Establishment of the symbiosis inConvoluta roscoffensis

Abstract

Of a wide range of algae tested, juvenileConvoluta roscoffensisingested onlyPlatymonas convolutae, the natural symbiont; related species of the generaPlatymonas, PrasinocladusandTetraselmis; andChlamydomonas coccoides. Platymonas convolutaewas not ingested to a greater extent thanPrasinocladus marinus, Tetraselmis tetratheleandTetraselmis verrucosa, or taken up in preference toT. verrucosawhen animals were exposed to a choice between the two species. Convoluta ingested fewer cells ofC. coccoidesthanP. convolutaeand related species. Uptake ofP. convolutaewas not affected by pretreatment of the cells with lectins or proteases, incubation in media of pH 5·0–9·0 or inhibition of algal photosynthesis, but was substantially reduced if the algae were killed.Cells ofP. convolutae, Pr. marinus, T. tetratheleandT. verrucosapersisted and divided in juvenile Convoluta. The algal population in the worms started to increase 2–3 d after ingestion and within 15–20 d the animals were uniformly green. These algae formed a viable symbiosis with Convoluta and promoted the growth of the animals. In contrast,C. coccoidescells did not persist in Convoluta for more than 12–24 h a nd were probably disrupted.P. convolutaecells lost their thecae within a few days of ingestion and before migration from the central to sub-epidermal region of the animal. Animal vacuoles surrounded recently ingested thecate algae. Structural studies of the adult symbiosis suggest that the algae were also intracellular and enclosed in vacuoles.It is proposed that Convoluta discriminates against algae unrelated toP. convolutaeon initial contact and in the central region of the host. The nature of the recognition mechanism(s) has not been established.