Life history of the deep-sea cephalopod family Histioteuthidae in the western Mediterranean

The life cycle of the two species of the deep-sea family Histioteuthidae inhabiting the Mediterranean Sea (Histioteuthis reversa and Histioteuthis bonnellii) was studied from monthly samples taken throughout the year during daytime hours by bottom trawl gears. A small sample of individuals found floating dead on the sea surface was also analyzed. Both species were caught exclusively on the upper slope at depths greater than 300 m. Their frequency of occurrence increased with depth and showed two different peaks, at 500–600 m and 600–700 m depth in H. bonnellii and H. reversa, respectively, which might indicate spatial segregation. Maturity stages were assigned using macroscopic determination and confirmed with histological analyses. Although mature males were caught all year round, no mature females were found, which suggests that their sexual maturation in the western Mediterranean takes place deeper than the maximum depth sampled (800 m). In fact, the increase in mean squid size with increasing depth in H. reversa indicates an ontogenetic migration to deeper waters. The individuals of both species found floating dead on the sea surface were spent females which had a relatively large cluster of small atresic eggs and a small number of remaining mature eggs scattered in the ovary and mantle cavity. The sizes of these females were clearly larger than the largest individuals caught with bottom trawls. A total of 12 and 7 different types of prey, belonging to three major taxonomic groups (crustaceans, osteichthyes and cephalopods), were identified in the stomach contents of H. reversa and H. bonnellii, respectively. In both species fishes were by far the main prey followed by crustaceans, whereas cephalopods were found only occasionally. The preys identified, mainly myctophids and natantian crustaceans, indicate that both histioteuthids base their diet on pelagic nictemeral migrators.     

The distribution and abundance of cephalopods on the continental slope of the eastern South Atlantic

During a cruise in March 1988 dedicated to investigation of the marine resources of South Africa's continental slope, 62 species in 22 families of Cephalopoda were collected from the Cape Canyon and Cape Point Valley. Multivariate analysis revealed a clear distinction between cephalopods of the upper and lower continental slope in both benthic and epibenthic habitats. Todaropsis eblanae and Todarodes angolensis were indicator species for the upper slope benthic, from 300 to 500 m, but on the lower slope, in 700–900 m of water, they were replaced by Histioteuthis miranda and Opisthoteuthis agassizii. In the epibenthic, both upper and lower slopes were dominated by Abraliopsis gilchristi and Lycoteuthis ?diadema, but other species characteristic of the lower slope were the oceanic species Mastigoteuthis hjorti, Ctenopteryx sicula and Taonius sp. A. Cranchia scabra indicated the presence of oceanic water at stations 700–900 m deep. Other species of significance in the catches were Todarodes filippovae, Histioteuthis macrohista, Rossia enigmatica and Bathypolypus valdiviae. Species affinities indicate that the cephalopod fauna of the southern African continental slope includes the following zoogeographic components: southern African endemic, circum-Subantarctic species associated with Antarctic Intermediate Water, circum-global southern tropical/subtropical, tropical Indo-Pacific and cosmopolitan tropical/subtropical.     

Dense infaunal assemblages on the continental slope off Cape Hatteras, North Carolina

