Terms used in the critter entries

The terms

aboral

also aboral surface

Of an echinoderm, the side opposite the mouth. On a sea star this is the upper surface, where the madreporite sits; the underside, with the mouth, is the oral surface.

Labelled on Sea star and urchin

aeolid

also aeolids, aeolid nudibranch

A nudibranch that carries cerata over its back in place of a plume of gills. Aeolids eat hydroids, anemones and soft corals, and most keep the stinging cells of that prey to defend themselves.

ambulacral groove

also ambulacral, ambulacra

The furrow along the underside of every sea star arm, from which the tube feet emerge. Commensal shrimps and parasitic snails living on a sea star are most often found in it.

Labelled on Sea star and urchin

anal fin

also anal fins

The single fin on a fish's underside, placed behind the vent. Identification keys routinely count its rays and the spines that precede it.

Labelled on Bony fish

antenna

also antennae

One of the pair of long, whip-like sensory appendages on a crustacean's head, positioned behind the antennules. Many shrimps have antennae several times as long as their bodies, and on a night dive these are often what a torch beam picks out before the rest of the animal.

Labelled on Decapod shrimp

antennule

also antennules

One of the shorter pair of sensory appendages set ahead of the antennae; most of a crustacean's chemical sensing happens here. Antennules are branched and antennae are not, which separates the two pairs.

Labelled on Decapod shrimp

aposematic

also aposematism

Boldly coloured to warn that the animal is venomous, toxic or otherwise a poor meal. In this reference the blue rings of Hapalochlaena show it most clearly.

aposematism

also aposematic, warning colouration

Warning colouration that signals toxicity or danger. Predators have to learn the signal before it protects anything, so unrelated toxic species tend to arrive at the same pattern (Mullerian mimicry), and harmless species sometimes imitate it.

ascidian

also ascidians, sea squirt, sea squirts, tunicate

A chordate that lives fixed in place and filters water, either alone or in colonies, taking water in by one siphon and expelling it by a second. Its larva swims with a tail and a notochord, both lost once it settles. Several nudibranchs and flatworms feed on ascidians.

Batesian mimicry

also Batesian, Batesian mimic

The copying of a defended animal's appearance by one that is harmless or edible, so that predators which have learned to leave the model alone leave the imitation alone as well. On these reefs the mimic filefish and Valentin's sharpnose puffer are the best-known pair. The deception holds only while the model outnumbers the mimic.

benthic

Living on or within the seabed instead of in open water. Almost every animal in this reference is benthic.

bony rings

also bony ring, trunk rings, tail rings

The body armour of seahorses and pipefish, in which bony plates are fused into rings in place of scales. Counts of trunk and tail rings are used in identification. The rings make a seahorse rigid to the touch and restrict its bending to the tail.

Labelled on Seahorse

brood pouch

also brood pouches

A closed pocket on a male seahorse's or pipefish's belly in which eggs are carried until hatching. The female places the eggs inside, and the male fertilises them in the pouch.

Labelled on Seahorse

bryozoan

also bryozoans, moss animal

A colonial animal made up of many minute feeding individuals, growing as lacy, mossy or branching crusts on hard surfaces. Tambja and related nudibranchs eat bryozoans and derive both their pigments and their chemical defence from them.

carapace

The hard shield over the front of a crustacean's body, running from behind the eyes back to where the abdomen begins.

Labelled on Decapod shrimpMantis shrimp

caryophyllidia

also caryophyllidium

Tiny stalked papillae covering the back of certain dorid nudibranchs, each with a ring of calcareous spicules and a sensory knob at the tip. They give Jorunna its velvety surface and the sea bunny its furry look.

Labelled on Dorid nudibranch

caudal blade

also caudal spine, caudal blades

The sharp, hinged spine at the tail base of a surgeonfish, which folds forward like a scalpel; unicornfishes carry a fixed pair of blades instead. The fish uses it on rivals and predators. It can cut deeply and is venomous in some species. The risk comes from handling the fish, not from swimming near it.

Labelled on Bony fish

caudal fin

also tail fin

The tail fin. Its shape, whether rounded, truncate, emarginate, forked or lunate, reflects the way the fish swims, and it is among the more useful features to record underwater.

