Zooplankton life under the perennial Antarctic Sea ice

Zooplankton was collected during the drift on the USA-Russia Ice Station Weddell 1 in the western Weddell Sea. Stratified tows up to 1,000 m depths were performed under perennial ice at a distance of ca. 300 km from the marginal ice zone from March to May 1992. Seasonal abundance, developmental stage composition and vertical distribution in populations of large dominant calanoid copepod species were studied. The abundance of Calanoides acutus changed inconspicuously and averaged 1,260 ind. m^-2; its developmental stage composition was characterised by the dominance of copepodites V (CV) and hardly changed in the course of observations. Calanus propinquus was at most stations in fairly low abundance and was represented by CV and adults; at the latest stations, it was not found. Rhincalanus gigas was also scarce; III-IV stages were present, but the first of them only at the southernmost stations. In contrast to this species, in the relatively abundant population of Metridia gerlachei (1,370 ind. m^-2 on average), a new generation was observed. There was a clear deepening of the median depth of occurrence of the above three species from March to early May, while in late May a shifting to upper layers was observed which remained unexplained. The data obtained were compared to the data collected in the ice-open eastern part of the Weddell Sea in the corresponding season. It can be seen that the differences in populations dynamics of the four species under the perennial ice depends on their trophic mode, i.e. capability to store lipid reserves in summer and/or to feed in winter, and with food spectra.     

Parasites of the Antarctic rattail <Emphasis Type="Italic">Macrourus whitsoni</Emphasis> (Regan, 1913) (Macrouridae,Gadiformes)

A total of 386 Macrourus whitsoni from Antarctic waters were examined for ecto- and endoparasites. Sixty-five M. whitsoni collected near Halley Bay (Weddell Sea) and 321 specimens from the continental slope off King George Island (South Shetland Islands) were studied for sphyriid copepods directly after being caught. A subsample of 25 specimens from the Weddell Sea and of 9 specimens from King George Island were studied for the presence of other metazoan parasites. Twenty-two species were found, including one myxozoan, six digeneans, one monogenean, three cestodes, seven nematodes, one acanthocephalan and three crustacean species/taxa. While Auerbachia monstrosa and Capillaria sp. are reported for the first time from around the Antarctic, the other parasites have been recorded earlier in the Southern Ocean. Many parasite species found have a wide zoogeographical range and a low host-specificity. The parasite fauna of M. whitsoni revealed several similarities with its congeners M. carinatus and M. holotrachys from Antarctic and sub-Antarctic waters. This can be explained by a wide host range of many macrourid deep-sea parasites, together with an overlap in distribution patterns of their hosts. Other supporting factors are host migration and a close phylogenetic relationship between the hosts, which enable the parasites to infest all three macrourids. Eight new host and 14 new locality records are established.     

Observations on population structure and reproductive features of <Emphasis Type="Italic">Laetmonice producta</Emphasis> Grube (Polychaeta,Aphroditidae) in Antarctic waters

The large scale-worm Laetmonice producta Grube 1877 is the most abundant aphroditid polychaete in Antarctic coastal waters. We investigated the demographic structure and some reproductive features of different L. producta populations from high-Antarctic (Weddell Sea) and Antarctic Peninsula (King George Island) shelf bottoms, collected in summer 1996 (ANT-XIII/3, EASIZ I cruise) and autumn 2000 (ANT-XVII/3, EASIZ III cruise). L. producta in the studied geographic areas showed a wide bathymetric range (200-850 m depth), and a different size distribution pattern with depth, characterised by a reduction of large specimens in the deepest stations. The species is gonochoric, with females more abundant in specimens of larger sizes. Eggs at different stages of maturation (ranging from 40 to 320 wm in diameter) were examined in 270 individuals from different stations and size classes. Egg size showed a slightly bimodal trend, with largely overlapping egg cohorts, suggesting a continuous reproduction, and a long-lasting gametogenesis. Significant differences (Kolmogorov-Smirnov test, P<0.05) in egg-size frequency distribution were detected only when data of the two geographic areas were compared (Weddell Sea vs King George Island), and not according to stations within each area, and females' size. The two sets of geographic samples were collected in different seasons and therefore it was not possible to assess if differences observed are due to sampling time or to geographic factors. Mature spermatozoa were recognisable only in autumn male specimens from King George Island, and showed a rounded nucleus and a short conical acrosome. Occurrence of an endosymbiont polychaete, Veneriserva pygoclava meridionalis (new sub-species of Dorvilleidae), was recorded in the coelomic cavity of 163 specimens of L. producta, 125 of which were from the deepest station of the Weddell Sea (stn. 14, 850 m depth). L. producta females with and without the endosymbiont did not show differences in egg-size distribution. The reproductive features of L. producta, together with its large size and slow growth, seem typical of a long-living predator species, and uncoupled from the typical summer environmental conditions and the pulsating system of coastal Antarctic waters.     

