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All animals are in constant interaction with other organisms. These interactions can be divided into two basic types: intra‐specific interactions and inter‐specific interactions.

Intra‐specific interactions are those that occur between organisms of the same species. They range between relatively loose associations such as those between members of a flock of sheep and highly complex interactions such as those seen in colonial invertebrates. For example, the adult (medusa) stage of the Portuguese man o’ war ‘jellyfish’ (Physalia physalis) may appear to be a single organism but is actually composed of colonies of genetically identical but polymorphic individuals. These colonies divide labour between themselves in a similar manner to that of organ systems within a non‐colonial organism. For example, some colonies are specialised for reproduction, whilst others are specialised for feeding. The term ‘jellyfish’ is in inverted commas because although P. physalis superficially resembles a jellyfish and is a member of the Phylum Cnidaria, it is taxonomically not a true jellyfish. Instead, it belongs to the order Siphonophora within the class Hydrozoa. The true jellyfish belong to the Class Sycphozoa within which there are several orders but in all of these, the medusa stage is a single multicellular organism.

Inter‐specific interactions are those that take place between different species of organism (Figure 1.1). As with intra‐specific interactions, the degree of association can vary between being extremely loose and highly complex. Odum (1959) classified these interactions on the basis of their effect on population growth using the codes ‘+’ = positive effect, ‘−’ = negative effect, and ‘0’ = no effect. This leads to six possible combinations (00, 0−, 0+ etc.), and these too can be broken down into further subdivisions. Some authors also include a consideration of the direction and extent of any physiological and biochemical interactions between the two organisms. Many terms have been suggested to compartmentalise these interactions (e.g., phoresis, mutualism, predation), but these are merely convenient tags, and they cannot be defined absolutely. This is because there is a huge diversity of organism interactions, and even within a single interaction there are many variables, such as the relative health of the two organisms, that will determine the consequences of the interaction for them both. It is therefore not surprising that there is a multiplicity of definitions in the scientific literature, and it is not unusual for two authors to use different terms for the same type of interaction between species. In this section, we will discuss symbiosis, commensalism, phoresis, mutualism and finally parasitism, with some examples of each.

Figure 1.1 Different species will sometimes co‐operate for mutual benefit.