Cavolinia tridentata (Niebuhr, 1775) forma affinis (d'Orbigny, 1836)
This is a shelled pteropod, 1.15 cm long, living at moderate depth. It is one of the smaller forms of this species. Dorsal surface strongly faulted, with three moderately developed central and two faintly developed lateral dorsal ribs. The caudal spine is straight or curved and relatively long. Upper aperture lip rounded, no keels along the lateral sides. Ventral side strongly vaulted in lateral view. Aperture triangular lunar-shaped. The sculpture consists of growth lines and many transverse striae on the ventral surface. Shell sculpture consists of faint growth lines and a very faint transverse striation. It is a mucus feeder on micro-zooplankton and phytoplankton. It lives in the SE-Pacific Ocean (Cavolinia t. affinis drawing).
Cavolinia tridentata forma affinis is small in relation to the other formae of the species, though the average size seems larger than in the forma occidentalis. The caudal spine is curved dorsal in some specimens, but is usually straight and projects slightly caudad, so that its axis makes an angle with the shell axis (Cavolinia t. affinis line drawing, Cavolinia t. affinis shell, Cavolinia t. affinis dorsal, Cavolinia t. affinis, Cavolinia t. affinis lateral, Cavolinia t. affinis adult). This oblique position is typical for all the Indo-Pacific formae though it always occurs to a lesser extent in the forma occidentalis. The usually opaque shell shows a slightly vaulted dorsal surface with typical longitudinal ribs. The dorsal aperture lip is rounded and the lateral sides has no keels. The lateral spine points are not, or only slightly, directed caudad. The arch-shaped rib at the base of the dorsal aperture lip is not as strong as in most of the other formae. The shell margin, between the caudal and lateral spines is concave. The aperture has a lunar shape. The ventral aperture lip is relatively small while the ventral side is strongly vaulted. This side is, however, not circular in lateral view as in forma occidentalis where the greatest shell thickness is found anterior to the middle. Growth lines and shell sculpture are very faint. There are numerous transverse striae on the ventral side. The shell colour is light purple brown hue over the whole shell.
The shell length varies between 8.5 and 11.5 mm, the width varies between 6.0 and 8.0 mm.
A special description is not available.
Cavolinia tridentata forma affinis is a protandric hemaphrodite.
Cavolinia tridentata forma affinis is phytophagous and epipelagic.
This forma occurs in the SE-Pacific, see the Cavolinia t. affinis map.
Hyalaea affinis d'Orbigny, 1836: 91 (1846) pl. 5, figs. 6-10.
The two syntypes a restored dry in the MHNP, the adult specimen (11.4 mm length, 7.2 mm width) is indicated as lectotype, the young specimen (11 mm length, 6.8 mm width) is indicated as paralectotype.
Type locality: South Pacific between 30° and 34°S and between 80° and 92°W.