1. Why Sea Slugs' Head Antennae Look Like Cow Horns
If you look at a sea slug tank in an aquarium, you might see a small, colorful creature moving with two projections sticking up from its head. If you look closely, it might seem like a tiny cow head. Encyclopedias, tourism guides from the Ministry of Land, Infrastructure, Transport and Tourism, and aquarium explanations all agree on one point: the name "umiushi" (sea slug) comes from these two antennae looking like cow horns [1][2][3]. The English version of the government’s explanation also states that the name "sea cow" is because the antennae resemble cow horns [2]. So, the answer to why they are called "cows" is not because they are cows, but because they have a part that looks like cow horns. It is not clear in these sources who first used this name or when.
There is also a chance of confusion with other animals. In Japanese, the characters for "sea cow" (kai-gyu) usually refer to marine mammals like dugongs and manatees [4]. The "cow" in "sea slug" has no relation to mammalian cows. So, what use are these "horns" to the sea slug?
2. The "Horns" Are Smelling Organs, Not Real Horns
Even though they look like horns, their function is closer to a nose. This is the interesting part about these creatures. The antennae of sea slugs are called rhinophores. This word comes from ancient Greek, combining "nose" and "carrier" [6]. The way they smell works in the water. Chemical substances that carry smells are dissolved in seawater. Sea slugs use their antennae to detect these chemicals, which helps them find food [6].
The shape of the antennae varies by species. Some have folds, some branch out, and others look like thin overlapping plates. These structures increase the surface area touching the water, which is explained as making it easier to detect chemicals [7]. This is similar to how a bare winter tree has more surface area exposed to the air than a utility pole. Some species can retract their antennae into sheaths, but not all species can do this [7]. This shows that the name "cow horn" comes from appearance, while the function is completely different. So, what kind of animal is this, if it is neither a cow nor a fish?
3. Sea Slugs Are Relatives of Snails That Lost Their Shells
Sea slugs are a group of marine animals related to snails with spiral shells, but whose shells have become small or disappeared. In classification, they mainly refer to the "nudibranchs" within the "opisthobranchs" [1][2]. At first glance, they look like sea cucumbers, but their closest relatives are actually snails, similar to land snails [2]. Because of this, they are often described as "snails without shells." However, aquarium explanations point out that this description does not apply to all types. One type of sea slug relative has a shell, and this type lacks secondary gills and even lacks the antennae that are the symbol of sea slugs [3]. Wikipedia also notes that while they generally do not have shells, some have trace remnants of shells [4]. This means that some sea slugs in this group do not have the antennae that gave them the name "cow." The origin of the name and the appearance of all group members do not perfectly match. What do other languages call this creature?
4. How English Names Like "Sea Slug" and "Naked Gills" Differ From the Japanese Name
In English, nudibranchs are sometimes called "sea slugs," a name that compares them to slugs, the land mollusks. "Sea slug" is a broader name that also covers some animals outside the nudibranchs [4][8]. Japanese compares the head antennae to cow horns, while English speakers compare them to slugs. This is an example of how different languages notice different similarities in the same creature.
There is another name used in classification. The English word "nudibranch" comes from Latin for "naked" and ancient Greek for "gill," meaning "naked gills" [8]. On the back of a sea slug’s body, there are secondary gills, and some species have gills that open like flowers [3].
When you line them up, the same creature has names based on cow horns, sea slugs, naked gills, and "nose-carriers" (rhinophores). These names reflect what the observer focused on. So, what range of animals does the name "sea slug" actually cover?
5. What Kinds of Animals the Name "Sea Slug" Covers
According to a glossary on a sea slug specialist website, "sea slug" is not a formal taxonomic term but a historical and customary group name [5]. It says that what is considered a sea slug varies depending on the person or context [5]. The pair of antennae that are said to look like cow horns are found in dorid nudibranchs, a specific type within the nudibranchs [4].
How many species are there? The English version of the Ministry of Land, Infrastructure, Transport and Tourism’s explanation states that about 1,400 species of sea slugs live in waters near Japan, and 384 of these are known from Tatsukushi Bay in Kochi Prefecture [2]. Dividing 384 by 1,400 gives about 0.27, meaning that just over a quarter of the species reported from waters near Japan are known from this one bay. However, these are figures from a tourism introduction, and the year of the survey is not known. The number of species counted changes depending on what is included in "sea slug." Because the counting range differs, these numbers cannot be directly compared with figures from other countries or regions. The shape of the "cow horn" antennae also differs by species. Can you compare them with your own eyes?
6. How to Compare Sea Slugs' Antennae and Gills in an Aquarium
If you have a chance to see sea slugs in an aquarium or in a book, first look for the two antennae on the head. They can be straight rods, have folds, or be branched [7]. Sketching the differences in shape can help you see the differences between species.
Next, look at the gills on the back of the body. Some species have gills that open like flowers [3]. If you encounter a type with a shell and no antennae, you have found a strange example where the part that inspired the "cow" name is missing [3].
When looking for them at the seaside, always go with an adult and observe the creatures without touching them. With names like cow horn, sea slug, naked gills, and nose-carrier, if you were naming this creature, what feature would you focus on?
SAFETY NOTE: When searching at the seaside, always go with an adult and observe the creatures without touching them.