Labelled on Bony fishFrogfish

caudal peduncle

also caudal peduncles

The slim section of a fish's body behind the dorsal and anal fins and in front of the tail. Many identifications depend on a marking in this area, such as a spot, bar, saddle or blade.

Labelled on Bony fish

central disc

also central disk

The body of a sea star or brittle star, with the arms radiating from it. A brittle star's disc is clearly separate from its arms, while in most sea stars disc and arms merge gradually.

Labelled on Sea star and urchin

cerata

also ceras

Finger-shaped projections on the back of an aeolid nudibranch. Each contains a branch of the gut and has a cnidosac at its tip.

Labelled on Aeolid nudibranch

cheliped

also chelipeds, chela, chelae

A crustacean leg that bears a claw; in most decapods it is the first pereiopod. The size, shape and armament of the chelipeds help with identification, and several shrimps and crabs have one clearly larger than the other.

Labelled on Decapod shrimp

chloroplast

also chloroplasts, kleptoplasty

The organelle in which a plant or alga photosynthesises. Sacoglossan sea slugs puncture algal cells and retain the chloroplasts, still functioning, in their own tissue for days to weeks, a process called kleptoplasty. Because the genes that keep a chloroplast running belong to the alga and not to the slug, the stolen chloroplasts eventually fail.

chromatophore

also chromatophores

A cell holding pigment that an animal expands or contracts to alter its colour. Cephalopods have them in layers, which lets a cuttlefish change its appearance in less than a second.

Labelled on Squid and octopus

ciguatera

also ciguatoxin

Food poisoning caused by eating reef fish that have built up toxins from dinoflagellates living on reef surfaces. The toxins become more concentrated at each step up the food chain, so large predatory fish are the main risk, and cooking does not destroy them. A contaminated fish looks no different from a safe one. The hazard applies to eating reef fish, not to diving among them.

cirri

also cirrus

Thin fleshy filaments, usually on the head or over the eye. They help to identify scorpionfish, blennies and certain nudibranchs in the field.

Labelled on Seahorse

cleaning station

also cleaning stations

A fixed spot on a reef where cleaner shrimps, cleaner wrasses or juvenile fish pick parasites and dead tissue off larger animals, which queue to be cleaned. Because it is the place and not the animal that stays fixed, a cleaning station offers the most dependable chance to watch a turtle, a manta or a big grouper keep still.

cnidosac

also cnidosacs

A sac at the end of each ceras of an aeolid nudibranch, holding stinging cells taken undischarged from its prey and used again in its own defence.

Labelled on Aeolid nudibranch

commensal

also commensally

Associated with a host species and gaining from it, with no clear harm or help to the host. On muck sites a large share of the subjects live commensally on urchins, sea cucumbers, sponges and corals.

corallite

also corallites

The skeletal cup built by one hard-coral polyp. Hard corals are identified by the size, spacing and wall structure of their corallites, which stay visible when the polyps have retracted.

Labelled on Cnidarian polyp

coronet

also coronets

The bony crown on top of a seahorse's head. Its shape and height help with identification, and in some species each individual's coronet differs slightly from every other.

Labelled on Seahorse

counter-illumination

also counterillumination

Emitting light from the underside so that the body matches the dim daylight above it and casts no silhouette to an animal looking upwards. This is why many deep-water and open-water species have photophores on the belly rather than on the back.

crinoid

also crinoids, feather star

A feather star, an echinoderm with a small central disc, a ring of feathery arms for filter feeding, and short cirri below for gripping. A squat lobster, a shrimp and a clingfish all live on crinoids, each matching the colour of its host's arms.

cryptic

also crypsis

Hidden by looking like the background. This differs from mimicry, in which an animal resembles another organism instead of its surroundings.

dactyl

also dactylus, dactyls

The final, movable segment of a crustacean's limb, forming the finger of a claw or the tip of a walking leg. On a mantis shrimp this is the striking part: barbed and folding in spearers, and enlarged into a calcified heel in smashers.

Labelled on Mantis shrimp

demersal

Living near the bottom but not permanently resting on it.

dermal appendages

also dermal appendage

Skin filaments and tassels that blur an animal's outline among algae or hydroids. Hairy frogfish, rhinopias and several scorpionfish are typical examples.

Labelled on Frogfish

digestive gland

also digestive glands

In molluscs, the branched gland that serves as a liver and carries out much of the digestion. In aeolid nudibranchs it sends a branch into each ceras, so the animal's colour reflects its recent food.

