Tuesday, December 25, 2012

Hard Corals (Phylum Cnidaria: Order Scleractinia) of Singapore

Hard corals (phylum Cnidaria, class Anthozoa, subclass Hexacorallia, order Scleractinia) are radially symmetrical animals with hard calcareous skeletons. Like other cnidarians, they possess explosive, harpoon-like cells called cnidocytes. Each cnidocyte contains a secretory organelle (cnidae), which can be a nematocyst that discharges a harpoon-like stinger carrying toxins, a ptychocyst that discharges sticky substances, or a spirocyst that discharges lasso-like threads. Hence while cnidocytes are often called "stinging cells", they do perform other functions apart from stinging.

Coral polyps
The coral animal, or polyp, has a simple body comprising a stomach (coelenteron) and a mouth surrounded by tentacles (where most of the cnidocytes are located), appearing like a little flower. The tentacles occur in multiples of six, and hence they are in the subclass Hexacorallia. It does not have an anus, and hence the mouth performs both functions of ingesting food and removing waste. The polyp resides in a calcareous structure called a corallite, and most species can clone themselves into huge colonies stretching many metres, comprising thousands of corallites side-by-side.

Coral Reef
The many colonies of hard corals occuring side-by-side form a coral reef. Most corals appear brownish, due to the unicellular algae, zooxanthellae, which live inside them. Corals with zooxanthellae are called hermatypic corals, and they have extensive calcareous skeletons which contribute to reef-building. Corals without zooxanthellae are called ahermatypic corals, and they do not contribute to reef-building. The zooxanthellae produce food through photosynthesis and share the food with the host corals, and in return get shelter and nutrients (waste products of the corals). While hard corals also actively feed on plankton with their tentacles, studies have shown that the zooxanthellae sometimes contribute to as much as 95 percent of the corals' nutritional needs.

While most corals appear brownish, there are some that are vividly coloured too. The bright colours mostly come from the corals' own colour pigments, believed to act as a "sun block" to protect the corals from ultraviolet rays. In some cases, the zooxanthellae may be brightly coloured as well.

Coral bleaching
Sometimes, the corals may expel their zooxanthellae or the latter may leave the coral due to environmental stress, resulting in a phenomenon known as coral bleaching. Without the zooxanthellae giving the corals their base colour, the corals turn white and appear bleached. One of the main causes of coral bleaching is the rise in water temperatures. If the situation does not improve and the corals cannot recruit new zooxanthellae to replace the lost ones, they may die.

Intratenacular budding and extratentacular budding
Hard corals can reproduce asexually or sexually. In asexual reproduction, the polyp can split into two (intratenacular budding), or a new clone may grow from the base of an existing polyp (extratentacular budding). Sometimes, part of a coral may be broken off and moved by the currents to another location, where it may attach itself to the substrate and form a new colony if the conditions are right. In sexual reproduction, some species broadcast their eggs and sperm into the sea in mass synchronous spawning, while others fertilise their eggs internally. The fertilised egg develops into a free-swimming larva called a planula, which will cement itself onto a suitable hard substrate, builds a corallite, secretes a hard skeleton, and eventually clones itself to form a colony.

Parts of a corallite
Singapore has more than 260 species of hard corals, and to identify them, it is important to know the parts of a corallite. Much of my knowledge on coral identification was gained from marine biologists Jani Thuaibah and Michelle Lee (thanks both!). My knowledge is certainly nowhere near to theirs, and I can only try to identify many of them to their genera. So if you find any mistakes below, do let me know!

Family Acroporidae

Acroporid corals are usually the dominant corals in pristine reefs, as they tend to be fast growing. However, they are highly dependent on the zooxanthellae for nutrition, and hence prefer clear water which allows more sunlight to reach the sea bed for the algae to photosynthesize better. As such, they are less abundant in Singapore's murky waters, though the diversity remains rich.

Acropora spp.
Acropora spp. often exhibit branching growth forms, and can be identified by a prominent corallite at the tip of each branch. The coral colony as a whole can be bush-like, antler-like or table-like with a flattish top. Some may be encrusting, forming a layer of finger-like structures over rocks or bigger shells. Due to the variety in growth forms, they have been given names such as table coral, staghorn coral, branching coral, and even finger coral.

Montipora spp.
Montipora spp. can be recognised by the papillate, granulated or spiny surface, and the corallite lacks a columella. The colonies may occur in branching, encrusting, or plate-like growth forms. Sometimes, the same colony may exhibit a few forms together, such as being plate-like at the edge of the colony, but have branching structures in the middle. They can appear rather similar to some Porities corals, but the latter has an obvious columella in the middle of the corallite, and does not appear "hollow" like the Montipora corals.

