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PACIFIC SCIENCE, Vol. XX, October 1966 
which renders them unattractive to fish. Since 
the five molluscs tested in Experiment III were 
all highly unpalatable, their surface pH when 
alive was measured, and that of all the other 
animals which were tested in Experiment III. 
Universal pH paper was used to measure the 
normal surface pH after the animal had been 
placed in a glass dish and allowed to drain for 
five minutes. Then the surface was wiped with 
a filter paper, stimulated gently with a glass rod, 
and the pH ions remeasured. 
Results 
The surface pH in all cases was approxi- 
mately 7.0. This result by no means rules out 
the possibility of acid secretion, but continuous 
acid secretion and secretion after gentle dis- 
turbance are unlikely. 
DISCUSSION 
Of 48 species tested, the 24 which were un- 
palatable to fish came from a wide range of 
animal groups. 
It is difficult to compare these results in detail 
with those of other workers. Stephenson and 
Grant (1957) worked on Plectroplites ambi- 
guus (Richardson), and marine molluscs occu- 
pied an equivalent position in the preference 
scale. However, marine Crustacea were less 
attractive for Plectroplites than were some 
TABLE 3 
Effect of Colour on Palat ability: 
Relation between Colour and Degree of 
Palatability 
COLOUR 
HIGHLY PALATABLE 
AND PALATABLE 
(number) 
UNPALATABLE 
(number) 
White ) 
Grey ) 
14 
2 
Cream ) 
Red ) 
Yellow ) 
8 
13 
Orange ) 
Brown ) 
Blue ) 
Green ) 
3 
5 
Purple ) 
Black 
0 
3 
marine molluscs, and the most attractive foods 
for Plectroplites (freshwater crustaceans and 
squid) were relatively less palatable in the 
present investigation. 
Steven (1959) compared the relative effec- 
tiveness of natural food substances in evoking 
exploratory feeding behaviour in Hepsitia stipes 
(Muller and Troschel) and in Bat hy stoma 
rimator (Jordan and Swain). Extracts of fresh 
plankton and of the muscle of the bivalve 
Area zebra were effective, while the echinoids 
Diadema antillarum and Lytechnis variegatus 
and a holothurian, Holothuria sp., also produced 
intense responses from less extreme dilutions. 
In the present work certain species of Holo- 
thuria were found to be unpalatable to fish. 
However, the presentation of a very small 
amount of food material, whether palatable or 
unpalatable (e.g., Holothuria') , caused explora- 
tory feeding reactions which lasted for one to 
two minutes. 
Three main factors could influence unpalata- 
bility: (1) ’Taste” due to diffusible chemical 
substances; (2) texture (though not unattrac- 
tive, the organism may be hard or tough, or 
may contain spicules) ; (3) colour, either in 
itself, or due to association with some other 
unpleasantness. 
In Experiment III, it was shown that un- 
palatable animals remained unpalatable when 
the texture was completely altered, and colour 
to some extent was altered. If colour was the 
sole cause of unpalatability, fish would ignore 
food of some particular colour without going 
near it or taking it into their mouths. Results 
indicate that this is not generally so ; and, 
while Table 3 indicates that differences in 
colour between palatable and unpalatable ani- 
mals are significant, there must be other factors 
contributing to unpalatability. 
Many sedentary, soft-bodied, "unprotected 5 ’ 
animals are unpalatable, and these include 
Porifera, Coelenterata, Nemertea, some Poly- 
chaeta, Opisthobranchiata, and Holothuroidea. 
Many of these animals have no obvious protec- 
tive devices: Holothuria scabra is just as 
unpalatable as H. leucospilota, but has no 
Cuvierian tubules; a few opisthobranchs eject 
purple ink, but many do not. Porifera and 
Nemertea have no obvious physical means of 
