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Cavolinia inflexa (Lesueur, 1813) forma inflexa (Lesueur, 1813)

Overview

This is a large shelled thecosomatous pteropod, 0.7 cm long, living in the upper water layers. It has a flat dorsal and ventral side. The caudal spine is very long and straight. The lateral spines are well developed and situated far above the middle of the shell. The shell sculpture consists of faint growth lines. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives mainly in the Atlantic Ocean but is also incidentally found in the Indo-Pacific (Cavolinia i. inflexa).

Taxonomic Description

In comparison to other cavoliniid species the shell is long and the ventral side is flat. The shell is hyaline with occasional brown or rose patches. The portion caudal to the lateral spines is large, and a true caudal spine is not distinguishable from the rest of the shell. The broadest portion is found between the lateral spines in the middle of the shell. The lateral spines are not elongated and only slightly developed. The small vault of the ventral side ends in a transverse groove under the ventral lip. This lip is also bent, but does not curl ventrally. Transverse ribs are absent on the ventral side, nor is there a flattened area near the lateral spines. The shell is bent dorsally in its posteriorly half. The dorsal side shows no longitudinal ribs, except for five wavy ribs near the upper lip. This upper lip is not separated from the dorsal side; it is truncated though it seems to be cut off in most specimens as its border is finely dentated. The aperture is, in comparison to other Cavolinia species, small and does not reach far posteriorly. The closing mechanism is present in the shell aperture, but it is not marked by a bulging of the dorsal side. Growth striae are clearly visible on the ventral and dorsal side. The embryonic shell is present in some adults, in others it is absent. In the latter, the sculpture is closed by a callus resembling a rounded embryonic shell. The real embryonic shell is, however, characterised by a small incision which separates it from the rest of the shell. The radula (formula 1-1-1) is composed of about 9 transverse rows of teeth.
Shell measurements: up to 7 mm long, up to 5 mm wide.

Morphology and Structure

The chromosome number is 24 (2N) (Thiriot-Quievreux, 1988)

Juveniles

The juveniles are dorso-ventrally compressed, pyramidally shaped in ventral view (Cavolinia i. inflexa juvenile). The whole shell is bent slightly dorsally, the aperture is oval, and forms in older stages two lateral slits in a posterior direction. Protoconch I and II are both transversely striated. In dorsal view the aperture rim is bent but less strongly than in Diacavolinia longirostris and it is not circular (Cavolinia i. inflexa protoc.2, Cavolinia i. inflexa protoconch).
A comparison of these juveniles with those of Cavolinia inflexa is given by Troost and Van der Spoel (1972). See the illustrations:development Diacavolinia, development Cavolinia for a full explanation of the development of these two genera

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is phytophagous. The temperature range is between 16°C and 28°C while the salinity range varies between 35.5 °/oo and 36.6 °/oo.

Distribution

The species as a whole, shows a warm water distribution while the forma labiata shows the tendency for a bisubtropical distribution, see the Cavolinia i. inflexa map. The forma inflexa is the most common form in the N-Atlantic and at the same time it is the most common representative of the genus Cavolinia in the Atlantic. The forma labiata is more abundant in the South Atlantic, South Indian and Pacific Oceans. The forma imitans is found in the Atlantic and near Zanzibar. The N-S range is the same in all formae. In the Atlantic they occur between 55°N and 45°S, while the species is absent in the Benguela Current and off the north coast of S America. The influence on the northern boundary by the Gulf stream is only small. In the Mediterranean a population of the forma inflexa occurs which does not penetrate into the E-Basin of the Mediterranean and the N-Adriatic Sea. The forma labiata is found also in the Mediterranean. In the Indian Ocean a population of labiata is found between 0°N and 40°S, and a smaller population occurs in the central and S-Arabian Sea. The S-Indian and Atlantic populations are connected by the population in the Agulhas Current, composed of a mixture of the formae inflexa and labiata. The population of the Indonesian Seas connects with the Pacific population which is most abundant north of New Zealand and in the W-Pacific. In the Pacific the species does not occur north of 40°N, in contrast to the Atlantic population which reaches 55°N. Only in the W-Pacific is the forma labiata found in the equatorial area, but this does not prove that it is not bisubtropically distributed as more records of other bisubtropical species are known.
Diurnal vertical migration is reported between a mean day depth of 88 m and a mean night depth of 98 m. A completely different vertical migration pattern and distribution is known for this formae in the Mediterranean.

Geological Record

This species is known from the Late Quaternary (last interglacial 100.000 BP) of the Red Sea, and Pleistocene from the Mediterranean. Probably this forma occurs during the Boreal in the Adriatic Sea

Types

Hyalaea inflexa Lesueur, 1813: 285, pl. 5, fig. 4.
The types could not be located.

Cavolinia inflexa inflexa