Astreopora spp.
Astreopora spp. often occur in massive boulder-like colonies, though smaller colonies may also occur in encrusting forms. The corallites are conical, and hollow-looking with deep-seated columella, and the coenosteum is granulated. Astreopora corals can be differentiated from the similar-looking Turbinaria corals by the granulated surface and deep-seated columella, as the latter has a smoother surface and a shallow columella.

Family Agariciidae

Agariciid corals generally have small polyps and corallites with indistinct walls. The corallites are situated within depressions or valleys separated by ridges. They generally have a leafy or plate-like appearance.

Pavona spp.
Pavona spp. can be recognised by their corallites, which are separated by prominent septo-costae forming intricate patterns with poorly defined walls. The colony usually exhibits a leafy or plate-like appearance.

Pachyseris spp.
Pachyseris spp. have prominent ridges that are mostly parallel to the edge of the colony. The two species recorded from Singapore are fairly easy to differentiate most of the time: Pachyseris rugosa (main picture) with irregular ridges running in various directions; and Pachyseris speciosa (inset) with concentric ridges.

Family Dendrophylliidae

Dendrophylliid corals generally have rounded and tubular corallites. They can tolerate low light conditions, and hence are commonly seen in local waters. Some species harbour zooxanthellae, while others lack the algae and feed on plankton.

Turbinaria spp.
Turbinaria spp. have big and round (usually tubular) corallites (wider than 0.5cm) with broad and compact columellae. They harbour zooxanthellae, and the colony can have leafy, plate-like, boulder like or columnar growth forms. In some species, the leafy or plate-like structures may twist and overlap each other, giving the colony a turban-like appearance, and hence they are sometimes called turban corals.

Tubastrea spp.
Tubastrea spp. often come in bright colours, which are due to the corals' own colour pigments since they lack zooxanthellae. They have cup-like, circular corallites, and the septa are not fused in adults. The polyps are large and fleshy. The species most commonly seen in Singapore is Tubastraea aurea, which is bright orange in colour. Tubastraea diaphana, which is black in colour, is more often seen in deeper waters. This group is not very well-studied in Singapore, and hence there are probably many species that are not recorded.

Dendrophyllia sp.
Dendrophyllia spp. can appear very similar to the previous species, as they can also come in bright colours, and may have circular to oval corallites. They can, however, be differentiated by their fused septa, forming Y-shaped patterns converging towards the middle of the corallite. The polyps are usually larger than those of the previous species as well. This group is also not very well-studied in Singapore.

Family Caryophylliidae

Caryophylliid corals have prominent polyps which conceal the skeleton when they are extended. The polyps are usually thick and inflated in the day, but deflated when it gets dark.

Euphyllia spp. have fleshy tentacles with U-shaped tips, rounded tips or branching tentacles with both. Those with U-shaped tips are often called anchor corals. The corallites lack a columella, and the septa are prominent and smooth-edged. The corallites are trumpet-like with separate walls, and have rounded, meandering or branching outlines.

Plerogyra sinuosa
Plerogyra sinuosa is the only species of its genus that has been recorded from Singapore. It has bubble-like vesiclesIt has trumpet-like corallites with separate walls. I have only seen this while diving in local waters, but never in the intertidal area.

Physogyra lichtensteini
Physogyra lichtensteini is the only species from its genus that has been recorded from Singapore. It has bubble-like vesicles, but often with pointed tips. The colonies have corallites with meandering outlines and short, widely spaced valleys. It appears to be uncommon in shallow waters, as I have only seen it a few times in the intertidal area.

Family Faviidae

This is the largest family of hard corals in the world with more than 25 known genera. Many faviids have massive, boulder-like colonies, while others can be encrusting, plate-like, columnar or even branching. The family name comes from the Latin word favus, which means "honeycomb", as many faviids colonies consist of tightly packed, small rounded or angular corallites which give them the "honeycomb" appearance.

Caulastrea spp.
Caulastrea spp. generally have phaceloid colonies (with trumpet-like corallites). It may resemble Lobophyllia corals, but unlike the latter it lacks the sharp, protruding teeth found on the septa of the latter. The polyps may have lighter-coloured stripes on them.

Cyphastrea spp.
Cyphastrea spp. generally have small, round, protruding coralllites (smaller than 0.5cm) with separate walls, prominent septa and columella. There are 10 primary septa (check for thick and complete ones).

Oulastrea crispata
Oulastrea crispata is the only species in its genus, and is easily recognised by the small encrusting colonies, black coenosteum and protruding corallites with white septa.

Diploastrea heliopora
Diploastrea heliopora is also the only species of its genus, and can be identified by the shallowly dome-shaped corallites, very thick walls, and small openings.

Favia spp.
Favia spp. have corallites with separate walls, though in some species (like above) they may be very tightly packed. The colony grows by intratentacular budding, and hence some corallites may appear peanut-shaped as they are splitting. They usually form boulder-like colonies.

