302 L. HILLIS- COLIN VAUX 



flushed, in its shallows at least, with flowing reef water. Both craters 

 were floored with very fine sediments, probably including clay-sized 

 particles. I saw no sediments of comparable fineness elsewhere in the 

 atoll. Both sediment bodies were contaminated with radionuclides. 

 These craters are particularly interesting as sites for colonizers, and 

 sites, moreover, with sediments of a size fraction not widespread in the 

 neighbourhood. Unfortunately it is reported that CACTUS crater, the 

 more interesting one, has been filled in as part of the recent clean-up 

 operation on the atoll. 



There were no Halimedae in LACROSSE, but a dense, pure stand 

 of a lax form of incrassata was found in the centre of CACTUS (Fig. 85). 

 No population count was made of this stand, but it had the appearance 

 of a density of 200 or more thalli per square metre. This stand was in 

 1 1 m of water, in the very centre of the crater, where light was poor 

 (visibility was about 1 m), and the water murky with clay or a colloid- 

 like suspension. The bottom was extremely soft and went into 

 suspension at the flick of a flipper. It was also billowed, apparently by 

 worm tubes. 



Away from the centre of the crater where the slope of the floor was 

 already apparent but at a depth of about 10 m, the straggling thalli of 

 incrassata thinned out and merged into a grove of Caulerpa ad serrulata. 

 Tube worms with chitinous tubes were collected in this community. No 

 Halimedae were found in shallower water, and a quick survey during 

 descent suggested that the sides of the crater were barren of plants. The 

 crater also contained a black-tipped shark. 



The dense incrassata population in CACTUS is of special interest 

 because we found this species to be rare at Enewetak, locating no 

 dense populations other than this one. The species incrassata is the 

 principal Rhipsalian Halimeda in the sandy and muddy shallows at 

 Glory Be in Jamaica. Possibly this species is restricted at Enewetak by 

 a paucity of fine substrates. But it certainly was able to disperse to this 

 unusual site at Enewetak, and this is one of the few pieces of data on 

 dispersal in Halimeda that we possess. It is possible that the dense 

 population was found in the deepest water at the centre of the crater 

 because a vegetative propagule settled there. Had the crater not been 

 filled in, it would have been extremely interesting to see if the resulting 

 clone spread up the sides. 



(e) The lagoon: pinnacles. With over 2000 coral knolls in the lagoon 

 (Emery et al., 1954) some considerable variation can be expected in 

 their biota. Of the three on which I collected (Fig. 97), Pole pinnacle, 

 north-west of Rex islet, was the most interesting for Halimeda, and 



