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In four experiments mussels were subjected to low food levels (mean 5-3 mg dry weight of cells animal" 1 day" 1 ) and constant temperature (15 °C) for prolonged periods. In the autumn, metabolic rate was low and the energy absorbed was equal to the energy demand: i.e. the animals were in 'energy balance'. In the spring, stress resulted in rapid decline of oxygen consumption, also leading to ' energy balance'. In contrast, in the winter, animals were out of 'energy balance' resulting in utilization of stored reserves.During stress the rate of amino-N efflux formed an important component of the energy budget.At temperatures above ambient in the winter the rate of gametogenesis was increased, which resulted in increased energy demand and high oxygen consumption. If food is scarce, part of this demand is met from stored reserves, primarily protein and carbohydrate. This ensures normal development of gametes in the winter. Later, when vitellogenesis is complete and reserves are depleted, stress results in rapid fall in oxygen consumption.There is a marked seasonal shift from a reliance on carbohydrate as the main energy reserve in the summer to a greater reliance on protein as the main reserve in the winter. I N T R O D U C T I O NIn a previous paper Bayne & Thompson (1970) showed that temperature and nutritive stress resulted in a decline in body condition of mussels, Mytilus edulis, when kept in the laboratory. Both carbohydrate and protein were lost from the body tissues but the losses (as a percentage of the initial values) were greater from the germinal (mantle) than from the somatic (non-mantle) tissues. In spite of the loss of body reserves, M. edulis was able to continue maturation of the gonad during the autumn to spring period. In the early summer, however, when the gametes were fully ripe, stress resulted in a recession of the gonad and a rapid loss of protein from the mantle tissues. A similar decline in condition index and loss of glycogen and protein has been reported for adult oysters, Ostrea edulis, when maintained under hatchery conditions (Gabbott & Walker, 1971).Recently, Widdows & Bayne (1971) have discussed the temperature acclimation of oxygen consumption rate, nitration rate and assimilation efficiency for Mytilus edulis in terms of an energy budget for the whole animal. Mussels were fed in excess of the * Present address: NERC Institute for Marine Environmental Research, Citadel Rd, Plymouth PLi 3AX. 270 P. A. GABBOTT AND B. L. BAYNE maintenance requirement (for acclimated animals) but were kept at temperatures 5°C above ambient. During the acclimation period (14 days) the energy assimilated did not meet the metabolic energy demand and body reserves must, therefore, have been utilized to maintain the energy balance until a new equilibrium was established. Animals maintained at low food concentrations were stressed both during and after temperature acclimation. It is evident that high temperatures and low food levels represent a severe physiological stress for M. edults.In a preceding paper Bay...

A micro-analytical scheme for the determination of protein, total carbohydrate, freereducing substances, total lipid, phospholipid and RNA levels in marine invertebrate larvae is described and discussed. The scheme is designed for use with 1–2 mg dry weight of tissue. Values of 171·0, 15·03, 50·4 and 11·58 μg/mg dry weight of larvae (flesh + shell) are given for the levels of protein, total carbohydrate, total lipid and RNA respectively, in newly released larvae of Ostrea edulis L.

In previous papers we have shown that high temperature and low food levels result in a decline in the body condition of mussels, Mytilus edulis, which have been kept in the laboratory (Bayne & Thompson, 1970; Gabbott & Bayne, 1973). In spite of the loss of body reserves, M. edulis is able to continue gonad development at an increased rate, when the temperature is above ambient (Gabbott & Bayne, 1973; Bayne, 1975). Although gametogenesis appears to be normal under these conditions, there is evidence that stress in the adult affects subsequent larval development in M. edulis (Bayne, 1972) and in the oyster, Ostrea edulis (Helm, Holland & Stephenson, 1973). In mussels, stress resulted in an increase in abnormal embryonic development during cleavage, gastrulation and development to the first shelled larval stage (Bayne, 1972). In oysters the viability of the larvae, assessed in terms of growth rate and percentage yield of spat, was less in larvae from adults kept at low ration than in larvae from adults kept at high ration. Irrespective of the feeding regime, the viability of the larvae fell as the length of the conditioning period increased, and this was related to a decline in the body condition of the adult oysters. The growth of the larvae in the 96 h period following liberation was significantly correlated with the lipid level in the newly released larvae (Helm, Holland & Stephenson, 1973).

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