Visual Target Discrimination in Blacktip Sharks ( Carcharhinus melanopterus) 
and Grey Sharks (C. menisorrah ) 1 
Albert L. Tester 2 and Susumu Kato 3 
Conditioned response experiments with juve- 
nile blacktip {Carcharhinus melanopterus ) and 
grey ( C . menisorrahy sharks (Schultz et ah, 
1953) were conducted at the Eniwetok Marine 
Biological Laboratory, Marshall Islands, during 
I960. Our objective was to investigate the 
sharks’ visual capabilities with regard to orien- 
tation, form, differential brightness, and color 
discrimination of targets. 
The conditioned response technique has been 
used only recently in the investigation of sen- 
sory perception in sharks. It was employed in 
auditory studies by Vilstrup (1951), Kritzler 
and Wood (1961), Olla (1962), Davies 
et al. (1963), and Wisby et al. (1964), 
in olfactory studies by Teichmann and 
Teichmann (1959), in electrical sensitivity 
studies by Dijkgraaf and Kalmijn (1963), and 
in visual studies by Clark (1959, 1961, and 
1963). Clark’s work on instrumental condition- 
ing of lemon sharks is particularly pertinent to 
the present study. 
Related studies on the anatomy and physiol- 
ogy of the visual system of elasmobranchs 
(Franz, 191 3 and 1931 ; Verrier, 1929; Gilbert, 
1963) and behavioral studies in the field (Hob- 
son, 1963) have provided some information on 
the visual capabilities of sharks. However, with 
the exception of Clark’s work, subjective 
methods utilizing training techniques have not 
been used to investigate vision in sharks. 
MATERIALS AND METHODS 
Subjects 
The sharks used were immature blacktips 
and greys 19 to 33 inches in total length (Table 
1 Contribution No. 245 of the Hawaii Institute of 
Marine Biology. Manuscript received August 10, 
1965. 
2 Department of Zoology, University of Hawaii, 
Honolulu, Hawaii. 
3 Presently with Bureau of Commercial Fisheries, 
La Jolla, California. 
1) captured from reef flats adjacent to the 
laboratory. 
Experimental Apparatus 
The experimental tank was located within a 
larger rectangular concrete tank which could be 
subdivided into 5-ft sections (Tester, 1963). It 
was housed in a building which excluded most 
light, and some extraneous noise. A booth adja- 
cent to the tank enabled the observer to view 
the sharks through a narrow slit 5 ft above the 
water without being seen by them. 
The design of the experimental tank is shown 
in Figure l, A. Dimensions were: width 4 ft, 
length 20 ft, and depth 3 ft. The ends were 
rounded with curved vertical sheets of galva- 
nized iron. Boundaries of the end compartments 
consisted of notches on the walls and dark lines 
on the bottom. All sides and ends were painted 
dull black, but the bottom was brown. 
A 6-inch square aperture was cut in the 
middle of each galvanized sheet, 12 inches 
below the water level. For some experiments 
a second square was cut with its upper edge 3 
inches below the bottom edge of the first aper- 
ture. Targets were mounted on panels which, 
guided by grooves located behind the apertures, 
were manipulated from the observation booth 
by means of cord and pulleys. In successive dis- 
crimination training, a single aperture was used. 
Two targets were clipped together, one above 
the other, and changes were made by lowering 
or raising the appropriate target to the level 
of the aperture. When two apertures were used, 
in simultaneous discrimination training, three 
targets were clipped in series, so that the middle 
and either the top or bottom targets were 
visible through the apertures. 
Paired electrodes were placed along the walls 
of both end sections L and R. Each electrode 
consisted of a brass rod to which were welded 
nine heavy copper wires spaced 6 inches apart, 
extending from the surface to within 3 inches 
461 
