Fig. 1. Identification of candidate mobile photosynthetic compounds by metabolite comparison. The algal cells are shown as the shaded oval within the animal (square). Control symbiosis: NaH¹⁴CO₃ is supplied to the symbiosis under photosynthesizing conditions; the mobile compound(s), M, labelled with ¹⁴C, is translocated from the algal cells to the animal, where it is metabolized to compounds A, B and C. Experimental symbiosis: ¹⁴C-labelled organic carbon source (S) supplied to the symbiosis under non-photosynthesizing conditions is metabolized to compounds X, Y and Z in the animal. When compounds X, Y and Z are similar to compounds A, B and C, the carbon source S is interpreted to be the same as, or allied to, the mobile compound M.