270 
Fishery Bulletin 99(2) 
Table 2 (continued) 
Diversity (H') 
No. of specimens with food 
Composition of diet 
1000- 
1425 m 
1425- 
1800 m 
1800-2250 m 
Size 1 
Size 2 
(Sizes 1 and 2) 
2.69 
94 
IRI %IRI 
(Sizes 1 and 2) 
2.33 
95 
IRI %IRI 
2.87 (2.93) 
25 
IRI %IRI 
3.16 
68 
IRI %IRI 
Nannastacidae 


1.6 
0.0 



_ 
Platysympus typicus 
— 
— 
2.2 
0.0 
65.6 
0.7 
14.7 
0.1 
Diastylis sp. 
— 
— 
0.2 
0.0 
7.6 
0.1 
9.1 
0.1 
Makrokylindrus sp. 
— 
— 
— 
— 
— 
- 
9.4 
0.1 
Mysidacea 
2112.3 
16.4 
1078.8 
7.4 
50.3 
0.5 
2269.5 
17.9 
Mysidacea unid. 
785.5 
7.0 
196.0 
1.5 
50.3 
0.6 
1117.3 
10.4 
Boreomysis arctics. 
108.4 
1.0 
47.0 
0.4 
— 
— 
2.2 
0.0 
Boreomysis sp. 
49.3 
0.4 
148.9 
1.1 
— 
— 
198.2 
1.8 
Parapseudomm a sp. 
1.2 
0.0 
— 
— 
— 
— 
— 
— 
Decapoda 
4.3 
0.0 
27.9 
0.2 
— 
— 
— 
— 
Decapoda unid. 
4.3 
0.0 
— 
— 
— 
— 
— 
— 
Larval Decapoda 
— 
— 
0.4 
0.0 
— 
— 
— 
— 
Decapoda Natantia 
— 
- 
17.4 
0.1 
— 
— 
— 
— 
Acanthephyra eximia 
— 
— 
7.2 
0.1 
— 
— 
— 
— 
Pontophilus norvegicus 
- 
— 
1.5 
0.0 
— 
— 
— 
— 
Osteichthyes 
— 
— 
27.0 
0.2 
— 
■ — 
— 
— 
Scales 
16.0 
0.1 
— 
— 
— 
— 
— 
— 
dance increases in the environment at 2250 m. The mouth 
position of Bathypterois makes it improbable that they can 
extract tanaids from the substratum, in which these crea- 
tures tend to live. Holdich and Jones (1983) observed that 
some tanaids can swim very fast for short periods and it 
is probable that Bathypterois mediterraneus capture them 
during these periods. At 1800-2250 m, adult Bathypterois 
mediterraneus consume mysids as secondary prey, which 
they capture swimming over the bottom. 
The scarcity of resources, which decrease even more 
below 1200-1400 m (Cartes and Sorbe, 1993), may force 
Bathypterois to diversify their diet at 1800-2250 m 
(H'=2. 93), extending to endobenthic prey, such as cu- 
maceans and tanaidaceans. This stenophagic decrease co- 
incides with Dayton and Hessler’s (1972) deep predator 
prototype. Less intense specialization in a type of resource 
leads deep predators to be more adaptable, whether due to 
a general scarcity of resources or to the abundance of an 
occasional single resource. 
Ontogenetic differences in the diet were small. Adult 
intestines mainly contained more and larger prey than 
did the intestines of juveniles, from all depths. Only at 
1800-2250 m were there clear differences in the prey 
captured by juveniles and adults, possibly reflecting pro- 
nounced qualitative changes in the decapod fauna of the 
western Mediterranean recorded around 2000 m by Cartes 
( 1993d). Bathypterois grail ator , the largest member of the 
genus (Sulak, 1977) from the Bahamas, also has an onto- 
genetic shift in diet (Crabtree et al., 1991), but Bathypter- 
Table 3 
Diet overlap (Schoener index) among the different bathy- 
metric and ontogenetic groups established. 
1425-1800 m 
1800- 
-2250 m 
Sizes 1 and 2 
Size 1 
Size 2 
1000-1425 m 
0.70 
0.50 
0.69 
1425-1800 m 
— 
0.47 
0.65 
1800-2250 m. (sizel ) 
— 
— 
0.60 
ois longipes and Bathypterois phenax do not significantly 
shift their diet ontogenetically. 
Our study goes beyond the preliminary data of Car- 
rasson and Matallanas (1990), indicating that benthope- 
lagic plankton, namely calanoid copepods, play an impor- 
tant dietary role in Bathypterois mediterraneus. Further, 
greater importance for mysids over amphipods (both ac- 
cidental prey), coincided with their relative abundance in 
the area (Cartes and Sorbe, 1993). Tanaids were found in 
the diet for the first time and seem important at depths of 
1800-2250 m. 
Bathypterois dubius, a related species found in the 
North Atlantic has a diet broadly similar to that of Bathyp- 
terois mediterraneus feeding exclusively on planktoben- 
thic copepods (Marshall and Merrett, 1977). However, Sal- 
