Situated some 32 kilometers (20 miles) off the Saudi city of Jubail is Jurayd Island, the southernmost of the chain of six coral-reef islands that exist here where coral islands should not exist. They support a unique ecosystem despite physical and chemical conditions that are normally considered to be too harsh to permit the varied interdependent life forms that have developed on them over the ages.
The Arabian Gulf is a narrow, shallow body of water. No rivers flow into it from the Saudi side; its single significant water exchange is with the Indian Ocean, and - estimates vary - it takes somewhere between 14 and 200 years for its entire volume to be replaced by intake and outflow through the Strait of Hormuz. Because of these characteristics and the severe climate, the marine environment of the Gulf is correspondingly severe. Water salinity and temperatures, greatly affected by meteorological conditions, are as extreme and changeable as anywhere in the world. Saudi coastal water temperatures range seasonally between 10 and 35 degrees centigrade (50 and 95°F). Salinity fluctuates seasonally at all Gulf locations, ranging between 38 and 70 parts per thousand (ppt) along the Saudi coast - up to double the 35-ppt salinity of "normal" open-ocean water.
Widely varying temperatures, and salinity that is excessive even by a few parts per thousand, create stressful conditions for most marine organisms, and the Arabian Gulf environment has thus usually been considered too harsh to support more than a minimal marine community, made up of a handful of particularly resistant species. At most localities near the coast, this is in fact the case - but not on the islands.
The Gulf islands are far enough offshore to be surrounded by relatively deep water and washed by constant currents. These two factors slightly reduce the large fluctuations of temperature and salinity that occur in the very shallow, near-stagnant coastal waters. The resulting conditions - temperatures between 15 and 33 degrees centigrade (59 and 93°F) and salinity ranging from 38 to 42 ppt - are still inhospitable to most organisms, but they are constant enough that an ecosystem has developed that far outstrips those of the mainland shores in complexity.
The six islands, south to north, are Jurayd, Jana, Kurayn, Karan, 'Arabiya and Harqus. Karan is the largest, at 1.3 square kilometers (half a square mile, or 320 acres) and Harqus the smallest at 0.2 square kilometer (less than 50 acres). Harqus, in fact, is so small and low that severe storms occasionally produce waves high enough to wash over it - and it is thus also the only island completely devoid of vegetation. All of the islands support slightly different biological assemblages, but all, even intermittent Harqus, have many physical features in common.
All the islands are coral-reef islands, formed when sand, produced by wave action and other factors, builds up on a submerged coral reef and finally breaks through the water's surface. Each island continues to grow as more sand accumulates, then a plant community gradually develops, and some permanency is achieved as sand is gradually cemented into intertidal beach rock. Harqus is the least developed of the islands.
The birds are the most prominent island inhabitants, especially to casual visitors. Winter and spring bring many species across the Arabian Peninsula on annual migrations between Europe or Asia and Africa (See Aramco World, November-December 1986), and the Saudi Gulf islands accommodate their share of these transients. On any given day during the peak of the migrations, a bird watcher there can never be sure what species he might see. More significant to island ecology, however, are the birds that rely on these islands for their existence, the birds that nest there.
Nesting seasons on the islands vary somewhat according to weather and food availability. Depending upon the bird species, however, they generally run from late winter to late summer. During this period, huge numbers of birds inhabit the islands.
The Socotra cormorants (Phalacrocorax nigrogularis) are the first nest-builders to arrive; they usually form only one modest colony of several hundred birds on Kurayn. They arrive during the cold of winter and, to create nests, scoop out depressions in the sand that are sometimes surrounded with small pebbles cemented into quasi-permanence by deposits of excreta. Several large, chalky blue eggs are laid in each nest, and incubation and the rearing of the young may continue for many months. Unfortunately, the cormorants bring with them hordes parasitic mites which infest the nesting area during their stay - and will also infes. the unwary photographer who ventures too close.
Four species of tern commonly nest on the islands: swift terns (Sterna bergii), lesser crested terns (Sterna bengalensis), bridled terns (Sterna anaethetus) and white-cheeked terns (Sterna repressa). The species normally arrive at the islands at slightly different times in the spring and early summer and tend to form separate nesting colonies, since each species favors slightly different areas of the islands (See Aramco World, September-October 1980). Occasionally the colonies are immediately adjacent to one another, and it is not too unusual for swift terns and lesser crested terns to intermingle, if preferred nesting areas are occupied.
The rest of the islands' wildlife is nocturnal, a way of life that brings several benefits. It permits creatures to escape the harsh sunlight and temperatures typical of the Saudi day, and it curbs predation to some extent by hiding animals from their enemies under the cloak of night. Unfortunately for human observers, it also means that the animals must be observed after nightfall, at the end of what is usually a long and trying day of getting to the islands and setting up a campsite.
Seasonally, the islands undergo population explosions of their largest warmblooded animal, the common house mouse (Mus musculus.) Mice were probably introduced to the islands as stowaways from passing boats, since sailors aboard local dhows occasionally stop at the islands to light a campfire and cook. Normally sparse food supplies ashore limit the mouse populations through most of the year, until spring - and the birds - arrive. Then, the abundance of eggs and chicks provides windfall food supplies of which the mice take full advantage. Though they are unable or unwilling to break eggs or kill chicks themselves, the raucous behavior of nesting birds provides sufficient cracked eggs and dead chicks for a veritable feast. The result is increased survival of young mice, and an upswing in their numbers.
