Issue #1 – Compiled by F. Meyerutdallas.edu/~rjstern/egypt/PDFs/General/Red Sea Field Guide.pdf · 2 • DGS Field Guides ... Saudi Aramco drilling results showed the Red Sea area

Issue #1 – Compiled by F. Meyer

2 • DGS Field Guides

TABLE OF CONTETS

LOCATION ...................................................................................................................................................3

ARAMCO’S RED SEA EXPLORATION ..................................................................................................3

OPENING OF THE RED SEA ....................................................................................................................5

RED SEA REEFS........................................................................................................................................12

GEOLOGIC CONSTRAINTS ......................................................................................................................12 REEF ZONES ...............................................................................................................................................12

PLEISTOCENE CLIMATE.......................................................................................................................13

GEOLOGICAL FIELD TRIP STOPS......................................................................................................16

REFERENCES ............................................................................................................................................19

* CORALS UNDER ATTACK: CAN THEY FIGHT BACK? ..............................................................20

INTRODUCTION.........................................................................................................................................20 CORALS AND SILTATION........................................................................................................................20 CAUSES OF SILTATION............................................................................................................................21 HOW CORALS FIGHT SILTATION ..........................................................................................................21 SEDIMENT CLEANING ENDURANCE....................................................................................................23 MUCUS DISRUPTION AND TISSUE DAMAGE......................................................................................24

Fighting Tissue Damage by Mucus Secretion ..........................................................................................24 Mucus Secretion and Its Disruption .........................................................................................................24 Tissue Damage and Repair.......................................................................................................................25

CONCLUSIONS AND SPECULATIONS ...................................................................................................25 REFERENCES..............................................................................................................................................26

Red Sea Costal Geology • 3

LOCATION

The geographical location of the Red Sea is between latitudes 12° and 30° N. This narrow sliver of ocean is about 2300 kms (1425 miles) long and reaches its maximum width of only 300 kms (185 miles) between latitude 16° and 20° N.

EGYPT

NubianShield

ETHIOPIA

GULF OF ADEN

Jizan

Ghawwas

Jiddah

Rayyis

Yanbu

Al Wajh

Wadi Azlam Duba

MidyanTabuk

Madinah

Hail

RafhaAl Jawf

Al JQurayyat

TaifJiddah

JORDANIRAQ

ARABIANSHIELD

Yanbu

R E D S E A

0 200 400

KMS

Figure 1 — Location map showing Red Sea, coastal plain, Arabian Shield and locations of various exploration areas.

ARAMCO’S RED SEA EXPLORATION

Saudi Aramco initiated exploration of the Red Sea region on September 9, 1990. Since then thousands of kilometers of seismic was acquired over all the basins. These efforts


focused on the Red Sea coastal area (Figure 1) and led to the identification of significant structures.

Figure 2 —Mosaic of 37 Landsat scenes shows the northern Red Sea area. The separation of the Arabian plate in a northeast direction away from Africa occurs along a line of transcurrent faulting known as the

Dead Sea rift system (A to B). Relative displacement along the fault is estimated to be approximately 105 kilometers (65 miles). In this mosaic the near parallelism of the opposing coastlines is clearly evident. Mountains on both sides of the Red Sea reach elevations of 1500 to 2000 meters (5000 to 7000 feet).

Saudi Aramco drilling results showed the Red Sea area to be very prospective. Exploratory wells were drilled in all the major basins. By 1995, 15 wells tested the various sedimentary sections. Two of these wells, the Yanbu North-1 and Yanbu East-1, were unproductive tests of the lower Miocene clastic section located in the central Red Sea area, but success came in the southern and northern areas.

Several wells detected hydrocarbons in the southern Red Sea area. Initial shows were noted in Ghawwas S-1, but subsequent drilling (Jizan North S-1) in the Jizan area led to the discovery of high gravity paraffinic oil and sweet gas from Lower Miocene sands.

Midyan Field includes gas/condensate production from two wells and oil/gas production from another well in the northern Red Sea. These hydrocarbons come from a Magna Group limestone reservoir of middle Miocene age.


OPENING OF THE RED SEA

The Red Sea and its related components, the Gulf of Aqaba and Gulf of Aden are of extreme geologic interest because they express a zone of incipient plate movement along a line of transcurrent faulting. In essence, these areas are an important part of the world rift system and more specifically, the crustal division of Africa into the Arabian and African plates. All available evidence suggests the fragmentation process began about 35 million years ago with the original line of separation approximately paralleling the axes of the Red Sea and Gulf of Aden. The rupturing process propelled the newly evolved Arabian Plate away from Africa is in a northwesterly direction. Today, the near parallelism of the opposing Red Sea coastlines offers strong support that the two plate fragments once were welded together (Figure 2).

Figure 3 —Hypothetical cross section of the Red Sea illustrating possible crustal relationships (from R.G. Coleman et al., 1977).

