Symbionts, Parasites, Hosts & Cooperation

Symbionts, Parasites, Hosts & Cooperation

Marine life species interact in a myriad of ways for protection, shelter, food, and more. Competition, symbiosis, commensalism, and parasitism are all categories in which these interactions occur.


There are two types of competition: interference or contest competition and exploitation or scramble competition. Interference competition occurs when one species keeps resources away from another. Scramble competition occurs when one species depletes a resource before another organism has a chance to use it. Competition is intensified when resources are limited and can occur within species or between species. When population sizes increase, competition is increased and organisms die, grow more slowly, and reproduction decreases. Some species will resort to resource partitioning where resources are divided among individuals for survival. Species may also adapt to new niches rather than expend energy on competing for resources.

Species with similar needs cannot live in the same place sustainably for an extended period of time. This principle is referred to as Gause’s competitive exclusion principle. Some organisms adapt to conditions by helping each other survive through a mechanism known as symbiosis.


Symbiotic relationships are non-competitive and include mutualism, commensalism, parasitism, and mimicry. All types of symbiosis are highly efficient and help to achieve a balance in the ecosystem. Mutualism is a relationship in which both species benefit. Commensalism is a relationship in which one species benefits and there is a neutral impact on the other. The term parasitism is generally used when one species benefits, but the other does not.

The basic types of species interactions are indicated by a (0), (+) or (-) indicating whether the two species involved experience zero effects, positive effects, or negative effects respectively. Commensalism is (+,0), Mutualism is (+,+), and Parasitism is (-,+). However, these are very simplistic descriptions and exceptions to the rule are common.

Mimicry is another symbiotic relationship less common than the others. Mimicry is a relationship in where one species mimics another, typically using color or pattern. For example, the harmless banded snake eel may imitate a more dangerous sea snake. The Mimic octopus changes shape to resemble a Lionfish, sea snake or a stingray. Camouflage is another form of mimicry and can be seen in seahorses and scorpionfish.

Whenever organisms share resources in the environment there will be competition for food and territory. Organisms are forced to occupy specific niches in the environment in order to avoid wasting energy in competition. Organisms will also avoid competition through cooperative relationships within the ecosystem. Fish are frequently found existing in more than one symbiotic relationship. For example, a fish can have parasites and be cleaned by another organism living on its body. The parasites on the fish are food for the organism cleaning the fish. It is important to note that symbiosis only takes place between two different species.


Commensalism is a symbiotic relationship where one species provides protection for another less mobile or more vulnerable species. The relationship between Clownfish and anemones is a well-known example of commensalism. Clownfish live in the stinging tentacles of sea anemones. They are coated in mucous, which protects them from the anemone’s stinging nematocysts. Other animals like crabs and shrimps also seek protection in anemones. The Anemone crab lives in the anemone’s tentacles and catches its food without ever leaving the safety of the tentacles. Another example of commensalism can be seen with the Man-of-War fish and the Portuguese Man of War jellyfish.

Cooperation within the sea abounds and sometimes takes a very unusual form. Some Imperial shrimps will actually ride on sea cucumbers, hopping off when they want to feed in certain areas. When the shrimp is ready to go to another area, it will hop back on the cucumber and be taken to the next place without using very much energy. Sometimes Imperial shrimp will ride on other animals like nudibranchs, and these animals offer protection to the shrimp because they are poisonous to other animals. Several species of sea cucumbers host the Pearlfish inside their intestines during the day. At night, the Pearlfish swims out of the anus of the sea cucumber to eat crustaceans. The sea cucumber doesn’t seem to mind this odd guest and the Pearlfish is relatively safe during the day.


More often than not, parasites are harmful to the host organism. Ectoparasites live on the outside of the host and endoparasites live on the inside of the host. Ectoparasites are often crustaceans in the order Isopoda or Copepoda. Isopods have adapted strong suckers, flat bodies, and sharp jaws used to attach to their host. They tend to molt in stages so that they remain latched on to the host. Some isopods will attach to the fish and cause no harm. In this case they eat particles of food that float by rather than feed on the host directly.


Mutualism is a symbiotic relationship in which both species benefit. For example, some anemones share a mutualistic relationship with Boxer crabs, Lybia tesselata. The Boxer crab holds the anemone in its claws to use its stinging tentacles to fend off predators. In turn, the anemone consumes the crab’s leftovers. There are also fish that spend their entire lives cleaning other fish. Gobies, wrasse and shrimps are well-known cleaners that man cleaning stations near coral reefs where rish go to have parasites, dead skin cells, and mucuous removed from their bodies. Cleaners are recognized as such by a characteristic horizontal line, which allows them to enter a larger fish’s mouth to clean it without being eaten. Fish in need of cleaning will often change color or swim in a vertical position to indicate they need to be cleaned. The color change may also help cleaners see parasites on the skin of the fish being cleaned.


EPA: Aquatic Biodiversity / Marine Ecosystems
Wikipedia: Biological_interaction
The Open Door Web Site: Ecology – The Competitive Exclusion Principle
Marine Ecological Processes, Ivan Vahiela; CH1, CH2, CH4, CH8, CH15
Deloach N, ‘Reef Fish Behaviour’, 1999, New World Publications; Allen GR & Steene R, ‘Indo-Pacific Coral Reef Guide’ 1999, Tropical Reef Research; Debelius H, ‘Crustacea Guide of the World’, 1999, Ikan.

Last update: February 16th, 2019