Search

Sediments of three different subtidal areas (15-705 m depth) of the Italian coasts (Manfredonia, Brindisi and Gallipoli) were investigated to study meiofauna and nematode composition. The nematodes were identified to the genus level and their abundances compared using multivariate analysis. Our data showed an evident depth gradient in meiofauna abundance: the shallowest sites had more diverse and abundant meiobenthic communities than the deeper ones. Nematodes were the dominant taxon (83-100%) at all sites, followed by Copepoda (0.5-8%). Sabatieria, Astomonema, Dorylaimopsis, Terschellingia and Daptonema were among the dominant nematode genera in the three areas. Nematode genus H’ diversities were not significantly dissimilar, though at community level some differences were detected among the study areas. The greatest differences were observed in the comparison of the communities from Manfredonia and Gallipoli. Furthermore, there was a difference between shallow (

The effect of an abundant sandy beach polychaete, Scolelepis squamata, on the colonisation of defaunated sediments by marine nematodes indicates that sandy beach fauna can be partially controlled by biological interactions within and across size groups. Experimental cores, equipped with windows allowing infaunal colonisation, were filled with defaunated sandy beach sediment containing two different treatments with and without S. squamata. These cores were inserted into microcosms filled with sediment with indigenous meiofauna collected from the field. The treatments were incubated in the laboratory at ambient temperature and salinity for 2, 7, 14 and 21 days, in order to follow the colonisation process of the defaunated sediments by the indigenous nematode fauna over time. Nematodes initially colonised both treatments, with abundances of up to 10% of the densities in the control; after 2 weeks, nematode densities in the cores without S. squamata surpassed the control densities. Nematode assemblages in both treatments were not species rich, and also differed in composition from the natural assemblages. The most successful colonising species, Enoplolaimus litoralis, was rare in the surrounding sediment, suggesting that colonisation was determined by species-specific characteristics such as body size, motility and feeding strategy. Initially the presence of macrofauna did not affect the nematode community composition, but after 2 weeks of the experiment, the presence of the polychaete seemed to facilitate the earlier establishment of non-opportunistic species.

Species assemblages are not randomly assembled from a local species pool; they often show segregated or aggregated distribution patterns. These patterns may be attributed to both biotic and abiotic factors. On a large scale abiotic factors may be important, while on a smaller scale other factors such as species interactions may become essential. Here we will focus on small-scale patterns in nematode communities. Species patterns are generally revealed by null models based on presence/absence data. Since there is an increasing chance of falsely rejecting the null hypothesis of a random assembled community with increasing matrix size, we used an algorithm generating independent null matrices and applied a large number of swap attempts to build a null matrix. Moreover, we applied an additional test to reveal the susceptibility of the analyses of checker and the C-, T- and Vscore to a Type I error for randomised data. To minimise the influence of the abiotic environment, we restricted the swapping algorithm of the null model to the replicate samples of one sampling event. Since stronger species interactions are expected for species of the same functional type, the nematode data was split according to the four feeding types defined by Wieser (1953). Our data indicate that species tend to aggregate and co-occur more often in some replicate samples than would be expected from a random species distribution of the local species pool. This is in accordance with the patchy distribution patterns known for nematode species. These aggregated patterns are also found for the different feeding types. The factors causing these aggregated patterns cannot be established since they are not included in the data, but the data do indicate that competitive exclusion is unlikely at the scale of a sample core.

Meiobenthic uptake of 13C labelled diatom Skeletonema costatum and Prymnesiophyte Phaeocystis was investigated in permeable and fine grained depositional sediments from the Southern Bight of the North Sea at different sediment horizons. Both the diatom and Phaeocystis-derived organic matter (OM) cascaded into meiobenthic biomass in low but similar percentages and label uptake was highest at the upper cm in both stations. Phaeocystis-derived OM might be as important as diatoms as a food source for the meiobenthos. Meiobenthic biomass in the coarse sediment was lower than in the fine sediment; however label uptake per unit of organism carbon was higher. This was explained as an adaptation to an environment where only low amounts of labile OM are present. In this station, the so-called "stout nematodes" (length to width ratio < 15, [Vanaverbeke, J., Soetaert, K., Vincx, M. 2004a. Changes in morphometric characteristics of nematode communities during a spring phytoplankton bloom deposition. Mar. Ecol. Prog. Ser. 273, 139-146]) showed lower uptake compared to the slender nematodes. Therefore their previously reported opportunistic response to a pulsed food supply must rely then on their life-history characteristics. In finer sediments Sabatieria showed the highest uptake at the 1-3 cm layer, reflecting migration to the sediment-water interface to feed on freshly deposited labile OM. Richtersia 13C uptake was fairly low, indicating the exploitation of another carbon source than the one derived from Skeletonema costatum or Phaeocystis. Enoploides presented the highest uptake, indicating that these predacious nematodes have another food source besides meiobenthic preys. Generally total uptake was low and not nearly sufficient to meet nematode carbon requirements.

