Re-appraisal of age composition,growth and survivorship of the deep-sea brittle star Ophiura ljungmani from size structure in a sample time series from the Rockall Trough

Samples were taken from a 2 900 m deep permanent station (centred on 54°40N; 12°16W) in the southern Rockall Trough, from November 1975 to September 1980, inclusive. Revision of juvenile size frequencies was necessary after removal of postlarvae of an annual non-viable settlement of Ophiocten gracilis (G. O. Sars) that were mistakenly included in a previous analysis. The present study of the revised data shows that settlement of Ophiura ljungmani (Lyman) occurs in summer when disk-size frequencies are dominated by the newly metamorphosed postlarvae. The likely age composition in larger sizes was suggested from the progression of modes along the size axis in juvenile size classes in consecutive were developed from Gaussian mixtures fitted by constrained computer search using, as starting values, component parameter values suggested from the frequency histograms. Choice between competing solutions was made on the basis both of goodness-of-fit and biological plausibility. However, identification of year-class structure amongst sizes >3 to 4 mm disk diameter remained conjectural owing to the low frequencies of adults in the samples. Comparison of the fitted component-means in the time series suggested a marked seasonal growth pattern, with maximal growth in spring. Probable age composition amongst adult sizes was extrapolated from von Bertalanffy growth parameters estimated from the means of juvenile year classes.Communicated by J. Mauchline, Oban     

Diets of some deep-sea brittle stars in the Rockall Trough

The stomachs of 1 165 individuals of the species Ophiacantha bidentata (Retzius), Ophiactis abyssicola (M. Sars), Ophiocten gracilis (O. Sars), Ophiura irrorata (Lyman), O. ljungmani (Lyman) and Ophiomusium lymani Wyville Thomson, collected from the general area of the Rockall Trough from 1973 to 1983, were examined; 47% contained material. This varied in nature and in volume between individuals, but overall, the diets of the 6 species showed quite high similarity. The few differences evident are interpreted as reflecting different lifestyles. Except for Ophiocten gracilis, the probably motile, epifaunal species (Ophiura irrorata, O. ljungmani and Ophiomusium lymani) showed the greatest variety in items, and the lowest similarity with the remaining species; they are probably unselective omnivores, eating whatever small prey or organic detritus they are able to find. Ophiacantha bidentata and Ophiactis abyssicola showed both the least dietary variety, and the highest similarity to each other. This probably reflects their more sedentary lifestyle, feeding on current-borne particles and small prey from perches on sessile fauna. The stomach contents of Ophiocten gracilis consisted mainly of amorphous organic material and mineral particles, indicating a microphagous feeding mechanism. With O. gracilis, the proportion of stomachs with food decreased markedly in summer, this coinciding with seasonal gonadal development when stomachs frequently appeared ruptured or compressed. The possible importance of seasonal sedimentation of fast-sinking particulates from the surface is discussed in relation to finds of flocculent organic material, often containing diatom frustules, in stomachs of all 6 species. Some doubts remain as to whether our results are biased by specimens egesting stomach contents on capture. As in shallow water, deep-sea ophiuroids seem to be trophic generalists lacking in dietary specialization. Brief notes on parasites encountered are also given.     

Reproduction, dispersal and settlement of the bathyal ophiuroid Ophiocten gracilis in the NE Atlantic Ocean

  Ophiocten gracilis is an ophiuroid found at bathyal depths in the North Atlantic Ocean. The adults show strong seasonal reproduction, with an ophiopluteus in the surface plankton. Settling postlarvae were collected in sediment traps moored at 1000 and 1400 m depth in the NE Atlantic during Julian Days 142 to 212 (May to July) in 1996. During this period, growth of postlarvae in the traps was linear and the diet consisted of phytodetritus and foraminifera. Experiments suggest that postlarvae sink at rates of up to 500 m d⁻¹, although this may well be slower in the natural environment. The high fecundity, seasonality and high population density resulted in high fertilization success, and many of the offspring were advected outside the normal adult range, where they were able to settle but did not survive to adulthood. Received: 8 October 1999 / Accepted: 8 May 2000     

Seasonal variation in some population parameters of Euchaeta species (copepoda: Calanoida)

