Linnaeus, 1767
Described form occurring in the area (Van der Spoel et al., 1997): Clio pyramidata Linnaeus, 1767 forma pyramidata Linnaeus, 1767
Description
A pelagic snail with a transparent uncoiled, pyramidal shell [C.p.pyramidata].
Shell thin, slender, straight and pyramidally shaped (see overview picture). The cross-section is triangular and the lateral sides are not gutter-shaped. 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.
Radula formula 1-1-1, with about 7 rows [C.p.pyramidata-r].
Juveniles: the embryonic shell is droplet-shaped and has a small cusp. 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 (Examples of C. pyramidata protoconchs of the subspecies lanceolata: C.p.lanceolata-prc C.p.lanceolata-prc2). Daily growth increment is much larger in the juvenile shell part than in the older parts of the teleoconch.
Colour
The shell is transparent. The reddish, dark brown visceral mass is seen through the shell.
Size
Shell length is about 20 mm; its width about 10 mm.
Ecology
Clio p. pyramidata is a good swimmer that feeds on epipelagic or mesopelagic phytoplankton and protozoa. Living in upper water layers, it can occur in mass blooms.
Remarks
Growth in this species and most Thecosomata is very quick since the shell reaches full size in about three weeks. Shell increments of one day are indicated in the illustration between the two lines (C.p.pyramidata-gl ).
Depth range
Epipelagic.
Distribution in the North Sea
Possibly in the northern North Sea, entered from the NE Atlantic.
World distribution
The subspecies Clio p. pyramidata is found only in the NE Atlantic from 40-70°N, but also off NW Africa and in the Mediterranean. It is absent in the coastal areas. The closely related Clio p. lanceolata is a worldwide (sub)species, occurring from 60°S to about 60°N. In the NE Atlantic, Clio p. lanceolata (not included here) is found west off the British Isles and might approach the northern entrance of the North Sea, from where it is never reported yet.
[After Van der Spoel et al., 1997]