The famed explorer Jacques-Yves Cousteau once described the underwater environment as “The Silent World”, titling a book and movie with that captivating phrase.

But divers who visit any of the world’s coral reefs and tropical seas know that the underwater world is anything but silent. Listen carefully and you’ll hear an ever-present cacophony of clicks, chirps, pops, crunches and crackles as you explore this environment. This background symphony is always there, even if divers become accustomed to it and unconsciously tune it out.

What is the source of all this noise? It’s an army of crustaceans, a kingdom of critters that are often hard to find and even more difficult to photograph.

Shrimp have 5 pairs of legs and a hard exoskeleton

Words & Photos by Michael Salvarezza and Christopher P. Weaver

It’s the myriad shrimp and crabs that inhabit the reef, often hidden from view, that are constantly chewing and disposing of the reef’s detritus. These creatures usually hide during the day, sometimes deep within crevices in the hard corals or rocks of the reef. Others hide in plain sight, beautifully camouflaged against sea fans, gorgonians, soft corals or crinoids. Some can be found beneath the protective tentacles of sea anemones, or inside the tubes of sponges.

And others don’t hide at all, making their presence known to passing fish who might be looking for a helpful parasite removal service.

This is the reef’s cleaning crew, a vital component of the reef’s eco-system. Without these crabs and shrimp perpetually consuming the waste of the reef, the corals would eventually die. Let’s take a closer look at the source of this underwater din and examine some of its quirky creatures in greater detail.

Both crabs and shrimp are crustaceans and are related to arthropods such as insects, spiders, and centipedes. Both arthropods and crustaceans have segmented bodies, external skeletons, and numerous, jointed limbs. However, unlike most arthropods which are strictly or mainly terrestrial, crustaceans are primarily marine aquatic creatures, although there are freshwater species as well.

Let’s first examine the crab.

Crabs are ubiquitous coral reef crustaceans but they can be found in all the world’s oceans, and can even be found in freshwater, and on land.

True crabs (Brachyura) have 10 legs – two pincer claws on the ends of the frontmost pair of legs and four pairs for walking. Some of these crabs are adapted for running and walking on the bottom, while others have a modified pair of paddle-shaped legs that help in swimming. Non-true or false crabs (Anomura) have three pairs of legs, with a fourth pair that is too small for walking and are often hidden under their body.

They first appeared during the Jurassic period, around 200 million years ago and today there are more than 7000 species that have been identified. Several species, including spider crabs and deep-sea red crabs, are adapted to live at extreme depths on the ocean floor, often near hydrothermal vents, where they can reach depths of several miles.

Many crabs, like this Spooner Crab, have formidable front claws

Those that dwell on reefs generally remain well hidden within the reef structure by day.

Most crabs are omnivores, feeding on a wide variety of available food items, including algae, worms, mollusks, bacteria, other crustaceans, fungi, and even reef detritus.

In addition to clearing the reef of dead organic matter, some crabs act as “cleaners” of hard coral colonies, removing and reducing the numbers of infesting parasites and other harmful organisms from the bodies of their coral hosts similar to the same way that cleaner shrimp benefit certain fishes.

For divers, one of the most entertaining types of crab is the Hermit Crab. In actuality, there are more than 800 unique species of Hermit Crabs. These crustaceans inhabit the empty and discarded shells of snails and other gastropods, using them as protection and crawling around the reef with their adopted homes on their backs. Divers will observe the crab’s eyes, front claws and tentacles poking out from the shell as the scamper around looking for food. Sensitive to movement and light, the crab will beat a hasty retreat into the shell if it feels threatened but will emerge fairly quickly if the danger is perceived to have gone.

This Hermit Crab looks like it was squirted with silly string

As hermit crabs grow, they require larger shells and sometimes will fight other crabs for their shells. Some hermit crab species are small, less than an inch in size, while others may measure a half a foot or more. Unlike other species of crabs, hermit crabs are some of the more easily observed crabs on the reef.

But some have taken the art of disguise to a whole different level. Known as Decorator Crabs, these ingenious creatures use living animals (like anemones) attached to their shells as an additional form of concealment.

In the central Indo-pacific region, keen eyed divers can spot the Orangutan Crab often hiding on bubble coral. A small crab with a carapace that measures only 0.8 inches in diameter, it has relatively long arms, which are thickly covered with reddish or reddish-brown fine hairs that make it resemble Doctor Zaius from the “Planet of the Apes” movie series.

