Meiofauna and nematode diversity in some Mediterranean subtidal areas of the Adriatic and Ionian Sea

Many studies on meiobenthic communities have been carried out in the western Mediterranean (Soyer et al., 1987; Soyer, 1971; Boucher, 1972, 1980; de Bovée, 1987, 1988) and more recent investigations have focused on deep-sea areas (de Bovée et al., 1990; Soetaert et al., 1991; Soetaert and Heip, 1995; Danovaro et al., 1995a). However, knowledge of the structure, composition and diversity of the meiofauna along the southern Italian coast is rather scarce and fragmentary. SCIENTIA MARINA 72(1) March 2008, 5-13, Barcelona (Spain) ISSN: 0214-8358

However, knowledge of the structure, composition and diversity of the meiofauna along the southern Italian coast is rather scarce and fragmentary.
Data generally relate to the intertidal zone and focus on particular taxa or on effects of anthropogenic disturbances on the whole community (Sandulli and de Nicola-Giudici, 1990;Sandulli and de Nicola, 1991).Studies on meiofauna of the southern Adriatic Sea and northern Ionian Sea are even scarcer.Some information about the distribution of meiofauna along the Apulian coasts (southern Italy) was given by de Zio Grimaldi et al. (1999), and Sandulli et al. (2002), who showed a strict relationship between meiofauna community composition and abundance and depth and sediment texture (with a clear preference for medium and medium-fine sands at 10-20 m depths).In previous studies, Mediterranean muds (below 50 m depth) have been shown to harbour a less diverse meiofauna than sandy habitats (Danovaro et al., 1995a(Danovaro et al., , 1995b(Danovaro et al., , 2000;;Sandulli et al., 2003Sandulli et al., , 2004)).
The aim of the present study was to gather further information on the structural characteristics of the meiobenthic and nematode communities along the southern Italian coasts located in the southern Adriatic Sea and northern Ionian Sea (Manfredonia, Brindisi and Gallipoli).
In particular, the major aims of this paper are: (1) to analyse the composition and biodiversity of the meiofauna and nematodes (up to genus level) of each Apulian area; and (2) to evaluate any similarities/dissimilarities between the three areas.

Study area
The study area (Fig. 1) includes three different zones along the Apulian coast: Manfredonia and Brindisi (Adriatic Sea) and Gallipoli (Ionian Sea).
The first study area, located inside the Gulf of Manfredonia, is sheltered and rather shallow, with depths never exceeding 100 m.It is characterised by low hydrodynamic conditions and increased phytoplankton sedimentation rates (Viel and Zurlini, 1986;Vaccarella and Paparella, 1998).The bottoms are essentially composed of silty sand sediments close to the shoreline (2-4 m depth), and muds in the central part of the Gulf (>8 m depth) and in the deeper zones (Colantoni and Gallignani, 1975).
The second area is located on the northwest of the Marine Protected Area of Torre Guaceto (Brindisi), between 60 and 300 m depth.From the shoreline to the open sea a sequence of different habitats is evident: sandy sediments occur down to 5-6 m depth, followed by meadows of Posidonia oceanica and coralligenous sediment (12-22 m depth).Muddy sediments prevail from 25 m depth onwards (Parenzan, 1979).
The deepest (200-700 m) and least sheltered area investigated is Gallipoli, situated on the northern Ionian Sea.Here, coastal sediments consist of coarse sand; followed by coralligenous sediment between 40 and 80 m depth, and silty mud down to 200 m.Deeper sediments consist of oligothrophic bathyal mud, according to Pérès and Picard (1964).

Sampling method and treatment of samples
Sediment sampling was carried out during three cruises from April to July 2004 aboard the oceanographic vessel Universitatis (CoNISMa).All samples were collected between 15 and 705 m depth.Seven sites within each area were selected (Fig. 1, Table 1).
Samples were obtained using a modified Van Veen grab.At each location, the grab was deployed three times.Sediment redox potential was measured immediately after sampling, using a microelectrode (Hanna Instruments 3131) connected to an Eh-pHmeter (Hanna Instruments 9023).Redox potential was measured from the top to 8 cm depth within the sediments.Subsamples for meiofaunal analysis were collected in triplicate using a Perspex core (6.2 cm 2 ) to a depth of 5 cm.
Samples were fixed in 5% neutral formaldehyde seawater solution.Meiofauna was extracted from muddy sediment by centrifugation with LUDOX AM solution (McIntyre and Warwick, 1984), retained on a 43 μm sieve and stained with Rose Bengal; finally the organisms were sorted and counted under a stereomicroscope (Higgins and Thiel, 1988).
Up to a maximum of 100 nematodes per sample were picked out randomly from the most abundant replicate sample, and permanent mounts in glycerol were made.Nematodes were identified to the genus level, using mainly the NeMys online identification key (Steyaert et al., 2005) and other relevant literature (i.e.Platt andWarwick, 1983, 1988;Soetaert and Vincx, 1987;Soetaert and Decraemer, 1989;Warwick et al., 1998).