Unusually dense assemblages of benthic infaunal invertebrates have been discovered in continental slope sediments off Cape Hatteras, North Carolina. Densities were highest on the upper slope, ranging from 24,055 to 61,244 (X¯=46,255) individuals m⁻² in nine samples taken at a 600-m site in 1984 and 1985, and from 15,522 to 89,566 (X¯=37,282) individuals m⁻² in single samples at 15 stations over a wider depth range of 530 to 1535 m in 1992. A lower slope station at 2000 m sampled six times in 1984–1985 and again in 1992, had densities consistently higher than 8500 individuals m⁻². Species richness and diversity are consistently lower on the Cape Hatteras slope than at other locations off North Carolina and elsewhere in the western North Atlantic. The 1992 studies indicated that the upper slope infaunal assemblages (600m) were dominated by oligochaetes, while the middle slope assemblages (800–1400 m) were dominated by the polychaeteScalibregma inflatum. This latter depth range could be defined into two assemblages based upon suites of less abundant species. At depths of 1500–2000 m, a lower slope assemblage dominated by various deposit feeding polychaetes and oligochaetes was found. Results from the 1984–1985 studies suggest seasonal or year-to-year patterns in the dominance ofS. inflatum andCossura longocirrata. Unusually high sedimentation rates and organic carbon flux have been recorded from the slope off Cape Hatteras and may account for the high infaunal productivity in the area. Most of the dominant infaunal organisms are species more typical of shallow, coastal habitats rather than deep-sea species that dominate other areas of the U.S. Atlantic continental slope. Parallel investigations regarding the nature of organic matter in the Cape Hatteras sediments have revealed a mixture of both marine and terrestrially derived carbon, only a small percentage of which is composed of the smaller molecular weight polyunsaturated fatty acids more typical of continental slope sediments. It is likely that the high percentage of refractory organic matter would favor the survival of preadapted shelf species over those from adjacent slope environments.     

Interactions between sediment chemistry and frenulate pogonophores (Annelida) in the north-east Atlantic

The small frenulate pogonophores (Annelida: Pogonophora a.k.a. Siboglinidae) typically inhabit muddy sediments on the continental slope, although a few species occur near hydrothermal vents and cold seeps. We present data on the distribution and habitat characteristics of several species on the European continental shelf and slope from 48°N to 75°N and show how the animals interact with the chemistry of the sediments. The environments inhabited include: shallow (30 m), organic-rich, fjord sediments; slope sediments (1000–2200 m) and methane seeps at 330 m depth. All the species studied obtain nutrition from endosymbiotic bacteria. They take up reduced sulphur species, or in one case, methane, through the posterior parts of their tubes buried in the anoxic sediment. We conclude that most species undertake sulphide ‘mining’, a mechanism previously demonstrated in the bivalves Lucinoma borealis and Thyasira sarsi. These pogonophores participate in the sulphur cycle and effectively lower the sulphide content of the sediments. Our results show that the abundance of frenulate pogonophores increases with increasing sedimentation and with decreasing abundance of other benthos, particularly bioturbating organisms. The maximum sustainable carrying capacity of non-seep sediments for frenulate pogonophores is limited by the rate of sulphate reduction.     

Demersal fish and habitat associations from visual surveys on the central California shelf

In 2004, we surveyed demersal fishes and habitats on the continental shelf off central California (65–110 m depth) using the occupied submersible Delta. Our objectives were to estimate the relative abundance of habitats and to examine demersal fish species composition, diversity, density, and sizes relative to these habitats. A total of 112 transects were completed covering 32 km of seafloor. A higher density of fishes was estimated in boulder and cobble habitats than in mud and brachiopod beds. More than 80% of the fishes were small, measuring 20 cm or less in total length. Species with small maximum size (primarily pygmy rockfish, Sebastes wilsoni, and blackeye gobies, Rhinogobiops nicholsii) accounted for nearly half (49%) of the total number of 12,441 fishes. Most fishes were immature, with only 4 of 20 harvested species having more than 50% of the individuals larger than the size at first maturity. Our study area on the continental shelf may be an ontogenetic transition zone for immature fishes before they move to their adult habitat on the slope. Alternatively, historical fishing pressure may have contributed to the lack of large, mature fishes in the survey area. Understanding the importance of these habitats to fishes at various life stages will improve our ability to assess these deepwater fish stocks and effectively manage these living resources on an ecosystem basis.     

Morphological and ecological parallels between sublittoral and abyssal foraminiferal species in the NE Atlantic: a comparison of Stainforthia fusiformis and Stainforthia sp.

