Deep-sea Nematode Research Articles

Cosmopolitanism and Biogeography of the Genus Manganonema (Nematoda: Monhysterida) in the Deep Sea

Simple SummaryThe deep sea comprises more than 60% of the Earth surface, and likely represents the largest reservoir of as yet undiscovered biodiversity. Nematodes are the most abundant taxon on Earth and are particularly abundant and diverse in the deep sea. Nevertheless, knowledge of their biogeography especially in the deep sea is still at its infancy. This article explores the distribution of the genus Manganonema in the deep Atlantic Ocean and Mediterranean Sea providing new insights about this apparently rare deep-sea genus.Spatial patterns of species diversity provide information about the mechanisms that regulate biodiversity and are important for setting conservation priorities. Present knowledge of the biogeography of meiofauna in the deep sea is scarce. This investigation focuses on the distribution of the deep-sea nematode genus Manganonema, which is typically extremely rare in deep-sea sediment samples. Forty-four specimens of eight different species of this genus were recorded from different Atlantic and Mediterranean regions. Four out of the eight species encountered are new to science. We report here that this genus is widespread both in the Atlantic and in the Mediterranean Sea. These new findings together with literature information indicate that Manganonema is a cosmopolitan genus, inhabiting a variety of deep-sea habitats and oceans. Manganonema shows the highest diversity at water depths >4,000 m. Our data, therefore, indicate that this is preferentially an abyssal genus that is able, at the same time, to colonize specific habitats at depths shallower than 1,000 m. The analysis of the distribution of the genus Manganonema indicates the presence of large differences in dispersal strategies among different species, ranging from locally endemic to cosmopolitan. Lacking meroplanktonic larvae and having limited dispersal ability due to their small size, it has been hypothesized that nematodes have limited dispersal potential. However, the investigated deep-sea nematodes were present across different oceans covering macro-scale distances. Among the possible explanations (hydrological conditions, geographical and geological pathways, long-term processes, specific historical events), their apparent preference of colonizing highly hydrodynamic systems, could suggest that these infaunal organisms are transported by means of deep-sea benthic storms and turbidity currents over long distances.

Deep-sea nematode assemblage has not recovered 26 years after experimental mining of polymetallic nodules (Clarion-Clipperton Fracture Zone, Tropical Eastern Pacific)

We investigated nematode assemblages inhabiting the 26-year-old track created by experimental deep-sea mining of polymetallic nodules, and two adjacent, undisturbed sites, one with nodules and one without nodules. The aim was to compare density, assemblage structure, and diversity indices in order to assess the process of recovery of the nematode assemblage inhabiting the disturbed site. This experimental dredging was conducted in 1978 by the Ocean Minerals Company (USA) in the area of a French mining claim in the Clarion-Clipperton Fracture Zone (Tropical Eastern Pacific) at a depth of about 5000 m. The nematode assemblage had not returned its initial state 26 years after the experimental dredging: the total nematode density and biomass within the dredging track were significantly lower than outside the track; the biodiversity indices showed significantly lower nematode diversity within the track; and the structure of the nematode assemblage within the track differed significantly from those in the two undisturbed sites outside the track. However, there were no significant differences in the mean body volumes of adult nematodes and adult–juvenile ratios between the track and reference sites. Parameters such as the rate of sediment restoration (which depends on local hydrological conditions) and the degree and character of the disturbance appeared to be of considerable importance for the recovery rate of the deep-sea nematode assemblages and their ability to recolonize disturbed areas. The rates of recolonization and recovery may vary widely in different deep-sea regions.

Deep-Sea Nematodes Actively Colonise Sediments, Irrespective of the Presence of a Pulse of Organic Matter: Results from an In-Situ Experiment

A colonisation experiment was performed in situ at 2500 m water depth at the Arctic deep-sea long-term observatory HAUSGARTEN to determine the response of deep-sea nematodes to disturbed, newly available patches, enriched with organic matter. Cylindrical tubes,laterally covered with a 500 µm mesh, were filled with azoic deep-sea sediment and 13C-labelled food sources (diatoms and bacteria). After 10 days of incubation the tubes were analysed for nematode response in terms of colonisation and uptake. Nematodes actively colonised the tubes,however with densities that only accounted for a maximum of 2.13% (51 ind.10 cm−2) of the ambient nematode assemblages. Densities did not differ according to the presence or absence of organic matter, nor according to the type of organic matter added. The fact that the organic matter did not function as an attractant to nematodes was confirmed by the absence of notable 13C assimilation by the colonising nematodes. Overall, colonisationappears to be a process that yields reproducible abundance and diversity patterns, with certain taxa showing more efficiency. Together with the high variability between the colonising nematode assemblages, this lends experimental support to the existence of a spatio-temporal mosaic that emerges from highly localised, partially stochastic community dynamics.

