Love thy Neighbour!?   Many of the animals that live on reefs have developed special relationships with other species to enhance their living strategies or decrease the risks of life they have to face. These interactions can occur on a number of diffferent levels. It may be as simple as whether you swim in a school or alone if you are a fish. If you live in a colony or are solitary if you are a cnidarian or bryozoan. Or if you have a symbiotic relationship with a completely different animal to help you live well. Symbiosis simply means 'living together'. Types of symbiosis include mutualism, where both species benefit from the relationship. Parasitism is where an animal lives in or on a host organism to obtain it's food. The host does not nessecarily benefit, in fact may suffer, from this union. Commensalism is a type of relationship where one or more species live together, maybe in the same burrow or shell without much interference to each other. Some symbiotic relationships are transient or opportunistic , meaning they only use each other for a short period of time, like lice on the skin of seals. However relationships may be permanent or obligate such as the coral polyp and it's zooxanthellae or the beautiful clownfish and it's anemone.	Commensal shrimp on crinoid   Photo by: Valerie Taylor
	Corals - Plant or animal?
	One of the most important symbiotic relationships on a coral reef is between the one between the coral polyp and microscopic algal cells, called zooxanthellae, that live inside them. Zooxanthellae are dinoflagellates, and there are several species - each species of symbiotic animal seems to have its' own species of zooxanthellae. The presence of zooxanthellae in the corals' tissue means that corals can act like a plant during the day - using the photosynthetic ability of the zooxanthellae to capture the suns' energy; and like an animal at night - feeding on suspended particles and plankton in the water that they capture with their tentacles and digest. Other animals live in symbiosis with zooxanthellae, such as giant clams (Tridacna) nudibranchs, soft corals and zoanthids (see 'Who's who'). The symbiosis is mutualistic as both species benefit from the relationship. The zooxanthellae supply much of the corals energy needs, by passing on organic carbon (glycerol, sugars and amino acids) that is produced by photosynthesis. Corals provide the zooxanthellae with a protected structure in which to live, as well as providing them with important inorganic nutrients such as nitrates and phosphates that the algae need for growth. There is then a tight recycling of nutrients in the symbiosis.The waste products of the host are then going straight to the algae, and not into the water column. Zooxanthellae also increase the rate of calcification (skeletal growth) of the coral. Corals usually grow into a form that maximises the exposure of zooxanthellae to sunlight, to increase photosynthesis and the production of "food". Zooxanthellae concentrations vary along the branches of the coral, often being denser in parts of the coral that are shaded. This symbiotic relationship between these tiny algal cells and their coral hosts is the basis of the success of coral reef ecosystems. The symbiosis allows corals and zooxanthellae to grow in water that is nutrient poor (see 'Oceans'), and is responsible for the high productivity and reef-building ability of corals,and therefore the success of the whole ecosystem.
	The Clownfish and the Anemone
	The clown fish is a brightly colored tropical fish that has an obligate relationship with the poisonous anemone whom it lives amongst. The fish is never found without the anemone but the anemone is often found without the fish. This relationship is a form of commensalism. The clownfish gradually coats itself with the mucus of the anemone which makes the anemone not recognise it as a foreign species but rather a part of itself. This means that the anemone does not fire it's poisonous nematocysts (Stinging cells) when the clownfish touches it. The clownfish is therefore protected by the anemone because it can dart into the tentactles whenever a predator approaches. The predator, not having the benefit of the mucus coating, can be stung by the deadly tentacles of the anemone, so generally chooses to leave the clownfish alone. The clownfish also uses the anemone as a safe nursery for them to lay their eggs. The anemone itself does not benefit from the clownfishes presence but also is not hindered.
	Service Stations of the reef.
	Another form of symbiosis that has developed in reef environments is the 'cleaning stations' . This is a particular area where creatures such wrasse and shrimps are known to be found. Fish travel for considerable distances to attend these 'cleaning stations' where the tiny cleaner wrasse or shrimp swim and hop in and out of their mouths and gill slits removing growths and parasites from the fish. The amazing thing about this relationship is that often these fish are predatory and would normally eat smaller fish and crustaceans like the cleaners. At the cleaning stations however ther is no risk of predation, as the fish patiently wait for the cleaning to be completed. Some instinct tells the larger predators that the cleaners are providing a service they cannot do without. In some areas where the cleaning stations where experimentally removed, there were sudden drops in general biodiversity in the area. The remaining fish often have fungal growths or bad parasitic infections. This is a another form of mutualism on the reef that does not require the pair to be biologically joined like the coral and the zooxanthellae but rather provides a mutual service that benefits both, the cleaners get to eat and the fish avoid parasitic infestations.
	Parasites
	Parasitism is one of the most commonly recognised forms of symbiosis. The parasite is the symbiont who dwells in or on the hosts body and receives benefits at the hosts expense. There are two types of parasitism. One is ectoparasitism where the parasite is external, such as the fish doctor. The fish doctor is an isopod crustacean that attaches itself to the fish under its fins, scales, or in it's gills. The fish doctor then sucks the blood of the host until its death, then moves on to a new host. The other type of parasite is an endoparasite. Endoparasites, such as viruses, bacteria, flatworms, roundworms and leeches live inside the body of the host. There are some cases of mesoparasites, these are parasites that live in a body cavity that is in reach of the environment such as the gill chamber or the anus of the animal. The pearl fish is one of these mesoparasites. This fish detects a chemical that comes from the anus of the sea cucumber. Sea cucumber achieve gas exchange by drawing in and expelling water through their anuses. When the cucumber breathes in the pearl fish hops in the current and is drawn into the anus of the cucumber tail first! This is not a beneficial thing for the cucumber and it often attempts to expell the parasite by ejecting most of their digestive tract out through their anus.
	A Lesson For Us All!
	Within the vast array of living things that inhabit reefs, there is a seemingly limitless amount of ways that they can all be interconnected. In order to live together, the animals of the reef employ the various forms of symbiosis. The above examples are only a few in the web of the reef. There are a multitude of connections and it is important to realise that all things and all processes on a reef , as in the rest of the world , rely on each other. Many of these processes we don't undersand, such as sponges and their bacteria. Some sponges have been found to have 40% of their soft tissue made of bacteria , while the seawater surrounding it is completely free of it. This relationship apparently dates back to the precambrian time , 1 billion years ago! It is not understood if the sponge gains anything from this relationship but there is probably an important reason that is simply not visible to us.The creatures of the reef are simply existing to the best of their ability. Disturbances on reefs can be natural or man made. When the web of life is not fully understood, interfering with this ecosystem could be very costly for those who live there.Coral reefs in tropical waters owe their existence to a symbiotic relationship, that between the coral polyp and the zooxanthellae. Without this mutually beneficial relationship, the whole web of life on a coral reef would fall apart.
	REN Links
	Oceans
	Energy Cycles
	Sex in the Sea
	External Links
	Coral Cay Conservation - Zooxanthellae
	Uni of Virgin Islands - Zooxanthellae
	Anemone fishes and their host sea anemones