Page 3 - Russo_Terlizzi_1998

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1992. Samples were collected at a single depth (-5m), in the --
Posidonia oceanica beds of S. Domino (Tremiti Islands, Adriatic ,.----
600
Sea), Gallipoli (Puglia Region, Ionian Sea), Lampedusa (Pelagian
r---
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
z
classification of GIRAUD ( 1977 ). All the prairies fell into the ,--
200
'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
r---
et t7l. (1985 b).
Molluscs were sorted and identified following SABELLI et al.
- b
(1990). The descriptive analysis was carried out taking into ,.---- ,.----
r--
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-
f--
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.
RESULTS
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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