Dead specimens of a minute fusiform rotaliid foraminifer are common in the 28–63 μm fraction of multiple corer samples from a 4850 m-deep site on the Porcupine Abyssal Plain (PAP). Their test morphology is remarkably similar to small specimens of Stainforthia fusiformis (Williamson, 1858), a species which is well known from coastal settings (intertidal to outer shelf) around NW Europe and North America. A detailed comparison of the PAP form with typical individuals of S. fusiformis from Norwegian waters (55–203 m depth), however, reveals slight but consistent morphological differences. The PAP specimens are smaller (test length 40–140 μm) than those from Norway (test length 80–380 μm), the chambers tend to be rather less elongate, the density of pores in the test wall is much lower, and there are differences in apertural features. We therefore conclude that the diminutive abyssal form is a distinct species, here referred to as Stainforthia sp. This interpretation is consistent with increasing evidence for genetic differentiation in deep-sea organisms, particularly along bathymetric gradients. Stainforthia sp. was previously illustrated by Pawlowski as Fursenkoina sp. and appears to be widespread and abundant in the abyssal North Atlantic (>4000 m depth). Stainforthia fusiformis, on the other hand, is most abundant in continental shelf and coastal settings. It extends onto the continental slope in the North Atlantic but has not been reported reliably from depths greater than about 2500 m.We suggest that the striking morphological convergence between these two species reflects the adoption of similar ecological strategies in widely separated habitats. Both are enrichment opportunists, a life-style which may explain the rather broad bathymetric range of Stainforthia fusiformis. This is a dominant species in organically-enriched and sometimes extremely oxygen-depleted environments on the continental shelf, and is a rapid coloniser of formerly azoic habitats. Live specimens of the abyssal form are typically found embedded within phytodetrital aggregates (organic material derived from primary production in the euphotic zone). It is presumably the availability of these organic-rich microhabitats, which enables this species to survive in the otherwise oligotrophic deep sea.     

Les observations de poissons tropicaux et le réchauffement des eaux dans l'Atlantique européen

A recapitulation of the records of tropical fishes from European Atlantic waters shows that 67.6% were fishes caught from the upper slope, between approximately 200 and 600 m; 19.8% were fishes caught from the continental shelf; and 13.5% were specimens caught from the middle slope, between 700 and 1 300 m. Since 1963, the upper slope species have made regular northward range extensions off of south Portugal to north-western Ireland (about 55° 30′N), more and less rapidly, about 30 years for Cyttopsis roseus and only 6 years for Sphoeroides pachygaster. The continental shelf species, observed from 1969 but mostly from 1981, have a northward range to south-eastern Ireland (about 52° N), but 65.2% of them have been caught off the south of the Bay of Biscay. The middle slope species, recorded only from 1991 according to the development of the deep fishery, were caught between 48° N and 60° N. The northward range extension of upper slope species and the higher frequency of records of continental shelf species from the southern part of the Bay of Biscay coincide with the investigations on the warming of the south-north current in the upper slope of northern Spain and of the south French Atlantic continental shelf.     

Patterns and drivers of asteroid abundances and assemblages on the continental margin of Atlantic Canada

Although continental shelf and slope environments typically exhibit high epifaunal biomass and have been subjected to increasing fishing pressure, ecological information on assemblages of non‐commercial invertebrate species from subtidal and bathyal areas remains limited. Sea stars (Echinodermata: Asteroidea), which are known to influence communities through their feeding habits, have received less attention than structural taxa such as corals and sponges. To better understand the ecological roles of asteroids on continental shelves, we investigated ~30 species and assessed their distributions and co‐occurrence with other benthic invertebrates on the shelf and slope of Eastern Canada. Using fisheries data and in situ video footage, we compiled a large dataset covering ~600,000 km² that included over 350,000 individual asteroid records (37–2243 m depth). Multivariate analyses revealed geographically distinct asteroid assemblages, with a maximal overall density at 400–500 m and the highest diversity at 500–700 m. The most abundant and densely occurring species was Ctenodiscus crispatus. We found that asteroids associate with corals, sponges, bivalves, and other echinoderms, and that depth and substrate influence these assemblages. We identified species likely to affect coexisting organisms by their burrowing behavior that can disrupt epi‐ and infauna (C. crispatus) and through predation on ecologically important corals (Hippasteria phrygiana). In addition to providing baseline distribution and ecological information for many bathyal asteroid species in the Northwest Atlantic, this work highlights the abundance and diversified roles of asteroids within continental shelf and slope ecosystems.     

