An Animal Not a Fruit

Sea Lemons are a type of Nudibranch - or sea slug. The scientific name Nudibranchia comes from the term "naked lung" - which may seem weird until you realize that the fluffy plumes on their backsides are their external gills and not just exciting decorations. The two horn-like nubs on their heads -called rhinophores- are sensory organs that allow them to sniff out food. Nudibranchs as a whole are marine gastropod mollusks which means they are a bit like oceanic siblings to terrestrial slugs and cousins to octopi and clams. Nudibranchs come in all sorts of fantastic shapes and colors- see some here:

While inspiring, the bright colors often indicate that the slugs are poisonous, thus deterring predators without needing a shell. Many sea slugs - like the tiny Blue Dragon (Glaucus atlanticus) - are kelptopredators and steal toxins from their prey and use them for themselves. The blue color in the blue dragon is from nematocysts (stinging cells) from nibbling on Portuguese man-o-wars. Some Nudibranchs even steal chloroplasts from the algae they predate which makes them photosynthetic and capable of gaining energy from just sunlight.

Sea Lemons eat Sea Sponges which are filled with tiny sharp glass toothpicks (called spicules) that are very hard to chew. Most predators can't get past the spicules, but as gastropods sea lemons have a unique organ called a Radula - think tongue but covered in tiny teeth. They use these spiked tongues to gobble up the sponges without much hassle. It's possible they incorporate chemical deterrents from the sponges into their own bodies as well, just like the Blue Dragon.

Sea Lemons themselves are fairly common and can be found all the way from Alaska to Baja in the intertidal zone - and yes, they really do look like lemons! They even smell a bit like citrus too (but not as much as the hooded nudibranch) My personal favorite species is the Noble Sea Lemon - Montereina nobilis.