planktonic microorganisms and following the largest individual which sometimes 

 breaks away from the group and goes foraging on the neighboring sand. On the 

 surface of the block, blennies grub among the algal covering, between the 

 retracted corals. On the north side a black mantis-shrimp goes quickly from 

 one nook to another, chattering its claws; it seems to be busy with nest build- 

 ing. Above it, brittle-stars can be seen, rather inactive in the fire-coral 

 cluster. On the other hand, Serpulids Spirobranchus spread their double color- 

 ful branchial crowns. The crab Stenorhynchus sits up under a small ledge near 

 the second mantis-shrimp which digs its burrow. A sea anemone with dichotomic 

 tentacles (sp. 2) and others with nodulous tentacles (sp. 1) are well spread 

 out; small specimens of Periclimenes are visible among the tentacles of the 

 latter. Sea-fans are also opened out. The squirrelfish ( Holocentrus ) often 

 stands under the larger of them ( Eunicea ) or swims around the block. In the 

 first days of observation, it often swam to the main crevice but owing to the 

 rebreathers, these manifestations of fear turned quickly to indifference. The 

 crevice is permanently occupied during daytime by the moray eel, which lives 

 harmoniously with the squirrelfish. Near the block, the young tilefish is 

 clearing its burrow .under a pebble, carrying small stones with its mouth; it 

 sometimes stops its work to swim on the west side in search of food. At that 

 time its swimming is very characteristic: it moves horizontally at 40 cm from 

 the bottom and stops every 30 cm for a few seconds, staying head down with a 45° 

 angle, and then from time to time it dives down blowing water and digs the sand. 

 It didn't seem to go father than 2-5 meters from its shelter. The two trigger- 

 fishes ( Balistes ) are absent. 



NIGHT: At first sight, the microcosm seems to be asleep, but this is a mis- 

 leading impression caused by the absence of the Pomacentrus and Thalassoma 

 ballet. In fact, activity is intense. All the corals are spread out, particu- 

 larly the Eusmilia which present huge polyps catching plankton attracted by the 

 light beam. Ophiurids erect their arms from many nooks and Terebellidae worms 

 spread their very long tentacles over the rock. Small Serpulids Hydro ides with 

 intricated tubes show the red clump of their branchiam crowns. Small sea 

 anemones (sp. 3) are opened out on algae Udotea and on a big sponge. The 

 ascidians and the sponges seem to be very turgescent. Crabs run on the base of 

 the fire-coral. This place is the center of intense activity, small Periclimenes 

 and Polychaetes climb on its branches, brittle-stars use this support to erect 

 their netting arms. Serpulids Sp ir obr anchus and Sabellids are opened out as 

 during daytime. Three beautiful long-antennaed shrimp Stenopus stand before the 

 opening of the main crevice. The red tentacles of a widely open Lima bivalve 

 are seen in a hollow. The arrow crab ( Stenorhynchus ) is absent, but several are 

 seen on the sand-flat. The main crevice is nearly entirely filled by two trig- 

 gerfishes sleeping, their tail turned towards the outside. The moray and the 

 squirrelfish are absent. Some sleeping damselfishes can be seen in the small 

 nooks; the Thalassoma , blennies and gobies, better sheltered, are not visible. 



Dawn and dusk are the most exciting periods for studying the behavior of the 

 microcosm fauna. A changeover from the diurnal fauna to the crepuscular- 

 nocturnal one, and conversely, happens during these periods in a rather short 

 space of time, with a very precise, always identical scenario. 



DUSK: 17:30. The whole diurnal fauna is still awake. The damselfishes and 

 the Thalassoma are active above the block. The moray appears at the entrance 

 of the crevice and shows much less timidity than during daytime. The 



VI-287 



