8 Fascinating Underwater Symbiotic Relationships

The incredible diversity of the world’s oceans and the complexity of its creatures allow symbiotic relationships to develop in sometimes surprising ways. Symbiotic relationships can be found across the planet (both above and below the waves), and they are a sign that an ecosystem is balanced and mature, allowing fascinating interspecies interactions to occur. We are going to look at the intriguing practice of symbiotic relationships in the ocean, along with eight examples.

What is a symbiotic relationship?

So, what is a symbiotic relationship and what different types occur in our oceans? There are three basic types of symbiosis found in the world’s oceans: mutualism, commensalism, and parasitism.

A symbiotic relationship can be any of several interactions between two organisms that can be beneficial, neutral, or harmful. We will look at the different types of symbiotic relationships and then give you some examples of each.

Do you love to learn about weird and wonderful marine life? Check out:  The Fascinating World of 8 Deep-Sea Creatures.


This kind of symbiosis benefits both parties (symbionts) involved. Benefits to the symbionts can include food, defense, and shelter. The best example of this in the marine world is the mutualism between coral polyps and their zooxanthellae algae, where the algae gain oxygen from the tissues of the living coral and they in return produce sugars via photosynthesis for the bulk of the coral’s food.


This kind of symbiosis describes when one animal gains a benefit without any significant impact on another symbiont. Commensalism can include transport (hitching a ride on fur, scales, and skin), housing, or a phenomenon called metabiosis where one animal uses an item another created after its death. Hermit crabs using gastropod shells are a great example of commensalism.


This kind of symbiosis is where one animal benefits while the host is harmed as a result. Parasites can be found internally, externally, and also intermittently as they utilize their host for their own benefit; often performing roles such as extracting blood or absorbing nutrition in the digestive tract.

8 symbiotic relationships in the ocean

The ocean hosts many different symbiotic relationships found all across the globe. We will look at a few examples found underwater, including the organisms involved and which category the relationship falls under.

1. Alpheid shrimps and gobies

This symbiotic relationship is quite a common sight for divers to see in the shallow sandy areas of many tropical destinations. Alpheid  shrimps are excellent burrowers and tunnel diggers and gobies have keen eyesight and act as alert lookouts for any passing dangers.

The shrimp’s eyesight is poor, so they rely on sensory antennae to keep in contact with a goby. Also, the goby can leave chemical cues to tell the shrimp not to exit the burrow. The goby gets a safe home, and the shrimp has his own security guard in this example of a mutualistic relationship.

Love sea turtles? Here are:  11 Amazing Facts About Turtles (and How to Tell Them Apart).

2. Groupers and octopi

Two very successful reef hunters in their own right make a formidable pairing when they team up to scour the reef in search of prey. The flexible octopus can reach areas of the reef inaccessible to the  grouper, and the grouper’s panic-inducing presence often sends small fish and crustaceans into the path of the  octopus

Working together as a team means the success rate of every hunt is higher and both animals get a good meal. This too is an example of mutualism in the ocean.

3. Megafauna and remoras

Large marine animals such as  manta rays, large  sea turtleswhales, and  whale sharks often have multiple remoras hanging off their underbelly. Typically these cause no harm to the animals although smaller creatures do seem to find them rather annoying!

Remoras provide a spa service to the larger creatures by eating parasites and dead skin. They use their modified dorsal fins as a sucker pad and get a free ride and a meal whilst keeping the other symbiont clean. This is a mutualistic relationship but there are commensalistic elements to it as well.

4. Isopod parasites in fish

These parasitic invertebrates can be found on many different fish species across the globe and are typically found around the heads of fishes affected. They will latch onto any areas they can sink their mouthparts into and start to gain nourishment directly from the fish’s bloodstream.

The gills, eyes, and even tongue of fish are prime locations for these creatures that look similar to a pill bug with a segmented body. The fish gain nothing from this and, as such, it is a parasitic relationship.

5. Hermit crabs using gastropod shells

Hermit  crabs are easily identified by their use of shells both as land crabs and in underwater settings. Shells are strong, transportable and durable offering protection on the move as well as being in ready supply near to or in the marine environment.

As hermit crabs only get access to these shells after the death of the gastropod mollusc, this is a commensalistic relationship. But more specifically, it is a metabiotic relationship as it is using the shell after the death of its owner.

6. Corals and zooxanthellae algae

One of the most famous symbioses on the planet, the relationship between coral and the vitally important algae found in its tissues has created the only structure made by a creature visible from space; the  Great Barrier Reef

The coral polyp has a calcium carbonate skeleton and is armed with stinging nematocysts on its feeding arms. This makes it a fortress for the zooxanthellae algae living within its tissues.

The algae, safe from harm out in the seawater, where it would be quickly grazed by zooplankton, provides up to 90% of the coral’s food from photosynthesis. Without doubt one of the most famous mutualistic relationships on the planet.

Do you know the difference between different types of corals? Take a look at  Hard Coral vs Soft Coral: What is the Difference?

7. Male deep sea anglerfish

One of the oddest interactions in our list, male anglerfishes in the deep sea have a hard task of finding a mate and so when they do, they need to take advantage. They bite onto the female and stay attached allowing their jaws to fuse to her body. She takes his sperm to use for fertilizing her eggs, but he stays alive and nourished from her body. A strange combo of parasitism with a bit of mutualism.

8. Cleaning fish/creatures and predators

The animals that provide cleaning services on the reef are seemingly untouchable because of what they provide to the animals around them. Primarily wrasse in the Indo-Pacific, and gobies or shrimps in the Caribbean, they swim into predatory fishes’ mouths and pick off parasites, dead skin cells, and detritus. They get a free meal and safe existence while the other fish get a spa / dental cleanup. Another mutualistic relationship in the sea.

These are just eight examples of the many symbiotic relationships happening in our oceans every day. 

If you enjoyed learning about these, check out the range of  SSI ecology courses. You will explore how ocean ecosystems function and learn how to dive safely with sharks, rays, marine mammals, and more.