Magda Vincx

Observations on the feeding ecology of estuarine nematodes

Observations on living estuarine nematodes show that previous feeding type classifications do not accurately represent the trophic structure of an intertidal mudflat in the Westerschelde Estuary (Netherlands). A new scheme with six major nematode feeding guilds is p roposed: (1) microvores; (2) ciliate feeders; and (3) deposit feeders sensu stricto are all nematodes without a distinct buccal armature. In the first two groups bacteria and protozoa, respectively are the major particulate food sources, while other items are included in the diet of the third. The three other categories are recognized among the nematodes with a buccal armature: (4) epigrowth feeders; (5) facultative predators; and (6) predators. Diatoms and other microalgae are an important particulate food for many epigrowth feeders. The importance of bacteria as a food source for these nematodes remains poorly documented. A strictly or mainly predatory behaviour has been described for only few species from the study area. Several nematodes, however, are facultative predators. The predatory strategy of Calyptronema maxweberi, as described in this paper, suggests the use of a paralysing or lethal secretion in prey capture, which, to our knowledge, is the first report for aquatic nematodes. Furthermore, the importance of sources other than particulate food in free-living aquatic nematodes is stressed. Our observatior., show that many aquatic nematodes are in fact opportunistic feeders, which may change feeding strategies in response to available food.

Meiobenthos of the deep Northeast Atlantic

This chapter throws the attention on the meiobenthos of the deep northeast Atlantic. The main purpose of this chapter is to summarize new results from an area lying between 15°N and 53°N and extending from the continental margin of western Europe and northwest Africa to the Mid-Atlantic Ridge. It considers first the nature and scope of meiofaunal research in the northeast Atlantic and then discuss the environmental parameters, which are believed to influence meiofaunal organisms. This chapter then discusses the various types and scales of pattern observed among meiofaunal populations within the study area, progressing from the large-scale bathymetric and latitudinal trends and then to small-scale horizontal patterns within particular areas. Faunal densities and faunal composition are considered separately and compared with data from other regions. This chapter also deals with the distribution of meiofauna within sediment profiles and the temporal variability of populations. This chapter concludes by discussing the recent review of deep-sea meiofauna, which focused mainly on the abundance and biomass data from different oceans and on the relationship between the biomass of the meiofauna and that of other faunal components

Nematode communities from the North Sea: environmental controls on species diversity and vertical distribution within the sediment

Nematode assemblages were sampled seasonally at three subtidal stations along the Belgian coast. The stations were characterized by muddy sediments (station 115), fine sand (station 702) and fine to coarse sand (station 790). The forces structuring vertical distribution were investigated by evaluating abundance, species composition, diversity and trophic composition, and relating these to sediment composition, redox state and food sources. The nematode assemblages at the two finer grained stations (115, 702) were dominated by Daptonema tenuispiculum and Sabatieria punctata . For both species, the vertical distribution in the sediment seemed not dependent on the redoxchemistry, as former believed for S. punctata , but primarily influenced by food availability. This feature could also be recognized for Ixonema sordidum and Viscosia langrunensis , the most abundant nematodes at the coarse sandy station (790). In general, nematode diversity was regulated primarily by sediment granulometry. Coarser sediments (station 790) yielded more diverse communities compared to the fine sediments (station 115, 702), however seasonal fluctuations and variations with depth into the sediment were not obvious. At the silty stations, when the sediment column was more oxidized in March, overall diversity was higher and showed a positive relationship to the mud content which varied with depth into the sediment. This positive relation is probably explained by an enhanced deposition of organic matter associated with the accumulation of fine particles near the river-mouths. Furthermore, the higher abundance, the lower diversity and the higher dominance found at the two silty stations of the eastern and the western part of the Belgian coast, pointed to a stressed, organically enriched environment. The results demonstrate that controls on nematode community structure are complex and that information at both species and community level are required to properly evaluate the effects of natural and anthropogenic impacts.