Favites spp.
Favites spp. have corallites with shared walls, and usually no paliformed lobes. They usually form boulder-like colonies.

Goniastrea spp.
Goniastrea spp. also have corallites with shared walls, but have well-defined paliformed lobes. The corallites may appear rounded or elongated, with a maze-like appearance.

Leptoria spp, Platygyra spp. and Oulophyllia spp.
Several other coral genera have maze-like appearance, and they include Leptoria spp, Platygyra spp. and Oulophyllia spp. Unfortunately I did not have closeup photos of the above corals, and hence could not identify them. Generally, Leptoria spp. usually has narrow (less than 0.4cm) but strongly parallel "valley and ridges", and the columella is a solid wall. Platygyra spp. generally have more ragged wall with exerting septa, have no paliformed lobes, and the columellae come in a continuous tangle of spines. Oulophyllia spp. have thinner septa, the valleys are short and wide, and paliformed lobes and/or columella are present.

Echinopora spp.
Echinopora spp. are rather different from most other faviid corals in local waters as they usually exhibit a plate-like and/or branching growth forms. They can be mistaken for Echinophyllia spp. or Oxypora spp., but has granulated coenosteum instead of the spiny ones found in the latter two species.

Family Fungiidae

Fungiid corals are mostly free-living and solitary as they mature, though there are several sessile and/or colonial species too. The juvenile free-living species are attached to the substrate by a stalk, and together with their gill-like septa, they somewhat resemble mushrooms, and hence they are often called mushroom corals. As they mature, they will eventually break away become free-living. They move by taking in more water into their body and shifting with the currents. Unfortunately I do not have clear photos of the sessile species, and hence will only highlight the motile species here.

Fungia spp.
Fungia spp. are solitary animals, i.e. , every “mushroom” is a single huge polyp sitting in its own corallite by itself. They are among the biggest coral animals, reaching up to 20 cm in diameter. Depending on the species, they can be circular or oval-shaped, and hence the common name "disk mushroom coral", but the mouth is usually located in the middle of the corallite. The tentacles are short, and are fully retractable.

Heliofungia actiniformis
Heliofungia actiniformis, commonly known as the sunflower mushroom coral, is also disk-shaped but has long tentacles with white tips that are always extended and never fully retracted. They are sometimes mistaken to be sea anemones, but unlike the latter, they have a hard skeleton and are free-living. This coral is the only species of its genus.

Ctenactis spp.
Ctenactis spp. were previously placed under the genus Fungia. The shape of the skeleton is oval or oblong, and they can have one or multiple mouths along the groove in the middle (called the fossa). The septa have obvious saw-like teeth.

Herpolitha limax
Herpolitha limax is a free-living colonial species, and each colony has several polyps in the axial groove in the middle, and smaller polyps in the grooves by the sides. They are commonly called boomerang corals as the colonies can be boomerang-shaped sometimes, but mostly they are oblong. Sometimes, Y-shaped or X-shaped colonies can also be found.

Polyphyllia talpina
Polyphyllia talpina is the only species in its genus. It is also a colonial species, with numerous polyps distributed over the upper surface of the colony, which can be oval, oblong or boomerang-shaped. The finger-like tentacles have white tips.

Family Merulinidae

Merulinid corals have corallites with fused walls, forming bumps or ridges. This family exhibits a variety of growth forms, and the colony may be massive (boulder-like), encrusting, plate-like, branching or columnar.

Merulina spp.
Merulina spp. usually form plate-like or encrusting colonies with short straight valleys running towards the edge, spreading out like a fan. In bigger colonies, branch-like or columnar structures can be seen extending out from the middle of the colony.

Hydnophora spp.
Hydnophora spp. have distinctive small ridges and bumps (or hydnophores) covering the surface of the colony, formed by fused walls of adjacent corallites. The tentacles are short and surround the hydnophores. The colonies may be branching, encrusting or boulder-like.

Family Mussidae

Mussid corals generally have thick and fleshy polyps, and large corallites with thick walls. The septa are lined with sharp, protruding septa teeth. They usually feed on zooplankton at night. Many mussids have thick meandering ridges on the surface of the colony, giving them a brain-like appearance, and hence these corals are also commonly called brain corals.

Symphyllia spp.
Symphyllia spp. are usually found in massive (boulder-like) colonies with wide valleys and thick meandering ridges (more than 1cm) form by fused walls. A narrow groove can usually be found on top and between the two fused walls. Sharp septa teeth can on the fused walls.

Lobophyllia spp.
Lobophyllia spp. have big and fleshy polyps and big corallites with separate walls. Like the previous genus, they have sharp septa teeth, differentiating them from other similar-looking corals with trumpet-like corallites forming submassive colonies.

