Four Types of Symbiosis
Most divers are aware of the relationship between clownfish and anemones. The colourful fish borrows the protection of the stinging tentacles, to which it is immune, and in return keeps the anemone clean and fends off potential enemies like butterflyfish by emitting a high-pitched sound.
This type of long-term symbiotic partnership is known as mutualism, since the biological interaction benefits both of the involved species. Hermit crabs that place anemones on their shell for mobile protection is another example – the anemone is rewarded with leftover food scraps when the hermit crab is feeding. Cleanerfish form a mutually beneficial relationship with fish, turtles, sharks and rays, even though they don’t share living space. They keep their skin healthy and free from parasites, and get a bite to eat in return.
Two other forms of symbiosis are parasitism and amensalism.
They describe relationships in which the host animal is harmed while the symbiont either benefits or is unaffected by the symbiosis. Parasitic copepods and isopods are well known on marine species, while amensialism is more uncommon. However, most interactions between marine animals and humans are amensal: Humans have negative effects on many marine species, but the effects of most of these species on humans are negligible.
The most common symbiotic relationship is commensalism, when one species obtains benefits like food or locomotion from another species, without giving any benefit or causing harm to the host. This is common among small crustaceans and fish, and is also frequently observed with remoras that hitch-hike on sharks to better their chances of survival. Even barnacles growing on whales, turtles and dugongs are commensal, although it may be discussed if the host truly is unaffected by this relationship.