Mollusks

The Half-naked Pen Shell

The Half-naked Pen Shell, Atrina seminuda (Lamarck, 1819), is the rarest among the three species of pen shells found in Southwest Florida. It differs from the more common Stiff Pen Shell, Atrina rigida, by having its large adductor muscle scar well within the boundaries of the iridescent (nacreous) area inside shell valve (arrow on shell image). The Stiff Pen Shell has that scar “protruding” beyond those boundaries. In addition, the color of the mantle in the living Half-naked Pen Shell is orang

The Rose Corbula

The Rose Corbula, Caryocorbula dietziana (C.B. Adams, 1852), is a small (about 8 mm, or 0.3-inch) locally found species belonging to a group of bivalves infamous for the difficult and confusing taxonomy, the family Corbulidae, also known as Basket Clams. Its shell has the posterior ridge typical of members of the family. Like in all other species of Corbulidae, the right valve is larger than the left one. The Rose Corbula may be rose, whitish, or cream-colored, sometimes showing flecks of gray o

The Coral-eating Mussel

The Coral-eating Mussel, Gregariella coralliophaga (Gmelin, 1791), is a coral-rock boring species, found locally within small coral branches strewn on the beach after storms. The common name is a misnomer, as these mussels do not eat coral, only use the coral skeleton as a substrate for boring. The 20-mm (about ¾ inch) shell is inflated, with a strong ridge on its postero-dorsal part. The shell margins are finely serrated. The shell interior is iridescent, bluish-white. The periostracum is thick

The Chalky Pitar

One of the least known local medium-sized clams, the Chalky Pitar, Pitar simpsoni (Dall, 1895) has a mostly whitish clam-shaped shell, with a polished surface and crowded growth lines. The Chalky Pitar generally resembles its gaudier cousin, the Lightning Pitar (Pitar fulminatus (Menke, 1828), also found locally), but lacks the strong brownish “lightning” markings seen on this latter species. The Chalky Pitar is also smaller, having a “higher” shell, with stronger hinge. The Chalky Pitar, Pitar

The Bipartite Caecum

The Bipartite Caecum, Caecum bipartitum de Folin, 1870, is one of about ten species of the family Caecidae known from our shores. Members of this family have small, tube-like shells that undergo a few changes in shape during the growth of the snail. The Bipartite Caecum grows only to 2 millimeter (0.08 inch) and has its shell festooned with narrow rings, which may sometimes be present only on the posterior (wider) part of the shell or, in some cases, be completely lacking. The two images are of

The Modest Triphora

Many believe that the Lightning Whelk, Sinistrofulgur sinistrum, is the only left-handed gastropod in our area. The fact is there is a handful of local species in the family Triphoridae that also display that feature. (Most species of Triphoridae are left-handed.) Of those, the most commonly found is the Modest Triphora, Marshallora modesta (CB Adams, 1850). The shell is small reaching at most 5mm (about 0.2 inch), and typically with three spiral rows of whitish beads set against a glossy-brown

The Grass Cerith

As implied by its vernacular name, the Grass Cerith, Bittiolum varium (Pfeiffer, 1840), lives in seagrass beds. The Grass Cerith is one of the most common species of mollusks in that particular type of ecosystem, where it supposedly grazes on the microalgae that grow on the seagrass blades. Its shell is small, rarely reaching beyond 4 mm (about 0.16 inch). The shell sculpture shows spiral lines and weak axial ribs that intersect forming raised nodules. The last whorl usually has one or two thick

The Milk Moon Snail

The Milk Moon Snail, Polinices lacteus (Guilding, 1834), is the least common among the shallow-water moon snails (Family Naticidae). The species has a broad distribution in tropical western Atlantic. In geographic areas where the species is more common, its shell may reach in excess of one inch, but the few samples found locally never exceeded 0.5 inch. The Milk Moon Snail has an egg-shaped, smooth, pure white shell with the umbilicus (the cavity on the base, or "lower" part of the shell) partia

The Varicose Cerith

The Varicose Cerith, Alaba incerta (d'Orbigny, 1841), is a small local gastropod that, at 5 mm (about 0.2 inch) in length, fits the definition of a micromollusk (a mollusk with its shell or body equal or smaller than 5 mm.) The Varicose Cerith shell is elegant and very delicate, with translucent walls and the occasional whitish thickening known as a varix. Varices are usually indicative of slower shell growth in gastropod mollusks. Varicose Ceriths are not very common on local beaches, being fou

The Costate Horn Snail

The Costate Horn Snail, Cerithideopsis costata (E.M. da Costa, 1778) is the smaller "sister" to the larger Ladder Horn Snail, Cerithideopsis scalariformis (say, 1825), a species covered in this column on January 13, 2017. Costate Horn Snails may grow to be a little more than 0.5 inch in length. The two species are the only local members of the Horn Snail family Potamididae, a group of marine snails highly adapted to living in brackish waters. Accordingly, the Costate Horn Snail is most commonly