Reef Invertebrates of Sri Lanka: Corals, Molluscs and More

Blue Dragon Nudibranch Regular

Pteraeolidia semperi

Blue dragons feed on hydroids and pass the undischarged stinging cells through the gut to the tips of their cerata, where they are stored and used defensively. They also farm captured algae in their tissues and gain energy from sunlight.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Chromodoris Nudibranch Regular

Chromodoris species

A small, soft-bodied sea slug patterned in vivid whites, blues, purples and yellows, with a pair of head tentacles and a feathery gill tuft on its back. It grazes sponges and stores their chemical defences.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Small Giant Clam Regular

Tridacna maxima

A ribbed clam wedged into coral or rock with only its mantle showing, in iridescent blue, green, gold or brown, closing with a visible flinch when a shadow crosses it.

Where: Wedged into shallow coral and rock on reef flats. Best: November to April on the west, May to September on the east. The true giant clam does not reach Sri Lanka. This is the one you will actually see. Also on: Kalpitiya, Trincomalee and Pigeon Island, Hikkaduwa and Unawatuna.

Varicose Phyllidia Regular

Phyllidia varicosa

Phyllidia have no exposed gills, breathing through folds under the mantle rim, and are hard and rubbery to the touch. They feed on sponges and concentrate the sponge's chemistry into a potent defensive secretion.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Wedge Sea Hare Regular

Dolabella auricularia

Wedge sea hares graze algae on shallow sand and seagrass, looking like a lump of rock until they move. The purple secretion works as a chemical decoy, overloading a predator's smell and taste receptors rather than acting as a smokescreen.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Bigfin Reef Squid Occasional

Sepioteuthis lessoniana

Bigfin reef squid hover in small groups just off the reef, hunting fish and crustaceans with a lightning strike of two long tentacles. Their skin carries chromatophores they use for hunting, courtship and threat, sometimes signalling to a mate and a rival simultaneously.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Broadclub Cuttlefish Occasional

Ascarosepion latimanus

A large cuttlefish that hovers over coral and mesmerises crabs with rippling colour before striking with two long tentacles. Males put on shimmering displays to court females.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east. Watch the skin. Colour and texture change faster than you can follow.

Coconut Octopus Occasional

Amphioctopus marginatus

Coconut octopuses collect discarded shells and coconut halves, stack them and stilt-walk across the sand on two arms while carrying the load. When threatened they pull the halves closed around themselves like a suit of armour.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Day Octopus Occasional

Octopus cyanea

A large reef octopus that hunts by day, flowing over coral and probing holes with its arms for crabs and shellfish. It changes colour and skin texture instantly for camouflage and communication.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east. Also on: Hikkaduwa and Unawatuna.

Giant Spider Conch Occasional

Lambis truncata

Spider conchs grow seven long finger-like projections from the shell lip as they mature, which prop the shell up and deter predators. Movement is a series of lurching hops powered by a sickle-shaped operculum jabbed into the seabed.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Pharaoh Cuttlefish Occasional

Sepia pharaonis

A large cuttlefish with a broad fin skirt and a buoyancy shell inside its body, able to flush shifting patterns across its skin. It stalks shrimp and fish, mesmerising them with pulsing displays before shooting out two long tentacles.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east. Watch the skin. Colour and texture change faster than you can follow.

Spanish Dancer Occasional

Hexabranchus sanguineus

One of the largest nudibranchs, deep red and often edged in white, it usually crawls the reef but can swim by flexing its whole body in flowing waves. It feeds on sponges and stores their toxins for its own defence.

Where: Reef and rubble after dark, on a night dive. Best: November to April on the west and south, May to September on the east. Swims by unrolling its mantle and undulating, usually only after dark.

Sri Lanka Tree Snail Occasional

Acavus phoenix

A large, glossy land snail with a heavy globose shell banded in chestnut, cream and orange, carried by a dark grey body up tree trunks and branches.

Where: On trunks and branches in wet zone forest, moving at night and after rain. Best: Year round, best just after heavy rain. A canopy animal. Scan trunks at head height and above, not the leaf litter. Also on: Sinharaja Forest Reserve, Kanneliya Forest Reserve, Kitulgala and the Makandawa Rainforest.

