Vermicularia fargoi Olsson, 1951 - Fargo's worm snail in Florida, USA.

The gastropods (snails & slugs) are a group of molluscs that occupy marine, freshwater, and terrestrial environments. Most gastropods have a calcareous external shell (the snails). Some lack a shell completely, or have reduced internal shells (the slugs & sea slugs & pteropods). Most members of the Gastropoda are marine. Most marine snails are herbivores (algae grazers) or predators/carnivores.

Shown above is one of the more bizarre gastropod shells around - this is Vermicularia, also called a worm snail. It’s one of the few snails that does not have a tightly coiled shell. Several snails from different families have shells somewhat like this (e.g., the vermetids & the turritellids). They all resemble the twisted mineralized shells made by some annelid worms, hence the common name “worm snails” or “worm shells”.

Despite the superficially very different-looking shells, malacologists have demonstrated that Vermicularia is very closely related to the high-spired snail Turritella (<a href="http://www.gastropods.com/Shell_Images/T/Turritella_terebra_2.jpg" rel="nofollow">www.gastropods.com/Shell_Images/T/Turritella_terebra_2.jpg</a>). Juvenile Vermicularia are free living, infaunal filter feeders that position themselves apex-down and aperture-up within the sediment. During this stage in ontogeny, the Vermicularia shell is tightly coiled, as is any ordinary gastropod shell. Later in life, the snail becomes an epifaunal, encrusting filter feeder (assuming hard or firm substrates are available), and its shell starts uncoiling. The advantage of an uncoiled shell in Vermicularia is generally inferred to be rapid upright growth (that’s desirable for a sessile, benthic filter feeder).

If hard or firm substrates aren’t available, Vermicularia generally doesn’t grow an unwound shell during growth, and it ends up looking like typical a Turritella shell. The degree of uncoiling also depends on the nature of the hard substrate (e.g., a ramose scleractinian coral vs. a hemispherical scleractinian coral vs. a bivalve shell). So, shell uncoiling is an ecophenotypic character.

Classification: Animalia, Mollusca, Gastropoda, Turritellidae

Provenance: marine beach from near the southern end of Cayo Costa Island, southwestern Florida, USA

Most info. synthesized from Morton (1953) and Gould (1968, 1969):

Morton (1953) - Vermicularia and the turritellids. Proceedings of the Malacological Society of London 30: 80-86.

Gould (1968) - Phenotypic reversion to ancestral form and habit in a marine snail. Nature 220: 804.