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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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