The role of oxygen in the vertical distribution of nematodes was investigated by means of an experiment in which different oxygen conditions were imposed on sediment from an intertidal area of the Oosterschelde (The Netherlands). To test our hypothesis that the vertical distribution of the nematode assemblages was not influenced by changing oxygen conditions (e.g. nematodes do not migrate to favourable oxygen conditions), 5 cm sediment was inversed and incubated for 5 days at the lab. In a first treatment, food (diatoms) was added to the bottom; in a second treatment oxygen and food were added to the bottom. For each case and a control treatment, fresh, well-aerated Oosterschelde water was added on top of the sediment. The analysis of the field situation showed that nematodes were the most abundant taxon. Highest densities were observed in the subsurface sediment layer (1-2 cm). The lower abundance in the oxygen and algae-rich superficial layer (0-0.5 cm) could be due to the time of sampling relative to the tides or to biotic factors (e.g. macrofaunal activity). The vertical distribution of the nematode assemblages in the experimental and control treatments proved to be significantly different. An obvious segregation existed between the nematode species assemblage from the superficial (0-0.2cm) and the deeper layers (0.2-1 cm and 4-5 cm). Characterising genera for the superficial sediment layers were Daptonema, Ptycholaimellus, Prochromadorella and Microlaimus; for the deeper layers Terschellingia and Microlaimus. The occurrence of the first species assemblage is determined by the presence of free oxygen. The second species assemblage is adapted to the reduced sediment; nevertheless, artificial addition of limited amounts of oxygen to the deeper sediment layers favoured the assemblage as higher abundances were recorded. In general, oxygen seems to be important in determining the vertical distribution of nematodes in this experiment.

Despite their rather barren and arid appearance, European sandy beaches harbour a highly diverse fauna and flora and some of them are even highly productive. In contrast to tropical sandy beaches little is known about the structural and functional diversity of the different benthic components. This study aims to investigate the structural diversity of the meiobenthos, emphasizing on free-living marine nematodes on a Belgian sandy beach.The samples were collected on the sandy beach of De Panne in the swept prism, i.e. the zone between highest and lowest watermark, subjected to tidal inundation. The beach, situated before the nature reserve "Westhoek Reservaat", is a macrotidal, dissipative, little exposed beach with fine-grained sediment. Within the meiobenthos, 11 taxa were recognized. Free-living Nematoda were the dominant (75.8 %-97.1 %) metazoans in all the stations, mostly followed by Turbellaria. The meiobenthic densities increased towards the low water mark as a consequence of macroscaled physical gradients. A patchy distribution of meiobenthos is primarily caused by gradients on micro- scale. Within the nematodes a total of 87 species, belonging to 67 genera, were recognized. Multivariate and statistical techniques in combinations with indicator species analysis revealed nematode communities associated with dry sand (Rhabditis sp.1 and Axonolaimus helgolandicus), upper (Trissonchulus sp.1 and Dichromadora hyalocheile) and lower littoral (Odonthophora phalarata, Odonthophora rectangular, Chaetonema riemanni and Cyartonema elegans).

The list was compiled by Guy De Smet, a non-specialist, who was assisted by nematologists Magda Vincx, Ann Vanreusel, Sandra Vanhove, Jan Vanaverbeke and Maaike Steyaert. Dr. F. Riemann gave useful advice for the genus Daptonema. The classification in the list is based upon Lorenzen (1979, 1994). Information on the geographic distribution in the list is based upon Gerlach and Riemann (1973, 1974), Platt and Warwick (1983, 1988) and Warwick et al. (1998).ReferencesLorenzen, S. Entwürf eines Phylogenetischen Systems der Freilebenden Nematoden, Kiel, 1979.Lorenzen, S. The Phylogenetic Systematics of Freeliving Nematodes, The Ray Society, 1994.Gerlach, S. A. and Riemann, F. The Bremerhaven Checklist of Aquatic Nematodes. Veröffentlichungen des Instituts für Meeresforschung in Bremerhaven, Supplement 4, (Heft 1 1973, Heft 2 1974).Platt, H. M. and Warwick, R. M. Free-living Marine Nematodes (Part I British Enoplids) Synopses of the British Fauna (New series) No. 28, 1983.Platt, H. M. and Warwick, R. M. Free-living Marine Nematodes (Part II British Chromadorids) Synopses of the British Fauna (New series) No. 38, 1988.Warwick, R. M., Platt, H. M. and Somerfield, P. J. Free-living Marine Nematodes (Part III Monhysterids) Synopses of the British Fauna (New series) No. 53, 1998.

The meiobenthos along a depth transect of oligotrophic sediments in the Arctic Laptev Sea was studied. The meiobenthos followed the general trends reported from other studies: densities decreased with depth in relation to the more limited supply of degradable organic matter at greater depths. Although the sediments along the transect were poor in organic matter in comparison with the NE Atlantic, the densities fitted well with the meiobenthic densities reported from the latter area. It is suggested that the meiobenthos in the cold polar waters is adapted to this extreme environment by a rapid response to short food pulses to the sediments. Nematodes were identified up to genus level and assigned to trophic groups. A total of 32 families comprising 95 genera were found along the transect. The communities were dominated by deposit feeders whose importance increased with depth. Both TWINSPAN and CCA analyses revealed a community shift along the depth transect: a shelf community dominated by Microlaimus and Chromadora could be distinguished from a slope community dominated by Monhystera and Leptolaimus. Generic diversity decreased with depth.