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Freeloaders, friends, or bloodsuckers? (Symbiotic) Relationships in the sea

Updated: Jun 15, 2021

anemone and clownfish in Nusa Penida Bali
You help me, I help you!

Just like in any community the ocean’s ecosystems are rife with interesting relationships and like with every relationship some a mutually beneficial, some benefit one of the participants but not the other and some are just toxic for one guy whilst the other gets all the benefits.

Here is an overview of the coolest and darkest symbiotic relationships in the sea.

The base of it all: zooxanthellae and coral

The very building blocks of a coral reef ecosystem are based on a mutually beneficial relationship, that of zooxanthellae and corals. Zooxanthellae are a species of dinoflagellates also described as a type of marine algae or plankton. They live inside the coral giving it its colour. Zooxanthellae are photosynthetic organisms containing chlorophyll. During the day they provide their host with the product of their photosynthesis such as sugars and amino acids. The coral in return provides them with an elevated position close to the light and with carbon dioxide and nitrogen compounds. If environmental stressors (like elevated temperatures, pollution, or salinity) are strong around the coral the zooxanthellae may leave the coral which causes bleaching. If the stressors are not removed the zooxanthellae do not return and the coral eventually dies usually smothered by “bad” algae.

The most famous couple: anemone and clown fish

Sea anemones have stinging cells, but they still need someone to clean and air their area. Clown fish are small fish with few defences in a big ocean full of predators. The two species decided there was definitely a mutually beneficial relationship (mutualism) to be explored here. The anemone fish has a special substance on its skin preventing it from being stung by the anemone, but no other predator would dare approach, keeping it safe. It is adding to its diet with its waste and food scraps whilst also keeping it clean and health due to aeration of its movement. Win-win!

Best friends forever: goby and pistol shrimp.

Probably the cutest relationship of all. The two species bond as juveniles and never leave one another, they are best friends for life. On a day to day basis the deal is this, the blind shrimp forages through the mud and builds a burrow; meanwhile, the fish is on the lookout for predators since the shrimp is exposed when it emerges with sand and debris. The shrimp keeps one antenna in contact with the fish’s body so if the fish darts into the burrow it does too. The goby uses the burrow to hide but also, importantly, to mate. The shrimp actually builds the goby a breeding chamber (and future stash spot for the eggs). The mating ritual involves a lengthy process and causes quite a bit of damage to the burrow (who knew gobies were such sex beasts), but the shrimp tidies up as the goby gets it on so the shenanigans can continue undistrurbed. That is one very very good friend if you ask me!

Freeloaders: shrimp, the blaggers of the sea.

Some shrimp do bring something to the table, they clean their hosts of dead skin, small parasites, and food debris. For some moray eels, the cleaner shrimp act as dentists/ teeth cleaners, getting a free meal for their hassle. But not all shrimp are such good guests. For the most part, they’re just in it for the free ride and food scraps. Most commensal shrimp, their name coming from their relationship with their host (commensalism: beneficial to one participant but not harmful or beneficial to the other), feeling a little exposed in an ocean full of shrimp loving predators, hop on to a bigger, scarier host and hide there. Urchins, sea cucumbers, Spanish dancers, crinoids, whip coral, anemones are all great rides or homes for shrimp, safe and often with the added bonus of free meals (think sea dancer poo, mmmm).

My friend, my disguise: crabs and sponges

Crabs like to look good. They are stylish dressers and will stop at nothing to have the coolest shells. Anyway, that’s what is seems like when you see the lengths they go to decorate their shells. Decorator crabs and hermit crabs both have mutualistic (mutually beneficial) relationship with other animals who live on their shells but for different reasons. The decorator crab grows sponges, hydroids and tunicates on its shell and thus camouflaged becomes practically invisible on the reef when it stops moving. The hermit crab will pop a couple of anemones on its back like a living stinging armour and even moves them to its new shell when it grows out of the old one. The organisms living on the shells get to move around to find new sources of food and get food scraps.

Parasite on fish
the stuff of nightmares via

Toxic evil: the isopod who ate my tongue

Unlike the mutualistic or commensal relationships described above, this is one is the summum of parasitism. There are many tick-like isopods in the sea, which latch on to their hosts and suck their blood forever but this particular one (Cymothoa exigua) takes the cake in terms of pure evil. The isopod enters the host fish through the gills. There may be more than one of them. Whilst the still-small males will stay in the gills, the females (who could have previously been males) will enter the mouth and sever the blood supply to the tongue which will eventually fall off. The parasite then latches on to the stub of the tongue and gets fist taste of anything the fish eats. They also may feed on the fish’s blood and mucus. Most fish with parasites are weaker and underweight. This is the stuff of nightmares.

There are many more symbiotic relationships on the reef, some clean, some use others to hide behind for protection or hunting, some just hitch a lift for life. Some even protect their hosts from terrible man made disasters. Have a peek on the web for more details. 

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