Various data suggest the initial rifting phase may have begun during the early Oligocene and certainly was in full motion during Miocene times. Basalt, the first indicators of ocean floor formation in the area of the rift (see Figure 3) give radiometric dates of Early Miocene age. Similar ages are obtained from radiometric dating of basic dikes in the area. Additionally, sedimentary sections suggest that terrigenous continental clastics began accumulating in pre-rift depressions during the Oligocene. A more comprehensive discussion of the entire rifting phase and the accumulation of sediments within the Red Sea system (Table 2) is given by Hötzl (1984), a portion of which is reproduced below.

“On the basis of the classification as Oligocene of the clastic continental series subjacent to the fossil-dated Miocene sediments, it must be assumed that there was a depression of



corresponding age at the apex of this updoming. The recent literature thus usually indicate of an Oligocene to Early Miocene beginning of the graben. C. DULLO, et al. (1983) could, however, show that some of the basal limestones previously classified as Miocene belong to the upper Oligocene, and therefore that the marine ingression in the graben trough thus took place before the Miocene.

The break up of a huge continental plate has to be caused by a supraregional stress field. It thus would not be surprising if the doming and rifting were essentially involved in the Mid--Tertiary event (uppermost Eocene - lowest Oligocene, 40-36 million years B. P.). This event was responsible for a number of movement processes on the edges of the plate fragments, and most particularly in the Mediterranean area.

The starting mechanism of graben depression is discussed differently. While I. G. GASS (1970) of J. D. LOWELL and G. J. GEMIK (1972) here speak first of graben collapse at the apex of the Arabian-Nubian updoming zone, R. G. COLEMAN et al. (1977) Suggest that there was first a broad, flexure-like downwarping as a result of the crustal extension. The letter is supported by certain geophysical findings, as well as by the gradually steepening dip of the Jurassic-Cretaceous strata series in the southern section of the graben.

Only after further tensional stress was there a complete break of the crust with intensive block faulting from the Oligocene or early Miocene. As a result of the related further subsidence there was then the marine ingression from the Mediterranean mentioned above and the subsequent deepening of the trough. Block faulting in the graben area and the simultaneous opening of fissures distinctly intensified volcanic activity. Fissures parallel to the rift spilled thick alkali basalts down the shoulders of the graben. At the same time, there was extensive magmatic activity in the trough itself. Special mention should be given to the numerous dikes on the edge of the southern Tihama near Jizan, where in addition to the dikes, the layered gabbro body of the Jabal at Tirf has also penetrated Tertiary sediments (M. GILLMAN, 1968; R. G. COLEMAN et al., 1977).

We still do not know whether in this first phase of rifting there was also formation of an oceanic crust via spreading processes in the axial trough. R, W. GIRDLER and P. STYLES (1974), as well as S. A. HALL et al. (1977) think there is a process of this sort in the lamellar anomalies in the magnetic field of the Red Sea. Based on magnetic measurements in the southern Red Sea (17° - 20° N lat.), they postulate the existence of Neogenic oceanic crust for the entire trough area. It is supposed to have developed during two spreading phases: The older one between 40-34 million years B. P., and a younger one (in the axial trough area) in the last 5 million years. With further measurements in the adjacent section to the north, the period for the older spreading phase was corrected to 20-30 million years B. P. (P. STYLES and S, A, HALL, 1960).

J. R. COCHRAN (1983) doubted the existence of an oceanic crust in the broad marginal area of the Shelf and used the result of borings and rock formation on individual islands to suggest a continental crust divided into horsts and special grabens. Block faulting parallel to the rift also could be responsible for the tonal arrangement of the magnetic field. Further study will, however, be required to provide definite information on the extent of the oceanic crust in the Red Sea.


The Sedimentation process also conforms with the development mentioned above of an initially flattish depression followed by breakage into blocks. The subsidence of the graben depression led to marine ingression in the Upper Oligocene and the continuation of this process finally brought about the transition from shallow-water to basin facies. The block faulting in the trough is also seen in the numerous coarse-clastic intercalations during the Aquitan and Burdigal. A decrease in tectonic activity toward the end of this first rifting phase is to be seen in the gradual transition from coarse- to fine-clastic sediments in the hanging layer, as well as ultimately in the transition to evaporitic sedimentation.

1.2.2.3. Spreading and shearing in the Plio- and Pleistocene

There is a reactivation of tectonic activity in the upper-most Miocene of lowest Pliocene. The evaporites are followed by coarse-to-fine clastic series, particularly in marginal areas and sometimes with distinct discordance. They attain thicknesses in excess of 1,000 meters. They reflect the recurring vertical tectonics that led to involvement, block faulting and tilting of the Miocene sediment sequence. At the same time, a breakthrough to the south developed that connected the Red Sea rift with the open ocean via the Gulf of Aden. The basin, which previously had been quite isolated, was again flooded. In the course of this movement there was also increased uplift of the graben shoulders, e. g. Sinai and the Asir highland, upon which the currently existent morphological opposite to the graben basin rests.

What is decisive for the present-day tectonic picture is the recurrence or renewal of the spreading process in the trough axis. Further extension there opens up the sediment filling, creating a graben in the graben. There, tholeiitic magma in the form of a mid-ocean ridge is brought up from the mantle and forms a new oceanic crust. Paleomagnetic measurements show a total opening of 80-100 kilometers for the last 4-5 million years, amounting to an average spreading rate of two centimeters/year.

