Similan Islands Marine Life: What You Will Actually See

Spinner Dolphin Regular

Stenella longirostris

A small, slender dolphin that leaps clear of the sea and spins like a top in mid-air.

Where: Open water on the crossings between islands. Best: Nov-Apr, morning crossings. Often bow-rides the dive boats on the run out to Koh Bon.

Pantropical Spotted Dolphin Occasional

Stenella attenuata

Calves are born unspotted and acquire their speckling through life, so you can read a dolphin's age off its skin.

Where: Open water on the outer crossings. Best: Nov-Apr, outer crossings.

White-bellied Sea Eagle Regular

Haliaeetus leucogaster

A huge coastal eagle in grey and white that snatches sea snakes and fish from the surface in a spectacular swoop.

Where: Over the island cliffs and Donald Duck Bay. Best: Nov-Apr, morning.

Black-naped Tern Occasional

Sterna sumatrana

Almost pure white apart from a narrow black band across the back of the head, like a headphone cable.

Where: Rocky islets and boat wakes around the islands. Best: Nov-Apr.

Banded Sea Krait Regular

Laticauda colubrina

A sea snake that hunts eels underwater but must come ashore to rest, drink fresh water and lay its eggs.

Where: Shallow reef and rubble at the island edges. Best: Nov-Apr, day. Highly venomous but placid; it is hunting eels, not you.

Green Sea Turtle Regular

Chelonia mydas

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

Where: Seagrass and shallow reef at Donald Duck Bay and Anita's Reef. Best: Nov-Apr, nesting Nov-Feb.

Hawksbill Turtle Occasional Critically Endangered

Eretmochelys imbricata

The reef's sponge specialist, its beautiful shell also the reason it was hunted to the edge.

Where: Coral bommies and sponge-covered walls throughout. Best: Nov-Apr.

Clark's Anemonefish Common

Amphiprion clarkii

A hardy, boldly banded anemonefish that will share its stinging anemone with a confidence bordering on cheek.

Where: Anemones on the shallow reef throughout. Best: Nov-Apr.

Common Lionfish Common

Pterois volitans

A flamboyant reef hunter fanned out in maroon and white plumes, every graceful spine tipped with venom.

Where: Overhangs and wrecks throughout. Best: Nov-Apr.

False Clownfish (Clown Anemonefish) Common

Amphiprion ocellaris

The famous orange and white anemonefish, immune to the stinging tentacles that shelter it and every group ruled by a single dominant female.

Where: Anemones on shallow reef at Anita's Reef and East of Eden. Best: Nov-Apr.

Moorish Idol Common

Zanclus cornutus

An unmistakable disc of black, white and gold trailing a long ribbon fin, one of the most elegant fish on any reef.

Where: Reef tops and bommies throughout. Best: Nov-Apr.

Powder Blue Surgeonfish Common

Acanthurus leucosternon

A stunning Andaman signature fish, powder blue with a yellow sail and a scalpel-sharp blade at the base of its tail.

Where: Surge zones and shallow reef edges. Best: Nov-Apr. A signature Andaman fish, and Similan is the best place in Thailand to see them in numbers.

Bluespine Unicornfish Regular

Naso unicornis

A large grey surgeonfish that grows a blunt horn on its forehead and carries bright blue blades at the base of its tail.

Where: Reef edges in current. Best: Nov-Apr.

Emperor Angelfish Regular

Pomacanthus imperator

A living work of art in electric blue and yellow stripes, its young disguised in a maze of blue and white rings.

Where: Coral heads and overhangs on the east side reefs. Best: Nov-Apr.

Giant Moray Regular

Gymnothorax javanicus

The largest moray on the reef, its gaping, fanged mouth a fearsome look that is really just how it breathes.

Where: Crevices at Elephant Head Rock and West Ridge. Best: Nov-Apr.

Palette Surgeonfish (Regal Blue Tang) Regular

Paracanthurus hepatus

The famous royal-blue tang of film fame, hovering over coral and diving into the branches when danger looms.