Thorny Oyster Occasional

Spondylus varius

Thorny oysters live glued to rock and coral, filtering plankton and snapping shut at any shadow. The mantle rim carries a row of simple eyes that detect movement, which is what triggers the closing reflex.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Tiger Cowrie Occasional

Cypraea tigris

Tiger cowries hide in crevices by day and graze algae and sponge at night. The living animal extends two lobes of speckled mantle over the outside of the shell, which is what keeps it free of the wear and encrustation other shells accumulate.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Geographic Cone Snail Rare

Conus geographus

Geographic cones hunt fish at night, either engulfing them in a distended mouth or harpooning them with a venom-loaded modified tooth. The venom is a cocktail of hundreds of peptides, and human deaths, while very rare, have occurred.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. The sting can kill. Never pick up a live cone shell, however good it looks.

Textile Cone Snail Rare

Conus textile

Textile cones hunt other snails at night, firing a harpoon-like radular tooth loaded with paralysing venom. The pattern of overlapping cream tents on a brown ground is produced by a wave-like chemical process along the growing shell edge.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. The sting can kill. Never pick up a live cone shell, however good it looks.

Triton's Trumpet Rare

Charonia tritonis

A large marine snail with a heavy, elegantly patterned conical shell up to half a metre long. A predator, it tracks down and devours sea stars, including the destructive crown-of-thorns starfish.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east. One of very few predators of the crown-of-thorns starfish, and collected for its shell, which is the problem.

Boulder Porites Common

Porites lutea

Boulder Porites form massive rounded domes that can exceed five metres and live for hundreds of years. Cores drilled from them show annual density bands that scientists read like tree rings to reconstruct past sea temperature and rainfall.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Brain Coral Common

Platygyra species

A massive, dome-shaped coral whose surface is covered in meandering ridges and valleys housing rows of polyps. Slow-growing and long-lived, it feeds via symbiotic algae and by capturing plankton at night.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Encrusting Zoanthid Common

Palythoa tuberculosa

Zoanthids form low, leathery mats of button-like polyps over rock in shallow, wave-washed water. Some Palythoa carry palytoxin at concentrations that cause serious poisoning through cuts or, in confined spaces, through vapour.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Mushroom Coral Common

Fungia species

A disc-shaped coral resembling the underside of a mushroom, radiating fine ridges from the centre. Unusually it is a single large polyp living free on the bottom rather than a fixed colony.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Staghorn Coral Common

Acropora species

A branching, antler-like coral formed by colonies of tiny polyps, growing in dense stands in shallow, sunlit water. It gains most of its energy from symbiotic algae and captures plankton with its polyps at night.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Both suffered severe bleaching mortality on Sri Lankan reefs in 1998 and again in 2016. Also on: Trincomalee and Pigeon Island, Kalpitiya, Hikkaduwa and Unawatuna.

Table Coral Common Near Threatened

Acropora hyacinthus

Table corals form broad horizontal plates up to two metres across on a single narrow stem, maximising light capture. That shape makes them the fastest growing reef builders and also the most easily smashed by storms, anchors and fins.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Both suffered severe bleaching mortality on Sri Lankan reefs in 1998 and again in 2016. Also on: Kalpitiya, Hikkaduwa and Unawatuna.

Blue Coral Regular Vulnerable

Heliopora coerulea

Blue coral builds a hard skeleton coloured deep blue by iron salts, unique among reef-building corals. The living colony looks unremarkable brown-grey, and the colour is only revealed when the tissue is stripped away.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. The skeleton is genuinely blue. Break a piece and you can see it, so do not.

Bulb-tentacle Anemone Regular

Entacmaea quadricolor

Bulb-tentacle anemones wedge their base deep into reef crevices and spread tentacles that develop a distinctive swelling below the tip. The bulbs form reliably in the presence of resident anemonefish and are usually absent without them.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Carnation Soft Coral Regular

Dendronephthya species

A soft, treelike coral without a hard skeleton, supported by water pressure and tiny mineral spikes, blooming in vivid pinks, reds and oranges. It filters plankton and has no symbiotic algae, so it thrives in shade and current.

Where: Deeper reef, wrecks and the Great and Little Basses reefs. Best: November to April on the west and south, May to September on the east.

Carnation Tree Coral Regular

Dendronephthya klunzingeri

These soft corals inflate with water to unfold into pink, red or purple trees, then collapse into a limp bag when the current stops. They feed entirely on drifting plankton, which is why they only thrive where water moves.

Where: Deeper reef, wrecks and the Great and Little Basses reefs. Best: November to April on the west and south, May to September on the east.