Meiobenthic distribution and nematode community structure in five European estuaries

Meiofauna from the intertidal zone of five European estuaries (Ems, Westerschelde, Somme, Gironde, Tagus) was investigated. Samples represented a cross section of various benthic habitats from near-freshwater to marine, from pure silts to fine-sandy bottoms. The meiobenthic community comprised everywhere a fauna strongly dominated by nematodes, with meiobenthic density increasing with increasing salinity. The Ems differed from the other estuaries due to the presence of a well developed community of Copepods, Gastrotrichs, large Ciliates and/or soft-shelled Foraminiferans in some sites. The Westerschelde stood out due to the near-absence of harpacticoid copepods and, as in the Tagus, the lower meiobenthic densities in the marine part of the estuary. For nematode community analysis, we also included data from the Tamar which were obtained from the literature (Warwick & Gee, 1984). This resulted in the enumeration of 220 species, belonging to 102 genera, each with a characteristic distribution along the salinity, sedimentary and latitudinal gradients. Using the multivariate technique CANOCO, a zonation along these different physicochemical determinants was observed as well although salinity and sediment characteristic (scale of hundreds of meters to kilometers) proved to be more important in explaining community structure than latitudinal differences (scale of hundreds of kilometers). Nematode diversity was nearly entirely determined on the genus level and was positively related to salinity. Deviations from this general trend in the Gironde and the Tamar were attributed to sedimentary characteristics or to low macrobenthic predation. The presence of a typical opportunistic colonizing nematode species Pareurodiplogaster pararmatus in the low-salinity region of the Gironde could indicate (organic?) pollution or disturbance of the intertidal mud-flats.

Meiofauna response to the Agip Abruzzo oil spill in subtidal sediments of the Ligurian Sea

Abstract The effects of the Agip Abruzzo oil spill (10 April 1991) on meiobenthic assemblages of the Ligurian Sea were investigated using samples collected from January 1991 to January 1992. Meiofauna appeared to be highly sensitive to hydrocarbon stress since the oil spill determined an observable decline in density, relative to the pre-pollution conditions. Such patterns were unusual since reported in a period in which, because of the usual seasonality of Mediterranean meiofauna, we would expect an increase in density. In particular, clear decreases in nematode, turbellarian and foraminiferan densities were observed. Oil-induced disturbance had no influence on copepods, which slightly increased after the oil spill, probably because they were dominated by epibenthic forms which were able to escape from the toxic effects of hydrocarbon pollution. The results of classification and ordination analyses as well as the univariate indices indicated that the level of the major meiofaunal groups was not sufficient for detecting oil-induced disturbance, but nematodes at the genus level proved to be highly efficient. The oil spill determined evident changes in terms of a decrease in N 1 diversity. Changes were also observed in richness and in the k -dominance curves of nematode genera, showing a decrease in overall community diversity. One month after the oil spill, diversity patterns were similar to pre-pollution conditions. In contrast, the nematode: copepod ratio and its modifications were not sensitive to the induced stress and proved to be inadequate for detecting hydrocarbon pollution. Similarly, no significant variations were observed when analysing the index of trophic diversity. However, the nonselective deposit feeding nematodes (1B) declined after the oil spill thus indicating possible changes in the functional characteristics of the meiobenthic assemblages. Meiofaunal response to oil pollution was extremely rapid. Meiofaunal structure was significantly affected only immediately after the oil spill (22 April 1991). Univariate and multivariate analyses indicated that the structural characteristics of meiobenthos recovered after only 2 weeks and were almost indistinguishable from pre-pollution conditions a few weeks later, indicating the high resilience of these meiobenthic assemblages.

The importance of fine-scale, vertical profiles in characterising nematode community structure

The spatial heterogeneity of the nematode community on an intertidal flat (the Molenplaat) in the Westerschelde estuary (SW Netherlands) has been investigated. The extent to which macroscale (km) variability was more important than microscale (m) variability was tested. In addition, the importance of vertical distribution profile in the sediment in explaining the horizontal macroscale variability was evaluated. Differences in the structure of the community were analysed at a kilometre scale at three sites that differed in chemico-physical features. The differences in geochemical and physical conditions on a horizontal scale were reflected in species composition and trophic structure of the nematode communities, and to a much lesser extent in their total abundance and species diversity. Detailed investigation of vertical depth profiles showed more pronounced differences between environmentally divergent sites. Sediment granulometry appears to be important in controlling the fauna in the upper sediment layers. At depth, similar faunal assemblages were found irrespective of sediment granulometry, suggesting that other environmental features are more dominant. Vertically, nematode species showed depth distributions that were indicative of sediment characteristics related to the site-specific hydrodynamic regime. Pronounced vertical segregation of nematode species was observed within sandy sediment under strong hydrodynamic and food-stressed conditions. A surface-dwelling nematode community of large predatory enoplids was separated from a deposit feeding xyalid-microlaimid community in deeper sediment layers (beneath 2 cm). Causal factors for this segregation are thought to be species interactions, feeding strategies and/or physical disturbance. In the finest sediments, with high silt content, almost all nematode species were confined to the upper sediment layers (1.5 cm). A sharp decline in density and diversity with depth was observed. Key factors for this distribution pattern are possibly related to the limited oxygen penetration in surface layers and the occurrence of sulphide in deeper sediment layers. At intermediate hydrodynamic and granulometric conditions, a gradual shifting of nematode community was observed with depth, with dominant nematode species maxima present at specific depth layers.