Where: Branching coral on the eastern reefs. Best: Nov-Apr.

Sixbar Angelfish Regular

Pomacanthus sexstriatus

A big, bold angelfish that grunts audibly when it is annoyed with you.

Where: Bommies and rubble slopes. Best: Nov-Apr.

Skunk Anemonefish Regular

Amphiprion akallopisos

A single white stripe down the spine, and a strict queue: when the female dies, the biggest male becomes her.

Where: Anemones on sheltered slopes. Best: Nov-Apr.

Titan Triggerfish Regular

Balistoides viridescens

The reef's bad tempered bulldozer, a heavy triggerfish that bites chunks out of coral to get at what is living underneath it.

Where: Reef tops throughout, nesting in sandy craters. Best: Nov-Apr. Nesting adults hold a cone of water above the nest and will see off fish far larger than themselves.

Bumphead Parrotfish Occasional Vulnerable

Bolbometopon muricatum

A hulking, bull-headed parrotfish that rams and bites living coral, turning reef rock into the white sand of tropical beaches.

Where: Reef tops at dawn on the outer islands. Best: Nov-Apr, first dive of the day. Best on the first dive of the day, before the schools break up.

Longnose Hawkfish Occasional

Oxycirrhites typus

A dainty fish gridded in red like graph paper, perched among the branches of a giant sea fan waiting to ambush prey.

Where: Sea fans on current-swept walls. Best: Nov-Apr.

Reef Stonefish Occasional

Synanceia verrucosa

The most venomous fish in the world, a living lump of encrusted rock that is almost impossible to see until it is too late.

Where: Rubble and reef base throughout. Best: Nov-Apr. Genuinely dangerous and near invisible. Never place a hand or knee on the bottom.

Regal Angelfish Occasional

Pygoplites diacanthus

One of the most beautiful fish on the reef, banded in orange and white with electric-blue edging like a living flag.

Where: Sheltered coral gardens. Best: Nov-Apr.

Ribbon Eel Occasional

Rhinomuraena quaesita

A ribbon of electric blue and yellow that sways from a hole in the sand, and remarkably changes sex and colour as it grows.

Where: Sand and rubble margins at Anita's Reef. Best: Nov-Apr. Blue adults and black juveniles occupy the same burrows for months.

Clown Triggerfish Rare

Balistoides conspicillum

Perhaps the most flamboyant fish on the reef, a black triggerfish spotted with huge white polka-dots and a saddle of gold.

Where: Deeper walls on the west side. Best: Nov-Apr. A prize for photographers; usually deep and moving.

Giant Grouper Rare

Epinephelus lanceolatus

The giant of the reef, a grouper that can grow larger than a diver and swallow prey the size of a small shark.

Where: Caves and deep ledges at Elephant Head Rock. Best: Nov-Apr.

Humphead Wrasse (Napoleon Wrasse) Rare Endangered

Cheilinus undulatus

A giant, curious reef fish with thick lips and a bulging forehead, able to crunch prey that few other animals can eat.

Where: Outer reef slopes at Koh Bon and Christmas Point. Best: Nov-Apr, best in calm Feb-Apr water. Large individuals are curious and will follow a diver.

Ornate Ghost Pipefish Rare

Solenostomus paradoxus

A dazzling relative of the seahorse that hangs head-down among crinoids, its frilled body vanishing into the feathery arms.

Where: Crinoids and sea fans on sheltered slopes. Best: Nov-Apr.

Painted Frogfish Rare

Antennarius pictus

A lumpy anglerfish that walks on hand-like fins and fishes for prey with a built-in rod and lure on its forehead.

Where: Sponges on the west-side walls. Best: Nov-Apr.

Indo-Pacific Sailfish Ultra rare

Istiophorus platypterus

The fastest fish in the sea, and it lights up electric blue across the sail when it is herding a bait ball.

Where: Blue water on the Koh Tachai and Richelieu crossings. Best: Feb-Apr.

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: Shallow reef flats and Donald Duck Bay. Best: Nov-Apr, all day.

