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Diacavolinia mcgowani (Van der Spoel, 1973g)

Overview

This is a medium sized, uncoiled thecosomatous pteropod, 0.4 cm long. It has a flat dorsal side with moderately developed ribs. The ventral side is vaulted. The caudal spine is absent and a caudal spine mark is not left as the ventral and dorsal sides grow together. The lateral spines are extremely long spade-like. Shell sculpture consists of faint growth lines and a faint transverse striation. The short dorsal lip has no notch, or constriction but a gutter is present. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in Pacific Ocean at shallow depths (Diacavolinia mcgowani 2).

Taxonomic Description

The hyaline shell has a faint yellowish hue; it is slightly triangular (Diacavolinia mcgowani line) but its shape and size are variable (Diacavolinia mcgowani). The relative short dorsal lip has no notch in the broad rostrum and no constriction, but is gutter-shaped. The ventral lip is normal sized and has a small median depression. The lateral spines are extremely long (Diacavolinia mcgowani lateral spine, Diacavolinia mcgowani lateral spine 2), not curved dorsally, but blunt and composed nearly entirely by growth of the ventral side; the gutter corners are small. The seven weak ventral ribs are line-shaped (Diacavolinia mcgowani ventral ribs). The lateral lines are convex and the flanks are not developed. The dorsal side is slightly convex, with weakly developed central and latero-dorsal ribs; the lock rib is small. The lock area is well developed, the main tubercle is well developed like the lunar one, and the minor tubercle is reduced. A second lock system is absent. The growth lines are faint. A hump is absent. The protoconch II area strongly projects with a marked caudal joint. In the holotype it is 1.04 mm; the caudal fold, 0.88 mm long, is nearly straight. There is no lip flap; the lip belly is small and the lip shoulder is small or absent. The aperture is wide. In the holotype the lip angle is about 144°, the lock angle is 65°, and the side angle is 62°. The holotype has a shell length of 4.20 mm and a width (spines included) of 4.82 mm.
The shell length ranges from 4.20 to 3.36 mm and the width from 4.82 to 2.80 mm.

Juveniles

A special description for this form is not available, see Diacavolinia longirostris.

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is phytophagous and epipelagic.

Distribution

Diacavolinia mcgowani is restricted to the E-Pacific Ocean, and only recorded from its type locality, in the oxygen minimum area, see the Diacavolinia mcgowani map.

Others

The type specimens, originally preserved in formalin, were put into alcohol in 1973 but still show decay. The paratypes are nearly dissolved, the ventral side of the holotype has partly detached.

Types

Cavolinia longirostris forma mcgowani Van der Spoel, 1973: 100, figs. 14.
Holotype (ZMUC. j 1973) and two damaged paratypes (ZMUC. j 1973) in ZMUC.
Type locality: 6°49'N 80°25'W, 0-100 m depth, CDAE. Stat. 1205 v, 14 January 1922.

Diacavolinia mcgowani