Diacria costata

Diacria costata (Pfeffer, 1879)

A central water and an equatorial form are distinguished in this species, but not given separately here (Leyen and Van der Spoel, 1982).


This is a small, uncoiled, shelled thecosomatous pteropod, 0.3 cm long. It is globular with rounded ventral and dorsal sides and well developed ribs. The lateral spines are small. The caudal spine is absent as, in juveniles, the protoconch is broken off, leaving only a mark. Shell sculpture consists of faint growth lines and more than four dorsal striae. Micro-zooplankton and phytoplankton are its food and it is a mucus feeder. It lives in the Pacific Ocean (Diacria costata 1, Diacria costata line drawing).

Taxonomic Description

The rather globular shell is red-brown and semi-transparent. The dorsal lip is arch-shaped and borders a very narrow shell aperture. The ventral and dorsal sides are both heavily vaulted, and show striae near the aperture lips. The caudal spine mark is oval. Between the small lateral spines and the corners of the spine mark the curved lateral sides are thickened. Usually a faint median depression is found on the ventral side and lip. The aperture lips are thickened. A pair of closing mechanisms, less developed than in Cavolinia, are present in the aperture. The radula formula is 1-1-1. The dorsal ribs are well developed (Diacria costata dorsal), transverse striation on dorsal and ventral surface is well developed, the ventral side is regularly rounded, lateral spines are less developed, not curved caudally (Diacria costata ventral). Five dorsal striae are present. The distance between these spines is relatively small, dorsal striae are imbricate and number more than four (Diacria costata).
A central water and an equatorial form are distinguished in this species, but not given separately here (Leyen and Van der Spoel, 1982) These forms are mainly separated on the base of the shell sizes.
Shell length 1.9-2.6 mm, shell width 1.72-2.24, diameter caudal spine mark 0.68-0.88 mm.


The Diacria costata juvenile is a long dorso-lateral flattened needle-like shell with the lateral sides only diverging anteriorly. The Diacria costata protoconch is rounded slightly egg-shaped. There is no sculpture on protoconch I or II. The lateral sides of protoconch II are thickened.


This species is a protandric hermaphrodite.


This species is phytophagous.


This species occurs in Central waters of the Pacific Ocean, see the Diacria costata map.


Hyalea quadridentata var. costata Pfeffer, 1879: 234, fig. 5.
Lectotype: HMNH (alcohol collection). Paralectotypes: three spec. HMNH (alcohol collection).
Type locality: restricted to Indian Ocean.