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PACIFIC SCIENCE, VoL XX, January 1966 
nearly the same, the difference can be attributed 
to the end products of digestion. 
DISCUSSION 
The easy tolerance of Opheodesoma to dilu- 
tion of its normal sea water environment (up to 
70% with no apparent harm) is puzzling. The 
animal has no particular structures devoted to 
osmoregulation; yet, the impermeability of the 
skin and the lack of rhythmic water circulation 
remove it from the category of osmoconformitv. 
There appears to be no direct adaptive signifi- 
cance in tolerance extending Opheodesoma' s 
environmental range. The animal Is usually con- 
fined to quiet reef living, and while the ability 
to resist dilution may have some survival value 
In times of heavy rainfall for animals in shallow 
water, they have been found at depths up to 40 
ft, presumably never exposed to fresh water. 
Opheodesoma is normally a continuous in- 
gester. This may call for a compensatory reduc- 
tion in the soluble salt content of the body fluids 
when the organic content is high. Then the in- 
flux of digestive end-products into the large 
coelomic compartment raise its osmotic pressure 
to nearly that of sea water. Lange ( 1963 ) , in 
his study on Mytelus , postulated a similar role 
for organic solutes acting as an osmotic buffer. 
Krogh ( 1939) cited Fredericks finding that the 
tissues of certain marine invertebrates, while in 
osmotic equilibrium with the surrounding water, 
might show considerable deficit in soluble salts. 
This deficit is made up by organic molecules of 
low molecular weight. 
Despite irregular water exchange with the en- 
vironment and variable organic molecule levels, 
the volume and salt level of the coelomic fluid 
of Opheodesoma in its natural environment ap- 
pear to have a certain stability. The values of the 
fluid samples taken through the body wall are 
probably more representative of actual concen- 
trations due to the relatively undisturbing nature 
of the method. The inference is made here that 
the concentration differences between drained 
and in situ coelomic fluids must be accounted 
for in the tissues (Fig. 3). Consequently, the 
values reported for tissue fluids are lower than 
they actually would be in vivo, as measurement 
is possible only on drained animals. These 
measurements really represent, then, a residual 
concentration after draining has removed some 
of the soluble substances which normally help 
keep osmotic equilibrium between tissue and 
coelomic fluids. 
It is suggested that tolerance to fresh water by 
the tissues is incidental and derives from ( 1 ) the 
slow "turnover” of the comparatively large 
volume of coelomic fluid with environmental 
water, and ( 2 ) the habit of steady ingestion of 
variable quantities of organic material by the 
animal, leading to oscillating levels of digestive 
end-products in the coelomic fluid. Cells within 
the small volume of tissue fluid exchanging with 
the coelomic compartment have time to equili- 
brate with a range of concentrations, and can 
tolerate them out of adaptive necessity. 
Submitting Opheodesoma to dilutions of sea 
water of the order they might naturally en- 
counter in terms of fresh water runoff creates 
no critical problem, particularly when ingestion 
is slowed or stopped. The resulting reduction in 
the level of organic molecules in coelomic fluid 
would lower its osmotic activity. New osmotic 
equilibrium would be established between tissue 
and coelomic fluids.' Then, as gradual exchange 
between coelomic and environmental water oc- 
curs via the mouth-anus route, reduction of co- 
elomic volume follows as a necessary condition 
for complete osmotic equilibrulm. 
ACKNOWLEDGMENTS 
This Investigation was performed while the 
author was on sabbatical leave from San Jose 
State College. He wishes to express his gratitude 
for the assistance and advice given by the Hawaii 
Marine Laboratory Staff. 
Use of the Fiske osmometer was made avail- 
able through the kindness of Dr. Ira Lichton and 
Miss Hester Kobayashi of the Department of 
Nutrition, University of Hawaii. 
REFERENCES 
Edmondson, C H. 1933. Reef and Shore Fauna 
of Hawaii. Bernice P. Bishop Museum Spec. 
Publ. 22. 295 pp. 
