THE SPONGES OF THE WEST-CENTRAL PACIFIC 273 



always arrive in the form of physico-chemical items, but these may well be 

 put in a special category. 



Mangrove thickets offer one of the best examples of such a biological 

 relationship. These thickets seem to occur in Micronesia exclusively where 

 the substratum is entirely or chiefly volcanic rock, avoiding predominately 

 calcareous regions. In the Bermudas, mangroves thrive where only calcareous 

 rock occurs, and much the same situation exists through the West Indian 

 regions. It may be that calcareous shore is quickly eroded, producing a depth 

 too great for mangroves. 



Near and within mangrove thickets, the water is commonly tinted brown, 

 due to some substances derived from the plants. In this discolored, often 

 dirty water, sponges thrive, as, for example, the genera Spongia, Biemna, 

 Adocia (turquoisia), and Anthosigmella. Many species which grow in the 

 mangrove vicinity are absent from other regions. 



There are scattered regions, often several acres in size, where there are 

 dense thickets of staghorn coral (Acropora) but little or no coral of any other 

 genus. I was not able to correlate the occurrence of these areas with ecological 

 factors, but such doubtless exist. It was very noteworthy, however, that 

 exceedingly few, if any sponges at all could be found in and about these 

 staghorn jungles. 



There are regions where it is strikingly obvious that many different 

 species of Anthozoan coral occur intermingled, especially the kind of coral 

 that forms horizontal or almost horizontal shelves or "table top" shapes. 

 Such regions were usually also teeming with sponge life. Clearly, that which 

 favors diversity of coral favors Porifera. Yet, one noteworthy exception 

 was found. 



The value of land drainage has been dwelt upon. Was this akin to fertil- 

 ization by manure? In the north portion of Ailing-lap-lap lagoon occurs a 

 small islet of about 4 or 5 acres, called Matien. Many hundreds of sea fowl, 

 chiefly terns, nest on this islet and roost there the year around even when not 

 nesting. Thus, there is a rich drainage of nitrogenous material into the 

 waters adjacent to Matien. I anticipated finding a correspondingly rich sponge 

 fauna but was disappointed. Further study, however, disclosed that whereas 

 only a moderate number of sponges occurred, say one per each 25 square 

 meters, practically every exposed centimeter upon which a sponge could 

 conceivably grow was occupied by a sponge. In other words, the coral was so 

 exceedingly vigorous, that the usual big patches of dead coral were wanting. 

 Sponges cannot attach to and grow on living vigorous Anthozoan coral. I 

 believe, if suitable substrates were provided about Matien, that sponges 

 would thrive there. This situation may be one of the few in which sponges 

 are crowded out by the superior competition of other sessile invertebrates. 



Throughout all the Micronesian collecting, the nearly invariable substrate 

 for sponge attachment was dead coral. A very few, such as Anthosigmella 



