The microcopepod fauna in the Eastern Mediterranean and Arabian Seas: a comparison with the Red Sea fauna

Multiple opening-closing nets of 0.05 mm mesh size were employed to study the community structure and vertical distribution of microcopepods at selected stations in the Red Sea, Arabian Sea and Eastern Mediterranean Sea down to a maximum depth of 1850 m. Calanoids, cyclopoids (Oithona and Paroithona) and poecilostomatoids (mainly Oncaea) were the 3 most abundant orders. In the epipelagic zone (0–100 m), these orders occurred at similar abundance levels, whereas in the meso- and bathypelagic zones the poecilostomatoid genus Oncaea dominated numerically by about 60–80% of all copepodids.The species diversity of Oncaea in the Red Sea is compared with preliminary results from the two adjacent regions. In the deep Eastern Mediterranean Sea, the number of species appears to be similar to that in the deep Red Sea and low as compared to the deep Arabian Sea. In this latter area an extremely speciose Oncaea fauna was found at depth below the oxygen-minimum-zone (900–1850 m). The results are related to the differences in the hydrographic conditions of these 3 areas.     

Biological observations on small cyclopoid copepods in the Red Sea

The breeding activity, sex ratio and variation in body lengthof a number of cyclopoid copepod species from the upper 450mof the Red Sea were analyzed. In most species the body lengthof adult specimens was in the lower range or less than thosereported from other pelagic regions. In the central Red Seamost species had some individuals actively breeding, duringboth autumn and winter. To the north, breeding was observedin fewer species. Only two species, Paroithona sp. and Agetuslimbatus, however, seemed to find conditions for reproductionbetter in the north than in the central area. The sex ratioswere quite consistent for all species within the families Sapphirinidaeand Corycaeidae (high percentage of males) and Oithonidae (lowpercentage of males). Within the Oncaeidae, the percentage ofmales varied from a few percent to >60% of adults. Thereappears to be a relationship between sex ratio and size. Populationsof the very small Oncaea species, O.ivlevi, O.prendeli and O.zernovi,include far fewer males than those of their larger congeners.The available information on the biology of Oncaea is summarized.Peculiarities in their life histories and reproduction are discussed.     

The zooplankton community in the vicinity of the ice edge,western Weddell Sea,March 1986

Summary The zooplankton community in the vicinity of the ice edge in the west central Weddell Sea was investigated in the late austral summer (March 1986). Sampling was done with two ships operating concurrently, one in the pack ice and the other in the adjcent open sea. Metazoan microzooplankton (<1 mm) was most abundant in the epipelagic zone. It consisted mostly of copepod nauplii and copepods of the genera Oithona, Oncaea, Ctenocalanus and Microcalanus. While species composition was similar in both areas, vertical patterns differed in that the microzooplankton had sparse populations in the upper 50 m under the ice. This may have been related to water temperature which in the upper 50 m under the ice was more than 1°C cooler than in the open sea. Zooplankton in the 1–20 mm size range was dominated by the calanoid copepods Metridia gerlachei, Calanus propinquus and Calanoides acutus which constituted half the biomass in the upper 1000 m. Their populations had highest densities in the upper 150 m, though much of the C. acutus population resided below 300 m. Metridia gerlachei and C. propinquus underwent diel vertical migrations in both areas whereas C. acutus did not migrate. Species diversity in the epipelagic zone was moderate and the fauna was characterized by species typical of the oceanic east wind drift. Diversity increased with depth and was due primarily to the appearance of circumpolar mesopelagic copepods in Weddell Warm Deep Water. Biomass of 1–20 mm zooplankton in the 0–1000 m zone was low (1.1–1.3 gDWm^-2) compared to other Southern Ocean areas investigated with comparable methods. It is suggested that this is related to Weddell circulation patterns and the resulting low annual primary production in the central Weddell Sea.     

