211 terms, defined in plain English. Every term used in a species entry is here and is linked the first time it appears on a page. The rest are the words a diver meets everywhere else: the parts of a shark, a jellyfish, a shell and a worm; how animals live together and where; and what the abbreviations in a scientific name mean.
Ten body plans, drawn with the parts named. Every label is a term from the list below and links to it, and every entry that appears on a plate says which one. The plates cover the groups whose anatomy is hardest to describe in words; the terms below cover the rest.
A dorid seen from above. The gill plume, the pair of rhinophores and the broad mantle are the three characters that place an animal here rather than among the aeolids or the flatworms.
An aeolid. No gill plume: the cerata do the breathing, carry a branch of the digestive gland, and end in a cnidosac holding stinging cells taken intact from the animal's prey.
The perch-like plan most reef fish are built on. Fin position, fin counts and what is marked on the caudal peduncle carry most of the weight in an identification key.
A frogfish. The first dorsal spine has become a fishing rod: the illicium carries the esca, and the shape of the esca is often the most reliable way to tell two frogfish apart.
An octopus has eight arms; a squid or cuttlefish has eight arms and two longer tentacles it shoots out to take prey. Both jet, breathe and ink through the siphon.
The decapod plan. Everything in front of the joint behind the carapace is head and thorax; everything behind it is pleon, and the pleon is what a shrimp flicks to shoot backwards.
A mantis shrimp. The raptorial appendage is the whole animal's argument: spearers flick out a barbed dactyl to seize soft prey, smashers strike with the heel.
Every character used to name a seahorse is on the outside: the coronet, the length of the snout, the count of trunk and tail rings, and the skin filaments some species carry.
Echinoderm anatomy is arranged around the mouth: the oral surface carries the ambulacral grooves and the tube feet, and the aboral surface carries the madreporite that lets water into the system that works them.
The polyp is the unit. An anemone is one large polyp; a hard coral is a colony of small ones, each sitting in a corallite of its own making.
The surface of an echinoderm facing away from the mouth: the upper side of a sea star, the side that carries the madreporite. The mouth side is the oral surface.
Labelled on Sea star and urchin
A small fleshy fin on the back between the dorsal and the tail, with no supporting rays. Among fish a diver meets it is mainly the lizardfish and the eeltail catfish that carry one.
A nudibranch with the back covered in cerata rather than carrying a gill plume. Aeolids feed on hydroids, anemones and soft corals, and most of them store their prey's stinging cells for their own defence.
Resembling something harmless in order to get close to prey. The false cleanerfish copies the cleaner wrasse and bites the fish that comes to be cleaned.
The channel running out along the underside of each arm of a sea star, carrying the tube feet. It is where the commensal shrimps and parasitic snails of a sea star are usually found.
Labelled on Sea star and urchin
A small crustacean flattened from side to side, with no carapace over the thorax. Includes the skeleton shrimps, which hold on to a hydroid with the rear legs and reach into the water with the front pair.
Jelly-filled pores in the snout of a shark or ray that detect the weak electric fields produced by muscle activity. They are how a shark finds a fish buried in sand, and are visible as a field of dark dots around the mouth.
The unpaired fin on the underside of a fish, behind the vent. Its ray count and the number of spines in front of it are standard characters in an identification key.
Labelled on Bony fish
The longer, whip-like pair of sensory appendages on a crustacean's head, set behind the antennules. In many shrimps they are several times the length of the body, and they are usually the first part of the animal a torch finds.
Labelled on Decapod shrimp
The shorter pair of sensory appendages in front of the antennae, carrying most of a crustacean's chemical sense. They branch; the antennae behind them do not, which is how the two pairs are told apart.
Labelled on Decapod shrimp
Conspicuously coloured as a warning that the animal is toxic, venomous or otherwise not worth eating. The blue rings of Hapalochlaena are the clearest example here.
Conspicuous colouring that advertises being toxic or dangerous. It only works if predators learn it, which is why several unrelated toxic species converge on the same pattern - Mullerian mimicry - and why a harmless species sometimes copies it.
The five-part jaw apparatus inside a sea urchin, worked by a set of muscles and carrying five self-sharpening teeth. It is what lets an urchin graze rock and, in some species, bore into it.
A sessile filter-feeding chordate, solitary or colonial, drawing water in through one siphon and out through another. The swimming larva has a notochord and a tail and gives them up on settling. Food for several nudibranchs and flatworms.
A ring of reef around a lagoon with no central island, formed as a volcanic island subsided beneath a reef that kept growing upward.
A coral that carries no symbiotic algae and feeds entirely by catching plankton. These are the corals that can live in caves, under overhangs and in deep or turbid water, and their polyps are usually out by day as well as at night.
A reef running parallel to the coast but separated from it by a lagoon deep and wide enough to be a distinct body of water.
