Diversification of chemosymbiotic bivalves: origins and relationships of deeper water Lucinidae

Although species of the chemosymbiotic bivalve family Lucinidae are often diverse and abundant in shallow water habitats such as seagrass beds, new discoveries show that the family is equally speciose at slope and bathyal depths, particularly in the tropics, with records down to 2500 m. New molecular analyses including species from habitats down to 2000 m indicate that these cluster in four of seven recognized subfamilies: Leucosphaerinae, Myrteinae, Codakiinae, and Lucininae, with none of these comprising exclusively deep‐water species. Amongst the Leucosphaerinae, Alucinoma, Epidulcina, Dulcina, and Myrtina live mainly at depths greater than 200 m. Most Myrteinae inhabit water depths below 100 m, including Myrtea, Notomyrtea, Gloverina, and Elliptiolucina species. In the Codakinae, only the Lucinoma clade live in deep water; Codakia and Ctena clades are largely restricted to shallow water. Lucininae are the most speciose of the subfamilies but only four species analyzed, Troendleina sp., ‘Epicodakia’ falkandica, Bathyaustriella thionipta, and Cardiolucina quadrata, occur at depths greater than 200 m. Our results indicate that slope and bathyal lucinids have several and independent originations from different clades with a notable increased diversity in Leucosphaerinae and Myrteinae. Some of the deep‐water lucinids (e.g. Elliptiolucina, Dulcina, and Gloverina) have morphologies not seen in shallow water species, strongly suggesting speciation and radiation in these environments. By contrast, C. quadrata clusters with a group of shallow water congenors. Although not well investigated, offshore lucinids are usually found at sites of organic enrichment, including sunken vegetation, oxygen minimum zones, hydrocarbon seeps, and sedimented hydrothermal vents. The association of lucinids with hydrocarbon seeps is better understood and has been traced in the fossil record to the late Jurassic with successions of genera recognized; Lucinoma species are particularly prominent from the Oligocene to present day. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 401–420.     

Biostratigraphic and paleoclimatic significance of a new Pliocene foraminiferal fauna from the central Arctic Ocean

A Pliocene benthic foraminiferal fauna containing a previously unknown species association was found in the basal section of a piston core collected from the crest of Northwind Ridge (NWR) in the central Arctic Ocean. The fauna is dominated by Epistominella exigua, Cassidulina reniforme, Eponides tumidulus, Cibicides scaldisiensis, Lagena spp., Cassidulina teretis, Eponides weddellensis, Bolivina arctica, and Patellina corrugata. The presence of Cibicides scaldisiensis in the assemblage and the occurrence of Cibicides grossus higher in the core are indicative of an early Pliocene age. The morphologically distinctive species Cibicidoides sp. 795 of McNeil (in press) which occurs in the NWR core sample was previously known only from Oligocene through Miocene deposits in the Beaufort-Mackenzie Basin of Arctic Canada. Ehrenbergina sp. A and Cibicidoides aff. C. sp. 795, also present in the core, are new and endemic to the Arctic late Miocene and early Pliocene. These species, and possibly others, are survivors of the late Miocene (Messinian) sea-level crisis, which caused a significant faunal turnover in the Arctic Ocean. The predominantly calcareous assemblage indicates deposition above the calcium carbonate compensation depth in an upper bathyal environment. Paleogeographic affinities for the bulk of the assemblage indicate probable connections between the Arctic and the North Atlantic Oceans, but the endemic species identify environmental differences or partial isolation of the western Arctic Ocean. The species association suggests a cold but milder paleoclimate than that which existed during Pleistocene glacial intervals.     