Phylogeography of the **Rhabditis (Pellioditis) marina** species complex: evidence for long-distance dispersal, and for range expansions and restricted gene flow in the northeast Atlantic

Pinpointing processes that structure the geographical distribution of genetic diversity of marine species and lead to speciation is challenging because of the lack of obvious dispersal barriers and the likelihood of substantial (passive) dispersal in oceans. In addition, cryptic radiations with sympatric distributions abound in marine species, challenging the allopatric speciation mechanism. Here, we present a phylogeographical study of the marine nematode species complex Rhabditis (Pellioditis) marina to investigate processes shaping genetic structure and speciation. Rhabditis (P.) marina lives on decaying macroalgae in the intertidal, and may therefore disperse over considerable distances. Rhabditis (P.) marina consists of several cryptic species sympatrically distributed at a local scale. Genetic variation in the COI gene was screened in 1362 specimens from 45 locations around the world. Two nuclear DNA genes (ITS and D2D3) were sequenced to infer phylogenetic species. We found evidence for ten sympatrically distributed cryptic species, seven of which show a strong genetic structuring. A historical signature showed evidence for restricted gene flow with occasional long‐distance dispersal and range expansions pre‐dating the last glacial maximum. Our data also point to a genetic break around the British Isles and a contact zone in the Southern Bight of the North Sea. We provide evidence for the transoceanic distribution of at least one cryptic species (PmIII) and discuss the dispersal capacity of marine nematodes. The allopatric distribution of some intraspecific phylogroups and of closely related cryptic species points to the potential for allopatric speciation in R. (P.) marina.

Meiobenthos of the central Arctic Ocean with special emphasis on the nematode community structure.

We investigated the abundance of the meiobenthos and the biomass and community structure of the nematodes in the central Arctic Ocean along two separate transects during 1991 and 1994. Meiobenthos abundances ranged from (100 to 600 individuals per 10 cm2, in the same order of magnitude as in other oligotrophic areas of the world’s deep ocean. Nematodes were the numerically dominant meiofaunal group at every station. Nematode biomass ranged from ( 1t o 48lg dry weight per 10 cm2. A combination of water depth and latitude explained 55% of the variability among stations in nematode biomass and 67% of the variability of total abundance, implying that both vertical and advective #uxes are important sources of food to the meiofaunal communities.The dominant nematode genus was Monhystera, a detrivorous/bacterivorous deposit feeder, suggesting that bacteria may play an important role in the food web of the meiobenthos in the Central Arctic. Multivariate analysis of genera abundances revealed di!erences among stations in the Eurasian and Amerasian Basins. During 1994, however, the deep stations in the Eurasian Basin were more similar to the other Amerasian stations, while the single deep station in the Makarov Basin was most similar to the other Eurasian Basin stations. The structure of meiofaunal communities in the central Arctic may provide insight into spatial variability in the Arctic Ocean. ( 2000 Elsevier Science Ltd. All rights reserved.