Structural characteristics of meiofauna
The Shannon-Wiener index (H') and Pielou index (J) were calculated to describe the diversity and evenness of the meiofauna and nematode communities.The genus nematode diversity data were analysed using the Kruskal-Wallis test by ranks since data did not meet the assumptions for ANOVA, even after transformation.
Non-metric multidimensional scaling (MDS) on relative nematode abundances was applied to spatial grouping based on the Bray-Curtis similarity index.Significance of differences among nematode genus composition between the three sampling areas were tested using the ANOSIM (analysis of similarities) according to Clarke and Green (1988) and Clarke (1993).The contribution of individual genera (cumulative contribution of 90%) to the average Bray-Curtis dissimilarity among ween the three areas was determined using the SIMPER "similarity percentages" routine (Clarke and Warwick, 1994).All multivariate analyses were performed using the Primer ® v.6 software package (Clarke and Warwick, 1994;Clarke and Gorley, 2006).

RESULTS
All examined samples were composed of silty mud sediment.Redox potentials of the upper two SCI.MAR., 72(1), March 2008, 5-13   centimetres were positive at all sites (Table 1), while Eh was always negative below 2 cm sediment depth.A total of 11 major taxa were found: 9 taxa were present in Manfredonia and Brindisi, and 5 taxa were observed in the Gallipoli area (Table 2).Generally, meiobenthic densities decreased with depth (Fig. 2).The highest values were recorded in Manfredonia (average density ranges from 144 to 1084 ind. 10 cm -2 ).Lower values were observed both in the Gallipoli (from 5 to 67 ind. 10 cm -2 ) and Brindisi areas (from 20 to 314 ind. 10 cm -2 ).
Figure 3 shows the average nematode diversity indices measured, Shannon diversity (H') and Pielou evenness (J), and average density per area.The Shannon diversity was not significantly different among areas (Kruskal Wallis test: p=0.083).The Pielou index values were high, between 0.6 and 1 (Fig. 3).

Multivariate analyses of nematodes
The multivariate analyses were performed to assess the variability of nematode communities in the different areas and at different depth ranges.All sites of the three study areas were in fact grouped into two categories: shallow sites (above the continental slope, 200 m depth) and deep sites (below 200 m depth).

