Observations on Osmotic Relationships in the Holothurian 
Opheodesoma spectahilis 1 
Paul Joel Freeman 2 
ABSTRACT: The means whereby Opheodesoma spectahilis, a holothurian confined 
to the quiet waters of Hawaii, can tolerate a dilution of 80% sea water were in- 
vestigated. The animal is notable for the ratio of coelomic to tissue fluid, about 4:1. 
Weight changes, osmometry, titrations, and sodium analyses on fluids before and 
after immersion of the animal in diluted sea water were some of the methods 
■employed. 
The animal was found to reduce its content of coelomic fluid when placed in 
dilute sea water for 24 hr. There is no rhythmic circulation or pumping of fluids. 
Water exchange and regulation of volume are by way of mouth and anus, as the 
body wall proved to be impermeable to water. Soluble salts of body fluids in both 
starved and feeding animals are below the concentrations found in sea water; but 
in feeding specimens, the coelomic fluid demonstrated osmotic activity approaching 
that of sea water. 
It is suggested that tolerance to fresh water by the tissues is incidental, and 
derives from ( 1 ) the slow replacement of the comparatively large volume of coelo- 
mic fluid with environmental water, and ( 2 ) the habit of steady ingestion of variable 
quantities of organic material, leading to changing levels of digestive end-products 
in the coelomic fluid. Cells within the small volume of tissue fluid exchanging with 
the coelomic compartment can tolerate and have time to equilibrate with a wide 
range of concentrations, out of adaptive necessity. Volume regulation occurs in the 
gradual exchange between coelomic and environmental water to further advance 
osmotic equilibrium. 
The sea cucumber Opheodesoma spectahilis 
is a holothurian whose habitat appears to be 
confined to the relatively placid waters of Kane- 
ohe Bay and Pearl Harbor of Oahu in the Ha- 
waiian Islands (Edmondson, 1933; Fisher, 1907). 
Although it is classed with the sea cucumbers, 
Opheodesoma differs from the more common 
types by having a thin, mucus-covered, smooth- 
muscle integument. This integument is usually 
stretched around an extraordinary amount of 
fluid contained in the coelomic cavity. This fluid, 
while it can be present in varying amounts from 
animal to animal and in the same animal from 
time to time, is most consistently found in a 
1 Contribution No. 237, Hawaii Institute of Marine 
Biology, University of Hawaii. Manuscript received 
August 3 b 1964. 
2 Department of Biology, San Jose State College, 
San Jose, California, and Hawaii Marine Laboratory, 
University of Hawaii, Honolulu. 
ratio of about four parts to one part of tissue 
fluid. This comparatively large amount of mo- 
bile fluid is one of the salient features of the 
animal and apparently plays a significant role in 
its function. 
In some other details of its physiology and 
anatomy, the animal is comparatively simple. It 
possesses a whorl of extensible-retractable ten- 
tacles representative of tube feet found in other 
echinoderms. These tentacles, usually 1 5 in num- 
ber, surround the mouth and help the animal to 
ingest its food. The unconvoluted intestine, lack- 
ing the associated respiratory tree found in most 
other holothurians, is usually packed with a 
sand-algae mixture. The organic portion is di- 
gested and the end-products pass through the 
intestinal wall into the coelomic fluid. This fluid 
appears to act as the sole medium whereby nutri- 
ents are diffused to the peripherally located cell 
population. 
60 