Diversity of reproductive features in some Antarctic polynoid and sabellid polychaetes, with a description of Demonax polarsterni sp. n. (Polychaeta, Sabellidae)

The Polynoidae and Sabellidae represent two polychaete families with contrasting life habits and different phylogenetic constraints. Species of both families studied in this work were collected on the shelf of the Eastern Weddell Sea (Antarctica) during the EASIZ-I (1996) and EASIZ-II (1998) expeditions. Among the Polynoidae, three different species of the genus Harmothoe were observed to brood eggs under the dorsal elytra, which is an uncommon feature in this family. The egg size of all brooding taxa ranged between 120 and 216 µm. Three Sabellidae species were studied: Myxicola cf sulcata is gonochoric with eggs among the largest (up to 1000 µm) recorded for the sabellids. Eggs, as well as larvae, are incubated within the branchial crown, a feature newly discovered for the genus Myxicola. Demonax polarsterni n. sp. (a newly discovered taxon) and Euchone pallida are gonochoric and possess relatively large eggs (350 µm and 270 µm, respectively). In comparison with cogeneric species of temperate areas, the Antarctic Sabellidae have larger egg dimensions and egg brooding that seems unconstrained by adult size.     

New species of benthopelagic hydromedusae from the Weddell Sea

Four medusa species were collected by an epibenthic sledge during the "Polarstern" ANT XV/3 cruise carried out from January to March 1998 in the eastern Weddell Sea. The specimens were collected in the benthic boundary layer at depths ranging between 1,583 and 2,034 m; 2 of the species collected are new to science. The narcomedusa Sigiweddellia bathypelagica gen. nov. et sp. nov. is characterised by two types of marginal tentacles and closed marginal statocysts. The trachymedusa Voragonema laciniata sp. nov. (known only from the single holotype) is characterised by the number and irregular shape of the centripetal canals. These findings are the first to report benthopelagic hydromedusae in deep Antarctic waters. Examination of several specimens of Benthocodon pedunculata (Bigelow 1913) leads us to move it to the genus Voragonema Naumov 1971 because of the clear presence of centripetal expansions in the ring canal.     

Ophiuroid biodiversity patterns along the Antarctic Peninsula

Benthic ecological surveys using standardized methods are crucial for assessing changes associated with several threats in the Southern Ocean. The acquisition of data on assemblage structure over a variety of spatial scales is important to understand the variation of biodiversity patterns. During the ANT XXIX/3 (PS81) expedition of RV Polarstern, three different regions at the tip of the Antarctic Peninsula were sampled: the northwestern Weddell Sea, the Bransfield Strait, and the northern boundary of the South Shetland Archipelago in the Drake Passage. The aim of this study was to characterize the distribution and biodiversity patterns of ophiuroid assemblages in these regions and depths. We quantified different community parameters in terms of the number of species, abundance, and biomass. Additionally, we calculated various components of species diversity (alpha, beta, and gamma diversity) over the three regions. Based on the benthic surveys, we collected 3331 individuals that were identified to species level (17 species). Overall, species diversity, as measured based on rarefaction, species richness and evenness estimators, was higher in the Bransfield Strait compared to the Weddell Sea and Drake Passage. Two deep stations in the Weddell Sea showed high dominance only of Ophionotus victoriae. Significant differences in the patterns of alpha diversity were found among the regions but not between depth zones, whereas beta diversity showed no differences. Regarding the resemblance among the ophiuroid assemblages of each region, there was a significant gradient from east to west with a maximum distance between the stations in the Drake Passage and the Weddell Sea. This study provides a baseline for detecting potential effects related to climate change, and it furnishes a basis for the implementation of monitoring schemes of Antarctic assemblages.     