DISCUSSION
Our knowledge of the distribution of meiobenthos in the bathyal zone of the Mediterranean Sea is mainly due to studies conducted in the Cretan, Aegean and northern Adriatic Seas (Danovaro et al., 1995b(Danovaro et al., , 2000;;Lampadariou, 2001), in the Gulf of Lion (de Bovée et al., 1990;Soetaert et al., 1991;Danovaro et al., 1999) and in Corsica (Soetaert et al., 1991).In the present study, qualitative and quantitative information is obtained on subtidal meiobenthic communities from less investigated areas of the Mediterranean, such as the southern Adriatic and northern Ionian Seas.
There are no data available from other studies on the meiobenthos in the southern Adriatic Sea, apart from data reported by Sandulli et al. (2002), who recorded average meiofaunal densities ranging between 180 ind. 10 cm -2 (50 m depth) and 5180 ind. 10 cm -2 (20 m depth) in sandy sediments.
In the Ionian Sea the only data on bathyal meiofauna are those provided by Danovaro et al. (1995), who reported densities of 290 ind. 10 cm -2 at 600 m depth, decreasing significantly with depth down to 4 ind.10 cm -2 at 1700-1800 m.Some sites of the  western and eastern Ionian Sea (600-1735 m depth) were investigated by Tselepides et al. (2004), who reported meiobenthic densities ranging from 220 to 797 ind. 10 cm -2 and from 93 to 218 ind. 10 cm -2 , respectively.Similar values were recorded off the Gulf of Taranto by Tselepides and Lampadariou (2004).
From the literature, there is evidence of a decreasing trend in meiobenthos abundance with depth (de Bovée et al., 1990;Tietjen, 1992;Vincx et al., 1994;Sommer and Pfannkuche, 2000).In fact, our results are consistent with previous data.Thus, the shallower sites of Manfredonia show a more diverse and abundant meiofauna while the deeper sites of Gallipoli have low meiofauna densities and a low number of taxa.In the Brindisi area, at sampling sites at a depth range common to both Manfredonia and Gallipoli, meiofauna composition was partly similar to that of the other two areas.The Manfredonia area is in fact characterised by a higher taxa diversity due to the contribution of Copepoda, Annelida, Kinorhyncha and Turbellaria.The same taxa are also present in Brindisi and at some sites of Gallipoli, but with a lower contribution to total diversity.Moreover, at three Gallipoli sites (Gl2, Gl4 and Gl7) only nematodes are present in very low densities.This might be due to the greater bathymetric depths of these sites (405-705 m depth), all below the continental shelf, where organic inputs from coastal areas are less relevant.The higher meiofaunal diversity recorded in Manfredonia could be attributable to the more sheltered and shallower environment, allowing a higher sedimentation rate and therefore a higher organic matter input in the sediment (Viel and Zurlini, 1986).The Brindisi area and-to a greater extentthe Gallipoli area are deeper and less sheltered than the Manfredonia area, showing greater hydrodynamism, which translates into lower sedimentation and organic accumulation rates (Parenzan, 1979).
In Manfredonia and Brindisi the dominant families of nematodes are Comesomatidae, with the genera Sabatieria and Dorylaimopsis; Xyalidae, with the genus Theristus; and Linhomoeidae, with the genus Terschellingia.In Gallipoli the dominant family is Syphonolaimidae, mostly represented by Astomonema.All these genera are typically present in enriched muddy sediments and show generally low abundances in sands (Vitiello, 1974).It is known that the ecological properties of sediments may be inferred on the basis of nematodes inhabiting them (Heip et al., 1985).The genus Astomonema, for example, is mostly present in association with reduced sulphur or methane seep sediments.This habitat, probably similar to our deep sites, would be a suitable environment for sulphur-oxidising bacteria symbiotic with Astomonema (Dando et al., 1991;Austen et al., 1993;Giere et al., 1995).Moreover, many species of the genera Theristus and Terschellingia are often found in muddy sediments rich in hydrogen sulphide (Warwick and Gee, 1984); the former genus lives even in the proximity of methane seepages (Jensen, 1995), and the latter is a known representative of the "thiobios" with Astomonema and Sabatieria (Giere, 1993).
One-way ANOSIM indicates a significant dissimilarity among the three Apulian areas.The dissimilarity is due to different abundance percentages of the most representative and common genera.This is more evident when sites in Manfredonia are compared with those in Gallipoli.Moreover, it is interesting to note that the dissimilarity between these two areas is mainly attributable to the contribution of two genera, Astomonema and Dorylaimopsis, the former being particularly dominant at the deeper sites of Gallipoli (0.43% in Manfredonia and 30.71% in Gallipoli) and the latter at the shallower sites of Manfredonia (28.86% in Manfredonia and 0.29% in Gallipoli).ANOSIM also shows that the nematode community structure between shallow sites (above 200 m depth) and deep sites (below 200 m depth) differs significantly, with Astomonema dominating the deep sites and Dorylaimopsis, Sabatieria and Terschellingia dominating the shallower sites.These four are the most abundant gen-era, representing an average of 60% of the total nematode community.
Our results are consistent with previous data by Vezzulli and Fabiano (2006) showing that the oligotrophic state of the sediment characteristic of the whole study area is responsible for the low meiofauna abundance and the genus composition of the nematode assemblage.Nevertheless, at a more detailed level, results of multivariate analysis show differences among the Manfredonia, Brindisi and Gallipoli sites, which may be mostly due to other environmental variables, such as depth, hydrodynamism and sedimentation rates.In particular, depth seems to be a relevant factor for explaining the structure of nematode assemblages since there is a significant distinction between shallow communities (above 200 m depth) and deep communities (below 200 m depth).Shallow sites are located within the continental shelf, where coastal influences are more relevant, while deep sites are all related to the continental slope, where terrigenous inputs are assumed to be much less important.
In the Mediterranean Sea, the knowledge of deep meiofauna is rather scarce compared to other areas, particularly with regard to the nematode community structure.Most studies on nematodes concentrate on the western part of the Mediterranean (Vitiello, 1976;Soetaert and Heip, 1995;Soetaert et al., 1995) and the eastern basin, particularly in the Aegean Sea (Lampadariou et al., 1997;Lampadariou, 2001;Buchholz and Lampadariou, 2002).Therefore, all data presented herein add further information on the subtidal Mediterranean meiofauna, and in particular provide valuable knowledge on the biodiversity of the nematode community of newly studied deep and oligotrophic sediment systems.

6
FIG. 1. -Study area showing position of transects and sampling sites.

TABLE 1 .
-Sampling data: location, water depth and redox potential (0-2 cm depth) for all sites.

TABLE 2
. -Total density and diversity indices of meiofauna, and percentage of taxa in the three Apulian areas.The average values between three replicate samples are represented.

TABLE 3 .
-Nematode genera identified in the three Apulian areas.Numbers represent abundance percentage per sample.

TABLE 4 .
-SIMPER analysis of raw abundance data showing the contribution (% cumulative=90%) of each genus to the mean Bray-Curtis dissimilarity.