1. Culture methods for the maintenance of symbiotic adult and juvenile Convoluta roscoffensis have been developed. The growth of adults but not juveniles is dependent on nitrogen enrichment of the medium. 2. Electron microscopic studies suggest that the algal symbionts in adult Convoluta are intracellular. 3. Structural studies of the gametes and early development of Convoluta are described. 4. Relevant aspects of the biology of the natural Convoluta symbiont, Platymonas convolutae, and a range of related algae have been studied. The growth of all Platymonas and Tetraselmis species tested except P.convolutae is stimulated by 0.1 M glucose. Glucose supports the growth of T.tetrathele, T.suecica and P.subcordiformis in darkness. P.convolutae and T.tetrathele have two uptake systems for uric acid, which conform to Michaelis-Menten kinetics; a high affinity system, operating in the concentration range 0.2-4.5 pM, and a low affinity system, operating at higher concentrations of uric acid. Uric acid uptake by P.convolutae is abolished by uncouplers of phosphorylation. Exogenous uric acid is accumulated in cells of Platymonas and Tetraselmis species up to internal concentrations of 0.3 M. P. convolutae metabolises (2-14C) uric acid to (14 C) carbon dioxide in the dark. 5. Aposymbiotic juvenile and symbiotic adult Convoluta, under standard culture conditions, contain solid uric acid. The uric acid content of juveniles declines on infection with P.convolutae and related species. Uric acid is utilised in the adult symbiosis, particularly under conditions of nitrogen demand. 6. Motile cells of P.convolutae and related species and Chlamydomonas coccoides aggregate around Convoluta eggs. These algae 'settle' on the eggs, by an interaction initiated by contact between the algal flagella and egg capsule surface. Living and killed algae are trapped by the capsules. No evidence that P.convolutae cells exhibit chemotaxis towards Convoluta eggs has been obtained. 7. Juvenile Convoluta ingest species of Platymonas and Tetraselmis, Prasinocladus marinus and Chlamydomonas coccoides but no other algae tested. Ingestion of P.convolutae is not affected by pretreatment of the cells with lectins or proteolytic enzymes, but is reduced if the algae are killed. 8. Cells of C.coccoidea, but not P.convolutae and related species, are disrupted in juvenile Convoluta. 9. Juvenile Convoluta form a viable symbiosis with P.convolutae, Pr.marinus, T.tetrathele and T.verrucosa under laboratory conditions. The structural changes of P.convolutae and T.verrucosa during the development of the symbiosis are described. 10. It is proposed that Convoluta discriminate against species unrelated to P.convolutae on initial contact and in the digestive region of the host. Evidence for a recognition mechanism discriminating between P.convolutae and related species is discussed. The nature of the recognition mechanism(s) has not been established.

1. Convoluta roscoffensis is an intertidel flatworm symbiotic with the green alga, Platymonas convolutae. Field studies of a population of Convoluta on Shell Beach, Herm, Channel Islands, involved the measurement of seasonal changes in numbers of Convoluta and in environmental conditions. Features of the habitat were measured, including the nutrient levels in the beach run-off water flowing over Convoluta colonies and the light energy available. 2. The position of the colonies on the beach meant that they received about 60% of available PAR (photosynthetically active radiation) i.e. 4883 E m-2 in 1977. The spacing of worms in the colonies (mean density 9.3 x 10 to the power 5 worms m-2) was such that they did not haye to shade each other. 3. Nutrient analyses revealed that the beach run-off water is rich in nutrients, and dominated by nitrate (mean concentration 6-16υg atoms N03-N 1to the power -1). Uptake of nutrients from the run-off water by Convoluta was not apparent. 4. The Convoluta population was most abundant in September and numbers were lowest in May and June. This pattern of fluctuation in numbers showed some relation to seasonal changes in light intensity but not to changes in nutrient levels. 5. Primary productivity measurements (using the 14C technique) of symbiotic and free-living Platymonas showed that both types of algae achieved assimilation numbers (mg carbon fixed (mg chlorophyll 'a')-1 h-1) which were in the range 1-3. Photosynthetic rate was higher in worms incubated in Herm run-off water than in offshore seawater. Comparisons between symbiotic and free-living Platymonas were made with respect to photoinhibiting light levels and the amounts of dark carbon fixation. 6. Extrapolation of primary productivity values to Convoluta in the field gave an estimated annual production of 872.9 g carbon m to the power -2 of colony for 1977. This is comparable with values for rich ocean waters and coral reefs. 7. Primary production measurements were also made on a tropical algal-invertebrate system, Zoanthus sociatus, at Discovery Bay, Jamaica. The oxygen technique was used, so the values of assimilation number were not directly comparable with those obtained for Convoluta, but the symbionts of Zoanthus (Gymnodinium microadriaticum) achieved assimilation numbers similar to those published for other free-living species of Gymnodinium. The symbionts isolated from Zoanthus were of similar size to and contained similar amounts of chlorophyll 'a' to the Platymonas symbionts of Convoluta. 8. Structural studies of the development of symbiosis in Convoluta showed that the host and symbiont are in very intimate contact. There was evidence for the controlled and integrated growth of host and symbiont in the distribution and orientation of the 20,000-70,000 algae inside Convoluta. The ratios of algal to host protein and cell volume were measured and compared with values for other algal-invertebrate systems to discover whether any general statements may be made regarding the proportions of algal to host tissue in established symbioses.