An edible or harmless animal taking on the appearance of a defended one, so that predators which have learned to avoid the model also avoid the copy. The mimic filefish and Valentin's sharpnose puffer are the clearest pair on these reefs; the copy only works while the model is the commoner of the two.
A ray, skate or guitarfish: an elasmobranch flattened from top to bottom, with the gill openings on the underside and the pectoral fins fused to the head to form the disc it swims with.
Living on or in the seabed rather than swimming in the water above it. Nearly everything in this reference is benthic.
The two-part scientific name of a species: genus first and capitalised, species second and lower case, both italicised. Common names vary between countries and languages; the binomial does not.
Light of one colour absorbed by an animal and re-emitted at another. Unlike bioluminescence it produces no light of its own and shows only under a blue light with a yellow filter over the lens.
Light produced by the animal itself, by a chemical reaction or by bacteria it keeps. Used for signalling, for luring prey and for counter-illumination, and best seen on a night dive with lights off.
A mollusc with a shell in two hinged halves: clams, scallops, oysters, pen shells and the giant clams. Most filter feed, and the giant clams also farm algae in their mantle tissue.
The loss of a coral's symbiotic algae, usually because the water has been too warm for too long. The coral turns white because the algae carried its colour. It is not dead at that point, but it is starving, and it dies if the water does not cool.
An isolated tower or mound of coral standing clear of the surrounding sand or reef flat. Small ones hold a complete community and can be worked all dive.
The armour of a seahorse or pipefish: bony plates fused into rings around the body in place of scales. Trunk and tail rings are counted in identification, and they are why a seahorse is stiff to the touch and why it bends only at the tail.
Labelled on Seahorse
The enclosed pocket on the belly of a male seahorse or pipefish that carries the eggs until they hatch. The female deposits the eggs into it and the male fertilises them there.
Labelled on Seahorse
A colonial animal forming lace, moss or twig-like encrustations on hard surfaces, each colony made of many tiny feeding individuals. Food for Tambja and its relatives, from which they take their pigments and their defence.
The bundle of tough threads a mussel, a pen shell or a juvenile giant clam spins to anchor itself. It is protein, secreted as a liquid that sets in seawater.
The cup at the top of a corallite, where the living polyp sits. Its diameter and depth separate species that otherwise look alike.
The hard shield covering the front part of a crustacean's body, from behind the eyes to the start of the abdomen.
Labelled on Decapod shrimpMantis shrimp
Minute stalked papillae, each ringed with calcareous spicules and topped by a sensory knob, covering the back of some dorid nudibranchs. They are what gives Jorunna its velvet texture and the sea bunny its fur.
Labelled on Dorid nudibranch
The hinged, forward-facing scalpel at the base of a surgeonfish's tail, and the fixed pair of blades in the unicornfishes. It is used against rivals and predators, is sharp enough to cut deeply, and in some species carries venom. It is a hazard to handling a fish, not to swimming past one.
Labelled on Bony fish
The tail fin. Its outline - rounded, truncate, emarginate, forked or lunate - follows how the fish swims, and it is one of the more useful things to note in the water.
A ridge along each side of the tail base on fast open-water fish and sharks. It stiffens the tail and reduces drag, and is one of the marks of an animal built for continuous swimming.
The narrow part of a fish's body between the rear of the anal and dorsal fins and the base of the tail. A great deal of identification hangs on what is marked there: a spot, a bar, a saddle or a blade.
Labelled on Bony fish
The body of a sea star or brittle star, from which the arms extend. In brittle stars it is sharply set off from the arms; in most sea stars the two grade into one another.
Labelled on Sea star and urchin
The flattened, sideways-extended head of a hammerhead shark. It spreads the ampullae of Lorenzini over a wider area and gives the animal a wider field of view.
The finger-like outgrowths along the back of an aeolid nudibranch, carrying extensions of the gut and, at their tips, the cnidosacs.
Labelled on Aeolid nudibranch
A whale, dolphin or porpoise. Air-breathing mammals fully adapted to water, with a horizontal tail fluke, and in the toothed species the ability to echolocate.
From the Latin for compare. Written between the genus and species name it means the animal resembles that species but the identification is not certain.
A clawed leg of a crustacean, the first pereiopod in most decapods. Its size, shape and armament are identification characters, and in several shrimps and crabs the two are markedly unequal.
Labelled on Decapod shrimp
The photosynthesising organelle of a plant or alga. Sacoglossan sea slugs pierce algal cells and keep the chloroplasts alive and working in their own tissue for days to weeks - kleptoplasty. The genes needed to maintain one belong to the alga, not the slug, which is why the arrangement eventually runs down.
A pigment-bearing cell that an animal can expand or contract to change colour. The layered chromatophores of cephalopods are what allow a cuttlefish to change appearance in under a second.