An insight into the feeding ecology of deep-sea canyon nematodes — Results from field observations and the first in-situ13C feeding experiment in the Nazaré Canyon

Submarine canyon systems provide a heterogeneous habitat for deep-sea benthos in terms of topography, hydrography, and the quality and quantity of organic matter present. Enhanced meiofauna densities as found in organically enriched canyon sediments suggest that nematodes, as the dominant metazoan meiobenthic taxon, may play an important role in the benthic food web of these sediments. Very little is known about the natural diets and trophic biology of deep-sea nematodes, but enrichment experiments can shed light on nematode feeding selectivity and trophic position. An in-situ pulse-chase experiment (Feedex) was performed in the Nazaré Canyon on the Portuguese margin in summer 2007 to study nematode feeding behaviour. 13C-labelled diatoms and bacteria were added to sediment cores which were then sampled over a 14-day period. There was differential uptake by the nematode community of the food sources provided, indicating selective feeding processes. 13C isotope results revealed that selective feeding was less pronounced at the surface, compared to the sediment subsurface. This was supported by a higher trophic diversity in surface sediments (Θ−1=3.50±0.2) compared to the subsurface (2.78±0.6), implying that more food items may be used by the nematode community at the sediment surface. Predatory and scavenging nematodes contributed relatively more to biomass than other feeding types and can be seen as key contributors to the nematode food web at the canyon site. Non-selective deposit feeding nematodes were the dominant trophic group in terms of abundance and contributed substantially to total nematode biomass. The high levels of ‘fresh’ (bioavailable) organic matter input and moderate hydrodynamic disturbance of the canyon environment lead to a more complex trophic structure in canyon nematode communities than that found on the open continental slope, and favours predator/scavengers and non-selective deposit feeders.

“A scientist, a nematode and a duck are sitting in a bar...”

A scientist, a deep sea nematode and a duck are sitting in a bar. The scientist turns to the nematode and asks: “How is it you can survive on the sea floor under kilometers of water where the press...

Abundance of small individuals influences the effectiveness of processing techniques for deep-sea nematodes

Nematodes are the most abundant metazoans of deep-sea benthic communities, but knowledge of their distribution is limited relative to larger organisms. Whilst some aspects of nematode processing techniques, such as extraction, have been extensively studied, other key elements have attracted little attention. We compared the effect of (1) mesh size (63, 45, and 32 μm) on estimates of nematode abundance, biomass, and body size, and (2) microscope magnification (50× and 100×) on estimates of nematode abundance at bathyal sites (250–3100 m water depth) on the Challenger Plateau and Chatham Rise, south-west Pacific Ocean. Variation in the effectiveness of these techniques was assessed in relation to nematode body size and environmental parameters (water depth, sediment organic matter content, %silt/clay, and chloroplastic pigments). The 63-μm mesh retained a relatively low proportion of total nematode abundance (mean±SD=55±9%), but most of nematode biomass (90±4%). The proportion of nematode abundance retained on the 45-μm mesh in surface (0–1 cm) and subsurface (1–5 cm) sediment was significantly correlated ( P<0.01) with %silt/clay ( R 2=0.39) and chloroplastic pigments ( R 2=0.29), respectively. Variation in median nematode body weight showed similar trends, but relationships between mean nematode body weight and environmental parameters were either relatively weak (subsurface sediment) or not significant (surface sediment). Using a low magnification led to significantly lower (on average by 43%) nematode abundance estimates relative to high magnification ( P<0.001), and the magnitude of this difference was significantly correlated ( P<0.05) with total nematode abundance ( R 2 p =0.53) and the number of small (≤250 μm length) individuals ( R 2 p =0.05). Our results suggest that organic matter input and sediment characteristics influence the abundance of small nematodes in bathyal communities. The abundance of small individuals can, in turn, influence abundance estimates obtained using different mesh sizes and microscope magnifications.

