Cittarium pica, common name the West Indian top shell or magpie shell, is a species of large edible sea snail, a marine gastropod mollusk in the family Tegulidae. This species has a large black and white shell.
This snail is known as "wilk" or "wilks" (or sometimes as "whelks") in the English-speaking Caribbean islands of the West Indies, where this is a popular food item. The word "will" or "wilks" can be used both as a singular form and a plural. This species is however not at all closely related to the species that are known as whelks in the U.S. and in Europe. In some Spanish-speaking parts of the Caribbean, when used as a food source Cittarium pica is known as bulgao, or simply as caracoles (snails, in Spanish). In Venezuela it is called quigua ; in Cuba it is called cigua.
Cittarium pica is considered the third most economically important invertebrate species in the Caribbean, after the spiny lobster (Panulirus argus ) and the queen conch (Eustrombus gigas ). It has gone locally extinct in some habitats due to overfishing and overexploitation.
The shell of this species can be up to 137 mm in maximum dimension. It is very thick and heavy, having an outline that is between trochiform and turbiniform in shape, with rounded shoulders and a somewhat low conical form.
The spire is conoidal. It contains about six convex whorls. The large body whorl is depressed-globose. The outer lip is simple. The lip is edged inside by black, or black and white. The columella is arcuate, produced above in a heavy porcellanous callous deposit, half-surrounding the umbilicus and deeply notched in the middle.
The shell of Cittarium pica presents a rather wide umbilicus, which is deep and devoid of sculpture, but spirally bicostate inside. The semicircular, oblique aperture is distinguishably nacreous inside as is the case in other Trochoidea, and is circular. The parietal callus is glossy and delicate, and has a node that projects towards the umbilicus. Juvenile individuals possess shells ornamented by spiral lines and strong cords, in contrast to the nearly smooth, homogeneous surface of mature specimens.
The lusterless color pattern is rather distinct, overall white with black zigzag flammules on each whorl. Those spots have a tendency to become axial lines in older, larger individuals. The upper surface is often entirely black. The aperture is commonly white, with an inner iridescence because of the nacre.
Young shells, or well-preserved adults, have the spire whorls sculptured by oblique folds, cut by a few spiral sulci. The periphery and the base in the half-grown shells are spirally lirate.
On some old, empty shells of large individuals, the black colored parts become slightly higher in relief, compared to the white areas surrounding it. This unusual morphology may be due to the action of blue-green algae, such as Plectonema terebrans, which continuously erode the surface of the white parts of the shell.
This species occurs rarely in the Florida Keys, and in the Caribbean coast of Mexico, Costa Rica, Panama, Colombia and Venezuela. It also occurs in the Bahamas, Cuba, the Cayman Islands, Jamaica, Haiti, the Dominican Republic, Puerto Rico, and the Lesser Antilles as far south as Barbados and Trinidad and Tobago. The species has been reintroduced to Bermuda.
This large snail is found on or under rocks, in exposed and moderately sheltered shores, both in intertidal and shallow subtidal zones. Cittarium pica generally does not live at great depths, though this has occasionally been reported. Most individuals are found at the water's edge, and have little tendency to disperse. Minimum recorded depth is 0 m. Maximum recorded depth is 7 m.
The west Indian top shell is known to be an herbivore, feeding on a large variety of algae, and sometimes also on detritus. They actively scrape the algal growths off rocks, and this tends to cause erosion over time. Feeding commonly occurs during the nocturnal period, when the snails are most active.
Cittarium pica is dioecious, which means each individual organism belonging to this species is distinctly male or female. The fertilisation in this species occurs externally. During the reproductive season, which normally occurs from June to November in the field, male individuals release their sperm into the water, as females simultaneously release their green colored unfertilised eggs. The encounter of those gametes produce yolky fertilised eggs, which will further develop into lecitotrophic (yolk feeding) larvae. They emerge from the egg capsules as shell-cap-bearing trocophores. These trocophore larvae do not spend much time in the plankton, because settlement occurs relatively soon, after 3.5 to 4.5 days.Individuals usually reach sexual maturity at shell lengths of 32–34 mm. The life span of this species is still unknown, but estimates for other top shells reach 30 years.