Labelled on Aeolid nudibranch

dorid

also dorids, dorid nudibranch

The largest nudibranch group, with a broad flat mantle, two rhinophores at the front and a ring or plume of gills on the back that can normally be withdrawn. Sponges are the usual food. Phyllidiids are dorids that have lost the dorsal gill and breathe beneath the edge of the mantle instead.

dorsal fin

also dorsal fins, spinous dorsal, soft dorsal

The fin running along a fish's back, split in many families into a front section with spines and a rear section with soft rays. Counts of these spines and rays are among the characters used most often to identify fish.

Labelled on Bony fishFrogfishSeahorse

esca

The lure on the end of an anglerfish's first dorsal spine, moved to attract prey close enough to take. Each species has its own esca shape, and it is frequently the surest way to separate one frogfish from another.

Labelled on Frogfish

eyestalk

also eyestalks, stalked eyes, stalked eye

A mobile stalk bearing the eye in crabs, shrimps and mantis shrimps, allowing the eye to be lifted, pointed and tucked away independently of the body.

Labelled on Decapod shrimpMantis shrimp

foot

also propodium

The muscular sole on which a mollusc crawls. In sea slugs it extends behind the body as a tail, and its front corners are sometimes pointed; in cephalopods it has developed into the siphon and the arms.

Labelled on Dorid nudibranchAeolid nudibranch

gape

The fully opened mouth. Relative to body size, frogfish and other anglerfish have the widest gape of any fish, and they open it so quickly that a strike lasts only a few thousandths of a second. For that reason it is very rarely witnessed.

Labelled on Frogfish

gill plume

also branchial plume

The ring or fan of feathered gills on a dorid nudibranch's back, which in most species can be pulled into a pocket. An animal without one belongs among the phyllidiids, the aeolids or the flatworms, not the dorids.

Labelled on Dorid nudibranch

gill rakers

also gill raker

Projections along the inner edge of a fish's gill arches, arranged like a comb. Long, fine rakers sieve plankton out of the water crossing the gills, the feeding method of fusiliers, mackerel and manta rays. Short, robust rakers sort and retain prey already captured.

Labelled on Bony fish

harem

also harems

A single adult male living with several females in a shared area, the normal arrangement in anthias, dwarf angelfish and many wrasses. In protogynous species the structure is built to change: when the male is lost, the dominant female turns into a male.

hectocotylus

also hectocotylised, hectocotylised arm

The altered arm a male cephalopod uses to transfer a sperm packet to a female. Which arm is modified, and in what way, differs between species; in a few octopuses the arm breaks off and stays behind.

Labelled on Squid and octopus

holothurian

also holothurians, sea cucumber

A sea cucumber, an echinoderm with a soft, elongated body and a ring of feeding tentacles round the mouth; many species can eject sticky defensive tubules. Several feed by working through sediment and leave coiled casts on its surface.

hydrozoan

also hydrozoans, hydroid colony

Any member of the class containing hydroids, fire corals and siphonophores. These are cnidarians more closely related to anemones and jellyfish than to true corals, and most can sting. Much of what divers take for weed or coral on a reef is in fact hydrozoan.

illicium

The altered first dorsal spine of an anglerfish, which holds the esca. If the esca is the bait, the illicium is the fishing rod.

Labelled on Frogfish

initial phase

also initial-phase

The first adult colour pattern of a parrotfish or wrasse, carried by females and by males that have not changed. It tends to be plain and often differs so much from the terminal male that the two are recorded as separate species.

ink sac

also ink gland

The organ where a cephalopod keeps its ink, which is expelled through the siphon. The ink serves as a smokescreen and also as a decoy, because mucus holds the cloud together at roughly the size and shape of the animal that released it.

Labelled on Squid and octopus

lateral line

also lateral lines

A row of pored scales along the side of a fish, leading into a canal of sensory cells that detect movement in the water. Where it runs, and where it breaks or drops to a lower row, help to identify the fish.

Labelled on Bony fish

lunate

Of a tail fin, shaped like a crescent: deeply forked, with long lobes and a curved trailing edge. The shape is linked to fast, sustained swimming, so it occurs in tunas, trevallies and the open-water unicornfishes and anthias, not in fish that stay close to the reef.

madreporite

also madreporites

A perforated plate on the upper side of a sea star or urchin, placed off-centre between two arms, which lets water into the animal's hydraulic system. It normally appears as a pale button.

