Notobranchaea tetrabranchiata (Bonnevie, 1913)
Overview
This is a large naked pelagic snail, up to 1.6 cm long, with an opaque body, the visceral organs may be seen through the body wall. The anterior tentacles are small. There are no buccal cones. The posterior gill has four well developed crests. It is a good swimmer that hunts zooplankton, and lives in all oceans (Notobranchaea tetrabranchiata).
Taxonomic Description
Head parts short and thick, no clear indication of a neck region. Skin smooth, unpigmented. Buccal cones and lateral gill absent. Posterior gill with four undulating but not fringed crests, which converge into a terminal knob. Foot composed of two broad based lateral lobes which are fixed horizontally near the neck region, and a rather long, narrow pointed posterior lobe. The very shallow vestibulum and buccal cavity are partly surrounded by thick circular muscles. The anterior tentacles are described as having a dirty green colour. Wings are, as normal in the genus, narrow at their base and broadly truncated at the end. The visceral mass does not reach completely to the caudal body pole. Radula formula is 6-1-6 or 5-1-5. (Notobranchaea tetrabranchiata a, radula after Bonnevie, Notobranchaea tetrabranchiata radula and jaw) The median plate has a semilunar shape with one or three small central cusps and dentation on its free border. These central cusps are, however, so small that it is best to consider them as a central group of denticles between the two lateral groups. There are about 30 transverse rows of teeth. The hooks are short, robust, curved and pointed, about 14 in each sac. The hook sacs are shallow with a weekly developed musculature. The hooks are hollow. The "clear cells" dorsally of the hook sacs are present. Massy (1917) gives a more detailed description of this buccal part. The jaw is composed of a row of conical, somewhat triangular denticles, about 16 in number. These dents are nearly straight, hollow and with "growth-lines" at their base. The Notobranchaea tetrabranchiata penis in both type specimens was evaginated and described by Bonnevie as forming a broad folded lobe. Along its margin a groove leads up to a short finger-like protrusion. Tesch (1950) described an accessory penial sucker which is perhaps the "finger-like protrusion" found by Bonnevie (1913). Along the border of the sucker, large glandular cells are present and the bottom of the sucker is partly provided with muscles, according to Tesch (1950). According to Pruvot-Fol (1926), the sucker of the penis is homologous with the median tubercle on the foot. Tesch (1950) described a penial sucker for Notobranchaea tetrabranchiata, while Meisenheimer (1905), Tesch (1913) and Pruvot-Fol (1942) described a median foot tubercle respectively for Notobranchaea valdiviae and Notobranchaea tetrabranchiata. In our opinion, Tesch (1950) gave a correct description, and the penis of Notobranchaea tetrabranchiata should be considered the same type as in Clione limacina. The "median tubercle" described by Bonnevie is in all probability a structure composed of three folds of the lateral and median footlobes. It is still amazing that these folds resemble the median tubercle of Notobranchaea grandis so clearly.
Notobranchia tetrabranchiata as described by Pruvot-Fol (1942) is considered to belong to another species as buccal cones are mentioned for these specimens from the Dana Expeditions.
Body length up to 16 mm.
Juveniles
A special description is not available.
Reproduction
This species is a protandric hermaphrodite.
Ecology
This species is a carnivore.
Distribution
This species distribution is comparable to species such as Pneumodermopsis ciliata. A tendency to a more neritic distribution may be present, though the records are scarce. It is found in the Atlantic, Indian, and W-Pacific Oceans between 60°N-45°S, see the Notobranchaea tetrabranchiata map.
Types
Notobranchaea tetrabranchiata Bonnevie, 1913 (1933): 63, pl. 8, figs. 61-62.
Lectotype: ZMVB 30432 sectioned (alcohol collection). Paralectotypes: ZMVB 17562, 1 spec. damaged (alcohol collection) and 3 slides with buccal organs.
Type locality: 48°29'N 13°55'W and 45°26'N 20'W, Coll.: CMSE, stat. 10 and 42.