Sea Cucumbers at Rongelap Atoll — Bonham and Held 
313 
reproduce by transverse fission, and that Holo- 
thuria difficilis Semper of the tropical Pacific 
commonly does so. 
In September 1959 in a warm pool of about 
37 C on the southwest shore of Burok I. two 
short specimens of H. atra were encountered 
lying end to end. The integument of the two 
adjacent terminal end portions was devoid of 
sand because of the absence of tube feet. The 
specimens were saved, and later examination 
revealed a calcareous oral ring in only one, in- 
dicating that the two individuals resulted from 
transverse fission. Another specimen in an ad- 
jacent pool was photographed in an advanced 
stage of constriction; but without disturbance 
by even so slight a stimulus as a shadow it vol- 
untarily relaxed the constriction and resumed 
normal shape within a period of about 5 min. 
During the September 1961 census, enough 
fissioning ( Fig. 7 ) and fissioned specimens were 
Fig. 7. H. atra, about 23 cm long, undergoing 
transverse fission in a pool 10-cm deep. Mellu I., sea- 
ward side near west end, Sept. 18, 1961, 1520 hr. 
encountered to verify that fission, or asexual re- 
production, does commonly occur in Holothuria 
atra . 
The importance of fissioning as a means of 
reproduction is emphasized in Figure 6 by the 
relative thickness of the small specimens and the 
consequent reduced slope (0.9) of their logarith- 
mic relationship of volume to length as com- 
pared to the usual cubic relationship (slope of 
3 ) for the larger specimens. 
If it may be assumed that sexual reproduction 
would be seasonal in nature, while fissioning 
would occur at all seasons, then the lack of ap- 
parent age classes or frequency modes in the 
volume-frequency histogram of Figure 5 is also 
consistent with the concept of reproduction by 
fissioning. 
SUMMARY 
1. The preferred habitat for H. atra and H. 
leucospilota on the seaward reef-flat is described 
and is linked with the presence of slabs or 
shelves of rock at the junction of the sandy 
beach with the reef-flat. On hot days runoff 
water from such areas is of relatively low tem- 
perature (because of the insulating effect of the 
slabs of rock). 
2. Estimates of numbers of these two sea 
cucumbers are based upon notes and photo- 
graphs made while traversing the peripheries of 
the larger islets and some reef and lagoon areas, 
resulting in an estimated total of about 5 X 10° 
H. atra and 2 X 10 5 H. leucospilota. 
3. The passage of sand through the gut of 
H. atra is at the rate of 1 g (dry weight) in 
from 5 to 10 min, resulting in an estimate for 
the atoll of about 2 X 10 8 kg of sand ingested 
and egested yearly. The primary ecological role 
of the sea cucumbers in this feeding action is 
to move the sand from one place to another. 
4. Length of H. atra observed ranged from 2 
to 60 cm. Measurements of 407 specimens in 
one pool showed that for specimens up to 15 
cm in length, volume increased slowly with 
length, while for those from 15 to 25 cm, vol- 
ume increased almost as the cube of length. This 
is construed as evidence of reproduction by 
fissioning. 
5. H. atra appeared healthy and feeding in 
small pools with temperatures up to 38.9 C. 
