Diacria trispinosa (De Blainville, 1821a) atlantica dark form Hilgersom and Van der Spoel, 1987
This is a medium sized, pelagic snail with typical colour patterns, 1.2 cm broad. The slender shell is uncoiled but bilaterally symmetrical with a long, sometimes broken off, caudal spine. It is almost completely chestnut brown ventrally with the same colour (except for a white spot on the anterior dorsal side) dorsally on the ribs and between the ribs. The lateral and caudal spine are colourless. It feeds on small plankton, mainly phytoplankton, in the epi- and mesopelagic zone of the Atlantic Ocean upwelling areas (Diacria t. trispinosa dark form).
It is almost completely chestnut brown ventrally with the same colour (except for a white spot on the anterior dorsal side) dorsally on the ribs and between the ribs (Diacria t. atlantica (DF)). The lateral and caudal spine are colourless (Diacria t. atlantica w/ caudal spine). The relative large shell is dorso-ventrally flattened and has three spines. Its overall colour is dark brown. The aperture is slit-like and runs between the lateral spines.
Shell length is 9.5 mm, width is 12 mm, ratio upper lip-spine tip/spine tip-membrane is 1.15, spine mark width is 1.05 mm, aperture height is 0.75 mm.
The juvenile has a long, dorso-lateral flattened needle-like shell with lateral sides diverging anteriorly only. Protoconch I is globular. There is no sculpture on protoconch I or II. The lateral sides of protoconch II are thickened.
This form is a protandric hermaphrodite.
This animals are phytophagous and coldwater epipelagic. The shell is frequently covered by hydropolyps. It swims with two wings and sinks with retracted wings.
This forma is restricted to the N-Atlantic Ocean in the upwelling waters off NW-Africa, see the Diacria t. atlantica DF map.
Diacria trispinosa forma atlantica dark form Hilgersom and Van der Spoel, 1987: 99, fig. 4. Types were not indicated.
Type locality: 21°45'N 18°05'W, Nectar exp. III stat 2, 10-3-1982.