Factors influencing the distribution of Subantarctic deep-sea benthic foraminifera,Campbell and Bounty Plateaux,New Zealand

We investigate the combination of environmental factors that influence the distribution patterns of benthic foraminiferal tests (> 63 μm) in a topographically varied region crossed by both the Subtropical and Subantarctic Fronts, south-east of New Zealand. Seafloor sample sites, extending from outer shelf (50 m) to abyssal (5000 m) depths, are bathed by five different water masses, and receive phytodetritus from Subtropical, Subantarctic and Circumpolar surface water masses. Eight mappable associations are recognised by Q-mode cluster analysis of the benthic foraminiferal census data. Similar associations are identified using cluster analysis based solely on the presence or absence of species. Canonical correspondence analysis and a correlation coefficient matrix were used to relate the faunal data to a set of environmental proxies. These show that factors related to water depth (especially decreasing food supply with increasing depth) are the most significant in determining the overall foraminiferal distribution. Other contributing factors include surface water productivity and its seasonality; bottom water ventilation; energetic state of the benthic boundary layer and resulting substrate texture; and bottom water carbonate corrosiveness. Three shallow-water associations (50–700 m), dominated by Cassidulina carinata, Trifarina angulosa, Globocassidulina canalisuturata, Gavelinopsis praegeri, and Bolivina robusta, occur in coarse substrates on the continental shelf, and on the crests and upper slopes of four seamounts under well-oxygenated, high energy regimes, and high food input. Three mid bathyal to upper abyssal associations (500–3300 m), dominated by Alabaminella weddellensis, C. carinata, and Epistominella exigua, occur in biopelagic sandy mud, beneath a region of strongly seasonal food supply, with their composition influenced by total food flux, ventilation (Oxygen Minimum Zone), and bottom current strength. An unusual lower bathyal association (1200–2100 m), dominated by T. angulosa and Ehrenbergina glabra, occurs in a belt of coarser sandy substrate that runs along the crest of the submarine plateaux slopes beneath the strongly-flowing Subantarctic Front-related currents. A deep abyssal association (3500–5000 m), dominated by Nuttallides umbonifer and Globocassidulina subglobosa, occurs on the abyssal plain beneath oligotrophic lower Circumpolar Water south-east of the Subantarctic Front and is strongly influenced by the cold, carbonate-corrosive conditions.     

流溪河水库敞水区轮虫多样性与群落的动态特征

流溪河水库是位于北回归线上的大型峡谷型供水水库,于2013—2014年对该水库敞水区轮虫群落及其环境因子进行观测,分析敞水区轮虫多样性与群落动态特征。流溪河水库介于贫营养到中营养水平之间,丰水期(5月—9月)的总磷、透明度均高于枯水期(10月—4月),两年的总磷的平均值为0.019mg/L,透明度为2.55m。两年共检出轮虫40种,单月检出物种数波动范围为8—19种,月均检出轮虫13种。螺形龟甲轮虫、真翅多肢轮虫、沟痕泡轮虫和胶鞘轮虫是主要优势轮虫。2013、2014年轮虫年均总丰度分别为68.9、66.9个/L,两年物种丰富度在丰水期大于枯水期,但枯水期轮虫群落Simpson多样性指数均大于丰水期。采用Bray-Curtis距离测度群落之间的相异性,计算表明两年的枯水期轮虫群落之间的平均距离均大于丰水期,即丰水期时轮虫群落相似性较高,枯水期时轮虫群落变异较大。丰水期的水温和水质参数的变化相对稳定,种类的优势度更为明显,导致丰水期群落之间更为相似。主成分分析表明,无柄轮属、胶鞘轮属、多肢轮属、异尾轮属、泡轮属、晶囊轮属和皱甲轮属的主要种类的全年分布有较大的差异,而其余多数轮虫种类全年分布差异较小,反映了我国南亚热带地区水温全年变幅小,可维持较多的全年性种类共存。群落的方差解析与RDA分析表明,相对于生物与化学变量,物理环境变量主导了流溪河水库敞水区轮虫群落的变异,就单个变量而言,水温和食物是影响流溪河水库敞水区轮虫群落结构的重要因素,降雨则是影响轮虫群落结构变异的宏观因素。绝大多数轮虫为滤食性的,处于食物链底端,个体小、生活史短,轮虫群落在对环境因子变化的响应上与浮游植物具有相似性。     

Ecology and Gametogenic Biology of the Genus Umbellula (Pennatulacea) in the North Atlantic Ocean

