The ecology, defensive behavior and toxicity of three species of reef flat holothtrrians (Actinvpyga mauritiana, Holothuria atra and Holothuria difficilis) were studied at Eniwetok Atoll, Marshall Islands. The average diurnal population density of H. di//icilis ranged from t.4 to 32 holothurians/ 900 cm*; resting respiratory rates (0.05ml O~/g wet wt/h) were comparable during day and night; nourishment in//. di]/icilis may be primarily from bacteria and foraminifera in which about 2% of the dry weight of sediment consumed is u~lized, and the species probably passes at least 3 g dry wt of sediment/ m~/day (> I kg/m2/year). A fundamental difference in energy flow is suggested: considerably more energy is pa~sed from benthic algae to grazing and browsing fishes to predatory fishes on coral reefs whereas, in extra-tropical latitudes, more energy is shunted from benthic algae to invertebrates to prodatom. The effects of holothurin leading to death in fishes are irreversible. Holothuria di//~cilis is best protecte~d from predation. Its body wall is toxic and it can accurately eject Cuvierian tubules, which are also toxic. The discharge of tubules was regulated by a circadian rhythm in May. Studies on holothurians and sponges suggest that many exposed coral reef invertebrates have evolved effective defensive mechanisms in association with high intensity predation.
~M~H~on
The present study originated from observations made during July t965 at Eniwetok, Marshall Islands (B~x~ys, 1967). At that time it was discovered that a small, brown, reef flat holothurian (Holothuria di]ficilis) discharged Cuvierian tubules when handled, and that fishes avoided the holothnrian. Later, an effo~ was made to determine to what extent certain, exposed, reef fiat sea cucumbers are adapf~l in protecting themselves from the numerous potential predators, particularly fishes, that coexist in the same areas. The small brown sea cucumber, Holothuria difficil~s (S~I~), was studied in detail, and information on toxicity and certain other aspects of biology was obtained for the black sea cucumber, Holothuria atra JXG~m, and the large brown holothurian, Actinopyga maur/t/ana (Quoy and G^rMAa])). Studies were carried out at the Eniwetok Marine Biological Laboratory (EMIL), Eniwetok Atoll, Marshall Islands, from April 25 to May 18, i967.
Materials and methods
Holothurians
were studied in the fie]d at the reef flats of Eniwetok, Parry, Rigfli, Igurin and Bogon Islands, representing north, south, east and west sections of Eniwetok Atoll, respectively. Observations were made by walMng, wading, and swimming under-* Supported by A.E.C. Contract AT (29-2)-226 with the Universit 7 of Hawaii.