Labelled on Squid and octopus
Poisoning from eating reef fish that have accumulated toxins originating in dinoflagellates growing on reef surfaces. The toxins concentrate up the food chain, so large predators carry most of the risk, and they survive cooking. Nothing about a fish's appearance indicates it. It is a hazard of eating reef fish, not of diving with them.
Found in tropical waters around the world. Used in the entries in place of a region when a species genuinely occurs in all three ocean basins.
Slender fleshy filaments, most often on the head or above the eye. A field character in scorpionfish, blennies and some nudibranchs.
Labelled on Seahorse
The paired rod on the inner edge of the pelvic fins of a male shark or ray, used in mating. Present or absent is the reliable way to sex an elasmobranch in a photograph.
A fixed place on a reef where cleaner wrasses, cleaner shrimps or juvenile fish remove parasites and dead tissue from larger animals that queue for it. Because the place is fixed rather than the animal, a cleaning station is the most reliable way to see a manta, a turtle or a large grouper hold still.
The cell that holds a nematocyst. Cnidarians are named for these cells, and they are the reason a jellyfish, an anemone, a hydroid and a fire coral all sting.
A sac at the tip of a nudibranch's cerata storing stinging cells taken intact from the animal's prey and reused for its own defence.
Labelled on Aeolid nudibranch
The sticky cell a comb jelly catches prey with. Comb jellies have no nematocysts and do not sting: they glue their food instead.
The central pillar of a coiled snail shell, visible down the middle of the opening. Its folds and ridges are diagnostic in several families.
One of the eight rows of fused cilia a comb jelly swims with. They beat in waves and split light as they do, which is the running rainbow seen along the animal. It is refraction, not bioluminescence.
Living on or with another species and benefiting from it, without obviously harming or helping the host. A great many muck subjects are commensal on urchins, sea cucumbers, corals and sponges.
The area covering Indonesia, Malaysia, the Philippines, Papua New Guinea, Timor-Leste and the Solomon Islands. It holds more reef fish and coral species than anywhere else on earth.
The cup of skeleton laid down by a single hard-coral polyp. The size, spacing and wall structure of the corallites is how a hard coral is identified, and it is what remains visible when the polyps are withdrawn.
Labelled on Cnidarian polyp
The crown of bone on the top of a seahorse's head. Its height and shape are used in identification, and in some species no two individuals carry quite the same one.
Labelled on Seahorse
Producing light on the underside of the body to match the faint daylight coming from above, so that nothing looking up sees a silhouette. It is why many open-water and deep-water animals carry photophores on the belly and not on the back.
Dark above and pale below. Seen from above the animal matches the dark water below it, seen from below it matches the bright surface. It is the commonest colour scheme in open water and it is camouflage, not decoration.
Active at dusk and dawn. The changeover period, when the day shift is going in and the night shift is coming out, and when several species mate.
A feather star: an echinoderm with a small disc, a ring of feathered filtering arms and short gripping cirri underneath. Habitat for a squat lobster, a shrimp and a clingfish, each coloured to match the arms of its own host.
Concealed by resembling the background. Distinct from mimicry, where an animal resembles another organism rather than the surroundings.
Small bottom-living fish that spend their lives hidden in rubble, sponge or coral. They are a large share of reef fish by number, are eaten constantly, and are mostly overlooked by divers.
Sticky white threads some sea cucumbers eject when disturbed, which tangle and immobilise a small attacker. They stick firmly to skin and to a camera port.
The last, movable segment of a crustacean limb: the finger of a claw, the point of a walking leg. In mantis shrimps it is the part that does the damage - barbed and folding in the spearers, swollen into a calcified heel in the smashers.
Labelled on Mantis shrimp
A crustacean with ten legs: the shrimps, crabs, hermit crabs, squat lobsters and lobsters. Most of the crustaceans a diver photographs are decapods.
Living close to the bottom without resting on it permanently.
The tooth-like scale that covers shark and ray skin, made of the same material as a tooth and pointing backwards. It is what makes shark skin feel smooth one way and rough the other, and it reduces drag.
Filaments and tassels of skin that break up an animal's outline against algae or hydroids. Characteristic of hairy frogfish, rhinopias and several scorpionfish.
Labelled on Frogfish
The branched gland that does the work of a liver and much of the gut in a mollusc. In aeolid nudibranchs a branch runs up into every ceras, which is why the animal takes its colour from what it has been eating.
Labelled on Aeolid nudibranch
Bold bands or blotches that break up the outline of an animal so the eye does not read it as one shape. The bar through the eye on many reef fish is the same idea applied to the one feature a predator looks for.
Active by day. Used in the entries alongside nocturnal and crepuscular to say when a species is worth looking for.