Bluespotted Ribbontail Ray Regular

Taeniura lymma

A small, brilliantly marked ray, olive gold studded with electric blue spots, tucked under coral ledges.

Where: Sand pockets and under table corals throughout. Best: Nov-Apr.

Whitetip Reef Shark Regular Vulnerable

Triaenodon obesus

A slender reef shark that rests motionless in caves by day and hunts the reef in packs by night.

Where: Under table corals and ledges at Christmas Point and Sharkfin Reef. Best: Nov-Apr, day. Sleeping under overhangs by day; look into the shade rather than the blue.

Zebra Shark (Leopard Shark) Regular Endangered

Stegostoma tigrinum

A gentle, spotted shark that lies on the sand like a giant lazy cat, striped as a pup and spotted as an adult.

Where: Sandy patches at East of Eden, Anita's Reef and Elephant Head Rock. Best: Nov-Apr, morning dives, best Dec-Mar. Rests on sand in the open; approach slowly from the side and it will usually stay.

Giant Manta Ray Occasional Endangered

Mobula birostris

The largest ray on the planet, an ocean wanderer that can span seven metres and loops through plankton clouds in slow somersaults.

Where: Koh Bon ridge and Koh Tachai pinnacle. Best: Feb-Apr, peak at Koh Bon. Koh Bon is the classic manta station; hang off the ridge and wait rather than chase.

Grey Reef Shark Occasional Endangered

Carcharhinus amblyrhynchos

The classic requiem shark of the drop-off, bold and inquisitive where currents sweep the reef.

Where: Deeper walls and current-swept corners of the outer islands. Best: Nov-Apr.

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 at Anita's Reef and Donald Duck Bay. Best: Nov-Apr.

Ocellated Eagle Ray Occasional Endangered

Aetobatus ocellatus

A dramatic ray that flies through open water on wings spangled with white spots, trailing a whip-like venomous tail.

Where: Drop-offs at Koh Bon and Christmas Point. Best: Nov-Apr, more frequent Feb-Apr. Usually passing in open water; look out into the blue on every safety stop.

Reef Manta Ray Occasional Vulnerable

Mobula alfredi

A four metre wingspan gliding overhead in slow, hypnotic loops that leave divers grinning for days.

Where: Koh Bon and Koh Tachai. Best: Feb-Apr, occasional from Jan.

Tawny Nurse Shark Occasional Vulnerable

Nebrius ferrugineus

A big, docile bottom shark that vacuums prey out of crevices with a powerful suck.

Where: Caves and overhangs at Elephant Head Rock. Best: Nov-Apr, day.

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: Koh Bon, Koh Tachai and the outer pinnacles on liveaboard itineraries. Best: Feb-Apr. The single most sought sighting on any Similan trip, and worth the liveaboard over a day boat.

Bowmouth Guitarfish Ultra rare Critically Endangered

Rhina ancylostomus

Half shark, half ray, with ridges of thorns above the eyes that look like a set of eyebrows.

Where: Sand beside reef on the outer islands and the Burma Banks crossings. Best: Feb-Apr. A once-in-a-career sighting for most divers in Thai waters.

Tiger Shark Ultra rare Near Threatened

Galeocerdo cuvier

The only shark with teeth built to shear turtle shell, and the least fussy eater in the ocean.

Where: Deep water off the outer pinnacles. Best: Feb-Apr.

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 safety stops and night dives. Best: Nov-Apr.

Chromodoris Nudibranch Regular

Chromodoris species

A tiny sea slug in outrageous colours that borrows toxins from the sponges it eats and flaunts them as a warning.

Where: Sponges on the west-side walls. Best: Nov-Apr.

Broadclub Cuttlefish Occasional

Ascarosepion latimanus

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

Where: Reef edges and sand margins. Best: Nov-Apr.

Day Octopus Occasional

Octopus cyanea

A shape-shifting reef genius that can change colour and texture in a blink and pour its boneless body through a coin-sized gap.

Where: Reef and rubble by day on the eastern sites. Best: Nov-Apr.

