Metabolic implications of modularity: studies on the respiration and growth of Electra pilosa

Abstract

The mass-specific respiration rate of Electra pilosa is independent of colony size and therefore there is no allometric metabolic constraint on colonial growth rate (modular iteration). Budding, however, is confined to peripheral zooids and so the amount of meristem per unit area of colony declines as the colony grows. Hence the rate of zooid production per individual is a decreasing function of colony size. E. pilosa partly compensates for this, first by increasing the budding rate of peripheral zooids as the colony grows and second by expanding the peripheral meristem into lobes. Among colonial invertebrates, a modular construction frees colonies from metabolic allometry and if modules retain their capacity for replication these may accumulate exponentially until restrained by extrinsic factors. Encrusting forms with strictly two-dimensional growth, however, are constrained by the peripheral location of the budding zone. This may be alleviated by faster budding, perhaps as a result of nutritional subsidies as the colony grows, and by departure from a circular shape.