Limacina (Munthea) bulimoides (d'Orbigny, 1836)
This is a small shelled pelagic thecosomatous pteropod with a left-coiled shell, 0.2 cm in diameter. The blunt spire is highly coiled. It has more than 6 colourless, transparent whorls. Thicker parts of the shell near aperture are brownish. There is no umbilicus. The shell is smooth except for faint growth lines. It is found in warm waters of all oceans where it feeds on phytoplankton and it is a mucus feeder (Limacina bulimoides 1).
The thin, transparent shell is highly conical (Limacina bulimoides 2). The whorls grow regularly. Sometimes a faint, interrupted longitudinal striation is present with the growth lines (Limacina bulimoides). It has a very small but distinct rostrum which is more pointed in older specimens than in young ones. The six whorls are separated by a clear suture which is sometimes light-brown. The thickened inner aperture is chestnut-brown. The umbilicus is very small. The relatively large operculum is rounded. Protrusions on the anterior wing border are absent (Limacina bulimoides soft parts).
Shell measurements: height to 2 mm, diameter to 1.4 mm.
The juveniles have a small, left coiled shell. Embryonic shell with rough ornamentation of irregularly ramified flat ridges (Boltovskoy, 1974). Growth rate is 0.15 mm per month (Wells, 1976).
The animal is a protandric hermaphrodite without brood protection. In the genital system the following organs were distinguished: gonad, gonoduct, receptaculum seminis, accessory gland with albumen and mucous gland, the genital aperture, seminal groove, penis and penial aperture. The sexual stages of the gonad in Limacina bulimoides are better separated than in Limacina retroversa (which may be due to the milieu, as well as, to specific differences). The following alignment of the stages can be recognised:
1. No sexual development; gonad represented by undivided pre-germinal cells.
2. Less than one gonadial whorl; an apical mass of lighter staining primary spermatogonia, which
in the lower part of the gonad have begun to divide into darker staining secondary
3. Less than two gonadial whorls; primary spermatocytes arranged in spherical clusters lying
freely in the lumen of the gonad. Penis and accessory sexual gland absent or very small.
4. More than two gonadial whorls; gonad contains primary spermatocytes and spermatids, penis
and prostate gland fairly large, female ducts present but usually not glandular.
5. Mature male.
6. Pure female, spermatogenesis ceased entirely.
The distribution of sexual stages among size groups is related to growth and sexual development.
The accessory sexual gland is subdivided into: the albumen gland-corresponding to G3 in Cavoliniidae-, the mucous gland-the G2 in Cavoliniidae-, the accessory mucous gland-the G1 in Cavoliniidae. The structure and histology do not differ essentially in Limacinidae and Cavoliniidae.
The seminal groove is an open ciliated groove running into the penis as also found in Cavoliniidae. In pure females the penis and prostate are lost, the mucous gland is active and the receptaculum seminis is filled. In the preceding male stage the receptaculum is hardly seen and the mucous gland is only moderately active.
The species is phytophagous and epipelagic. A temperature range of 13.8°-27.8°C was measured and the salinity range was 33.5°/oo-36.7°/oo. The diurnal vertical migration shows a mean day level at 90 m and a mean night level at 80 m.
The present species has, as far as known, a very discontinuous distribution. Probably the distribution is tropical/subtropical and not bisubtropical. Its typical oceanic character is demonstrated by its absence near coasts. A rare record west of Ireland suggests an influence of the Gulf Stream, but the straight northern boundary in the Atlantic proves this influence to be incidental only. The species is also found in the Central and W Mediterranean Basin and the Aegean Sea.
Via the Agulhas Current the species "penetrates" into the Indian Ocean where it is rarely found. The Indonesian Archipelago population is rich. In the Gulf of Aden it is found in deposits, but it is absent in the deposits of the Red Sea, though it is recorded from this area (Rampal, 1990), see the Limacina bulimoides map.
This species was known from the Late Quaternary, it occurs in the last interglacial, 10.000 BP and disappears in the Wurm glacial, but it also found from the Younger Dryas into the early Holocene layers of the Red Sea, and the Pleistocene of the Mediterranean. In the Pre Boreal it occurs in the Adriatic Sea.
Atlanta bulimoides d'Orbigny, 1836: 179 (1846) pl. 12, figs. 36-38.
Types are not present in MHNP or BMNH where they should be present (Gray, 1854).