Limacina (Thilea) lesueuri d'Orbigny, 1836
This is a small, shelled, pelagic thecosomatous pteropod with a left-coiled shell, 0.1 cm in diameter. The spire is depressed, the body whorl increases very quickly in diameter. It has 4.5 colourless, transparent whorls. There is a narrow but deep umbilicus, spiral striation is found around the umbilicus and the remaining parts the shell are smooth. It is found in warm waters of the all oceans, where it feeds on phytoplankton, and it is a mucus feeder (Limacina lesueuri 1, Limacina lesueuri apical, Limacina lesueuri oral).
The transparent thin shell is flatly coiled and is a very smooth. It is composed of about 4.5 whorls and the last whorl occupies about 3/4 of the total shell. The shell is significantly broader than high (Limacina lesueuri). The inner border of the aperture is straight. The umbilicus is very narrow (Limacina lesueuri umbilicus). Some faint spiral lines are found on the shell around the umbilicus. In young specimens the umbilicus is relatively large and deep. The suture is very well marked. (Limacina lesueuri apical, Limacina lesueuri oral) On the anterior wing border protrusions are absent (Limacina lesueuri softparts).
Shell measurements: average diameter 1.l mm, height to 0.8 mm, diameter to 13 mm. Maximum diameter of the operculum is 0.6 mm and its length/width ratio is 2/3.
The juveniles have a small left coiled shell. Embryonic shell has a rough granulated surface (Boltovskoy, 1974).
The species is a protandric hermaphrodite, There is no brood protection.
The species is phytophagous and epipelagic being most abundant in Central waters, but found in the entire warm water belt. The temperature range is 13.8-27°C. Diurnal vertical migration is present, the mean day time level is 103 m and the mean night level is 85 m.
This typical bisubtropical species is sometimes compared with Limacina inflata but, though in the latter some central areas are scarcely populated, its bisubtropical character is not as clear as in L. lesueuri. The the theory that the E-Atlantic is richer in pteropods than the western part is indicated by this species. From the Sargasso Sea no records are known. The records east of Ceylon probably belong to a larger North Indian population.
In contrast to most other tropical species it is absent in the Indonesian Archipelago. A record NE of Scotland is due to the action of the Gulf Stream, as are the exceptional records in the English Channel. Other exceptional records are known from the West Mediterranean Basin. Two records in the South Indian Ocean deal with specimens transported southwards by the West Wind Drift, see the Limacina lesueuri map.
This species is probably known from the Pliocene in the Mediterranean (Geronimo, 1975). Late Quaternary of the Red Sea.
Atlanta lesueurii d'Orbigny, 1836: 177 (1846) pl. 20, figs. 12-15.
Syntypes: BMNH 18184.108.40.206, cat. no. 64, 5 spec. bad condition (dry collection originally alcohol).
Type locality: Atlantic and Pacific Ocean. Coll.: CVAM.