Studies of the biology of oceanic copepods are few relative to those of coastal species. Females of the genus Euchaeta have spermatophores attached to the genital somite by the male and carry their broods of eggs attached to this somite, so defining the breeding season. Populations of E. norvegica in the fjordic environment of Loch Etive (collected between 1971 and 1974 and from 1978 to 1979), Scotland and in the marginal oceanic region of the Rockall Trough, northeastern Atlantic Ocean (collected between 1973 and 1976) and of a further eight species of Euchaeta in the Trough are examined. Seasonal changes in the incidence of egg masses and spermatophores attached to the females, sex ratio and population numbers are determined. Sampling errors in the estimation of these parameters are larger in the oceanic Rockall Trough. The Loch Etive populations of E. norvegica produce two generations per year with a proportion, variable between years, of the population producing a third generation. This species produces a single annual generation in the Rockall Trough as do E. acuta and E. pseudotonsa. Two generations per year are probably passed through by E. gracilis in the Trough while the large bathypelagic E. sarsi may produce a single generation in the autumn of every second year. The other bathypelagic species, E. barbata, E. scotti, E. abbreviata and E. longissima, breed continuously throughout the year and no estimate of their generation times is possible. Consequently, this approach to population analysis is useful where breeding is seasonal, but resolving generation time in bathypelagic crustaceans remains a problem.     

Observations on the reproduction and postlarval morphology of pourtalesiid sea urchins in the Rockall Trough area (N.E. Atlantic Ocean)

Gametogenesis was studied histologically in the deep-sea species Pourtalesia jeffreysi (Wyville Thomson), P. miranda (A. Agassiz) and Echinosigra phiale (Wyville Thomson) collected during 1973–1983 from various positions at depths of 1 040 to 2 921 m in the Rockall Trough. The gonads are small and no synchrony in terms of the stage of gametogenic development was evident in females of the latter two species collected at different times of the year, indicating nonseasonal breeding. Males appear to be ripe at all times of the year. Follicle cells were well developed around the developing oocytes. Both the size range of the yolky ripe eggs (173 to 357 m diam) and potential fecundity within the range of 1 000 to 4 000 oocytes per individual, are indicative of an abbreviated, indirect, lecithotrophic mode of development occurring in all three species. Pourtalesiid postlarvae occurred also in fine-meshed trawl hauls, and their morphology is described. It was not possible to distinguish between P. miranda and E. phiale at sizes <3 mm in length.     

Growth and recruitment of the deep-sea urchin Echinus affinis

Large samples of the sea urchin Echinus affinis Mortensen were obtained from a time-series of Agassiz trawlings from a 2 200 m-deep permanent station (Station M), and at neighbouring positions, in the Rockall Trough (North-east Atlantic Ocean) over a period of 7 yr (1978 to 1985). Counts of growth zones visible in the skeletal elements of the test were used to age individuals. Various growth functions were fitted to counts from a full range of the sizes available. Functions giving a sigmoidal growth curve fit the early phase of growth better than the von Bertalanffy model, although the latter provided better fit amongst larger sizes. The fit of a robust and flexible model recently developed by Preece and Baines to describe the human growth curve overcame this limitation. Skeletal banding is thought to result from seasonally varying growth as a result of annually pulsed fallout of phyto-detrital food to the deep-sea floor. Early stages were found in only a few of a time series of samples obtained with a fine-meshed epibenthic sledge, suggesting that recruitment to the population from its annual breeding may only occasionally be successful. Postlarval growth was estimated from samples taken soon after presumed settlement and later in the year. The fitted growth curve showed good agreement to that obtained from annual banding, and corroborates an initially exponentially increasing growth rate. Postlarval survivorship was estimated, by means of computer simulation, from a sample that included postlarvae along with a range in juvenile size, to be about 10% amongst postlarvae after settlement and thereafter about 90% yr^-1. Adults are inferred to be up to about 28 yr old. A markedly uneven representation of ages in a large subsample of aged adults of even frequency in size is interpreted by means of simulations as probably reflecting multi-year cycles in recruitment success. The possible causes of a varying size structure amongst large samples of adults, which showed some spatial segregation in relation to the track of the trawl on the bottom at Station M, are discussed.     

