Two Reefs in the Maldives

Part 1: "A shallow lagoon at Kanifinolhu "

By Alf Jacob Nilsen, Norway

Reprinted with permission from Das Aquarium

Maldives.....tropical dreams in the Indian Ocean. A long way from the Norwegian wet and windy summer. Coco-nut palms, crystal clear water and lots of exotic water life - it sounds like a dream.

Not many years ago these coral islands had not been discovered by tourism, and was poorly known. Today nearly every tourist-magazine has the Maldives as an exotic goal. This is both good and sad. As the tourism has increased, the effects on the reefs are undoubtedly negative. They are badly ruined at some islands. But still some wonderful reefs can be seen, and for those doing SCUBA diving, a fantasy world beyond the reach of any snorkeler is still intact.

However, to many people visiting the Maldives the coral reef is something totally new - a strange world of "flowers", easy to pick and bring back home for the window ornament. The easy picking and careless swimming may be a result of a lack of understanding for the reef biology. Just by the use of mask and fins it is possible to observe much of the fantastic biology of a coral reef and to observe the most peculiar form of life.

The purpose with our tour in the summer of 89 was to have a pleasant holiday, do a lot of diving and do some simple biological reef-research. As we were gone for three weeks, we would like to stay at two very different island. We did a lot of planning prior to the travel, and finally ended up by choosing "Kanifinolhu" in the North Male Atoll and "Embudu Village" in the South Male Atoll.

In these two articles we shall analyse two particular reef spots at these two very different islands in order to increase our knowledge on reef biology.

KANIFINOLHU

The island of Kanifinolhu is situated in the North Male Atoll, and is simply called "Kani". It lies on the eastern side and at the very edge of the atoll. Like many Maldivian tourists resorts it is build in a friendly style with small cabins directly on the beach. The inhabitants are very friendly too, but must have been very wondered at our luxurious way of living. The island have a well equipped diving center and a diving boat leaving for excellent diving spots twice a day. However, Kanifinolhu is not the right island to pick if one wants to do snorkeling only. The house reefs at Kanifinolhu is situated far from the beach, they are poorly developed and not very interesting. Like many of the island situated on the edge of an atoll, Kanifinolhu is surrounded by large sandflats and on the eastern side heavily exposed to waves and wind.

Luckily we were given three nice cabins at the northern side of the island. In front of us was a shallow lagoon with the maximum depth of 2 meters of water at high tide. This lagoon reached app. 150 meters from the beach. On the outer edge huge waves washed ashore the whole day long.

Although we were a bit disappointed with the house reefs, we soon found this lagoon to be most interesting, and were going to spend several hours each day for the next 10 days exploring it.

Abiotic factors influencing the lagoon.

Some very distinct factors did have their influence on this shallow lagoon.

First it is the shallow water depth. With a maximum depth of app. 2 meters at high tide, many of the animals were exposed to an intense sunlight. We could not measure the illumination, but it must have exceeded 200,000 lux at the surface on mid day.

Secondly the tide was regular with a low tide in the morning and the highest tide occurring in the afternoon. The difference between high and low tide was about 0.75 meter.

This meant that some of the patch-reefs in the lagoon were regularly exposed to open air.

Thirdly there was a strong and changing current flowing in or out of the atoll. The current shifted twice a day and was sometimes so strong that swimming in the lagoon was very hard. The currents made the water in the lagoon quiet muddy, and many of the corals were for this reason covered with particles from time to time.

Fourthly the outer part of the lagoon was exposed to a huge wave action, which also brought particles into the lagoon.

It is all these factors that have influenced the growth of the hermatypic corals in the lagoon which in turn have given it its shape.

Different zones.

It was very easy to see that the lagoon was naturally divided into different zones with their own typical fauna. We use the term "zone" in these article as the name of a limited area in the locations that clearly differed from another area either in animal and/or plant life or in one or more abiotic factors. There was of course a great deal of overlapping between each zone. The organisms mentioned here as "typical", should be regarded as "the most dominant species of each zone" as it looked to us who, however, were unable to carry out a detailed investigation of the zonation.