With the activation of the spreading process, the active tectonics shifted to the central trough. On the edges and flanks of the old graben, the seismic and tectonic activity is subsiding considerably. Deep-reaching gravity block movement on sliding planes, as apparently already seems to be the case in the southern part of the Red Sea, may increasingly contribute to a gradual stabilization of the steep edges of the graben flanks.

The northeast drift of the Arabian plate caused by the spreading process is overlaid by a pronounced sinistral shear movement along the Aqaba - Dead Sea system. This plate shift has already been recognized and correctly interpreted by L. PICARD (1937) and others. More recent work confirms the previously calculated total movement sum of more than 107 kilometers. R. FREUND et al. (1970) postulate a two-phase movement, which they compare chronologically with the two rifting periods in the Red Sea.

Y. BARTOV et al. (1980), however, used the absolute age of dikes parallel to the rift to show that the total movement is younger than 22 million years. Our own work in the Midyan region also indicates a post-evaporitic beginning of the shear movement (H.-J. BAYER et al., 1983). The connection between drift and lateral movement, and spreading and shearing, is the subject of new work in progress.


With the reactivation of tectonic processes at the beginning of the Pliocene, volcanism also picked up. We find young alkali basalts in the graben, in the present-day coastal plain, and on the flanks, as well as on the graben shoulders, and there as far as 200 kilometers away from the graben margin. The latest activity continuing up to the present, particularly on the Arabian side of the Red Sea rift. This shows off the large influence of the rift mechanism for the Quaternary geology of this area.”

COASTAL GEOLOGY

Major directional changes in the Arabian Red Sea coastline have structural controls related to the rifting process. The westward protrusion of the coastline about 50 kilometers north of Yanbu is a good example. There, the coastline turns abruptly to the west before resuming its northwesterly strike (see guidebook cover). Approximately 25 kilometers further north; the coastline turns back abruptly to the east before resuming its northwesterly trend on line with that at Yanbu. This protruding block-like middle section of the coast defines a large fault block bounded by more or less east-west trending faults. The northern fault lineament can readily be seen extending landward and into the crystalline basement complex from the offshore reefs.

The coastal sedimentology can be summarized an interplay between continental and marine sedimentation over the 120 kilometer section we will be traveling. As a whole, continental sediments dominate the 5-20 kilometer expanse of coastal plain accumulations and relegate the marine deposits to the immediate (1-2 kilometers) seaboard.

Striking features of the marine sequences are the numerous marine terraces (Figure 4). These morphologically distinctive benches record both constructional and erosive episodes of various Plio-Pleistocene sea levels. The following section on the Yanbu Cement Plant by Al-Sayari, et al. (1984), provides a detailed account of the sequence of terraces typical of the area.


Figure 4 —Schematic cross sections showing typical terraced marine sequences. A) Western escarpment of northern Jabal Al Jarra, B) East coast of Al Hasani; 1 calcareous sand, 2 slope debris, 3 recent coral reef, 4 Pleistocene coral reef, 6 dipping of the limestone, 7 thick mature duricrust (from Jado and Zötl 1984).

YANBU CEMENT PLANT

“The Yanbu Cement Plant takes young carbonate rock from 15 kilometers north of Sharm Al Khawr for the manufacture of cement. Here this occurrence of marine sediments is limited to a strip of coast with a width of two-four kilometers. Marginal erosion has again dissected out the occurrence as a table-like ridge. With its orientation parallel to the graben, the ridge is of tectonic origin similar to the position of Jabal Jarra in the north. It forms the border of a block-like section of the coast which is 25 kilometers long and projects especially far out; it is limited by the bay of Sharm Al Khawr and Marsa Maqbarah.

Near the cement plant, the ridge reaches a height of 40 meters; both the top planation and the various terrace steps were formed by previous sea levels.

Here as well, in the vicinity of the cement plant, the two-meter level forms the older reef limestone; its maximal breath here is 50 meters, but in many places it is completely missing. On the steep front edge this step has a height of 1.7-2 meters, and 2.5 meters at the rear. Young corals related to the formative phase of this erosional depth could only be identified beyond doubt in occasional pockets or niches.


The next-highest terrace step is six meters a.s. 1. Here it has a width of up to one kilometer and is generally still a uniform, even surface. Only on the edges toward the lower two-meters step, or right next to the sea, are there shallow erosional depressions or channels, so that the upper edge in places is barely four meters a.s.1.

Farther back, this six-meter planation may be limited by a steep rise, or a distinct flat upslope. This is, however, always characterized by pronounced weathering formations such as excavations and most especially a more intensive duricrust formation.

Proceeding upward, there are further terrace levels at 8 to 10 meters, and about 15 meters above m.s.l. Its surface already shows a small relief and is covered with a duricrust. There are additional terrace steps at about 20 meters and 25 meters above m.sl. They vary considerably, however, in distinctness. As a rule, the edges are highly overformed and flattened; only for short stretches do they also appear as steep edges. One generally has the impression of a considerably more pronounced and mature duricrust formation as compared to the 15-meter level, which is especially noticeable upon the top planation of this ridge at about 40 meters above m.s.l.