The largest group of nudibranchs: a broad flat mantle, a pair of rhinophores at the front and a ring or plume of gills on the back that can usually be retracted. Most feed on sponges. The phyllidiids are dorids that have given up the dorsal gill and breathe under the mantle margin instead.
The fin along a fish's back, in many families divided into a spinous first part and a soft-rayed second. Spine and ray counts here are among the most-used characters in fish identification.
A dive made by letting the current carry you, with the boat following the surface marker. The alternative to swimming against water that will win.
The point where a reef ends and the bottom falls away steeply. Big animals patrol along it, the water is usually clearer off the edge, and depth is easy to lose track of.
Finding objects by making a sound and listening to the echo. Dolphins produce clicks through the nasal passages, focus them with the melon, and hear the returns through the lower jaw.
The horny capsule an oviparous shark, skate or catshark lays, usually with tendrils that anchor it to a sea fan or a rock. Empty cases wash up; a full one held to a light shows the embryo.
A shark, ray or skate. The group has a skeleton of cartilage rather than bone, no swim bladder, and skin covered in tooth-like denticles rather than scales. Without a swim bladder a shark either keeps moving or rests on the bottom.
Found only in one region and nowhere else. The Red Sea and the Galapagos both have a high proportion of endemic fish, which is a large part of why divers go to them.
The lure at the tip of an anglerfish's first dorsal spine, waved to draw prey within reach. Its shape is species-specific and is often the most reliable way to tell frogfish apart.
Labelled on Frogfish
The ejection of part of the gut by a sea cucumber under stress. It is a defence, and the animal regrows what it lost, but it is a serious cost and is one reason not to handle them.
The hard external skeleton of a crustacean or other arthropod. It cannot expand, which is why growth happens in steps at each moult rather than continuously.
A movable stalk carrying the eye, found in crabs, shrimps and mantis shrimps. It lets the eye be raised, aimed and folded away independently of the body.
Labelled on Decapod shrimpMantis shrimp
The horizontal tail of a whale, dolphin or dugong. It beats up and down, unlike a fish tail, which beats side to side.
The muscular sole a mollusc creeps on. In sea slugs it runs out behind the body as a tail and its front corners are sometimes drawn into points; in cephalopods it has become the arms and the siphon.
Labelled on Dorid nudibranchAeolid nudibranch
A reef growing directly out from the shore, with no lagoon between. The commonest reef type and the one most shore dives are on.
Torpedo-shaped: thickest a third of the way back and tapering at both ends. The body plan of a fish that swims continuously in open water, such as a tuna, a jack or a barracuda.
The mouth at full opening. Frogfish and the other anglerfish have the largest gape relative to body size of any fish and expand it fast enough that the strike is over in a few thousandths of a second, which is why it is almost never seen.
Labelled on Frogfish
A snail or slug. The group includes the shelled sea snails, the sea hares, the pleurobranchs and the nudibranchs, and the shell has been reduced or lost independently many times within it.
The ring or fan of feathery gills on the back of a dorid nudibranch, usually retractable into a pocket. Its absence places an animal in the phyllidiids, the aeolids or the flatworms rather than among the dorids.
Labelled on Dorid nudibranch
The comb of projections along the inner edge of a fish's gill arches. Long, fine rakers strain plankton from water passing over the gills, which is how fusiliers, mackerel and manta rays feed; short, stout ones sort and hold prey that has already been caught.
Labelled on Bony fish
The boundary between water of different salinity, seen as an oily shimmer. It occurs where fresh water meets salt, at a river mouth or in a cavern.
A group of one adult male and several females holding a shared area, usual in anthias, dwarf angelfish and many wrasses. In the protogynous species the arrangement is unstable by design: lose the male and the dominant female becomes one.
The modified arm with which a male cephalopod passes a sperm packet to a female. Which arm it is, and how it is modified, is a species character; in a few octopuses it detaches and is left behind.
Labelled on Squid and octopus
A sea cucumber: an echinoderm with a soft elongate body, a ring of feeding tentacles at the mouth and, in many species, sticky defensive tubules it can eject. Several process sediment for a living and leave coiled casts on the surface.
A cnidarian that spends most of its life as a branching colony of polyps fixed to the bottom, often looking like a fern or a feather. Several sting sharply, and many nudibranchs and shrimps live only on them.
A member of the class that includes the hydroids, fire corals and siphonophores. They are cnidarians, related to jellyfish and anemones rather than to the true corals, and most of them sting. A great deal of what a diver takes for coral or weed on a reef is hydrozoan.
The modified first dorsal spine of an anglerfish that carries the esca — the rod, to the esca's bait.
Labelled on Frogfish
The first adult colouring of a wrasse or parrotfish, worn by females and by males that have not yet changed. It is usually plain and is frequently different enough from the terminal male to be logged as another species.
