Snorkelling and Diving Around Koh Samui: What You Will See

Fishing Cat Rare Vulnerable

Prionailurus viverrinus

A wild cat that dives for fish, with partially webbed toes and a genuine love of swimming.

Where: Mangrove and marsh on the mainland coast opposite. Best: Year round, night.

Indo-Pacific Finless Porpoise Rare Vulnerable

Neophocaena phocaenoides

A shy, finless little porpoise with a smiling face that surfaces without a splash.

Where: Calm shallow coastal water. Best: Year round, best Feb-Sep.

Indo-Pacific Humpback Dolphin Occasional Vulnerable

Sousa chinensis

Born dark grey and turning pink with age, the colour coming from blood vessels under unpigmented skin.

Where: Shallow coastal water and river mouths. Best: Year round, best Feb-Sep.

Indo-Pacific Bottlenose Dolphin Regular

Tursiops aduncus

The classic bottlenose of warm coasts, sociable, clever and quick to ride a bow wave.

Where: Open water between the islands. Best: Year round, best Feb-Sep.

Bryde's Whale Occasional

Balaenoptera edeni

A warm-water baleen whale that lunges up through bait balls with its mouth wide open.

Where: Open Gulf water on the crossings, and the upper Gulf feeding grounds. Best: Sep-Nov. Lunge-feeding at the surface in the upper Gulf is one of the great Thai wildlife sights.

Spoon-billed Sandpiper Rare Critically Endangered

Calidris pygmaea

One of the world's rarest and most charismatic shorebirds, tiny and armed with an unmistakable spatula bill.

Where: Inner Gulf salt pans north of here. Best: Nov-Mar.

Far Eastern Curlew Rare Endangered

Numenius madagascariensis

The longest bill of any wader in the world, curved like a sickle and used to reach crabs deep in the mud.

Where: Mainland mudflats. Best: Nov-Mar.

Milky Stork Rare Endangered

Mycteria cinerea

A rare, ghostly white stork of the mangroves, its bare face flushing from yellow to red in the breeding season.

Where: Coastal mangrove on the mainland. Best: Year round.

Nordmann's Greenshank Rare Endangered

Tringa guttifer

One of the world's rarest waders, a stocky greenshank that winters on Thai mudflats among thousands of commoner shorebirds.

Where: Inner Gulf mudflats. Best: Nov-Mar.

Great Knot Occasional Endangered

Calidris tenuirostris

Flies from Siberia to Australia with a single refuelling stop, and losing that one mudflat has nearly finished the species.

Where: Mainland mudflats. Best: Nov-Mar.

Chinese Egret Rare Vulnerable

Egretta eulophotes

Hunts by sprinting across the mudflat with wings half open, a running dash that no other egret uses.

Where: Tidal flats on the mainland shore. Best: Nov-Mar.

Malaysian Plover Rare

Anarhynchus peronii

Nests in a scrape on open beach so well camouflaged that the greatest danger is being stepped on.

Where: Quiet sandy beaches. Best: Year round.

Brown Booby Rare

Sula leucogaster

Dives from twenty metres and hits the water at over sixty kilometres an hour, with air sacs in the skull absorbing the shock.

Where: Offshore rocks and open water. Best: Year round.

Bridled Tern Occasional

Onychoprion anaethetus

Spends most of the year entirely at sea, feeding on fish driven to the surface by tuna hunting below.

Where: Rocky islets and boat wakes. Best: Year round.

Lesser Frigatebird Occasional

Fregata ariel

Cannot land on water despite living over it, and stays airborne for weeks at a time by riding thermals in its sleep.

Where: Offshore on the crossings. Best: Year round.

Pacific Reef Heron Regular

Egretta sacra

Comes in two entirely different colours, charcoal grey and pure white, and both can hatch in the same nest.

Where: Rocky shores at low tide. Best: Year round.

Green Sea Turtle Regular Endangered

Chelonia mydas

The great grazing turtle of the tropics, the only sea turtle that turns vegetarian as an adult.

Where: Seagrass and shallow reef around Koh Tao and the Ang Thong islands. Best: Year round, best Feb-Sep.

