THE CORAL REEFS OF ANDAMAN AND NICOBAR ISLANDS

Hydrographic surveys of the seas around Andamans and Nicobars w"re made from 1889 to 1907 and again in 1926 and 1956 by the Marine Survey of India. Oceanographic work in the seas around these islands was largely that of I.N.S. Investigator (1887-1901) limiting mainly to deeper waters beyond reef limits. The teAt of these surveys was compiled in the Bay of Bengal Pilot. Though these surveys provide a useful guide for contours, depth and for general oceanographic and meteorological conditions, these add very little to the knowledge of reef characteristics and growth. Sewell (1938) and Scheer (1971) briefly mentioned the occurrenCe of shore reefs.

The Andaman group of Islands are separated from the Nicobar group by a 296 km wide channel.The Nicobars are set obliquely towards the south-east.The northernmost is the Car Nicobar and the Southernmost, the Great Nicobar.The larger islands in the central group are Katchal, Camorta, Teressa, Bompoka and Tillachong while Battimalv and Choura are smaller islands located north of the Central group.

GEOMORPHOLOGY
The geomorphology of islands, reefs and other associated structures can be studied in two ways.viz.by visual observation and boring data.The following geomorphic structures are visible in these areas.
Reef platforms : A feature common to these islands, as elsewhere, is the development of benching condition formed by the peneplanation of s!1ores by waves.These benches or platforms are usually absent on steep shores but are more often found on sloping landscapes.The shore rocks of sandstone, conglomerates and plutonic rocks are less porous and less soluable.The platforms formed around these rocks show more features of abra'iion.The shore ~ocks consisting of Jimestone (PI.IVa) and calcareous clays are more soluable and therefore show more features of solution.The structural and biological differences of the platforms in the two groups of islands are enumerated below.
Platfornls of Andaman Islands (PI.IIa): Usually platforms of about 500 m in width from shore occur.Shore rocks appear as relicts on these platforms, some in a pinnacle form and others in a vertically bedded state~ Wide erosion channels, some 20 m in width intersect the platforms at various places.Surge channels of the type found in the Nicobars are absent.The platform edges are flat on the channel side and sloping on the windwc.lrdside.Reef edges contain mostly the species of Porites, Favia, Pocil/opora and Acropora.On the leeward shores.the bouJders though usually small, are rounded.The alcyonarians, Sarcoplzytol?, Lobophytum and Sclerophytum and the gorgonids Isis hippuris, M elithea phillippinensis and Chalcogorgia sp.occur in shallow shoals and at reef edges.The slow rise of tide on leeward shores pIovides a good habitat for the growth of the fragile species of Acropora and the sea cups, Phyllospongia calciformis.Andaman platforms favour thick growth of corals at reef edges and patchy growths at various places on the platforms.
Pla~forms of Nicobar Islands: (PI.VI a; IV b) Platforms up to 1000m in width from shore occur.These appear to be formed mainly by the dissolution of shore rocks.The platforms develop prominent land-Ward and seaward inc1inations.Surge channels of 1 m in depth and width at reef edges are found on the windward side.The ,reef edges are outressed and trenc;hed.The slippery nature of reef edses is mainly d .. to the presence of algae.Porites, Pavia, Acropora, Poci//opota, H~/;opora, Tubipora and Montipora are the most common among the boulder forming coral genera.Alcyonarians and gorgonaceans also occur.
Most shores of Andaman and Nicobar Islands contain the associated reef structures such as 'Pinnacles which are mainly the submerged rocks of the island shores.At some places there are rubble terraces of t.4 m thick which are formed usuaUy by strong monsoonal winds or cyclones.Some of them may also be formed by wave action during high tide.
BORINO DATA Boring data on these islands are very few and are limited to bays where there is least coral growth as required for harbour development.One boring made on the 30 m width of the flat indicated a 6 m thickness of coral bed followed by 10m thick hard clay bed (Shri Raju, Personal communication).Bore holes in a 4" size inner casing are known for four places (2 in Andamans and 2 in Nicobars) in the off reef areas to a depth of 15 m below the sea bed.The base rock is covered by a 10m thick coarse sand which is again covered by a 3 m thick clay bed, containing loose coral boulders.The topmost layer near the surface is a 3.5 m cover of silt with occasionaJ clay lumps.In the off-reef areas ofLittJe Andamans at a depth of 15 m, the top layer of 7 m consisted of fine sand, followed ~y 5 m of loose clay and 3 m of hard clay.This hard clay was described as a very stiff grey silty clay Which is said to have formed the base of the coral bed.°No data of borings on the reef platforms of Nicobars are available, but examination of growing reef fronts show not less than 10 m where huae columns of growing corals are visible as arms extending out into the sea, dipping down to the bottom from the reef rim.Boulders of Montipora, Acropora, Porites and Favia lare seen rising from these extended areas upwards towards the sea leve).This situation Can be visualised in bright sunlight and calm weather all along the windward reef front at Car Nicobar.
Geomorphologically the following profiles of platfornls may be clistinguisl\ed especially in the Nico bar Islands.
1. High level conglomerate flats with prograding s.l1ores.2. Unconsolidated conglomerate fiats 3. Consolidated flat conglomerate 4. Unconsolidated irregularly shaped fiats S. Exposures of old beach rock, base rock.recent or sub recent upraised reefs under severe erosion and beach retreat.
SedimentologicalIy, the following environments of deposition may alto be distinguished as in the other tropical reefs.
Records of the Zoological Survey 0/ India 1. Reef front or reef waH 2. Brecciatic zone 3. Reef-flat shoals or back reef shoals 4. Beach sediments or beach deposits.
Biologically, the following zones may be distinguished.

