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Limacina (Thilea) inflata (d'Orbigny, 1836)

Overview

This is a very small shelled pteropod, coiled nearly in one level. The aperture has a blunt tooth prolongated on the last whorl by a faint keel. Occasionally it occurs in large swarms especially in more neritic waters. It is found in all oceans in the warm water belt. It is an ovoviviparous species which feeds on microplankton (Limacina inflata 1).

Taxonomic Description

The planorboid shell is coiled in one level Limacina inflata detail, Limacina inflata dorsal), and has up to three whorls (Limacina inflata). Last whorl is swollen with an aperture tooth on the outer border which gives the shell aperture a heart-shape, the tooth is mostly supported by a strong rib (Limacina inflata line drawing). A very thin glassy membrane is formed between the tooth (Limacina inflata tooth) and aperture margin, which is perhaps comparable with the columellar membrane ("lame vitree" described by Souleyet); sometimes it is absent. The rostrum, or aperture tooth, is variable in shape. A rib runs over the dorsal side of the last whorl and protrudes to form the tooth; or the rib starts at the upperside of the last whorl and continues into the middle of the whorl; in some specimens the rib starts at the underside of the whorl; sometimes even two ribs, one starting at the underside and one at the upperside of the last whorl, are found, running parallel to the aperture border and coming together in the middle of the last whorl to form the tooth. Brown spots occur in the vicinity of the tooth and umbilicus. The umbilicus is deep (Limacina inflata umbul.) and distinct though not extremely wide, but the inner whorls are all seen. The operculum is transparent, very thin and horny, coiled sinistrally, and slightly conical. When the shell is 1 mm, the operculum has 4.5 whorls in the operculum. The nucleus is rather centrally placed and sometimes growth striae are found. The Limacina inflata radula formula is 1-1-1 (Limacina inflata median tooth, Limacina inflata lateral tooth).
Maximum diameter of the shell to 1.5 mm, average is 1.3 mm.

Morphology and Structure

When the animal is retracted the operculum does not completely close the relatively large shell aperture. The size relation of shell and operculum of an average sized specimen are as follows: shell diameter 0.86 mm, aperture length 0.68 mm; aperture breadth, 0.50 mm; diameter of operculum 0.31 mm. The attachment of the operculum is only by a small triangular part of the posterior footlobe. On the operculum this attachment forms a triangle with the centre at top and the last growth line at its base. The shell/operculum ratio expressed as the diameter of the operculum x 100/diameter of the shell, varies between 32 and 80 in shells of 1.00 mm to 0.21 mm diameter. The relation of the smallest to the greatest shell diameter is always 0.8, independent of the shell diameter itself. The discontinuity in the logarithmic graphs of the ratio of diameter of the operculum and shell length, when the shell length is about 0.43 to 0.57 mm has nothing to do the sexually mature of the specimens. Although it is still obscure which influences are responsible for this irregular growth, it is important for growth studies in general.
The chromosome number is 20 (2N) (Thiriot-Quievreux, 1988).

Juveniles

The Limacina inflata veliger has a small left coiled shell. Embryonic shell has a rough granulated surface both on proximal zone and first whorl (Boltovskoy, 1974). Growth rate is 0.12 mm per month (Wells, 1976). (Limacina inflata protoc.).

Reproduction

The species is a protandric hermaphrodite and ovoviviparous. The juveniles develop in the mantle and there is brood protection.

Ecology

The species is phytophagous and epipelagic. The temperature range is 8.5-28°C and the salinity range is 33.5%o-37.0%oS
Diurnal migration is distinct; in the Florida Current it occurs between 0-700 m with a mean day level at 236- 300 m and a mean night level at 76-232 m. The diurnal migration ranges over 195 m in the Bermuda area. The species is associated with L. bulimoides and L. lesueuri.

Distribution

This warm-water species is circumtropical. Most typical of its range is its absence in the Sargasso Sea and Canary Current, and its rare occurrence in the Central Indian Ocean and the E-Pacific. In the Indo-Pacific and Atlantic Ocean the southern limit is near the subtropical convergence. Its rare occurrence in the southern part of the Benguela Current is due to the fact that this area is mainly populated with animals from the Indian Ocean. It is common in the Central and W Basin of the Mediterranean, but only immigrants are found in the E Basin. In the Red Sea some exceptional records are known. The influence of the Labrador Current and the Gulf Stream on the northern boundary of this species is small, though sometimes a specimen is transported far north by the Gulf Stream. This oceanic species is only seldom recorded from coastal regions and it is absent in the North Sea, Gulf of Mexico, the Yellow and China Seas. The few records from the Caribbean concern specimens transported by the North Equatorial Current. The exceptional records near Iceland, Cape Horn, Tasmania and near the Ross Sea are due to current influences of the Gulf Stream, the turbulences between the West Wind Drift and Brazil Current, the Eastern Australian Coast Current and the West Wind Drift, respectively, see the Limacina inflata map. The temperature range of this species is about 14°-28°C, and the salinity range about 35.5-36.7°/oo.

Geological Record

This species is known from the Late Quaternary (it is nearly continuously present from the start of the Riss into present but it is rare or absent during the Older Dryas) of the Red Sea, and the Pleistocene of the Mediterranean. In the Older Dryas it occurs in the Adriatic Sea. This species is known from the Miocene of Australia.

Types

Atlanta inflata d'Orbigny, 1836: 174 (1846) pl. 12, figs. 16-19.
Syntypes: BMNH 1854.12.4.35, cat no. 61 about 82 spec. bad condition (dry collection, originally alcohol).
Type locality: Atlantic and Pacific Ocean 36°N-6° (S? or W?). Col. CVAM.
Limacina scaphoides Gould, 1852: 485.
Types never indicated, described after a figure by Mr. Dana.

Limacina inflata