Life history of the bathyal octopus Pteroctopus tetracirrhus (Mollusca,Cephalopoda) in the Mediterranean Sea

The life cycle of the deep-sea octopus Pteroctopus tetracirrhus was studied from monthly samples obtained throughout the year in different areas of the western Mediterranean (mainly around the Balearic Islands and along the coast of the Iberian Peninsula). A total of 373 individuals (205 females, 168 males) were analyzed; females ranged from 4.5 to 14.0 cm mantle length (ML) and males from 4.5 to 11.5 cm ML. There were few small-sized octopuses (<7 cm ML) in the samples, which might indicate that these individuals inhabit rocky grounds that are not accessible to trawlers or waters deeper than the maximum depth sampled (800 m). The species occurred more frequently around the Balearic Islands than along the Iberian Peninsula as they appeared in 20% and 7%, respectively, of the hauls in these areas. The octopus inhabits the lower continental shelf and upper slope in both areas, primarily between 200 and 500 m depth. Modal lengths were followed from autumn, when recruits were caught by trawlers, to summer, when reproduction took place. Females grew from 8 to 10 cm ML from winter to spring, but this modal size did not increase further in summer; males grew from 7 to 9 cm ML from winter to spring. The total disappearance of large individuals after summer suggests a life cycle lasting a single year. The evolution of the monthly mean sizes showed that the growth was best described by log-linear functions in both sexes. The length at first maturity was clearly higher in females (12 cm ML) than in males (8 cm ML). A total of 30 different prey items, belonging to four major taxonomic groups (crustaceans, osteichthyes, cephalopods and gastropods), were identified in the stomach contents. The diet of the octopus was based on crustaceans and teleosts, which accounted for 75% and 23% of the prey items, respectively. Cephalopods and gastropods were accessory prey as they only represented 1.6% and 0.7%, respectively, of the total. The octopus showed a marked preference for the benthic fish Symphurus nigrescens and the endobenthic crustacean Alpheus glaber. The bathymetric distribution of P. tetracirrhus coincides with those of these two main prey, which suggests that the distribution of the octopus might be strongly linked to its trophic resources.     

Density,biomass and community structure of demersal fishes off the Pacific coast of northeastern Japan

Demersal fish communities were studied on the lower continental shelf and the upper continental slope along the Pacific coast of northeastern Japan. Species composition, number and weight of each species were examined based on otter trawl samples at 45 stations. Mean density and biomass of demersal fishes were 131 ha^–1 and 21 kg ha^–1, respectively. The ten most abundant species comprised of about 95% of total number and weight of overall catch indicating simple species composition. Gadiform fishesTheragra chalcogramma, Gadus macrocephalus andPhysiculus maximowiczi were the most important species by number, weight and frequency of occurrences, and three main community types represented by the three key species were recognized.Theragra-dominant community showed higher density and biomass, and lower diversity thanPhysiculus-dominant community did. Species diversity of demersal fish community was negatively correlated to density and biomass. Density and biomass of demersal fish community were high on the uppermost slope, and the high abundance resulted from low-diversity communities dominated byT. chalcogramma andG. macrocephalus.     

Reproduction in rare bathyal octopods Pteroctopus tetracirrhus and Scaeurgus unicirrhus (Cephalopoda: Octopoda) in the east Mediterranean as an apparent response to extremely oligotrophic deep seas

Reproductive patterns of two benthic bathyal octopods, Pteroctopus tetracirrhus and Scaeurgus unicirrhus have been studied in extremely nutrient-poor environment of the deep-sea Eastern Mediterranean. Both species were found to exhibit a reproductive tactics of producing eggs much larger than in the western part of the sea which likely results in larger hatchlings with higher viability. P. tetracirrhus exhibited a typical “deep-sea” spawning strategy of simultaneous maturation of a single batch of large eggs with atresia of excessive oocytes, whereas reproductive strategy of S. unicirrhus is particular for shelf octopodids: asynchronous maturation of numerous batches of small eggs with no obvious regulatory atresia. Existence of these two types of ovary development and utilisation of fecundity are closely related to two types of evolutionary stable reproductive strategies based on existence of either very large or very small eggs with a few species occupying the “intermediate” position.     

