5 341 



West Indies, and the change in the vohime of water caused by the tide does 

 not therefore take place. With higher seas the waves naturally wash in higher 

 degree over the reef, and the algæ, which grow behind it, are naturally exposed to 

 a rather strong motion. The bottom, which is commonly rather fast, consists of 

 dazzling white coral-sand often mixed with larger or smaller pieces of coral; also 

 stationary rocks or coral-reefs are to be found ; the Caulerpas occur on sand bottom 

 as well as on the rocks and corals. When the Caulerpas grow on the corals I 

 think the latter are always dead; on living corals Caulerpas or other algæ most 

 probably do not occur, though one would think that where the corals live luxuri- 

 antly the algæ would also grow well there. The corals occur especially on more 

 exposed localities, where we have some current and the renewal of the water is 

 thus constant ; even if some localities are perhaps too exposed for the Caulerpas 

 other algæ might be able to grow there, but they are also commonly wanting. 

 SvEDELius has also mentioned this fact (29, pag. 192 — 3); but at the same time he 

 points out, that the Caulerpas among the algæ are just the most suited to live 

 there and that they are also able to live betveen living corals. But as far I have 

 seen in the West Indies it is only on dead corals that the Caulerpas grow; how 

 far the Caulerpas may be able to fix themselves on living corals I consider as 

 rather doubtful, and just the fact that at places where the corals grow especially 

 luxuriant and quite occupy the bottom we do not find any Caulerpas or other 

 algæ speaks against it; it is most probably only in localities less favourable for 

 the corals, where dead corals also occur, that the Caulerpas are able to introduce 

 themselves. In this connection I may also mention that certain Corallinaceæ seem 

 to me to be pioneers in just as high degree as the Caulerpas with regard to 

 getting foothold among the corals. 



The water being steadily renewed is clean and quite clear and the light is 

 therefore strongly reflected when it further meets the whitish bottom. Yet I need 

 only point out that the water in the smaller basins can naturally be heated up 

 somewhat by the sun, but commonly the renewal of the water is so great that 

 this has only less importance. 



The algæ which occur here in these localities are thus exposed to some surf 

 and intense sunlight; the water is clear and fresh and the substratum sand bottom, 

 rocks and coral-reefs. 



Going now from these localities to the more sheltered, such as we find e. g. 

 in the bays and lagoons, we meet with quite difierent conditions of life. Here is, 

 firstly, complete shelter; the water can only seldom be in movement worth men- 

 tioning as the entrances to these places are commonly sheltered by off lying coral- 

 reefs or sand-banks or what is the most common, the vegetation of the mangroves 

 has gradually pushed from the shores into the shallow water and nearly closed the 

 entrance to the lagoon, so that there is only connection with the sea through a 

 narrow passage. The water in the lagoon is therefore only renewed to a small 

 extent; it is commonly very unclear, filled up with quite small mud particles and 