First record of Boulenger's anthias Sacura boulengeri (Heemstra 1973) in the Red Sea

In November 2020, we observed several individuals and collected one juvenile of an unidentified anthiadine fish (Serranidae) between depths of 250 and 307 m near vertical walls of rocky reefs in the northern Red Sea. Further morphological and molecular analyses revealed that the collected specimen matches Sacura boulengeri, a species previously reported only from the Gulf of Oman to India.     

Two new Oncaea species (Copepoda: Poecilostomatoida) from the Red Sea

Two new Oncaea species, O.umerus and O.hawii, are describedfrom females taken in the central Red Sea. They are closelyrelated to each other and to a group of Oncaea similis-likespecies, whose morphological characteristics are reviewed forcomparison.     

Vertical structure of small metazoan plankton, especially noncalanoid copepods. I. Deep Arabian Sea

The abundance and vertical distribution of micro-metazoans sampledwith fine nets of 0.05 mm mesh size were studied at three stationsin the Arabian Sea during the intermonsoon period (April/May1987) and down to 1850 m depth. In the epipelagic zone (0–100m). values of biomass and metazoan abundance tended to be higherthan those reported for other tropical oceanic areas. In themesopelagic zone, which is characterized by an extreme oxygendeficiency between 100 and 1000 m depth, the abundance of metazoantaxa and species numbers of non-calanoid copepods were largelyreduced. However, intermediate abundance maxima occurred withinthis zone, which were dominated by specific metazoan taxa (copepods.appendicularians) and species of non-calanoids (Oncaea sp. C).In the bathypelagic zone below 1050 m, the species diversityof the dominant copepod family Oncaeidae increased substantially.Two-thirds of a total of 69 oncaeid species recovered were confinedto this layer. As most of them were small in size and occurredin low abundance only, the increase in total oncaeid densityand/or plankton biomass was less conspicuous. Dominant Oncaeaspecies in the bathypelagic zone were O.longipes and O.brodskii.The results are compared with published data from the ArabianSea and other tropical oceanic areas with and without an extrememesopelagic oxygen minimum zone Possible causes of the intermediateabundance maxima within the oxygen deficiency zone are discussed.     

The distribution of the Euchaetidae (Copepoda: Calanoida) around South Georgia

Summary Fourteen species of the genus Euchaeta (Copepoda: Calanoida) were encountered during two cruises undertaken around South Georgia during November–December 1981 (Summer) and July–August 1983 (Winter). All 14 species were present in summer but only nine in winter. The distribution of the majority of species centered on the mesopelagic (500–1000 m) and bathypelagic (1000–2000 m) depth horizons. During both cruises, four species, Euchaeta Antarctica, E. biloba, E. rasa and E. farrani were numerically dominant. Although all four species had a wide depth distribution, E. Antarctica and E. biloba had distributions centered around the 250–500 m (epi-mesopelagic) and mesopelagic depth horizons. E. rasa was most abundant in the mesopelagic whereas E. farrani, the largest species encountered, was centered on the mesopelagic and bathypelagic depth horizons. Two species, E. Antarctica and E. biloba commonly occurred in the surface 250 m, the former being far more abundant especially over the shelf and shelf break areas, with a high abundance of copepodite stages IV and V being present in summer and adults in winter.     

Thaliacean distribution and abundance in the northern part of the Levantine Sea (Crete and Cyprus) during the eastern Mediterranean climatic transient, and a comparison with the western Mediterranean basin

First results are presented on the composition, abundance and vertical distribution of the thaliacean fauna in the Levantine basin obtained from stratified tows at three deep-sea sites in 1993: SE off Crete, and SW and NE off Cyprus. Samples with a 10 m²-MOCNESS (mesh size 1.67 mm) were poor in species and specimens as compared to samples with a 1 m²-double-MOCNESS (0.333 mm). Of the 12 species identified, six species belonged to the most abundant Doliolida, predominated by the phorozooids of Doliolum nationalis, five species belonged to the Salpida and one to the Pyrosomatida. Thaliaceans, most abundant by species and numbers SE off Crete, comprised ≤0.2% of the local mesozooplankton standing stocks. Presumably, they did not contribute substantially to the vertical flux generating the locally increased biomass and activity of the microbial benthos at the 4,300 m deep Cretean site. Most doliolids and salps were collected from the epipelagic and upper mesopelagic layers, and appeared to be most abundant close to the deep maximum of chlorophyll-a. Samples below 150 m were rarely rich in specimens, although two species performed diel migrations from the mesopelagic zone into the surface layers where some vertical segregation was evidenced. The aspect of niche separation is discussed.     