Long-term changes in reproductive patterns of the holothurian<Emphasis Type="Italic"> Oneirophanta mutabilis</Emphasis> from the Porcupine Abyssal Plain

Time-series studies (1989–2002) in the NE Atlantic have shown large-scale changes in the composition and structure of the benthic community on the Porcupine Abyssal Plain. Radical changes in the abundance of some species in 1996 led to a significant shift in the way in which organic matter was reprocessed at the seabed. This article examines the reproductive processes of the holothurian Oneirophanta mutabilis collected during the time series. The reproductive biology of O. mutabilis is reviewed. No males were evident in any of the samples. The sex ratio (females: no gender) was biased significantly towards no-gender individuals. The maximum egg size was 650 m and there was no evidence of synchrony in reproduction. Significant changes in the oocyte-size distribution and the fecundity of O. mutabilis were noted through time, coinciding with the time of greatest faunal change in the benthic community. There was an increase in the proportion of previtellogenic oocytes and a decrease in the proportion of mature vitellogenic oocytes in 1997 and 1998, in parallel with a significant decrease in fecundity of the post-1996 samples. Samples from 2002 showed a reversal in the reproductive trends, with an increase in the proportion of mature vitellogenic oocytes and fecundity. The results are discussed in relation to large changes in abundance of the epibenthos on the Porcupine Abyssal Plain. It is suggested that the superabundance of certain megafaunal species on the abyssal seafloor affected the availability of trophic resources for O. mutabilis, leading to the changes in its reproductive effort.     

Deep-sea bottom-living fishes at two repeat stations at 2 200 and 2 900 m in the Rockall Trough,northeastern Atlantic Ocean

Repeated sampling of deep-sea bottom-living fishes was conducted at two stations in the Rockall Trough at depths of 2 200 and 2 900 m. The 2 200 m station (M) was sampled 16 times by an Agassiz trawl between April 1978 and April 1985 and yielded 473 fish belonging to 17 species. The 2 900 m permanent station (PS) yielded 781 fish belonging to 11 species from 9 Agassiz trawls, 2 small box otter trawls, 4 semi-balloon otter trawls and 7 epibenthic sledges between March 1975 and April 1985. Macrourid fishes were numerically dominant at both stations with Coryphaenoides guentheri and Coryphaenoides (Nematonurus) armatus being the most abundant species at M and PS, respectively. In terms of biomass the morid, Antimora rostrata, was dominant at M but C. (N.) armatus was dominant at PS. The only species that showed clear evidence of a seasonal reproductive cycle was C. guentheri. Many species showed no indication of reproducing in the Rockall Trough. The diets of almost all the species are described and considered in relation to the food resources exploited. Investigations on the pelagic and benthic invertebrate faunas at these stations should reveal the extent to which the bottom-living fishes exploit the available resources.     

The reproductive biology of echinothuriid and cidarid sea urchins from the deep sea (Rockall Trough,North-East Atlantic Ocean)

The reproductive biology of 5 species of echinothuriid (Phormosoma placenta, Calveriosoma hystrix, Araeosoma fenestrum, Sperosoma grimaldii and Hygrosoma petersii) and 2 species of cidarid (Cidaris cidaris and Poriocidaris purpurata) sea urchins from the deep sea (Rockall Trough) has been examined from samples collected during 1973–1983. In all species the gonads lie within the interambulacrum attached to aboral gonopores and when fully developed occupy most of the test not occupied by the gut or Aristotle's lantern. In all the species, initial oocyte development takes place along the germinal epithelium embedded in nutritive tissue. In all the echinothuriids and in Poriocidaris purpurata, the oocyte grows to ca. 200 to 450 m, at which stage vitellogenesis begins. Oocyte growth continues until a maximum egg size of 1 100 to 1 500 m is attained. In the echinothuriids, two types of nutritive tissue are found. In the carly stages of gametogenesis the oocyte is surrounded by well-structured periodic acid Schiff (PAS)-positive tissue. As the oocyte grows this tissue becomes vacuolated, suggesting that there is a transfer of nutriment to the developing oocyte. In Phormosoma placenta, unspawned oocytes are phagocytosed. There is no evidence of seasonality in any of the echinothuriid species or in Poriocidaris purpurata. Extrapolation with shallow-water echinothuriids suggests that larval development is lecithotrophic, omitting any planktotrophic phase. Of the species examined, only Cidaris cidaris has a reproductive strategy which produces a known larva, although the limited samples did not permit any determination of seasonality in this deep-sea population.     