The gland in which a cephalopod stores ink, released through the siphon. It works as a screen and also as a decoy, since a cloud held together by mucus is about the size and shape of the animal that left it.
Labelled on Squid and octopus
A crustacean flattened from top to bottom, with seven pairs of similar legs. The marine ones a diver notices are parasites riding on fish, in the mouth or on the flank.
Sheltered water inside a reef or an atoll. Usually shallow, often sandy, and calmer and warmer than the outside of the reef.
The line of pored scales along a fish's flank, opening into a canal of sensory cells that read water movement. Its course, and the point at which it is interrupted or steps down, is an identification character.
Labelled on Bony fish
Crescent-shaped, of a tail: deeply forked with the lobes drawn out and the trailing edge curved. It goes with sustained fast swimming, which is why it is found in trevallies, tunas and the open-water anthias and unicornfishes rather than in reef-bound fish.
The perforated plate on the upper surface of a sea star or urchin, set off-centre between two arms, through which water enters the animal's hydraulic system. It usually shows as a pale button.
Labelled on Sea star and urchin
Salt-tolerant trees growing in the intertidal zone, whose submerged roots hold juvenile fish, sponges and anemones. The water is usually green and shallow.
The body wall of a mollusc, which secretes the shell where there is one. In a dorid nudibranch it is the broad flat skirt covering the back and overhanging the foot, and its margin is where the phyllidiids breathe. In a cephalopod it is the muscular sac behind the head enclosing the organs, and mantle length is the standard measurement, since arms stretch and contract.
Labelled on Dorid nudibranchSquid and octopus
One of the appendages between a crustacean's mouthparts and its walking legs. In porcelain crabs the third pair carries long feathered setae and is swept through the water to strain out plankton.
Labelled on Decapod shrimp
The free-swimming, bell-shaped stage of a cnidarian: what everyone calls a jellyfish. Many cnidarians alternate between a fixed polyp stage and a drifting medusa stage, and a few skip one of the two.
The fatty structure in the forehead of a toothed whale or dolphin that focuses the clicks used in echolocation. It is what gives the animal its rounded profile.
One of the sheets of tissue that divide the inside of an anemone or coral polyp, carrying the digestive filaments and the reproductive tissue. Their number and arrangement is used in classification.
Labelled on Cnidarian polyp
The jelly layer between the two cell layers of a cnidarian. It is most of the volume of a jellyfish and is what makes the animal transparent.
The shedding of the old exoskeleton that lets a crustacean grow. The animal is soft and defenceless for hours afterwards and normally hides. A complete empty shell on the sand, with the legs and antennae intact, is a moult rather than a dead animal.
Diving on soft bottoms of sand, silt or volcanic sediment rather than on reef. The ground looks empty and holds a high number of specialised animals, most of them small, buried or living on a host.
A bag of mucus that many parrotfishes secrete around themselves before sleeping in a crevice. The working explanation is that it masks the fish's scent from night-hunting predators, particularly moray eels. Night divers find them wrapped this way regularly; the cocoon takes about half an hour to build and is abandoned at dawn.
A relationship in which both species gain. The shrimpgoby and its snapping shrimp share a burrow the shrimp digs and the goby watches; the anemonefish defends the anemone that shelters it.
The iridescent inner layer of some shells, laid down in sheets thin enough to split light. The same material coats an intruding grain of sand and forms a pearl.
The stinging capsule of a jellyfish, anemone, hydroid or coral. Each is a single cell holding a coiled, barbed thread under pressure, fired by touch. Aeolid nudibranchs eat cnidarians, pass the nematocysts through the gut undischarged, and store them in their own cerata to use.
Labelled on Aeolid nudibranchCnidarian polyp
A third eyelid that sweeps across the eye of some sharks to protect it, most obviously at the moment of a bite. Sharks without one, such as the white shark, roll the eye back instead.
Active at night. A large share of the animals in this reference are nocturnal, which is why the same site gives a different list after dark.
Restricted to one way of life, with no alternative. An obligate associate is found only on its host and cannot live independently, as opposed to a facultative one, which can.
An eye-like spot of colour. Used to startle predators or to draw an attack away from the real head.
A lid. In tube worms, the hard plug that seals the tube behind the withdrawing animal; in a fish, the bony cover over the gills.
Labelled on Bony fish
A brittle star: five thin arms sharply set off from a small central disc, moving by whipping rather than by tube feet. The arms detach at pre-formed breakage planes and regrow.
The thick, frilled arms hanging from the centre of a jellyfish bell, carrying the mouths. They are shorter and heavier than the tentacles around the rim.
The flat upper surface of an anemone or coral polyp, with the mouth at its centre and the tentacles around its rim.
