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Diacria major (Boas, 1886a)

Overview

This is a medium sized, pelagic snail with typical colour patterns, 1.1 cm broad. The shell is uncoiled but bilateral symmetrical with a long, sometimes broken off, caudal spine. The colour pattern of the shell covers a relatively smaller part of the larger shell surface so that the shell has a white, hyaline appearance. Only the dorsal aperture rim and the ventral side, along the aperture, are brown/red. It feeds on small plankton, mainly phytoplankton, in the epi- and mesopelagic zones of the Central waters of all oceans (Diacria major 1).

Taxonomic Description

The most important difference between Diacria major to other species of this group, is the more caudal direction of the lateral spines (Diacria major line drawing). The cranial shell portion, or permanent shell is relatively larger, the ventral side is flat and the aperture lips are well developed (Diacria major drawing, Diacria major dorsal, Diacria major ventral, Diacria major living). The colour pattern of the shell occupies a relatively smaller part of the larger shell surface so that the shell has a white, hyaline appearance. Only the dorsal aperture rim and the ventral side along the aperture are brown/red.
The shell length may reach 8.5 to 13 mm, the caudal spine not included, the width is 11 mm, the ratio upperlip-spine tip/spine tip-membrane is 1.65 mm, the spine mark width is 1.2 mm, the aperture height 1.0 mm.

Juveniles

The juvenile has a long dorso-lateral flattened needle-like shell with the lateral sides only diverging anteriorly. The protoconch I is globular. There is no sculpture on protoconch I or II. The lateral sides of protoconch II are thickened. After the juvenile stage a minute stage occurs (Diacria major minute stage) in which the caudal spine (=protoconch I + II) may be still present (Diacria major). In the adults it is thrown off (Diacria major without caudal spine).

Reproduction

Diacria major is a protandric hermaphrodite.

Ecology

Diacria major is phytophagous. The shell is frequently covered by hydropolyps. It swims with two wings and sinks with retracted wings.

Distribution

Diacria major lives in the Central water masses of all oceans, it is epi- to mesopelagic and less common than the related Diacria species, see the Diacria major map.

Types

Hyalaea trispinosa var. major Boas, 1886: 95, 210.
Lectotype: ZMUC (dried out), Paralectotypes: 2 spec. (dried out), selected from material used by Boas in describing var. major.
Type locality: restricted to 25°N 35°30'W Coll.: Andréa.

Diacria major