Coral reefs cover only 0.2 per cent of the world’s ocean floor but support over 25 per cent of all known marine life. This diversity isn’t just about charismatic characters — coral, sharks, turtles, colourful reef fish — a large chunk of it is tiny, lesser-known, easily hidden creatures. These include both sessile (fixed in one place) and mobile reef invertebrates, as well as the itty bitty reef fish.

Coral reefs are considered the rainforests of the sea. In both systems, diversity is largely from invertebrates (animals without a backbone, like worms, insects, and crustaceans). While diversity in a rainforest is built in, on, and around plant life, coral reef diversity primarily resides in, on, and around sessile animals such as hard corals, hydroids, anemones, soft corals, sponges, tunicates, and others.

Coral reefs have lots of tiny nooks and crannies. A large variety of creatures remain hidden on a coral reef, including fish, shrimp, crabs, worms, molluscs etc. There are so many tiny (< 5 cm) reef fish and invertebrates that scientists feel there are at least eight million undiscovered species globally. In fact, it’s these small easily-hidden fish that have the greatest impact. Recent studies show that 60 per cent of the fish biomass preyed upon in a reef comprises the small cryptic bottom-dwellers like blennies and gobies. While research on Andaman’s cryptic reef life is just beginning to take form, underwater photographers taking photos with a macro lens help us piece together their incredible diversity and function.

Survival for these little fellows is not just about defence. There is a lot more cooperation. Many reef species have co-evolved and need each other to exist and thrive. The symbiosis between hard coral and zooxanthellae is a prime example of such a mutualism, where both organisms benefit from the association — the coral get food and the zooxanthellae get a home. Commensalisms or associations between two critters in which one benefits and others are neither harmed nor benefited are also very common. Such associations are commonly observed on soft corals, sea stars, feather stars etc. Often the partner species for commensal shrimp, fish, crabs, etc., are evident from their name (e.g. whip coral shrimp).

“Fun-sized” coral reef animals are invertebrates and fish that measure less than 5 cm in length. Once your eye catches these vibrantly coloured reef creatures, it’s hard not to miss them, despite their small size. Nudibranchs, like this chromodorid, are good examples of that. Their bright colours and flamboyant form attract seasoned as well as novice scuba divers. Unless it’s moving, it can be pretty hard to tell a nudibranch’s head from its tail. The head on this one is to the right, where you see the two antennae (or rhinophores), and halfway to its tail is its tuft of gills.  Cover photo: A tiny goby peers out of its grove of eight-tentacled (octocoral) polyps.

“Fun-sized” coral reef animals are invertebrates and fish that measure less than 5 cm in length. Once your eye catches these vibrantly coloured reef creatures, it’s hard not to miss them, despite their small size. Nudibranchs, like this chromodorid, are good examples of that. Their bright colours and flamboyant form attract seasoned as well as novice scuba divers. Unless it’s moving, it can be pretty hard to tell a nudibranch’s head from its tail. The head on this one is to the right, where you see the two antennae (or rhinophores), and halfway to its tail is its tuft of gills.
Cover photo: A tiny goby peers out of its grove of eight-tentacled (octocoral) polyps.

Stare long enough on a portion of the coral reef, and you are guaranteed to catch some movement, either a tiny fish or tiny invertebrate. Small (< 5 cm) bottom-dwelling, cryptic species like these blennies and gobies account for half the fish species found on coral reefs. Abundant and highly biodiverse, these species mature early, produce many offspring, are a vital food source for other marine creatures. Blenny cuteness and character is definitely enhanced by the fleshy branching appendages (cirri) found above their eyes and nostrils. The biological role of cirri remains unclear.

Stare long enough on a portion of the coral reef, and you are guaranteed to catch some movement, either a tiny fish or tiny invertebrate. Small (< 5 cm) bottom-dwelling, cryptic species like these blennies and gobies account for half the fish species found on coral reefs. Abundant and highly biodiverse, these species mature early, produce many offspring, are a vital food source for other marine creatures. Blenny cuteness and character is definitely enhanced by the fleshy branching appendages (cirri) found above their eyes and nostrils. The biological role of cirri remains unclear.

The peacock-tail anemone shrimp (Ancylocaris brevicarpalis), as its name describes, lives in close association with sea anemones. The anemone protects the shrimp from predators, and the shrimp helps keep the anemone clear of debris, parasites, and unwanted slime. The shrimp is unaffected by the anemone’s stinging cells thanks to its hard exoskeleton. Female peacock-tail anemone shrimps are larger in size and have larger and more numerous white spots than males.

The peacock-tail anemone shrimp (Ancylocaris brevicarpalis), as its name describes, lives in close association with sea anemones. The anemone protects the shrimp from predators, and the shrimp helps keep the anemone clear of debris, parasites, and unwanted slime. The shrimp is unaffected by the anemone’s stinging cells thanks to its hard exoskeleton. Female peacock-tail anemone shrimps are larger in size and have larger and more numerous white spots than males.

