Crustaceans

ShrimpsCaridea

Infraorder49 species

There is an animal on this reef that shoots with sound, and it is a shrimp.

A pistol shrimp has one claw grown out of proportion to the rest of it, built as a hammer and anvil rather than a pincer. When it snaps shut, the water jet leaves so fast that the pressure behind it drops below the vapor pressure of seawater and a bubble tears open in the liquid. The bubble collapses a fraction of a millisecond later, and the collapse produces a shockwave that stuns prey, a flash of light too brief to see, and a momentary temperature inside the bubble that has been measured in thousands of degrees. The sound is loud enough that colonies of these shrimps show up as permanent static on sonar, and they have interfered with submarine detection.

That is one shrimp. The group covers everything from that to the cleaner shrimp that will climb onto your hand and work at your cuticles, to species the size of a grain of rice living inside a single coral head.

What makes them worth looking for here is that most of them live with somebody else. A snapping shrimp digs and maintains a burrow and shares it with a goby that cannot dig but can see, and the shrimp keeps one antenna on the goby's tail as a tripwire while it works outside. Others live among the tentacles of anemones, immune to the stings, or between the spines of urchins, or inside sea cucumbers, or on a single species of coral and no other. Almost every conspicuous invertebrate on this coast has a shrimp somewhere on it.

They also have to shed their skeleton to grow. A shrimp splits its shell, backs out of it, and spends the next few hours soft and defenseless while the new one hardens, which is when most of them are eaten, and it is why so many of them do it inside a hole.

Macro photographers work this group harder than any other, because the shrimp does not run, the background is an anemone, and the subject is a centimeter long.

How to recognise shrimps

InvertebrateNot a fish - coral, mollusc, crustacean, echinoderm and other invertebrate groups.

small size, typically 1-8 cm body length
slender, somewhat laterally compressed body
prominent rostrum and stalked eyes
two pairs of long antennae
walking legs with small pincers, varied colour patterns

49 species in Shrimps

Actinimenes

Anemone shrimps that live on the tentacles themselves, walking over stinging cells without being fired at, usually as a pair holding one anemone. Small and close to transparent, with the markings confined to a line down the back.

Anemone Shrimp

Actinimenes inornatus

Ornate Anemone Shrimp

Actinimenes ornatus

Alpheus

The snapping shrimps, and the sound on the reef is theirs. One claw is built as a hammer and anvil rather than a pincer, and the snap tears a bubble open in the water whose collapse does the damage. Most of them are also landlords: they dig and maintain a burrow they cannot see out of and share it with a goby that can, keeping one antenna on the goby as a tripwire while they shovel. A coral head that clicks continuously has them in it.

Coral Snapping Shrimp

Alpheus lottini

Djeddah Snapping Shrimp

Alpheus djeddensis

Fine-striped Shrimp

Alpheus ochrostriatus

Marbled Snapping Shrimp

Alpheus rapax

Red-spotted Goby Shrimp

Alpheus rubromaculatus

Tiger Pistol Shrimp

Alpheus bellulus

Cinetorhynchus

Hingebeak shrimps, named for a rostrum that folds upward on a joint. Deep red, marked with white spots or bands, in caves and under ledges by day and out over the reef after dark - and the eyes throw a bright point back at a torch, which is how they are found at night.

Hendersons Hingebeak Shrimp

Cinetorhynchus hendersoni

Reticulated Hingebeak Shrimp

Cinetorhynchus reticulatus

Cuapetes

Glass shrimps. The body is transparent enough to read text through, and they get away with that by never being alone - each one lives on a host nothing wants to bite, an anemone, a soft coral or an urchin. Two defenses, neither enough by itself.

Elegant Anemone Shrimp

Cuapetes elegans

Glass Shrimp

Cuapetes tenuipes

Saron

Marble shrimps: bodies covered in dense tufts of short bristles that read as moss rather than as an animal, marbled green, brown and red, and redder at night than by day. The males carry the first pair of walking legs drawn out to extraordinary length, so the sexes look like different species.

Pinecone Marble Shrimp

Saron inermis

Urocaridella

Cleaner shrimps that hover instead of perching. They hang in open water at a cave mouth, which almost nothing else in the family does, and advertise by rocking the body rapidly back and forth - a signal that has been tested: hungry shrimp rock more, and fish choose the ones that do.

Cave Cleaner Shrimp

Urocaridella pulchella

Clear Cleaner Shrimp

Urocaridella antonbruunii

Zenopontonia

Shrimps that ride other animals: sea cucumbers, sea stars, big nudibranchs. They take the color of whichever host they are on - blue on a blue sea star, red on a crown-of-thorns - so the search is for the host, and then for a shape on it that does not belong.