From Aggregates to Integrates: Physiological Aspects of Modularity in Colonial Animals

Physiological adaptations of colonies in the areas of resource-sharing, defensive behaviour and locomotion are reviewed and discussed. Exchange of nutrients between zooids is a fundamental attribute of true colonies, although the transport systems involved vary and, in some cases, complex junctional structures appear to regulate passage of materials between zooids, as in the parallel case of interconnected cell systems. It is not known if regulatory molecules, as well as nutrients, pass via these interzooidal pathways. Almost all colonies show some form of behavioural coordination but, again, the pathways employed vary considerably. Examples of coordination by nerves, by conducting epithelia and by photic signals, are given. Zooids typically retain a high degree of local autonomy and the interzooidal pathways serve more as labile links between the action systems of the zooids than as centres where behaviour is initiated. Siphonophores are exceptional in having nerves, in their stems, which initiate as well as relay behaviour. Defensive responses are the commonest form of coordinated activity in colonies, but some colonies show coordinated locomotion. The most striking examples are those salps and siphonophores that regulate the activity of their locomotory modules either in terms of frequency of pulsation or in terms of direction of locomotion, or both. Modularity has theoretical mechanical advantages in animals swimming by jet propulsion. The requirements of pelagic locomotion have led planktonic colonies to diverge markedly from the pattern seen in sessile colonies. Pelagic colonies, unlike sessile ones, are typically linear and unbranched, show an anterior-posterior axis, are determinate in form, at least at the anterior pole, and lack much of the plasticity and regenerative ability that characterize sessile colonies.