Loggerhead Turtle Ultra rare Vulnerable

Caretta caretta

Jaws powerful enough to crush a queen conch, and it navigates ocean basins by reading the earth's magnetic field.

Where: Offshore water. Best: Year round, best Feb-Sep.

Thorny Seahorse Rare Vulnerable

Hippocampus histrix

Covered in long sharp spines and able to shift colour to match whatever it has wrapped its tail around.

Where: Sponge rubble and seagrass margins. Best: Year round, best Feb-Sep.

Mandarinfish Rare

Synchiropus splendidus

Its colours are not pigment but structural blue, a pigment cell type known from only one other genus of fish on earth.

Where: Rubble shallows at dusk, Koh Tao. Best: Year round, best Feb-Sep, dusk. A dusk dive at a known rubble patch is the only realistic way to see the mating display.

Blue-ringed Angelfish Occasional

Pomacanthus annularis

Named for a single electric blue ring above the gill, painted on a fish that is otherwise swept with gold.

Where: Granite boulder reef. Best: Year round, best Feb-Sep.

Golden Trevally Occasional

Gnathanodon speciosus

The bright yellow juveniles ride shotgun with sharks and rays, sheltering inches from the mouth of the thing that could eat them.

Where: Around pinnacles, often shadowing larger fish. Best: Year round, best Feb-Sep.

Longhorn Cowfish Occasional

Lactoria cornuta

A box of armour with horns front and back, that hunts by blowing jets of water into the sand.

Where: Seagrass and rubble. Best: Year round, best Feb-Sep.

Saddleback Anemonefish Occasional

Amphiprion polymnus

The one anemonefish that lives out on open sand rather than the reef, in a host buried up to its mouth.

Where: Carpet anemones on sandy shallows. Best: Year round, best Feb-Sep.

Zebra Lionfish Occasional

Dendrochirus zebra

A pocket lionfish that herds shrimp into a corner with fanned pectoral fins before swallowing them whole.

Where: Overhangs and wrecks. Best: Year round, best Feb-Sep.

Orbicular Batfish Regular

Platax orbicularis

A tall silver disc of a fish that drifts up curiously to inspect its own reflection in your mask.

Where: Around pinnacles and mooring lines. Best: Year round, best Feb-Sep.

Pyjama Cardinalfish Regular

Sphaeramia nematoptera

Looks like three different fish stitched together, and the male holds the whole clutch of eggs in his mouth for a week.

Where: Branching coral in the shallows. Best: Year round, best Feb-Sep.

Longfin Bannerfish Common

Heniochus acuminatus

A black-and-white pennant fish trailing a long white banner over the reef.

Where: Coral heads and boulders. Best: Year round, best Feb-Sep.

Crocodile Flathead Occasional

Cymbacephalus beauforti

Flattened into the sand with only two lacework eyes showing, waiting for something to swim within a body length.

Where: Sand and rubble. Best: Year round, best Feb-Sep.

Largetooth Sawfish Ultra rare Critically Endangered

Pristis pristis

A seven metre ray with a chainsaw for a nose, and almost certainly gone from Thai waters.

Where: Historically the shallow Gulf and river mouths. Best: No current season. Almost certainly extirpated from Thai waters.

Whitespotted Wedgefish Ultra rare Critically Endangered

Rhynchobatus australiae

Its fins are the single most valuable in the global shark fin trade, which is why it has all but disappeared.

Where: Sand beside reef, historically. Best: Year round, best Feb-Sep. Effectively gone from the Gulf. Recorded here for honesty rather than expectation.

Whale Shark Rare Endangered

Rhincodon typus

The biggest fish in the ocean, a spotted, bus sized filter feeder and the undisputed holy grail of tropical diving.

Where: Sail Rock and the pinnacles between Samui and Koh Tao. Best: Year round, best Feb-Sep, peak Mar-May. Sail Rock is the Gulf's whale shark site. Day boats run from Samui, Phangan and Koh Tao.

Whitespotted Bamboo Shark Occasional Endangered

Chiloscyllium plagiosum

A small, slow shark that walks along the bottom on its fins and can survive hours in a tide pool with almost no oxygen.

