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Diacria trispinosa (De Blainville, 1821a) forma trispinosa (De Blainville, 1821)

Overview

This is a medium sized, pelagic snail with typical colour patterns, 1 cm broad. The shell is uncoiled but bilateral symmetrical with a long, sometimes broken off, caudal spine. The rim of the aperture is chestnut brown, the rest of the shell is slightly transparent and chestnut brown on thicker places such as the lateral ribs and the tops of the longitudinal ribs. It feeds on small plankton mainly phytoplankton in the epi- and mesopelagic zone of all temperate and warmer oceans (Diacria t. trispinosa dorsal, Diacria t. trispinosa 1,
Diacria t. trispinosa 3)

Taxonomic Description

This form has a long, slit-like aperture (Diacria t. trispinosa ventral lip, Diacria t. trispinosa aperture part) in the strongly dorso-ventrally compressed shell. It is not separated into three parts by a closing mechanism but is continuous (Diacria t. trispinosa vent aper area, Diacria t. trispinosa). The rim of the aperture is chestnut brown, the rest of the shell is only slightly transparent and chestnut brown is present at thicker places such as the lateral ribs and the tops of the longitudinal ribs (Diacria t. trispinosa 1). Specimens darker in colour may be intermediates with Diacria t. atlantica (Diacria t. intermediate). At the place where the closing mechanism is found in Cavolinia the aperture borders in the present species almost join. The intact shell has a extremely long caudal spine with a rounded embryonic shell. The lateral spines are long extensions of the lateral ribs, perpendicular to the shell axis. The aperture border is thickened especially dorsally, the ventral border is bent outwards ventrally. The dorsal aperture border protrudes above the ventral one. The dorsal side of the shell is rounded but less swollen than the ventral side. Three ribs are present ventrally, and five ribs dorsally. The juveniles differ slightly from those of the other species of the genus. The most typical difference is that the embryonic shell is perfectly round in the present species and more oval in Diacria quadridentata. The Diacria t. trispinosa radula formula is 1-1-1, it is composed of about 9 transverse rows of teeth.
Shell length is 8 mm, the width is 10 mm, the ratio upperlip-spine tip/spine tip-membrane is 1.3, spine mark width is 1.5 mm, the aperture height is 0.9 mm.

Juveniles

The juvenile has a long dorso-lateral flattened needle-like shell with lateral sides diverging only anteriorly. The protoconch I is globular. There is no sculpture on protoconch I or II. The lateral sides of protoconch II are thickened.

Reproduction

This form is a protandric hermaphrodite.

Ecology

This form is phytophagous form living in the epi- to mesopelagic zone with distinct vertical migration. Diurnal vertical migration of adults is found between 0 and 500 m. The shell is frequently covered by hydropolyps. It swims with two wings and sinks with retracted wings.

Distribution

Found between 40°N-40°S in all oceans. In Central water masses it is less abundant, as it is in these waters replaced by Diacria major, see the Diacria t. trispinosa map.

Geological Record

This species was known from the Late Quaternary (last interglacial: 100.000 BP) of the Red Sea, and the Pleistocene of the Mediterranean, and Adriatic. From the Adriatic this forma disappeared in the Atlanticum. It penetrated the Adriatic already in the Older Dryas. It was found in the Mid Pliocene of Italy.

Others

The hydroids Laomedea striata (Clarke, 1907) or Kinetocodium danae Kramp, 1922 are sometimes found on these shells (Diacria t. trispinosa w/hydroid), and Campaniclava clionis is also mentioned as an epibiont (Campan.clionis).

Types

Hyalaea trispinosa De Blainville, 1821: 82.
Types are not located.

Diacria trispinosa trispinosa