Labelled on Sea star and urchin

mantle

also mantle margin

The body wall of a mollusc; where a shell is present, the mantle produces it. On a dorid nudibranch it forms the wide, flat skirt over the back that overhangs the foot, and phyllidiids breathe along its edge. On a cephalopod it is the muscular sac behind the head that holds the organs. Mantle length is the standard measurement for cephalopods, because the arms stretch and shrink.

Labelled on Dorid nudibranchSquid and octopus

maxilliped

also maxillipeds

One of the limbs located between a crustacean's mouthparts and walking legs. Porcelain crabs have a third pair fringed with long feathery setae, which they sweep through the water to catch plankton.

Labelled on Decapod shrimp

mesentery

also mesenteries

One of the tissue partitions inside an anemone or coral polyp, bearing the digestive filaments and reproductive tissue. Their count and pattern are used to classify the animal.

Labelled on Cnidarian polyp

mucus cocoon

also mucus envelope, mucus cocoons

A mucus envelope that many parrotfishes produce around themselves before sleeping in a crevice. The current explanation is that it hides the fish's scent from predators hunting at night, moray eels in particular. Parrotfish wrapped in cocoons are a regular sight on night dives; building one takes about thirty minutes, and it is left behind at dawn.

nematocyst

also nematocysts, stinging cells

The stinging capsule of a coral, hydroid, anemone or jellyfish. It is one cell containing a coiled, barbed thread held under pressure and triggered by contact. Aeolid nudibranchs feed on cnidarians, move the nematocysts through the gut without firing them, and keep them in their cerata for their own use.

Labelled on Aeolid nudibranchCnidarian polyp

obligate

also obligately

Limited to a single way of life. An obligate associate lives only on its host and cannot survive alone, in contrast to a facultative associate, which can.

ocellus

also ocelli

A spot of colour resembling an eye, used to startle a predator or to divert an attack from the actual head.

operculum

also opercula

A lid or cover. On a fish it is the bony plate over the gills; on a tube worm it is the hard plug that closes the tube once the animal has pulled back inside.

Labelled on Bony fish

ophiuroid

also ophiuroids, brittle star

A brittle star, with five slender arms clearly separated from a small central disc. It moves by lashing its arms rather than with tube feet, and the arms break at pre-formed planes and grow back.

oral disc

also oral discs, oral disk

The flat top of an anemone or coral polyp, with the mouth in the middle and tentacles around the edge.

Labelled on Cnidarian polyp

oral tentacles

also oral tentacle

Two short sensory tentacles next to a sea slug's mouth, used to feel and taste the surface it moves over. They are separate from the rhinophores, which are further back and detect scent in the water.

Labelled on Dorid nudibranchAeolid nudibranch

ossicle

also ossicles

A tiny calcareous piece set in the body wall of an echinoderm. In sea cucumbers, ossicle shape is a key identification character; in the synaptids the ossicles are shaped like anchors and snag on a wetsuit.

Labelled on Sea star and urchin

palaemonid

also palaemonids

A member of the Palaemonidae, the family that contains most of the small commensal shrimps living on sea cucumbers, anemones, corals and urchins.

papillae

also papilla

Small fleshy skin projections. Frogfish and octopuses raise or flatten them to alter their outline and texture, so a hairy octopus can look hairy at one moment and smooth at the next.

Labelled on FrogfishSquid and octopus

parapodia

also parapodium, parapodial

Paired side flaps found on some sea slugs and worms. Plakobranchus folds them over its back like the covers of a book and spreads them flat to face the light.

pectoral fin

also pectoral fins

The paired fin just behind the gill cover, equivalent to a front limb. Wrasses and parrotfishes swim mainly with it, and in frogfish it is lobed and jointed and used for walking.

Labelled on Bony fishFrogfishSeahorse

pedicellariae

also pedicellaria

Minute stalked pincers among the spines of a sea urchin, which keep the surface clean and discourage settling organisms. Some fire urchins have venomous pedicellariae, one reason an urchin is a well-protected home for a shrimp.

