Creseis acicula (Rang, 1828b) forma acicula (Rang, 1828b)
Overview
This is a shelled pelagic snail, up to 4 cm long, with a transparent uncoiled shell. The cross-section is round. The surface is smooth. The shell is extremely long and narrow, tube-shaped and not curved. The visceral mass is seen through the shell. It is a clumsy swimmer that feeds on phytoplankton and protozoa. It lives in the warmer waters of all oceans in the upper water layers (Creseis a. acicula 1).
Taxonomic Description
These extremely long shells have a rounded aperture. The shell is not perfectly straight as there are many small aberrations in its shape caused by irregular growth. The columellar muscle runs free through the shell lumen over a very long distance; there is a great distance between the point where it is attached and the point where the muscle disappears into the visceral mass (Creseis a. acicula 2). The embryonic shell differs in shape from that of the related forma clava, as the caudal end is composed of an oval shaped embryonic part with one or two ring-shaped swellings cranially. Between the oval part and the swelling, the shell is somewhat thicker and more chestnut-brown than in the other shell parts. The embryonic shell is sometimes, besides the swelling, separated by a clear transverse ring. The radula formula is 1-1-1, there are about 7 transverse rows of teeth. The rear angle of the shell is very small, 13-14°. Wing protrusions at the anterior border are present with a wing gland opposite of it. This gland differs in the related forma clava, as it does not touch the base of the protrusion, while in the forma clava it evidently touches.
The shell is needle-like, up to 33 mm long and 1.5 mm wide.
Morphology and Structure
The pallial gland resembles that of Hyalocylis striata by having a broad zone of cuboid cells with a zone of jigsaw-puzzle cells caudally with a zone of jigsaw-puzzle cells above. The chromosome number is 20 (2N) (Thiriot-Quievreux, 1988).
Juveniles
The cylindrical juvenile consists of a long, blunt ending protoconch I and a somewhat broader cylindrical protoconch II and only very faint incisions separate protoconch I and II and teleoconch (Creseis a. acicula 3). See also the description above.
Reproduction
The forma is a protandric hermaphrodite. Twenty chromosomes are found in the diploid. The spermatozoids are of two types, the first has 10 chromosomes, the other 9. In the first cleavage the 20 chromosomes are divided into two pairs of 10, in the second cleavage the 9 larger chromosomes are divided into two pairs of each 9, the smaller ones do not split in making one of the two pairs complete up to 10.
Ecology
In Triest the juveniles are found in July and in the Gulf of Naples in September. Breeding in the subtropics takes place in the summer. The species is phytophagous and epipelagic. It occurs in swarms and is sometimes abundant in neritic waters (Pillai and Rodrigo, 1979). The temperature and salinity ranges are about 10.0°-27.9°C and 35.5-36.7°/oo S, respectively. In the Mandapan Lagoon (9°17'N-79°E), this species lives between 26°-33°C and a salinity between 25-45°/oo. Mass swarming in coastal waters is a normal phenomenon. Swarming is sometimes synchronised with that of Penilia avirostris and blooms of Trichodesmum (Sakthivel and Haridas, 1974).
Distribution
This form is circum-global 50°N-45°S, see the Creseis a. acicula map. It is clear that in the distribution of the species the formae are spread regularly and there is no indication that the formae have separate ranges. There is doubt on the taxonomic value of these formae, which however, can be morphologically distinguished. The species as a whole shows a continuous distribution in warm waters. In the Sargasso Sea and the Benguela Current only a small number of samples of this species have been recorded and in the centres no records are known. In contrast to most species, Creseis acicula even breeds in the North Atlantic. The range is that of a warm water species that is not clearly bisubtropical. However, when abundance is considered, a bisubtropical tendency is clear. This species is, in general, less stenotherm than Creseis virgula and penetrates further north and south than Creseis virgula. In the east tropical Atlantic and close to the equator it seems less abundant than Creseis virgula.
Geological Record
This form is found in the Pleistocene and the Pliocene (Plaisancien) of the Mediterranean; and in the Late Quaternary (from 140.000 BP to present) of the Red Sea. It penetrates the Adriatic already in the Older Dryas.
Others
Neurosecretion is described for this species.
Type
Cleodora (Creseis) acicula Rang, 1828: 318, pl. 17, fig. 6.
Lectotype: MHNP (dry collection). Paralectotypes: MHNP, 2 spec. and 18 fragments (dry collection).
Type locality: "Mer des Indes" (Indian Ocean). Coll.: Rang [1837].