Updated 3 September 2023, originally published on 3 March 2019 | Part of the “A Pioneering Cruise Line” Anthology of Stories
Corals from the Raja Ampat Archipelago, Indonesia To South-East Cape, Tasmania
It is appropriate to have an expert in the field write about corals given their inclusion in our cruise company’s name and the predominance of corals and coral built reefs in the places we visit around the West Papuan islands and each of the northern Australian sites. Dr Terry Done is a retired Senior Principal Research Scientist from the Australian Institute of Marine Science where he worked mainly in Townsville and concentrated on issues related to the Great Barrier Reef. In retirement, Terry is a Guest Lecturer on several cruise itineraries. Terry’s comprehensive article on corals follows and provides a real insight into the diversity and lives of these organisms.
You see plenty of corals on Coral Expeditions’ cruises in tropical seas. With the warm sun on your back, you see them through your face-mask as you glide across a shallow reef, and you see their bleached skeletons on the beach beneath your feet as you step ashore. Coral reefs are like cities in the sea, the corals themselves building complex structures that are home to thousands of species of marine plants and animals.
Who has not marvelled at the spectacle that television brings to us of the life, movement, colour and light of tropical reefs? Nothing epitomises the splendour of coral reefs more than the reefs of the benign, warm shallow waters of Indonesia’s Raja Ampat or Australia’s Great Barrier Reef. But corals are a diverse lot of creatures, and while most of them are confined to the warmth of tropical seas, others of a hardier constitution are capable of building reefs even in cold and deep waters, far from the tropics, in places like the seamounts off Scandinavia, Alaska, northern Japan and south-east Tasmania.
What are corals, these creatures that create nature’s most vibrant marine ecosystems? Take a close look at coral skeletons you come across on the beach, and you will begin to get the idea. What you are seeing is a sunbleached and eroded fragment of a limestone skeleton that, in life, supported the soft tissues of the coral animal. With over 700 species of ‘stony’ reef-building corals worldwide, chances are you will see a variety of shapes from saucer-shaped skeletons that you can hold in your hand, to vases, sheets, cobbles, and branches as small as a toddler’s finger or as long as your own arm, that have broken from some undersea coral bush or tree. Scattered among the bleached white coral skeletons of these ‘stony’ corals are often to be found pieces of a different shape, colour or texture. These are skeletons of other coral groups that have evolved different means of housing their polyps and presenting them to the passing currents. Typically, your find may include brilliant red organ pipe corals, brown gorgonian fans or ebony twigs of a black coral tree.
A coral grows from a microscopic larva that fixes itself onto something substantial on the sea floor and then transforms itself into a polyp, similar to miniature sea anemone, with a ring of stinging feeding tentacles surrounding a central gut cavity. Unlike a true anemone, to which corals are closely related, this single polyp deposits a small limestone cup beneath itself.
Under the microscope, the cup is seen to contain elaborate scaffolding that supports the equally complex internal tissue structure of the polyp. Depending on which of the 700 species it is, this tissue carries the genetic code that determines what happens next, whether it grows into a vase, a bush, a tree, a fan, a whip, a plate, a head, a slipper or a saucer! While the growth form destination varies amazingly, the first step is the same for almost all corals. The original polyp grows, and upon reaching a certain size, it divides into two, four, eight (and so on, ad infinitum) interconnected polyps draped across the surface of the skeleton that, over periods from years to centuries, individual coral colonies deposit beneath themselves. The genetic code instructs the tissues on polyp shape and size (from millimetres to centimetres across) and the general skeleton shape that the colony adopts, be it robust wave-resistant forms up in the surf zone, or porcelain thin vases and delicate filigree lacy forms in sheltered parts of the reef. Examine any piece of coral on the beach, and you will see that its surface texture is caused by the pattern of limestone cups (called calices, singular calyx). Fresh specimens are sharp in texture but become smooth and rounded after years to decades, been rolled up and down that beach by the waves.
The process of seemingly unending replication of polyps and limestone skeleton is the basis for an increase in the size of most, but not all, corals. In the case of the mushroom coral, the polyp and its single cup grow bigger and bigger. On a beach specimen, you can clearly see that the supporting skeleton takes the form of low ridges radiating from a central slit that, in life, surrounded the polyp’s mouth into which tentacles, aligned along the seams, pass microscopic food matter. Mushroom corals are very common on reefs, sometimes just scattered among other types of corals, but often occurring in aggregations of thousands upon thousands of individuals. They are nonetheless referred to as ‘solitary corals’, in recognition of their mode of growth, and in contrast to the ‘colonial’ corals that build through replication.