Vertical distribution and abundance of mesoplanktonic medusae and siphonophores from the Weddell Sea,Antarctica

The composition, abundance and vertical distribution of mesoplanktonic cnidarians collected along a transect across the Weddell Sea have been analysed. The transect was characterized by a thermocline, approximately between 200 and 100 m, which deepened significantly towards the shelf edges. In total, 10 species of medusae and 18 species of siphonophores were identified. The most abundant medusae were Pantachogon scotti (up to 11,671 specimens/1,000 m³) and Arctapodema ampla (up to 960 specimens/1000 m³). The most abundant siphonophores were Muggiaea bargmannae (up to 1,172 nectophores/1,000 m³) and Dimophyes arctica (up to 230 nectophores/1,000 m³). Five assemblages of planktonic cnidarians were distinguished: (a) epipelagic species located in and above the thermocline; (b) epi- and upper mesopelagic species located in, above and just below the thermocline; (c) epi- and mesopelagic species located in and below the thermocline; (d) mesopelagic species; (e) lower mesopelagic species. Differences in the depth distribution of the various species gave rise to a clear partitioning of the mesoplanktonic cnidarian population throughout the water column. This vertical partitioning was related to the existence of a thermocline, the structure of the water column and the vertical distribution of prey.     

Breaking the ice: large-scale distribution of mesozooplankton after a decade of Arctic and transpolar cruises

Mesozooplankton collected during five summer expeditions to the Arctic Ocean between 1987 and 1991 was analysed for regional patterns in biomass and species distribution, distinguishing between an epipelagic (0–100 m) and a deeper (0–500 m) layer. A total of 58 stations was sampled mainly in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean and in areas of the Greenland Sea, West Spitsbergen Current and Barents Sea. Results from the different expeditions were combined to create a transect extending from the Fram Strait across the Eurasian Basin into the Makarov Basin. Mesozooplankton dry mass in the upper 500 m decreased from 8.4 g m⁻² in the West Spitsbergen Current to less than 2 g m⁻² in the high-Arctic deep-sea basins. In the central Arctic Ocean, biomass was concentrated in the upper 100 m and was dominated by the large copepods Calanus hyperboreus and C. glacialis. In contrast, the mesozooplankton in the West Spitsbergen Current was more evenly distributed throughout the upper 500 m, with C. finmarchicus as the prevailing species. The distribution of abundant mesopelagic species reflected the hydrographic regime: the calanoid copepod Gaetanus tenuispinus and the hyperiid amphipod Themisto abyssorum were most abundant in the Atlantic inflow, while Scaphocalanus magnus was a typical component of the high-Arctic fauna. The relatively high mesozooplankton biomass and the occurrence of boreal-Atlantic species in the central Arctic Ocean are indicators for the import of organic material from allochthonous sources, especially from the northern North Atlantic. Hence, in spite of its enclosure by land masses, the Arctic Ocean is characterized by an exchange of water masses and organisms with the North Atlantic, and advection processes strongly influence the distribution of plankton species in this high-latitude ecosystem. Received: 18 December 1997 / Accepted: 11 April 1998     

Vertical gradients in species richness and community composition across the twilight zone in the North Pacific Subtropical Gyre