Overrunning of shelf water in the southern Mid-Atlantic Bight

Analyses of two years (1992 and 1993) of high-resolution sea surface temperature satellite images of the southern Mid Atlantic Bight (MAB), showed that unusually extensive overhang of shelf water occurs episodically, and coherently over along shelf distances of several 100 km. These episodes are dubbed overrunning of the Slope Sea by shelf water. The overrunning volume has a “face” and a “back” (southern and northern limit). It transports substantial quantities of shelf water southward, and does not retreat onto the shelf, but eventually joins the western edge of the Gulf Stream in the vicinity of Chesapeake Bay. The combined analyses of satellite imagery and various in situ data further demonstrated that the shelf waters overrunning the Slope Sea were not mere surface features but reached depths between 40 and 60 m. Results confirm previous concepts on shelf circulation, shelf–slope exchange and fate of shelf water. They also shed new light on shelf water budget: overrunning of the Slope Sea and southwest transport by upper slope current constitutes an important conduit for shelf water transport. Winter storms move the shelf–slope front, and with it shelf water, offshore to distances 10–40 km. The offshore displacement of shelf water can be related to the onshore veering of the Gulf Stream near Cape Hatteras, producing a blocking effect on the shelf circulation. Such a blocking effect of the southwestward flow of shelf water in the MAB appeared to be the reason for the overrunning of shelf water off New Jersey. In addition, the excess fresh water discharge from the St. Lawerence was also observed to be related to the overflow of shelf water off New Jersey.     

Community structure and dynamics of deep-water decapod assemblages from Le Danois Bank (Cantabrian Sea, NE Atlantic): Influence of environmental variables and food availability

The community structure of the decapod crustaceans inhabiting Le Danois Bank (Cantabrian Sea, NE Atlantic Ocean) was studied on two cruises performed in October 2003 and April 2004. Otter and beam-trawls were used to collect this fauna. At depths ranging between 455 and 1048 m, we found distinct decapod assemblages on the bank summit and deeper in the inner basin (between the bank and the continental shelf). The faunal discontinuity between these groups appeared at around 600 m (e.g. between 612 and 642 m in the basis of species replacement). The summit assemblage was characterized by low diversity (in terms of number and relative abundances of species) and by the dominance of hermit crabs (Pagurus alatus, Anapagurus laevis, Pagurus excavatus), the crangonid Pontophilus spinosus and the squat lobster Munida sarsi. Species characterizing the deeper assemblage, which was richer in terms of diversity, were Munida tenuimana, Parapagurus pilosimanus, Pontophilus norvegicus, the crab Geryon trispinosus and a number of bathypelagic shrimps (Sergia robusta, Acanthephyra pelagica and Pasiphaea tarda). Changes in decapod composition characterized by multidimensional scaling analyses were correlated with different variables, e.g. %mud and %organic matter (OM), temperature and salinity close to the bottom. Among those the %OM and %mud in sediments affected decapod distributions the most. The summit of the bank was covered by fine sediments with low proportion of mud (13.9–29.3%) and OM (2.55–3.50%). By contrast, sediment of the inner basin had a higher proportion of OM and mud (64.1–84.2%; 6.26–7.00%, respectively). The low proportion of mud at the summit of Le Danois Bank may explain the absence or scarcity of burrowing species (e.g. the lobster Nephrops norvegicus, the shrimps Calocaris macandreae and Alpheus glaber and the crab Goneplax rhomboides), that are dominant at similar depths (400–500 m) in the upper muddy assemblage on the mainland-continental slope in the Bay of Biscay (44–46°N). The dominance of certain species on the summit of submarine mounts can probably be related to their biology and feeding ecology. For example, pagurids are deposit feeders, even consuming marine snow (e.g. Chl-a identified in guts of Pagurus alatus), and they have low gut fullness (probably indicating a capacity to withstand long periods under starvation), that would favour their adaptability to a rather unpredictable habitat such as Le Danois Bank summit. Regarding prey availability, zooplankton/micronekton and infauna distributed around Le Danois Bank showed different depth-related patterns. Among zooplankton, mesopelagic decapods, mysids, and fishes were absent at the bank summit, while euphausiids exhibited high abundances over the summit. No significant trends with depth were found for infauna abundance, and for instance polychaete densities were similar both at the summit and the inner basin. Therefore, prey availability was lower for summit assemblages regarding zooplankton/micronekton. Patterns in mean size vs. depth were species specific for decapods, and the possible role of the bank summit as a recruitment area was not general for the whole decapod assemblage.     