Where: Under coral ledges at night around Koh Tao. Best: Year round, best Feb-Sep, night.

Cowtail Stingray Occasional Vulnerable

Pastinachus sephen

Named for the broad flap of skin hanging under its tail like a cow's tail, unique among rays.

Where: Sandy shallows and seagrass. Best: Year round, best Feb-Sep.

Jenkins Whipray Occasional Vulnerable

Pateobatis jenkinsii

A big sand-flat ray with a ridge of enlarged thorns down its spine and a tail twice the length of its body.

Where: Sand flats between coral heads. Best: Year round, best Feb-Sep.

Blacktip Reef Shark Common Vulnerable

Carcharhinus melanopterus

The neat little reef shark of the shallows, its black-tipped fins often cutting the surface over sun-lit sand flats.

Where: Shark Bay on Koh Tao and shallow reef flats. Best: Year round, best Feb-Sep. Shark Bay is a snorkel, not a dive, and the sharks are there most mornings.

Indo-Pacific Horseshoe Crab Rare Endangered

Tachypleus gigas

Not a crab at all, closer to a spider, and its blue blood is used worldwide to test every injectable drug for contamination.

Where: Muddy shores on spring tides. Best: Around the full and new moon.

Anchor Coral Occasional Vulnerable

Euphyllia ancora

Every tentacle ends in a fat hammer-shaped tip, so the whole colony looks like a field of tiny anchors.

Where: Sheltered bays. Best: Year round, best Feb-Sep.

Blue Coral Occasional Vulnerable

Heliopora coerulea

The only coral on earth with a blue skeleton, hidden under brown tissue until the animal dies.

Where: Shallow high-energy reef. Best: Year round, best Feb-Sep.

Table Coral Common

Acropora hyacinthus

Grows outward faster than it grows up, spreading a flat platform that shades out every competitor beneath it.

Where: Shallow reef around Koh Tao. Best: Year round, best Feb-Sep.

Painted Spiny Lobster Occasional

Panulirus versicolor

Has no crushing claws at all, and defends itself instead by rasping out a screech you can hear underwater.

Where: Crevices under boulders. Best: Year round, best Feb-Sep, night.

Mimic Octopus Ultra rare

Thaumoctopus mimicus

Impersonates lionfish, flatfish and sea snakes, choosing the right disguise for whichever predator is bothering it.

Where: Silty sand near river mouths. Best: Year round, best Feb-Sep.

Berry's Bobtail Squid Rare

Euprymna berryi

Farms glowing bacteria in a light organ and uses them to erase its own shadow from the moonlit sand below.

Where: Sand at night. Best: Year round, best Feb-Sep, night.

Blue-ringed Octopus Rare

Hapalochlaena

A thumb-sized octopus whose glowing blue rings warn of venom with no antidote.

Where: Rubble and shell debris in the shallows. Best: Year round, best Feb-Sep. Small, beautiful and capable of killing an adult. Never handle anything in rubble here.

Bobbit Worm Rare

Eunice aphroditois

A three metre worm buried in the sand that strikes so fast it can cut a fish in half.

Where: Silty sand at night. Best: Year round, best Feb-Sep, night.

Boxer Crab Rare

Lybia tessellata

Carries a live sea anemone in each claw and waves them at threats, and if it loses one it tears the other in half to clone a replacement.

Where: Rubble at night. Best: Year round, best Feb-Sep, night.

Coconut Octopus Rare

Amphioctopus marginatus

Carries a coconut shell around under its body and reassembles it into a shelter, one of the few clear cases of tool use in an invertebrate.

Where: Silty sand and shell debris. Best: Year round, best Feb-Sep.

Flamboyant Cuttlefish Rare

Metasepia pfefferi

The only cuttlefish known to be poisonous to eat, and it advertises the fact in pulsing purple, yellow and red.

Where: Silty sand on muck dives. Best: Year round, best Feb-Sep.

Zebra Mantis Shrimp Rare

Lysiosquillina maculata

The largest mantis shrimp in the world, and it pairs for life in a burrow it may hold for twenty years.