On the inland side, the ridge breaks down into vertical walls on an erosional margin. On the Upper edge there is the overhanging hard crust, and thereunder are the slightly backweathering limestones, including marly limes in places. The erosive nature of this margin is underscored by isolated erosion remnants, as well as occasional pillars that are still jointed. The steps visible in the wall are irregular and in some places limited by bed boundaries. Their surfaces also bears a mature duricrust.

Planation levels as remnants of older sea levels are visible in the adjacent erosion zone to the east. Most especially, remnants of the 10-meters- and 20-meters-terraces are to be seen in the subsequent depression directed toward Sharm Al Khawr in the southeast. These terrace remnants are laced with occasional faults parallel to the graben, which are responsible for the downwarp of individual blocks.

The marine carbonates here also show distinct lateral and vertical facies differences. An understanding of the structure of the entire sequence is best obtained from the erosion remnants on the eastern edge of the ridge. Here, unlike the west side, where entire coral colonies are to be seen on the individual steps, they usually form only the very thin upper crust. Underneath, there are relatively thick arenitic limestones with occasional large fossils. Snail and sea -urchin impressions as well as large pina and venus shells are also worthy of note here.

Some three meters under the upper crust there is an oyster layer 0.6-0.8 meter thick. Below that, there is again the lagoonal development, and the fossil shells mentioned previously become less evident. The extent to which this sequence contains deposits of varying ages, or the entire stratigraphic extent of this limestone sequence with a thickness of at least 40 meters belonging to the Quaternary, are matters that cannot be resolved with reference only to the macroscopic fossil content.


While higher gravel fans north of the cement plant also reach as far as the marine-sediment ridge, the southern area, including the part with the road to the cement plant, still belongs to the erosion zone extending from Sharm Al Khawr. The younger clastic sediments have again been removed there, so that various sequences of the Tertiary Raghama Formation are exposed. Immediately south of the road, for example, Miocene gypsum is being excavated.”

RED SEA REEFS

GEOLOGIC CONSTRAINTS

Extentional tectonism associated with the opening of the Red Sea graben exerts a profound influence on reef development. Block faulting relegates reef development to the flanks of the graben because the fault displacement results in significant submarine relief. Coastal areas plunge abruptly into water depths below the photic zone. Scleractinians, the major reef builders during the Tertiary, are hermatypic corals, meaning they possess symbiotic algae and therefore can only live within the photic zone. Hence, scleractinian reefs of past and present are exclusively the fringing reef type in the Red Sea. Fringing reefs develop along the coast and extend from mean sea level outward into water no more than 50 meters deep. Those of the Red Sea typically exhibit a shear vertical or steeply sloping walls that are interrupted by a succession of sandy shelves.

Figure 5 —Schematic of a hypothetical, zoned, marginal reefs showing the environmental zonation and the spectrum of depositional textures. (from James 1984).

REEF ZONES

Reef growth segregates the coastal region into a series of distinctive geomorphologic features and environmental reef zones. These are schematically shown in the accompanying zonation diagram (Figure 5). A shore zone extends from the beach


seaward to the reef crest. This region encompasses a shallow water area generally referred to as the back reef lagoon. Water depths deepen from the shoreline before shallowing again close to the reef. Water depths of back reef lagoons in the Red Sea may reach a meter or more, but generally are only a fraction of a meter deep. Little coral growth normally exists in this setting except in places close to the reef. Typically, the coral development consists of scattered or small clumps of colonies.

The line of surf identifies the location of the reef crest, a zone inclusive of a number of subzones. One easily identifiable component in this zone is the reef flat, an interval dominated by coral rubble. This site of coral boulder accumulation begins at the surf line and extends a variable distance in a landward direction. Seaward of the surf line lies the zone of shallow water coral growth. Much of the most vigorous coral development exists in this area. This region of active reef growth receives the full force of the waves, a condition that limits the diversity of coral growth.

Subzones of the fore-reef include a terrace, an escarpment, a slope and vertical cliff. Maximum coral diversity occurs in this part of the reef typically above 25 meters. Coral diversity and marine life in general decrease perceptibly below 30 meters.

GENERAL CHARACTERISTICS

A diversity of coral types but little zonation and little predominance of one species over another in terms of the number of coral heads characterize a fringing reef. A limited number of typical reef organisms observed on the living reefs are shown in the accompanying plates.

PLEISTOCENE CLIMATE

Significant climatic fluctuations punctuated the Pleistocene and presumably had a significant impact on the sedimentology of the coastal region of the Red Sea. The following article by Hötzl and Zötl summarizes this information.




