Palatability of Marine Invertebrates — Russell 
459 
TABLE 4 
Tests of Palatability when Texture and Colour Are Masked by Gelatin 
GROUP 
TEST SPECIES 
PART OF BODY 
TESTED 
CONCENTRATION 
NECESSARY TO 
MAKE GELATIN PLUS 
PRAWN UNPALATABLE 
1 
Clymene 
whole animal 
50% 
(Slightly 
Cirratulus 
whole animal 
50% 
unpalatable) 
2 
Torquigener 
muscle 
30% 
(Unpalatable) 
Holothuria 
whole animal 
30% 
pardalis 
cuvierian tubule 
30% 
Black sponge 
whole animal 
30% 
mucus from surface 
30% 
Catostylus 
whole animal 
30% 
Alcyonium sp. 
whole animal 
30% 
3 
Nemertine 
whole animal 
20% 
(Highly 
Glossodoris 
whole animal 
20% 
unpalatable) 
Aplysia 
mantle and foot 
20% 
Notarchus 
mantle and foot 
20% 
Dolabella 
mantle and foot 
20% 
Umbraculum 
mantle and foot 
20% 
protection, although the texture of a sponge 
may be sufficient protection. 
The most unpalatable animal seemed to be 
the pleurobranch Umbraculum, which most fish 
did not touch. McNae (1962) notes that 
"when handled the animal ( Umbraculum ) 
gives off a characteristic scent reminiscent of 
that of aplysiids. This scent may have some 
defensive function.” The author has noted a 
characteristic odour in many opisthobranchs, 
notably Aplysia and Glossodoris. 
Thompson and Slinn (1959) noted an acid 
secretion which was apparently responsible for 
making Pleurobranchus distasteful to predators. 
They recorded that the predators "tasted” but 
discarded it immediately and, in the case of fish, 
often violently. Although Pleurobranchus is 
brightly coloured, this did not seem to warn the 
fish in any way. 
In the present experiments, animals with 
bright, supposedly warning, colouration re- 
mained unpalatable when their pattern was 
destroyed and their colour partly masked by 
gelatin and Metapenaeus. Some cryptically 
coloured, well camouflaged animals were also 
unpalatable, notably Dolabella, No tare bus, 
Aplysia, Holothuria scabra, and H. pardalis. 
These experiments and those of Thompson 
(I960*, b) do not support the idea which 
seems to have originated with Herdman 
(1890*, b), Garstang (1889, 1890), and 
Crossland (1911) that cryptically coloured, 
camouflaged animals were acceptable to fish as 
food, while brightly coloured ones were rejected 
because of their warning colouration. 
While the results point to the importance of 
chemical defense mechanisms, the presence of 
an acid secretion does not seem to be the cause 
of unpalatability in any of the animals whose 
external pH was measured. Of all the gastro- 
pods tested by Thompson (i 960 b), only five 
produced an acid secretion, while the remainder 
had similar glands in the epidermis which pro- 
duced a secretion which presumably also had 
a defensive function. In this connection, one 
must mention the "characteristic odour” of 
Umbraculum, Aplysia, and Dolabella. 
The complex phenomena of learning must 
play some part in the selection of food by fish, 
and it is at this level that colour may be im- 
portant, but learning cannot determine the 
causes of unpalatability. It is suggested that 
chemical defense mechanisms may be wide- 
spread among invertebrates and that a search 
