Gamete production, somatic growth and reproductive effort were measured in sea urchins maintained on a mixture of kelp and mussel flesh at three ration levels. Urchins on low ration were able to maintain an output of gametes half that of urchins fed ad libitum. This was achieved at the expense of somatic production, which was negative in the low ration groups. Despite the reduction in the output of gametes, reproductive effort was greater in the low ration urchins than in the well fed ones. The lipid content and the energy content were greater in eggs released by urchins fed ad libitum than in eggs produced by urchins at low ration.
Several indices of reproductive effort were derived from the data and their advantages and disadvantages evaluated. All showed the same trend towards an increase in reproductive effort as the food supply was depleted. In order to obtain some of these indices, it was necessary to measure oxygen uptake, ammonia excretion, ingestion rate and absorption rate, but a direct partitioning of energy between growth and reproduction was found to be the most ecologically meaningful approach to the problem, and does not require measurement of physiological variates. In situations where growth cannot be measured, however, reproductive effort may be expressed as the production of gametes divided by the energy ingested or absorbed.
A shift in the allocation of resources from growth to reproduction is seen as an appropriate response to a depletion of the food supply in an environment in which the duration of adverse conditions cannot be predicted. Under these circumstances, investment in growth may be unprofitable, yet the urchin may retain the capacity to restore the normal energy balance should conditions improve.