Labelled on Sea star and urchin

pelvic fin

also pelvic fins, ventral fin, ventral fins

The paired fin on the underside of a fish, equivalent to a hind limb. Gobies have the pair joined into a disc that grips the bottom, and frogfish use theirs as props.

Labelled on Bony fishFrogfish

pereiopods

also pereiopod, walking legs, walking leg

The five pairs of thoracic legs of a decapod crustacean. The first pair bears claws, sometimes the second pair as well, and the remaining pairs are used for walking.

Labelled on Decapod shrimp

pharyngeal teeth

also pharyngeal jaws, pharyngeal mill

A second set of grinding plates in the throat, behind the jaws. They allow wrasses, parrotfishes and emperors to eat hard-shelled prey despite mouths that look too weak for it, with the jaws grabbing and the pharyngeal teeth crushing.

photo-identification

also photo identification, photo ID

Identifying an individual animal by a natural marking that stays the same throughout its life, and comparing it with a catalogue. Manta belly patterns, whale shark flank spots and dolphin fin notches are used in this way. Most knowledge of where these animals travel comes from photographs taken by divers, not from tags.

planktivore

also planktivorous, planktivores

An animal that eats plankton carried past in the water, not food fixed to the reef. This explains much about where fish are seen. Planktivores hold position in the water column on the side of a slope exposed to current and retreat into cover when threatened, so a wall with current holds fish that a sheltered bay lacks.

pleon

A crustacean's abdomen, the muscular, segmented section behind the carapace that bears the pleopods and ends in the tail fan. A shrimp flexes it sharply to dart backwards.

Labelled on Decapod shrimp

pleopods

also pleopod, swimmerets, swimmeret

Paired limbs beneath a crustacean's abdomen, used for slow swimming and, in females, for carrying eggs until hatching.

Labelled on Decapod shrimp

polyclad

also polyclads, flatworm

A free-living flatworm of the sea. It is thin enough to ripple as it moves, has two folded pseudotentacles on its front edge, and lacks both a gill plume and rhinophores. Those absences separate a flatworm from the nudibranch it imitates.

polyp

also polyps

The single animal of a hydroid, anemone or coral, a column attached at the base with tentacles ringing a mouth on top. Many polyps share one skeleton in a hard coral colony, whereas an anemone is one large polyp.

Labelled on Cnidarian polyp

prehensile tail

also prehensile

A tail that can grip. Seahorses, pygmy seahorses and pipehorses wrap theirs around hydroids, gorgonians and weed, which lets them stay put in current that carries everything else away.

Labelled on Seahorse

proboscis

also probosces

A feeding tube that can be extended far beyond the body and then pulled back. Cone shells fire their harpoon through one, nemertean worms hunt with one, and several sea slugs push one into their prey.

protandrous hermaphrodite

also protandrous, protandry

An animal that first matures as a male and may later become female. Both anemonefish and ribbon eels do this, so the biggest fish in an anemonefish group is the female, and a yellow ribbon eel was previously blue.

protogynous hermaphrodite

also protogynous, protogyny

An animal that starts out female and may later turn male. Wrasses, parrotfishes, anthias and many groupers follow this pattern, and it explains why one species can look like two on the same reef: the terminal male began as a female and bears no resemblance to her. The reverse order is protandrous hermaphroditism.

radula

also radulae, radular, radular tooth

The ribbon of teeth a mollusc uses to feed, scraped across food like a rasp. In sea slugs its tooth shape and count are primary identification characters. Two groups on this site have altered it beyond recognition: sacoglossans keep one row of piercing teeth for puncturing algal cells, and melibes have no radula at all.

Labelled on Dorid nudibranch

raptorial appendage

also raptorial appendages, raptorial claw

The folded strike limb carried beneath a mantis shrimp's head. Spearers shoot out a barbed dactyl to catch soft-bodied prey, while smashers hit with a calcified heel that can break shells, striking so fast that the water behind it cavitates.

Labelled on Mantis shrimp

rhinophores

also rhinophore

The two sensory tentacles on a nudibranch's head, which detect scent carried in the water. Whether they are smooth, annulate or lamellate is a primary identification character.

Labelled on Dorid nudibranchAeolid nudibranch

rostrum

The projection at the front of a crustacean's carapace, extending between and past the eyes. Identification relies on its shape, its length and how many teeth it bears.

