Critical thermal maxima and oxygen uptake in Elysia viridis (Montagu, 1804), a sea slug capable of photosynthesis

Photosynthetic animals produce oxygen internally, providing an ideal lens for studying how oxygen dynamics influence thermal sensitivity. The sea slug, Elysia viridis, can retain functional chloroplasts from its food alga Bryopsis plumosa for months, but retention is limited when fed Chaetomorpha sp., limiting potential oxygenic benefits. We fed slugs each alga and exposed them to 17{degrees}C (their current yearly maximum temperature) and 22{degrees}C (the increase predicted for 2100), to examine plasticity in thermal tolerance and changes in oxygen uptake when fed and starving. We also examined slugs under increased illumination to examine a potential tradeoff between increased oxygen production, and a faster rate of chloroplast degradation. Following exposure to these conditions, we performed ramping trials, subjecting them to acute thermal stress to determine their thermal tolerance. We also measured oxygen uptake before and after ramping. We observed increases in thermal tolerance for specimens exposed to 22{degrees}C, indicating they acclimated to temperatures higher than they naturally experience. Fed slugs exhibited higher rates of oxygen consumption before exposure to acute thermal stress, and suppressed their oxygen uptake more after it, than starved slugs. Under higher light, slugs exhibited improved thermal tolerance, possibly because increased oxygen production alleviated host oxygen limitation. Accordingly, this advantage disappeared later in the starvation period when photosynthesis ceased due to chloroplast digestion. In conclusion, E. viridis can suppress metabolism to cope with heat waves, however, starvation influences a slugs thermal tolerance and oxygen uptake, so continuous access to algal food for chloroplast retention is critical when facing thermal stress. Summary StatementOxygen has been implicated in determining an ectotherms thermal sensitivity. Examining photosynthetic (and therefore oxygen-producing) sea slugs under various conditions helps elucidate how oxygen and other factors impact thermal tolerance.