Influence of mesh size and core penetration on estimates of deep-sea nematode abundance, biomass, and diversity

It is essential to maximize the information that can be gathered in deep-sea studies by thoroughly assessing sample processing methods. Nematodes are commonly used for the study and monitoring of deep-sea floor habitats, but the potential effects of different methods on the quantification of community attributes remain to be quantified. Here, we consider key methodological elements by comparing the effect of sediment depth and mesh size (63, 45, and 32 μm) on: (1) estimates of nematode community attributes, and (2) the sampling effort required to detect changes in these attributes at a bathyal site on the Chatham Rise, south-west Pacific Ocean. The 63 μm mesh retained most (95%) of the nematode biomass but a lower proportion (53–71%) of the nematode abundance. Retention efficiency of common species on this mesh ranged from 12 ( Hapalomus sp.) to >88% ( Comesomatidae spp.). The 63 μm mesh yielded significantly lower diversity estimates than the finer meshes, and failed to detect differences in community structure observed using the 45 and 32 μm mesh sizes. Sediment depth had a substantial effect on all measured community attributes, highlighting the importance of sufficient core penetration into the sediment (≥5 cm) for adequately characterizing nematode distribution. Power analysis showed that using a 32 μm mesh and deepest core penetration led to relatively few (3–8) samples being required to detect significant changes in nematode diversity indices relative to coarser mesh sizes. Characterization of nematode diversity and community structure using appropriate and robust methods of sampling is suggested as a sensitive and efficient tool for the assessment of anthropogenic impacts on deep-sea ecosystems.

Preferred use of bacteria over phytoplankton by deep-sea nematodes in polar regions

The present study explored the selective feeding properties of Antarctic and Arctic deep-sea nematodes within an experimental setup. Nematodes are assumed to play an important role in the carbon flux within the polar bathyal food webs, but knowledge about their natural diets is limited. For the first time, deep-sea multicore sediment samples from both polar regions were incubated aboard research vessels with either 13 C-labelled bacteria or diatoms to determine whether the nematode community prefers freshly settled phytodetritus to a bacterial food source. The cores were collected at 2112 to 2400 m water depth and incubated onboard for 1, 3 and 6 d in the Arctic (Hausgarten) and for 1, 7 and 14 d in Antarctica (Kapp Norvegia). Natural carbon isotope signals of nematodes and organic sedimentary carbon showed a clear average offset (+3.2‰). The contribution of bacteria to the diet of nematodes explained this 13 C offset and observed natural 13 C isotopic signatures. The nematodes showed a clear, relatively rapid (maximum at 6 to 7 d) and sig- nificant selective response to the pulse of 13 C enriched bacteria in surface sediments of both regions. This indicated that bacteria were preferred over fresh phytoplankton as a carbon source for both Arctic and Antarctic deep-sea nematode communities. The results also suggest that bacteria may provide a path through which unused detritus may enter the traditional metazoan food web by microbial reworking of organic matter. At the same time, uptake rates of nematode communities were minimal, which suggests the contribution of nematodes to benthic mineralisation of freshly deposited organic matter may be limited in deep polar seas.

The state of knowledge on deep-sea nematode taxonomy: how many valid species are known down there?

All available information from literature sources dealing with deep-sea nematode species was analyzed, in order to obtain an overview of the state of knowledge in deep-sea nematode taxonomy and answer the question of how many valid nematode species are known from the deep sea so far. One hundred and twenty-seven taxonomic and ecological literature sources reported a total of 638 valid species belonging to 175 genera and 44 families, from 474 deep-sea stations at depths of 400–8,380 m. This number is less than 16% of all known marine nematode species, whereas the deep sea comprises about 91% of the ocean bottom. Of these species, 71% were initially described from the deep sea. Most of the valid species have been reported from the North Atlantic, including the Mediterranean. The rest of the World Ocean, including the Pacific, Indian, Arctic and Antarctic oceans, is considerably less studied. The largest numbers of valid species were reported from the continental slope and the abyssal plains, while information on valid species from trenches, deep-sea canyons, and seamounts is extremely scanty. Some deep-sea families are much more investigated than others in proportion to their relative species abundances in the deep sea, i.e., the percentage of valid species from these families among all valid deep-sea species is much higher than the real percentage of species from these families reported in faunistic studies (e.g., Desmoscolecidae, Comesomatidae, Sphaerolaimidae, Benthimermithidae, Leptosomatidae, and Draconematidae). On the other hand, the families Xyalidae, Oxystominidae, and Monhysteridae were recognized as the most “underinvestigated,” as, in spite of their high species abundance in the deep sea, there are quite a few taxonomic studies on these taxa. Some deep-sea nematode species were reported from two or three oceans, and can be considered probable cosmopolitan species. Some number of probable eurybathic species were also found (the difference between minimum and maximum depth was from 1 km to more than 5 km).

