Slug. Or Plant.

Tuesday, July 15, 2025

Slug. Or Plant.

Today’s special animal friend is Elysia timida, a solar-powered sea slug who doesn’t need any fossil fuels, thank you very much. If only we, too, could photosynthesize! E. timida’s taxonomy is full of eldritch vocabulary such as “sacoglossan” and “opisthobranch.” Try throwing these into your daily conversations, and tell your friends Cynthia sent you.

E. timida is about half an inch long. It has two rhinophores at the front — those are the things like tentacles — with an eye at the base of each and two parapodia, sort of arm-ish extensions of its body. It waves these to move through the water and can also fold them up over its back.

Native to the Mediterranean Sea and parts of the Atlantic Ocean, E. timida lives in shallow water. It eats two species of algae, Acetabularia acetabulum and Padina pavonica. These look like mushrooms. In a practice called kleptoplasty, the sea slug ingests the chloroplasts from these algae and maintains them in its body for up to 45 days.

The green cells give the slug its color, and they also continue to make sugars using sunlight, just as they did when they were part of a plant. The slug manages its food production by covering and uncovering its green back using the parapodia.

Like many other snail or slug species, E. timida is hermaphroditic. Each individual of a mating pair deposits sperm to fertilize the other’s eggs. The fertilized eggs are laid in the spring in a small, white coil. The eggs hatch into planktonic larvae which quickly assume their adult forms.

E. timida produces an unpleasant secretion to deter predatory fish. It is typical, in these situations, for some predator species to evolve a tolerance for the prey’s defensive chemicals. It is possible that information about successful predation can be found at the Sea Slug Forum or the Solar Sea Slug Blog, publications whose existence delights me.

Elysia chlorotica, a western Atlantic relative, looks just like a leaf!