Although metazoan animals in the mesopelagic zone play critical roles in deep pelagic food webs and in the attenuation of carbon in midwaters, the diversity of these assemblages is not fully known. A metabarcoding survey of mesozooplankton diversity across the epipelagic, mesopelagic and upper bathypelagic zones (0–1500 m) in the North Pacific Subtropical Gyre revealed far higher estimates of species richness than expected given prior morphology‐based studies in the region (4,024 OTUs, 10‐fold increase), despite conservative bioinformatic processing. Operational taxonomic unit (OTU) richness of the full assemblage peaked at lower epipelagic–upper mesopelagic depths (100–300 m), with slight shoaling of maximal richness at night due to diel vertical migration, in contrast to expectations of a deep mesopelagic diversity maximum as reported for several plankton groups in early systematic and zoogeographic studies. Four distinct depth‐stratified species assemblages were identified, with faunal transitions occurring at 100 m, 300 m and 500 m. Highest diversity occurred in the smallest zooplankton size fractions (0.2–0.5 mm), which had significantly lower % OTUs classified due to poor representation in reference databases, suggesting a deep reservoir of poorly understood diversity in the smallest metazoan animals. A diverse meroplankton assemblage also was detected (350 OTUs), including larvae of both shallow and deep living benthic species. Our results provide some of the first insights into the hidden diversity present in zooplankton assemblages in midwaters, and a molecular reappraisal of vertical gradients in species richness, depth distributions and community composition for the full zooplankton assemblage across the epipelagic, mesopelagic and upper bathypelagic zones.     

Cnidaria from the Croker passage (Antarctic Peninsula) with a special focus on Siphonophorae

Studies of “gelatinous” zooplankton are rather rare, and little is known about the biology and ecology of Antarctic Cnidaria, especially for siphonophores. More investigations are necessary for complementing the current information on the “gelatinous” zooplankton inhabiting this important but little know biogeographical region, especially because siphonophores very likely play a significant role in the Antarctic food chains. The species composition, abundance and vertical distribution of planktonic cnidarians in the Croker Passage were evaluated using the data obtained from three expeditions. Zooplankton were sampled with a double plankton net between 0 and 1,200 m water depth in both summer and winter seasons. In total, ten species of siphonophores and four species of medusae were identified. Siphonophore assemblages were dominated by Dimophyes arctica, Pyrostephos vanhoeffeni, and Diphyes dispar. D. arctica and D. dispar occurred in greatest numbers in summer, mainly in the upper mesopelagic zone. P. vanhoeffeni, a cryophilic species, occurred most abundantly in winter (in the lower mesopelagic zone), when they probably reproduce. Restricted circulation from the Bellingshausen Sea over the continental shelf into Croker Passage may exclude the deeper-living oceanic fauna from the area of investigation, as indicated by the low abundances of Crystallophyes amygdalina, Muggiaea sp., and Heteropyramis spp. Four species of medusae were identified. The highest abundance was noted for Solmundella bitentaculata and Arctapodema sp. These occurred most abundantly in the middle (January) and end (April) of summer.     

Oceanic micronektonic/macrozooplanktonic community structure and feeding in ice covered Antarctic waters during the winter (AMERIEZ 1988)

Summary Fifty-seven species of oceanic micronekton and macrozooplankton were collected under pack ice during the winter in the vicinity of the Weddell-Scotia Confluence with a modified opening-closing Tucker trawl. The majority of the 57 species did not vertically migrate and lived deeper during the winter than during the spring or fall. However, despite the short day length, several of the most common mesopelagic fish and crustaceans did migrate. Fish moved into shallower depths at night but apparently most did not continue into the near-freezing upper mixed layer, leaving that zone to the migratory crustaceans. In the upper 1000 m, the dominant species were, in order of decreasing biomass, Euphausia superba, the cnidarian Atolla wyvillei, the ctenophore Beroe sp., and the mesopelagic fish Electrona antarctica, Bathylagus antarcticus and Gymnoscopelus braueri. Thysanoessa macrura and Salpa thompsoni were biomass subdominants. The majority of the dominant species showed little seasonal differences in biomass. However, the biomass of gelatinous species varied considerably with A. wyvillei and Beroe sp. being most abundant and S. thompsoni least abundant during the winter. Incidence of food in the stomachs in several important species was low, suggesting a low impact on their Zooplankton prey. Specimens of S. thompsoni had high quantities of food in their guts but this species was uncommon so its net impact would also have been low. Euphausia superba and the three common mesopelagic fish had significantly lower stomach fullness ratings during the winter than during the fall, suggesting an overall decrease in feeding activity of dominant species during the winter.     

