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Physiological and metabolic aspects of experimental heterochrony in colonial hydroids

Hydractiniid hydroids display a range of morphological variation from sheet‐like to runner‐like forms, thus exemplifying the patterns of heterochrony found in many colonial animals. Experimental studies of clonal replicates of a Podocoryne carnea colony demonstrate a relationship between these heterochronic traits and physiological and metabolic factors. While colonies exposed to dilute solutions of 2, 4‐dinitrophenol (an uncoupler of oxidative phosphorylation) for 4 hours per day show a strong dose‐response relationship between within‐colony gastrovascular flow rate and the timing of morphological development, colonies exposed for 20 hours per day fail to show this relationship. Spectrofluorometric measures of redox state suggest that the degree of “loose‐coupling” of oxidative phosphorylation is proportional to the exposure time. Thus colonies exposed for 20 hours have significantly diminished ATP production relative to colonies exposed for 4 hours. Two underlying factors may be responsible for the breakdown of the dose‐response relationship between flow rate and development: (1) colonies exposed to a high dose of uncoupler for 20 hours per day allocate a larger‐than‐normal share of their total energy budget into maintaining the minimal flow rate required physiologically, and (2) colonies' redox state has a direct effect on the timing of morphological development. The latter effect is particularly intriguing, given the extensive literature relating “metabolic activity” to development. In general, these results suggest a relationship between energy allocation, metabolism, and development in these hydroids, and these interrelationships may be central to the evolutionary process of heterochrony.