Ecological Observations on the Sea Cucumbers Holothuria atra 
and H. leucospilota at Rongelap Atoll, Marshall Islands 
Kelshaw Bonham and Edward E. Held 1 
In a distributional study of the black sea cu- 
cumbers, Holothuria atra (Jager) and H. leu- 
cospilota ( Brandt ) , at Rongelap Atoll ( Fig. 1 ) 
the beaches of most of the larger islets were 
surveyed during September 1959, March 1961, 
and September 1961. Estimates of abundance 
and size were made, and observations on tem- 
perature tolerance, predation, asexual reproduc- 
tion by fission, and some other ecological aspects 
were considered. 
These two species of sea cucumbers are fre- 
quently, although not always, found together in 
the intertidal area of the seaward sides of the 
islets at the edge of the reef-flat nearest shore. 
They occupy pools of water from a few centi- 
meters to about one-half meter deep at low tide 
(Figs. 2, 3), or are occasionally entirely exposed. 
H. atra usually covers itself with a coating of 
light coral sand held in place by tube feet, al- 
though occasionally it is naked and black. The 
integument is thick and firm. This species lacks 
the Cuvierian organ and thus may be handled 
without the annoyance of adhesive threads. H. 
leucospilota is always naked and almost black, 
has a softer integument with fine protuberances 
giving it a prickly appearance. The intricate, 
microscopic calcareous deposits in the integu- 
ment differ markedly in the two species: in H. 
atra (Fig. 4 A) the "tables” are tall and there 
are no "buttons,” while in H. leucospilota (Fig. 
4B ) the tables are short and squat, and there are 
many buttons. 
H. atra also occurs in deeper water in the 
lagoon, where it does not coat itself with sand, 
and to an undetermined extent on the vast areas 
of reef-flat between islets. At high tide the water 
at the seaward shores of the islets usually covers 
both species to a depth of I-IV 2 m, and they 
1 University of Washington, Laboratory of Radia- 
tion Biology, Fisheries Center, Seattle 5, Washington. 
Manuscript received December 20, 1961. 
are then frequently subjected to wave action 
from the surf whose initial force has been 
broken by the outer reef. A conspicuous feature 
of this habitat is the churning agitation and 
aeration of the water. H. atra appears to main- 
tain its position by inhabiting depressions and 
holding onto the relatively smooth reef-flat by 
means of its tube feet. H. leucospilota usually 
anchors the posterior portion of its body under- 
neath a stable rock or in a hole in the reef floor. 
In the absence of other cover it may find its way 
under a cluster of H. atra (Fig. 3). At appro- 
priate times it extends its highly mobile and 
extensible anterior end outward for feeding. 
Concentrations of these two species almost 
always occur where large slabs of beachrock are 
situated on the lower edge of the sandy beach 
slope bordering the inner edge of the reef-flat, 
and where, after the recession of the tide, the 
sea water may be observed to issue from the 
sand onto the reef-flat below the slabs of rock 
until the next tide. Below a sandy beach without 
slabs of rock no such prolonged runoff is evi- 
dent, and the sea cucumbers usually are few or 
lacking. It is in the pools fed by the relatively 
cool water from underneath the rock slabs that 
H. atra and H. leucospilota thrive. Concentra- 
tions of sea cucumbers were occasionally seen 
in the absence of slab-rock, and rarely slab-rock 
was found without sea cucumbers. They occupy 
an ecological niche almost devoid of other 
macroscopic organisms; occasionally small snails 
occur, but frequently the sea cucumbers are 
alone. There is no obvious food in the sand they 
ingest. 
CENSUSING OF SEA CUCUMBERS 
The surveying procedure consisted of observ- 
ing, photographing, and taking notes on the 
abundance of Holothuria while walking around 
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