Some shrimp prefer to associate with anemone relatives (cnidarians) like coral. Bubble coral (Physogyra sp.) are hard coral that possess bulbous tentacles that are extended during the daytime giving the coral a bubbly appearance. (Left) An unidentified shrimp extends itself out of its bubble coral home to filter feed; developing eggs are clearly visible under its transparent abdomen. (Right) Squat shrimp (Thor sp.) are often called sexy shrimp because of their provocative dance. Their quirky abdominal movements, often in sync with one another, are amusing to observe and can captivate divers for long underwater minutes; it’s as if they’re performing in a talent show.

Some shrimp prefer to associate with anemone relatives (cnidarians) like coral. Bubble coral (Physogyra sp.) are hard coral that possess bulbous tentacles that are extended during the daytime giving the coral a bubbly appearance. (Left) An unidentified shrimp extends itself out of its bubble coral home to filter feed; developing eggs are clearly visible under its transparent abdomen. (Right) Squat shrimp (Thor sp.) are often called sexy shrimp because of their provocative dance. Their quirky abdominal movements, often in sync with one another, are amusing to observe and can captivate divers for long underwater minutes; it’s as if they’re performing in a talent show.

Commensals are often hard to spot. Here we have images of whip coral stalk along with its fish and shrimp associates. Whip coral is a kind of black coral (Order: Antipatharia) that grows like a whip or wire. The outer live portion of whip corals are yellow-white to red/blue/green in colour and contain small six-tentacled polyps. The colony can grow up to 2 m or more in height. Whip coral goby (top left) and whip coral shrimp (top right) can be traced by searching for their eyes. Catching a glimpse of these creatures on this magnificent animal is tricky, leaving you swimming round and round the whip, and is much like chasing a lizard up a tree. In such encounters, only patience prevails, and if you are lucky, you get a shot of a whip coral shrimp and whip coral goby on opposite sides of the whip (above).

Commensals are often hard to spot. Here we have images of whip coral stalk along with its fish and shrimp associates. Whip coral is a kind of black coral (Order: Antipatharia) that grows like a whip or wire. The outer live portion of whip corals are yellow-white to red/blue/green in colour and contain small six-tentacled polyps. The colony can grow up to 2 m or more in height. Whip coral goby (top left) and whip coral shrimp (top right) can be traced by searching for their eyes. Catching a glimpse of these creatures on this magnificent animal is tricky, leaving you swimming round and round the whip, and is much like chasing a lizard up a tree. In such encounters, only patience prevails, and if you are lucky, you get a shot of a whip coral shrimp and whip coral goby on opposite sides of the whip (above).

There is no shortage of animals masquerading as plants on the coral reef. Take, for example, feather stars (crinoids), close relatives of sea stars, urchins, and sea cucumbers (echinoderms). Feather stars look like beautiful flowers with many arms with rows of filaments that help trap food particles and a bird claw-like foot to anchor themselves onto the bottom. They offer protection and lack unpleasant secretions or chemical defences. This makes them ideal candidates for commensalism with marine invertebrates like crabs, shrimps, and worms. Crabs (left) and shrimp (right) enjoy prime locations on the feather star’s arms, near the food grove where filtered particles are transported towards the mouth. Feather stars are so conducive to commensalism with small invertebrates that they tend to harbour the highest density of commensals compared to other spikey-skinned animals (echinoderms).

Echinoderm-shrimp commensalism is common on coral reefs. Shrimps live on the surface of the host, which offers protection and a steady food source, and in turn, the shrimp keeps the host free of parasites and dead cells. The 2-cm emperor shrimp (Periclimenes imperator) confidently walks on the prickly sea cucumber skin; its bright colours may attract others, but if danger approaches, it simply ducks for cover to the underside of the cucumber. Here, another shrimp species (likely Periclimenes soror) roams the bumpy skin of a cushion sea star (Culcita sp.). The living textures occupied by these commensal creatures can inspire landscapes in outer space sci-fi movies. While we still do not know enough about these creatures’ interactions, we do recognise their importance. These tiny reef creatures may play significant roles in controlling parasites and disease, helping maintain reef health from the bottom-up.

Echinoderm-shrimp commensalism is common on coral reefs. Shrimps live on the surface of the host, which offers protection and a steady food source, and in turn, the shrimp keeps the host free of parasites and dead cells. The 2-cm emperor shrimp (Periclimenes imperator) confidently walks on the prickly sea cucumber skin; its bright colours may attract others, but if danger approaches, it simply ducks for cover to the underside of the cucumber. Here, another shrimp species (likely Periclimenes soror) roams the bumpy skin of a cushion sea star (Culcita sp.). The living textures occupied by these commensal creatures can inspire landscapes in outer space sci-fi movies. While we still do not know enough about these creatures’ interactions, we do recognise their importance. These tiny reef creatures may play significant roles in controlling parasites and disease, helping maintain reef health from the bottom-up.

Mahima Jaini
Mahima Jaini

is a marine biologist, broadly interested in reproductive ecology, fisheries and conservation.

Dhritiman Mukherjee
Dhritiman Mukherjee

is one of India's most prolific wildlife and conservation photographers. His work has been featured in leading publications. He is also a RoundGlass Ambassador, and an RBS Earth Hero awardee.


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