The roles of habitat heterogeneity in generating and maintaining biodiversity on continental margins: an introduction

(introduction to Special Issue: The roles of habitat heterogeneity in generating and maintaining biodiversity on continental margins A Contribution to the Census of Marine Life)

Low endemism, continued deep-shallow interchanges, and evidence for cosmopolitan distributions in free-living marine nematodes (order Enoplida)

BackgroundNematodes represent the most abundant benthic metazoa in one of the largest habitats on earth, the deep sea. Characterizing major patterns of biodiversity within this dominant group is a critical step towards understanding evolutionary patterns across this vast ecosystem. The present study has aimed to place deep-sea nematode species into a phylogenetic framework, investigate relationships between shallow water and deep-sea taxa, and elucidate phylogeographic patterns amongst the deep-sea fauna.ResultsMolecular data (18 S and 28 S rRNA) confirms a high diversity amongst deep-sea Enoplids. There is no evidence for endemic deep-sea lineages in Maximum Likelihood or Bayesian phylogenies, and Enoplids do not cluster according to depth or geographic location. Tree topologies suggest frequent interchanges between deep-sea and shallow water habitats, as well as a mixture of early radiations and more recently derived lineages amongst deep-sea taxa. This study also provides convincing evidence of cosmopolitan marine species, recovering a subset of Oncholaimid nematodes with identical gene sequences (18 S, 28 S and cox1) at trans-Atlantic sample sites.ConclusionsThe complex clade structures recovered within the Enoplida support a high global species richness for marine nematodes, with phylogeographic patterns suggesting the existence of closely related, globally distributed species complexes in the deep sea. True cosmopolitan species may additionally exist within this group, potentially driven by specific life history traits of Enoplids. Although this investigation aimed to intensively sample nematodes from the order Enoplida, specimens were only identified down to genus (at best) and our sampling regime focused on an infinitesimal small fraction of the deep-sea floor. Future nematode studies should incorporate an extended sample set covering a wide depth range (shelf, bathyal, and abyssal sites), utilize additional genetic loci (e.g. mtDNA) that are informative at the species level, and apply high-throughput sequencing methods to fully assay community diversity. Finally, further molecular studies are needed to determine whether phylogeographic patterns observed in Enoplids are common across other ubiquitous marine groups (e.g. Chromadorida, Monhysterida).

A-, β-, γ-, δ- and ε-diversity of deep-sea nematodes in canyons and open slopes of Northeast Atlantic and Mediterranean margins

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 396:197-209 (2009) - DOI: https://doi.org/10.3354/meps08269 α-, β-, γ-, δ- and ε-diversity of deep-sea nematodes in canyons and open slopes of Northeast Atlantic and Mediterranean margins R. Danovaro*, S. Bianchelli, C. Gambi, M. Mea, D. Zeppilli Dipartimento di Scienze del Mare, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy *Email: r.danovaro@univpm.it ABSTRACT: Meiofaunal biodiversity, with a special focus on nematodes, was investigated in 6 submarine canyons and 5 adjacent open slopes along bathymetric gradients (from ca. 200 to 5000 m depth) from 3 deep-sea regions (northeastern Atlantic, western and central Mediterranean) spanning >2500 km and across a wide gradient of trophic and physicochemical conditions. The analysis of local (α) diversity at equal depths showed the presence of similar values in the NE Atlantic and Mediterranean deep-sea sediments. The comparison of the α diversity between different deep-sea habitats (canyons versus adjacent open slopes) revealed the lack of significant differences in species richness in most of the investigated systems. However, the analysis of nematode species composition showed the presence of major differences among different sampling depths (i.e. 500 versus 1000 versus 2000 m depth) and habitats. Turnover (β) diversity was high in all of the investigated deep-sea systems, but was higher in the NE Atlantic (87%) than in the Mediterranean margins (range 51 to 60%), resulting in higher values of regional (γ) diversity in the Atlantic margin. Turnover diversity among regions (δ diversity) was highest (~91%) between the NE Atlantic and western Mediterranean, but still extremely high between the western and central Mediterranean margins (~80%), thus leading to similar values of biogeographical diversity (ε) in the NE Atlantic and Mediterranean deep biogeographical provinces. The results suggest that biogeographic differences in deep-sea species composition are related to differences in β and δ diversity and not to differences in α diversity, and that the analysis of the factors driving β diversity are crucial to understand the spatial patterns of biodiversity in the deep sea. KEY WORDS: Deep-sea nematodes · Biogeography · Canyons · Open slopes · Mediterranean Sea · Atlantic Ocean Full text in pdf format PreviousNextCite this article as: Danovaro R, Bianchelli S, Gambi C, Mea M, Zeppilli D (2009) α-, β-, γ-, δ- and ε-diversity of deep-sea nematodes in canyons and open slopes of Northeast Atlantic and Mediterranean margins. Mar Ecol Prog Ser 396:197-209. https://doi.org/10.3354/meps08269Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 396. Online publication date: December 09, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Deep-sea nematode assemblages of the Clarion-Clipperton Nodule Province (Tropical North-Eastern Pacific)