GEOLOGICAL FIELD TRIP STOPS

STOP 1 (in transit)

Road cut east of Sharm Al Khawar. Here we see a portion of the continental sequence typical of the Tertiary coastal sedimentation along the Red Sea. Also present is weathered basalt, an extrusive that flowed to the coast from Jabal An Nabah some time during the Plio- to Pleistocene. Photograph (J. Filatoff, 1999)

STOP 2 (all day)

Al Hassi Coast Guard Station. Here we will see a good example of both a modern fringing reef and a raised reef developed during a previous Pleistocene highstand. The living reefs are developed along the coast and very near the shore. The present morphology of the living reefs is probably the result of erosion from the last period of glaciation.


The beach region is characterized by sand derived from the migration of dunes. It is a good place to see a mixed clastic carbonate transition. Biogenic components such as coral and mollusks are present in the terrigenous clastics of the beach. These become increasingly more abundant at the expense of the terrigenous clastics in the sediments of the lagoon.

Low outcroppings of Pleistocene reefs interrupt the sandy beach. These exposures are made up of the same faunal components observed in the modern fringing reef tract. Corals are particularly well preserved.


STOP 3 (Optional)


REFERENCES

Al-Sayari, H. Hötzl, H. Moser, W. Rauert, and J. G. Zötl, 1984, The Quarternary from Umm Lajj to Yanbu Al Bahr, in A. R. Jado, and J. Zötl, eds., Quaternary Period in Saudi Arabia, Wein, New York, Springer-Verlag, p. 62-95.

Coleman, R. G., 1977, Geological background of the Red Sea, in L. S. Hilpert, ed., Red Sea Research 1970-1975, Saudi Arabian Dir. Gen. Mineral Resources Bull., p. C1-C9.

Hötzl, H., 1984, The Red Sea, in A. R. Jado, and J. Zötl, eds., Quaternary Period in Saudi Arabia, Wein, New York, Springer-Verlag, p. 13-25.

Hötzl, H., A. R. Jado, H. J. Lippolt, and H. Puchelt, 1984, The Quaternary from Umm Lajj to Yanbu Al Bahr, in A. R. Jado, and J. Zötl, eds., Quaternary Period in Saudi Arabia, Wein, New York, Springer-Verlag, p. 82-106.

Hötzl, H., and J. Zötl, 1984, Middle and Early Pleistocene, in A. R. Jado, and J. Zötl, eds., Quaternary Period in Saudi Arabia, Wein, New York, Springer-Verlag, p. 332-335.

Jado, A. R., and J. Zötl, 1984, Quaternary Period in Saudi Arabia, Wein, New York, Springer-Verlag, p. 354.

James, N. P., 1980, Shallowing-upward Sequences in Carbonates, in R. G. Walker, ed., Facies Models: Geoscience Canada Reprint Series 1, Kitchner, Geological Association of Canada, p. 109-119.


* CORALS UNDER ATTACK: CAN THEY FIGHT BACK?

Franz O. Meyer

* Presented at 1989 NAUI IQ, Reprint Published in Conference Proceedings

INTRODUCTION

Broken, scarred, ghostly white and algal infested coral skeletons are among the visual casualties reefs bear in their struggle for existence. Examples of such coral damage occur on all reefs and are part of a reef building process that has operated form more than 400 million years. But the amount of damaged coral is significantly greater on reefs that are popular dive sites as opposed to reefs that are virgin areas. And that should concern all divers.

While the relationships between diving and reef damage is obvious, reasons given for what causes the damages are debatable. Professional divers working at reef resorts and on live-aboard dive boats believe corals are ultra-sensitive to silting and touching by recreational divers. Their observations suggest divers fanning silt and disrupting the mucus membrane of corals are to blame for the spread of death across the reefs. Active dive conservationists and dive publications strongly support their contentions. Together they ask divers to avoid fanning silt on corals and to refrain from touching these organisms to stem the advancing tide of reef damage. But their claims are not supported with documented studies, and it may be that reef damage has other causes. An important question now is: Are corals actually sensitive to silting and contact or can they fight back? Existing research provides some instructive answers, but documentation needs to be initiated of diver activities and their impact on reefs.

This paper presents a summary of research studies on how corals respond to siltation and disruptions of their mucus and tissues. It identifies commonly held misconceptions on the subjects and provides a basis for divers to properly assess the ecological impact their activities may have on coral reefs.

CORALS AND SILTATION

A general misconception among divers is that corals are defenseless against sedimentation. Many influential and vocal divers suggest this unfortunate view partly because they consider corals to be filter feeders. Corals are not limited to filtering water for nourishment. Rather they are voracious carnivores who actively capture prey by using tentacles armed with snares, toxic harpoons, or sticky threads (nematocysts). Every diver who has played a dive light beam on a coral on a night dive has observed the aggressive and frenzied slaughter of plankton drawn to the light. While corals do draw water into their body cavities, the process is not analogous with filter feeders such as sponges. So the effect of mud, silt or sand (sediment) on corals is not of the siltation problem. Most corals being attached to the bottom must be able to shed sediment because they live in environments where siltation is frequently a problem.


CAUSES OF SILTATION

Environmental conditions make it virtually impossible for corals to escape siltation. At least three conditions exist which collectively contribute to a fall of sediment: (1) sediment is continually being formed, (2) gravity constantly acts to pull sediment down the reef, and (3) rough seas stir-up the bottom.

