Studies on a Nudibranch that Contains Zooxanthellae I. Photosynthesis, Respiration and the Translocation of Newly Fixed Carbon by Zooxanthellae in Pteraeolidia ianthina

Zooxanthellae of the genus Symbiodinium are present in the tissues of the aeolid nudibranch Pteraeolidia ianthina. Individuals with widely differing densities of zooxanthellae are found living off the mid-central coast of eastern Australia. Nudibranchs with low densities of zooxanthellae (0.02-0.5 × 10⁶ cells mg^-1 protein) are common in winter populations, whereas those with comparatively higher densities (1-3 × 10⁶ cells mg^-1 protein) are the most common form at other times of the year. Zooxanthellae at all densities in the host exhibit the capacity for photosynthesis, and they grow while resident in the host. The mitotic indices of the zooxanthellae were found to vary as a function of the population density of the algae in the host and were highest in those animals with the lowest densities of zooxanthellae. Oxygen flux was measured as a function of irradiance. Maximum photosynthetic capacity (P_max), light utilization efficiency (α ) and compensation irradiance (I_c) varied between animals, although most of the variation was attributable to differences in the density of zooxanthellae between animals. P_max and α were positively correlated with density (r² = 0.82 and 0.88 respectively); I_c exhibited an exponential decrease with the density of zooxanthellae in the host (r² > 0.81). I_c for animals with 1-2 × 10⁶ cells mg^-1 protein had values of I_c between 30 and 50 μ E m^-2 s^-1 (1 μ E = 6 × 10¹⁷ photons), similar to values of I_c measured for shade-adapted corals in other studies. Between 25 and 50% of the total photosynthetic products synthesized over 1 h were shown to move from the alga to the animal in that hour. The total respiratory rate of the association was positively correlated with the density of zooxanthellae (r² = 0.94). This relation was used to estimate the respiratory rate of a zooxanthella in vivo. Estimates were, however, unreasonably high; this result suggests that zooxanthellae may directly influence the respiratory rate of the host, possibly by increasing the supply of respiratory substrates.