Page 2 - Russo_Terlizzi_1998

Giovanni Fulvio Russo & Antonio Terlizzi

This latter aspect seems to be of particular importance in
assessing the bionomic 'status' of seagrass systems as, in the
most complete study carried out so far, which took imo account
several Posidonia beds of a well defìned geographic area (Apulia),
BIANCHI et al. (1989) found such a hig h structural variability in
the macrobenthic associations as to hypothesize that this system
represents a sort of ecologie 'cross-roads', in which four com -
partments which are 'eco-ethologically' independent of each
other are assembled in a 'poli-biocoenotic' complex.
The present paper is the second part of a study comparing a
number of mollusc associations from different geographical
areas of the Mediterranean. In a previous comparative analysis, a
high quali-quantitative variability in the mollusc assemblages
was observed (TERLIZZI & Russo, 1995 ). This characteristic of
the associations was interpreted as a product of the sampling
method utilised, the suction-sampler, which was considered to
be effective mainly on the substrate and at the base of the sea-
g rass leaves. This is the most variable portion of the habitat rep-
resented by the seagrass (i.e. ranging from rock to mud passing
Figure l. Map with rhe fìve sampling sires. throug h g ravel and sand), the leaf stratum being far more
homogeneous in its constitutive components (leaf shoots).
Therefore, the observed coenotic variability may be consid-
Molluscs are one of the best-represented groups in the vagile ered 'topological', being related to 'intrinsic' factors such as the
fauna of Posidonia beds (GAMBI et al. , 1992) and have for some physical characteristics of the sites, rather than to 'extrinsic'
time been the object of si necological studies. The results ones such as the biologica! accommodations to large spatio-tem-
obtained from recent research indicate a notable structural vari- poral scales, from which biogeographical and coevolutive com-
ability in the malacological community, despite the early stud- munity patterns originate.
ies ofLEDOY.ER (1962, 1966), utilised in the bionomic model of The complementary sampling method of the hand-towed net,
PÉRÈS & PICARD 0964), which recognize a single 'biocoenosis' more effective on the leaf stratum, was considered in the present
inhabiting the Posidonia beds of the Mediterranean. srudy and compared with the suction sampler. The aim was to
The 'intra-site' variability of the mollusc community has gather more information about both the overall coenotic variabili-
been studied both in time, taking imo consideration diel and ty and the contribution made to i t by the different strato-coenoses.
year cycles (Russo et al., 1984; 1991 a), and in space, taking
imo account environmental gradients related to water move- MATERIALS ANO METHODS
ment (Russo et al., 1983). The 'inter-site' variability is poorly The investigations were part of the oceanographic emise
understood, as few comparative analyses on a large geographic 'MAREVIVO' (o/s 'Minerva'), in the Adriatic and Ionic Seas and
scale are available (e.g. Russo et al., 1985 a, 1991 b). in the western Mediterranean, during summer Quly-August)

Figure 2. Collecting methods: (a) suction-sampler (after Russo et al., 1986); (b) hand-rowed net (after Russo et al., 1985 b).

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