Sediment community oxygen consumption in the deep Gulf of Mexico

Sediment community oxygen consumption (SCOC) has been measured from the continental shelf out to the Sigsbee Abyssal Plain in the NE Gulf of Mexico (GoM). SCOC rates on the continental shelf were an order of magnitude higher than those on the adjacent continental slope (450–2750 m depth) and two orders of magnitude higher than those on the abyssal plain at depths of 3.4–3.65 km. Oxygen penetration depth into the sediment was inversely correlated with SCOC measured within incubation chambers, but rates of SCOC calculated from either the gradient of the [O₂] profiles or the total oxygen penetration depth were generally lower than those derived from chamber incubations. SCOC rates seaward of the continental shelf were lower than at equivalent depths on most continental margins where similar studies have been conducted, and this is presumed to be related to the relatively low rates of pelagic production in the GoM. The SCOC, however, was considerably higher than the input of organic detritus from the surface-water plankton estimated from surface-water pigment concentrations, suggesting that a significant fraction of the organic matter nourishing the deep GoM biota is imported laterally down slope from the continental margin.     

Structure and resilience of overwintering habitats of Calanus finmarchicus in the Eastern Norwegian Sea

Very high concentrations of overwintering Calanus finmarchicus were found in the eastern Lofoten Basin of the Norwegian Sea close to the shelf break in January 2001–2002. A coupled 3D hydrodynamic and ecological model was used to study the formation of this deep overwintering aggregation and its stability. The ecological model includes nutrients, phytoplankton and microzooplankton in addition to a stage-structured model of C. finmarchicus. Using a Eulerian approach, the model was initiated with an overwintering stock evenly distributed in the oceanic regions of the Norwegian Sea, i.e. where depths>600 m. Spawning and development of the new generation take place in response to vertical mixing and phytoplankton development. Animals are assumed to begin their descent to overwintering depths of 700–1000 m as late stage Vs. Model results show that, in late summer, high concentrations of animals were found at overwintering depths near the shelf break north of the North Sea, off the northeastern Vøring Plateau and in the eastern Lofoten Basin along the slope of the Barents Sea shelf. They remained there for months due to deep eddies and southward, deep currents along the Norwegian shelf. The simulation experiments indicate that the combined effect of deep anticyclonic circulation and vertical migration behavior of the animals may explain the high concentrations of overwintering C. finmarchicus found in field surveys in the Eastern Lofoten Basin, close to the shelf break.     

Observation of behavior of a sergestid shrimpSergestes lucens Hansen by underwater photography

Some behavior ofSergestes lucens by day was observed by underwater camera with an electronic flash. A number of species was snapped at about 2–60 m above the steep continental slope where the bottom ranged between 341 and 437 m depths. They were often abundant from 3–4 to 10 m above the bottom. It suggests that the shrimps scatter widely along the continental slope by day. The maximum concentration of the shrimp was 1.2 individuals/m³. None of the shrimps was found on the bottom, and the majority were swimming in horizontal position.     

Depth-related distribution and abundance of seastars (Echinodermata: Asteroidea) in the Porcupine Seabight and Porcupine Abyssal Plain,N.E. Atlantic

