Aquatic ecology and Biodiversity

Unit 8- Animal associations and interactions

8.6. Commensalism
Commensalism is an interaction between two different species of living organisms, where one partner or species benefits and the other species is neither benefited nor harmed. The term commensalism derives from the Latin word commensa, meaning sharing a table as guest messmates. The term commensalism was coined by Van Beneden. In this relationship, there is no physiological interdependence among the partners and the partners are referred as commensals. The purpose of this relationship is to get food, space, shelter, defense and transport. Further, based on the position or location of the commensal on the host species, it is grouped into
Endocommensals and Ectocommensals. Commensalism is of the following two types based on the level of association between the commensals.

• Permanent commensalism
• Temporary commensalism

Permanent commensalism
In this type of commensalism, the commensals remain more or less in permanent contact with their hosts. E.g. Barnacles are permanently attached on the body surface of whales and these barnacles are not obtaining any food from the tissues of the whales (host). The barnacles are called as ectocommnsals as they do not obtain any nourishment from whales and in turn the whales are helping barnacles in providing substratum for attachment. Many micro-organisms such as protozoa and saprophytic bacteria and fungi live within the tissues or cavities of higher plants and animals without causing any harm to the host. These are known as endocommensals
Temporary Commensalism
In this type of association, the commensals remain only in temporary contact with each other. This intermittent contact between the animals for food, shelter or for breeding sites is seen in many marine animals. Examples - Sucker fish and shark
Temporary association or contact between two animal commensals is seen in sucker-fish and the sharks or whales. Sucker-fish (Echeneis) is found attached by means of its dorsal sucker to the underside of a shark or other large animal.
1. The decapod crustacean, Polyonyx lives in the U-tube of the marine annelid, Chaetopterus. It obtains food particles and oxygen supply from water entering the tube due to the pumping action of the worm’s parapodia.
2. Portuguese man-of-war and fish
Further, based on the position or location of the commensal on the host species, it is grouped into Endocommensals and Ectocommensals.
Physalia physalis is commonly known as Portuguese man-of-war. It is capable of killing almost all small sized fishes with the help of its tentacles and consuming them. But the small fish, Nomeus sp. lives and moves in between the tentacles of the Portuguese man-of-war freely without getting affected. It also takes shelter, protection and food from the Portuguese man-of-war.
Types of commensal relationships

• Epizoism
• Endoecism
• Inquilinism

Epizoism: Many sedentary marine animals are “epizooic” organisms setting facultative or even obligatory on others, particularly in those with hard shells. Eg : Ciliates, hydroids, bryozoans, sedentary polychaetes and cirripides settle on lamellibranches, gastropods, crustaceans
Endemism: Endemism is a partnership in which animal habitually takes shelter within the tube or burrow of another. Here, shelter or protection is the factor behind the association. However, in some cases food sharing relationship. Examples-
• Burrowing and tubicolous annelids, echiuroids, echinoderms, crustaceans etc., are host for many commensals. The burrow of an echinoid warm (Urechis alepo) is shared with an arrow goby (Clevelandia ios), a scale worm (Hespernoe adventor) a clam (Cryptomya sp.) and a pea crab (Scleroplax sp.)
• Commensal (Nereis fucate) live in the whelk shells inhabited by hermit crab (Eupayurus sp.) for protection as well as food.
Inquilinism: This commensal association in which one lives within the other, the former utilizing the host animal mainly as refuge. Here protection is the factor behind association. Examples
• Hydroid Colony lives by attaching to the Lamellae of the ctenidia of wood boring molluscs such as Teredo furcifera, Martesia striata.
•The pearl fish (Carapus acus) dwells in the intestine of a sea cucumber