Clio convexa

Clio convexa (Boas, 1886a)


This is a shelled, uncoiled, pyramidally shaped, pelagic snail, up to 0.8 cm long. The shell is transparent, triangular in shape and cross-section. The reddish or dark brown visceral mass is seen through the shell wall. Growth lines are prominent, the lateral sides are gutter-shaped in cross-section. It is a good swimmer that feeds on phytoplankton and protozoa. It lives in the warmer waters of the Indian and Pacific Oceans in the upper and mesopelagic depths where it can occur in mass blooms (Clio convexa 1).

Taxonomic Description

The conical hyaline shell is much smaller and the rear angle of the shell is smaller than in the forma lanceolata. The lateral ribs are not thickened as in the other formae but they are double with a small flat surface in between. These lateral ribs diverge and run nearly straight in the caudal part of the shell, in the cranial part they may converge (Clio convexa), this is, however, not pronounced. Most typical in this forma is the embryonic shell which constantly differs from the other formae, being less oval and having with a sharp cusp. The normal longitudinal ribs and the transverse striation are identical to those in the forma pyramidata. The present forma is sexually mature when the shell is 7.2 mm long and the body 5.2 mm long, while the nearest related formae have not yet developed their gonads when the body is the same size. The cross-section through the shell near the aperture is more triangular than in forma lanceolata. The wings are colourless, the lips near the mouth are brown to reddish. The radula is of the same type and structure as in the forma lanceolata. The lateral dentation of the median plate is perhaps finer and the median cusp more slender, but these are variable characters. The radula of the forma lanceolata, taken from a specimen of the same size as a full-grown specimen of the forma convexa shows, however, distinctly less lateral dentation and larger dentation on the median plate. In Red Sea specimens the lateral sides are more strongly curved dorsally than in the open ocean specimens.
Shell measurements: up to 8 mm long, width up to 4.5 mm.


The Clio convexa protoconch I is oval, pointed and broad at the anteriorly. Protoconch II is very narrow and the lateral ribs of the adult shell begin nearly directly after the incision between protoconch I and II.


This species is a protandric hermaphrodite.


This species is phytophagous and epipelagic to mesopelagic and executes strong diurnal vertical migration.


This species is an Indo-Pacific form of warm tropical and subtropical waters, see the Clio convexa map. The record of this species for the Adriatic Sea (Van Straaten, 1985) is based on a misidentification and the one for the Mediterranean is doubtful as this species is illustrated by Vergnaud-Grazzini and Herman Rosenberg (1969) but not mentioned in the text.

Geological Record

This species was known from the Late Quaternary (last Interglacial 100.000 BP) and Holocene of the Red Sea.


Cleodora pyramidata var. convexa Boas, 1886: 73, 203, pl. 9, fig. 97a-d.
Lectotype: ZMUC, one damaged spec. (alcohol collection). Paralectotypes: ZMUC, 2 spec. (alcohol collection).
Type locality: (restricted) 17°20'N 88°12'E, 1845. Coll.: CGLE no. 654, Reinhart.

Original description

Var. convexa. Fortil sædvanlig temmelig bred, bagtil smal, Siderandene bagtil konkave, fortil konvexe. Midtkjølen ikke meget skarpt afsat, svagt krummet. Størrelse ringe (indtil c. 8 mm). - Sydlige Del af det Indiske Ocean (fra c. 24°SBr. til 40°SBr.). Den af Pfeffer (Berl. Mon. 1879) under Navnet «Cleodora australis» afbildede og beskrevne Skal er uden Tvivl en smal Cleod. pyramidata; med australis har den ingensomhelst Lighed. Uforstaaelig er mig P.'s Angivelse (hvortil ogsaa hans Fig. 9 passer), at der paa Undersiden befinder sig en Kjøl, der fortil skulde gaa over til at blive en Fure - en Karakter, der hverken tindes hos pyramidata eller australis, og som overhovedet er fremmed for Sl. Cleodora. - Samme Forf.'s «Form A. Pyramidata» (Abhandl. Naturw. Ver. Hamb. 7.Bd. p.94) er = vor Var. lata, hans «Form B. Martensii» = vor Var. Angusta; hans «Form C. Sulcata» er jeg ikke i Stand til at tyde med Sikkerhed.