Ecological Observations on the Sea Cucumbers Holothuria atra and H. leucospilota at Rongelap Atoll, Marshall Islands KELSHAW BONHAM and EDWARD E. HELD! IN a distributional study of the black sea cu­ are then frequently subjected to wave action cumbers, H olothuria atra (Jager ) and H. leu­ from the surf whose initial force has been cospilota ( Brandt) , at Rongelap Atoll ( Fig. 1) broken by the outer reef. A conspicuous feature the beaches of most of the larger islets were of this habitat is the churning agitation and surveyed during September 1959, March 1961, aeration of the water. H. atra appears to main­ and September 1961. Estimates of abundance tain its position by inhabiting depressions and and size were made, and observations on tem­ holding onto the relatively smooth reef-flat by perature tolerance, predation, asexual reproduc­ means of its tube feet. H. leucospilota usually tion by fission, and some other ecological aspects anchors the posterior portion of its body under­ were considered. neath a stable rock or in a hole in the reef floor. These two species of sea cucumbers are fre­ In the absence of other cover it may find its way quently, although not always, found together in under a cluster of H. atra (Fig. 3). At appro­ the intertidal area of the seaward sides of the priate times it extends its highly mobile and islets at the edge of the reef-flat nearest shore. extensible anterior end outward for feeding. They occupy pools of water from a few centi­ Concentrations of these two species almost meters to about one shalf meter deep at low tide always occur where large slabs of beachrock are (Figs. 2, 3), or are occasionally entirely exposed. situated on the lower edge of the sandy beach H . atra usually covers itself with a coating of slope bordering the inner edge of the reef-flat, light coral sand held in place by tube feet, al­ and where, after the recession of the tide, the though occasionally it is naked and black. The sea water may be observed to issue from the integument is thick and firm. This species lacks sand onto the reef-flat below the slabs of rock the Cuvierian organ and thus may be handled until the next tide. Below a sandy beach without without the annoyance of adhesive threads. H. slabs of rock no such prolonged runoff is evi­ leucospilota is always naked and almost black, dent, and the sea cucumbers usually are few or has a softer integument with fine protuberances lacking. It is in the pools fed by the relatively giving it a prickly appearance. The intricate, cool water from underneath the rock slabs that microscopic calcareous deposits in the integu­ H . atra and H. leucospilota thrive. Concentra­ ment differ markedly in the two species: in H . tions of sea cucumbers were occasionally seen atra (Fig. 4A) the "tables" are tall and there in the absence of slab-rock, and rarely slab-rock are no "buttons," while in H . leucospilota (Fig. was found without sea cucumbers. They occupy 4B) the tables are short and squat, and there are an ecological niche almost devoid of other many buttons. macroscopic organisms; occasionally small snails H. atra also occurs in deeper water in the occur, but frequently the sea cucumbers are lagoon, where it does not coat itself with sand, alone. There is no obvious food in the sand they and to an undetermined extent on the vast areas ingest. of reef-flat between islets. At high tide the water at the seaward shores of the islets usually covers CENSUSING OF SEA CUCUMBERS both species to a depth of 1-1 Y2 m, and they The surveying procedure consisted of observ­ 1 University of Washington, Laboratory of Radia­ tion Biology, Fisheries Center, Seattle 5, Washington. ing, photographing, and taking notes on the Manu script received December 20, 1961. abundance of Holotburia while walking around 305 306 PACIFIC SCIENCE, Vol. XVII, July 1963 " ' . .. ,." ~ I ' . ~ of() rhi.. I J fl r.....,." ..." RONGELAP ATOLL :\~.:: :: ::: .., +i' w..