The deep-sea nematode assemblages of the French mining claim area in the Clarion-Clipperton Nodule Province (the Northeastern Pacific Basin, 14°N, 130°W, depth 4,947–5,046 m) were studied at species, genus and family levels. Material was obtained using multicorers and box corers. In total, 2,174 individuals from 14 stations and 21 samples were examined. The nematode assemblages from the nodule-bearing (NB) part of the sampling area differed significantly from those at the nodule-free (NN) section. The average nematode density was 103 inds/10 cm2. The average nematode densities in the NB and NN parts were 69 inds/10 cm2 and 137 inds/10 cm2 respectively. Altogether, at least 325 putative species, 97 valid genera and 33 valid families were found. The real number of nematode species for the sampling area was estimated at about 510. The richest genera were Acantholaimus (33 putative species) and Thalassomonhystera (23 putative species). The proportions of juveniles, males and females among nematode individuals were about 44%, 23% and 33% respectively. About 2/3 of all nematodes were deposit-feeders and around 1/3 were epistrate-feeders. The nematode assemblage of the studied area may be considered to be a Theristus discolensis–Thalassomonhystera sp.3 assemblage with variation of the dominant species. The dominant species at the NN site were Theristus discolensis (12.2%) and Thalassomonhystera sp. 3 (5.3%). In contrast, at the NB site, Thalassomonhystera sp. 3 was most abundant (14.1%), followed by T. discolensis (6.2%). At genus level, Thalassomonhystera (17.9%), Theristus (13.0%) and Acantholaimus (12.8%) dominated at the NN site, whereas the abundance of Theristus was half of that (6.4%) at the NB site. The most dominant families at the NN and NB sites were Xyalidae (28.1% and 16.6%, respectively), Monhysteridae (17.9% and 25.3%) and Chromadoridae (16.2% and 22.5%).

Deep-sea nematodes of the family Microlaimidae from the Clarion-Clipperton Fracture Zone (North-Eastern Tropic Pacific), with the descriptions of three new species

The description of six species of family Microlaimidae (Nematoda), from 5,000 m depth at the Clarion-Clipperton Fracture Zone (North-Eastern Tropic Pacific), is given. Three previously described species (Microlaimus discolensis Bussau et Vopel, 1999; M. porosus Bussau et Vopel, 1999; and Caligocanna mirabilis Bussau et Vopel, 1999) were found about 5200 km far from the area where the type specimens were originally discovered (the Peru Basin, South-East Pacific). Of the new species, Aponema martinezi sp. n. is closest to two other Aponema species, A. minutissima Kovalyov and Miljutina, 2008 and A. nanum (Blome, 1982). However, it differs from them in having a non-set-off head, amphids located quite far from the non-annulated cephalic capsule, and by possessing the gubernaculum with the apophysis. Microlaimus abyssalis sp. n. belongs to a group of Microlaimus species with dorsocaudal apophyses of gubernaculum. It differs from other four Microlaimus species which have apophyses of similar shape (M. crassiceps Gerlach, 1953; M. decraemerae (Muthumbi &amp; Vincx, 1999); M. mnazi (Muthumbi &amp; Vincx, 1999); and M. undulates Gerlach, 1953) by lacking of supplementary precloacal organs in males and some other body parameters. Microlaimus parviporosus sp. n. possesses four submedian rows of pores along its entire body length. In this feature it resembles three other Microlaimus species (M. cyatholaimoides de Man, 1922; M. discolensis Bussau and Vopel, 1999; and M. porosus Bussau and Vopel, 1999). The new species differs from M. cyatholaimoides and M. porosus by the ratio of the length of the outer labial setae and of the cephalic setae (approximately of equal length in the new species vs. the much longer cephalic setae in two latter species) as well as some other parameters. The new species differs from M. discolensis by its shorter head setae of two rings (1.2 μm vs. 6–9 μm) and some other parameters.

