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Hyalocylis striata (Rang, 1828b)

Overview

This is a shelled pelagic snail, up to 0.8 cm long, with a completely transparent uncoiled, fragile shell. The cross-section is round. The surface has transverse undulations. The shell is long tubular shaped and only slightly curved. The visceral mass is seen through the shell. It is a good swimmer that feeds on phytoplankton and protozoa. It lives in the warm upper waters of all oceans, where it can occur in mass blooms (Hyalocylis striata 1, Hyalocylis striata 3, Hyalocylis striata drawing, Hyalocylis striata black and white photograph).

Taxonomic Description

Shell is curved faintly dorsally, fragile and cylindrical. The cross section of the shell is circular. The embryonic shell is thrown off in adults and a closing membrane is formed near the place of rupture (Hyalocylis striata soft parts, Hyalocylis striata 3). The concave closing membrane is punctuated at the surface. The embryonic portion leaves a completely circular mark at the top of the adult shell (Hyalocylis striata 2). The adult shell is transversely striated (Hyalocylis striata 1, Hyalocylis striata struct.) by broad, thickened bands. The distance between the successive bands increases towards the aperture. The shell is round in cross-section. The Hyalocylis striata radula formula is 1-1-1.
The rear angle of the adult shell is 24°, while the shell length is about 8 mm.

Morphology and Structure

The chromosome number is 28 (2N) (Thiriot-Quievreux, 1990)

Juveniles

Juveniles are not frequently reported from the plankton, and adults with parts of the protoconch attached to the shell are extremely rare. The juvenile consists of an oval protoconch I that is somewhat narrower posteriorly than anteriorly (Van der Spoel and Newman, 1990). A drawing of the Hyalocylis striata juvenile and a fossil juvenile shows the relative small size of the embryonic parts. A gradual incision separates the protoconch I from II. There is no difference in structure or shape between protoconch II and teleoconch. In juveniles, above the protoconch I, the shell is broad conical and shows about 8 transverse rings of the same type as in the adult shell. At the end of the larval stage a septum is formed that separates the adult shell from the protoconch which is later lost. Only a round mark (Hyalocylis striata anterior shell part) of the protoconch is left at the caudal end of the shell. The soft parts develop through a skinny or minute stage into the adult. (Hyalocylis striata closing membrane). Creseis chierchiae is not a juvenile of the present species as was incorrectly postulated by Van der Spoel (1967).

Reproduction

The species is a protandric hermaphrodite.

Ecology

The species is phytophagous, epipelagic. The temperature range is 17.4-27.8°C, the salinity is 36.00°/oo S.

Distribution

This species occurs in all oceans between 40°N and 40°S. The Mozambique Current has an important influence on the southward penetration of the species into the W-Indian Ocean, the Agulhas Current transports the population around Cape Agulhas and thus effects a far southward distribution in the Atlantic as well. In the Indian Ocean, directly north of the equator the species is rather rare. West of Chile and west of Panama an isolated record was found. Records off Panama are not known, but this does not necessarily indicate that the species is absent, as this area has been poorly investigated. It is known from the Mediterranean and Red Sea, see the Hyalocylis striata map.

Other

The protoconch described for Hyalocylis striata by Grecchi and Bertolotti (1988) does not belong to this species.

Geological Record

This species is known from the Late Quaternary (from 140.000 BP) to the middle of the Holocene of the Red Sea, and the Pleistocene of the Mediterranean. It is found in the Mid Pliocene of Italy.

Types

Cleodora (Creseis) striata Rang, 1828: 315, pl. 17, fig. 3.
Syntypes: MHNP, fragm. 8 spec.; damage too serious to select lectotype (dry collection).
Type locality: Indian Ocean. Coll.: Rang.

Hyalocylis striata