In addition to consuming eggs and chicks, the mice roam the entire island and eat whatever animal or vegetable material they can find that will sustain them. Visitors who moor boats by tossing an anchor into the low vegetation at the top of the beach are apt to find their craft seething with hungry mice when they return to it. Summer campers on the island often become very aware of the mice during the nights, when their campsites are overrun with furry visitors searching for groceries.
In normal times, early summer signals the onset of sea-turtle nesting. Adult hawksbills (Eretmochelys imbricata) - endangered by demand for tortoiseshell made from their beautiful carapaces - are the first to appear, gathering around the islands in late spring to begin mating activities. By mid-May the females begin crawling ashore at night to deposit eggs in large pits they dig on the upper beach -the only reason sea turtles, true deep-water creatures, ever come ashore. Within a month after the first hawksbills appear, the larger green turtles (Chelonia mydas) show up. They too are endangered, over-exploited worldwide for their meat, hides and eggs. From skeletal remains found, it is suspected - but not verified - that loggerhead turtles {Carreta carreta) also occasionally nest on the islands. Mating and nesting continue for most of the summer.
By September, turtles begin leaving island waters. The only traces they leave behind are their large nest pits on the beaches and the nightly hatching of their young, with their dramatic and dangerous trek across the beach to the water where they will spend their lives. To a limited extent, sea turtles also nest on the Saudi mainland, but predation and increasing competition for habitat have driven most turtles off mainland beaches. Well in excess of 90 percent of the Saudi sea-turtle population nests on the islands, and of the six, Karan is the favored nesting ground for these endangered animals.
Intertidally - that is, in shore areas between the low - and high-tide marks - the beaches support the largest and most diverse permanent community of animals on the islands. But though sandy beaches surround most of every island, they are not where the majority of animal life is found: Rocky intertidal areas are. There are no large animals permanently residing here, but some of the small ones are spectacular in their ecological roles.
Ghost crabs (Ocypode samtari) are fast, agile predators that favor the sand beach, but cover most of the island in their quest for food. They prowl much the same areas as the mice, but are much bolder and more aggressive, and tend to ignore vegetable matter in favor of live prey, which they capture with their speed and their powerful pincers. All other small animals on the island serve as food for the ghost crabs. They often construct conical mounds of sand at the entrances of the burrows in which they spend the daylight hours; counting the mounds thus gives a daytime clue to their population density.
Another animal at home on all areas of the island beach is the land hermit crab (Coenobita sp.). Though they prefer rockier beach and tend not to venture as far inland as mice and ghost crabs, hermit crabs are found on all types of beaches. These small crabs usually tuck their vulnerable rear ends into the shell of some dead gastropod, or snail, but they can use any available item with a suitably-sized hole as a residence - including bottle tops discarded from passing tankers. They eat both plants and animals but have very limited ability to catch and overpower prey.
The rocky beaches have one undis-putedly dominant animal, the predatory rock crab (Eriphia sebana smithii). Cryptic coloration, inconspicuous shape and power - not speed and agility - allow this crab to dominate the rocks at night. Its hunting technique is the ambush: It hides motionless and strikes unwary passers-by with strong crushing claws. Rock crabs can catch and subdue any member of the island community, and no creature -including other rock crabs - is safe from their attack.
Perhaps the most spectacular-looking rock dweller is the sally-lightfoot crab (Grapsus tenuicrustatus). Large and often gaudily colored, these agile crabs scuttle sidewise over the rocks. Opportunistic feeders, they are usually seen picking through the algae growing on the rocks, but they will eat a wide variety of food. Adult males are easily differentiated from females by their larger, showier claws.
Shell-less snails are the most frequently encountered of the marine grazing community that depends on algae growing on the rocks for its sustenance. These creatures are sea slugs (Onchidium peronii). They can remain out of the water for extended periods thanks to a modified mantle cavity that acts as a lung. Far from eye-catching in color or shape, these cryptic slugs are often discovered only when the unwary exylorer steps on one. Their unassuming appearance, however, belies their complexity, for these slugs are hermaphrodites, with male and female organs in the same body.
Self-impregnation is not advantageous, however, so sea-slug reproduction is a somewhat complicated process, accomplished by mutual hypodermic impregnation with a calcareous spike. In the mating ritual, one slug pursues and overtakes another; the two then circle each other, rear up over each other's back, and lunge with their spikes; they thus simula-taneously impregnate their partners and are impregnated.
Perhaps the most surprising animal found on the beach rock, however, is a fish of the blenny family, Istiblennius lineatus. Fish are not normally intertidal animals, but these fish can retain water in their bodies, which keeps their gills wet and makes brief land excursions possible. The blenny squirms its way onto the rocks at night to graze on the algae growing there, sometimes several meters from the water. Nocturnal, dry-land foraging allows the blenny to avoid wave surges and the high visibility of daylight that would make him easy prey for some predators.
The Arabian Gulf islands provide numerous subjects for observation and study. The animal community they support changes from day to night, full moon to new moon, season to season and year to year. The ecological parameters that allow the creatures to dwell on the islands are often near the animals' tolerance limits, and slight changes in conditions might produce sweeping changes in island ecology. For areas of such restricted size, the coral islands of Saudi Arabia play a large and important role in the overall ecosystem of the Arabian Gulf.
Brock Stanaland was born in Dhahran, Saudi Arabia, and returned there as an adult to work as a marine biologist. He is now a commercial fisherman in Jupiter, Florida.
