Species diversity patterns of free-living marine nematodes in the Aegean Sea

Deep-sea nematode from the Aegean Sea

Nematodes are the most numerous multicellular animals on earth inhabiting almost every possible environment. A handful of soil will contain thousands of the microscopic worms, many of them parasites of insects, plants or animals. Free-living marine forms are also very abundant, including nematodes that feed on bacteria, fungi, and other nematodes, yet the vast majority of species encountered are poorly understood biologically. 

Today, we can only speculate how many species of nematodes exist. For example, there are estimates that they constitute up to 80% (Bongers, 1988) or 90% (Jairajpuri & Ahmad, 1992) of all metazoan on earth. More recently, Lambshead (1993) estimated the number of nematode species in the deep sea to as high as 1x10⁸. It becomes evident that if these estimations are pragmatic, then, we only know a tiny fraction of this rich and successfully resistant to environmental degradation taxon. Consequently, our interest lies more in what we can learn from the nature itself since nematodes will probably be one of the last groups that will disappear from the planet.

Over the last few years, the Institute of Marine Biology of Crete (IMBC) has undertaken several studies on the diversity of free-living marine nematodes in the eastern Mediterranean, thus fostering a better understanding of local and regional biodiversity patterns.

Fig. 1. Rarefraction curves for the different zones of the Aegean Sea.

There is a long tradition in marine ecology to separate the environment in different zones where certain assemblages occur, which are different from those occurring in other zones. The benthic environment is usually separated in zones according to two different criteria, depth and latitude. 

In the following analysis of diversity, the collected samples were separated in six different zones: 1) littoral zone of the south Aegean (LS), 2) upper sublittoral zone of the south Aegean (USS), 3) lower sublittoral zone of the south Aegean (LSS), 4) bathyal zone of the south Aegean (BS), 5) lower sublittoral zone of the north Aegean (LSN), and 6) bathyal zone of the north Aegean (BN). 

Data from the littoral and the upper sublittoral zone of the north Aegean do not exist. In marine terminology, littoral is the tidal zone. In areas such as the Mediterranean, where tides are practically non-existent, the limits of the littoral zone correspond to the upper and lower level of the swash zone. The sublittoral zone extends from the lower watermark to 200 m depth whilst the bathyal zone extends from 200 m to 2,000 m. Deeper, the abyssal zone (2,000 to 6,000 m) and the hadal zone (> 6,000 m) occur. However, these two zones do not exist in the Aegean Sea. 

In the analysis that follows, the sublittoral zone was further separated in two sub-zones, the upper sublittoral (from the lower water mark to 5 m depth) and the lower sublittoral (from 5 to 200 m depth). In the Aegean, the upper sublittoral zone is generally considered as a high-energy zone, in terms of water movements, whereas the lower sublittoral is considered as a low-energy zone.

The analysis of nematode diversity in the Aegean Sea showed that there was a definite bathymetric pattern, which was not linear. The rarefaction curves (Sanders, 1968) for each zone are presented in Fig. 1. There is a gradual increase of diversity as depth increases. The littoral zone (0 metres) shows the lowest diversity, whereas the highest diversity values were found at the bathyal zone (200-2,000 m). 

Fig. 2. Mean x standard errors for some diversity indices for each zone.

Apart from the rarefaction curves, several other diversity indices were calculated, both weighted for species richness and equitability. A similar pattern of increasing diversity with depth is derived by these indices (Fig. 2). These results were further supported by ANOVA, which demonstrated that these differences were highly significant.

Nonlinear patterns of diversity have been found in several studies. For example, Rex (1983) and Paterson et al. (1995), found parabolic distribution patterns of macrobenthic diversity with pick values near the bathyal zone in the Pacific and the Atlantic respectively. Similarly, Boucher & Lambshead (1995), combining data from the literature and Dinet & Vivier (1979), analysing data from the Atlantic found parabolic diversity patterns for marine nematodes.

It appears, however, that macrofauna in the Aegean Sea does not follow the above trend. As have been shown by Tselepides (1992) and Karakassis & Eleftheriou (1997), the macrofauna diversity decreases with depth, something they attributed to the decrease of food availability with depth. Therefore, it appears that in the Aegean the two benthic components show an inverse diversity response to depth. 

It would be very interesting to examine whether the observed increase of nematode diversity in the Aegean follows the general parabolic pattern found by others. The IMBC has recently started collecting samples from several sites of the eastern Mediterranean around the 3,500 isobath, since the abyssal and hadal zones are absent from the Aegean Sea. These samples are currently being analysed in order to test whether the nematode diversity starts to decline after the bathyal zone. Nevertheless, abyssal depth is the limit that can be reached for the Mediterranean since no hadal environment exists in the entire basin.

Bibliography

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Boucher, G. & Lambshead, P.J.D. (1995). Ecological biodiversity of marine nematodes in samples from temperate, tropical and deep sea regions. Conservation Biology 9, 1594-1604.

Dinet, A. and M. H. Vivier (1979). Le meiobenthos abyssal du golfe de Gascogne. II. Les peuplemets de nematodes et leur diversite specifique. Cahiers de Biologie Marine 20: 109-123.

Jairajpuri, M.S. & Ahmad, W. (1992). Dorylaimida. Free-living predaceous and plant-parasitic nematodes. Oxford & IBH Publishing, New Delhi.

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Tselepides, A. (1992). Ecological study of the bathyal ecosystem of the Aegean Sea. PhD Thesis, Biology Department, University of Crete, Heraklion.