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Cephalobrachia macrochaeta Bonnevie, 1913 (1933)

Overview

This is a rather large naked pelagic snail, up to 1 cm long, oval to barrel-shaped and mainly transparent. The posterior footlobe and lateral lobes are small. There are no gills. The hook sacs are very large, with large gold coloured hooks. It hunts zooplankton as food. It lives in the Atlantic and Indian Oceans in the upper water layers (Cephalobrachia macrochaeta drawing).

Taxonomic Description

Body barrel-shaped, gills are absent. In young specimens circular ciliated bands are present. The mostly blackish visceral mass fills the whole body (Cephalobrachia macrochaeta). When the hook sacs are invaginated, both body poles are rounded. The small lateral footlobes coalesce in front and form a single lobe, fixed anteriorly. The posterior footlobe is small and pointed, when present; all footlobes are unite. A median tubercle is not found on the foot. The wings are large, narrow at the base and also tapering at the free tips. The radula is placed on a large conical odontophore, it has the formula 3-1-3 and about 12 transverse rows are present. The lateral teeth are horn-shaped and in some laterals the basal plate is obscure or absent. The median plate has at its free border a blunt cusp and a number of small denticles. The radula teeth are extremely large. Buccal cones or other appendages are absent. A jaws are absent. The hook sacs are very large and rich in muscle tissue. The hooks, about 40 at each side, are of different sizes, the largest hooks may reach a length equal to one fourth of the body length. When evaginated the hook sacs form two powerful arms, diverging from the buccal cavity. A lip of glandular cells, ventrally to the radula, is homologous to the jaw. This lip forms a bow across the ventral side of the proboscis, at the base of the odontophore between the hook sacs.
Body length up to 10 mm.

Morphology and Structure

Brownish to black ink (see below) may be delivered through the mouth. The intestinal system in C. macrochaeta shows two typical structures, viz. a blind sac near the connection of stomach and hindgut and a glandular sac in the middle of the hindgut. The blind sac close to the beginning of the hindgut is also described for Hydromyles globulosa by Meisenheimer (1905); an epithelium similar to that of the liver-stomach complex is observed by Meisenheimer. Lalli and Gilmer (1989) mentioned this blind sac as the possible source of ink. In Cephalobrachia macrochaeta the blind-sac entrance is covered with a ciliated epithelium that is continuous with the hindgut epithelium. The inner epithelium is similar to that of the liver-stomach cavity. The only remarkable structure of this blind sac is that it has a circular closing muscle around the opening to the stomach. It is difficult to believe that the epithelia of this blind sac produces an 'ink-like substance' as they are so similar to liver cells. This blind-sac was hitherto considered unique in Hydromules globulosa, which is not correct.
The middle of the hindgut is slightly swollen in many Gymnosomata (Meisenheimer, 1905; Tesch, 1913), but in the literature no special epithelia are described from this part of the hind gut. In Cephalobrachia macrochaeta there is a large sac in the middle of the hind gut covered with a characteristic glandular epithelium. It consists entirely of flat glandular epithelial cells with large secretion granules in the active cells. Stained in Mallory-Heidenhain these spherules are orange. It is also remarkable that the opening of the hindgut to the stomach and the anus are both provided with circular closing muscles. The delivery of the contents of the blind-sac and of the glandular sac, together with the hindgut, is effected by contraction of the transverse body muscles that are very numerous in this area. Regulation of the ejection through the anus as well as through the mouth is possible as there are closing muscles at the openings.

Juveniles

A special description is not available.

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is a carnivore. Ink ejection in molluscs is well known in the Cephalopoda, it also occurs in Cephalobrachia macrochaeta. A specimen was held for some hours in the aquarium and production of an ink cloud at the moment the specimen was photographed with flash light was observed. See the illustration Cephalobrachia macrochaeta producing ink.

Distribution

It is bathypelagic occurring in warmer waters (65°N-10°S) below 1000 m and in colder areas also between 400 and 750 m. The distribution as represented here cannot be a complete picture as it is discontinuous. In all probability this is a cosmopolitan deep-sea species, see the Cephalobrachia macrochaeta map.

Types

Cephalobrachia macrochaeta Bonnevie, 1913 (1933): 61, pl. 7, figs. 53-58.
Lectotype: ZMVB 17563 1) (alcohol collection) Paralectotypes: ZMVB 30431.2), 1 spec. (alcohol collection) and 2 slides with buccal organs.
Type locality: 48°29'N 13°55'W l), 48°74'N 36°53'W 2) Coll.: CMSE, stat. 82 2), 92 1). Specimens mentioned as syntypes from CMSE station 88 are lost.

Cephalobrachia macrochaeta