Active colonisation of disturbed sediments by deep-sea nematodes: evidence for the patch mosaic model

An on-board experiment was performed during a research cruise to investigate the abil- ity of deep-sea nematode species to actively colonise defaunated sediments. Small cylinders of 500 µm wire mesh filled with defaunated sediment were inserted into microcosms containing sediment with indigenous meiofauna collected from 1300 m depth in the Arctic Ocean. The defaunated sediments were either enriched with the diatom Thalassiosira weissflogii, or remained unenriched. Samples from the defaunated sediment were taken after 9 and 17 d. As controls, microcosms with sediment contain- ing the indigenous meiofauna but without an internal cylinder were also sampled at each time inter- val. Nematodes colonised both enriched and unenriched sediments with abundances of up to 20% of the controls. Irrespective of the time of sampling, abundance and number of species were significantly higher in the enriched treatment, suggesting that the presence of food enhances colonisation and re- silience. Nematode assemblages in the defaunated sediments were species-rich and differed from the controls. The majority of colonising species were rare or undetectable in the controls, suggesting that episodic disturbances may be necessary for their persistence in deep sea sediments. Colonisation was in part determined by species characteristics such as size and motility. At the same time, a large number of different species colonised the empty cylinders, resulting in poor similarity in community composition between replicates, particularly between samples enriched with diatoms. Our results in- dicate a poor predictability of community composition of recently disturbed sediments despite highly reproducible abundance and diversity patterns and lends experimental support to the existence of a spatio-temporal mosaic that emerges from highly localised colonisation patterns.

Effects of megafauna exclusion on nematode assemblages at a deep-sea site

Caging experiments were performed at the Arctic deep-sea long-term observatory HAUSGARTEN to investigate the effects of megafauna exclusion on nematode assemblages. Six experimental cages were deployed at 2500 m water depth and sampled after 4 years for meiofauna and a series of sediment parameters. With the exception of chlorophyll a and phaeopigments, which were significantly higher inside the cages, sediment parameters did not differ between inside and outside the cages. Yet multivariate analysis of the sedimentary environment showed a higher variability outside the cages, indicating higher environmental heterogeneity in the presence of megafauna. Nematode densities (aver. 1516 ind 10 cm −2) were significantly higher in sediments from inside the cages, whereas total number of species (aver. 70 sp.), Shannon–Wiener diversity (aver. 3.4) and Pielou evenness (aver. 0.8) did not differ between the two treatments. Also differences in faunal multivariate structure between cage and control samples were not significant. Nevertheless, Fisher's alpha diversity, the number of nematode genera and taxonomic distinctness showed significantly higher values outside the cages. Since univariate measures are often insensitive to changes in community structure, and interpretation of the multivariate analysis was confounded by the large number of rare species, we further analysed our data by dividing the species into three groups of frequency (frequent, intermediate and occasional) and into three groups of abundance (dominant, sub-dominant and scarce). Results from this analysis revealed that the higher nematode abundances inside the cages were mainly due to an increase in density of scarce species that were either frequent or intermediate in frequency. In contrast, occasional species were adversely affected. These results lead us to believe that it is mainly the frequent species occurring in low densities that take advantage of favourable conditions (i.e. lack of disturbance or increased phytodetritus) in the deep sea. The higher taxonomic distinctness, Fisher's diversity, and number of genera at the control sites, together with the fact that they were environmentally more heterogeneous, suggest that megafaunal organisms play an important role in creating microhabitats in the sediment and significantly influence deep-sea nematode assemblages. However, the lack of any changes in species richness inside the cages after 4 years suggests that they cannot fully account for the maintenance of the remarkable high levels of species coexistence in the deep sea. The key is more likely to be found in evolutionary history than actual ecology.