Sand, silt, and virtually all mud which cloud coral environments are formed by plants and animals that live and die on reefs. White lime sand beaches and sand channels on reefs are nothing more than the skeletal remains of organisms. A variety of algae (Halimeda, Penecillus, Udotea) are among the most prolific producers of particles ranging in size from mud to coarse sand. Sponges, soft corals and a host of other reef dwellers contribute skeletal particles to the formation of sand blankets. All of these organisms grow on the reefs, but some tiny forms of algae (coccoliths) and single celled animals (foraminifera) not only produce stony skeletons but also live and die in the water column above the reefs. Skeletons formed by these plankton are a major source of mud and sand. As with many other organisms, the duration of life is not long, insuring corals receive a constant hail of fine particles.

Rough seas put sediment back into the water column to cause a heavy rain of sediment. Major wave surge reaches depths of 60 feet or more (Bascom, 1964) stirs up loose mud and sand on the bottom to create very turbid conditions. Hurricanes are perhaps the most dramatic example of violent wave surge that reaches depths below 60 feet. Such storms cause catastrophic bottom erosion. Studies of reef environments following hurricanes document areas on reefs where corals were either buried or undermined of as much as three feet of sand (Woodley, 1981). Major storm surge is well known to create severe siltation problems as evidenced recently by hurricane Gilbert, but even minor storms or surge caused by large swells are sufficient to stir up the bottom to depths of 30 feet. Divers need only to look at sand ripples found at these depths for proof.

Sediment reaching the sea floor poses problems for corals in yet another way. Gravity does its share in silting corals. Its force moves sediment down the reef, causing silt and sand to cascade on corals living at successively lower levels. Its pull also causes floating skeletal materials to rain down on the sea floor and everything that lives there.

Siltation is a natural problem all corals experience daily. Like people who live in snow belts, survival by reef corals demands they have a way of moving silt.

HOW CORALS FIGHT SILTATION

Cleaning the living surface of sediment is a task coral engage both passively and actively. Passive sediment shedding is accomplished through certain growth forms and currents (Horn, 1971). Thin stick forms like the yellow pencil coral (Madracis mirabilis) or staghorn coral (Acropora cervicornis) are ideally suited passive shedders. Both corals have little surface available for silt accumulation. Staghorn corals also have polyps that are widely separated, so the chance of sediment clogging oral areas is decreased even more than in branching forms with closely spaced polyps.


Another excellent architectural design for passive sediment rejection is the thin, platy and upright growth habit exhibited by lettuce leaf corals (Agaricia tenuifolia) in shallow water. Like branching corals, only a small area is present at the top of each plate for sediment accumulation. This form, coupled with an erect growth habit, is very effective in letting sediment slide passively off the colony. Despite having an excellent adaptive design, lettuce leaf corals actively rid their colonies of sediment,

Corals have four ways of sweeping silt off their living surfaces. Included are processes of: (1) polyp inflation, (2) mucus entanglement, (3) ciliary action and (4) tentacular action (Hubbard and Pockock, 1972; Schuhmacher, 1977).

Polyp inflation is a process whereby corals can puff themselves up like a balloon. All corals achieve this by drawing enough water into their bodies to cause soft tissues to expand. Polyp enlargements generally no more than doubles the normal body size. But spectacular polyp volume increases are reported for species like the fungiid, Fungia actiformis, which can enlarge the polyp about five times its normal size (Schuhmacher, 1977). Inflated polyps give coral n appearance of having polyps with bloated stomachs, but it is an effective means of shedding sediment. The swelling action causes large and small grains to simply roll off the ballooned coral tissue.

Considerable variation exists among corals in their ability to pump themselves up and so does their ability to shed silt. Corals with large inflation ratios are among the best sediment rejecters (Schuhmacher, 1977). Many are small forms living attached or loose sand bottoms. Excellent examples are the common rose coral (Mencina areolata) in Caribbean waters and the fungiid coral Diaseris in Indo-Pacific waters. Frequently covered by sediment and overturned by waves, these corals not only use polyp inflation to move sediment but also to right themselves (Hubbard and Pockock, 1972)

The ability of corals to shed sediment by inflation perhaps is exemplified most dramatically by the free-living fungiid coral Diaseris. Some species of this coral can actually escape burial even when covered by a layer of sand half an inch thick. Individuals can be seen blasting their way through layers of fine sand following siltation in an aquarium. Unlike most corals, some species of Diaseris can deflate rapidly by forceful expulsion of water. The explosive force with which this is done jettisons not only the sediment above the coral but also lifts the colony several millimeters above the sediment floor.