Where: Burrows in silty sand. Best: Year round, best Feb-Sep.

Box Jellyfish Occasional

Chironex indrasaksajiae

Has twenty four eyes, including four that form proper images, and it is the only animal in Thai waters that has killed swimmers.

Where: Inshore water, especially after rain. Best: Jun-Oct. Genuinely dangerous in the Gulf. Heed local warnings and swim where there is vinegar on the beach.

Broadclub Cuttlefish Occasional

Sepia latimanus

A big reef cuttlefish that hypnotises prey with bands of colour pulsing down its arms.

Where: Reef edges and sand. Best: Year round, best Feb-Sep.

Fire Urchin Occasional

Asthenosoma varium

Its spines glow orange-red at the tips and deliver a burning sting, and a pair of tiny shrimp live on it in perfect safety.

Where: Silty sand and rubble. Best: Year round, best Feb-Sep. Venomous spines, and often carries commensal shrimp worth looking for.

Haddon's Carpet Anemone Occasional

Stichodactyla haddoni

A metre-wide sticky carpet on open sand that will genuinely hold onto a bare hand, and can pull a small fish under.

Where: Sandy shallows. Best: Year round, best Feb-Sep.

Hairy Squat Lobster Occasional

Lauriea siagiani

Lives only on giant barrel sponges, and is so small and so pink that most divers swim past a dozen without noticing.

Where: Inside barrel sponges. Best: Year round, best Feb-Sep.

Sea Pen Occasional

Pteroeides sp.

Plants itself in the sand like a quill pen, glows blue-green when touched, and can pull itself completely underground.

Where: Sand flats at night. Best: Year round, best Feb-Sep, night.

Slate Pencil Urchin Occasional

Heterocentrotus mamillatus

Its spines are thick blunt cylinders, and they were once used as writing implements on slate, hence the name.

Where: Reef crevices. Best: Year round, best Feb-Sep.

Variable Neon Slug Occasional

Nembrotha kubaryana

Eats toxic sea squirts, keeps their poison for itself, and then advertises it in green and orange.

Where: Sponges on the granite boulders. Best: Year round, best Feb-Sep.

Wedge Sea Hare Occasional

Dolabella auricularia

Ejects a cloud of purple ink that does not hide it but jams the chemical senses of whatever is attacking.

Where: Seagrass at low tide. Best: Year round, best Feb-Sep.

Bigfin Reef Squid Regular

Sepioteuthis lessoniana

Talks in colour: it can run one message down its left side and a different one down its right at the same time.

Where: Midwater on night dives and under boats. Best: Year round, best Feb-Sep.

Durban Dancing Shrimp Regular

Rhynchocinetes durbanensis

Packs into caves in the hundreds, and the whole mass sways together when a torch beam crosses them.

Where: Overhangs and crevices. Best: Year round, best Feb-Sep.

Upside-down Jellyfish Regular

Cassiopea andromeda

Lies on the seabed with its tentacles pointing at the sky, farming algae inside its own tissue like a coral.

Where: Shallow sandy lagoons and mangrove edges. Best: Year round, best Feb-Sep.

Bulb-tentacle Anemone Common

Entacmaea quadricolor

The tentacles only swell into bulbs when an anemonefish is living in it, and nobody is quite sure why.

Where: Reef flats. Best: Year round, best Feb-Sep.

Giant Barrel Sponge Common

Xestospongia testudinaria

Filters tens of thousands of litres a day and can live for well over a century, which is why divers call them the redwoods of the reef.

Where: Deeper boulder slopes. Best: Year round, best Feb-Sep.

Pincushion Urchin Common

Tripneustes gratilla

Decorates itself with shell, weed and rubbish held on with its tube feet, apparently as a sunshade.

Where: Seagrass and rubble. Best: Year round, best Feb-Sep.

Synchronous Firefly Regular

Pteroptyx malaccae

Mangrove trees hung with thousands of fireflies, all flashing in perfect unison like living fairy lights.

Where: Riverside mangroves on the mainland coast. Best: Year round, best in the rains, after dark. Evening boat trips run from the mainland rather than the island itself.