Species diversity and distribution within the deep-sea nematode genus Acantholaimus on the continental shelf and slope in Antarctica

Species diversity and distribution—in relation with biogeography, bathymetry and environmental characteristics—within the genus Acantholaimus were investigated on the shelf and slope (182–2,009 m) in the Weddell Sea (Kapp Norvegia and Vestkapp) and near the Antarctic Peninsula (Drake Passage and Bransfield Strait). The occurrence of the otherwise typical deep-sea genus Acantholaimus in high densities and diversity on the continental shelf is a unique feature of the Southern Ocean. Fifty-five Acantholaimus species (morpho-types) were distinguished in our study area, of which 39 were found on the shelf. The Acantholaimus communities were characterised by a high number of congeneric species and the presence of many rare species, similar to typical deep-sea communities. Fourteen species had a distribution extending from the shelf to the lower slope. High local and regional diversity was obvious and a considerable level of endemism was suggested. A negative impact of chlorophyll a concentrations, reflecting the availability of phytoplankton, was found on the Acantholaimus densities and species diversity.

Taxonomy and biogeography of Molgolaimus Ditlevsen, 1921 (Nematoda: Chromadoria) with reference to the origins of deep sea nematodes

Molgolaimus is a genus of free-living marine nematodes which is found in high densities (10–35% of the total community) up to 2000 m depth. Its occurrence is often associated with organically enriched and recently disturbed areas. Currently, only 16 species have been described, mainly from shallow waters. The present study contributes 17 new species mainly from the Weddell Sea but also from the Pacific Ocean, and provides an illustrated polytomous identification key to species level. The 33 Molgolaimus species described can be identified based on just a few morphometric features: spicule length, body length, anal body diameter, tail length and pharynx length. A first insight into the biogeography of this deep sea genus at species level is presented. A comparison of morphometric characteristics between species suggests that the most similar species co-occur in the same geographical region, rather than within the same bathymetric zones or similar ecosystems separated over long distances. These observations suggest that deep sea nematodes may not have a common origin but might have derived “recently” from shallow water taxa. Therefore, global distribution of nematodes could be explained by means of palaeogeographical events.

Rhaptothyreus typicus Hope &amp; Murphy, 1969 (Nematoda: Rhaptothyreidae): an anatomical study of an unusual deep-sea nematode

Abstract Descriptions are given for the males of Rhaptothyreus typicus, a morphologically unique and incompletely investigated deep-sea nematode, using light microscopy, TEM, and SEM. The cephalic sensilla are arranged in two circles: an anterior circle consisting of six pairs of papilliform sensilla and a posterior circle consisting of four papilliform sensilla. The fovea of the giant, oblong, amphid is covered with cuticle bearing minute perforations. The actual amphidial aperture, revealed only by TEM, is minute and slitlike. The apical oral aperture is reduced to a simple pore and the vestigial pharynx is devoid of an internal lumen and musculature. The mid-gut is a trophosome consisting of a solid mass of irregularly-shaped cells filled with rod-shaped structures that are assumed to be intracellular symbiotic bacteria. There is no evidence of even a vestige of a reproductive system except for the presence of the single spiculum. The phylogenetic relationships of Rhaptothyreus remain unknown.

Meiofauna communities of continental slope and deep-sea sites off SE Brazil

There is little meiofauna data from the southwest Atlantic, and especially lacking are studies examining deep-sea communities. In this study, the structure of meiofauna communities, particularly nematodes, was analyzed from data derived from 101 samples (48 samples on the continental-slope and 53 samples at a deep-sea site) collected at Campos Basin, SE Brazil. Differences in the meiofauna densities and in the number of taxa between continental slope and deep-sea sites depended on the taxonomic level examined. While total meiofauna abundance did not differ significantly between sites, nematode densities were significantly higher in the deep sea (mean of 157 inds.10 cm −2) than on the slope (mean of 129 inds.10 cm −2). The number of meiofauna taxa was significantly higher at the continental slope site. Yet for the most abundant meiofaunal group, the nematodes, whilst the number of families did not differ between continental slope and deep sites, the number of genera was significantly higher in the deep sea. The remarkable resemblance between the dominant nematodes ( Halalaimus, Acantholaimus, Daptonema, Theristus and Sabatieria) from SE Brazil and other deep sea studies confirmed earlier suggestions of a typical deep-sea nematode community with a broad geographical distribution. Multivariate analysis derived from meiofauna and nematode data showed that the structure of the fauna differed significantly between sites. Correlations detected between meiofauna and some sediment properties, such as grain size and sediment heterogeneity, although significant, were very low.