Half Plant, Half Animal

The Eastern Emerald Elysia is a green sea slug.  Pictured above, one may mistake it for a leaf.   In doing so, one would, perhaps, be half correct.

Officially Elysia chlorotica, the creatures do something no other animals do: they can produce chlorophyll, the green pigmentation which is a key ingredient in allowing plants to photosynthesize sunlight into useful energy.  As Sidney Pierce, a biologist researching the creature, notes, “[Elysia] can make their energy-containing molecules without having to eat anything,” which is “the first time that multicellular animals have been able to produce chlorophyll.”  Previously, and still true otherwise, this part of the photosynthesis process was unique to the plant world.

The slugs can’t entirely carry out photosynthesis themselves, however.  Photosynthesis requires chlorophyll (the pigmentation) which captures the light and chloroplasts (an organelle) which processes it, converting it to energy.  After eating enough algae, Elysia have somehow stolen the genes needed to produce chlorophyll and can produce it themselves — and pass it to their offspring.  However, thus far, the slugs haven’t been able to do the same for chloroplasts, so slugs young and old need to eat enough algae to steal the necessary organelles.  Once they do, however, eating becomes optional — Pierce has, in a laboratory, kept the slugs alive for months without food.  Instead, they get 12 hours of direct light each day and photosynthesize for themselves the energy needed to survive.

Bonus fact: While slugs are typically thought of as gooey, vile creatures, some have more redeeming physical qualities.  Other than the leaf-like one above, there are also “sea angels,” sea slugs which look like creatures from Narnia, not our planet’s seas.  Sea angels grow, at most, to be 5 cm long and are gelatinous, mostly transparent hermaphrodites — egg fertilization occurs internally, and a free-floating viable egg is expelled into the water where they float freely until hatching.

From the Archives: Perpetual Jellyfish: Another great sea creature.

Related: A miniature, self-contained ecosystem.  For your desk.