Four putative species of the seapen genus Umbellula, U. lindahli, U. thomsonii, U. durissima and U. monocephalus, are found at bathyal and abyssal depths in the Porcupine Seabight and Abyssal Plain in the NE Atlantic Ocean and at bathyal depths off the Bahamas. The most common species, U. lindahli, has a cosmopolitan distribution in the world ocean. In situ observations, from bathyal depths in the Bahamas, of the posture of U. lindahli show that the polyps are spread out to maximize particle capture from the water column. In addition, the individuals of U. lindahli from the Bahamas suggest that the tentacles are cropped by a predator, but there is no evidence of predation in the NE Atlantic specimens. Reproduction in U. lindahli is typical of that previously observed in pennatulids. Fecundity is high, although most of the oocytes present are previtellogenic with only a few developing through to maximum size at any one time. Some specimens of U. lindahli appear to be infested with a small flatworm.     

Fauna of Prosobranch Gastropods of Peter the Great Bay,Sea of Japan,and the Biogeographical Composition

The complete list of prosobranch gastropods comprising 195 species classified in 97 genera and 38 families was compiled for Peter the Great Bay (northwest of the Sea of Japan) and the adjacent water areas. In general, in the bay low boreal species prevail, which supported its assignment to the Manchurian (North Japanese, Sea of Japan, Asian) Low Boreal Pacific Subregion of the Boreal Pacific Region. Shallow water thermotropic species mostly prevailed in the littoral and upper sublittoral (to a depth of 50 m) zones of the bay; psychrotropic species were found in the lower sublittoral and upper bathyal (50–500 m) zones; and bathyal thermotropic species inhabited the lower (more then 500 m) bathyal zone.     

海水来源的548株真菌的近海及远海分布模式及其抑菌活性分析

[目的]海洋真菌是新颖天然产物的理想来源.本研究旨在加深对可培养海洋真菌多样性的了解,以及挖掘具有应用潜力的海洋真菌.[方法]采用膜过滤法分离近海和远海海水样品中的真菌,通过菌株分离纯化、ITS基因序列测序,分析浅海带、半深海带和深海带海水样品中可培养真菌的多样性.在此基础上,采用固体平板法筛选具有抑菌活性的菌株.[结...     

Late Quaternary paleoceanography as recorded by benthonic foraminifera in Strait of Sicily sediment sequences

At specific times during the late Quaternary, a widespread low-salinity surface water layer was produced over the eastern Mediterranean which created stagnant conditions and associated deposition of sapropelic muds in the deep basins. The Strait of Sicily has been positioned above the anoxic deeper water masses and is therefore devoid of sapropel deposits. Sediment sequences in the region contain a valuable record of upper bathyal (200–1000 m) benthonic foraminiferal change during times of deep-basin stagnation.Quantitative examiniation of benthonic foraminiferal assemblages in Strait of Sicily Quaternary cores generally reveals changes coeval with sapropels S1, S2, S4, S5, and S6. The benthonic foraminiferal expression is seen as increases in frequency ofGlobobulimina affinis, Globobulimina pseudospinescens, Chilostomella mediterranensis, Bolivina dilatata, and otherBolivina species during intervals corresponding to the episodic abyssal stagnation events. Some cores also display a decrease inCassidulina carinata and an increase inArticulina tubulosa.Similar assemblage changes have been previously documented to be closely associated with the sapropel layers in the deep basins of the eastern Mediterranean (the actual sapropel layers usually do not contain benthonic foraminifera). However, in the Strait of Sicily area, the species exhibit less drastic changes than in the deep eastern basins because of the less severe anoxic conditions. In the deep eastern basins, benthonic foraminifera almost totally disappeared in sapropel layers; at the same time, there was a general reduction in foraminiferal numbers in the shallower Strait area.     

