Limacina (Munthea) trochiformis (d'Orbigny, 1836)
Overview
This is a small, shelled, pelagic thecosomatous pteropod with a left-coiled shell, 0.1 cm in diameter. The spire is highly coiled, the body whorl increases quickly in diameter. It has 5 colourless, transparent whorls. There is a narrow but deep umbilicus. The shell is smooth except for faint growth lines. It is found in warm waters of the all oceans where it feeds on phytoplankton and it is a mucus feeder (Limacina trochiformis 2).
Taxonomic Description
The shell is composed of 5 rapidly growing whorls separated by a clear suture. The apical angle is 75-96° (average of 84°). The colour of the shell is white to light-purple, the thicker parts along columella and upper aperture lip, are purple-brown. The shell aperture is relatively small. The operculum is less elongated than in Limacina retroversa. The umbilicus is narrow but deep (Limacina trochiformis umbilicus). The columellar aperture border is slightly concave in contrast to the straight border as found in Limacina retroversa. The shell surface is smooth except for the last whorl which has some spiral lines (Limacina trochiformis, Limacina trochiformis 1). Wing protrusions are absent.
Shell measurements: height to 1 mm, width to 0.8 mm.
Juveniles
The Limacina trochiformis veligers have a small left coiled shell. Embryonic shell has spiral ornamentation of irregularly ramified flat ridges, a depression separates protoconch I and II (Boltovskoy, 1974). The protoconch I is smooth, the protoconch II shows growth lines (Limacina trochiformis protoconch). Growth rate is 0.10 mm per month (Wells, 1976).
Reproduction
The species is a protandric hermaphrodite and there is no brood protection.
Ecology
The species is phytophagous and epipelagic. The temperature range is 13.8-27.9°C and the salinity range is 3.5-3.7%S. Vertical migration is found with a mean day level of 165 m and a mean night level at 99 m.
Distribution
A clearly discontinuous, bisubtropical distribution has been found for this species. A single record is known from the South China Sea. The species is found in the Gulf of Aden, but it does not normally penetrate into the Red Sea. The species is considered to be bisubtropical but is also found in the tropics, see the Limacina trochiformis map.
Geological Record
This species is known from the Late Quaternary (it is nearly continuously present from the start of the Riss into Present) of the Red Sea, and the Pleistocene of the Mediterranean.
Types
Atlanta trochiformis d'Orbigny, 1836: 117, (1846) pl. 12, figs. 29-31.
Types are not present in MHNP or BMNH where they should be present (Gray, 1854).