As is the case for all of life on Earth, the ‘Goldilocks’ principle is at work in determining where corals occur. For corals, it is a matter of not too hot, not too cold, but just right! The ‘just right’ environment for most types of coral occurs in tropical seas, with their vast areas of shallow sea floors, island coasts and clear warm water, in which corals thrive. But some corals also do occur, and indeed, build reefs, in waters that are deep and cold, such as on the seamounts near Tasmania. Dark and cool are just right for these temperate corals. What is the difference between tropical and temperate corals? In shallow tropical seas, the coral animal lives in a mutualistic relationship with tiny single-celled algae that occur in densities of 2–5 million per square centimetre within the polyps’ tissue. Being plants, these algae (referred to as zooxanthellae) undertake photosynthesis, using sunlight to convert dissolved carbon dioxide into carbohydrates and evolving oxygen in the process. When water temperature and clarity are just right, the plants produce a massive excess of carbohydrate that is used by the coral colony to drive the biochemical processes that grow and maintain its tissues and skeleton and to produce gametes (sperm and eggs). The polyps, as their contribution to the mutualistic relationship, provide nitrogen and phosphorus through their capture of dissolved organic matter, bacteria, protozoans and both zoo- and phytoplankton.
There can be no such cosy arrangement in the cold, deep aphotic zone, where all light, the critical resource for zooxanthellae, reduces to zero through the upper 100 metres or so of the water column. Thus, these azooxanthellate corals of the cool deep are white, lacking the 2 – 5 million zooxanthellae per centimetre that masks the whiteness of the skeletons of their tropical counterparts. The paleness of deep sea corals is in marked contrast to the many-hued colours of the shallow zooxanthellae-bearing corals of the tropics. Deepsea corals, lacking the carbohydrate subsidy from the zooxanthellae and relying entirely on the food that they capture with their tentacles, grow and build reefs very slowly. A primary source of food for deep sea corals is the detritus of plankton and microbes produced at the ocean surface and subsequently fall to the ocean floor.
When circumstances permit, corals can build coral reefs. Because tropical corals have their internal food factory of zooxanthellae, they have a prodigious capacity to grow fast and build reefs at a rapid rate. The essential ingredients are lots of time, lots of corals, an existing high point on the seafloor on which corals can settle, and a pattern of waves and currents that are not too strong, not too weak and that tend to contain rather than disperse the developing reef framework.
Given that sea levels rose after the last ice age and that tropical corals are confined to the upper 60 metres, we find all the Earth’s surface-breaking reefs are therefore quite young. The sequence of events that is common to many coral reefs (including the deepsea coral reefs) is the continued occupation of the one place by successive generations of corals, with the limestone skeletons of earlier generations providing settlement places for following generations. In shallow reefs, periodically, the structure is patchily broken and corals dislodged by storm waves, thereby compacting and extending the reef across the adjacent shallow sea floor. You can see these processes at work along the shallow edges of the reefs of Indonesia’s Raja Ampat and the Great Barrier Reef, where a low tidal range allows corals to thrive in the shallow back reef, up the reef slopes and onto the reef flat.
At Montgomery Reef in the Kimberley, there is an interesting variation on this theme. Here the corals live on, in and around a vast rock platform and are uncovered to varying degrees as the water partially drains from the reef ’s huge impounded lagoons during low tides. Water drainage from the lagoon is impeded by large containment banks around the perimeter formed primarily from rhodoliths, which are coral-like stony structures but formed by red algae. They are unattached, up to 12 centimetres in diameter and accumulate at the perimeter of the lagoons through currents and tidal movements. On Montgomery Reef, encrusting coralline algae are the only marine life physically and physiologically tough enough to build reef framework in that area’s huge intertidal zone, down to around 10 metres. Stony corals that do settle in the intertidal zone have a relatively short life expectancy and are therefore small. Soft corals that lack a stony skeleton but instead support themselves with hydrostatic pressure and a scaffold of spicules are common and often quite large in Montgomery Reef ’s shallows within the lagoons. Because they are filled with sea-water, they do not desiccate when the reef is high and dry and exposed to the hot Kimberley sun. By contrast, the tissues of stony corals are thin and prone to desiccate. Both types produce sunscreens and mucus to protect themselves, with the mucus washing off the reef in the region’s world famous cascades adding to the striking foamy patterns that surround expeditioners as we explore this amazing place.