A first attempt at determining larval growth in three Antarctic fish from otoliths and RNA/DNA ratios

Otolith growth and RNA/DNA ratios of larval stages of Notolepis coatsi, Gobionotothen gibberifrons and Trematomus scotti, three Antarctic fish, were studied during early 1998. RNA/DNA ratios were significantly different between the notothenioids and paralepidids (P<0.01), but similar among notothenioids (P>0.05). The ratios were independent of larval total length and water temperature. Recent otolith growth (based on the last five increments) and biochemical indices correlated slightly, but not significantly. N. coatsi inhabited deeper and colder waters of the Weddell Sea and showed less growth than the other species, which are associated with the Antarctic Peninsula. In all three species, some larvae had very good growth rates though most were rather poor. Recent growth indices might allow the detection and back-dating of growth changes in Antarctic larvae, and provide insight into a crucial yet poorly understood life-phase of these fish.     

Larval necrophilia: the odd life cycle of a pandeid hydrozoan in the Weddell Sea shelf

Colonies of an athecate hydroid were found at six stations in the high Antarctic (Weddell Sea) growing on dead specimens of Flabelligera mundata (Annelida, Polychaeta). All living specimens of F. mundata at the same stations were free of epibionts. Transplantation experiments showed that hydropolyps did not produce stolons on substrates other than the epidermal jelly coat and chetae of dead F. mundata specimens. The largest colonies (>1,000 polyps) producing medusa buds were cultured until medusa liberation: growth of medusae was then surveyed for the next 5 weeks, but development of adult features was extremely slow. Young medusae were ascribed to the suborder Pandeida by the presence of two main characters, namely (1) hollow marginal tentacles, and (2) a mouth with four simple lips. Considering polyp and young medusa features, this species is acknowledged as newly recorded for the Southern Ocean, and assigned to the genus Neoturris (Hydroidomedusae, Pandeidae).     

Mobile epiphytic invertebrates inhabiting the brown macroalga, Desmarestia chordalis, under the coastal fast ice of Lützow-Holm Bay, East Antarctica

The species composition of mobile epiphytic animals inhabiting the brown alga, Desmarestia chordalis Hooker and Harvey, in Lützow-Holm Bay, East Antarctica, was investigated. A total of 797 individual invertebrates in 22 taxa were collected from 3 clumps of algae. The assemblage was dominated by two amphipod crustaceans, Prostebbingia sp. and Haplocheira plumosa Stebbing, which composed 70.3% of the total number of individuals, followed by a gastropod species, Skenella paludinoides (Smith). The species composition differs from those found in previous studies conducted in Prydz Bay and Ross Sea, suggesting the existence of various types of epifaunal communities along the East-Antarctic coast.     

The role of iceberg scours in niche separation within the Antarctic fish genus Trematomus

Species of the Antarctic fish genus Trematomus occupy different trophic niches. It is not clear, however, whether small-scale variability in benthic community structure affects niche separation. Therefore abundance and biomass of fish were determined and stomach content and food composition were compared in areas affected by iceberg scours and unaffected areas in the Weddell Sea. Trematomus eulepidotus, T. lepidorhinus and T. scotti dominate undisturbed areas, whereas T. nicolai and especially T. pennellii dominate disturbed areas. Total stomach content and number of prey taxa per fish are higher in preferred than in non-preferred areas. These findings indicate that small-scale horizontal patterns caused by iceberg scours play a distinct role in Trematomus niche separation.     

Bryozoans of the Weddell Sea continental shelf,slope and abyss: did marine life colonize the Antarctic shelf from deep water,outlying islands or in situ refugia following glaciations?

