Pyramidellidae

Family of sea snails

Pyramidellidae, common name the pyram family, or pyramid shells, is a voluminous taxonomic family of mostly small and minute ectoparasitic sea snails, marine heterobranch gastropod molluscs. The great majority of species of pyrams are micromolluscs. The pyram family is distributed worldwide with more than 6,000 named species in more than 350 nominal genera and subgenera. This family of micromollusks has been little studied and the phylogenetic relationships within the family are not well worked out. There is an absence of a general consensus regarding which species belong to a specific genus or subgenus, contributing to much confusion. Schander (1999) names more than 300 supraspecific names. As there has been no serious generic revision of the genera worldwide, generic polyphyly can be expected to be rampant throughout the family. However, the family itself is deemed monophyletic. However a study in 2011 seems to indicate that this family is deeply nested within the Pulmonata instead of the Heterobranchia. The family is currently divided into 11 subfamilies (Ponder & Lindberg 1997). Chrysallidinae Saurin, 1958
Cingulininae Saurin, 1959
Cyclostremellinae Moore, 1966
Eulimellinae Saurin, 1958
Odostomellinae Saurin, 1959
Odostomiinae Pelseneer, 1928
Pyramidellinae J. E. Gray, 1840
Sayellinae Wise, 1996
Syrnolinae Saurin, 1958
Tiberiinae Saurin, 1958
Turbonillinae Bronn, 1849

This family is found worldwide, but many species are only found in relatively small geographical ranges. The species found at the Cape Verdes are mainly endemic species.

The length of the slender, elongated (turreted or conical) shells varies between 0.5 mm and 3.5 cm, but most species in the family have shells which are smaller than 13 mm. The texture of these shells is smooth or sculptured in various forms such as ribs and spirals. Their color is mostly white, cream or yellowish, sometimes with red or brown lines. The shell of these snails has a blunt, heterostrophic (i. e. whorls appear to be coiled in the opposite direction to those of the teleoconch) protoconch, which is often pointed sideways or wrapped up. The teleoconch is dextrally coiled, but the larval shells are sinistral. This results in a sinistrally coiled protoconch. The columella has usually one, but sometimes several, spiral folds. The aperture is closed by an oligogyrous operculum. The operculum is ovoid and paucispiral, with the apex anterior, a thread like arcuate ridge on the proximal side, the inner margin notched in harmony with the plaits of the pillar when prominent. The species are characterized by the lack of jaw or radula, because they are ectoparasites (mostly on polychaetes or other molluscs).

The Pyramidellidae are ectoparasites, feeding mainly on other molluscs and on annelid worms, but some are known to feed on peanut worms and crustaceans. A few species in the family Pyramidellidae, such as Otopleura mitralis, are symbiotic with sea anemones, such as Neoaiptasia morbilla. They do not have a radula. Instead their long proboscis is used to pierce the skin of its prey and suck up its fluids and soft tissues. The eyes on the grooved tentacles are situated toward the base of the tentacles. These tentacles have a concave surface. Between the head and the propodium (the foremost division of the foot), a lobed process called the mentum (= thin projection) is visible. This mentum is slightly indented in midline. These gastropods are hermaphrodites, laying eggs in jelly like masses on the shell of their hosts. Some species have spermatophores.

The name of this taxonomic family comes from the shape of the shell, which is like a pyramid. More specifically, the shape is like a right circular cone, which is equivalent to a right pyramid whose base has many sides.