Macrobrachium rosenbergii (de Man) is an important commercial species whose larvae develop through several stages in brackish water, after which they metamorphose and the postlarvae migrate into fresh water. Modelling the multiple factor interactions which may affect the ontogeny of physiological adaptation during larval development provides an opportunity to evaluate Alderdice's concept (Alderdice, D.F., 1972. Factor combinations: responses of marine poikilotherms to environmental factors acting in concert. In: Kinne, O. (Ed.), Marine Ecology, vol. 1, part 3, Environmental Factors. Wiley-Interscience, London, pp. 1659-1722) that the integral mobility of continuously unfolding fluid salinity-temperature-time response surfaces can identify important underlying adaptive phases in the life of an animal. Weight-specific oxygen consumption and nitrogen excretion were measured in up to seventeen larval stages of Macrobrachium rosenbergii, reared in 12 combinations of salinity (10.1-17.1 parts per thousand) and temperature (24-33 degrees C). Changes in larval weights, respiration, excretion, net growth efficiencies (K-2) and O:N ratios were modeled by polynomial regression and examination of the unfolding of the phases in the resulting nonlinear three dimensional response surfaces along the fourth or time/larval stage dimension. Larval growth rate was sigmoid and divided into three phases each comprising several instars: a post embryonic low or no-growth phase, an exponential-growth phase and a pre-metamorphosis slow growth phase. Low K-2 and O:N values during the low or no-growth phase indicate that growth is not the priority and available energy from stored Lipoprotein may be channeled into programmed tissue reorganization. Histology suggests that the focus is dedifferentiation and reconstruction of the hepatopancreas. A sharp discontinuity, followed by a rotation and translation of the oxygen consumption response surface at this point suggests a physiological shift to a new domain of attraction. This was related to the commissioning of the hepatopancreas and the changeover to reliance on an exogenous food supply. In the exponential growth phase increasing net growth efficiency and low O:N values show that anabolic processes predominate. During the final pre-metamorphosis phase, net growth efficiency decreases-but energy utilization remains high. The weight specific ammonia-nitrogen excretion response surface shows a tendency towards increasing excretion as salinity decreases. This suggests that the new physiological attractor is the development of an osmoregulatory mechanism based on counter-ion exchange. This may be a driving force for postlarval migration back to freshwater. This study suggests that the integral mobility of response surfaces during larval development can identify homeostatic adjustments associated with crossing the apparent boundary between one physiological domain of attraction and another. (C) 1999 Elsevier Science B.V. All rights reserved.
