412 PERCY SLADEN TRUST EXPEDITION 



IV. ALDABEA. FRINGING REEF— REEF FAUNA. 



The fringing reef outside the islands of the atoll-rim varies in width, at its widest 

 measuring about 500 yards, though near Point Vaquois it is hardly 200 yards between 

 the cliffs and the breaking edge. In the composition of its surface there are two extremes 

 (though with every intermediate grade) found respectively on the north and south coasts. 

 To simplify description I give the chief characters of the reef in two places, first near the 

 northern section across the land-rim, and secondly in the south near the Takamaka 

 section. The reef to the north, bounded by the cliffs previously described, may be divided 

 into four somewhat ill-defined zones. The first consists of a narrow depression in the 

 rock at the foot of the cliffs : it is usually 5 or 6 yards wide and 6 feet deep, and owes its 

 origin to the backwash from the waves which at high tide beat with great violence against 

 the cliffs. 



Outside this zone is the reef-flat, a broad flat zone with a surface consisting of 

 a coarse rubble of broken coral fragments, all much encrusted with lithothamnia ; in 

 places are small accumulations of sand, which usually support a vigorous growth of the 

 cotyledonous plant, Cymodocea. To seaward of the flat, which forms the greater part of 

 the reef, is a narrow boulder zone, 20 — 30 yards wide, the boulders consisting of broken 

 coral colonies rarely more than a foot high and hardly deserving the name "boulder"; they 

 always become encrusted with lithothamnia, Squamariacese, and rarely Polytrema. Out- 

 side the boulder-zone is a rather indefinite buttress-zone, the buttresses being composed 

 of coral rubble, while the intervening channels contain scattered and sickly coral colonies, 

 mostly of the genus Pocillopora. Outside the breaking edge I was only able to make 

 scattered observations, but the slope appeared to be steep and to support scattered colonies 

 of massive corals. I append sections kindly taken for me by Captain Parcou, F.R.G.S., on 

 the outer slopes of Assumption and Astove, which reefs appeared to have similar slopes to 

 that of Aldabra. (Plate 29.) 



The reef to the south of Takamaka differs from the reef of the north of the atoll in 

 being very completely covered with sand, which is usually bound down by extensive beds 

 of Cymodocea : areas covered with the usual coral rubble are scarce. The boulder-zone is 

 almost absent, and the buttress-zone ill-defined, though it was interesting in that the 

 buttresses were composed of sand held together by Cymodocea, the rhizomes of which 

 were coated with lithothamnia and in a few cases with an encrusting form of Millepora, 

 the sand being thus bound together by calcareous ropes. Live coral was naturally almost 

 absent. 



The outer slope seemed to be gradual and to be covered with sand, with here and 

 there a coral colony or large mass of some colonial alcyonarian. Again I would call 

 attention to the outer slopes to the south-east of Astove and Assumption which are 

 probably similar. 



These two types of reef cover almost all that are found round Aldabra, though 

 intermediate stages are frequent. The reef on the north coast as far east as Point Hodoul 

 is of the first type, while southwards it becomes more sandy as far as Dunes Jean Louis 

 and Dunes de Maistre. After this it becomes less sandy round the west of Main Island 



