As water temperatures rise, ectothermic (cold blooded) sea dwellers face a perfect storm. With warming temperatures their metabolic rates will rise just as their supply of oxygen begins to dwindle – oxygen diffuses 300,000 times more slowly in water than in air – leaving them at risk. But one group of sea slugs has a trick up their sleeves. By gobbling up seaweed containing green photosynthetic chloroplasts, some sea slugs can absorb the oxygen-producing chloroplasts into their own bodies, providing an internal source of oxygen to augment their environmental supply, as well as additional nutrition when food is scarce. ‘You could say that the sea slugs are solar powered’, says Ellie Laetz from the University of Groningen, The Netherlands. But how much protection might kidnapped chloroplasts offer starving sea slugs in a warming world?
To find out, the team went in search of emerald sea slugs (Elysia viridis) in a shallow reef off the coast of the southern Netherlands, bringing the small invertebrates back to the lab in Groningen where they fed them on hen pen seaweed (Bryopsis plumosa) – whose chloroplasts the sea slugs can retain for several months – while keeping the sea slugs in warm aquaria at 17°C. Then Laetz and colleagues transferred half of the sea slugs to even warmer aquaria at 22°C, the temperature that the North Sea is predicted to reach in 2100. Once the sea slugs had settled in their respective homes for several weeks, the researchers began feeding half of the 17°C and half of the 22°C sea slugs on sea emerald seaweed (Chaetomorpha), so that the sea slugs would replace their hen pen chloroplasts with sea emerald chloroplasts, which were thought to be retained for just a matter of weeks when slugs are deprived of food.
Having prepared the four groups of sea slugs, the team then withdrew the animals’ food, and over the next 8 weeks monitored various factors to find out how the sea slugs coped with sudden warm periods while depending solely on their hijacked chloroplasts for sustenance. Impressively, the sea slugs that had been living at higher temperatures (22°C) coped better with a sudden heat wave than the slugs that had been living at 17°C, and smaller animals seemed to weather the heat better, probably because they can absorb more oxygen from the water than larger slugs with a smaller surface area to body mass ratio. And slugs that had accumulated sea emerald chloroplasts seemed to deal with heat waves better than slugs that dined on hen pen. The team also checked how much oxygen the animals breathed from the water and realised that even at the beginning of the period of starvation, the internalised chloroplasts did not provide enough oxygen to satisfy the sea slugs’ demands; they also had to consume oxygen from the environment.
Sea slugs that had dined on hen pen at 17°C under brighter light seemed to cope better with heatwaves, possibly because their chloroplasts were in better condition and able to provide more oxygen. However, the sea slugs that were living at 22°C became yellower over the period of food deprivation, as the chlorophyl in their chloroplasts broke down, suggesting that sea slugs require a steady seaweed supply to maintain a healthy chloroplast population. ‘We predict that emerald sea slugs will be able to adapt to the rising temperatures resulting from climate change’, says Laetz, but only if their seaweed food supply is able to survive as well.
