212
L. Lopiano et al.
Thble 3. Feeding Index (IA) of main food items in the diet of D. puntazzo (D.p.) and S. salpa juveniles (8.s.)
Items
8am
12 am
4pm
apm
12 pm
4am
D.p. S. s.
D.p. S.S.
D.P.
Amphipoda
9.4
0.2
7.4
0.0
9.5
Ascidiacea larvae
0.0
0.1
45.3
0.7
42.1
Bryozoa
0.0 10.5
0.0
0.1
0.0
Copepoda
0.2
1.0
2.5
0.3
4.2
Fecal pellets
0.0
0.1
0.0
0.7
0.0
Hydrozoa
0.0
7.1
0.0 31.2
0.0
Isopoda
0.0
0.0
1.4
0.0
1.3
Mollusca larvae
4.6
0.0
0.0
0.0
7.7
Ostracoda
0.0
0.1
0.0
0.0
0.0
Phycophyta
0.0 68.2
0.2 61.7
0.2
Polychaeta
0.0
0.1
0.0
0.0
0.0
Tanaidacea
4.0
0.0
O.S
0.0
0.0
Unidentified eggs
8.7
0.1
3.1
0.2
0.0
negligible prey", and at 12 pm, when they represented a "fundamental prey". Other "fundamental prey" were taxa Hydrozoa at 12 am and
Ascidiacea larvae at 4 pm, while the other items
may be considered "secondary prey",
Gastric evacuation of D. puntazzo juveniles
fitted a square root model with a rate R of 0.011
± 0.001 g DW h- 1 (r = 0.84; P < 0.001; n = 28). The
amount of food consumed daily, calculated
according to Pennington's model, was 0.0113 g
DW g-l body DW, equal to 1.3% of the mean dry
weight of the fish. Also gastric evacuation of S.
salpa juveniles was best described by the square
root model, with R values of 0.014 ± 0.001 g DW
h- 1 (r = 0.89; P < 0.001; n = 42). Daily food ration
was 0.0159 g DW g-1 body DW, equal to 1.6% of
the mean BDW.
Values of standardised Levins' index of niche
breadth (B) and of simplified Morisita's index
niche overlap (C H ) between D. puntazzo and S.
salpa juveniles, in the different sampling times,
are reported in Table 4. Juveniles of D. puntazzo
Thble 4. Standardised Levins' index (BJ and simplified
Morisita's index (C H ), in the different sampling times
Time
BA
C H
D.p.
S.s.
Sam
0.2
0.1
0.1
12 am
0.1
0.1
0.1
4pm
0.2
0.1
0.9
Bpm
0.1
0.1
0.1
12 pm
0.2
0.1
0.1
4am
0.0
0.0
0.0
&.s.
D.p. &.s.
D.p. &.s.
D.p. 8.s.
0.0
56.9
0.2
29.4
0.0
0.0
0.0
53.9
3.5
1.5
10.8
4.4
0.0
0.0
3.4
0.0
0.8
0.0
0.0
0.0
0.0
1.3
14.8
0.9
1.4
0.4
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
0.0
4.2
0.0
4.8
0.0
1.4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.2
0.3
0.0
0.0
1.5
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
0.0
15.0
0.6 72.0
0.3 43.6
0.0
0.0
0.1
0.0
0.2
0.0
0.0
0.0
0.0
0.0
2.4
0.0
3.1
0.0
0.0
0.0
0.8
1.2
2.4
1.4
1.4
0.0
0.0
had a niche breadth larger that S. salpa at most
sampling times. The analysis of C H values showed
that juveniles of two species presented a niche
overlap at 4 pm (C H = 0.9).
Discussion
The qualitative analysis of the gastric contents of
D. puntazzo in the Stagnone di Marsala allowed
to identify these juveniles as micro-carnivorous
mainly feeding on small Amphipoda and
Tanaidacea, that in turn are associated to
Cymodocea and Caulerpa. Another important
contribution to their diet was provided by planktonic prey. such as Ascidiacea larvae, planktonic
eggs and Copepoda Harparcticoida. The D. puntazzo diet appeared to be highly selective, due to
the proportion of Ascidiacea in their stomachs
compare to their density in the Stagnone di
Marsala (Campolmi 1998). Copepoda were intensively preyed during the entire diel cycle.
Similarly, several studies carried out in coastal
and estuarine areas (Knox 1986; Coull 1990) have
shown that juveniles of different fish species
largely feed on Harpacticoida which, due to their
high fatty add content (Volk et al. 1984) are characterised by high calorific values (Watanabe et al
1983). Our results conflrmed previous results on
food preference of juvenile D. puntazzo from the
same area (Mirto et al1994). Algae and marine
seagrasses did not represent important food
items in the diet of these juveniles. and this is in
contrast with results on food preferences for
adults (Porcile et aL 1987; Sala and Ballesteros