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).