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Structural patterns in the Posidonia oceanica beds rnolluscs

   sample-points  were  more  closely  arranged  than  the  inter-site   ical aspect is of particular interest and may be a promising fleld
   ones. As  a  conseguence,  the hypothesis  that  the 'inter-site' vari-  of investigation.  In  fact,  up to now most  malacological  research
   ability  of the  malacological  associations  is  the  effect  of a  bias   activity  has  focused  on  investigating  the  historical  aspects  of
   introduced  by  the  sampling  method  should  be  excluded.  The   single  species,  such  as  their  evolution  and  biogeography,  with
   large-scale  variability  of the  malacofauna  of the  seagrass  beds   little or  no  attention  paid  to the study of the  history of assem-
   seems  to  be  so  intrinsic  to  the  associations,  and  the  structural   blages they form.
   'signal'  so  strong,  that  they  are  recognisable  independently of   Last, but not least, the large variability in the composition of
   the  sampling  method  utilised.  Consequently,  the  strong struc-  the  assemblages  described  in  the  present  paper  implies  a  great
   tural  differences  between  the  Mediterranean  sites  seem  to  be   difflculty in  recognising a unique and well-defined  malacologi-
   related  mainly  to  the  edaphic  factors  characterizing  each  sea-  cal  taxocoene for  the Posidonia  beds.  Species co-occurring  in  the
   grass bed. The density of the prairie seems to have an  important   different beds consti tute a very low percentage of the total.
   role  in  determining  the  strong srructural  differentiation  of the   Therefore,  the  present  analysis  of one  of the  main  faunistic
   malacological assemblage of the Medas, with respect to all other   components  of the  vagile  fauna  does  not  agree  with  the  model
   sites.  In  this site, in fact,  the air-lift was  far  more efficient  than   of PÉRÈS  &  PICARD  (1964),  w h ere  the  Medi terranean  Posidonic1
   the  hand-net,  as  indicated  by  the  high  presence  of molluscs   beds  are  considered  as  a  single  biocoenotic  and  bionomic  unit
   more  strictly  related  to  the  substrate,  such  as  bivalves  and,   (HP).  Present a-ecologica!  data agree  better with  the  'poly-bio-
   among  gastropods,  the  families  of Alvaniidae  and  Marg inelli-  coeni tic'  hypothesis (BIANCHI  et  al.,  1989),  although  b-ecology
   dae.  Apart  from  the vegetal  cover of the substrate (e.g.  density   needs to be focused  in the future in order to gain a better under-
   of plant shoots),  the guality of the substrate  itself, its exposure   standing  of the  ecologica!  processes  underlying  the  structural
   to water movements and the related sedimentation rates may be   patterns described.
   among the main edaphic factors  affecting the quali-quantitative
   composition  of the  malacological  associations  and  determining   CONCLUSIONS
   the variety  of functional  adaptation.  In  fact,  the  dominance  of   - The suction-sampler and the hand-towed net discriminate the
   herbivore-deposit  feeders  (e.g.  Bittùtm  spp.)  is  typical  of sea-  intra-site  variability, as  they sample  different parts of the  habi-
   grasses under conditions of low environmental energy, while, on   tat produced  by  the seagrass.  This gives  rise  to 'method-depen-
   the contrary, brusher-herbivores (e.g.} u;ubin/IS spp.  and Tricolia   dent' patterns which correspond to a real  'topological' stratiflca-
   spp.), are abundant in conditions of high water movement.   tion  of the  malacological  association  within  the  seagrasses.
      In addition,  the information  collected suggests the presence   Therefore,  the  study  of this  type  of variability  needs  sampling
   of a  latitudinal  coenotic  gradient.  This  is  fairly  evident  in  the   by both the above complementary technigues.
   ordination  model,  where  the  station  points  show  saturation   - When inter-site comparisons are performed, a new and stronger
   along  Fl  consistent  with  their geographical  (i.e.  longitudinal)   structural  variability is  added, against which  the discriminating
   positions. There are no species with narrow biogeographical dis-  power of the  two  methods  becomes  ineffective,  as  they produce
   tributions affecting  the composition  of the assemblages.  There-  similar  patterns.  This  new  variability  may  be  considered  as
   fore,  this  latitudinal  pattern  is  likely  to  be  of the  structural   'edaphic',  being  related  to  local  factors  affecting  the  seagrasses.
   type, a product of the way widely distributed species are  sorted   Consequently, for  the study of this second type of variability, one
   together,  rather  than  the  result  of the  presence  of endemie   of the two techniques may be adopted indifferently.
   species in a narrow region of the Mediterranean. This geograph-  - A  longitudinal trend was observed even  if no species with nar-
                                                        row biogeographical distributions were  recorded.  Therefore  this
                         air-lift                       pattern  is  more likely to be related to the 'history' of the assem-
                                                        blage (e.g. successional stages) rather than to the history of some
                                                        of the species belonging  to them (e.g. evolutionary ecology).
                                                        - The  high  variability  in  the  composition  of the  malacological
                                                        assemblages  does  not  fit  with  the  model  by  Pérès  and  Picard
               Mcda.c; (57 _(,Ci%)
                                                        which considers  the Mediterranean  systems of Posidonia  oceanica
                                                        beds  as  belonging  to  a  sing le  bionomic  unit;  on  the  contrary,
                                                        our  findings  represent  additional  evidence  in  support  of the
                                                        hypothesis  considering  the system as  an  'ecologica! cross-roads'.

               Meda> (40  15%)

                                                        BIANCHI  C.  N., BEDULLI  D, MORRI  C., 0CCHPINTI AMBROGI  A.,  1989
                   Marctt1mo (l 7.42(1/u)
                                                              - L'herbier de posidonies:  ecosysteme  ou  carrefour ecologiqueì
                                                              International  \Vorkshop  on  Posidonia  Beds,  Boudoresque  C.F.,
               Figure 5. Samples  by air-l1ft: (a) quaiiranve   Meinesz  A. ,  Fresi  E.  &  Gravez  V.  edit.,  GIS  Posidonie  Pubi. ,
             and (b) quanritative dominances among rhe sires.   Fr., 2:  257-272.

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