FISHERY BULLETIN: VOL. 80. NO. 2 



are generally true, the fish, which appear to be 

 preferentially attracted, would be more effi- 

 ciently digested and converted than crustaceans. 



Chin barbels (and the first dorsal ray of Chau- 

 liodus) are not fully developed until after meta- 

 morphosis; for most species covered here, the 

 smallest specimens examined (Tables 6, 7) are 

 roughly the size at which the barbel appears 

 fully developed. The Astronesthes spp. are quite 

 small at metamorphosis, and it is not surprising 

 that they apparently eat some of the zooplankton 

 encountered regardless of whether or not they 

 possess a barbel. Likewise, newly metamor- 

 phosed C. sloani would be expected to encounter 

 so many more appropriate-sized crustaceans 

 than fish that it would include some of the former 

 in its diet. Bertelsen (1951) has similarly sug- 

 gested that juvenile ceratioids eat some items as 

 a result of visual detection and capture rather 

 than by use of their lures. (The Bathophilus and 

 Photonectes spp. are almost as small at metamor- 

 phosis as C. sloani, but, perhaps because they can 

 handle relatively larger prey, appear to feed in 

 the passive mode immediately after acquiring a 

 barbel.) Astronesthes indieus, however, continues 

 to feed like the barbelless A. "cyaneus" i.e., as 

 would be predicted for an actively searching spe- 

 cies, until at least up to ca. 60 mm. Astronesthes 

 indieus is unique in that the barbel is not fully 

 developed shortly after metamorphosis but 

 changes considerably as the fish approaches 

 adult size (Gibbs 1964); consequently, it may not 

 begin to feed passively until later. 



Thysanactis dentex has a well-developed bar- 

 bel, metamorphoses at rather large size, and 

 appears to capture fish as frequently as the other 

 species with barbels; but it also feeds on zoo- 

 plankton until it is fairly large and includes rela- 

 tively large crustaceans in its diet at all sizes. It 

 thus appears to feed as an actively searching 

 visual predator as well as by using the barbel. In 

 spite of the advantages suggested for passive 

 feeding, T. dentex is obviously successful at com- 

 bining both methods; except for Idiacanthus 

 fasciola, it is by far the most common species of 

 the barbelled stomiatoids in the study area 

 (Table 9). 



Assuming that the above hypotheses are valid, 

 then evidence for selective feeding by some spe- 

 cies indicates that interspecific differences in 

 barbel morphology and, perhaps, methods of de- 

 ployment have evolved to specialize in attraction 

 of a restricted type of prey, i.e., some of the spe- 

 cies may be analogous to devoted aficionados of 



fly fishing in Homo sapiens. Although even the 

 "general ist" T. dentex showed some evidence of 

 preference for certain fishes, most evidence of re- 

 stricted diets was from Eustomias, which is the 

 most speciose genus considered and also has the 

 most varied and ornate barbels. If sufficient data 

 become available, it would be pertinent to com- 

 pare the degree of preference in Eustomias with, 

 e.g., Bathophilus or Aristostomias, in which all 

 the species have similar and rather plain bar- 

 bels. Regardless of whether the barbel is used or 

 not, the advantages of specialization in diet, such 

 as increased efficiency of capture, must be great. 

 The overall density of prey in the study area is 

 low compared with other oceanic areas, and 

 much current ecological theory (e.g., Schoener 

 1971; Werner and Hall 1974) would predict 

 broad diets rather than restriction to a single 

 prey type such as Scopelosaurus spp., whose den- 

 sity is <1% of the already low total fish density. 



The Photostomias spp. have no obvious fea- 

 tures that would predict a diet restricted almost 

 totally to sergestids. Though they lack a barbel, 

 this would not explain the total absence of fish 

 from the diet. The absence of caridean shrimps 

 from their diet, as well as from those of the other 

 species, may be related to stouter exoskeleton 

 and heavier spines in these shrimps than in ser- 

 gestids, but there are no such features to suggest 

 why the very abundant large euphausiids and 

 penaeid shrimps are also nearly completely 

 ignored. Photostomias must either be able to 

 attack and detect only sergestids or have some 

 method of luring only them into proximity. 



Malacosteus niger is the only "nekton-eating" 

 predator which does not vertically migrate 

 (Clarke 1974). Zooplankton densities within most 

 of its depth range are low both day and night, but 

 by day it overlaps with several abundant ver- 

 tically migrating fishes as well as sergestids 

 (Walters 1977) and large euphausiids (Hu 1978). 

 Since this species is apparently well adapted for 

 ingestion of large prey, has one of the largest 

 gapes of all stomiatoids (Morrow 1964), and is so 

 poorly adapted for small prey — no gill rakers or 

 floor to the mouth, it is all the more perplexing 

 that it had eaten so few relatively large items. It 

 appears to subsist on a rather odd assortment of 

 copepods. 



Among the nekton-eating species, there were 

 few differences between day- and night-caught 

 fish in the incidence of intact prey, and none of 

 the differences were sufficiently large to allow 

 any inference about feeding chronology. There 



302 



