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The Bivalvia — Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. (Edited by Brian Morton). Hong Kong University Press, Hong Kong, 1990. FEEDING AN D DIGESTIO N I N BIVALV E LARVA E A. Luca s Laboratoire de Zoologie, Faculte des Sciences et Techniques, 29287 Brest Cedex, France ABSTRACT During the larval life, three nutritional stages can be distinguished: endotrophy (or lecithotrophy ), mixotroph y (o r intermediat e trophi c stage ) an d exotroph y (o r planktotrophy) . I n Mytilus edulis, for example, reared at 17-20°C, the first stage lasts 2 days, the second about 6 days and the third about 20 days. This model is valid for most oviparous species of bivalves belonging to the planktotrophic group. Many authors assume that dissolved organic matter can be absorbed from sea water at any time during the larval life. But a net gain in weight resulting from flux and influx of amino-acids has not been clearly demonstrated. In a longterm experiment on Mytilus edulis larvae, a circadian rhythm of release and absorption often amino acids was observed. This suggests that short-term experiments cannot resolve this problem. For particulate matter, no clear information i s available on inert particles. Three types of living particles can be distinguished: nannoplankton, picoplankton and bacteria. The first is well known and represents the only food supplied in hatcheries. The second, which has a high productivity, can be ingested by larvae, but has previously been ignored. The same applies for bacteria: using clearance techniques and the grinding of larvae, bacterial ingestion has been shown to be a constant phenomenon and, by TEM observations, the presence of lysed bacteria in the digestive tract, including the crystalline style, has been demonstrated for Vibrio, Pseudomonasand some Gram + strains. In addition to clearance rate and radioisotope methods, observations by epifluorescence microscopy gave data on phytoplankton ingestion and digestion: quantity ingested, time of digestion, rhythm of feeding, selection of algae. Information gathere d by this method allowed comparison of larvae living in experimental conditions (continuous feeding, large quantities of food, but poor variety) and in the wild (discontinuous feeding, limited food, but high variety). Enzymatic analysis has shown for Pecten maximus hatchery larvae, an activity which was high for amylase and laminarinase, when the larvae were fed and lower when the larvae were unfed, whereas the activity of a lisozyme-like enzyme, which could break bacterial membranes, appeared to be high in both fed and unfed larvae. 178 A.LUCAS INTRODUCTION Yonge (1926) gave the first overvie w of the anatomy and physiology of the feedin g organs of a bivalve larva. It is noteworthy that by using simple techniques such as observing larvae under the microscope, histology an d the discerning use of specific stain s (bromothymol blue, carmine indian ink, iron saccharate) major advances were made. Since 1950 , empirica l method s hav e bee n establishe d fo r larva l rearin g an d spa t production i n laboratorie s an d i n commercia l hatcheries . Man y biologists , especiall y Loosanoff and his staff at Milford laboratory (Connecticut, U.S.A.) and Walne at the Conway laboratory (Gwynedd, G.B.) worked on larval biology of the more commonly reared species of oysters, clams, mussels and cockles. Important results were summarized in two publications : Loosanoff an d Davis (1963) and Walne (1966). In addition to these fundamental researches in aquaculture, two other topics improved studies o f larva l biology . First , th e us e of bivalv e larva e a s bio-assays i n problem s o f toxicology an d pollution. For example, Mulinia lateralis (Calabrese and Rhodes, 1974) , Mytilus edulis (Lucas, 1975 ; Le Pennec and Le Roux, 1979) , Crassostrea virginica and Mercenaria mercenaria (Calabrese etal, 1977) , Crassostrea gigas (Robert and His, 1985) and Spisula solidissima (Zoto and Robinson, 1985). Second, researches on the recruitment determinism of fisheries stock, by understanding the causes of larval and postlarval mortality, in order to connect results on spawning (studied in adults) to those of recruitment (studied in juveniles). From 1985 , a French national programme has been set up on three bivalves: Pecten maximus, Crassostrea gigas and Ostrea edulis. There are many publications on the larval biology of bivalves, especially feeding and growth. Gabbott (1983) gave information on energy metabolism, enzyme studies, fatty acid nutrition during larval development and metamorphosis. Bayne (1982) synthesized information on feeding, respiration and growth of bivalve and gastropod larvae. These two publications...

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