Osmotic Relationships in Opheodesoma spectabilis — Freeman 
61 
Opheodesoma may be found in large numbers 
quietly feeding in the shallow waters over coral 
reefs. During heavy rains, when large amounts 
of fresh water run off from land to reefs located 
close to shore, the animal is unaffected by in- 
fluxes of fresh water that decimate populations 
of other reef-dwelling invertebrates. 
The primary objective of this work was to 
establish by what means these holothurians were 
able to function despite marked dilution of their 
sea water environment from time to time. 
Evaluations of changes due to water loss or 
uptake were made by comparing weights before 
and after the experimental immersion period. 
Osmotically active particles present in environ- 
mental water, coelomic fluids, and tissue fluids 
were determined by means of a Fiske cryo- 
scopic osmometer. Sodium concentrations were 
measured by means of flame photometry. Titra- 
tions for chlorosity of sea water and body fluids 
as a measure of salt concentration followed 
Knudsen’s procedure (U. S. Hydrographic Of- 
fice, 1955). 
MATERIALS AND METHODS 
Animals of an average length of 45 cm were 
collected from the Coconut Island reef, where 
at low tide they can be found as little as 10 cm 
below the surface of the water. The animals were 
kept in tanks supplied with a constant flow of 
aerated sea water. They were starved for 48 hr 
prior to experimentation. This provision was 
thought necessary because Opheodesoma is a 
detritus-algae feeder and the animals, upon first 
being taken from the water, differ from each 
other in the amount of sand and algae present in 
their intestines. After 48 hr the intestines were 
cleared in all animals. 
Preliminary experiments indicated that the 
animals could survive in as low a dilution as 
50% sea water for at least 24 hr. However, upon 
being returned to 100% sea water, death usually 
occurred within a matter of a few days. When 
placed in dilutions of 80% and 70% sea water 
for 24 hr, then removed to 100% sea water, 
animals survived up to three weeks. For further 
experimentation 80% sea water was chosen as 
representing the amount of dilution the animals 
might naturally encounter without being irre- 
parably harmed. 
Animals chosen for study were placed one 
each to a 5 -liter glass tank containing a known 
ratio of sea water to distilled water. At least 
three animals were used in all dilution experi- 
ments, along with three control animals in 
100% sea water. The tanks were aerated and 
topped with aluminum foil to prevent evapora- 
tion. Immersions were for 24-hr periods. The 
majority of experiments were carried out at 
room temperatures (23°-27° C). 
RESULTS 
Initial experiments were devoted to determin- 
ing whether Opheodesoma could be categorized 
as either an osmoconformer or an osmoregulator. 
Following a 48-hr starvation period, intact 
animals with coelomic fluid formed while in 
100% sea water were weighed and placed in 
80% sea water for 24 hr (Fig. 1). 
It is evident from the data that the whole 
animal does not act as an osmometer. On the 
contrary, when in environmental water more 
dilute than body fluids, the animals lose rather 
than gain weight. Apparently some form of 
regulation does occur which runs counter to the 
simple osmotic relationship that would be ex- 
pected in the experiment described. That is, if 
the integument of Opheodesoma is permeable to 
water as described for another holothurian by 
f 
GROUP 
I 
Control k 
GROUP / 
2 
Effect 
of I 
Dilution x 
On removal from 
100% sea water 
After 24 hours in 
100% sea water 
On removal from 
100% sea water 
After 24 hours in 
80% sea water 
0 
100 
200 
weight in grams 
Fig. 1. Effect on animal weight after immersion in 
dilute sea water for 24 hr. 