Aim At the height of glaciations such as the Last Glacial Maximum (LGM), benthic life on polar continental shelves was bulldozed off nearly all of the Antarctic shelf by grounded ice sheets. The origins of the current shelf benthos have become a subject of considerable debate. There are several possible sources for the current Antarctic shelf fauna, the first of which is the continental slope and deep sea of the Southern Ocean. The high levels of reported eurybathy for many Antarctic species are taken as evidence supporting this. A second possible source for colonists is the southern margins of other continents. Finally, shelves could have been recolonized from refugia on the continental shelves or slopes around Antarctica. The current study investigates whether the patchily rich and abundant biota that now occurs on the Antarctic continental shelf recolonized from refugia in situ or elsewhere. Location Weddell Sea, Antarctica. Methods We examined bryozoan samples of the BENDEX, ANDEEP III and SYSTCO expeditions, as well as the literature. Using similarity matrices (Sørensen coefficient), we assessed similarities of benthos sampled from around Antarctica. By assessing numbers of species shared between differing depths and adjacent shelf areas, we evaluated the origins of cheilostome bryozoan communities. Results Bryozoans decreased from 28, 6.5 and 0.3 colonies per trawl, and 0.16, 0.046 and 0.0026 colonies per cm² of hard surface from shelf to slope to abyssal depths. We found little and no support for recolonization of the Weddell Sea shelf by bryozoans from the adjacent slope and abyss, in the scenario of LGM faunal wipe‐out. The Weddell Sea shelf bryozoan fauna was considerably more similar to those on other Antarctic shelves than to that of the adjacent (Weddell Sea) continental slope. The known bryozoan fauna of the Weddell Sea shelf is not a subset of the Weddell Sea slope or abyssal faunas. Main conclusions We consider that the composition of the current Weddell Sea bryozoan fauna is most easily explained by in situ survival. Thus we consider that at least some of the Weddell Sea fauna persisted throughout the LGM, although not necessarily at the same locations throughout, to recolonize the large area currently occupied.     

Effects of ultraviolet radiation and dissolved organic carbon on the survival of subarctic zooplankton

High intensities of ultraviolet radiation are known to be harmful to aquatic biota, especially for species living in shallow, clear water bodies. Zooplankton species from such habitats are good model organisms to study the effect of changes in UV radiation, and how animals deal with this. We tested experimentally the effect of natural UV radiation, which was controlled by different filters and varying concentrations of UV-screening dissolved organic carbon (DOC), on the survival of the cladocerans, Daphnia longispina and D. pulex, and a calanoid copepod, Eudiaptomus graciloides. All species originated from subarctic Fennoscandia where underwater UV intensity is influenced by ozone depletion, changes in DOC and timing of ice break-up. Measured as mortality, all species were affected by both UVB and UVA radiation. Survival was highest and similar between species in the dark controls and photosynthetically active radiation exposures. Under each UV filter, the highest DOC concentration provided significant and best shelter from UV radiation and led to best survival. Variation in survival was observed between species. E. graciloides responded more readily to changes in UV radiation than did the daphnids. In natural environments, species' previous exposure to light and different protection strategies (pigmentation, vertical migration) are probably as important factors controlling the survival of zooplankton as radiation intensity and optical properties of water.     

Review on the distribution and biology of Antarctic Monoplacophora, with first abyssal record of Laevipilina antarctica

Records of extant Monoplacophora are still scarce, often limited to single specimens or empty shells. Little is known about monoplacophoran diversity, distribution and biology. This study summarizes the present distributional knowledge of all Antarctic monoplacophorans, adding new records from the Eastern Weddell Sea. The record of Laevipilina antarctica from over 3,000 m depth extends its previously known bathymetrical range from 210 to 644 m down to abyssal depths. Special symbiosis with bacteria might contribute to this remarkable eurybathy that is unique amongst extant monoplacophoran species. L. antarctica now is known from several stations along the shelf and slope of the Eastern Weddell and Lazarev Seas. Micropilina arntzi seems limited to the shelf and upper slope of the Lazarev Sea. An undescribed Laevipilina species is known only from a single station at the upper slope of the Eastern Weddell Sea. Distributional patterns are discussed and correlated to environmental conditions and available biological information.     