Vertical mesozooplankton abundance and distribution in the deep Eastern Mediterranean Sea SE of Crete

Mesozooplankton from the Levantine Basin, Eastern Mediterranean,were analysed for composition and vertical distribution fromthe surface to 4000 m, with special emphasis on calanoid copepods.Copepods were dominant, ostracods and chaetognaths ranked next.Zooplankton abundance and biomass were highest in the top 100m layer. A secondary concentration maximum was between 450 and750 m. The exponential decrease of zooplankton with depth wasstronger than in the open ocean, but less strong than in theRed Sea. Similarly to the Red Sea, three genera among calanoidcopepods were predominant in the deep sea. As with the WesternMediterranean, but different from the Red Sea, populations ofmesopelagic and one bathypelagic species were found to occurwell below 1250 m to depths of at least 3000 m. Similarly toboth seas, long-range vertical migrators were rare, those presentmainly belonging to the genus Pleuromamma, and diel migrationswere restricted to the upper 600 or 1000 m. Due to the hightemperatures in the deep water bodies of the Levantine and RedSeas, the decomposition of sedimentary particles is assumedto be accelerated. This prevents much detritus from reachingthe sea floor which may explain the anomalous low abundancesof deep-sea zooplankton and benthos. Though the similaritiesto the Red Sea seem to be strong, the Levantine Sea is likelyto hold an intermediate position between the open ocean andthe Western Mediterranean on one side and the Red Sea on theother side.     

Diel vertical migration of the loosejaw dragonfishes (Stomiiformes: Stomiidae: Malacosteinae): a new analysis for rare pelagic taxa

The diel vertical migration (DVM) of three genera of the stomiid subfamily Malacosteinae (Photostomias, Aristostomias and Malacosteus) was analysed from capture records of nearly 300 specimens in the Atlantic Ocean. To account for broad temporal and geographic scales encountered in this study, local time of capture was transformed to a corrected time representing position in a solar day. Species of Photostomias and Aristostomias undertake asynchronous DVMs characterized by a residence in the mesopelagic zone during the day and separate migrating and non‐migrating subpopulations at night. Species of Photostomias displayed an asynchronous DVM pattern characterized by a residence in the lower mesopelagic zone (>500 m) during the day and a segregated distribution at night. Specimens of Photostomias guernei captured at night in the mesopelagic were nearly identical in size to those captured in the epipelagic; however, day epipelagic specimens were stratified by size. In species of Aristostomias, few specimens were caught in the mesopelagic zone during the day and only small specimens were captured in the mesopelagic zone at night, indicating that sampling depth may not have been adequate to capture the bulk of mesopelagic daytime residents and the entire size range of the non‐migrating night‐time residents. In contrast, Malacosteus niger was distributed below 600 m, did not regularly migrate to the epipelagic zone and was stratified across the 700 m isobath. From these data, relationships between DVM patterns, morphology and foraging ecology are inferred and biases and applications of this method are discussed.     

