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The Common Limpet (Patella vulgata)
Limpets are gastropods with simple cone-shaped shells. The majority of limpets are herbivores which live by grazing up films of algae by means of radula—an organ which is a tongue covered in rows of teeth. The most salient feature of limpets is their incredible tenacity. They cling to rocks and other haed surfaces with incredible tenacity—to such an extent that they have become synonymous with tenacity and obduracy. Brute force will not compel a limpet to stop clamping to a favored surface and the animal will allow itself to be destroyed before giving up its grip.
There are a number of limpet-like gastropods but the true limpets are named Patellogastropoda. The majority are tiny—less than 8 cm (3 inches) although a few larger species are known. Some limpets have a “home scar” on their favorite rock—a niche which fits them carefully and where they are perfectly camouflaged. Such limpets tend to be territorial and will fight for the grazing rights to algae near their scar.
Although limpets are most familiar to people at the tidal line (where rockbound limpets survive above the low tide line by adhering to rocks and tightly maintaining moisture) the creatures exist in numerous different marine ecosystems, including the depths of the sea, coral reefs, whale skeltons, seagrass forests, black smokers, and cold seeps. The deepwater limpets are probably detritivores. Limpets begin sexual life as males, but when they mature to a couple of years old they turn female. Young limpets undergo a brief larval stage before clamping down on a hard surface and stating to graze or forage.
A limpet shell pillbox
Clione or “sea angels” swimming in a Tokyo aquarium (Photo by REUTERS/Kimimasa Mayama)
In terms of taxonomical diversity the gastropods are second most diverse class of animals on Earth (outnumbered only by the teeming class Insecta of the other great invertebrate phylum Arthropoda). This means that there are some deeply strange arthropods out there. While we traditionally think of gastropods as snails and slugs there are odd subcategories of these creatures, like the subject of today’s post, sea angels (of the clade Gymnosomata).
A “Sea Angel” (Clione limacina)
Sea angels consist of six different families of pelagic marine opisthobranch gastropod molluscs. Gastropods are named for their famous foot (the name means “stomach-foot”–a misnomer since gastropods all have true stomachs elsewhere) however the name is even more inappropriate for sea angels. In these free-smimming predators, the gastropod foot, so familiar to us as seen on snails, has evolved into a pair of delicate wings for swimming through the water. Sea angels are very small: the largest species only reach 5 cm (2 inches) in length and most varieties are much more miniscule. They prey on other tiny creatures swimming among the plankton—particularly other smaller slower species of gelatinous mollusks.
A hunting sea angel (photo by Alexander Semenov)
Adult sea angels lack any sort of shell—which they discard when they metamorphose into adulthood. Their feeding apparatuses can be strangely complicated—pseudoarms and tentacles which recall their cousins the cephalopods. Sea angels are numerous in the oceans but some scientists are concerned that the acidification of the world’s oceans will cause substantial problems for the tiny translucent gastropods.
Sea Angel (Platybrachium antarcticum)
