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A Kid's Guide to Symbiotic Relationships of the Sea

Written By: Peter Whiting

Symbiotic relationships are interactions between two different types of living things or organisms. When two different organisms interact with one another over long periods of time, they can form a relationship. In some relationships, both organisms benefit from the interaction while only one may benefit in others. These relationships include mutualism, commensalism, parasitism, and competition. What type of symbiotic relationship two organisms have depends on whether one of them benefits and whether one is hurt.

What is a Symbiotic Relationship in the Animal Kingdom?

There are four main types of symbiotic relationships: parasitism, commensalism, mutualism, and competition. In parasitism, one organism (the parasite) benefits while the other is hurt or weakened. Parasites often live with, in, or on the host to feed. Either by taking food away from the host or using the host as a food source. In commensalism, or a commensal relationship, one organism benefits while the other is unaffected. Because the creature benefitting from the relationship is not a source of food or danger, it is often ignored. This allows it to benefit from the food, safety, or home the other organism provides. Mutualism is where both organisms benefit from the relationship. In this relationship, both creatures are welcome and often sought out by the other. Lastly, competition symbiotic relationships are where two organisms live together and compete for the same resources. In this relationship, the health of an environment can depend on who, if anyone, wins the ongoing competition. From food and protection to health and cleaning, creatures in our oceans look to one another to survive.

Sea Anemone and Clownfish

A great example of mutualism is the relationship between a sea anemone and a clownfish. In this relationship, both organisms benefit from living with one another. Sea anemone's brightly colored tentacles are armed with stinging cells and harpoon-like stingers that help it capture food and stay safe from predators. Clownfish are covered in thick mucus layers that allow them to live within the anemone unharmed. Because the clownfish does not try to eat the anemone, it also does not trigger an attack from the harpoon-like stingers. This allows the clownfish to use the sea anemone as a source of protection and a home. Although the sea anemone can sting potential predators, some fish often try to tear off pieces of the nutrient-rich tentacles. A clownfish hiding within the sea anemone will dart out to scare these predators, keeping his home safe and intact. The clownfish also eat any harmful parasites that try to attach to the sea anemone, keeping its home clean and healthy. As the clownfish consumes food provided by the anemone, sea anemones also receive food and nutrients from the clownfish in the form of nutrient-rich waste.

Barnacle and Whale

In the relationship between barnacles and whales, only the barnacle benefits but the whale is unharmed. This makes their symbiotic relationship a perfect example of commensalism. While you may have seen whales of all shapes and sizes, barnacles are notoriously shy creatures that hide within its shell at the first sign of danger. So, what are they? Barnacles are sticky crustaceans, related to crabs and shrimp, that can create a strong glue. This glue allows them to stick to sturdy objects like boats, rocks, and underwater volcanoes that sit close to food sources. In this case, the whale provides the perfect home for these sticky crustaceans. By attaching to the whale's head and chin, barnacles catch their food as the whale swims and hunts. Barnacles feed by sticking out feather-like appendages that catch microscopic organisms floating by. Since the barnacles do not harm the whale in any way, the whale allows them to attach to their skin and live alongside them.

Coral and Sponge

A healthy ecosystem provides enough food and space for both corals and sea sponges to grow. However, if either thrives and begins to outgrow the other, they can take valuable resources from the other. This competition over resources can lead to a competitive symbiotic relationship. Corals eat algae, fish, and zooplankton, or small drifting organisms, while sponges are filter feeders who prefer plankton, viruses, bacteria, and detritus (small floating debris or waste). While these two organisms do not necessarily compete for the same food sources, they do compete for something equally important, space. One of the most important resources in the health of a coral reef is room to grow. When coral reefs are healthy, they provide a bounty of food and resources for a multitude of creatures. In this thriving environment, some sea sponges may begin to spread quickly. Without room to grow, the coral can be hurt. Since the coral provides food, homes, and protection for several organisms that both the coral and sponge depend on, an imbalance can lead to less food and room to grow for both organisms.

Cleaner Fish

Living in the ocean can be a dirty business. Sea animals of all types depend on various types of cleaner fish to help rid themselves of algae, parasites, dead skin, and debris. Cleaner fish can be found everywhere from fresh or brackish water to marine water. In these symbiotic relationships between cleaner fish and sea animals, ranging from sharks and rays to sea turtles and eels, both organisms benefit from the relationship (mutualism). Because these cleaner fish are well-known, they are often sought out by other fish and sea life. Anyone needing cleaning can swim up to a cleaner fish, with their mouth open wide, and wait. Since these cleaner fish rid larger fish or animals of harmful parasites and debris, they can enter the larger animal's mouth unharmed. Cleaner fish eat the dead skin, algae buildup, and parasites they remove from the animals they clean. In this symbiotic relationship, both organisms benefit because one gets clean while the cleaner fish gets easy access to food, as many fish and animals will seek out cleaner fish for help.

Goby Fish and Pistol Shrimp

The goby fish and pistol shrimp have a mutual symbiotic relationship where both organisms benefit from one another. In this relationship, these two creatures share both an environment and a home. The pistol shrimp builds its burrow and allows the small goby fish to live with them. Outside this burrow, the goby uses its keen eyesight to watch out for predators and warns the shrimp of danger with a flick of its tail. Since the pistol shrimp is nearly blind, this early warning system gives both creatures time to retreat to the burrow. The goby allows both to search for food while the shrimp's ability to dig provides them with a place to hide from predators.

Spider Crab and Algae

Another mutual symbiotic relationship occurs between spider crabs and algae. In this relationship, both organisms benefit from the relationship as the spider crab provides the algae with a place to grow while the algae allows the crab to blend in with its environment. Shallow waters are often warmer and receive more sunlight than deeper water. Since algae require sunlight to grow, the spider crab's shell provides a sturdy surface on which to thrive. Because warmer, shallow water is a great source of food for many types of sea creatures, it can be a dangerous place to live. Allowing the algae to grow over its back for easy access to food and sunlight, the spider crab can hide from predators and hunt for its own food in relative safety.

Remora and Shark

A surprising form of mutualism is also found between the remora fish and sharks. If you've seen pictures or videos of sharks swimming in the open ocean, you've probably noticed a fish stuck to its belly. Remoras, also known as suckerfish, have a long, flat suction cup on the top of their head that they use to attach under a shark. By attaching to the shark's belly, a remora can save energy, get easy access to food, and be protected from other predators. Sharks are messy eaters and can leave a lot of leftover bits of food in their wake. Remoras often take advantage of this and feed off of the bits of food that comes by. By allowing the remora to attach to them, sharks gain a companion that provides a much-needed cleaning service. Remoras can easily detach from the shark's belly or chin by swimming forward. The remora can then pick parasites off of the shark's body and remove skin or uneaten food from the shark's mouth, keeping the shark healthy and clean.

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