The sea mouse is a really strange polychaete. Polychaetes are worms, usually very long and thin, and include the common clam worm which is often used as bait by recreational fishermen and may reach a length of up to ten inches. There is even an Australian species called the Bobbitt Worm (named after Lorena Bobbitt because after mating the female bites off the male's reproductive organ and feeds it to their young) that can grow to nearly two feet long. However, the vast majority of marine worms are much, much smaller. Most species are nearly microscopic and live interstitially (between grains of sand). The sea mouse departs from the normal "worm-like" body of the average marine polychaete because it is both very large, but also very compact.
Taxonomists are clever people. Many species’ scientific name comes from the Latin or Greek words for the discoverer of the animal, the behavior of the animal or something the animal resembles. In this case the sea mouse, whose scientific name is Aphrodite aculeata, is named after the Greek goddess of love because it looks like a certain human anatomical structure. A commenter on my blog suggested it should be named a Sea Merkin instead of Sea Mouse. I had to look up that term and it gives a whole new meaning to Peter Sellers character in Dr. Strangelove, President Merkin Muffley. Oh, man. This diary is getting ugly. Let’s get to some family friendly facts.
Sea mice get their nom de plume from the furry body, which tends to hide the segments associated with most annelids. As the ventral picture above shows, they are indeed segmented. The size of this typical one, although only about five inches long, is notable in the width relative to the length. It is about three inches wide and almost two inches thick. That’s actually pretty astonishing for a worm.
Before moving on, let’s make the distinction between polychaetes and oligochaetes, both of which are annelids. You’re all familiar with oligochaetes. These include smooth, mainly terrestrial and freshwater animals like earthworms. Polychaetes have bristles. Both groups have segmented bodies, but polychaetes have little extensions branching off each segment known as parapodia, which is Latin for "almost feet". Extending from these parapodia are bunches of hair-like structures called setae. The setae are hairy bristles that serve to protect the soft-bodied worm from predators. Although earthworms don’t have parapodia, they do have setae, but they are microscopic. For our discussion they don’t count.
The sea mouse is found in the fairly deep waters of the Northern Atlantic ocean. Being a benthic (bottom-dwelling) animal it relies on its camouflage for protection, as well as the irritating dorsal bristles. It will often travel just below the surface of the sand, allowing particles to cling to its setae, and still remain camouflaged when it resurfaces.
Aphrodite feeds mainly on other worms, sucking them out of the sand as it travels. It can feed on annelids several times longer than it is, sucking them up like spaghetti.
You’ll notice in the photo above, some of the bristles appear to be green and blue. This is an illusion. The bristles are actually made up of hexagonal cells which form a crystal-like structure within the hairs that are incredibly efficient at trapping light photons, bouncing around the particles within the cells and causing them to emit these unusual color frequencies. This sends a warning signal to predators that the "spines" (which are actually harmless hairs) are poisonous.
If you do any further research on this animal, don’t be confused if your searches come up with this nudibranch. Although they share a common name, nudibranchs are mollusks, related to snails and clams. The real sea mouse, as I’ve defined it, is a worm.
Pronunciation guide: Polychaetes and oligochaetes rhyme with parakeets.
Setae is pronounced "see-tay".
Other diaries in this series can be found here.