Metabolic maintenance costs of the suspension feeder Styela plicata

The bioenergetic basis of the biannual reproductive cycle of the solitary tunicate Styela plicata was investigated in order to evaluate hypotheses concerning the lack of larval settlement in summer. The rate of ingestion and absorption efficiency were measured in order to provide an estimate of the rate at which material was made available for maintenance, growth, and reproduction. At a given temperature the rate of ingestion was proportional to the 0.7 power of wet mass. the ingestion rate increased rapidly with increasing temperature between 12° and 18°C (Q₁₀3), but was independent of temperature between 18° and 28°C. Absorption efficiency was independent of temperature and body size and averaged approximately one-third for both carbon and nitrogen. Metabolic maintenance costs were estimated from measurements of oxygen consumption and excretion of ammonia and urea reported for s. plicata. These require only 18±11% of the carbon and 37±22% of the nitrogen absorbed from the gut of S. plicata over the temperature range 12° to 28°C. Metabolic maintenance makes no excessive demands on the material absorbed in the gut at a particular time of year, and a surplus of carbon and nitrogen substrate is available throughout the year for growth and reproduction. Predation on larvae and young adults may be responsible for the low rate of settlement observed in summer months.