The Limits to Indeterminate Growth: An Optimal Size Model Applied To passive Suspension Feeders

A model was developed that can be used to analyze the relationship between body size of indeterminately growing animals and habitat suitability. The model states that the amount of energy available for reproduction in a given season is maximized by maximizing the difference between energy intake and energetic cost as functions of individual mass. Two problems of energy allocation to growth and reproduction are considered as different forms of the model, one where space to store energy during the season is limiting within the organism and a second where it is not. The general model predicts that optimum size will increase with habitat suitability (more prey or lower physiological stress) and that asexual reproduction (colony or clone formation) can often be more energetically favorable than growth to the largest possible individual body size. Relationships between body size, morphology, prey capture, and energetic cost were examined for a passive suspension feeder, the sea anemone Anthopleura xanthogrammica in Washington State. Prey capture was measured in the field for naturally occurring prey and for marked prey released in field experiments. Energetic cost was measured as mass loss during starvation in aquaria. Results show that: (1) prey capture depends on the surface area of the feeding structure, (2) growth isometric in this species, and (3) energetic cost increases as a higher power of mass than does prey capture. Observed patterns of size distribution and prey capture rate in the field are as predicted by the model. Larger individuals are found in habitats having more prey or lower stress. For example, Anthopleura xanthrogrammica increases in size from the high to the low intertidal. Low intertidal habitats have both more feeding time and generally lower summer temperatures and are thus more energetically suitable than those in the high intertidal.