Geographical and ecological distribution of marine halacarid genera and species (Acari: Halacaridae)

At the end of 2002, the number of marine halacarid species was 1018, that of genera 51. A single genus, Copidognathus contains 33% of all species (336). Eleven genera are monotypic. Geographical provinces with a large number of species are the tropical western Pacific, temperate northeastern Atlantic, temperate southeastern Pacific, and Mediterranean-Black Sea. Most records of halacarid species are from temperate and tropical areas; 10% of species are known from polar zones. On a generic level, 29 genera are recorded from tropical and temperate but not from polar provinces, five genera are restricted to the tropics, and none to polar regions. The majority (920 species or 90%) of all species live in the upper 200 m. Records of genera with exclusively algivorous or brackish/fresh water species are bound to littoral habitats; all the other genera occur in more than one depth zone. Arenicolous genera, though most abundant in the littoral zone, have representatives in the bathyal. Four marine genera (Copidognathus, Halacarellus, Isobactrus, Lohmannella) have representatives in coastal fresh water, and three genera, Acarothrix, Caspihalacarus and Peregrinacarus, are predominantly inhabitants of diluted brackish and fresh water. None of the free-living halacarid genera of the world's oceans appears to be endemic to one geographical province.     

Influence of temperature,oxygen and food availability on the migrational activity of bathyal benthic foraminifera: evidence by microcosm experiments

Foraminifera are a dominant group of amoeboid protists in the deep-sea and play possibly a significant role in decomposition processes of incoming organic matter. In order to study the poorly known ecology of these protozoans, microcosm experiments with living bathyal benthic foraminifera have been conducted. Foraminifera from 2880 m and shallower water depths were successfully maintained and their movement patterns investigated. By video documentation, high mean migration speed of 20,02 (N = 22) and 24,48 m min^-1 (N = 10) at 4 °C were found for species such as Hoeglundina elegans and Quinqueloculina seminula from 1847 m and 1921 m water depth, respectively. The results demonstrate that some bathyal foraminifera have migration speeds comparable to those of shallow water species. Environmental factors such as temperature, food concentration and oxygen content showed a marked influence on the migration of some species. An increase in temperature from 10 °C to 15 °C resulted in an increase of 35% in the migration speed of Allogromia spp. However, other species reacted differently. Higher food concentration resulted in a decrease in speed of some species. While in Quinqueloculina lamarckiana speed was not greatly affected by a low oxygen content in the sediment, other foraminifera responded to oxygen depletion by migration to the surface layers. Observations of benthic foraminifera in the laboratory microcosms are discussed in relation to microhabitats and the fate of organic matter on the sea floor and in the sediment.     

Osmotic and ionic hemolymph concentrations of bathyal and abyssal amphipods of Lake Baikal (Siberia) in relation to water depth

We studied the adaptive variations of the hemolymph concentrations in relation to water depth and pressure using deep-dwelling amphipods from Lake Baikal. Hemolymph osmolality was determined in six bathyal and abyssal species immediately after capture when values come closest to the habitat concentrations. In three species, hemolymph osmolalities correlated positively with depth of capture. Prevalent ions in the hemolymph are sodium and chloride. Lactate, our indicator for capture stress, was highest after trawling (2–6 mM) and lowest after retrieval from cages (0–0.6 mM). Acclimation to different pressure was studied by exposing the specimens to different water depths over several days. Hemolymph concentrations did not change after acclimation to surface pressure in the sublittoral Acanthogammarus albus, a native also to shallow water, but decreased by 30–80 mosmol/kg H₂O in the bathyal and abyssal species Acanthogammarus grewingki, Acanthogammarus reicherti, and Parapallasea lagowskii. Similarly, hemolymph osmolality decreased in A. reicherti and P. lagowskii originating from deep water, when acclimated to reduced water depth, and, in A. reicherti hemolymph osmolality reached its original high value when returned to the depth of capture. Higher hemolymph osmolalities and NaCl concentrations, demonstrated here for the first time, may provide selective advantages to abyssal species. Accepted: 24 August 2000     

Patterns in the Distribution of American Beachgrass (<Emphasis Type="Italic">Ammophila breviligulata</Emphasis>) and the Density and Reproduction of Annual Plants on a Coastal Beach