Labelled on Decapod shrimp

sacoglossan

also sacoglossans, sap-sucking slug

A sea slug that feeds by puncturing algal cells and sucking them out, not by biting. Some keep the algae's chloroplasts working in their own bodies and take nutrition from them. Sacoglossans are not nudibranchs, although they live alongside them and divers routinely group the two together.

scute

also scutes

A horny or bony plate in the skin or shell of a reptile. Scute arrangement distinguishes a hawksbill from a green turtle: the hawksbill has overlapping plates and a serrated rear margin, and the green has flat plates that meet edge to edge and a smooth margin.

sequester

also sequestered, sequestration

To obtain a chemical defence from food and store it instead of producing it. Sea slugs sequester compounds from bryozoans and sponges, aeolids sequester stinging cells, and pufferfish sequester tetrodotoxin taken in through their diet.

setae

also seta, setal

Hairs or bristles on a worm or crustacean. Some crabs have hooked setae that hold decoration in place, and in filter feeders the setae form fans that sieve plankton from the water.

siphon

A cephalopod's muscular funnel, through which water is forced out for breathing, jet propulsion and aiming ink.

Labelled on Squid and octopus

snout

also snouts

The section of a fish's head ahead of the eye. Snout length compared with head length is a standard measurement. Seahorses, pipefish and seamoths have the snout extended into a tube through which they feed.

Labelled on Bony fishSeahorse

spawning aggregation

also spawning aggregations

A gathering of one species to spawn at a set place and time, frequently the same reef corner within a few days of the same moon every year. It is both the most productive and the most vulnerable point in the animal's year, because the timing is predictable; fishing on aggregations has reduced several groupers and the bumphead parrotfish throughout their range.

spicule

also spicules

A tiny mineral rod set in the tissue of a soft coral or sponge, giving it support. Certain nudibranchs take the spicules of their prey into their own bodies.

telson

also telsons

The final segment of a crustacean's abdomen, in the middle of the tail fan with the uropods on either side. A mantis shrimp's telson is armoured, and the keels and spines on it are counted for identification.

Labelled on Decapod shrimpMantis shrimp

tentacle

also tentacles

A flexible appendage without joints or skeleton, used for feeling, catching or holding. Cnidarians have tentacles around the mouth, armed with stinging cells. For cephalopods the word has a narrower meaning: an octopus has eight arms and no tentacles, whereas a cuttlefish or squid has eight arms plus two longer tentacles that shoot out to seize prey.

Labelled on Squid and octopusCnidarian polyp

terminal male

also terminal phase, terminal-phase

The last colour phase of a male parrotfish or wrasse, usually reached when a female changes sex. It is generally bright, frequently unlike the initial phase, and carried by a single dominant fish in a group whose other members look quite different.

test

The stiff internal skeleton of a sand dollar or sea urchin, built from fused calcareous plates, to which the spines and pedicellariae attach.

Labelled on Sea star and urchin

tetrodotoxin

also ttx

A powerful neurotoxin that stops nerve signals, produced by bacteria and accumulated by the animals that carry it, including pufferfish and the blue-ringed octopus. No antidote exists, and treatment is supportive.

tube feet

also tube foot, podia

Small hydraulic feet in rows beneath each sea star arm, or among an urchin's spines. Water pressure extends each one, and most finish in a sucker. Working together they move the animal and pull its prey open.

Labelled on Sea star and urchin

tubercle

also tubercles

A raised, rounded bump on skin or shell. The pattern of tubercles on a nudibranch is frequently diagnostic.

Labelled on Dorid nudibranch

uropods

also uropod

Flattened paired appendages on each side of the telson, forming the tail fan of a shrimp, lobster or mantis shrimp together with it.

Labelled on Decapod shrimpMantis shrimp

vertical migration

also diel vertical migration

The movement each night of open-ocean animals from the depths up towards the surface, returning before dawn. It is the largest daily animal movement on Earth. A blackwater dive depends on it: for a few hours deep-water fauna rises to depths within a diver's reach and then descends again.

zooxanthellae

also zooxanthella

Single-celled algae that live in the tissues of corals, giant clams, some anemones and a few sea slugs, supplying sugars to the host in return for shelter and nutrients. When a host expels them, it bleaches.

Labelled on Cnidarian polyp