Labelled on Cnidarian polyp
The pair of short sensory tentacles beside a sea slug's mouth, used to touch and taste the ground it is crossing. They are distinct from the rhinophores, which sit further back and read scent carried in the water.
Labelled on Dorid nudibranchAeolid nudibranch
A small calcareous element embedded in the body wall of an echinoderm. Their shape is a primary identification character in sea cucumbers, and in the synaptids they are anchor-shaped and catch on a wetsuit.
Labelled on Sea star and urchin
Carrying eggs. A female shrimp, crab or lobster holds the egg mass under the abdomen, and it is often visible through the shell before it is visible outside.
Laying eggs that develop outside the mother. Among sharks it is the catsharks, bamboo sharks and horn sharks, which leave a tough egg case attached to the bottom.
Producing eggs that hatch inside the mother, so the young are born live but were fed by the yolk rather than by the mother. Common among sharks and among scorpionfish.
Belonging to the Palaemonidae, the family holding most of the small commensal shrimps found on urchins, corals, anemones and sea cucumbers.
Small fleshy projections of the skin. Octopuses and frogfish raise and lower them to change outline and texture, which is what makes a hairy octopus hairy one moment and smooth the next.
Labelled on FrogfishSquid and octopus
Paired flaps along the sides of some sea slugs and worms. In Plakobranchus they fold over the back like a book and are opened flat to the light.
An animal that lives on or in another at its expense without normally killing it. The isopods on a cardinalfish and the barnacle inside a crab are the examples in this reference.
The paired fin behind the gill cover, answering to a forelimb. It does most of the swimming in wrasses and parrotfishes, and in frogfish it is lobed, jointed and used to walk.
Tiny stalked pincers scattered between a sea urchin's spines, used to clean the surface and deter settlers. In several fire urchins they are venomous, which is part of why an urchin makes a defensible home for a shrimp.
Labelled on Sea star and urchin
The paired fin on the underside, answering to a hind limb. In gobies the two are fused into a disc that holds the fish against the bottom; in frogfish they prop it up.
The five pairs of legs on the thorax of a decapod crustacean. The first pair, and sometimes the second, carry the claws; the rest walk.
Labelled on Decapod shrimp
A second set of grinding plates in the throat, behind the jaws. It is what lets wrasses, parrotfishes and emperors take hard-shelled prey with a mouth that looks incapable of it: the jaws seize, the pharyngeal teeth crush.
Recognising an individual animal from a natural marking that does not change over its life, and matching it against a catalogue. The belly pattern of a manta, the flank spots of a whale shark and the fin notches of a dolphin are all used this way; most of what is known about where these animals go comes from divers' photographs rather than from tags.
A seal, sea lion or walrus. Sea lions and fur seals have external ear flaps and can bring the hind flippers forward to walk; true seals cannot.
One of the small side branches along a feather star's arm, giving it the feathered look. Food is caught on the pinnules and passed down a groove in the arm to the mouth.
An animal that feeds on plankton drifting past rather than on anything attached to the reef. It explains a great deal of where fish are found: planktivores hang in the water column on the current-exposed side of a slope and drop into cover when threatened, which is why a wall in current holds fish that a sheltered bay does not.
The underside of a turtle's shell, joined to the carapace at the sides. Its pattern and the number of scutes on it are used in identification, and it is where a turtle's sex is judged from in adults.
The abdomen of a crustacean: the segmented muscular part behind the carapace, carrying the pleopods and ending in the tail fan. It is what a shrimp flicks to shoot backwards.
Labelled on Decapod shrimp
The paired limbs under a crustacean's abdomen, used for slow swimming and, in the female, for holding the eggs until they hatch.
Labelled on Decapod shrimp
Toxic when eaten. A puffer is poisonous, not venomous: it injects nothing, and it is only dangerous to something that tries to eat it.
A free-living marine flatworm. Thin enough to ripple, with a pair of folded pseudotentacles at the front margin and no gill plume and no rhinophores - which is how a flatworm is told from the nudibranch it is imitating.
The individual animal of a coral, anemone or hydroid: a column fixed at its base, with a ring of tentacles around a mouth at the top. A hard coral is a colony of them sharing one skeleton; an anemone is a single large one.
Labelled on Cnidarian polyp
A tail used for gripping. Seahorses, pygmy seahorses and pipehorses hold onto gorgonians, hydroids and weed with theirs, which is how they keep station in current that moves everything else.
Labelled on Seahorse
An extensible feeding tube that can be pushed out well beyond the body and drawn back in. A cone shell fires its harpoon from one, a nemertean worm hunts with one, and several sea slugs use one to reach into a prey animal.
The lobe in front of the mouth of a polychaete worm, carrying the eyes, antennae and palps. On a bobbit worm it is the part that shows above the burrow.