The first zone is the beach itself with pure, clean coral sand. Zone 1 was never covered with water, while zone 2 was above the water at low tide and partly overflown at the highest tide, when small waves washed ashore. The only major organism here was the Stalk-Eye-Crab Ocypode saratan from the family Ocypodidae, and a common species in the Indian Ocean and in the Red Sea. It is a very interesting species as the males, being much larger than the females, build 15 cm high sand mounds in front of their burrows in order to attract the females. Even if their look is quite furious the crabs are harmless. They were most active in early morning, late afternoon and in the night when they searched for food along the tide line.

Further out, approximately 15 meters from the shore, there was a zone with sand and a few boulders and dead coral-fragments scattered on the bottom. The most easily organism to observe here was the pistol shrimps from the genus Alpheus together with their gobies belonging to the genus Amblyeleotris . They were very shy and one had to lay very still for several minutes in order to get a glimpse of this very interesting symbionts. If one turned one or two of the boulders which were scattered on the sandy bottom, a variety of small crustaceans appeared. They became the excellent meal for Thalassoma hardwickei , one of the many species of lippfishes which occurred in most parts of the lagoon and gathered around us all the time.

At nighttime strange anemones living in tubes in the sand were especially abundant in zone 3. It might have been the species Pachycerianthus mana (order Ceriathipatharia), which is quite common on sand flats on Indian Oceans reefs, but only expanded at night.

This zone was never exposed to open air as long as we was there, but as the depth at shallow tide was only about a foot, the illumination was extremely high. There was many particles floating in the water when small waves rolled upon the coral beach. Besides this the tourists swimming from the beach would easily disturb any life in this zone.

In the outer edge of this zone and in the next zone we found the first stony coral, Stylophora pistillata in the family Pocilloporidae. This is an extremely hardy species. Larvae can attach themselves to floating objects and in this way being transported while they are growing. For this reason the species are widely distributed in the Indio-Pacific; from South Africa to the Red Sea further east to Japan and the Tuamoutu Island in the eastern part of the Pacific Ocean. The colonies can have different colours from white to red, but in our lagoon they were all yellow. The common fish Dascyllus aruanus seeked protection among their branches.

This zone was also the playground for "Humu-humu-nuku-nuku-apuaa" which of course means "The fish which carries a needle and has a snout and grunts like a pig" - the national fish of Hawaii, the triggerfish Rhinecanthus aculeatus . The species is widely spread in the Indo-Pacific. In our lagoon it seemed to love very shallow and highly light-exposed sandy flats. Here it hide itself in small holes in the sand when we approached.

The very strange Zoanthidea Palythoa tuberculosa was quiet common in the outer part of zone 4. The polyps have joined in a common coenenchym that became an efficient adaptation to the strong sunlight. The colonies can easily be mistaken for soft corals, but very few species of soft corals can live in the extreme illumination in a shallow lagoon. We did find a few colonies of Sarcophyton trochelioporum , but soft corals were very rare. Palythoa tuberculosa is a very good example of a organism that is adapted to a life in the lagoon. We did also find some colonies at 10 meters on the reefs between Kanifinolhu and Lohifushi when diving there.

The most peculiar animal in zone 4 was, however, several colonies of sponges which were growing in the most intense illumination. This is quiet uncommon to sponges. Although many sponges contain symbiotic blue green algaes, they cannot accept high illumination, but is very abundant from 15 - 30 meters depth in most reefs. Here we did have a species, perhaps from the genus Phakellia, which seemed to love strong sunlight.

The zones that have the maximum depth in the lagoon which is not more than 2 meters at high tide. Here microatolls were abundant. A microatoll is a small structure resembling a model of an atoll which is formed by corals growing towards the surface. In shallow lagoons and reef flats they soon reach the surface and then the top of the growth dies due to ultraviolet radiation and exposure to open air. The living tissue continues to grow sidewards and eventually small structures resembling atolls are formed.

At extremely low tide occurring in the tropical morning, the highest parts of the microatolls were fully exposed to open air and the corals must have suffered a lot from high UV-radiation. When the water level rose again, an excellent alternating water movement swept over these zones.

