A symbiotic relationship can take on many forms, which are classified depending on how the creatures share their living space or use each other. Understanding what sorts of relationships may exist in the natural world between species will help you to better understand the type of aquarium inhabitants you might acquire. The specialized defenses of certain species are often used by those that have little or none of their own, and is called inquilinism. A defenseless little reef fish would want to take advantage of the natural protection provided by a sea-urchin neighbor’s razor-sharp, and sometimes venomous, spines. In our aquaria an example would be Bangaii cardinalfish and long spine sea urchin. Clownfish, having covered themselves in mucus that makes them invisible to their host anemone are able to dance among the battery of the anemone’s stinging cells without harm. Some animals like the Carrier Crab, are highly creative in locating their defenses.
Some creatures use others as camouflage. Decorator crabs snip pieces off of sponges and other nearby organisms and embed them into their shells, sometimes even carving the sponge into a cap that neatly fits on their carapace. Other crabs plant sea anemones onto their shells devising a built-in self-maintaining shield of stinging cells – or hold one in each claw, and like a boxer, attempt to punch the offender with its borrowed battery. Scientists believe inquilinistic relationships merely evolved from creatures living in close proximity with one another.
Endoecism refers to animals that live in the shelters created by their host, most typically burrows. Proximity is also a likely reason for such a relationship to grow over the millennia, combined with a frequent hunt for shelter by what eventually becomes the symbiont. The arrow goby of North America is commonly found in the burrows of various invertebrates, favoring crab holes. The symbiont usually benefits the most in this relationship; however having a little visitor has its advantages. Should the goby find a chunk of food too large for it to handle, it will give it to the crab. The crab chops it up as it devours its free snack, allowing the goby to take a few shredded pieces back. Sometimes the live-in symbionts feed on the hosts’ waste products, earning their space as housekeepers. In the goby and blind shrimp relationship, the goby is unable to dig a burrow and the blind shrimp is unable to see potential predators.
The two most obvious symbiotic relationships involve food associations (commensalism) and associations in which both host and symbiont benefit (mutualism). These two are very close, but in commensalism, the issue is only food and it’s usually only the symbiont that benefits directly. Many shrimp, crabs and copepods, for example, live on the surface of corals and other cnidarians, eating their mucus coating, dead skin or any adhering organic particles. The crustaceans usually cause no harm to their coral hosts – and may even pluck off an occasional parasite.
Mutualism is the most well-known type of symbiosis, and is marked by an interdependence of host and symbiont. In most mutualistic relationships, one could not survive without the other, which makes these sorts of relationships among the most fascinating. Hermatypic (reef-building) corals and their algal symbionts, zooxanthellae, are a common example of such crucial interdependence. The zooxanthellae live inside the coral’s tissue, taking advantage of its waste products and transforming them into substances the coral can use to grow and maintain its calcium carbonate home. During times of high stress, such as prolonged high temperatures, the coral will eject zooxanthellae into the water column. Although the coral can live with a small amount of algal symbionts over short periods of time, a long absence of their usual population of symbionts spells certain death.
Cleaning symbiosis is similar, but the symbionts don’t live within their hosts’ tissues. A large fish will literally pull into a “cleaning station,” which is nothing more than an area where cleaner shrimp and fish live. Like Indy 500 pit mechanics, the cleaners scramble from their crevasses and overhangs, picking off parasites such as isopods (not ich however), algae, and detritus for the fish, while scoring an easy meal. Often times the fish signals it willingness to be cleaned by its sidewise positioning.
Many reef species are relegated to settling on a surface before adulthood, where they remain and live out their life. But these “sessile” organisms can sometimes become mobile should they settle down on the right spot. When one animal uses another for transportation, the symbiotic relationship is called phoresis. Barnacles, for example, may settle on a crab carapace, snail shell, or on the back of a whale. I call this the “hitchhiker relationship”.
Coleman shrimp are normally found in pairs on the toxic sea urchin (occasionally available from Drs. Foster and Smith and others), Asthenosoma varium, also called a fire urchin, with the female being the larger of the two. Coleman shrimp move amongst the poisonous spines and pedicellaria without incurring harm but they usually clear an area of these obstructions where they perch.
The white and dark brown striped Urchin crab is another guest of the fire urchin often occurring on the same animal as the Coleman Shrimp. Whereas the Coleman Shrimp only lives on the fire urchin, Urchin crabs live in association with a variety of urchins. The last segment of its leg forms a hook to hold onto the spines of the sea urchin. It can be found singly or in pairs.
Often found on the sea cucumber Actinopyga, the swimmer crab is commensally associated, receiving transportation and protection. It is often found with the Emperor Shrimp discussed below. While the swimmer crab benefits from the relationship, the sea cucumber does not.
Emperor shrimp hitch rides on various partners; I have seen them on sea cucumbers of various species and nudibranchs of various species including Risbecia tryoni, Ceratosoma trilobatum, and Spanish Dancers. Their coloration varies according to the “transportation partner” they ride on, but I am uncertain if they are able to change coloration if they happen to change transportation. Not content merely to relax and enjoy the ride, they are constantly on the move, normally perched near the substrate looking for food to pass by; I guess this is the marine version of “meals on wheels”?
Hopefully this discussion of relationships, relationship types, and especially hitchhikers on the reef will give you an idea for interesting aquaria subjects.