Giant Clam Occasional Critically Endangered

Tridacna gigas

The largest shellfish on Earth, a metre-wide clam that farms algae in its own rippling, jewel-coloured flesh.

Where: Shallow reef tops at Anita's Reef and Donald Duck Bay. Best: Nov-Apr.

Spanish Dancer Occasional

Hexabranchus sanguineus

A giant crimson sea slug that unfurls its frilled body and swims through open water like a flamenco dancer's skirt.

Where: Night dives on rubble and reef base. Best: Nov-Apr, night.

Triton's Trumpet Rare

Charonia tritonis

A giant sea snail with a beautiful spiral shell that is one of the very few animals to hunt and eat the coral-killing crown-of-thorns.

Where: Reef base and rubble. Best: Nov-Apr.

Brain Coral Common

Platygyra species

A boulder of living stone grooved in winding ridges and valleys, so like a brain that the name writes itself.

Where: Reef tops throughout. Best: Nov-Apr.

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 and rubble margins. Best: Nov-Apr.

Magnificent Sea Anemone Common

Heteractis magnifica

A great carpet of stinging tentacles that is home and fortress to the anemonefish, a partnership at the heart of the reef.

Where: Shallow reef tops throughout. Best: Nov-Apr.

Staghorn Coral Common

Acropora species

The fast-growing branching coral that builds the living thickets of the reef, nursery and shelter for a whole community.

Where: Shallow eastern reef flats. Best: Nov-Apr.

Table Coral Common Near Threatened

Acropora hyacinthus

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

Where: Eastern slopes at East of Eden. Best: Nov-Apr.

Carnation Soft Coral Regular

Dendronephthya species

An explosion of pink, red and orange bloom that inflates with water to feed, turning shaded walls into hanging gardens.

Where: Shaded overhangs and swim-throughs at Elephant Head Rock. Best: Nov-Apr.

Giant Sea Fan (Gorgonian) Regular

Annella species

A great lattice fan spread across the current, a soft coral that farms the flow for plankton and shelters seahorses and hawkfish.

Where: Current-swept walls at Koh Bon and Elephant Head Rock. Best: Nov-Apr.

Feather Star (Crinoid) Common

Comatulida species

A living bouquet of feathery arms that clings to high points on the reef by day and can crawl, or even swim, to a better spot.

Where: Sea fans and coral heads in current. Best: Nov-Apr.

Long-spined Sea Urchin Common

Diadema setosum

A pincushion of needle-sharp black spines with a glowing orange ring, grazing algae and quietly shaping the reef.

Where: Rubble and reef base, especially at night. Best: Nov-Apr.

Banded Coral Shrimp Regular

Stenopus hispidus

A candy-striped shrimp with impossibly long white antennae that it waves to advertise its parasite-cleaning service.

Where: Cleaning stations under overhangs. Best: Nov-Apr.

Peacock Mantis Shrimp Occasional

Odontodactylus scyllarus

A jewel-coloured reef brawler that punches with the speed of a bullet and sees the world in colours we cannot imagine.

Where: Sand and rubble burrows at Anita's Reef. Best: Nov-Apr.

Harlequin Shrimp Rare

Hymenocera picta

A shrimp painted like fine porcelain that hunts as a devoted pair and eats nothing but living starfish.

Where: Under ledges on rubble slopes. Best: Nov-Apr. Found by guides who know the site; almost never spotted alone.

Coconut Crab Ultra rare Vulnerable

Birgus latro

The largest land invertebrate on earth, with a grip strong enough to crack a coconut and hold ninety times its own body weight.

Where: Forested interiors of the outer uninhabited islands. Best: Nov-Apr, at night. Access is restricted; this is a park ranger sighting rather than a visitor one.

Christmas Tree Worm Common

Spirobranchus giganteus

Pairs of tiny spiral trees in every colour, studding the coral and vanishing in a flash when a shadow passes.

Where: Embedded in massive corals on the reef tops. Best: Nov-Apr.

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 slopes on the west side. Best: Nov-Apr.