Beach communities of eastern North America are commonly dominated by the native perennial grass Ammophila breviligulata. It typically co-occurs with a variety of annual grasses and herbs in the pioneer zone. To determine the potential significance of this vigorous clonal species to the annual community, density, spatial distribution, seed production and potential seed rain were quantified for three focus species (Triplasis purpurea, Cenchrus tribuloides, and Heterotheca subaxillaris) in a 45×40 m section of coastal beach on Staten Island, New York, USA. During autumn 2000, five 40 m transects (10 m apart) were established perpendicular to shore; ramet density for Ammophila, and densities and per capita seed production for the three species, were estimated in 150 contiguous quadrats (35 cm × 30 cm) per transect. Both Triplasis and Cenchrus were more likely to occur in quadrats without Ammophila. Per capita seed production of Cenchrus and Heterotheca showed a curvilinear decrease with increasing numbers of Ammophila ramets. Mean seed rain per m² was significantly reduced in the presence of Ammophila for all three focus annuals. In this beach community, Ammophila acts as an ecosystem engineer, but has a mostly negative influence on the distribution, density, and reproduction of the co-occurring annuals. Results suggest that continual, deliberate planting of A. breviligulata could be detrimental to the abundance and diversity of native annual herbs on the eastern coasts of North America.     

Vertical and lateral variations in coccolithophore community structure across the subtropical frontal zone in the South Atlantic Ocean

In this study the coccolithophore compositions of 11 plankton depth stations along a N–S transect from the South Atlantic Subtropical Gyre to the Subantarctic Zone were examined qualitatively and quantitatively. The lateral and vertical distribution patterns of not only the most abundant taxa but also of the morphotypes of distinct species complexes, such as Calcidiscus leptoporus, Emiliania huxleyi, and Umbellosphaera tenuis were the focus. Geographic variation among morphotypes mirrors different ecological affinities of the members of a species complex. Multivariate statistics were used to infer the relationship between a set of known environmental data and species concentrations. The results of the detrended Canonical Correspondence Analysis (CCA) revealed the presence of distinct species assemblages. The Subtropical Gyre assemblage within the upper 50 m of the photic zone is mainly composed of Umbellosphaera irregularis, U. tenuis types III and IV, Discosphaera tubifera, Rhabdosphaera clavigera, S. pulchra and E. huxleyi var. corona, adapted to warm and oligotrophic conditions. In the deeper photic zone abundant Florisphaera profunda, Gephyrocapsa ericsonii and Oolithotus spp. are encountered, benefiting from higher nutrient concentrations in the vicinity of the nutricline. A well-defined Subtropical Frontal Zone (STFZ) association is clearly dominated by E. huxleyi types A and C throughout the upper 100 m of the water column. Secondary contributors in the upper photic zone are Syracosphaera spp. (mainly S. histrica, S. molischii), Michaelsarsia elegans, Ophiaster spp. and U. tenuis type II. This assemblage is associated with cooler, nutrient-rich waters. E. huxleyi type B is found deeper in the water column. Here it is accompanied by Algirosphaera robusta, G. muellerae, and S. anthos indicating a tolerance of lower light availability in environments with elevated productivity. C. leptoporus spp. leptoporus shows relatively high cell numbers in all sampled water levels throughout the STFZ. Interestingly, its coccoliths are often smaller 5 μm in lith diameter. The mean coccolithophore assemblages of a station were compared to the underlying surface sediment assemblages. For the most part, the distribution of the morphotypes is reflected in the sedimentary archive, thus proving their potential as paleoecological proxies.     

Explaining bathymetric diversity patterns in marine benthic invertebrates and demersal fishes: physiological contributions to adaptation of life at depth

