THE BIOLOOY OB CORAL REEF8
221
channels. (17) The reef slope with coral growth notably decreasing
at about 10 fathoms but without the local terracing of the corresponding windward depths.
The further analysis of the more windward environments in terms
of the coral species most chraoteristic of the different zones on Bikini
ie supplied by W e b (1954) and is illustrated in Fig. 2. With numbers
indimtkg the same zones aa in the preceding figure, the major areas
are : A (=l), the seaward slope ; B, the seaward reef; C, the island ;
D, the lagoon. Wells distinguished the following zones (indicated by the
corresponding lettering in Fig. 2) in terms of their dominant corals.
(a) Sclerhelia-DendrophyUia zone of ahermatypic oorals below 80
fathoms. (b) Leptmeris zone (between 50 and 80 fathoms) " marked by
special species or a facies which differs from those of the surface reef
forms but is not truly ahermatypic ". (0) Echimphylliu zone from 10
to 50 fathoms and the lowest to which " surface reef species extend and
grow with any degree of strength". Then on the upper surface of the
reef in regions of decreasing exposure come (d) the Acropora: cuneuta
zone, (e) the Acropora digitifera zone, (f) the Acrqma palifera zone,
(g) a highly characteristic Heliopora zone with large colonies of this
blue octocorallian, and (h) the Porites lutea zone. The three laat occupy
the greater part of the just-submerged reef flat (449, a region which
Wells describes as that of microatolls (z). Their formation is the result
of limited possibilities of upward growth causing the colonies, killed
off at their summits, to enlarge peripherally around a dead and often
partly excavated centre. The sheltered margins of the lagoon are
characterized by (if a Porites udrewsi zone and then (j) an Acropwu
reticukda zone found both on the lagoon reef flat (9) and at c o r n -
ponding shallow depths near the summits of the coral patches. This
overlaps with (k) an Acrqorufomzosa zone which extends generally
over the lagoon floor and terr9ce (1) an Acropru ruyneri subzone entirely
confined to the floor.
The highly significant environmental differences between the reef
margins on the windward and leeward sides of reefs, already noted
elsewhere, e.g. in East Indian reefs (Umbgrove, 1947) and at Low Isles
and reefs of the Great Barrier series, have been described in great
detail for these atolls, notably by Tracey et al. (1948), Mu& and
Sargent (1964) and Wells (1954, 1957). The windward margin, fully
exposed to the oceanic surf, invariably possesses a mmive algal ridge
composed largely of Porolithon (= Lithothumnh) which extends seaward to a depth of up to 10 fathoms in the form of a series of buttresses
or spurs with deep intervening grooves or surge channels (Fig. 3).
Especially in regions of greatest exposure the upper end of these
221
channels. (17) The reef slope with coral growth notably decreasing
at about 10 fathoms but without the local terracing of the corresponding windward depths.
The further analysis of the more windward environments in terms
of the coral species most chraoteristic of the different zones on Bikini
ie supplied by W e b (1954) and is illustrated in Fig. 2. With numbers
indimtkg the same zones aa in the preceding figure, the major areas
are : A (=l), the seaward slope ; B, the seaward reef; C, the island ;
D, the lagoon. Wells distinguished the following zones (indicated by the
corresponding lettering in Fig. 2) in terms of their dominant corals.
(a) Sclerhelia-DendrophyUia zone of ahermatypic oorals below 80
fathoms. (b) Leptmeris zone (between 50 and 80 fathoms) " marked by
special species or a facies which differs from those of the surface reef
forms but is not truly ahermatypic ". (0) Echimphylliu zone from 10
to 50 fathoms and the lowest to which " surface reef species extend and
grow with any degree of strength". Then on the upper surface of the
reef in regions of decreasing exposure come (d) the Acropora: cuneuta
zone, (e) the Acropora digitifera zone, (f) the Acrqma palifera zone,
(g) a highly characteristic Heliopora zone with large colonies of this
blue octocorallian, and (h) the Porites lutea zone. The three laat occupy
the greater part of the just-submerged reef flat (449, a region which
Wells describes as that of microatolls (z). Their formation is the result
of limited possibilities of upward growth causing the colonies, killed
off at their summits, to enlarge peripherally around a dead and often
partly excavated centre. The sheltered margins of the lagoon are
characterized by (if a Porites udrewsi zone and then (j) an Acropwu
reticukda zone found both on the lagoon reef flat (9) and at c o r n -
ponding shallow depths near the summits of the coral patches. This
overlaps with (k) an Acrqorufomzosa zone which extends generally
over the lagoon floor and terr9ce (1) an Acropru ruyneri subzone entirely
confined to the floor.
The highly significant environmental differences between the reef
margins on the windward and leeward sides of reefs, already noted
elsewhere, e.g. in East Indian reefs (Umbgrove, 1947) and at Low Isles
and reefs of the Great Barrier series, have been described in great
detail for these atolls, notably by Tracey et al. (1948), Mu& and
Sargent (1964) and Wells (1954, 1957). The windward margin, fully
exposed to the oceanic surf, invariably possesses a mmive algal ridge
composed largely of Porolithon (= Lithothumnh) which extends seaward to a depth of up to 10 fathoms in the form of a series of buttresses
or spurs with deep intervening grooves or surge channels (Fig. 3).
Especially in regions of greatest exposure the upper end of these