NUDIBRANCHS NUDIBRANCHS NUDIBRANCHS!

TAVISHI

I love nudibranchs, as any other ocean enthusiast would. Nudibranchs are incredible in color, shape, and more. While their color is often indicative of aposematism, most times, nudibranchs are just silly little harmless guys.

Above is Chromodoris willani, one of my personal favorite species of nudibranchs. Nudibranchs are an order of sea slugs, and are the most common animals which are referred to as sea slugs. Two other small groups include other sea slugs, but nudibranchs are much more prominent. The defining characteristic of nudibranchs is their lack of shell following the larval stage: instead, they are these vibrant gastropods which often have oscillatory movement.I cannot help but think of sky eels from The Stormlight Archive, a book series by Brandon Sanderson, when I think of nudibranchs. However, I chalk this up to the mental image in my head of their movement.

Nudibranchs experience bilateral symmetry, and are relatively primitive organisms. While they do have eyes, these eyes are very simple, only consisting of about five photoreceptors and ultimately, not suuuuper useful. Rather, nudibranchs rely on other sensory organs, such as rhinopores, to investigate the world around them. Rhinopores are sensory organs shaped like tentacles used for smell and taste. Nudibranchs also have oral tentacles which play a role in taste.

1 represents the oral tentacles, and 2 represents the rhinopores.

Nudibranchs are split into two groups: dorids and aeolids. The main differences between dorids and aeolids are the presence of cerata on the mantle of aeolids and a branchial plume near the anus in dorids. Cerata are the projections on the mantle of aeolids only, and have a variety of functions. Not only do they play a role in respiration for the organism, but in some, they store the cnidosacs of their prey. Cnidosacs contain nematocysts, which are stinging cells from their harmful prey.

Most famously known for its wing-like cerata, Glaucus atlanticus, or the blue dragon sea slug, isn't just all pretty colors. The blue dragon sea slug feeds on the Portuguese man o' war, one of the most deadly hydrozoans packing a powerful punch with its stings. And because the blue dragon sea slug steals its cnidosacs, its stings are even more concentrated and harmful than the original owners. In aeolids, the cerata function as gills of a sort (not actually gills, diffusion across epidermis), but in dorids, the branchial plume is a gill, and that's how gaseous exchange takes place. Gills are centralized into one branchial plume around the anus of the nudibranch.

This is one of my favorite dorid nudibranch species- the orange spotted nudibranch, Gymnodoris rubropapulosa! As you can see, near the anus of the slug, there's a lil branchial plume!

Other differences among dorids and aeolids include the structure of the digestive gland. The digestive gland of the aeolids are branched, allowing extension into cerata, thus allowing cnidosacs to sting people albeit they are in the digestive gland.