Cardiapoda placenta (Lesson, 1830)
This is a large naked pelagic snail with a completely transparent cylindrical body with one swimming fin, up to 5 cm long. The shell is very small and embedded in the visceral nucleus. Only the broadly rounded visceral nucleus is darkly pigmented, the large eyes, with a black retina, and the mouth organs are clearly visible. The intestine is frequently full and visible. It hunts large prey such as fishes, shelled pteropods and other heteropods. It lives mostly in the upper layers in the warm waters of all oceans (Cardiapoda placenta shell).
It is a robust, cylindrical animal, and the visceral nucleus is not protected by a shell (Cardiapoda placenta 1). A small shell is found composed, as in Carinaria, out of a proto- and teleoconch on top of the nucleus. The protoconch is regularly coiled and dextral with about 3 whorls with slightly elevated apex. The teleoconch is flat, roughly triangular with a membraneous shell growing perpendicular to the aperture of the protoconch. The visceral nucleus is on a long stalk and projects posteriorly to the fin which is attached opposite the origin of the stalk (Cardiapoda placenta). The muscles in the fin are fused to form a homogeneous mass. The fin is round and bears a sucker in both sexes. The visceral nucleus is rounded and encircled by a row of gills. There are numerous gills. The tail is short and ends in a star-shaped expansion, probably homologous to the clasper in Carinaria lamarcki. The eyes are relatively small, slightly triangular and are slightly broader than the lens. The position of the retina is slightly oblique to the axis of the eye. The left and right tentacle are present. The cutis of the body is thin and smooth except for some tubercles in fully grown specimens. The Cardiapoda placenta radula formula is 2-1-1-1-2. The (Cardiapoda placenta radula 1) shows a tricuspoid median plate.
Maximum body length 50 mm.
The juveniles have a small dextrally coiled shell (Cardiapoda placenta juv.). The shell has three whorls in the juvenile which form a very flat spire. One spiral line, close to the suture, follows half a whorl. The width of the last whorl is half the shell diameter. The velum has 6 lobes with a light brown rim and two brown spots each. The tentacles are brown. The rest of the animal is nearly colourless.
In this species the sexes are separate.
This species is carnivorous and epipelagic.
When comparing the disjunct occurrence of this species with Cardiapoda richardi it is apparently obvious that C. placenta is neritic and Cardiapoda richardi more oceanic. Cardiapoda placenta occurs between 40°N and 40°S. Its absence in equatorial areas differs than in e.g. Carinaria cristata which is absent in equatorial currents, but probably present in subtropical areas. Cardiapoda placenta is frequent along the coast and thus absent in the centre of oceans. See the Cardiapoda placenta map.
Pterotrachea placenta Lesson, 1830: 253, pl. 3, fig. 2.
Type locality: New Guiney.