Fire Coral Regular

Millepora platyphylla

Fire coral forms smooth mustard-yellow blades and encrustations with white tips, covered in fine hair-like stinging polyps. Brushing against it causes an immediate burning welt that can last for days.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Not a coral and it stings. Brush past it in a wetsuit gap and you will remember.

Giant Sea Fan (Gorgonian) Regular

Annella species

A large, fan-shaped soft coral with a fine branching mesh, oriented across prevailing currents to maximise feeding. Its polyps filter plankton from the passing water.

Where: Reef edges and drop-offs, including the Trincomalee canyon rim. Best: November to April on the west and south, May to September on the east. Also on: The Great and Little Basses Reefs.

Haddon's Carpet Anemone Regular

Stichodactyla haddoni

Haddon's carpet anemone spreads a broad, densely tentacled oral disc across silty sand and seagrass, retracting entirely into the substrate when threatened. The sting is strong enough to adhere firmly to human skin.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Magnificent Sea Anemone Regular

Heteractis magnifica

A large sea anemone with a broad column and hundreds of thick, often bulb-tipped tentacles, coming in shades of green, blue, purple and red. It stings and captures prey but also hosts anemonefish and shrimp immune to its venom.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Also on: Hikkaduwa and Unawatuna.

Orange Sun Coral Regular

Tubastraea coccinea

Sun corals lack the symbiotic algae almost all reef corals depend on, and instead catch zooplankton with long translucent tentacles at night. By day the colony closes to a cluster of hard orange knobs.

Where: The Trincomalee and Passikudah wrecks and deeper rocky reef. Best: November to April on the west and south, May to September on the east. Opens at night and on shaded wreck overhangs by day.

Toadstool Leather Coral Regular

Sarcophyton trocheliophorum

Leather corals form thick mushroom-shaped stalks and caps with retractile polyps over the upper surface. Periodically they withdraw all polyps, secrete a waxy film and slough it off, taking accumulated algae and sediment with it.

Where: Deeper reef, wrecks and the Great and Little Basses reefs. Best: November to April on the west and south, May to September on the east.

Anchor Coral Occasional Vulnerable

Euphyllia ancora

Anchor coral extends thick fleshy tentacles day and night, each tipped with a T or anchor-shaped swelling. It deploys long sweeper tentacles after dark to sting and clear space around the colony.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Black Coral Occasional

Antipathes dichotoma

Black corals grow as feathery bushes and whips on deep walls and in caves, their tissue white, brown or grey over a jet black skeleton. They grow extraordinarily slowly, which is why a modest bush may predate written history.

Where: Reef edges and drop-offs, including the Trincomalee canyon rim. Best: November to April on the west and south, May to September on the east. Also on: The Great and Little Basses Reefs.

Bubble Coral Occasional Near Threatened

Plerogyra sinuosa

Bubble coral expands water-filled vesicles during daylight to maximise the surface exposed to sunlight for its symbiotic algae. After dark the bubbles shrink and long sweeper tentacles emerge to catch plankton and to sting neighbouring corals.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Sea Pen Occasional

Pteroeides sp.

Sea pens anchor a muscular bulb in soft sediment and unfurl a feathered rachis of polyps to feed on plankton at night. Disturbance triggers waves of blue-green bioluminescence, and a serious threat makes the whole colony deflate and withdraw into the seabed.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east.

Sea Whip Coral Occasional

Junceella fragilis

Sea whips grow as long, flexible, unbranched cords anchored to rock, oriented across the prevailing current to intercept plankton. They propagate clonally by shedding fragments that root where they land.

Where: Reef edges and drop-offs, including the Trincomalee canyon rim. Best: November to April on the west and south, May to September on the east.

Tube Anemone Occasional

Cerianthus filiformis

Tube anemones live buried in soft sediment inside a tube woven from mucus and spent nematocyst threads. Two rings of tentacles emerge at night, long outer ones for capture and short inner ones for handling prey.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east.

Blue Sea Star Common

Linckia laevigata

A five armed sea star of a striking deep blue, sometimes purple or orange, resting on sand, rubble and reef flats. It grazes films of algae and detritus, and can regenerate from a single severed arm.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Also on: Trincomalee and Pigeon Island, Hikkaduwa and Unawatuna.