REEFS
In the geometric classification of reefs as fringing, barrier and.atoll, Andamans and Nicobars belong mostly to the fringing type.A barrier was reported earlier on the west of Andaman ridge (vide Sewell, 1938) but atoll reefs are absent.The following c~tegories of reefs may be distinguished further in the Andaman and Nicobar Islands.(d) Knoll reefs (coral knolls) : These are found in the midst of channels or to one side adjoining the main reef.These are circular topogra phic highs arising from about 20 m depth in these islands (Reddaih et ai, 1974).Some of them are also found in the off-reef areas.The knoB is flat-topped with two hollows and two rims.It is a hard and compact hillock.Porites and Favia are the chief reef builders.Alcyonarian growths of Sarcophyton, Lobophytum and Scierophyt'Um are found on top of the knoll.Acropora grows on the periphery e. g., channels of the Richie's archipelago.
(e) Patch ree.fs : These are also circular growths developed on the inner side of the main reef usually half to one fathom in low spring tides.Though they resemble knoll reefs, these are not compact.Almost all corals found in the vicinity but mostly the fragile forms grow indiscriminately and lie loose on top of the patch.Most of them colJapse or break while walking over them.Alcyonarian growths are abundant.
The patch may grow a foot higher than the adjoining main reef.The.patch reefs are apparently the formative stages of knoll reefs.These are found in the back r.eef shoals of Nicobar Islands.e. g., Camorta.Other reef builiers : Molluscs are also important reef builders on these shores.Most boulders of Favia and Porites in the Nicobars and in the Richie's archipelago contain 10 to 20 specimens of Tridacna erocea or sometimes T squamosa.This genus is an important molluscan reef builder at the reef-flat level.Other species mo~tl)' Turbo: Trochus, LIlmbis.Conus and Cypraea occur at reef edges and flats.Sea cucumbers fOIm an important constituent of leeward reefs, some species occurring inunusually large numbers and sizes.Foraminifera also occur in emer~ sediments and beach deposits.
phyceae, also contribute in the formation of coral reefs.Though coraJlin.alge occur, the part played by them is not so marked as in atoll reefs.Lithothamnion sp. and Lithophyllum amplostratum are conspicuous on the windward side at reef edge~, forming important constituents of the butress zone.On the leeward shores, theIr presence is insignificant.Less wave action, lack of suitable base for their settlement and growth are some factors accounting for their absence but these are recorded at considerable depths by Sewell (1938).Turbinaria sp.occurs profusely on windward reef fronts but its part in reef ecology is not known.Halimeda opuntia is a very conspicuous semi-cJacareous alga found on almost all reef-flats.Ulva and A vernvillia sp.grow in front of mangrove fiats and act as sediment binders.Sargassum, Lamin~rj(l, Codium, Gelidium etc. and a number of other algal species occur on leeside of islands 'where there are deep Shoals.In the shallow shoal~ near the shores, Amphiroa occurs abundantly.