The Zooplankton community of Croker Passage,Antarctic Peninsula

Summary Zooplankton species composition, abundance and vertical distribution were investigated in the upper 1000 m of Croker Passage, Antarctic Peninsula during the austral fall (March–April, 1983). 106 species were identified, many being mesopelagic and reported previously from the Southern Ocean. The most numerous species (>1000/100 m³) were the copepodsMetridia gerlachei, Microcalanus pygmaeus, Oncaea antarctica andOncaea curvata. Oncaea curvata alone constituted half the zooplakton population. Zooplankton biomass was dominated by three copepod species,Metridia gerlachei, Calanoides aculus andEuchaeta antarctica,which comprised 74% of the biomass. Size analysis revealed most of the zooplankton numbers were in the >1 mm fraction. The biomass distribution was polymodal with major maxima in the >1 mm and the 4–4.9 mm size classe. The >1 mm peak, exclusive of protozoans, was primarily copepod nauplii and copepodites ofOncaea, Metridia andMicrocalanus. The 4–4.9 mm peak was mostlyCalanoides acutus andMetridia gerlachei.All of the principal species had broad vertical distributions both day and night. There was some suggestion of diel vertial migration byMetridia gerlachei andEuchaeta antarctica, with segments of their populations migrating into the upper 100 m and 200 m, respectively, at night. Most of the dominant and subdominant species were concentrated below 200 m,with only the subdominantOithona similis having its maximum in the epipelagic zone. The occurrence of zooplankton at winter depths appears to have been earlier in Croker Passage in 1983 than has been generally reported for waters south of the Polar Front.Total standing stock of net-caught zooplankton (>15 mm) in the upper 1000 m was estimated at 3.1 gDW/m², which is somewhat higher than values reported for the West Wind Drift and for open ocean areas of temperate to tropical latitudes.Euphausia superba (17–52 mm) dominated the pelagic biomass, exceeding zooplankton standing stock under a square meter of ocean by a factor of 15. This is in contrast to lower latitudes where zooplankton biomass is usually greater than macrozooplankton-micronekton.     

Vernal abundance,structure and development of epipelagic copepod populations of the eastern Weddell Sea (Antarctica)

Summary Pelagic copepod populations under the pack ice of the Antarctic Weddell Sea were sampled with a 50 m net between October 2 and December 7, 1986, to study their abundance and developmental stage composition before and at the onset of the vernal phytoplankton bloom. Subadult stages and adult females were incubated to estimate rates of development and egg production. Copepod densities in the upper 200 m were highest for the small-sized species Oithona similis, Oncaea curvata and Ctenocalanus citer. About 95% of the copedite stages belonged to these species, dominated by Oithona. The copepodids were outnumbered by the nauplii in all species, except in Oncaea. The stage distribution in the small-sized species was bimodal with peaks in N3 and C4. The larger species Calanus propinquus and Metridia gerlachei showed peaks in nauplii only. Eggs were relatively abundant in all small and large species. Animals smaller than 1 mm not only were more abundant than the larger ones, but also had a higher total potential respiration. Eggs were produced by incubated females in sea water virtually without food at 0°C. Eggs hatched, and Oithona nauplii developed at a rate of about 7 days per stage. Copepodite stages did not develop significantly. Reproduction in the most abundant species commonly occurred before the algal spring increase when food levels were very low. Maintenance of a stable stage distribution at the expense of a high juvenile mortality seemed to be characteristic for the overwintering strategy of Antarctic copepods.     

Micronekton and macrozooplankton in the open waters near Antarctic ice edge zones (AMERIEZ 1983 and 1986)

Summary Micronekton and macrozooplankton assemblages (0–1000 m) were sampled from the open ocean in the vicinity of marginal ice zones in the southern Scotia and western Weddell Seas using midwater trawls. Small regional differences in species composition were found in the differing hydrographic settings with the Scotia Sea being slightly more diverse. Most species exhibited broad vertical ranges with no distinct pattern of vertical movement. Exceptions were mesopelagic fish and Salpa thompsoni which undertook diel vertical migrations. Biomass was high (2.4–3.1 g DW/m²), comparable to Pacific subarctic waters. Euphausia superba and Salpa tompsoni were the numerical and biomass dominants, representing over 50% of the total numbers and standing stocks. In terms of biomass, euphausiids were the most important group at shallow depths (0–200 m) but were surpassed by salps in the Scotia Sea and mesopelagic fish in the Weddell Sea when all depths down to 1000 m were considered. Pelagic fish biomass (3.3–4.4 g WW/m²) greatly exceeded published estimates for birds (0.025–0.070 g WW/m²), seals (0.068–0.089 g WW/m²) and whales (0.167 to 0.399 g WW/m²), making mesopelagic fish the most prevalent krill predators in the Antarctic oceanic system.