An animal that matures as a male and can change to female later. Anemonefish and ribbon eels both do it, which is why the largest fish in an anemonefish group is the female and why a yellow ribbon eel was once a blue one.
An animal that begins life as a female and can become male later. It is the rule in wrasses, parrotfishes, anthias and many groupers, and it is why the same species can appear on one reef as two: the terminal male is a changed female and looks nothing like her. The counterpart, male first, is a protandrous hermaphrodite.
One of the feathered feeding filaments that make up the crown of a fan worm or a Christmas tree worm. They filter food from the water and carry the eyes that trigger the animal to withdraw.
The toothed ribbon a mollusc feeds with, drawn over the food like a rasp. Its tooth shape and count are primary identification characters in sea slugs. Two groups on this site have modified it past recognition: the sacoglossans have a single row of piercing teeth for opening algal cells, and the melibes have lost it entirely.
Labelled on Dorid nudibranch
Breathing by swimming forward with the mouth open so that water is forced over the gills. Sharks that use it cannot stop swimming for long; those with spiracles or an active gill pump can rest on the bottom.
The folded striking limb of a mantis shrimp, carried under the head. Spearers flick out a barbed dactyl to seize soft prey; smashers strike with a calcified heel hard enough to break shells, and the strike is fast enough to cavitate the water behind it.
Labelled on Mantis shrimp
The pair of sensory tentacles on a nudibranch's head, used to detect scent in the water. Their shape — lamellate, annulate, smooth — is a primary identification character.
Labelled on Dorid nudibranchAeolid nudibranch
The forward projection of a crustacean's carapace, between and beyond the eyes. Its length, shape and the number of teeth along it are standard identification characters.
Labelled on Decapod shrimp
A sea slug that feeds by piercing algal cells and sucking out the contents rather than by biting. Several keep the algae's chloroplasts alive inside themselves and draw nutrition from them. Not nudibranchs, though they are found alongside them and filed with them by everyone who dives.
The gland that lets a sea turtle, a sea snake or a seabird drink seawater by excreting the excess salt. In turtles it empties beside the eye, which is why a nesting turtle appears to be weeping.
A group of fish swimming in the same direction at the same speed and spacing, turning as one. It makes a single fish hard for a predator to pick out.
A small hard element, usually of calcium carbonate, embedded in the soft tissue of a soft coral or gorgonian, giving the colony its stiffness and its texture. Their shape is used to identify species that look identical alive. A sponge's equivalent is a spicule.
A bony or horny plate in a reptile's shell or skin. The arrangement of the scutes is what separates a green turtle from a hawksbill: overlapping plates and a serrated rear edge in the hawksbill, flat abutting plates and a smooth edge in the green.
A flowering plant, not an alga, growing rooted in sand in shallow water. Seagrass beds are nurseries for juvenile fish, and are what a dugong feeds on.
An underwater mountain rising from deep water without reaching the surface. Currents striking it push nutrients up, which is why seamounts hold large animals.
One of the radiating vertical plates inside a coral corallite. The number and arrangement of septa is a standard character in coral identification and shows clearly in a close photograph of a bare skeleton.
To take a chemical defence from food and store it rather than manufacture it. Sea slugs sequester sponge and bryozoan compounds, aeolids sequester stinging cells, and pufferfish sequester tetrodotoxin from their diet.
Bristles or hairs on a crustacean or a worm. In crabs they can be hooked, for holding decoration; in filter feeders they form the fans that strain plankton out of the water.
A loose group of fish staying together for company and safety, swimming at their own speeds and in their own directions. A school is the tighter, coordinated version of the same thing.
The muscular funnel through which a cephalopod expels water, used for jet propulsion, respiration and directing ink.
Labelled on Squid and octopus
A colony of specialised individuals that functions as a single animal, each part doing one job: floating, swimming, catching prey or reproducing. The Portuguese man o' war is one, which is why it is not a jellyfish.
A dugong or a manatee: a slow, plant-eating marine mammal with no dorsal fin. The dugong is the only sirenian in the Indo-Pacific and feeds entirely on seagrass.
The part of a fish's head in front of the eye. Its length against the head length is a standard measurement, and in seahorses, pipefish and seamoths it is drawn out into a tube that the animal feeds through.
A segmented, flexible, usually branched fin support. Counting soft rays against spines in each fin is one of the standard ways a fish is identified in a photograph.
Written after a genus name, it means the species has not been determined. Used in this reference where an animal is confidently placed in a genus but the species is unsettled or undescribed. The plural is spp.
A gathering of a species at a particular place and time to spawn, often the same reef corner within a few days of the same moon each year. It is the most productive moment in the animal's year and the most dangerous: an aggregation is predictable, and several groupers and the bumphead parrotfish have been reduced across their range by being fished on one.