Cushion Star Common

Culcita novaeguineae

Cushion stars start life as recognisable five-armed juveniles and inflate as they grow until the arms are absorbed into a domed pentagon. They feed by everting the stomach onto coral and algae and digesting it externally.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Granulated Sea Star Common

Choriaster granulatus

Granulated sea stars have short, very thick blunt arms and a smooth pale pink to cream surface with a darker centre. They graze detritus, algal film and dead coral, everting the stomach over the substrate.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Long-spined Sea Urchin Common

Diadema setosum

A black sea urchin with very long, fine, venom-tipped spines and a distinctive orange ring on the body. It shelters by day and grazes algae from the reef at night, its spines swivelling toward any shadow.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Also on: Hikkaduwa and Unawatuna.

Pincushion Urchin Common

Tripneustes gratilla

Pincushion urchins graze seagrass and algae in shallow water, often in dense aggregations. They pick up debris and hold it over the upper surface, which is thought to reduce ultraviolet exposure and to break up their outline.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Slate Pencil Urchin Common

Heterocentrotus mamillatus

Slate pencil urchins wedge themselves into crevices during the day using their heavy triangular-sectioned spines, and graze algae at night. The spines are solid enough to be collected for wind chimes and jewellery.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Blue Brittle Star Regular

Ophiothrix purpurea

Brittle stars have a small central disc and five long, flexible, spiny arms used for walking rather than tube feet. They shelter inside sponges and under rubble by day and extend arms into the current to filter feed at night.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Feather Star (Crinoid) Regular

Comatulida species

An ancient relative of starfish with many feathery, branching arms radiating from a central body, gripping the reef with clawed appendages. It spreads its arms into the current to filter plankton.

Where: Reef edges and drop-offs, including the Trincomalee canyon rim. Best: November to April on the west and south, May to September on the east. Also on: Trincomalee and Pigeon Island, Hikkaduwa and Unawatuna, The Great and Little Basses Reefs.

Pineapple Sea Cucumber Regular Endangered

Thelenota ananas

Pineapple sea cucumbers process enormous volumes of sand, extracting organic matter and excreting clean sediment, which keeps reef substrate turned over and oxygenated. The high value of dried teatfish has driven serial depletion across the Indo-Pacific.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. Sea cucumbers are fished hard in the Gulf of Mannar for the export trade.

Sea Cucumber Regular

Holothuria species

A soft, elongated relative of starfish and urchins, lying on sand and reef floor, feeding by passing sediment through its gut to extract organic matter. Some defend themselves by expelling sticky or toxic threads.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. Sea cucumbers are fished hard in the Gulf of Mannar for the export trade.

Crown-of-thorns Starfish Occasional

Acanthaster planci

A large starfish with up to twenty arms bristling with sharp venomous spines, coloured red, grey or purple. It feeds by everting its stomach over coral and digesting the polyps in place.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Outbreaks have done real damage to Sri Lankan reefs. Spines are venomous.

Fire Urchin Occasional

Asthenosoma varium

Fire urchins have a soft, flexible leathery test rather than a rigid shell, and short venomous spines tipped with bulbous orange sacs. Coleman shrimp and zebra crabs live on the upper surface, clearing a bare patch to sit in.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. Both are venomous. The flower urchin is genuinely dangerous.

Flower Urchin Occasional

Toxopneustes pileolus

Flower urchins are covered not in long spines but in hundreds of flower-like pedicellariae, tiny three-jawed pincers loaded with venom. Envenomation causes severe pain, and paralysis and respiratory difficulty have been recorded.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. Both are venomous. The flower urchin is genuinely dangerous.

Basket Star Rare

Astroboa nuda

Basket stars are brittle stars whose five arms divide repeatedly into thousands of fine tendrils. At night they climb sea fans and whips and open into a large parabolic net facing the current, curling captured plankton back to the mouth.

Where: Reef edges and drop-offs, including the Trincomalee canyon rim. Best: November to April on the west and south, May to September on the east. Unfurls only at night.

Fiddler Crab Common

Austruca annulipes

Fiddler crabs emerge in thousands on mangrove mudflats at low tide, sifting sediment for organic matter and rolling the cleaned sand into pellets. Males wave a single hugely enlarged claw in a species-specific rhythm to attract mates and warn rivals.

Where: Lagoons, estuaries and mangrove creeks at Puttalam, Negombo and Kalametiya. Best: Year round.

Giant Mud Crab Common

Scylla serrata

Mud crabs live in burrows among mangrove roots, emerging on the tide to hunt molluscs, crustaceans and carrion. Large males reach 3 kg with claws powerful enough to break heavy shell.

