Photosynthetic activity of the solar-powered lagoon mollusc Elysia timida (Risso, 1818) (Opisthobranchia: Sacoglossa)

Abstract

The sacoglossan Elysia timida (Risso, 1818) is typical of shallow sea areas and coastal lagoons. It retains symbiotic functional chloroplasts derived from its algal food Acetabularia acetabulum (Linnaeus) for 45 days. However, some aspects of the photosynthetic behaviour of the mollusc-chloroplast association are unknown. This work studies the photosynthetic characteristics of E. timida from a lagoon environment (Mar Menor), defined between 25 and 600 µmol quanta m–2 s–1 by the photosynthetic-irradiance curve. Experimental data were fitted to different exponential models and models based on Michaelis-Menten kinetics. Among the latter, the model that bests fits to the experimental data was identified. Furthermore, it incorporates respiration values and allows an estimation of compensation irradiance (Ic: 11.89 µmol quanta m–2 s–1). This model showed a rapid saturation of the photosynthetic apparatus at relatively low irradiance values (Ik: 31.33 µmol quanta m–2). Excess light seems to be modulated by the mollusc's opening and closing of their parapodia. Mean P/R values (Production versus Respiration relationship) are above 0.91; considered high in coralzooxanthellae symbioses. The photosynthetic efficiency of the symbiotic chloroplasts is fairly high, taking into account that sacoglossans are more active than corals and have a higher respiration rate.