ZONATION OF CORALS
Detailed zonation patterns for the corals are difficult to obtain as the corals have the tendency to grow irregularly in any concievable place where there is some water in motion, Though transect studies have been attempted, these have not shown a clear pattern of zonation.CeJ..tain broad gene1!aIisations can be made.On th~ leeward shores, corals grow more freely as these are met with less interference from waves.Boulders of Favia and Porites are found growing for about 200 m up to almost near the reef rim.Their tops are fiattish as their upward growth is checked by sub-aerial exposure (Pl.III b).These are rounded boulders when young but assume a fiat top due to weathering and solvent action of rain water during sub-aerial exposure.These features can be seen in growths developed in front of the mangroves.
From about 20 m from the reef edge, there appears a succession of coral species running parallel to the reef edge, each species covering about 50 m in width (PI.VI a).Thus it may be inferred that on windward shores, the reef edge zonation appears parallel but the zonation on the remaining parts of the reef flat is difficult to arrive at as these are metamorphosed after death and decay.Sometimes these may be present in shoals.It may be assumed tha.tPorite~ and Favia are the chief fiat builders growing perpendicularly to the shore for about half the' width of the reef fiat but after, a succession of various coral species, one after the other run parallel to the reef edge.In very wide shoal areas, Acropora grows in thickets and it is the shallowness and calm water that deter .. mine the extent of the fragile species of Acropora.Certain hardy species of Acropora such as A. humilis are encrusting on windward reef fronts and arc: capable of refisting the most forceful wave effect.More inforllltion is needed before conclusi ve evidence is presented on zonation or corals in these shores.

MANGROVES
A unique feature of Andamans ,and Nicobars is the presence of mangroves in open shores also, while these are quite common in every channel and bay in sheltered places.These are absent, however, on certain islands like Car Nicobar which may be due to the severity Of wave action that prevents settlement and growth.When erosion had cut back a number of open shores and channels, shoals may have developed close to shore by deposition seaward.These shoals today are filled with corat growths.Whether mangroves have settled first or the corals is difficult to conclude.More likely that the mangroves did so earlier s:lpported by the appearance of mangrove seedlings in the spits form,d in front of S'.lch rec~nt shoals.e. g., Tapong, Nancowry.Almost in every m3.ngrove flat in op~n shores, bays and channels, corals are seen growing in front and behind mangroves so long as a few feet of water is present.Mangroves provide a she!tered environment for the corals from severe wave action and from abrupt deposition of 'land drawn sediment.The massive forms of Porites and Favia usually grow here to a § much as 5.5 xn and 2 m in diameter respectively.Where wave act~on is severe and deposition by flood is more, these defoliate and vanish.
I. Recent or sub-recent Reefs: Emergent reefs : These are ",idt;)y reported on many islands by earlier authors.At Car Nicobars, this reef is extensive on the north-east.Coral and Tridacna beds are found 2 m above the present sea level, and exposed insitu on the island rim at Keating point and all along the beach face.Tridacna maxima and a number of other bivalves occur in quarry pits of 3 m deep, some Tridaena reaching 1 m in length.Such reefs indicate negative eustatic movements.II.Modern Reefs(a) Windward reefs: (PI.IV b).Reefs growing in the direction of prevailing winds are called windward reefs.Andamans and Nicobars lie in the monsoonal belt.So these reefs grow chiefly in the direction of monsoon winds.Swell brought by other sources generally have a considerably less wave force, but may carry food supply to the reef.A reef growing windward is bevelled or sloped.The character of slope varies from place to place depending upon the oceanographic conditions.Such reefs are found on most islands on the windward side.(b) Channel reefs: (PI.II a; VI b) : Channels present in these i~lands are usually protected from winds due to the presence of islands on either side.These are therefore referred as leeward reefs.Because of less wind action, less wave action follows and flat growth of reef is developed.The reef edges are usually steep, not sloping as in windward reefs.e. g., Channels of South Andamans and Kamorta-Nancowry complex.(c) Bay reefs : These reefs are subjected to similar wind and wave action as the channel reefs but sediment deposition plays a large part ia the srowth and death of these reefs.the bay is filling in nature both by upward coral growth and by sediment deposition and ultimately brinsing the bay reef to the level of the channel reef.Porites and Favia aro the chief reef builders.Alcyonarian gro~ths are present but not abundant.Fragile Acropora occur in deeper bay~.ego Bays of Nancowry.
Most coelenterate groups occur.Of these, Scleractinia form the most important constituent among the reef builders in these shores.The Octocorallia and Mi11iporina are well represented.Hydroids occur mostly in deeper waters.Zoantharaia and Scyphozoa are less prominent.Of about 75 species of Scleractinian corals studied (vide table I) about 15 % are boulder forming, hard and wave resistant.Some species of Acropora (PI.V a) and Porites (PI.II b) are enctu~ ting in nature.Most of these corals grow in the direction of prevailing winds.