The following points highlight the three different types of symbiosis seen in animals. The types are: 1. Mutualism 2. Commensalism 3. Parasitism.
Symbiosis: Type # 1. Mutualism:
Mutualism can be defined as a relationship between two different species in which both partners are benefitted or the association in which mutual metabolic dependency occurs.
Mutualism is of two types:
(i) Facultative mutualism and
(ii) Obligatory mutualism.
(i) Facultative mutualism:
In this mutualistic relationship one species can live normally independent of another.
(ii) Obligatory mutualism:
In this type of mutualism the relationship is obligatory for the existence of single or both species.
Various examples of mutualism can be cited by plant-plant association, plant- animal association and animal-animal association. In plant-plant association, the various species of algae and fungi are closely associated to form lichens in which algae synthesize certain organic compounds and these compounds are utilized by the fungi.
In return, the fungi provide the algae with water, minerals and protection. This is an example of obligatory mutualism because if the two species are separated they will die.
Another example of mutualistic relationships (plant-animal) can be cited by the unicellular green algae (Zoochlorellae) and brown or yellow algae (Zooxanthellae) which live in the protective ectoderm of certain hydras (Hydra sp.), molluscs and worms.
The algae can manufacture their own food by photosynthesis and give off O2 benefitting the animals which is taken by the animals for the respiration and in turn supply CO2 and N2 to the plants that are benefitted to the host. This is an example of plant-animal’s mutualistic relationship.
Another example of animal-animal mutualism is the relationship between flagellated protozoans (e.g., Trichonympha) living in the gut of wood eating termites. The protozoans live entirely depending on carbohydrate, collected from wood chips which are ingested by their host-termites. But the host cannot digest these wood chips because they have no cellulase-digesting enzyme.
For this the hosts depend on totally Trichonympha which are capable of digesting wood chips by secreting cellulose digesting enzyme-cellulase. In return the flagellates get food and protection. Here the protozoans act as mutualists and termites act as hosts.
Symbiosis: Type # 2. Commensalism:
Commensalism means “eating at the same table”. It is a kind of association between two or more species where there is no physiological dependence between them and the activities of the partners are centered around the food. In commensalism one partner is benefitted while the other partners are neither benefitted nor harmed.
The benefitting animal which takes part in this association called commensal, the relationship between such organisms, is called commensalism. Kendeigh (1974) has stated that commensalism is used in broader sense in recent years centring on cover, support, protection and locomotion other than food.
Commensals are of two types:
(i) Ecto-commensals and
(ii) Endo- commensals.
An example of commensalism can be cited as the relationship between remora fish (the ectocommensal) and the sharks and rays (the hosts). The dorsal fin of remoras (Remora sp.) or shark suckers (Echeneis sp.) is modified to form a dorsal sucker and is found to be attached to the host. The shark suckers or remoras feed on copepods that are found on the host’s body and also the residue of the prey of host’s periodically.
Other than shark and rays, the tarpons, barracudas, sail fishes, marlins, sword fishes, sunfish, sea turtles, whale and dolphins are associated with shark suckers and remoras. Certain commensals (shark suckers and remoras) have a preference toward certain hosts. The pea crab, pinnotheres lives within mantle cavity of sea mussels. The pea crab as an endocommensal collects food particles from the host mussels.
Symbiosis: Type # 3. Parasitism:
Parasitism is described as the hetero- specific association in which one partner derives all possible benefits and the other is affected adversely. The benefitted member is called a parasite and the losing member accommodating a parasite is recognised as the host. Between the two partners, the parasite is always smaller in size and metabolically dependent on the host.
Nature of parasitism:
Crofton (1977) lists the following features of parasitism:
(i) That the parasite is physiologically dependent upon the host;
(ii) That the parasite has a greater reproductive potential than the host;
(iii) That it is capable of killing the heavily infected host, and
(iv) That the infection process tends to increase the chance of dispersion of the parasites within the host population.
Types of parasites:
Generally there are two types of parasites on the basis of relationship:
(i) Temporary or partial parasites and
(ii) Permanent parasites.
(i) Temporary or partial parasites:
The organisms which spend only a part of their life-cycle as a parasite. For examples, the duration of the feeding of mosquitoes and leeches on the host’s blood. Another example may be provided as the glochidium larvae of some freshwater bivalves which spend the earlier part of life on the skin of fish as a parasite and after few weeks the larvae transform as young ones and lead an independent life.
(ii) Permanent parasites:
When the organisms spend their entire life as parasites. The adult liver flukes (Fasciola hepatica) as permanent parasites spend in the bile duct of the liver of sheep, cows and pigs and occasionally found in man.
Based on their location in relation to host, the permanent parasites are divided into the following:
(i) Ectoparasites:
When the parasites spend on the outside of their host as in the case of sucking lice (e.g., Pediculus sp.) of mammals, fleas (e.g., Pulex, Ctenocephalus) on birds and mammals, Gyrodactylus, a monogenean ectoparasite of fishes and frogs.
(ii) Endoparasites:
When the parasites live within the body of host, such as intestine, body cavities, organs or in the blood. The members of trichomonad flagellates (e.g., Trichomonas, Tritrichomonas), opalinids (e.g. Opalina), sporozoans (e.g., Monocystis, Gregarina, Eucoccidium, Eimeria, Plasmodium, etc.), Myxosporideans, Microsporideans, digenetic trematodes, cestodes, nematodes, etc. are endoparasites.
Incidental parasite:
When a free-living animal meets accidentally an unnatural host and leads a parasitic life for certain period.
Periodic or Sporadic parasite:
An organism which visits its host intermittently to obtain some metabolic requirements, is called periodic or sporadic parasite.
Facultative parasite:
An individual leads a parasitic life if he gets a chance or he spends a free-living life as usual.
Obligatory parasite:
If a parasite fails to meet a suitable host to complete its life cycle, the parasite must die.
Phoresis:
The term is derived from Greek phoros or phoron which means bearing or bearer. It is a symbiotic relationship in which one member of the association that is usually smaller, is mechanically carried by another organism (that is usually larger) of a different species.
In this relationship the smaller one is called ‘phoront’ and the larger one is called ‘host’. An example of phoresis is the transport of a crustacean ectoparasite, BopyruS on the gills of sea fish.
Zoonosis:
It is an infectious disease which is transmissible from vertebrates or invertebrates (sometimes by a vector) to humans.
Hyperparasitism:
When a parasitic species is parasitized by an another parasite species. Such parasite is called hyperparasite, as in the case of Nosema notabilis which is a hyperparasite on a myxosporidian, Sphaerospora polymorpha, a parasite of the urinary bladder of the toad fish.
Sexual Parasitism:
When a male becomes a parasite on the opposite sex or vice versa. For example, the female Bonellia gives the shelter of minute male Benellia in its uterus. The male is dependent entirely in females for food and shelter.
Types of Host:
Reservoir hosts:
The animals which are infected and serve as a source from which another animals can be infected. The source animals are called reservoir hosts.
Transfer or paratenic host:
An individual that gives a temporary shelter to a parasite and helps the parasite in reaching to a definitive host called the transfer or paratenic host.