Dietary constraints on reproductive periodicity in two sympatric deep-sea astropectinid seastars

Examination of the diet of two sympatric species of seastar, Bathybiaster vexillifer and Plutonaster bifrons from 2200 m depth in the Rockall Trough, NE Atlantic Ocean (57°18'N; 10°28'W), suggested that diet may determine the different reproductive patterns found between these two species. In the non-seasonally breeding B. vexillifier, the diet showed a high Shannon-Wiener prey diversity index, the dominant prey being the irregular echinoid Hemiaster expergitus together with a variety of prosobranch gastropods and protobranch bivalves. By contrast, the prey diversity in the seasonally breeding P. bifrons was significantly lower than that of B. vexillifer. In addition, organic carbon content in the sediment residue in the stomachs of P. bifrons displayed a seasonal cycle, while no such seasonality was detected in B. vexillifer. The stomachs of P. bifrons also contained a higher proportion of scavenged material, including the seasonally available remains of mesopelagic blue whiting. These data, together with Bathysnap (time-lapse camera) observations of feeding behaviour in both species, suggest that B. vexillifer is a predator feeding deep in the sediment, whereas P. bifrons feeds close to the sediment surface where it is affected by the seasonal availability of phytodetritus and fish carcasses.     

The near-bottom shrimp faunas (Decapoda: Natantia) at two abyssal sites in the Northeast Atlantic Ocean

Bottom and near-bottom decapod shrimp communities from two northeast Atlantic abyssal sites (ca. 4 040 and 5 440m) were examined in 1984 and 1985. In both cases, an increase in abundance was observed in the bottom 90 m of the water column, with the largest numbers of shrimps occurring closest to the sediment surface (10 to 25 m off the bottom). The benthopelagic communities were composed of species drawn from both pelagic and benthic habitats rather than being specific and discrete. Hymenodora glacialis was the dominant species at the shallower site but it was absent from the near-bottom samples at the deeper one. H. acanthitelsonis, however, occurred at both sites. Several new depth records have been established, and there are two recordings of species previously unknown to the eastern Atlantic basin. Vertical migration and differences in modes of reproduction are discussed.     

Life-history biology of Peniagone azorica and P. diaphana (Echinodermata: Holothurioidea) from the north-east Atlantic Ocean

Aspects of the reproduction and population biology of two elasipodid holothurians collected during the period 1973–1984 from the north-east Atlantic Ocean were examined. The reproductive biology of both the benthic species Peniagone azorica and the benthopelagic P. diaphana are similar. A primary oocyte forms from an oogonium and grows to about 100m diameter, whereupon it undergoes vitellogenesis and increases to 300m before being spawned. A high proportion of primary oocytes are not spawned and undergo a complicated breakdown process resulting in the formation of an amorphous sac in the ovary wall. It is possible that when an ovarian tubule is full of these sacs it atrophies and drops off, allowing other tubules to develop. The maximum egg size for both species suggests abbreviated larval development. In P. azorica, juveniles reach sexual maturity for the first time at about 30 mm length. The population structure suggests that the adults grow slowly, although it is possible that the observed unimodal distribution in size-frequencies results from one or several recruitments. Recruitment to the population is probably infrequent and may occur irregularly. Comparison of the population structure of closely spaced samples suggests a patchy distribution on the bottom. The benthopelagic lifestyle of adult P. diaphana suggests that the larvais also planktonic.     

Diets and bathymetric distributions of the macrourid fish of the Rockall Trough,northeastern Atlantic Ocean

The Macrouridae are the most common fish caught in demersal trawls on the continental slope and rise of the Rockall Trough. They represented 41% of all fish caught, with Coryphaenoides rupestris amounting to 28% of the catch. No previous study of the trophic interaction of these fish has been made over a wide bathymetric range. Samples were obtained at 250-m intervals of depth between 400 and 2 900 m in the period 1975 to 1981. The stomachs of 5 326 fish belonging to 12 species were examined to define their diets. Eight species are primarily benthopelagic feeders while four are primarily epibenthic feeders. The bathymetric centres of distribution of the populations of the benthopelagic feeding Trachyrhynchus murrayi, Malacocephalus laevis, Coryphaenoides rupestis, C. guentheri, Nematonurus armatus, Chalinura brevibarbis, C. leptolepis and C. mediterranea are different from each other. Similar differences in bathymetric distribution occurred among the epibenthic feeding Nezumia aequalis, Coelorhynchus coelorhynchus, C. occa and Lionurus carapinus. These differences decrease competition among species that exploit similar resources.     

