Sea aneamones are very similar creatures to coral, but unlike the latter, do not possess a calcareous external skeleton. A constant circulation of water through the central cavity of the anemone brings them a host of minute marine organisms to dine upon, although like corals, sea anemones receive most of their nutrition from photosynthetic algae living in their tissues. To assist them to stun their larger victims, sea anemones are equipped with a vicious armoury of stinging cells which discharge a barbed, poison-tipped thread into anything that comes into contact with them. Not that this deters the passing sea slug who feasts on sea anemones without undue discomfort. Indeed, when an anemone is eaten by a sea slug, its stinging cells are not digested butare passed through the sea slug’s body to be incorporated in the latter’s tentacle-like projections (cerata) as part of the sea slug’s own defence system.
Coral reefs afford a number of different habitats, each of which is hometo a characteristic set of fish species. Each type of fish exploits different aspects of their habitat so that a great many species can occupy the same ecological niche without undue competition. Some, like the ubiquitous parrotefish, are herbivores and graze on the thin film of algae that grows on all bare surfaces or else nibble at the wavingfronds of more leafy types of algae. Others, like the butterflyfish, dine on the coral itself. Puffer fish are able to digest even the calcareous exoskeleton of the coral polyp and have no trouble with other variously armoured invertebrates such as sea urchins, crustacea and stariish. Most species of reef fish, however, are carnivores, ranging from the diminutive gobies who munch on minuscule crustacea, to the mightiest shark species which prey on large fish, turtles and even other sharks. Many of these killers are especially active at dawn and dusk when the crepuscular light beneath the waves makes them less visible to their potential prey.
The Ocean Depths
Many creatures ofthe open sea find their daily sustenance in the countless billions of tiny photosynthetic organisms known as phytoplankton that fill the world’s oceans. Only the upper waters of the ocean receive sufficient light to enable the phytoplankton to go about their businessand as one might expect, phytoplankton are especially abundant in tropical seas which receive plenty of sunlight—the waters off Bali are among the world’s richest in this respect. This invisible ‘forest’ of phyroplankton provides food for an equaly numerous army of microscopic animals,the zooplankton, which feedon them. Together, they stand at the bottom of an oceanic food chain which feeds some of the mightiest creatures living today—the whales. These behemoths of the ocean waves dine on plankton, squid, small fish and krill (tiny crustacea) whichthey filter from the sea by means of agiant sieve-like structure in their mouths, consisting of hundreds ofslender whalebone plates, fringedwith bristles, which are suspended from their upper jaws. Some 18 species of whale occur in Balinese waters, though they are unlikely to be seen from the shore. Dolphins, however, are quite a common sight, especially from boats.
Marine turtles are quite at home in the deepest of waters but must come onto dry land to lay their eggs. Often, they will cross thousands of kilometres of ocean to return to their natal nesting beaches—it is thought that scent trails may assist them in this remarkable feat of navigation. The seas around Bali are frequented by three species of turtle. They are the green turtle (Cheloniamydas), the learherback (Dermochelyscoriacea) and the hawksbill (Eretmochelysimbricata). They generally lay their eggs in sandy nests on the beaches of Bali’s northwestern coastline. Hawksbill turtles also come ashore on the southern beaches of Nusa Dua, while green turtles visit the island of Nusa Penida. Marine turtles nest all year round, but seem to prefer to lay their eggs during the rainy season from September to December
Squid are literally jet-propelled creatures who use a powerful combination of muscles working in opposition to each other to expel water from their body cavities, thereby effecting a forward motion, They are shaped like torpedoes for minimum drag, and lacking any inherent buoyancy, must keep swimming or else sink—position-maintenance speed is one tenth of the maximum possible. Squid are also able to change their colour at will. Colour pigments, linked to muscle fibres, expand or contract in response to nerve signals, and when angry, alarmed or sexually aroused, many species pulse with vivid bands of colour of startling intensity, which sweep over the creature in iridescent waves.
Oceanic Whitetip Shark
The oceanic whitetip shark(Carcharhinuslongimanus) cruisesthe surface layers uf the open seadown to a depth of 500 metres. Itgrows to some 2 metres in length and distinguished by its distinctivewhite-tipped fins. It is :1 ferociouspredator and is extremelydangerous.