94 DAHLAK 



between the cold currents and the warm, temporarily stag- 

 nant waters. In fact, every time we explored these improvised 

 lagoons, where an hour earlier innumerable sea birds and some- 

 times lines of dromedaries had been walking, we encountered 

 black-fin sharks of three feet or more in as little as three span 

 of water. There were also guitar-fish four or five feet long, and 

 many rays and sting-rays weighing some dozens of pounds. 



The reason for this invasion into new waters is evident. 

 The immense, shallow and muddy beaches offer the rays, the 

 sting-rays and the guitar-fish an incalculable quantity of 

 molluscs which they can nose up methodically from the 

 bottom. The black-fins, on the other hand, hunt for mullet 

 and other small lagoon fish. It must be noted however that 

 the regular and persistent entry of the black-fin shark and 

 sometimes of the dusky shark into the low-tide waters has 

 only been half explained by us. 



The madreporic formations in the channels are few and 

 often practically non-existent. Besides, it is well known that 

 coral needs the open sea for its existence, and a constantly 

 changing water carrying with it fresh and plentiful forms of 

 nutrition. It is true that the water of the channels is always in 

 movement and extremely rich in plankton, but coral needs 

 water at a constant temperature and not below 77°-86° F — a 

 condition which does not obtain in the channels. The proof 

 of this observation lies in a study of the position of the few 

 existent madreporic formations. All, without exception, lie in 

 restricted tranquil zones, protected from the direct flow of the 

 current, in water that has a chance of becoming tolerably 

 warm. In addition, madreporic formations need pure, clear 

 water. The water of the channels was unsuitable in this 

 respect since it carried sand washed away from the lagoons, 

 beaches and sea-bed. 



We studied this vast zone for twenty days. A total of 



