Harmony on the Reef

In the natural world, day to day life within an ecosystem can be a hostile affair. If you’re considered prey, a momentary lapse of concentration could be a death sentence. In among the complex community of a reef, it seems like it’s a case of every fish for himself. Remarkably, however, this is not true, the quest for survival has led to some fascinating relationships being formed. Different species join forces to work together in order to beat the odds and make it through another day. How they sit down and figure this stuff out is beyond me but some of the most mind blowing things I have seen underwater are demonstrations of such alliances we call symbiotic relationships.

The first time I really got to understand this whole symbiosis thing was in the Philippines. While out on a dive, a guide pointed out a small fish in the sand. I could tell it was a goby, pretty cool to look at, but something about his behavior seemed a little shady, like he had something to hide. I remember wondering why the guide had bothered to point it out, I mean there were thousands of fish within the immediate vicinity, what was special about this one? As we continued across the sandy bottom, I became aware that something kept happening on the edge of my vision, by the time by brain would catch up I’d find myself looking at another one of these gobies, kinda poking his head out of a hole, looking as if to say “What… there’s nothing to see here, move along.” Something was up but I couldn’t put my finger on it.

It was about five minutes later when I finally got the opportunity to see the bigger picture. Up ahead was a little shrimp in the sand digging around and right next to him was one of the very same gobies, except, unlike the others, this one was fully out in the open. I started to slowly swim towards them but as I was approaching, the goby turned his head towards me. At first nothing happened, he just stared but then I started to exhale and the sound of my bubbles clearly startled the goby as he shot, seemingly backwards, into his hole. The strange thing was that the nearby shrimp also shot down the same hole, both species shared the same refuge.

On land I asked the guide about this and was informed that the goby and the shrimp live together. They work together using each other’s strengths to protect themselves. The shrimp is an excellent ground worker and efficient at digging holes to live in as well as finding food in the sand. Unfortunately for him though, he is virtually blind so unable to detect danger till it is too close. The goby therefore acts as a lookout for the shrimp and upon noticing a threat, sends a vibration through the water from his fins which the shrimp recognizes before running for cover into the hole. And there they live together in harmony.

If you’d like to get specific on the subject, the kind of symbiotic relationship in the example above would be referred to as mutualism. By definition, this means that two species gain something from their longstanding interaction together, our friend the goby gets a nice house and easy food out of the deal whereas Mr Shrimp gets the defence warning that his own bad vision can’t provide. Some scientists argue that this is the only true form of symbiosis but in recent years the definition has been confirmed to include any kind of persistent biological interaction. Under this broad heading, there are two other common definitions – parasitic & commensalistic.

Upon looking into mutualistic symbiosis, I realized that I had seen countless relationships of this kind in the past but never made the connection. A perfect example that’s always remarkable to watch is that of the cleaning station. Small organisms like shrimp and little fish hang out in these places and wait for bigger fish to turn up. When the big fish arrive they can chill out at the station to let the smaller guys clean their gills, skin & teeth.

Everybody is in on the deal, the big fish know that if they open their mouth, little dudes like cleaner shrimp will jump on in and start eating through the nasty stuff in the fish’s teeth. The big fish knows he can’t disrupt this relationship by taking a cheap meal and eating the shrimp, as to damage the relationship would soon result in disease. The little dudes take the free lunch and enjoy the security of knowing that they are not going to get attacked for a while, not in the cleaning station at least.
A cleaning station can be easy to spot or sometimes not so. A good tip for finding them in the Caribbean is to look for anemone as it stands out against the typically harder surroundings of the coral on the reef. Corkscrew anemone which often gathers under rocks or in other cracks in the reef is a great place to find Pederson cleaner shrimp as well as pistol shrimp.

Any time a diver is treated to seeing a larger creature underwater like a shark, manta ray or even a big turtle, there is a good chance they can see a remora as part of the package. The deal here is very much the same kind of thing as the cleaning station except when you’re a boss like a shark you don’t need to travel to get your skin cleaned, the cleaners live with you by sucking onto your body, cleaning off the parasites as you glide through the water, pretty cool huh?

So what about these other kinds that may or may not be true forms of symbiosis? One in particular is a little more unequal than the rest – parasitism. This defines a deal which has kinda been imposed upon “the host” who would no doubt prefer to live without their “guest” if possible. In the Caribbean we can see a fairly common example of this which is the isopod that often attaches itself to the head of a soldierfish. From this position, the nasty, woodlouse looking parasite latches onto the fish and feeds from its blood and bodily fluids. It may just be my imagination, but whenever I see a soldierfish that has one of these long term passengers tagging along for the ride, it makes him look even sadder than his buddies (they usually look pretty sad). They also appear to be more recluse than their buddies, hiding under rocks as if they are ashamed of the monstrosity that lives on their head, and who could blame them?

Commensalism is defined as a deal where only one of the parties gain, but this can be an interesting topic for discussion because how do we know that this is truly the case? If two species hang out together and we can clearly see benefit to one but not the other, does that mean that there really is only one benefit, or is it the case that there is only one benefit which we understand?
The relationship between the stingray and the fish, often a bar jack, that swims above them is usually referred to as commensalism. The idea being that stingrays are messy eaters, as they cruise around sucking through the sand for goodies, they fire the bits they don’t want through the two holes in the top of their heads, a fish hanging out there may well get some leftovers that the ray accidentally spat out. Because there is no perceived gain for the ray, this kind of symbiotic relationship is referred to as commensalism. Some may argue however that the jack is a lookout for the ray who has a restricted field of vision in which case it would be another version of mutualism, I’ll let you decide.

The big picture of a marine ecosystem is made up of tiny details like an intricate pattern woven by nature. Symbiosis is a perfect example of one of these tiny details, without which, the bigger picture would not be seen. It’s a fascinating thing to imagine how such relationships are formed. The sight of a moray eel languishing on the reef with a couple of banded coral shrimp picking their lunch out of his mouth will always remind me of how lucky I am to witness such an astonishing act of nature. Finding one cool creature underwater is awesome but finding a couple interacting in such a way is out of this world.

Harmony on the Reef from iDive Global on Vimeo.

About the Author: Drew McArthur is a professional dive instructor and boat captain currently working at Divetech on Grand Cayman. Since his first dive over 20 years ago, he has found himself in all kinds of underwater environments from golf ponds on the side of a snowy Welsh mountain to the tropical paradise he has now become accustomed to. A fanatic of all things tech, one of the main things Drew loves about Grand Cayman is the accessibility of such premier dive sites. Check out Drew’s blog here.

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