Bathymetric biodiversity patterns of marine benthic invertebrates and demersal fishes have been identified in the extant fauna of the deep continental margins. Depth zonation is widespread and evident through a transition between shelf and slope fauna from the shelf break to 1000 m, and a transition between slope and abyssal fauna from 2000 to 3000 m; these transitions are characterised by high species turnover. A unimodal pattern of diversity with depth peaks between 1000 and 3000 m, despite the relatively low area represented by these depths. Zonation is thought to result from the colonisation of the deep sea by shallow‐water organisms following multiple mass extinction events throughout the Phanerozoic. The effects of low temperature and high pressure act across hierarchical levels of biological organisation and appear sufficient to limit the distributions of such shallow‐water species. Hydrostatic pressures of bathyal depths have consistently been identified experimentally as the maximum tolerated by shallow‐water and upper bathyal benthic invertebrates at in situ temperatures, and adaptation appears required for passage to deeper water in both benthic invertebrates and demersal fishes. Together, this suggests that a hyperbaric and thermal physiological bottleneck at bathyal depths contributes to bathymetric zonation. The peak of the unimodal diversity–depth pattern typically occurs at these depths even though the area represented by these depths is relatively low. Although it is recognised that, over long evolutionary time scales, shallow‐water diversity patterns are driven by speciation, little consideration has been given to the potential implications for species distribution patterns with depth. Molecular and morphological evidence indicates that cool bathyal waters are the primary site of adaptive radiation in the deep sea, and we hypothesise that bathymetric variation in speciation rates could drive the unimodal diversity–depth pattern over time. Thermal effects on metabolic‐rate‐dependent mutation and on generation times have been proposed to drive differences in speciation rates, which result in modern latitudinal biodiversity patterns over time. Clearly, this thermal mechanism alone cannot explain bathymetric patterns since temperature generally decreases with depth. We hypothesise that demonstrated physiological effects of high hydrostatic pressure and low temperature at bathyal depths, acting on shallow‐water taxa invading the deep sea, may invoke a stress–evolution mechanism by increasing mutagenic activity in germ cells, by inactivating canalisation during embryonic or larval development, by releasing hidden variation or mutagenic activity, or by activating or releasing transposable elements in larvae or adults. In this scenario, increased variation at a physiological bottleneck at bathyal depths results in elevated speciation rate. Adaptation that increases tolerance to high hydrostatic pressure and low temperature allows colonisation of abyssal depths and reduces the stress–evolution response, consequently returning speciation of deeper taxa to the background rate. Over time this mechanism could contribute to the unimodal diversity–depth pattern.     

Daily Fish and Zooplankton Abundances in the Littoral Zone of Lake Texoma,Oklahoma-Texas,in Relation to Abiotic Variables

Many studies have shown the effects of yearly or monthly environmental conditions on the structure of fish and zooplankton communities. Environmental conditions can also vary greatly on much shorter time scales. We tested the effects of abiotic conditions on the daily abundance of fish and zooplankton in the littoral zone of Lake Texoma, Oklahoma-Texas. After date was removed statistically from the analysis, no environmental variables were particularly important in determining the daily abundance of zooplankton, whereas, numbers of fish in the littoral zone were related to changes in wind velocity and wave height. Regressions of daily fish abundance against wave height showed that the response differed among species and among life-history intervals within species. Numbers of juvenile Dorosoma petenense, Notropis atherinoides, and Morone chrysops were positively correlated with wave height, whereas juvenile Menidia beryllina were negatively correlated with wave height. We suggest that changes in the abundance of particular species may be associated with (1) avoidance of inshore areas to escape possible physical damage by wave induced turbulence, (2) attraction to inshore areas to feed on prey organisms suspended in the water column by wave induced turbulence, and (3) avoidance of inshore areas to escape high predator abundance and increased possibility of being eaten due to turbulence.     

Relationship of surface sediments and benthic foraminiferal distribution patterns in the Norwegian Channel (northern North Sea)

Our study of the distribution of benthic foraminifers in surface sediment samples from the Norwegian Channel has shown that eight species and five different assemblages are important. The assemblages are dominated byUvigerina peregrina, Bolivina skagerrakensis, Cassidulina laevigata, Elphidium excavatum andBulimina marginata, respectively.Hyalinea balthica, Melonis barleeanum andTrifarina angulosa are also abundant. The first four assemblages and their dominant species can be related to certain water masses with known characteristics. Two species are related to certain types of substrate;M. barleeanum seems to prefer a fine-grained substrate, whileT. angulosa occurs most abundantly where the content of silt and clay is low.B. skagerrakensis andH. balthica occur most abundantly in sediments with a high content of organic carbon, whileT. angulosa shows inverse correlation to the organic carbon content.     

