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   1992.  Samples  were  collected  at  a  single  depth  (-5m),  in  the                          --
   Posidonia  oceanica  beds  of S.  Domino  (Tremiti  Islands,  Adriatic                      ,.----
   Sea), Gallipoli (Puglia Region, Ionian Sea), Lampedusa (Pelagian
   Islands, Channel of Sicily), Marettimo (Egadi Islands, Channel of   ,.----
   Sicily) and Meda Grande (Medas Islands, Balearic Sea) (Fig.  1).   -o  400
     The  density  of the  prairies  was  recorded  according  to  the   lo.                            a
   classification  of GIRAUD  ( 1977 ).  All  the  prairies  fell  into  the   ,--
   'very dense'  category (more than 700 shoots/m2), except for  that                  -
   of the  Medas,  whi ch  was  classified  as  'dense'  (500-700   r---  H
   shoots/m2).                                            o                   1---
      For  each  station  two  different  samples  were  collected  by
   SCUBA divers.  The first  sample was collected  by an 'air-lift' (or   80
   'suction-sampler')  (Fig.  2a) over  a  surface  of one  square  meter;
   the second  was  obtained  by  a  'hand-towed  net'(Fig.  2b),  over  a                      ,.----
   surface  of about  twenty  square  meters,  according  to  the  tech-
                                                         60                                          -
   nique described by LEDOYER  (1962)  and standardized by  Russo
   et t7l.  (1985 b).
     Molluscs  were  sorted  and  identified  following  SABELLI  et  al.
                                                                                                     -  b
   (1990).  The  descriptive  analysis  was  carried  out  taking  into   ,.----       ,.----
   account  the  presence  of species  and  their  relative  and  absolute
   abundances.  The comparison of the  results obtained  by  the  two   ,...---        r---
   collecting  methods  in  the  different  beds  allowed  a  preliminary   20  f--  1-  -  -  t--  f---  r-
   evaluation of their efficiency with respect to the different species.
     In order to identify coenotic patterns on a large geographical
   scale,  a  structural  comparison  of the  malacological  associations   o
                                                            S.  Dommo   Galhpoh   Lampedusa   Marettimo   Medas
   was performed starting  from  a matrix of raw data and using the
   multivariate  ordination  technique  of Correspondence  Analysis   IDair-lift  Ohand-net l
   (CA)  (PIELOU,  1984).  The  significance  of the  axes  was  test ed
   according  to the method proposed by FRONTIER  (1974).
                                                        Figure 3. Values of (a) abundance and (b) specie richness per sampling method,
                                                                          in the fìve stations.
                                                          As  regards  the  samples  collected  by  air  lift,  the  station  of
   Descriptive analysis                                 Medas showed the highest values  of species richness (53 species,
   Samples yielded  2122 specimens of Molluscs:  1847  Gastropods   69 %);  lower values of qualitative dominance were observed  for
   (87%) and 275  Bivalves (13 %).  907 specimens (43 %) were col-  Gallipoli  (31 %),  Marettimo  (30%),  Lampedusa  (22 %)  and  S.
   lected by suction sampler,  1215 (57 %) by hand-towed net.   Domino  (19% )  (Fig.  Sa).  T he  highest  values  of abundance
     With  the  exception  of the  Medas,  in all  other  stations  the   obtained  by air-lift  were  recorded  in  the station  of Medas  (523
   net  collected  many  more  individuals  than  the  suction  sampler   individuals  58%),  the lowest  for  Gallipoli (6%) and Lampedusa
   (Fig.  3a).  On the contrary, the air lift sampled more species than   (5%);  intermediate  values  were  observed  for  Marettimo (19% )
   the net, except from the station ofLampedusa (Fig.  3b).   and S.  Domino (12 %) (Fig.  5b).
     Overall,  85  species  were  collected,  of which  62  were  Gas-  Regarding  the  samples  collected  by  hand-towed  net,  the
   tropods  (73%) and  23  Bivalves  (27 % ).  34  species  (40%)  were   ranking  of stations  according  to  species  richness  has  an almost
   collected by both sampling methods, 43 (51 %) were exclusively   opposite  trend:  Lampedusa and  S.  Domino showed  the  highest
   sampled by air-lift and  just 8 (9%) exclusively by hand-net.   values  (55  species and 48% ),  intermediate values  were observed
     The  number  of species  yielded  by  the  two  sampling  tools   for  Marettimo (36%) and Medas (36%) while  the lowest values
   was  quite  different  in  all  the  stations.  In S.  Domino,  about   were observed  in the prairie of Gallipoli (only  12 species,  29%)
   50%  of species  were  exclusively  collected  by  net,  while  in   (Fig.  6a).  The  highest  values  of abundance  using  the  hand-
   Medas about  70%  of species were  exclusively  collected  by suc-  towed  net  were  recorded  for  the  prairies  of Marettimo  and  S.
   tion sampler.                                        Domino (35 %  and  32%, respectively),  while lower values  were
     The  number  of species  collected  by  both  sampling  tools  is   observed for  Lampedusa (15 %),  Gallipoli  (10%)  and the Medas
   quite high  in Lampedusa and Marettimo (about 40%),  and very   (8%) (Fig.  6b).
   low in the other three  stations (about 20%).  By considering  the   As  regards  the distribution of single species  in  the samples,
   exclusive and the common species,  it is  once  again evident that   quite  a  high  number of species  (18  species,  21 %) were  'single-
   in  all  the  stations  the  suction  sampler  collected  many  more   tons'  (e.g.  present  in only one  station with a single individua!).
   species than the net (Fig. 4).                       O nly  Bittùmz  latrei!lii  was  present  in all  the stations and  in all

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