Inter-specific variations in distribution, abundance and possible life-cycle patterns of Themisto spp. (Amphipoda) in the Barents Sea

Amphipods from the central and northern areas of the Barents Sea were studied by a series of MOCNESS profiles between 1990 and 1997. Themisto abyssorum, though dominant in warmer Atlantic water, was also present in Arctic water. In contrast, T. libellula was a typical Arctic species and penetrated very little into the Atlantic water masses. T. compressa was seldom found in the studied area. Linear regression analysis showed a statistically significant negative relationship between abundance of T. libellula and the variability in the Atlantic inflow. The abundance of this species seems to be, to a large extent, determined by the amount of Arctic water in the Barents Sea. The sub-Arctic species, T. abyssorum, only has a 1-year life-cycle, with the peak in release of young (2-3 mm) occurring in May and June. A few individuals may survive to be older. The Arctic species, T. libellula, seems to release the young earlier and the length-frequency distributions seem to indicate a 2-year life-span. The spring phytoplankton, which usually blooms during April in the Barents Sea, followed by high abundance of Calanus spp. and krill species, are regarded as important factors that influence the release of the amphipods' young.     

Assemblages of sea stars (Echinodermata: Asteroidea) and brittle stars (Echinodermata: Ophiuroidea) in the Weddell Sea (Antarctica) and off Northeast Greenland (Arctic): a comparison of diversity and abundance

Composition and distribution of asteroid and ophiuroid assemblages were investigated by means of Agassiz trawl catches at 34 stations in 220- to 1,200-m depth in the Weddell Sea and at 17 stations in 90- and 830-m depth off Northeast Greenland. A total of 86 species (48 sea stars, 38 brittle stars) were identified in the Weddell Sea whereas off Northeast Greenland a total of 26 species (16 sea stars, 10 brittle stars) were recorded. In both study areas, brittle stars were numerically more important than sea stars, and abundances generally decreased with water depths. Multivariate analyses revealed a conspicuous depth zonation of sea and brittle stars off Greenland. Very high abundances of Ophiocten sericeum and Ophiura robusta characterized the assemblages on shallow shelf banks whereas in greater depths Ophiopleura borealis, Ophioscolex glacialis and Ophiacantha bidentata became dominant, albeit at significantly lower densities. Mass occurrences of brittle stars, such as those recorded on Greenlandic shelf banks, have not been discovered in the Weddell Sea, where distinct assemblages were discriminated in deep shelf trenches as well as on the eastern and southern shelf. Ophioplocus incipiens, Ophiurolepis martensi and Ophiurolepis brevirima were the most prominent species on the eastern shelf. Ophiacantha antarctica, Ophiurolepis gelida and Ophionotus victoriae on the southern shelf, and Ophiosparte gigas as well as the asteriod Hymenaster sp., in the shelf trenches. Overall, the Weddell Sea housed conspicuously more asterozoan species than the waters off Greenland. Higher species diversity was also evident at both a regional and local scale, especially for the eastern Weddell Sea shelf. However, because many species from the Weddell Sea are closely related, the Weddell Sea assemblages were not significantly different from the Greenland ones in terms of taxonomic diversity and distinctness. Received: 29 April 1996/Accepted: 10 June 1996     

Electrophoretic investigation of genetic variation in two krill species Euphausia superba and E. crystallorophias (Euphausiidae)

Summary Specimens of Euphausia superba and of E. crystallorophias from different locations were analyzed electrophoretically for protein variation. The present study extends previous genetic studies on E. superba by analyzing samples from the East Wind Drift and repeat samples from the Bransfield Strait, Elephant Island and the Weddell Sea. E. crystallorophias was collected in the Weddell Sea and around the Antarctic Peninsula in order to provide information on the breeding structure of the species in this region. For all loki taking all sampling sites together for both species except GPI in E. crystallorophias no significant deviation of phenotype distributions from random mating expectations was observed. Furthermore, the allele distributions in all polymorphic loci for both species were found to be homogeneous. Estimates of genetic distance between samples within each species are low (0.0001 to 0.0003 in E. superba and 0.0001 to 0.0002 in E. crystallorophias), and are consistent with results expected for samples from a single interbreeding population. Estimate of genetic distance between these two species was 0.9729. These results suggest that for each species specimens from all locations investigated in the Bransfield Strait and Weddell Sea belong to a single genetically homogeneous population. A possible mechanism for maintaining such homogeneity and the implications for fishery management are discussed.     

