Symbiosis in the Coral Reef Ecosystem

Coral reefs are amongst the most diverse ecosystems in the world. Coral reefs comprise less than 1% of the world’s oceans, yet, they are the habitat of approximately one quarter of the Earth’s marine species. Coral reefs are very complex marine ecosystems, comprising shading areas with cavern-like formations and towering structures, providing home for hundreds of marine plants, fish and invertebrate species, most of which live in a symbiotic association in order to survive.

Symbiosis refers to a close association between two organisms that can benefit either one or both organisms. There are three types of symbiosis; mutualism, comensalism and parasitism. Mutualism is the most common type of symbiosis and is characterized by an interdependence of host and symbiont in which one organism is unable to survive without the other. One such example is the symbiotic relation between reef-building corals and their algal symbiont.

Mutualistic relation between coral polyps and zooxanthelae

Coral polyps, the building blocks of a coral reef, live in a mutual association with a photosynthetic algae. Inside each coral polyp lives a green algae called zooxanthelae. The algae supplies oxygen to the polyp through the process of photosynthesis and provides the energy needed for the polyp to live. The polyp in return provides the algae with carbon dioxide and nitrogen. The algae also benefits from the coral’s protective skeleton. Coral polyps usually develop close to the ocean surface water. This permits the algae to use the sunlight for photosynthesis.

Clown fish and sea anemone

Another example of symbiotic mutualism is the clownfish and sea anemone. The anemone’s tentacles are deadly to most fish; however, the clown fish develops a mucus coating which makes them immune to the anemone. In this way, the clown fish can live among the anemone obtaining protection from predators. The clown fish feeds on organisms that could harm the sea anemone and the fecal wastes from the clown fish serve as nutrients to the sea anemone. Gobies live in a symbiotic relation with burrowing shrimps. They both live in a burrow in the sand which is maintained by the shrimp. Since the shrimp is blind, the goby maintains contact with the shrimp by flicking its tail when in danger, while the shrimp uses an antennae.


In a comensal relationship, one organism benefits while the other neither benefits or gets harmed. cardinal fish are small enough to hide among the spines of the sea urchin and the star fish. these small fish use the spines to protect themselves from predators. Remoras attach themselves to the bodies of larger fish, such as sharks and turtles and as they do, they feed on the food left behind by the host animal.


In a parasitism association, one organisms lives in a second organism called the host. In this association the host is harmed, while the parasite benefits from it. There are two types of parasitism; ectoparasitism and endoparasitism. Ectoparasitism involves external parasites and endoparasitism is when the parasites live inside the body of the host. The fish doctor, which is a type of isopod crustacean, attaches itself to the gills, scales and fins of fish, sucking its blood until the fish dies. The pearl fish detects the chemicals secreted by the sea cucumber during which it enters the sea cucumber. The sea cucumber tries to expel the pearl fish by expelling their digestive tract through their anus, causing lethal damage.

Cleaning symbiosis is a type of comensalism in which cleaner shrimp feeds on algae, parasites and detritus from the fish. Sometimes the fish will signal its willingness to be cleaned by adopting a peculiar positioning. On occasions, a coral reef animal will utilize another animal as a means of transportation in a symbiotic relation known as phoresis. Coleman shrimp are usually found in pairs on top of the toxic sea urchin, where they perch and hide from predators.

The emperor shrimp benefits from riding on the tops of various marine species, such as the nudibranchs and sea cucumbers and as they do, they feed on other small marine animals. Small sardines are often attracted by jelly fish, where they find protection from predators under the jellyfish’s bone or even in between the stingy tentacles.

Coral reefs are one of the most biological diverse ecosystems on Earth. Marine species rey on the reef ecosystem for survival and many other animals and plants inhabiting the coral reef ecosystem have developed symbiotic relationships in order to survive. Coral reefs posses an intricate biological and economic value. According to, coral reefs and the marine organisms that inhabit them are in danger of disappearing if actions are not implemented to protect them.