Observations on the Heart Shell, Corculum cardissa (L.) , 
and Its Associated Zooxanthellae 
Siro Kawaguti 1 
introduction 
The heart shell, Corculum cardissa (L.), 
has a peculiar type of shell. It is greatly com- 
pressed antero-posteriorly, resulting in a dis- 
tinct heart shape, which is the source of the 
common name. Moreover, it has many zoo- 
xanthellae in its gill filaments, labial palps, 
mantles, and liver. The peculiar shape and 
habits of this mollusk may be explained by 
this association with zooxanthellae. 
This is the second case of association with 
zooxanthellae known in the lamellibranchs. 
It is interesting to compare it with the other. 
Tridacnidae, which is fully described by 
Yonge (Rept. Great Barrier Reef Expedition 
1928-1929. 1: 283-321, 1936). 
These observations and experiments were 
carried out at the laboratory of the Palao 
Tropical Biological Station and the reef fiat 
in its vicinity in 1936 and 1940. A prelimi- 
nary report was published in 1941 in Science 
of the South Sea ( Kagaku Nanyo ) 3: 179- 
180 [in Japanese]. 
HABITAT 
At Palao Corculum is found on reef flats 
under strong sunshine. It usually lies on the 
sand flat where dead shells and coral frag- 
ments have accumulated, and its anterior end 
faces the ground. It lies in a roughly hori- 
zontal position, excavating the bottom to the 
form of its shell, which is sometimes greatly 
produced anteriorly, as is shown in Figures 2 
and 3. There is usually no attachment to the 
bottom but exceptionally there are several 
short threads of byssus. 
It is very difficult to find the shell in its 
habitat, as the upper surface is frequently 
covered with filamentous algae or muddy 
depositions. In an active state the shell opens 
by 2-3 mm. at the ventral end and a vigorous 
current is produced from an exhalant siphon 
which is projected slightly at the middle of 
the shell. An inhalant siphon is situated 
ventrally and on the same level of the shell. 
At the ventral end of the shell the mantle 
is separated. Very thin, small papillae appear 
at the distal portion. 
THE SHELL 
One specimen of shell is seen in Figures 1 
and 2, from the top (or posterior in anatom- 
ical point of view ) and left sides, respectively. 
The boundary of the shell rests, usually, on 
a plane but it is frequently slightly convex 
or concave. The height is not the same from 
1 Department of Biology, Okayama University, 
Okayama, Japan. Manuscript received January 28, Fig. 1. Upper (posterior) surface of heart shell. 
1949. (Natural size.) 
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