Mucus, a slimy but sticky organic film (Dumas and Thomassin, 1977) secreted by corals, is a natural defense against many kinds of stresses including siltation. Its sticky, web-like characteristic entangles particles whereas its slimy property allows the entire mess to slide off the coral colony. Most corals along with other cleansing processes use mucus entanglement. Two patterns of mucus entanglement were noted during siltation experiments. Corals, dependent mainly on mucus to remove silt, produce copious amounts of it, whereas those corals, which use additional ways to remove sediment, produce mucus in sparing amounts. A good example of a mucus-dependent rejecter is the meandrine brain coral (Meandrina meandrites). Strands of mucus, carrying concentrations of mud, silt and sand, can be observed sliding off their colonies shortly after siltation. Boulder corals (Montastrea annularis) exemplify the other pattern of


mucus rejection. They use ciliary and mucus action to shed particles. In these corals clots of sand bound with mucus could be seen falling off the colony after experimental silting with fine sand.

Ciliary action, movement created by vibrating hair-like growths on coral cells, accompanies more or less all sediment clearing activity. It is the moving force that sweeps sediment from coral living surfaces. Some of the fungus corals and the lobed star coral (Solenastrea hyades) depend on ciliary action alone to rid the colony of sediment. The sweeping action of these corals is organized like an assembly line with each polyp taking a turn whisking silt outward. Unfortunately, ciliary transport is quite sensitive to grain size. Removal of grains larger than half a millimeter (0.5 mm) is a sever task for most coral and for some entirely impossible.

Coral tentacles may also be used to clear silted colonies. Tentacular clearing action is especially effective for removing large particles. The process, similar to ciliary action, involves moving sand off the colony by passing it from one polyp to the next. Surprisingly, few corals can use their tentacles to remove sediment. Club finger (Porites porites) and mustard hill coral (Porites asteroides) are two notable exceptions.

SEDIMENT CLEANING ENDURANCE

Corals differ considerably in their stamina to actively clean themselves of sediment on a continual basis. Among the most hardy coral species are those which can dramatically enlarge their polyps. Inflators not only are capable of moving sediment continuously, but they can also endure siltation rates 5 to 10 times higher than those found on coral reefs (Schuhmacher, 1977).

Corals using ciliary or mucus action are more sensitive to continuous siltation as shown by experimental work. Schuhmacher (1977) demonstrated that some of these corals simply quit their cleaning action after a short period of repeated sedimentation. A continuous rain of sediment temporarily exhausts both the mucus-secreting and ciliary drive for a period of one to two days. Recovery is possible only if siltation stops during the recovery period. Continued siltation after the coral is exhausted results in death.

Another example of a coral capable of surviving heavy silting is the shallow-water starlet coral (Siderastrea radians). Living on the reef flats in shallow, high-energy environments, colonies of this coral are regularly silted or overturned by wave surge. Not only can these corals successfully contend with frequent silt on their living surface, but they also manage to survive having their polyps stuck repeatedly in the sand. During storm conditions colonies for these corals, up to six inches in diameter, actually tumble around like “rolling stones” (Glynn, 1974) on the sea floor. Despite tissue injury, the corals survive, heal and continue to grow. Their exceptional hardiness is documented in their skeletons, which consist of perfect spheres and have a radial arrangement of polyp living chambers (corallites).


MUCUS DISRUPTION AND TISSUE DAMAGE

Depicted as a delicate protective shield, the disruption of mucus by divers touching living coral is often cited as a major contributor to reef damage. Contrary to popular view, mucus is not a magical, easily damaged shield. Instead, corals form and replace mucus rapidly in response to a variety of natural processes that remove it.

Touching reef corals may not be an inquisitive probe but rather an unplanned landing for many divers. Protected by skins, wet suits, and gloves, divers that crash on corals remove mucus and inflict lesions to coral tissues. Brutal assaults with fins or kneepads cause skeletal damage. Although severe, various ecological and experimental studies suggest corals are able to confine and heal such tissue and skeletal destruction.

Fighting Tissue Damage by Mucus Secretion

Although familiar with names, shapes and the appearance of coral polyps, most conservation conscious divers know surprisingly little about the coral animal itself. This is a real problem since individuals concerned with reef conservation must know at least basic coral biology to address coral ecology issues. A basic knowledge of mucus secretion and coral tissues is especially critical in considering how their localized disruptions impact the colony.

Stony corals on reefs are nothing more than limestone boulders or branches covered by a thin veneer of living animal cells. Tiny, delicate polyps, residing in skeletal depressions, are obvious parts of the animal tissue on most corals, especially on night dives. A mucus film covering the coral surface is also well known to most divers who have touched corals with their skin. What is not apparent is the thin layer of tissues that extends between the polyps over the entire skeletal surface but beneath the mucus film. These tissues are only a few cells thick, but they perform many functions including two of concern to this discussion. The bottom tissue contains cells that make the skeleton between the polyps. The upper tissue layer includes mucus glands and hair-like cilia important in removing sediment. Hidden beneath a mucus coating and resting on the hard stony skeleton, these tissue areas seem hard and tough, but in reality they are easily damaged, delicate membranes.

Mucus Secretion and Its Disruption

Mucus is a complex organic secretion (Dumas and Thomassin, 1977) that may be compared to mucus found in congested sinuses. It is formed by corals in response to a variety of chemical and physical stimuli. Tissue damage always triggers an outpouring of mucus, but exposure to air, abnormal salinity, high temperatures, exposure to chemicals, silt, and pollutants also induce mucus secretion (Dumas and Thomassin, 1977; Patton, 1976). Every reported condition that stimulates mucus secretion of corals represents a stressful situation. So it may be inferred that mucus is a first line defense against tissue damage. Although mucus may act as a protective organic barrier during periods of stress, its disruption at other times does not appear to be a significant problem.