In the Caribbean, divers will often be intrigued by the uniquely shaped Arrow Crab. These shy creatures will often be found in crevices, nooks and crannies of the reef. Their distinctive triangular shape makes them easy to identify and they can grow to have a carapace of more than 2.5 inches along with long and thin 4-inch legs. Females will carry their egg cases beneath their bodies until they are ready to hatch.

Various species of Spider crabs are found in both cold and warm water environments. These creatures, with bodies so slim that they contain some of their internal organs in their legs, inhabit such diverse eco-systems as the reefs of the Indo-Pacific and the icy waters of Antarctica.

Spider crabs are found in tropical seas as well as Antarctic waters

Indeed, crabs come in a wide array of sizes, shapes and colors. They are essential omnivores and are critical to helping keep the reef environment healthy.

Their counterparts on the reef cleaning crew are the shrimp.

Shrimp are swimming crustaceans that have a long narrow muscular abdomen with a hard shell, long antennae, and a fanlike tail. Shrimp are members of the decapods order within the class Malacostraca, meaning they have 10 legs, which include crabs, lobsters, crayfish and prawns. Worldwide, there are more than 3000 species of shrimp inhabiting all oceans and water environments, including the deep ocean where several species can be found as far down as 16,000 feet.

On the reef, shrimp can be found in rocky crevices, underneath coral outcroppings, beside stinging anemones, clinging to crinoids and whip corals, and even hiding on soft corals. Many species of shrimp have striking coloration but many others have translucent bodies so clear that internal organs can easily be seen.

This Yellow-Spotted Anemone Shrimp also has a translucent body

Their diet varies, with some species adapted to feed primarily on algae and vegetation, while others clean fish and other organisms of parasites. Shrimp are opportunistic and will feed on dead animals, and some, like the Mantis Shrimp, are predatory and use their powerful claws to kill small fish and organisms.

Divers in Caribbean waters will often spot the dazzling blue colors of the Pederson’s Shrimp if they search near anemones such as the Corkscrew Anemone. This species has a transparent abdomen and divers can see the insides of the animal if they look closely.

Also known to frequent the homes of stinging anemones is the Caribbean Squat Anemone Shrimp. These shrimp benefit from the protection of the Anemone while helping clean the area of detritus.

Look inside a sponge, or beneath a coral crevice and you might find a pair of Banded Coral Shrimp tucked away with their long antennae extended as an advertisement to passing fish. The Banded Coral Shrimp, found in all tropical seas, is a cleaner shrimp and once a fish is attracted, the shrimp delivers its service by scouring the body of the customer for parasites and dead skin. If the fish is willing, it may even open its mouth wide in a signal to the shrimp that it’s safe to actually go inside and scrape the teeth and mouth for parasites…a potentially dangerous situation for the shrimp. But somehow, the reef creatures have developed an unspoken code through the millennia that enables the customer fish to gain relief by signaling to the shrimp that it won’t get devoured as it delivers it’s service.

Look very closely and you might even see eggs attached to the underside of the body of the female.

In the tropical Pacific, divers often marvel at the gaudy shapes and colors of a number of species of shrimp. Here, we find several types of shrimp that have evolved to camouflage so well on their hosts that they can exist nowhere else. For example, there are species of shrimp that live on specific species of whip corals, gorgonians and even soft corals because their bodies so perfectly mimic the shapes of these organisms.

Shrimp can be tiny or grow as large as a foot depending on the species. The Mantis Shrimp, found in the Indo-Pacific, reaches these impressive sizes and sometimes can be seen guarding a huge ball of eggs or small shrimp fry at the entrance of their burrow in the sand.

This Mantis Shrimp from the Maldive Islands is guarding a giant ball of eggs

Like crabs, shrimp are also found in cold water environments. Often growing larger in the frigid waters, these species of shrimp serve the same purpose: clean the reefs of debris, dead animals and harmful parasites. From the Arctic to the Antarctic, and everywhere in between, Shrimp have become the reef’s essential cleaning crew workers.

So next time you splash into the water and begin exploring the reef, stop for a moment and listen to the sound of the reef’s cleaning crew.

It’s a background noise that indicates a vibrant ecosystem at work. And look closely in some of the smallest crevices, alongside colorful anemones, on the strands of whip corals and soft corals, and you may spot these industrious creatures lying in wait for their next meal. They are some of the most magnificently colored and uniquely shaped creatures in the marine world.

And without them, the reefs would not exist.

Words & Photos by Michael Salvarezza and Christopher P. Weaver

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Words & Photos by Michael Salvarezza and Christopher P. Weaver