Acanthastrea sp.
Acanthastrea spp. form massive but flattish colonies. The corallites have shared walls, and are circular or angular in shape. The septo-costae, which are marked with sharp teeth, are thicker nearer to the wall, but thinner near the columella. The polyps are big, thick and fleshy, and usually marked with bead-like fleshy structures on the surface. They can sometimes be confused with corallimorphs, but can be differentiated by their hard skeleton and septa teeth, which corallimorphs lack.

Family Oculinidae

Oculinid corals have crown-like corallites with sharply pointed septa. The tentacles have prominent white tips, somewhat resembling a galaxy of stars, and hence they are commonly called galaxy corals. They appear to be quite tolerant of sedimentation, and can occur in large colonies on our reefs.

Galaxea spp.
Galaxea spp. are represented by two species in Singapore: Galaxea astreata (main picture) with uniformly circular corallites; and Galaxea fasicularis (right-bottom) which has both round and oblong or oval corallites. Both species have crown-like corallites with protruding septa teeth, and white-tipped tentacles.

Family Pectiniidae

Pectiniid corals often exhibit a plate-like or leafy appearance. Many species produce a lot of mucus, possibly to gather and remove sediment from the colony. Most species have long sweeper tentacles for feeding.

Pectinia spp.
Pectinia spp. are often called lettuce corals for their leafy, lettuce-like appearance. The surface is irregularly covered with numerous thin septa.

Oxypora spp. or Echinophyllia spp.
Oxypora spp. and Echinophyllia spp. are two very similar-looking groups of corals from this family, and I am still unable to differentiate them. They usually form encrusting or plate-like colonies with round or oval corallites. The surface of the colony is usually covered with spiny structures, instead of the granulated surface found in the similar-looking Echinopora spp.

Family Pocilloporidae

Pocilloporid corals often exhibit branching growth forms. The polyps are small, and when they are extended they form a "furry" layer over the colony

Pocillopora damicornis
Pocillopora damicornis is one of two species of this genus that can be found in Singapore. This species can be recognised by the scar-like patterns of the corallites. The other Pocillopora species generally have verrucae (cone-like bumps) on the surface of the colony.

Family Poritidae

Poritids corals generally have small corallites with shared walls.The columella is present.

Porites spp.
Porites spp. have tiny and inconspicuous polyps and corallites forming tiny depressions on the surface of the colony with complete septa and columellae. They usually exhibit massive, columnar or branching growth forms.

Goniopora spp.
Goniopora spp. are sometimes mistaken for sea anemones due to their long and fleshy polyps concealing the skeleton. Each polyp have 24 tentacles, and the colonies may be submassive, massive or encrusting. They are very similar to corals of the genus Alveopora from the same family, but the latter has 12 tentacles per polyp.

Family Siderastreidae

Siderastreid corals may occur in leafy, plate-like, columnar or encrusting forms. The corallites have poorly defined walls and the septa are usually partially fused to form fan-like groups.

Psammocora spp.
Psammocora spp. have corallites which form tiny petal-like structures on the surface of the colony. Depending on the species, the colony may be columnar, encrusting, submassive, or leafy in appearance.

Pseudosiderastrea tayami
Pseudosiderastrea tayami is the only species in its genus. It usually forms small, encrusting (at most slightly dome-shaped) colonies. The corallites have shared walls, and are polygonal in shape. The septa are usually fused with each other in fan-like groups.

Family Trachyphylliidae

Trachyphylliid corals have huge polyps, large paliformed lobes and fine septa teeth.

Trachyphyllia geoffroyi
Trachyphyllia geoffroyi is the only member of this family in Singapore. It has thick fleshy polyps and big corallites with thick walls. The juvenile is attached to the substrate, after which it will clone itself to form a colony and break away to become free-living. It is sometimes called the open brain coral, due to the many gaps among the brain-like colony.

  • Huang, D., K. P. P. Tun, L. M Chou and P. A. Todd. 2009. An inventory of zooxanthellate sclerectinian corals in Singapore including 33 new records. Raffles Bulletin of Zoology Supplement No. 22: 69-80.
  • Chou, L. M. 1998. A Guide to the Coral Reef Life of Singapore. Singapore Science Centre. 128 pp.
  • Erhardt, H. and D. Knop. 2005. Corals: Indo-Pacific Field Guide. IKAN-Unterwasserachiv, Frankfurt. 305 pp.
  • Ruppert, E.E. and R.D. Barnes. 1991. Invertebrate Zoology (International Edition). Saunders College Publishing. U.S.A. 1056 pp.
  • Veron, J. and M. G. Stafford-Smith. 2011. Coral ID. www.coralid.com version 1.1. Australia. Retrieved Dec 23, 2012, from http://coral.aims.gov.au.

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