Coral reefs are widely known for their incredible biodiversity and species richness. While many people associate coral reefs with their incredible fish diversity, the majority of species on coral reefs are actually invertebrates!

What Is An Invertebrate?

An invertebrate is an animal that does not have a vertebral column, derived from the notochord. Humans and dogs, for instance, do have a vertebral column or spine, and they are therefore vertebrates. Octopi, sponges, jellyfish and crabs, however, do not have a vertebral column and therefore are known as invertebrates. Invertebrates are highly diverse, and more than 90% of all animal species are classified as invertebrates.


Marine invertebrates are a large component of the coral reef ecosystem, and many rely on the corals as a source of food, shelter, spawning and breeding sites. In fact, corals themselves are invertebrates and the building blocks of these incredible ecosystems. In particular, stony corals secrete a hard calcium carbonate exoskeleton which forms the bedrock of the coral reef and builds up the reef structure over time. 


There are many types of invertebrates which are grouped into families, known as phylum. Scientists group similar species together into phyla, in a field of study known as taxonomy, because it makes them easier to study, as each group share special distinguishing features. Corals are part of the phylum known as Cnidaria, which also includes jellyfish, anemones and sea pansies. The members of this group are distinguished by specialized cells called cnidocytes or stinging cells. These are used to defend the organism and for prey capture. Anemones, and other coral types, such as soft corals, develop on the hard skeletal structures of the stony corals and add to the overall complexity of the system. 


Some species of sponges, phylum Porifera, also rely on reefs as a habitat, and they themselves form part of the reef structures and provide shelter for other invertebrates, such as shrimps and crabs. 

Platyhelminthes and Nemertea

Members of the phyla Platyhelminthes and Nemertea, such as flat worms and ribbon worms, rely on the coral reefs for habitat and protection. They live in the cervices created by the corals. Many flatworms are carnivorous and feed on other inhabitants of the coral reefs, such as sea squirts, mussels and oysters. However, some flatworms are scavengers, and others eat algae. Some species of flatworms have elaborate mating rituals, which take place within the coral reefs.


Another group of marine worms is the phylum Annelida, which includes species such as the parchment worm, fire worm, spaghetti worm and the polychaetes. Some species of polychaetes, such as feather duster or Christmas tree worms, bore into the calcium carbonate skeleton of the corals to find a home. Some Annelids are filter feeders, while others feed on detritus, and some are carnivorous, and feed on snails and worms living in the corals. 


The phylum Mollusca is largely comprised of benthic (or bottom-dwelling) invertebrates, but some species, such as squids, octopuses and cuttlefish, are free swimming. Despite being free swimming, many species of octopuses, cuttlefish and squids rely on coral reefs for protection, shelter and as a hunting ground. Other molluscs, such as marine snails, chitons and nudibranchs, are largely herbivorous and rely on the coral reefs as a grazing ground.

They eat primary producers, such as algae and seaweeds, which grow on the reef. Some species of marine snails are actually predatory and bore into the shells of sessile invertebrates to prey on the organisms inside. Gastropods also rely on the reefs for camouflage and hiding places, as many fish species prey on these marine invertebrates. Bivalves are another group of mollusks that are commonly found on coral reefs. These creatures are normally identifiable by the matching twin shells which surround their bodies. Common examples include clams and scallops, which rely on the reefs as an important habitat and concealment from predators, such as reef fish and sea stars. 


Arthropoda is a diverse phylum containing many familiar reef creatures, such as crabs, shrimps, lobsters and other crustaceans. Many crustaceans rely on the coral reefs for protection, and spend many daylight hours hiding out in rock crevices or in between coral branches. They often wander more freely at night in search of food. Crabs are generally omnivorous and feed on a variety of other reef associated species, such as algae, worms, molluscs, fungi, other crustaceans and also detritus. They are also thought to be beneficial to other coral species, as they remove infesting parasites and other harmful species from the corals themselves. Some species of shrimp are fascinating as they act as cleaners and remove parasites from reef associated fishes or other invertebrates. They are incredibly important to the coral ecosystem and contribute to the health of the surrounding organisms. Other crustaceans act as scavengers and eat decaying matter, thereby cleansing the reef.


Echinoderms are generally mobile, although fairly slow. Common examples, include sees stars which prey on reef associated molluscs and other invertebrates, including the reef itself, and sea urchins which are active grazing herbivores. Coral reefs provide shelter and habitat to both sea urchin and sea stars, as well as to other echinoderms, such as sea cucumbers and starfish. Sea cucumbers are decomposers and help keep the reef healthy by processing and preventing the build-up of waste.


In addition, to providing food and shelter, coral reefs also provide breeding sites and act as nurseries to many invertebrate species. Like corals, many invertebrates breed by spawning and release large quantities of eggs into the ocean at one time. In order for this to be successful, large numbers of individuals need to release their eggs at once, so that consumers are overwhelmed by the sheer number of eggs and so unable to eat all of them, ensuring that some eggs survive. Because corals reefs are such complex systems, species can exist the densities that are required for spawning to be successful. Corals also provide places for where the eggs can land and develop into juveniles.

In conclusion, coral reefs are an important habitat for many invertebrate species, which rely on the reef for shelter, protection, food, and reproduction. Invertebrates are incredibly important members of the food web, and also provide many ecosystem services to other species, such as cleaner shrimp, which removes parasites, or sea cucumbers, which act as decomposers of organic material.