Defensive strategies of modular organisms

Abstract

Convergences concomitant with the occurrence of modular growth among systematically remote plant and invertebrate taxa not only reflect similar optimal ways of exploiting resources such as space, but also common defensive requirements among such organisms. This paper analyses the kinds of unfavourable interspecific interactions, principally predation, epibiosis, and endobiosis, which are found among the major aquatic invertebrate groups that may be considered to be modular (Porifera, Bryozoa, and some of the Coelenterata and Tunicata). Most of the organisms are also non-locomotory, and in extreme cases, virtually immotile. The defence mechanisms of organisms exhibiting the opposing traits of (i) modular and unitary organization, and (ii) motility and immotility, are compared and contrasted. There is a more widespread occurrence of defence (i) by means of consolidated and unconsolidated skeletal reinforcement, and (ii) by actively and passively dispensed secondary substances, in less motile than in more motile organisms. These defensive modes represent alternatives to 'fight' and 'flight' responses seen within the more motile invertebrates. Lack of motility is of greater significance in correlating defensive modes than is modularity. The balance between physical and chemical mechanisms used in defence can vary, even among closely related taxa. A more particular pattern of significance is the more widespread occurrence of defence by the use of passively dispensed chemical substances within modular, rather than unitary non-locomotory invertebrate groups. This may be a response to the increased risks of pathogenic infection which modular biota face through their susceptibility to frequent large scale wounding and partial mortality.