experiments demonstrated that aggregations are an effective defense against some predators, and that the presence of crabs increases the tendency of large urchins to aggregate. We hypothesize that healthy Laminaria spp. beds persist because kelp bed associated predators keep urchins at low densities and in hiding. A reduction in predation pressure permits urchin densities to increase to the point where they form aggregations, which provide better defense than hiding. These aggregations then graze destructively on Laminaria spp., forming barrens. These barrens seem to be a new, stable configuration of the system.     

Genetic evidence of cryptic speciation within hammerhead sharks (Genus Sphyrna)

Surveys of genetic variation within cosmopolitan marine species often uncover deep divergences, indicating historical separation and potentially cryptic speciation. Based on broad geographic (coastal eastern North America, Gulf of Mexico, western Africa, Australia, and Hawaii) and temporal sampling (1991–2003), mitochondrial (control region [CR] and cytochrome oxidase I [COI]) and nuclear gene (lactate dehydrogenase A intron 6 [LDHA6]) variation among 76 individuals was used to test for cryptic speciation in the scalloped hammerhead, Sphyrna lewini (Griffith and Smith). CR and COI gene trees confirmed previous evidence of divergence between Atlantic and Indo-Pacific scalloped hammerhead populations; populations were reciprocally monophyletic. However, the between-basin divergence recorded in the mtDNA genome was not reflected in nuclear gene phylogenies; alleles for LDHA6 were shared between ocean basins, and Atlantic and Indo-Pacific populations were not reciprocally monophyletic. Unexpectedly, CR, COI, and LDHA6 gene trees recovered a deep phylogenetic partition within the Atlantic samples. For mtDNA haplotypes, which segregated by basin, average genetic distances were higher among Atlantic haplotypes (CR: D _HKY=0.036, COI: D _GTR=0.016) than among Indo-Pacific haplotypes (CR: D _HKY=0.010, COI: D _GTR=0.006) and approximated divergences between basins for CR (D _HKY=0.036 within Atlantic; D _HKY=0.042 between basins). Vertebral counts for eight specimens representing divergent lineages from the western north Atlantic were consistent with the genetic data. Coexistence of discrete lineages in the Atlantic, complete disequilibrium between nuclear and mitochondrial alleles within lineages and concordant partitions in genetic and morphological characters indicates reproductive isolation and thus the occurrence of a cryptic species of scalloped hammerhead in the western north Atlantic. Effective management of large coastal shark species should incorporate this important discovery and the inference from sampling that the cryptic scalloped hammerhead is less abundant than S. lewini, making it potentially more susceptible to fishery pressure.     

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.     

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.     

Growth and DNA damage in young Laminaria sporophytes exposed to ultraviolet radiation: implication for depth zonation of kelps on Helgoland (North Sea)

Growth as an integrative parameter of all physiological processes was measured in young sporophytes of temperate Laminaria digitata, Laminaria saccharina and Laminaria hyperborea exposed in the laboratory to irradiance consisting of either only photosynthetically active radiation (PAR) or to a spectrum including ultraviolet radiation (UVR) (PAR+UVA+UVB) by use of cut-off glass filters. Size increment was measured every 10 min over a period of 18–21 days using growth chambers with on-line video measuring technique. In the chamber, plants were grown at 10±2°C and 16:8 h light–dark cycles with 6 h additional UVR exposure in the middle of the light period. Tissue morphology and absorption spectra were measured in untreated young sporophytes while chlorophyll a content and DNA damage were measured in treated thalli at the end of the experiment. Sensitivity of growth under UVR was found to be related to the observed upper depth distribution limit of the upper sublittoral L. digitata, upper to mid sublittoral L. saccharina and lower sublittoral L. hyperborea. Tissue DNA damage is, however, dependent on thallus thickness which minimizes UVR effect where outer cell layers shade inner cells and provide longer pathlength for UVR. Exposure to UVR causes cellular, enzymatic and molecular damage. Presence of UV-absorbing compounds further reduces effective UVR from reaching physiological targets. The cost of producing higher amount of UV-absorbing compounds and effective DNA repair mechanism can, however, divert photosynthate at the expense of growth. Tissue chlorophyll a content was not significantly different between treatments suggesting a capacity for acclimation to moderate UVR fluence. Growth acclimation to repeated UVR exposure was observed within a period of 12 days while growth inhibition was observed after a longer UVR exposure period of 21 days. The results give further insight into the effects of UVR on the cellular level and show how ecological parameters such as the upper depth distribution limit are dependent on cellular processes.