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Clio pyramidata Linnaeus, 1767 forma pyramidata Linnaeus, 1767

Overview

This is a shelled pelagic snail, up to 2 cm long, with a transparent uncoiled shell. The shell is slender and triangular. The cross-section is triangular and the lateral sides are not gutter-shaped. The reddish, dark brown visceral mass is seen through the shell. It is a good swimmer that feeds on phytoplankton and protozoa. It lives in the N-Atlantic in the upper water layers where it can occur in mass blooms (Clio p. pyramidata drawing).

Taxonomic Description

The shell is hyaline and sometimes has a reddish hue, it is thin, straight, pyramidally shaped (Clio p. pyramidata). The two lateral ribs are thickened and only slightly diverging and bent. Transverse striation and growth lines are distinct. Cross-section triangular in all parts except most posteriorly. The dorsal rib protrudes slightly above the shell aperture. The latero-dorsal sides both have three swellings, and one longitudinal rib runs over the ventral shell surface. The greatest width of the shell, the aperture width, is found above the middle of the shell. The lateral ribs of the shell are rounded, especially near the embryonic shell. The embryonic shell is droplet-shaped and has a small cusp. The Clio p. pyramidata radula formula is 1-1-1, with about 7 rows.
The shell length is about 20 mm; its width about 10 mm.

Morphology and Structure

The chromosome number is 22 (2N) (Thiriot-Quievreux, 1988). Growth in this species and most Thecosomata is very quick since the shell reaches full size in about three weeks. Shell increments of of one day are indicated in the illustration between the two lines (Clio p. pyramidata growth lines). The mantle is shown in a shell-less specimen (Clio p. pyramidata (mantle). In this mantle a muscle system is found separate from the body muscles as seen in the next illustration (Clio p. pyramidata body muscles).

Juveniles

Protoconch I is slender oval and continues into protoconch II with only a small incision. After protoconch II the juvenile shell, triangular in cross section, begins to grow (Clio p. lanceolata protoconchs, Clio p. lanceolata). Daily growth increment is much larger in the juvenile shell part than in the older parts of the teleoconch.

Reproduction

The form is a protandric hermaphrodite, and it sometimes shows strobilation (Clio p. pyramidata strobilation). The strobula or aberrant stage, except for the wings, is composed of egg cells (Clio p. pyramidata aberrant gonad). A rudimentary intestine is found and large quantities of sexual products (Clio p. pyramidata aberrant intestine). The wings are normal in structure though the major part is formed of interstitial cells in the newly hatched aberrant stage (Clio p. pyramidata aberrant wings). A large duct with male products is present beside the egg mass (Clio p. pyramidata aberrant musc). See also Clio p. lanceolata.

Ecology

This form is phytophagous, and epipelagic or mesopelagic.

Distribution

The species is found from the Antarctic continent to 65°N in the Atlantic and up to 50°N in the Pacific. In the N-Atlantic the forma pyramidata is found between 45°N and 65°N, it is absent in the coastal areas. It does not penetrate into cold water in general, as is demonstrated by its absence in the Labrador Current, see the Clio p. pyramidata map. Between 45° and 50°N intermediates between the forma pyramidata and the forma lanceolata are present. In the Atlantic between 40°S and 50°N the forma lanceolata is found. The absence of the species in the Pacific north of 50°N is due to the fact that the forma pyramidata does not occur in the Pacific. In the Sargasso Sea the forma lanceolata is scarcely recorded, thus the absence in the Central Indian Ocean and Pacific cannot be the result of the poor productivity of these waters alone; perhaps a bisubtropical distribution is the reason for its rarity.

Geological Record

This form was found in the Pleistocene of the Mediterranean and Adriatic. From the Adriatic this forma disappeared in the Atlanticum. It is found in the Late Pliocene of Italy.

Types

Clio pyramidata Linnaeus, 1767.
Types and type locality unknown.

Clio pyramidata pyramidata