Skeletal growth markers in the deep-sea brittle starsOphiura ljungmani andOphiomusium lymani

Growth bands have been found in the calcitic vertebral arm ossicles of the commonly occurring deep-sea brittle starsOphiura ljungmani Wyville Thomson andOphiomusium lymani (Lyman) (Echinodermata: Ophiuroidea) trawled from the Rockall Trough (N.E. Atlantic) at 2 200 and 2 900 m depth from 1973 to 1982. InOphiura ljungmani, the study of ossicle microstructure by SEM shows that growth bands reflect differences in stereom porosity and surface relief, similar to that previously found amongst shallow-water brittle stars. The pattern inOphiomusium lymani was much less clear from the microstructure, but could be revealed by heating the ossicle to 450 °C and clearing in xylene. The bands showed up as fine, translucent rings separated by more opaque, wider zones, perhaps reflecting differences in organic material incorporated within the calcite. Both growth-banding patterns probably reflect an annual cycle in skeletal growth rate. On this assumption, the number and spacing of the banding in the two species indicates contrasting growth strategies. WithOphiura ljungmani, a rather regular annual growth increment and perhaps shorter lifespan (up to ca. 10 yr) thanOphiomusium lymani is indicated. The latter shows a relatively wide spacing of early bands, followed by tight clustering of the outermost bands corresponding to adult sizes. This growth pattern is characteristic of species escaping from predation by rapid growth to relatively large adult size. AdultO. lymani probably grow slowly, some perhaps reaching 20 yr of age. Growth curves were fitted to size-at-age corresponding to measurements of the size and ordering of growth bands. These corroborate age structure previously estimated from analysis of size frequencies in time series from the stations sampled in the present study. Such skeletal growth markers should be of value in analysis of the demography of deep-sea populations.     

Food habits and intestinal parasites of deep demersal fishes from the upper continental slope east of Newfoundland,northwest Atlantic Ocean

Stomach contents and intestinal parasite faunas of 471 individuals of demersal fishes in 14 species were examined from the Carson Canyon region (Lat. 45°30N; Long. 48°40W) of the upper continental slope of the Grand Banks off Newfoundland, Canada. Individual species tended to feed either on benthic or on pelagic/benthopelagic organisms, but pelagic prey assumed the greatest importance overall. Data from stomach contents were supported by the parasite information. Prevalence of parasites was higher in benthic feeders (53.1%) than in pelagic feeders (28.9%), and relative abundance by major group was: Digenetic Trematoda 5.8% benthic vs 27.8% pelagic, Nematoda 53.1% vs 72.2%, and Acanthocephala 40.9% vs 0%. Of the dominant fishes, there were more species of benthic feeders (5) than pelagic feeders (3), but pelagic feeders were numerically more abundant (pelagic 70.9%, benthic 20.5%). Benthic feeders were on average larger (=270.6g) than pelagic ones (=130.6g), but pelagic feeders represented a larger proportion of the biomass (pelagic 43.3%, benthic 25.9%). The results of this study combined with those from other areas suggest that feeding from the pelagial by demersal fishes at upper continental slope depths is probably the general rule.     

Reproductive strategies and diet in deep-sea nuculanid protobranchs (Bivalvia: Nuculoidea) from the Rockall Trough

Although the taxonomy of deep-sea protobranch bivalves is becoming better known, relatively little information is available on their reproductive biology and whether or not populations show reproductive periodicities. We have examined the reproductive biology of three common sympatric species as part of a long-term time-series of samples taken from 2900 m in the Rockall Trough from 1973 to 1983. Malletia cuneata Jeffreys, 1876 produces a maximum of 30 oocytes at any one time and these grow to a maximum size of 240 m. Maximum fecundity of Ledella pustulosa (Jeffreys, 1876) and Yoldiella jeffreysi (Hidalgo, 1877) is 174 and 360, respectively, and both species produce an egg of 120 m in diameter. These data indicate lecithotrophic early development in L. pustulosa and Y. jeffreysi, but direct development in M. cuneata; however, evidence from the prodissoconch length of M. cuneata suggests lecithotrophic development. L. pustulosa and Y. jeffreysi also differ from M. cuneata in having a distinct reproductive cycle with spawnout in the winter months. Both the continuously breeding M. cuneata and the seasonally breeding L. pustulosa ingest diatoms, coccoliths and foraminiferans, but whereas the diet of M. cuneata appears to be constant throughout the year there is an apparent reduction in the feeding activity of L. pustulosa concomitant with the deposition of phytodetritus on the deep-sea bed.     