Zone 5 differed very much from any other zone in the lagoon by the almost monotypical growth of the beautiful, "blue coral" - the octocoral Heliopora corulea . The colonies were growing in rows with clean, white coral sand lying between them. These strange structures were obvious shaped by the current that most of the day was flowing at high speed through this zone. It was a glorious sight as many colonies have grown to an almost circular shape. We observed them to have several of the polyps expanded even at daytime, but at night they were all fully expanded. Among the branches lived several red crabs which hide completely during the day.

Mixed with the "blue coral" were small populations of the stony corals Pocillopora damicornis , Acropora formosa and Acropora divaricata . Between them were numbers of solitary corals from the family Fungiidae, mostly the species Fungia fungites . Their yong polyps are called "acanthocauli" and live attached to boulders and corals. In our lagoon they could be counted in numbers. Dense populations of the coraline algae Halimeda opuntia and the brown algae Turbinaria ornata were also quiet abundant in zone 5.

The small and not very colourful fish Stegastes fasciolatus , were numerous together with the barsh Ephinephelus aurolineatus . Several groups of Chromis sp. and of course the very common lippfish and some species of butterfly fishes, especially the beautiful Chaetodon trifasciatus , which is impossible to keep in captivity, seemed to love this place. As we approached the next zone the number of fish species increased further.

If we turned some boulders (being very careful to return them to their normal position), we found a beautiful and diverse growth of sponges, mussels, worms, crustaceans and calcareous algaes. We did even find one species of Caulerpa , perhaps C. peltata , growing on the downward side of some boulders. Here it was protected from the strongest light. This reminds us once again of the harmful UV-light that few animals can tolerate in larger amounts.

During nighttime the outer part of the lagoon were densely populated with sea urchins, coursed by the relative high growth of algae on dead corals and boulders. The species Echinometra mathai was the dominating one, but the beautiful Echinometra molaris with long, purple spines did also occur in large numbers. It drilled small holes in the boulders where it spent the day and went hunting at night. We did also see the lovely "pencil-urchin" Heterocentrotus mamillatus , and another long-spined species from the genus Echinothrix .

The next zone had the highest diversity of stony corals. At least three species of Acropora were abundant. An identification of Scleractinian corals is not easy, but we were almost sure that the dominating species of Acropora were A. tenuis , A. formosa and A. divaricata . Among them were colonies of Goniopora sp., Porites sp., Galaxea sp. and Pocillopora damicornis and several other stony corals which were not so abundant and hard to identify.

Both species of giant clams that exist in the Maldives were present here. A few numbers of the large Tridacna squamosa lived on the sandy bottom. Their mantel was scattered brown. None of them were attached, but some were totally surrounded by Acropora colonies and could not move at all. In the dead coral boulders hundreds of Tridacna maxima were burrowed. Their mantel were beautiful crystal blue, but the individuals were quiet small, not more than 7-10 cm in length. Their only food is delivered from the symbiotic algaes, and the intense light in a shallow lagoon is perfect for giant calms.

The fishes in this zone were very numerous. We did see large schools of Acanthurus triostegus and Gnatodentex aurolineatus , the latter belongs to the family Lutjianiidae and is a very common species in all Maldivian reefs. Several species of Scaridae were common and some of them joined in large groups from time to time. Among the branches of Acropora sp. lived Stegastes fasciolatus and Dascyllus aruanus . A large murena from the genus Gymnothorax showed up one morning, and beneath the Acropora colonies several Myripristis sp. and the beautiful Sargocentron spinniferum were hiding. The only member of the genus Centropyge that was common in the lagoon was C. multispinis , and this was in fact the only Centropyge species we did see during the three weeks of snorkeling and diving.

As we approached the reef edge the depth decreased rapidly and the growth of corals stopped. It was impossible to swim as far out as to the reef edge. The wave-action was to great and the strong current would have carried us away. This influenced the growth of all sessile animals and almost nothing living was seen - the zone was a desert of crushed corals.

When we dived outside the lagoon we found the most beautiful reefslope with an abundant coral and fishlife. However, 10 days on a tropical island is not much - especially when the sun is shining from a crystal clear sky all the time. Our days at Kanifinolhu came to an end and we headed for the next island...."Embudu Village" in the South Male Atoll.