The filtration rate of a range of sizes of five species of bivalves has been measured by estimating the abundance of natural particles in sea water before and after it had flowed past the animals. Preliminary experiments showed that the filtration rate was positively correlated with the flow rate and for comparative purposes the flow had to be held to a constant amount. The flow rate also affected the heart rate of Ostrea edulis L., and comparative trials with small Ostrea and Crassostrea showed increased growth at a flow of 180 ml./min compared with 70 ml./min. The specific filtration rates at 19-21 °C of Ostrea and Crassostrea gigas Thunb. were two to three times greater than those of the other three species studied. Crassostrea and Mytilus were the least affected by temperature, Ostrea was intermediate, while Venerupis decussata (L.) and Mercenaria mercenaria (L.) showed a very marked reduction in filtration rate as the temperature was lowered. I N T R O D U C T I O NThe rate of water filtration by bivalves has long excited interest, but it has in practice proved difficult to measure in conditions where the animal is relatively free from constraint. Its estimation is important from a number of aspects: feeding studies; as an indicator of the animal's reaction to its environment; and for predicting the flow of water required for the culture of economically important species. The work reported in this paper started as part of the general programme on shellfish culture in progress at this laboratory. During the development of a suitable method for studying the water requirements it became clear that one factor, water current, had a more important influence than has been generally recognized.In the past, this work has usually been done by one of two methods: either by observing the rate of removal of particles from sea water or by separating the exhalant current from the general water flow and measuring its rate of production. On this basis a distinction is often made between pumping rate -the movement of water through the mantle cavityand filtration rate. Strictly, the first method measures filtration rate and, unless evidence can be produced that all the particles are being retained, this can be less than the pumping rate. On the other hand, it can be questioned whether a wide difference between pumping and nitration rates is part of the normal behaviour pattern if reasonably large particles are used. Pumping water consumes a good deal of energy (Collier, 1959, shows that an oyster pumping 10 l./h is consuming 5 mg oxygen/h). Estimations of the percentage of oxygen removed during the passage of water are usually in the range of 3-10% (Ghiretti, 1966), which suggests that the very large volumes of water moved are primarily for feeding rather than respiration.

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Six species of algae, Chlorella marina Butcher, Dunaliella tertiolecta Butcher, Isochrysis galbana Parke, Monochrysis lutheri Droop, Phaeodactylum tricornutum Bohlin and Hemiselmis virescens Droop were fed to oyster larvae in bacteria-free culture. In addition, cultures of Chlorella stigmatophora Butcher, Isochrysis galbana and Phaeodactylum tricornutum containing bacteria were also used. Cultures of galbana containing bacteria were used as controls in all series of experiments. In each series various densities of food, expressed both as cell numbers and as packed cell volume, were fed to the larvae of Ostrea edulis L. for 22–46 h and the change in the mean size of the larval population was used as the index of the value of the food.The experiments with Chlorella confirmed previous work that organisms of this genus have little value as food for larvae. With Dunaliella and Phaeodactylum growth was better at low food densities with some (but not all) batches of larvae compared with Isochrysis. The comparison of cultures of Isochrysis and Phaeodactylum with and without bacteria showed no consistent improved growth with either food. Monochrysis had about the same food value as Isochrysis. With Hemiselmis some growth was obtained but not as much as with Isochrysis.

Preliminary experiments had suggested that the extent of the development of the bacterial flora which develops when sea water is confined in small vessels is an important factor in the laboratory culture of oyster larvae. In the experiments reported in this paper the growth and settlement of oyster larvae was compared in controls of normal sea water with those in which the bacterial flora was controlled with antibiotics.A concentration of 50 i.u. of the sodium salt of penicillin G per ml. suppressed bacterial growth for at least 2 days and, in a series of experiments using fifteen different broods of larvae, significantly (P>0·001) more spat were obtained in those to which penicillin had been added than in the controls.A mixture of 50 units of penicillin G and 0·05 mg streptomycin sulphate was tried in three series of experiments. This completely suppressed the development of bacteria for at least nine days. In all three series many more spat were obtained than in the controls, but in only one series was there more spat than in the comparable penicillin experiment. It is suggested that when the bacterial population is very vigorous, or perhaps when certain species are present, this mixture will be more useful than penicillin alone. Experiments with other concentrations of penicillin, and with streptomycin and chloromycetin alone were not successful.

A bloom of the dinoflagellate Gyrodinium aureolum Hulburt in the eastern Irish Sea during September and October 1971 was accompanied by unusual mortalities of Arenicola marina L. and Echinocardium cordatunt Pennant. Observations in the field and laboratory suggest that the mortalities may not have been directly related to the occurrence of Gyrodinium but could have been a consequence of unusual climatic conditions which resulted in oxygen depletion in benthic substrates. The decomposition of the bloom possibly aggravated the situation.