The depth-related distribution of seastar (Echinodermata: Asteroidea) species between 150 and 4950 m in the Porcupine Seabight and Porcupine Abyssal Plain is described. 47 species of asteroid were identified from ∼14,000 individuals collected. The bathymetric range of each species is recorded. What are considered quantitative data, from an acoustically monitored epibenthic sledge and supplementary data from otter trawls, are used to display the relative abundance of individuals within their bathymetric range. Asteroid species are found to have very narrow centres of distribution in which they are abundant, despite much wider total adult depth ranges. Centres of distribution may be skewed. This might result from competition for resources or be related to the occurrence of favourable habitats at particular depths. The bathymetric distributions of the juveniles of some species extend outside the adult depth ranges. There is a distinct pattern of zonation with two major regions of faunal change and six distinct zones. An upper slope zone ranges from 150 to ∼700 m depth, an upper bathyal zone between 700 and 1100 m, a mid-bathyal zone from 1100 to1700 m and a lower bathyal zone between 1700 and 2500 m. Below 2500 m the lower continental slope and continental rise have a characteristic asteroid fauna. The abyssal zone starts at about 2800 m. Regions of major faunal change are identified at the boundaries of both upper and mid-bathyal zones and at the transition of bathyal to abyssal fauna. Diversity is greatest at ∼1800 m, decreasing with depth to ∼2600 m before increasing again to high levels at ∼4700 m.     

Distribution and diversity patterns of asellote isopods (Crustacea) in the deep Norwegian and Greenland Seas

Distribution and diversity patterns of asellote isopods (Crustacea) of the deep Norwegian and Greenland Seas are described. The asellotes show the same pattern of rapid faunal change across the upper continental slope as commonly described elsewhere. Here the rate of species replacement is maximum at depths of 800–1000m, but decreases towards greater depths. The distribution of the asellotes shows some correlations to the distribution of sediment types. Species diversity is maximum at 800m and decreases with depth. The species diversity pattern is related here to heterogeneity of the sediments and different species immigration rates into shallow and deep Arctic waters.     

Nepheloid layer distribution in the Benguela upwelling area offshore Namibia

The distribution of nepheloid layers across the outer shelf and upper continental slope off Namibia was studied during a cruise with R.V. Meteor in late austral summer 2003. Optical measurements, carried out with a transmissometer and a backscattering fluorometer, are correlated with suspended particulate matter (SPM) and particulate organic carbon (POC) values from water sample filtration. Conductivity-temperature-depth and oxygen data are used to relate the nepheloid layers to hydrographic structures. The particle content of surface water at the continental slope is controlled primarily by the offshore extension of highly productive upwelling filaments. A pronounced bottom nepheloid layer (BNL) covers the entire area of study with maximum intensity above the outer shelf and at the shelf break—an area where erosional forces dominate. The detachment of this BNL at the shelf break feeds a major intermediate nepheloid layer (INL) at 25.5°S. This INL is positioned at 250–400 m depth, at the lower boundary of an oxygen minimum zone, and is likely connected to the poleward flow of South Atlantic Central Water (SACW) across the shelf break. Together, these strong subsurface nepheloid layers are indicators of intensive lateral particle transport from the outer shelf towards a depocenter of organic matter on the upper continental slope.     

Interranual variability in horizontal patterns of larval fish assemblages in the northeastern Aegean Sea (eastern Mediterranean) during early summer

Larval fish community structure was studied in the northeastern Aegean Sea (NEA) over an area influenced by the advection of Black Sea water (BSW). Sampling was carried out in early summer during a period of 4 years (2003–2006). Taxonomic composition and abundance presented high variability in space that remained relatively constant among years. Tow depth and indicators of trophic conditions in the upper water column (i.e., zooplankton displacement volume, fluorescence) explained significantly the structure of larval assemblages during all surveys. The northern continental shelf (Thracian and Strymonikos shelf), where a large amount of enriched, low salinity BSW is retained, was dominated by larvae of epipelagic species, mainly anchovy (Engraulis encrasicolus). Interannual changes in horizontal extension of the BSW seemed to match closely observed changes in the distribution of anchovy larvae. Mesopelagic fish larvae were particularly abundant beyond the continental shelf (over the North Aegean Trough) where a strong frontal structure is created between the low salinity waters of BSW origin and the high salinity waters of the Aegean Sea. Larvae of certain mesopelagic species (e.g., Ceratoscopelus maderensis) may occasionally be transported inshore when the prevailing current meanders towards the coast or feeds anticyclonic gyres over the continental shelf.