~ l l r r .f . ... ~ r .o(rr·..·..·, .:t··,·,,,·,. ~ FIG. 1. Map of Rongelap Atoll showing islet names and numbers. the periphery of 16 islets completely, and 5 The places of concealrnerrtrunder overhanging others partially, thus covering 67 % of the sea­ ledges were inspected , but rocks were seldom ward shores of all islets 'in the atoll. The islets lifted to check for specimens . are numbered clockwise on the map of Rongelap Table 1 shows the distribution of H. atra and Atoll shown in Figure 1 starting at the north­ H. leucospilota at the localities surveyed. Section west extremity. Only about 2% of the inter-islet numbers representing 100-m units of seaward reef area and shallower lagoon bottom regions shoreline appear under the appropriate fre­ supposedly suitable for H. atra was censused. quency group. The central frequency groups in­ Distances were measured by chain on the crease by a factor of 10 while smaller ranges of seaward shores of four islets, Aniiru, Kabelle, numbers delimit the extremes. Along with the Namoen, and Rongelap, while at other islets range in the column headings are the means in distances were either determined by pacing, or, parentheses-geometric except for the first, in most cases, estimated. On the east rim of the where the zero limit precludes use of the geo­ atoll high-altitude vertical, and elsewhere low­ metric mean. Geometric and arithmetic means altitude oblique, aerial photos were employed to were tested for agreement with totals of the aid in orientation. Photographs taken on the last two columns showing sums by localities reef-flat or undisturbed sea cucumbers were taken directly from a previous tabulation, which sometimes used in the estimations of abundance. also supplied the data for the body of Table L Sea Cucumbers at Rongelap A toll-BON H AM and HELD 307 TABLE I D ISTRIB UTION OF H . atra AND H . leucospilota ON THE SEAWARD SHORES OF ISLETS BY IO O·M SECTION N UMB ERS (The body of the table shows section num bers in which the abunda nce of sea cucumbers fell within the range indic ated in the column heading. Sections for each islet were num bered clockwise starting at the coun rerclock­ wise extremity (left-hand end when facing seaward) of the seaward shore.) et ate l OO-m s e c t i o n numb er Rema rks To tal i nd i vidu- No Na me .... .... I I I I I I cc l ~ t a a I s obse rved '"~ '" Numbers of !!. a tra Number s o f li. leuc10spi o r e s t ima t ed .c ., 0-2 3- 1 0 11-10 0 1 0 1 - 100 1 - 0 - 2 3 - 10 11-100 1 OJ.- 1 0 0 1- ., u (1 ) • Co .. Co ( 1 )' 5 .62 ) (3 3 .2) 100 0 30 0 0 (5 . 62 ) ( 3 3 .2) 1000 3000 H. un ll.w.= ~ ~ . (3 18) ( 1730) (318 (1 730 ) '" '" li..ll1.12.til 2 Nae o 15. '" 19 1 - 5 1-5 0 0 22 3 W. Yug ui 15 0 0 4 pigan i- va rcvec 15 19 1 - 4 5 6 1-4 5 6 E . shore , many 2 1 0 2 000 5 Aerik. 2 2 1 - 3 1-2 3 12 atra o n W. sho re 12 4 0 6 E . Yuqu i 22 1 -4 2-4 1 8 ~ on E . 16 on W. 24 6 4 s ho re 8 Lomui l al 14 1 - 5 1 - 5 0 0 ce l e n 14 1- 4 1 -4 lJ on 13 1 3 Inner 1 2 (1 0 . 0 0 0 atra in lagoon . 1. 13 & 1. 1 5 ) 100 0 0 0 13 1 3 Outer 12 17 1 2 2 6 0 24 0 18 Re e f a rea 13 (700 me t e r s north o f Ani iru I. had 8 3 , 0 0 0 a tra,26 0 0 l euco.and 4 300 S t ichopus 83000 2600 e h loranatu s ) 18 Ani iru 1 2 12 1 - 2 1 -2 200 60 1 1 9 Kabel Le 1 0 1 1 2 2 7 3 .8.9 0 .1.4 . (1 0) 2-1 0 0 . 1 20 0 atra & 100 30 00 1 50 I 13 6 l euc o . in N.E . channel I 20 Namoe n 1 2 2 1 2 1 4 ~ . S . s i d e 2300 16 29 Mellu 1 7 18 2 -5 1. 15 6 - 9 . 13 10-12 . 1-2 3 3 atra , 3 l e uc o .,E 35 00 13 14, 16 - 19 . 20 . c hanne l 18 . 21 22 .2 3 31 Gog an 18 1-8 3-6 1-2 . 8 ili,2.. E . c h an ne l . 3 30 32 7 -8 s lab r o c k a t 1 -3 & 5. 34 Kies h l - 18 1-6 1-" 600 0 echi 39 Enia e t ok 8 9 2 -3. 1 . 1 2 , 4 , 9 11 , 14 1 0 1 - 9 , 10 ,14 0 5 ~ & 1 leuco . 3600 2 4 5 - 8 13 .15 - 11-13 N. s i d e 2 3 15 - 2 3 5 3 Rong elap 5 8 - 1-2 . 3- 5 . 6 .7 .15 . 28 -30 , 1 -24 . 25 . 4 1 4 6-48. some a t r a , l ag oon 34 00 1)90 lL 8-14 . 36. 4 2 .