A group of very similar species that have not been fully separated, or that are separated on characters a photograph does not show. The entries say so where an identification is limited by this rather than by the photograph.
A microscopic mineral rod embedded in the tissue of a sponge or soft coral, giving it structure. Some nudibranchs incorporate the spicules of their prey.
An opening behind the eye of a ray, an angel shark or a wobbegong that draws water in for the gills. It lets a bottom-resting animal breathe without taking sand in through the mouth.
A mantis shrimp. Not a shrimp and not closely related to one: a separate group with the second pair of limbs modified into either a club for smashing or a barbed spear for catching fish.
Whatever the animal is sitting on or in: sand, silt, rubble, rock, coral or another animal. Used in the entries when the exact material matters to where a species is found.
The back-and-forth movement of water caused by waves passing overhead, strongest in the shallows. It moves a diver bodily and is the main reason a shallow macro subject is hard to photograph.
The gas-filled organ that lets a bony fish hold its depth without swimming. Fish that live on the bottom often have a reduced one or none at all, and sharks have none.
Two species living in close association. It covers relationships that help both, help one without harming the other, or help one at the other's expense; the specific words are mutualism, commensalism and parasitism.
The last segment of a crustacean's abdomen, sitting at the centre of the tail fan between the uropods. In mantis shrimps it is armoured, and its keels and spines are counted in identification.
Labelled on Decapod shrimpMantis shrimp
A flexible appendage with no joints and no skeleton, used to feel, catch or hold. In cnidarians the tentacles ring the mouth and carry the stinging cells. In cephalopods the word is narrower: an octopus has eight arms and no tentacles, while a squid or cuttlefish has eight arms and two longer tentacles that are shot out to take prey.
Labelled on Squid and octopusCnidarian polyp
The final colouring of a male wrasse or parrotfish, usually reached by a female changing sex. It is typically bright, often bears no resemblance to the initial phase, and is worn by one dominant fish among many that look like something else entirely.
The rigid internal shell of a sea urchin or sand dollar, made of fused calcareous plates and carrying the spines and pedicellariae.
Labelled on Sea star and urchin
A potent neurotoxin that blocks nerve signalling, made by bacteria and concentrated by the animals that carry it - pufferfish and the blue-ringed octopus among them. There is no antidote; treatment is supportive.
The boundary between two layers of water at different temperatures. It is visible as a shimmer, like heat haze, and the water below it is usually colder and often clearer.
The rows of small hydraulic feet along the underside of each arm of a sea star, or between the spines of an urchin. Each is pushed out by water pressure and most end in a sucker; together they move the animal and hold its prey open.
Labelled on Sea star and urchin
A rounded raised bump on the skin or shell. In nudibranchs the arrangement of tubercles is often diagnostic.
Labelled on Dorid nudibranch
The place the specimen used to describe a species originally came from. It is a historical fact about the name, not a statement about where the animal lives now.
Deep water rising to the surface, bringing nutrients with it. It makes cold, often green water and a great deal of life, and it is why some of the richest diving is not the clearest.
The paired flattened appendages either side of the telson. With it they make the tail fan of a shrimp, lobster or mantis shrimp.
Labelled on Decapod shrimpMantis shrimp
The swimming larval stage of most molluscs, carrying a ciliated lobe it both swims and feeds with. Blackwater dives are one of the few places a diver sees them.
Injecting a toxin through a spine, a fang or a harpoon. A stonefish, a lionfish and a cone shell are venomous. It is not the same as poisonous, and the distinction decides what first aid applies.
The nightly movement of open-ocean animals from deep water towards the surface, and back down before dawn. It is the largest daily movement of animals on the planet, and it is what a blackwater dive is: the deep-water fauna comes up to a depth a diver can reach, for a few hours, and then leaves.
The stiff facial whiskers of a seal or sea lion, sensitive enough to follow the wake a fish left in the water some seconds earlier.
Bearing live young that were nourished directly by the mother during development, rather than by a yolk. Among sharks it includes the requiem sharks and the hammerheads.
The network of fluid-filled canals that works an echinoderm's tube feet. It is filled from the sea through the madreporite, and it is the reason a sea star moves slowly and grips hard.
One complete turn of a snail's shell. Counting whorls and reading how tightly they are coiled is basic to shell identification.
An early swimming larval stage of a crab or other decapod, usually spiny and nothing like the adult. Common on blackwater dives.
One individual within a colonial animal: a siphonophore, a bryozoan, a colonial ascidian or a coral. In many colonies the zooids are not identical, and each kind does one job for the whole.
Single-celled algae living inside the tissue of corals, giant clams, some anemones and a few sea slugs, passing sugars to the host in exchange for shelter and nutrients. Bleaching is the host expelling them.
Labelled on Cnidarian polyp