We all know a worm. We all know a snail. Or at least, we think we do.
But nature often blurs the lines we draw so confidently. In the case of vermetid snails, also known as “worm snails,” appearances can be deceiving. These creatures look like worms but are, in fact, true snails that have abandoned the for a tubular one.

Vermetid snails were long mistaken for worms because of their irregular, tube-like shells instead of classic spiral shells, a confusion that puzzled scientists for over years. First named by the French naturalist Rafinesque in 1815, these “worm snails” bounced between genera and even families until modern anatomy and molecular studies revealed their true snail identity.

Unlike other snails, whose coiled shells are perfect for crawling, vermetids have evolved elongated tubes resembling those of serpulid worms. The difference lies in the details: vermetid shells are shiny inside and built from three layers, while worm tubes are dull and have only two. The family Vermetidae includes about 13 genera and over 200 species found in tropical and temperate seas, from shallow waters to depths of 300 meters. Some species even form dense reefs of winding tubes that reshape the habitats they inhabit.

Vermetids with their tubes create microhabitats for other marine species and can alter local water flow, influencing reef structure and nutrient cycling. In aquaria, however, they’re often seen as pests for overgrowing corals with their mucus webs.

From Crawlers to the Anchored Life

Once juvenile vermetids settle, they cement themselves permanently onto rocks, coral, or other hard surfaces and never to move again. This sessile lifestyle has driven remarkable adaptations, from their feeding strategies to their reproduction. Inside the tube, the body twists to follow the shell’s shape. The head carries tentacles and a mouth with a radula, and the foot, which in most snails is used for crawling, is greatly reduced. The mantle cavity, containing the gills and reproductive organs, lies deep within, protected by an operculum from predators and currents. The operculum that is present in some species acts like a “trap-door” sealing the shell, while some species have lost it entirely through independent evolutionary events.

The Mucus Net Strategy

Forget the slow-grazing garden snail; vermetids are oceanic innovators!
They use nets to catch their food. Using mucus nets that vermitids drift or spread in the water, they capture microscopic food. Some spin delicate webs that cling to their surface, others release fine threads that merge into floating nets. After trapping plankton and organic particles, the snail reels in the net and consumes its catch. For example, Dendropoma maxima uses glands in its head and foot to produce strong, tear-resistant mucus threads that form a mesh like underwater flypaper. When enough particles collect, the vermetid retracts and swallows the entire net.

Remarkably for sedentary animals, vermetids have separate sexes. Males release sperms into the water, and females capture them with their feeding nets, turning the net into a mating opportunity. Fertilized eggs are brooded inside the female’s tube, and the young emerge as tiny crawling or swimming larvae ready to start their own lives.

Evolution and Phylogeny

Molecular studies confirm that Vermetidae form a monophyletic group within Littorinimorpha, closely related to slit worm snails (Siliquariidae) and turret snails (Turritellidae). Genetic analyses reveal two main lineages within the family, with multiple independent losses of the operculum.

By combining behavioral innovation, physiological specialization, and ecological influence, vermetid snails showcase evolution’s creativity. Next time you explore a rocky shore or coral reef, look closely at the twisted tubes encrusting the surface. You might find these hidden engineers quietly weaving their invisible nets beneath the waves.

References

• Lee, Y., Kim, K. B., Choi, E. H., & Hwang, U. W. (2024). Complete mitochondrial genome of the worm snail Thylacodes adamsii (Littorinimorpha: Vermetidae) from South Korea. Mitochondrial DNA Part B, 9(6), 753–757. https://doi.org/10.1080/23802359.2024.2368209
• Golding, R. E., Bieler, R., Rawlings, T. A., & Collins, T. M. (2014). Deconstructing Dendropoma: a systematic revision of a world-wide worm-snail group, with descriptions of new genera (Caenogastropoda: Vermetidae). Malacologia, 57(1), 1-97. https://doi.org/10.4002/040.057.0103
• Joshi, D. M., & Mankodi, P. C. (2016). The Vermetidae of the Gulf of Kachchh, western coast of India (Mollusca, Gastropoda). ZooKeys, (555), 1. https://doi.org/10.3897/zookeys.555.5948
• Vermetidae – Wikipedia
• https://www.inaturalist.org/
• WoRMS – World Register of Marine Species – Vermetidae Rafinesque, 1815
• OpenAI. (2023). ChatGPT (Mar 14 version) [Large language model]. https://chat.openai.com/chat, 2025.10