locating, even during night dives. Its measurements were: length 120 cm, 

 width 80 cm, height 60 cm. Its main axis was N-S. The depth was 21 meters. 

 This little block was formed by organic construction built principally by 

 corals, bivalves, serpulid worms, and calcareous algae. Its relief offered to 

 the motile fauna two kinds of shelter: large sized shelters, i.e., a large 

 transferse crevice (E-W) with accessory chambers and a transverse tunnel in the 

 vicinity of the sediment; small sized shelters: very numerous, hollows in the 

 rock or interstices in the epifauna buildings. Around the small patch reef, 

 the bottom was coarse sand strewn with organic pebbles carrying sea-fans, 

 sponges, large algae like Sargassum or Udotea . On this flat the coral blocks 

 were regularly scattered every 10 meters. 



Partial Faunal Inventory 



In spite of the smallness of this coral block, the diversity of the sessile and 

 motile organisms is great; they form a "microcosm" largely independent of the 

 surrounding benthic biotopes. Faunal identifications were made from in situ 

 drawings, photos, and by means of some small samples. In this limited inventory, 

 symbolic letters mark the period of activity for most species and, for motile 

 fauna, the range of action as these elements were noted during this field work 

 (Table 1). 



Table 1. Symbols keynoting period of activity and 

 range of motile fauna 



Diurnal activity D 



Crepuscular or nocturnal activity N 



Reduced diurnal activity, maximum activity during the night.. dN 

 Activity independent from the nycthemeral rhythm, constant 



or depending on other factors, such as currents C 



Motile fauna: 



Movements restricted to the microcosm I 



Movements overstepping the microcosm area E 



The flora of the block consisted of a covering of small algae, in Dyctyotales , 

 and in four big Udotea at the top of the block. 



Very close to the coral islet, within a radius of one meter, a diurnal sedentary 

 fauna could be seen inhabiting the sediment. There was a mantis shrimp sp.2, 

 in its burrow against the east side of the block; a family of six gobies 

 ( Gnatholepis thompsoni ) inhabiting the pebbles emerging from the sand in the 

 sourthern base of the islet; two jawfishes ( Opisthognathus aurifrons ) NW of the 

 block, one of them incubating eggs in its mouth; and a sand tilefish ( Malacanthus 

 plumieri ) , its shelter beneath a pebble in the north end. 



Life History of the Microcosm 



NOON: The first thing perceptible to the eyes is the dance of the damsel-fishes 

 (Pomacentrus) and of the Thalassoma above the block. The first have a very 

 jumping, active swimming. They catch suspended particles or browse the surface 

 of the rock. The small Thalassoma swim in clusters around the islet, swallowing 



VI-286 