Studies on the feeding habits of various reef organisms show coral mucus is an important food source and can be harvested without ill effects to the coral. Crabs commonly feed on organic-rich grains that settle on corals; when the particles are not present they feed on mucus whose production they induce by scratching the coral (Patton, 1976). Members of the file, damsel, and butterfly fish groups are other mucus feeders (Benson and Muscatine, 1974). A well known example is the ornate butterfly fish (Chaetodon ornatissimus) who uses a blunt snout to scrape away mucus that was concentrated over abrasion on coral surfaces (Hobson, 1975). Certain fishes such as the multiband butterfly fish (Chaetodon multicinctus) feed on mucus and coral tissue (Hobson, 1975). Unlike mucus film disruption, tissue damage is a more serious problem.

Tissue Damage and Repair

Coral tissues, however, are easily damaged and their repair takes a long time. According to Schuhmacher (1977), large sand grains or excessive amounts of silt cause minor tissue damage if corals are unable to remove them. Extensive tissue damage involving the chemical removal of all tissue from a several inch square area may be inflicted by corals competing for space with one another (Lang, 1973). But even in cases involving tissue damage, corals survive either by healing damaged areas completely or by preventing their spread to the rest of the colony. Staghorn corals are exceptional among corals in their ability to survive despite extensive tissue damage. These corals can be broken repeatedly, yet fragmented portions of the colony continue to grow separately.

Not all corals recover from tissue damage as well as staghorn corals. Experimental studies by Bak, Brouns and Heys (1977) on leaf coral (Agaricia agaricites) and boulder coral suggest it takes healthy corals a long time to recover. They found it took about 30 days for half of the individuals they tested to completely heal tissue lesions 1/2 inch in diameter. After 80 days, only 60 to 80 percent of corals they tested healed completely. The rest took up to half a year to recover or they lost the injured portion of the colony to other organisms.

CONCLUSIONS AND SPECULATIONS

Although studies on the causes of reef damage at popular dive sites need to be initiated, existing and present research suggests silting and mucus disruptions by divers may not be as severe a problem as popularly portrayed. Corals commonly are subjected to natural rates of siltation far more than those produce by divers. The also have effective ways of moving silt off colonies when coral tissues get covered. However, corals can die from exhaustion during prolonged periods or heavy siltation events and divers should be careful not to silt corals after such events.

Mucus disruptions are normally not a problem for corals either. Divers who gently touch corals with their skin will disrupt the mucus film but normally will not harm the coral. Exceptions may occur when corals are exposed to environmental extremes and the mucus is essential protection.

Any other form of contact with living coral appears to create tissue damage that takes a long time to heal or may result in partial coral mortality.


Although corals can defend against may forms of stress, including tissue damage, it is the regularity of coral tissue damage and perhaps the addition of nutrients from sewage discharge by dive boats that is turning our reefs into algal meadows. Like the worn path in a lawn, popular dive sites can recover in time but traffic and coral tissue damage by divers must be reduced.

REFERENCES

Bascom, W., 1964, Waves and Beaches: Garden City, New York, Anchor Books Doubleday and Company, Inc., 267 p.

Benson, A. A., and L. Muscatine, 1974, Wax in coral mucus: Energy transfer from corals to reef fishes: Limnology Oceanography, v. 19, p. 810-814.

Dumas, R., and B. A. Thomassin, 1977, Protein fractions in coral and zoantharian mucus: possible evolution in coral reef environments: Proceedings: Third International Coral Reef Symposium, Biology, p. 517-524.

Glynn, P. W., 1974, Rolling stones among the Scleractinia: Mobile coralliths in the Gulf of Panama: Proceedings of the Second International Symposium on Coral Reef, p. 138-198.

Hobson, E. S., 1975, Feeding patterns among tropical reef fishes: American Scientist, v. 63, p. 382-392.

Horn, H. S., 1971, The Adaptive Geometry of Trees: Princeton, Princeton University Press, 129 p.

Hubbard, J. A. E. B., and Y. P. Pockock, 1972, Sediment rejection by scleractinian corals: A key to paleo-environmental reconstruction: Geologishe Rundschau, v. 61, p. 598-626.

Lang, J. C., 1973, Interspecific aggression by scleractinian corals. II Why the race is not always to the swift: Bulletin of Marine Science, v. 23, p. 250-279.

Patton, W. K., 1976, Animal associates of living reef corals, in O. A. Jones, and R. Endean, eds., Biology and Geology of Coral Reefs, p. 1-36.

Schuhmacher, H., 1977, Ability of fungiid corals to overcome sedimentation: Proceedings: Third International Coral Reef Symposium, Biology, p. 503-510.

Woodley, J. D., 1981, Hurricane Allen’s impact on Jamaican coral reefs: Science, v. 214, p. 749-755.

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