Reproductive activity of two dominant Antarctic copepod species, Metridia gerlachei and Ctenocalanus citer, in late autumn in the eastern Bellingshausen Sea

The main reproductive period of herbivorous Antarctic copepods is coupled to the phytoplankton bloom in spring, while omnivorous or carnivorous species apparently reproduce year round. However, our knowledge of the reproduction during autumn and winter is limited. Therefore this study during a cruise with RV "Polarstern" aimed to measure reproductive activity in two dominant copepod species, Metridia gerlachei and Ctenocalanus citer, in late autumn (April/May). For this purpose, gonad development stages were determined from both species, taken with Bongo net in the eastern Bellingshausen Sea (Antarctic Peninsula region), and related to the ambient feeding conditions represented as chlorophyll a concentration. In addition, experiments were conducted with M. gerlachei to study the influence of feeding and starvation on gonad development. Gonad maturity of M. gerlachei differed considerably between stations and decreased over time. The correlation of chlorophyll a and the proportion of mature females, though significant, was weak and highly variable. In experiments, the gonads of feeding females were in better condition than those of their starving counterparts, showing that reproductive activity in M. gerlachei was related to ambient feeding conditions. Mature females of C. citer were found at each station (17-67%). The gonad stage composition was rather similar at all stations, showing no clear temporal and spatial trend. In this species, the proportion of mature females was not related to the chlorophyll a concentration, indicating either that the ambient phytoplankton stock was sufficient to fuel maturation or that other food sources were used. Our study shows that both species are reproducing during the austral autumn, indicating that their reproduction is partially decoupled from the spring phytoplankton bloom.     

The reproductive behaviour of Pogonophryne scotti confirms widespread egg-guarding parental care among Antarctic notothenioids

In this paper, the first documentation of egg-guarding behaviour in an artedidraconid species, Pogonophryne scotti, through in situ photographic imagery obtained during video transects is provided. The male specimen was observed closely guarding a well-defined multi-layered egg mass deposited on the sea floor at 240 m on the southern South Orkney Islands shelf in the northern Weddell Sea. Egg-guarding parental care is present in species that are distributed among all of the major lineages of Antarctic notothenioids; however, lack of information on egg-deposition behaviours in Bovichtidae and Pseudaphritis prevents assessment of whether parental care originated prior to the origin of the Antarctic notothenioid radiation.     

Phytoplankton dynamics at the ice-edge zone of the Lazarev Sea (Southern Ocean) during the austral summer 1994/1995 drogue study

Phytoplankton dynamics in the open waters of the ice-edge zone of the Lazarev Sea were investigated over 12 consecutive days during a drogue study conducted in austral summer (December/January) 1994/1995. Throughout the study, the upper water column (<30 m) was stratified with a well-defined pycnocline evident. Although a subsurface intrusion of colder, more saline water into the region was recorded on days 5-9 of the experiment, its effect on the water column structure was negligible. Total surface chlorophyll a biomass doubled between days 1 and 5 (from 0.82 to 1.62 mg m^-3), and then showed a tendency to stabilise, while the depth-integrated chlorophyll a standing stock displayed an increasing trend during the entire experiment. All changes in biomass were associated with an increase in microphytoplankton. Flagellates and picoplankton dominated cell counts, while diatoms composed most of the phytoplankton biovolume. Results of the study indicate that, during the period of investigation, average cell abundance decreased. Coupled with this decrease was an increase in the biovolume and average size of the phytoplankton. Phytoplankton succession was observed in the ice edge during the drogue study. Typical ice-associated species of genera Haslea, Fragilariopsis and Chaetoceros, which dominated at the beginning of study, were replaced by open-water species of genera Corethron, Dactyliosolen and Rhizosolenia. The shift in phytoplankton species composition and size can likely be related to high light intensities and grazing by microzooplankton. The intrusion of colder, more saline water on day 5 appeared to modify the diatom succession, indicating extreme variability in phytoplankton dynamics in the near ice-edge zone of the Lazarev Sea.