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Symbiosis in the Marine Environment

There are many inter species relationships in the world beneath the waves, and if you are a regular diver, fish identification enthusiast or simply a log book fanatic, you’ll probably already have noticed a fair few. Some are not so surprising, but others can be found in the most unexpected of places!

Any kind of biological interaction between living things is known as symbiosis, and there are three main types of relationship: mutualism, commensalism and parasitic. People often mistake mutualism and symbiosis for the same thing, however this is not always the case. While mutualism is certainly the cutest and most appealing form of symbiosis, and the one which divers tend to know the most about, there is no denying the fair share of commensal and parasitic relationships that exist in the marine world. So with no more ado, let’s dive in (pun totally intended) and find out a bit more about each of these.


What is Mutualism?

Mutualistic relationships refer to pairings who live and work together for mutual benefit. Now that’s what we call underwater love! Most of these pairings will live in close quarters to each other and include some form of protection for one species, and some form of energy transference for the other. However while the majority of underwater mutualistic relationships involve services provided on a local level, mutualism can also involve the transference of benefits over larger distances. For example back on land, birds and bees are in a mutualistic relationship with the flowers and trees they feed upon, as in return their seeds are spread over a much larger area. Some common examples of marine mutualism include:


Corals and Zooxanthellae

”What is Zooxanthellae?!” we hear you say – or try to! Maybe we need to sound that out! So before getting started on this one, a mini biology lesson to get you up to speed… Phonetically pronounced ‘Zoo-Zan-Thell-Ee’, these are single celled organisms that contain chlorophyll. Corals form their calcium skeleton and the zooxanthellae lives within it. The coral benefits from the energy the zooxanthellae create through photosynthesis, while the zooxanthellae is protected from predators by the coral skeleton.

It is the zooxanthellae that gives corals their yellow/brownish colour, as coral itself has no natural colouring. This is why in periods of high water temperatures much of the coral you will see appears white/bleached. It’s not a case of the coral changing colour, just that the zooxanthellae can no longer survive in such a hot conditions, so has left the coral to find a cooler place to call home.



Goby Fish & Shrimp

A true watchman and housekeeper relationship, these fish and their partner shrimps live in burrows on sandy sea beds. The shrimp is almost blind, and without the Goby fish to help and protect it, would struggle to navigate and/or survive out in the open. It is afforded protection and a safe place to live in return for cleaning & maintenance of the Goby’s hole.

If you take some time to observe the interaction between these two unlikely roommates, you’ll notice that when outside of the burrow, the shrimp will have one of its antenna rested permanently on/under the Goby fish’s tail. If a predator approaches, the fish will move its tail, sending a message to the shrimp to retreat back into the burrow and avoid danger.

Goby Fish


Anemone and Anemonefish

Any diver who has ever brushed against an anemone will tell you that the sting can be irritating, as it normally results in an itchy red swollen patch of skin where the contact was made. Fun fact: current research estimates that only around 1 in every 2000 humans are immune to anemone stings. So keep up your efforts on buoyancy control, as that lucky one in 2000 is probably not you!

To most marine life however, the sting is much more toxic, hence why you will not see any other fish inside an anemone. The anemonefish has developed a protective layer on their outer scales that protects them from the sting of the anemone tentacles, which enables it to live safely within it. The anemonefish feeds on leftover fish and algae from the anemone, and is provided with protection from predators. The anemone receives a cleaning service, as well as improved water circulation from the anemonefish swimming around within it.



Cleaner Fish & Cleaner Shrimp

Often referred to as the dentists of the marine world, cleaner wrasse and cleaner shrimps keep the skin/scales, gills and mouths of the fish they tend to bacteria free, and in return never need to go without a meal. You will see many ‘cleaning stations’ around Koh Tao dive sites where you can witness this. The variety of species that cleaner wrasse and cleaner shrimp tend to is varied, including Parrotfish, Giant Groupers, Moray Eels and even Damselfish.



Remoras and Sharks

Remoras feed on parasites on the shark’s skin, eat leftover food scraps that their apex predator hosts discard, and also get to hitch a ride through the ocean and be afforded protection from predators. Sounds a bit one sided so far, we know. But the shark also benefits as it is kept clean of parasites that could affect its health, and because the remora keep the water surrounding them clear of food waste, the shark is also less at risk from unhealthy organisms that may otherwise present themselves.

Whale shark in Koh Tao waters


What is Commensalism?

A commensal relationship is one where where one party in the relationship benefits, and the other is neither helped nor harmed by the process. In most cases this involves one species using the other for either easy transportation, housing, or the sharing of food. Some common examples of marine commensalism include:


Barnacles & Whales

Barnacles will attach themselves to the bodies of whales in order to be transported to where food is readily available. Aside from food and a free journey, being attached to such a big ocean giant also means they are protected from potential predators. The whale receives no benefits in this relationship, but comes to no harm as a result either.



Flatworm & Crab

Flatworms regularly latch on to hermit crabs in order to eat it’s food. There is still enough food for the crab itself too, however, so no the hermit does not lose out in this process.


Hermit Crabs

Hermit crabs are themselves a happy recipient on a commensal relationship. They have to find shells for shelter and protection, and therefore use various snail shells, or shells discarded by larger brothers or sisters. These shells are available because the snail has died, or the crab that lived inside previously, has upgraded to a larger shell, so the previous inhabitant is not affected.



Emperor Shrimp

The emperor shrimp is all in for a free ride, and uses modes of transportation such as sea cucumbers to move from one place to another. While not solely reliant just on sea cucumbers, the shrimp does normally choose a host which is either poisonous or inedible, therefore gaining maximum protection from predators.


What is Parasitism?

Parasitism is an altogether one way relationship, where only one species benefits while the other is harmed – and there are some pretty grizzly examples! There are way more parasitic relationships out there than you’d imagine – and we’re not just talking about your lazy family members! Also this is not something confined to the marine environment; there are myriad examples to be found on land too. The most common marine parasites include:



This is a type of louse which enters a fish through its gills, eats its tongue and then replaces it with its own body. So, yeah, pretty gross! In doing so, this slimy critter secures a safe place to live, but also bags itself a guaranteed meal every time the fish eats.



Rhizocephalan Barnacles

This unusual type of barnacle (which remains in larva form rather than developing a hard outer shell like its more civilised cousins) clamps itself over the reproductive organs of its host, simultaneously rendering the crab unable to reproduce, and also allowing itself internal access. Here’s where it gets creepy…from its base on the underbelly, the barnacle then grows inside the crab, eventually taking over most of its body, drinking in the crabs bodily fluids for nutrients as it goes. We’ll skip out on showing you a pic of this one, we’re fairly confident you’ll be OK with that! 😉


Tongue eating beasties and body invading barnacles aside, you begin to see the extent of adaption in response to changing conditions, and just how far some species are prepared to go to survive. You have to give it to some of these guys, regardless of their specific type of symbiosis, they’ve gotten pretty inventive!

Can you think of any more examples of symbiosis? Hopefully not the revolting parasitic kind, but perhaps once you start thinking, you’ll be able to come up with a few more mutualistic and commensal examples. Plus if you pay close attention on your next dives, you’re sure to spot more and more symbiotic relationships on every dive!



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