Temperature and salinity constraints on the life cycle of two brackish-water nematode species

The present study investigates the influence of salinity and temperature on the life history of two estuarine bacterivorous nematode species, Pellioditis marina and Diplolaimelloides meyli, isolated from the mesohaline zone of the Westerschelde Estuary, SW Netherlands. Gravid females and adult males were inoculated in petri dishes containing agar layers of nine (for P. marina) or five (for D. meyli) different salinities, from almost freshwater to higher than marine, and incubated at a temperature of 20°C, to study the impact of salinity; agar layers with a salinity of 20‰, incubated under each of six different temperatures from 5 to 30°C, served to study the effect of temperature. Daily and total fecundity, development time and sex ratio were quantified, and preadult mortality was estimated. The results are compared to those of a partner study on the influence of salinity and temperature on respiration, assimilation and scope for production in the same nematode species. Salinity had relatively minor effects on fecundity, development times and sex ratio in both species, but strongly impacted juvenile viability at the extremes of the salinity range: at salinities close to 0 and 40‰, preadult mortality was more than 80% in P. marina; it was 100% at 5‰ in D. meyli. Both species had an (near) optimal fitness at salinities of 10 to 30‰. Temperature had a pronounced influence on both nematodes over the entire range studied. Diplolaimelloides meyli still reproduced and matured at temperatures exceeding 30°C, while P. marina had an upper temperature limit for reproduction of 25°C. Development times of D. meyli were more temperature-dependent than those of P. marina: the mean development time from adult to adult for the latter nematode ranged from 2 days at 25°C to 7 days at 9°C. The development time of D. meyli increased from 7 days at 25–30°C to 63 days at 10°C, temperature below which no reproduction occurred. Female-biased sex ratios were found in D. meyli at low temperatures and in P. marina under optimal salinity conditions. The life history results largely agree with the predicted scope for production, but discrepancies were found near the extremes of the abiotic range of both species. It is emphasized that the ranges observed are characteristic of populations, not of species; they may to an extent have been influenced by culture conditions. A comparison of the present results with literature data on other P. marina populations demonstrates that some populations of this species may still reproduce successfully under conditions which are lethal to other populations, raising the question as to whether cryptic species rather than populations of a single species are involved.

ON THE CULTIVATION OF FREE-LIVING MARINE AND ESTUARINE NEMATODES

Although a large body of literature exists on the systematics and ecology of free-living marine and brackish-water nematodes, key questions on the nature and magnitude of interactions between nematodes and other organisms in the benthos remain unanswered. Relatively few authors have investigated live nematodes in food web studies or in experiments dealing with the nematodes’ response to a varying environment. It is mainly for the latter purpose that attempts have been made to maintain, rear and cultivate selected species. This paper describes the methodology used for the maintenance, rearing, and eventual permanent agnotobiotic cultivation of a variety of estuarine nematodes. Spot plates, where small samples of sediment or macrophyte material are inoculated on a sloppy agar layer, have been used for the purpose of maintenance and initial cultivation. Those species that reproduce on spot plates are then selected for monospecific cultivation on agar layers with different nutrient enrichments and with micro-organisms cotransferred from the spot plates as food. Mixtures of bacto and nutrient agar prepared in artificial seawater were specifically suitable for the xenic cultivation of nine bacterivorous and, when supplied with Erdschreiber nutrients, two algivorous/bacterivorous nematode species. Up to three generations of five other nematode species have been reared under laboratory conditions, and several more were kept alive and active for variable periods of time on agar. Generation times observed on spot plates forAdoncholaimus fuscus andOncholaimus oxyuris were substantially shorter than previously published estimates and suggest a correspondingly higher predatory and scavenging potency for these and related enoplids. A procedure for the long-term storage of nematodes at −80°C with glycerol as a cryoprotectant was successfully used forDiplolaimella dievengatensis, Panagrolaimus sp. 1, andPellioditis marina, but not forDiplolaimelloides meyli. The authors have also summarized the existing literature on the cultivation of marine and brackish-water nematodes. Continuous cultivation appears to have been successful mainly for Aufwuchs and epiphytic nematodes; only few sediment-dwellers have been established in permanent culture. Of only just over 30 species that have ever been cultivated, more than half belong to one family (Monhysteridae) and three are Rhabditida, an order poorly represented in the marine environment. Four species have been grown in monoxenic and one in axenic culture, the latter though with limited success. It is concluded that our understanding of the basic nutritional requirements of marine nematodes is as yet insufficient, and that the culture techniques which have so far mainly deployed agar or liquid substrates, while being suitable for the cultivation of Aufwuchs and epiphytic nematodes, do not accurately enough mimic gradients specific of the natural habitat of many sediment-dwellers.