Bathyal ostracodes from the Florida-Hatteras slope,the Straits of Florida,and the Blake Plateau

Epibathyal ostracodes from the Florida-Hatteras slope, the Blake Plateau and the Straits of Florida were studied to determine the relationship of numerous genera and species to bottom-water environmental conditions such as dissolved oxygen and bottom-water temperatures. From a total of 100 samples, 44 samples evenly distributed between 200 and 1100 m water depth and having an average of 325 specimens were examined in detail.Using occurrence data from the adjacent continental shelf, carapace preservation, Rose Bengal staining and population data, indigenous death assemblages were distinguished from transported or reworked fossil specimens. The percent of transported specimens varied as follows: Blake Plateau < 1%; Straits of Florida 10–60%; Florida-Hatteras slope 1–15%. Indigenous death assemblages contained between 10 and 61 species per sample, averaging 33.3 species.Krithe, Argilloecia andPseudocythere occur in > 90% of the samples and usually constitute 10 to 30% of each.Trachyleberidea, Bairdoppilata, Saida, Paranesidea, Ambocythere, Bythocypris, Cytherella, Bradleya, Henryhowella, and Polycopidae occur in 45 to 80% of the samples in varying percentages.The upper depth limits of 39 taxa occur at or just below the thermocline suggesting a relationship to temperature.Australoecia, Quasibuntonia, Cytheropteron, Ruggieriella, Saida, Ambocythere, Trachyleberidea, Macrocypris, Krithe, “Thalassocythere”, andCytherella are most common or restricted to the O₂ minimum zone. Conversely,Anchistrocheles, Bradleya, Henryhowella, andRockallia are most common below 750 m in well oxygenated water with temperatures below 8°C.The results show that: (1) ostracodes display a narrow depth zonation controlled by dissolved oxygen and water temperature; (2) species diversity is very high for a bathyal zone; (3) ostracodes can be used to identify the source of sediment that has been transported downslope; and (4) some taxa are useful in recognizing low oxygenated water in Cenozoic deposits.     

Pliocene benthic foraminifera from the Blake Plateau: Faunal assemblages and paleocirculation

Relative abundance of benthic foraminifera have been analyzed from core V26-145 from the Blake Plateau. The investigated sequence represents the time interval between 1.8 and 4.6 Ma. In order to determine how different sieve sizes influence the relative abundance patterns, three sediment size fractions were studied separately. It becomes difficult to maintain consistent taxonomic concepts in the fraction 63–125 μm, partly because this fraction contains high abundances of juvenile forms. However, the 63–125 μm fraction holds high abundances of the important small speciesEpistominella exigua. Due to these reasons only the two larger fractions (125–250 μm and >250 μm) were considered meaningful to analyze for relative abundance patterns. An analysis of the two larger fractions (>125 μm; >250 μm) shows no consistency in relative abundance patterns.The relative abundance patterns for the 34 most common species in the size fraction >125 μm were analyzed by means of correspondence analysis. Three benthic foraminiferal assemblages (I, II, and III) were recognized and these can be associated with water masses. Assemblage I is associated with the Florida Current and consists of shallow water species (Amphistegina gibbosa, Compressigerina sp. A,Discorbinella biconcavus, Islandiella teretis, Reussella atlantica, andSiphonina pulchra). Assemblage II contains key species for North Atlantic Deep Water (NADW) (Cibicidoides kullenbergi, Epistominella exigua, Globocassidulina subglobosa, Lenticulina peregrina, Oridorsalis umbonatus, andPlanulina wuellerstorfi). The third assemblage (III) contains species associated with the Antilles Current (Bolivina rhomboidalis, Cassidulina obtusa, Cassidulina vortex, andNuttallides umbonifera). The correspondence analysis reveals an alternation in dominance between Assemblage I and Assemblage II prior to 3.3 Ma, suggesting lateral oscillations between the Florida Current and NADW. At about 3.3 Ma Assemblage I disappears and Assemblage III increases in importance, suggesting an increasing influence of the Antilles Current in the upper part of the record.