Behavioural responses of competent lobster postlarvae to odor plumes

We determined the ability of competent laboratory-reared Homarus americanus postlarvae to use odor plumes as a potential mechanism of orientation during the habitat selection process at settlement. In a Y-maze apparatus, both swimming direction (upstream or downstream) and arm selection (control or experimental) were strongly influenced by the nature of the chemical stimulus tested. In control conditions (unconditioned water in both proximal arms of the experimental device), postlarvae swam predominantly upstream (61:39) but displayed no arm preference (48:52). When submitted to a metabolite solution of adult conspecifics or macroalgae, postlarvae swam consistently upstream (73%) and showed a significant preference for the arm providing these stimuli (69%). In contrast, postlarvae submitted to cunner (Tautogolabrus adspersus) metabolites swam predominantly downstream (68:32), and those that did swim upstream significantly avoided the experimental arm (30:70). In repeated trials with the same individuals, postlarvae behaved similarly in 80% of the assays, suggesting a low intra-individual variability in behavioural responses. It is concluded that distance chemoreception may play a role in habitat selection by lobster postlarvae at settlement.     

New patterns of diurnal vertical migration of some deep-water copepods in the Tyrrhenian and Adriatis Seas

The diel vertical migration of copepods from intermdiate water layers of the Tyrrhenian and Adtiatic Seas has been studied on the basis of plankton samples made from depths down to 1000 m, with a Nansen net, mesh size 250 , diameter 113 cm. The species considered display two different patterns of migration: (1) nocturnal ascent, e.g. Pleuromamma abdominalis and Euchaeta acuta; (2) nocturnal descent, e.g. species of the genera Spinocalanus, Scaphocalanus, Temoropia, Mormonilla and Oncaea. These two patterns are compared on the pasis of the dimensions of the species and of their different capacities for active movement. Ililumination is considered to be the most important factor influencing vertical movements. The behaviour of Pleuromamma gracilis is also described, the adult population of which is divided in two discrete sections, each one showing a different pattern of migration.     

Growth and production of<Emphasis Type="Italic"> Ophiocten gracilis</Emphasis> (Ophiuroidea: Echinodermata) on the Scottish continental slope

Growth and production of the bathyal ophiurid brittle star Ophiocten gracilis was studied from skeletal growth bands and disc size frequencies of specimens collected in sled and trawl samples taken on the continental slope off Scotland. Growth bands showed up in SEM examination as ring-like zones in surface relief and texture of the stereom microstructure of the intervertebral muscle insertions on the arm ossicles. Seasonal variability in somatic growth, presumed to underlie this growth pattern, may reflect reproduction and/or a possible non-feeding period during gonad maturation. Disc size-at-age was back-calculated from size-at-age interpreted from growth-band series on the vertebral ossicles from arms of O. gracilis. Pooled growth-band frequency data and normal-distribution mixtures based on size-at-age data were used to test for overgrowth of early growth bands on the ossicles from larger individuals. Von Bertalanffy and Gompertz growth models were fitted to the finalised back-calculated disc size-at-age data. These were used along with the modal structure of the observed disc size frequencies to develop a demographic model based on normal-distribution mixtures constrained by the growth model. These and other defining parameters were fitted by non-linear regression to size structure observed in a sample of the breeding population from 997 m depth on the Hebrides Terrace. Recruitment was estimated according to available data from sediment-trap time series. A ratio of somatic production/biomass, P_S/B, in the range of 0.43–0.54 was estimated using a fitted size/mass relationship and the increment summation method (ISM) applied to the fitted growth models. A narrower, but otherwise similar, range in estimated P_S/B ratios (0.48–0.49) was obtained in a parallel approach using the mass-specific growth rate method (MSGRM), whereby the same size/mass relationship was applied to the observed frequencies and growth parameters fitted to growth banding. Using previously obtained data on population density, a standing crop of 4.8 g wet weight (~0.58 mg AFDW) m^?2 would provide annual wet weight production in the range of 1.9–2.4 g (~0.23–0.29 mg AFDW) m^?2 in the population between ca. 700–1000 m depth. Somewhat greater production estimates (P_S/B=0.73–0.98) were obtained from MSGRM by pooling the sample with size frequencies from other large samples in which postlarval sizes were more numerous, but larger sizes less numerous. Similarly high production was estimated by MSGRM from a box corer sample from the Wyville-Thomson Ridge. Explanations for variability in size structure are discussed, but even the lower estimates are comparable to boreal shallow-water brittle stars. The high rate of growth and production by accepted deep-sea standards may be related to a capability for interface feeding.     

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