Sea slugs are a diverse group of marine mollusks that inhabit oceans around the world. Many sea slugs have a shell in their larval stage, but most species lose their shell when they mature. Instead of a shell, most sea slugs have a soft, unsegmented body. Some species can be very small, under an inch long. These tiny sea slugs often float on the ocean’s surface. There are a few main families of small floating sea slugs, including bubble shells, sea butterflies, and some nudibranchs.
Bubble Shells (Family Aplustridae)
Bubble shells, also known as aplustrum or boat snails, are a family of small floating sea slugs. There are over 100 species of bubble shells. They inhabit warm, tropical waters and are most diverse around Australia and the Indo-Pacific islands. As their names suggest, bubble shells have a bubble-shaped shell that covers part of their body. The shell is small, usually under an inch long. It floats on the water’s surface due to a raft of bubbles the sea slug secretes.
Here are some key facts about bubble shells:
| Scientific Name | Aplustridae |
| Common Names | Bubble shells, boat snails |
| Size | Under 1 inch long (0.5-25 mm) |
| Habitat | Tropical oceans |
| Distinctive Feature | Bubble-shaped shell |
The bubble shell floats upside down on the ocean surface, with its shell facing up and body hanging below. The shell has a small opening that allows the slug to retract most of its body inside for protection. The slug secretes a bubbly mucus raft that makes the shell buoyant so it can float.
Bubble shells feed on plankton and other small organisms by filtering water. They have a horn-like mouthpart called a proboscis to catch food particles. Most bubble shell species are hermaphroditic, meaning each individual has both male and female reproductive parts. They lay eggs in a mass of bubbles or slimy raft on the ocean surface. After hatching, the free-swimming larvae develop shells and eventually mature into buoyant adults.
Some of the most widespread and commonly encountered bubble shells include:
– Hydatina physis: Also called the red-lined bubble shell, has a light brown shell with reddish brown dotted lines. Found in the Indo-Pacific.
– Aplustrum amplustre: The greater boat bubble, has an elongated shell marked by dark spiral bands. Native to the Caribbean sea.
– Bulla occidentalis: The western bubble shell, is small and clear with an iridescent shine. Common along the Pacific coast of North America.
Sea Butterflies (Family Pneumodermatidae)
Sea butterflies, known scientifically as pneumodermatids, are a family of small marine slugs. There are about 50 species, all less than 0.5 inches long. Sea butterflies inhabit oceans worldwide, but are most abundant in colder northern waters. These tiny slugs are called “butterflies” because their two wing-like parapodia resemble insect wings.
Here are some key characteristics of sea butterflies:
| Scientific Name | Pneumodermatidae |
| Common Name | Sea butterflies |
| Size | Under 0.5 inches long (2-12 mm) |
| Habitat | Oceans worldwide |
| Distinctive Feature | Two “wing-like” parapodia |
Sea butterflies float upside down on the ocean surface, with their parapodia spread out like wings on either side of their body. The “wings” are extensions of the slug’s foot. Rows of cilia beat synchronously to propel sea butterflies through the water. They steer by adjusting their wing position. Sea butterflies feed on plankton and marine snow particles by filtering water through mucous nets secreted from their mouth. The slugs are either male or female (unlike most other slugs which are hermaphrodites). During mating, the male deposits sperm packets on the female’s body. She stores them until her eggs mature, then lays the fertilized eggs in a gelatinous string. Like other sea slugs, sea butterflies lack a shell as adults, having shed it after their larval stage.
A few widespread sea butterfly species include:
– Limacina helicina: Abundant in the Arctic and northern seas. Has a spiral shell as a larva.
– Clione limacina: Also called the naked sea butterfly, it lacks a larval shell. Common in boreal waters.
– Cavolinia inflexa: Has sharply angled “wings.” Native to the North Atlantic and Pacific oceans.
Nudibranchs
Nudibranchs are a large group of marine slugs that shed their shell after their larval stage. Most nudibranchs are bottom dwellers rather than surface floaters. However, there are a few families of tiny nudibranchs, under an inch long, that live floating upside down on the ocean’s surface film. These include species in the family Plyllidiidae.
Here are some key facts about surface-dwelling nudibranchs:
| Scientific Name | Various families like Plyllidiidae |
| Common Name | Nudibranchs |
| Size | Under 1 inch long (2-25 mm) |
| Habitat | Ocean surface film |
| Distinctive Feature | Lack a shell as adults |
These tiny nudibranchs live on the surface tension layer of the sea, floating upside down with their foot facing up. Species like the blue sea slug (Glaucus atlanticus) and the purple sail nudibranch (Glaucilla bennettae) use a raft of bubbles, mucus, or digested siphonophores to stay afloat. Others like the crystal aeolis (Crystalaeolis annulata) have a flattened foot adapted for surfacefloating. They propel themselves by rhythmic foot undulations.
Surface nudibranchs feed on floating hydrozoans, ctenophores, salps, and other plankton. Most are specialist predators on a few specific prey types like the Portuguese man o’ war. Their colors and feathery gills serve as camouflage to avoid predation by fish below. Reproduction varies, with most laying a ribbon-like egg mass that floats on the surface.
Some common surface-dwelling nudibranchs include:
– Glaucus atlanticus: The blue ocean slug, preys on Portuguese man o’ war.
– Fiona pinnata: The Fried egg nudibranch has a flat, white body with orange tentacles.
– Cuthona nana: The dwarf Cuthona floats upside down in tropical oceans worldwide.
Conclusion
In summary, small floating sea slugs include bubble shells, sea butterflies, and some surface-dwelling nudibranchs. These tiny marine mollusks inhabit tropical to polar oceans worldwide. They float upside down by various means like gas-filled shells, parapodia flaps, or mucus rafts. Though often overlooked, these mini slugs fill an important ecological niche feeding on plankton at the sea surface. Learning more about these beautiful and bizarre invertebrates provides insight into marine biodiversity.