Where: Lagoons, estuaries and mangrove creeks at Puttalam, Negombo and Kalametiya. Best: Year round.

Horn-eyed Ghost Crab Common

Ocypode ceratophthalmus

Ghost crabs dig spiral burrows above the tideline and emerge at dusk to scavenge, sprinting sideways across open sand. Males produce a rasping stridulation from inside the burrow to warn off rivals.

Where: Rocky shores, tide pools and the intertidal on both coasts. Best: Year round. The crab that sprints across the sand at Rekawa while you are waiting for a turtle. Also on: Tangalle and Kalametiya, Hikkaduwa and Unawatuna, Rekawa and Kosgoda.

Banded Coral Shrimp Regular

Stenopus hispidus

A striking red-and-white banded shrimp with long white antennae and slender pincers, living in reef crevices, usually in pairs. It picks parasites and dead tissue from fish that visit its cleaning station.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Durban Dancing Shrimp Regular

Rhynchocinetes durbanensis

Durban dancing shrimp aggregate in dark overhangs and caves by day, emerging at night to scavenge. The hinged rostrum tips upward, which is unusual among shrimp and gives the group its name of hingebeak shrimp.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

White-spotted Hermit Crab Regular

Dardanus megistos

White-spotted hermit crabs are large, hairy and vivid scarlet with black-ringed white spots, occupying big gastropod shells. They scavenge, hunt and occasionally attack live snails to acquire a better home.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east.

Boxer Crab Occasional

Lybia tessellata

Boxer crabs hold a small anemone permanently in each pincer, using the stinging tentacles as both weapon and mop for collecting food. Experiments show a crab deprived of one anemone will split the remaining one, and both halves regenerate.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Emperor Shrimp Occasional

Zenopontonia rex

Emperor shrimp live commensally on large sea cucumbers, Spanish dancers and other slow-moving invertebrates. The host provides transport and protection while the shrimp picks food from the sediment it disturbs.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Giant Freshwater Prawn Occasional

Macrobrachium rosenbergii

A very large, translucent blue-grey prawn, the males carrying a pair of enormously elongated blue claws that can exceed the length of the body.

Where: Shallow river and lagoon margins after dark, by torchlight. Best: Year round, after dark. Torchlight picks up the eyes first. The claws are what you remember. Also on: Gal Oya National Park, Arugam Bay and the East Coast Lagoons, Kitulgala and the Makandawa Rainforest.

Ornate Spiny Lobster Occasional

Panulirus ornatus

Ornate spiny lobsters are the largest in the region, reaching 60 cm and 8 kg. Adults undertake mass breeding migrations, forming queues in which each animal keeps antennal contact with the one in front.

Where: Reef and rubble after dark, on a night dive. Best: November to April on the west and south, May to September on the east. Heavily fished. Numbers on accessible reef are a fraction of what older divers describe. Also on: The Great and Little Basses Reefs.

Painted Spiny Lobster Occasional

Panulirus versicolor

Painted spiny lobsters shelter under coral heads by day with only their long white antennae protruding, and hunt molluscs and worms at night. Rubbing the base of the antenna against a file on the shell produces a loud stick-slip rasp that works even underwater against a wet predator.

Where: Reef and rubble after dark, on a night dive. Best: November to April on the west and south, May to September on the east. Heavily fished. Numbers on accessible reef are a fraction of what older divers describe.

Peacock Mantis Shrimp Occasional

Odontodactylus scyllarus

A stocky, brilliantly coloured crustacean in green, red and blue with club shaped forelimbs. It strikes prey with a blow so fast it boils the water, and its eyes carry more colour receptors than any known animal.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. The strike is one of the fastest movements in the animal kingdom and can crack a finger.

Porcelain Anemone Crab Occasional

Neopetrolisthes maculatus

A small, smooth crab, cream spotted with red or brown, living among the tentacles of large sea anemones. It sweeps the water with feathery mouth appendages to catch plankton.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Slipper Lobster Occasional

Scyllarides squammosus

Slipper lobsters have no long antennae at all; the second pair is modified into wide flat plates used for digging and defence. They hide under ledges and in sand by day and feed on molluscs at night, prising bivalves open with the edge of the carapace.

Where: Reef and rubble after dark, on a night dive. Best: November to April on the west and south, May to September on the east. Heavily fished. Numbers on accessible reef are a fraction of what older divers describe.

Tiger Pistol Shrimp Occasional

Alpheus bellulus

Pistol shrimp close a specialised claw so fast that the water jet cavitates, collapsing with a bang loud enough to stun small prey. The collapsing bubble produces a flash of light and momentary temperatures of thousands of degrees.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Zebra Mantis Shrimp Occasional

Lysiosquillina maculata

Zebra mantis shrimp are spearers rather than smashers, firing barbed raptorial claws to impale passing fish from a burrow mouth. Pairs are monogamous, sharing and defending the same burrow for decades.

Where: Sand and rubble patches between coral heads. Best: November to April on the west and south, May to September on the east. The strike is one of the fastest movements in the animal kingdom and can crack a finger.

Harlequin Shrimp Rare

Hymenocera picta

A small, flattened shrimp, cream splashed with blue-edged pink or purple blotches, with broad petal-like claws and paddle antennae. It lives in pairs and preys exclusively on sea stars.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east. Eats starfish and nothing else. Sri Lanka's animal is Hymenocera elegans, the Indian Ocean form.

Sri Lanka Tree Crab Very rare

Perbrinckia scansor

A small, dark, long-legged freshwater crab with an unusually high-domed carapace, found in water-filled cavities well above the forest floor.

Where: Rainwater held in hollows and rot holes of large wet zone forest trees. Best: Year round; females at ground level February and March. Do not go pulling apart tree hollows to look. The best chance is a guide who already knows a tree, or a female on the ground in the breeding months. Also on: Sinharaja Forest Reserve, Kanneliya Forest Reserve.

Christmas Tree Worm Common

Spirobranchus giganteus

A small tube-dwelling worm that extends two brightly coloured, spiral crowns from a burrow bored into living coral. The crowns filter plankton and also serve as gills, and snap out of sight instantly when disturbed.

Where: Shallow reef flats and coral heads at Bar Reef and around Pigeon Island. Best: November to April on the west and south, May to September on the east. Wave a hand near one and the whole colony vanishes at once. Also on: Trincomalee and Pigeon Island, Hikkaduwa and Unawatuna.

Giant Barrel Sponge Regular

Xestospongia testudinaria

Giant barrel sponges pump water through a network of chambers, straining out bacteria and organic particles and expelling clean water from the central opening. A large individual processes many times its own volume every minute.

Where: Deeper reef, wrecks and the Great and Little Basses reefs. Best: November to April on the west and south, May to September on the east.

Persian Carpet Flatworm Occasional

Pseudobiceros bedfordi

Persian carpet flatworms glide over the reef on a film of cilia, engulfing tunicates and small invertebrates. Mating takes the form of penis fencing, in which each animal tries to inseminate the other and avoid being inseminated, since bearing the eggs is the costlier role.

Where: Coral reef and boulder slopes on both coasts. Best: November to April on the west and south, May to September on the east.

Portuguese Man o' War Occasional

Physalia physalis

The man o' war is a siphonophore, a floating colony in which separate zooids handle flotation, feeding, defence and reproduction. A gas-filled blue float catches the wind while tentacles up to thirty metres long trail below.

Where: Rocky shores, tide pools and the intertidal on both coasts. Best: November to April on the west and south coasts. Strandings come in on the monsoon wind. The sting works on the beach too.

Upside-down Jellyfish Occasional

Cassiopea andromeda

Upside-down jellyfish rest bell-down on shallow mud and seagrass, exposing frilled oral arms packed with symbiotic algae to the sun. They also release clouds of stinging mucus into the water, which causes the itching that swimmers call stinging water.

Where: Lagoons, estuaries and mangrove creeks at Puttalam, Negombo and Kalametiya. Best: Year round.

Bobbit Worm Rare

Eunice aphroditois

Bobbit worms line a burrow in sand or rubble and wait with only five antennae exposed, seizing passing fish with scissor-like jaws. The strike is fast enough to sever prey outright, and the worm drags what it catches underground.

Where: Reef and rubble after dark, on a night dive. Best: November to April on the west and south, May to September on the east.

Box Jellyfish Rare

Chironex indrasaksajiae

Box jellyfish have a cube-shaped bell with tentacles trailing from each corner, and swim actively rather than drifting. The venom attacks skin, nerves and heart simultaneously, and severe stings can be fatal within minutes.

Where: Rocky shores, tide pools and the intertidal on both coasts. Best: Year round. Medically serious. Stings are reported from Sri Lankan beaches, mostly in the wet months.