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Clione limacina (Phipps, 1774) subsp. limacina (Phipps, 1774) forma minuta Pruvot-Fol, 1926

Overview

This is a naked pelagic snail, up to 0.5 cm long, with a mainly transparent body that only shows orange/red colour in the tail, tentacles and mouth organs. The reddish brown visceral mass is seen through the body wall. It is a rather quick swimmer that hunts shelled pteropods as food. It lives in the N-Atlantic in the upper water layers where it can occur in mass blooms (Clione l.l. minuta 2).

Taxonomic Description

Body more droplet-shaped than in the forma limacina, no gills present in this very small form. The reddish and yellowish colour of the visceral mass is less intense than in the forma limacina. The wings are broad and large. Lateral footlobes moderately developed, the posterior footlobe is rather short with a thin, terminal filament, sometimes reduced by contraction. There are three pairs of buccal cones. The radula formula varies between 6-1-6 and 9-1-9. The median plate is variable but more distinctly dentated than in the forma limacina, and a median cusp or an irregularity is absent (Clione l.l. minuta radula, Clione l.l. minuta radula 2, Clione l.l. minuta radula 3). The number of transverse rows is smaller than in the forma limacina. The hook sacs are large and rather deep, bearing at each side 15 to 20 hooks.
Body length up to 5 mm.

Morphology and Structure

The note on anatomy covers the whole genus. Gills are lacking and respiration is effected through the whole body surface, as the whole body forms a wide blood-filled haemocoele. The circulatory system as a consequence is very simple. A heart is found with very thin pericard posteriorly to the visceral mass on the right side. The renal opening is near the anus. The eyes are reduced and difficult to trace in serial sections. The buccal cones are adhesive tentacles extruded by the pressure of the body fluid and retracted by muscles which are attached to the buccal wall. In the lumen of the cones there are unicellular glands besides the muscle cells. The hook sacs are surrounded by longitudinal and circular muscles. The radula sac, behind and posterior to the hook sacs, is a groove above the two rod-like cartilaginous structures forming the odontophore. The salivary glands are long strips opening at the sides of the radula sac. Subepithelial mucous glands open through the sides and the floor of the buccal cavity in front of the odontophore. The narrow oesophagus opens into a reduced stomach. The intestine is narrow and strongly ciliated. Relatively small faecal pallets are passed without any mucus binding.
The hermaphroditic reproductive system is a primitive opisthobranch Bauplan. The gonad is found as a conical "sac" beneath the visceral mass, the central part of which is occupied by male products. In older animals the periphery is crowded with small acini containing developing oocytes. The gonoduct is narrow and may distend like a bursa seminis. The accessory gland is divided into two parts. Through the first part, in which the gonoduct opens, both ova and sperma pass. It is lined with large squarish cells staining pink at the base and with the secretion spherules blue after Azan stain. At some places scattered ciliated cells lie around the lumen. This gland evidently lays down the thin nutritive coating around the eggs before they pass through the mucous gland. This gland is a large horseshoe-shaped caecum, blind at either end, and opening narrowly towards the first gland. The dimensions of the gland corresponds well with the oblong gelatinous egg strips (1 to 1.2 mm long) with 30 to 40 eggs. From the albumen gland a narrow vagina runs to the body wall at the right side. At the side of the vagina a receptaculum seminis is present. The penis is well developed. Normally it is stored in the glandular retractor sheet. When evaginated, a long stalked sucker is seen with the proper penis at its base. The sucker is disc-shaped, supported by a long stalk surrounded by a transparent mantle. The penis is composed of a pyramidally shaped central part in which a central and a lateral sustentacular bar is seen composed of vacuolated, cartilaginous-like tissue. Along this penis the seminal groove runs from the base to the top in a S-shaped curve. At both sides of the tip two ciliated lobes are found. At the body side of the penial base a tuberculated lobe is found while at the opposite side a glandular lobe surrounds the base. The sucker is connected to the body integument with muscles, while the penis is attached by an irregular muscle to the stalk of the sucker. The sheath of the penis is partly glandular and functions as a prostate, though the real prostate is attached as a glandular tube to the retractor sheath.

Juveniles

A special description is not available.

Reproduction

The present forma is a protandric hermaphrodite. It is mature when 3 mm long, while the forma limacina is mature when 12 mm long. The forma minuta spawns in the open ocean, while the forma limacina tends to spawn over shallow waters. Spawning occurs in summer and each animal may spawn more than once. In the laboratory the eggs are laid overnight. The eggs are embedded in a transparent oblong or square ribbon, of 1 to 1.2 mm. The colourless eggs are placed irregularly in the ribbons. After two or three days the larvae emerge. Newly hatched larvae have thimble-shaped shells of slightly irregular shape, 0.15-0.16 mm long and 0.11 mm wide. These shells are transparent and pointed at the top. The bilobed velum is edged with larger cilia and below this row a single row of smaller cilia is found. These two rows form a groove to the mouth. In this stage the animal is capable of complete retraction into its larval shell. In the body a ciliated foot, stomach, intestine, liver and two statocysts are visible. About three days after hatching the shell has grown to a length of about 0.18 mm and the aperture margin shows fine longitudinal striation. After some days the shell probably cast off, the whole body is then covered by fine cilia. After 18 days, when the body length is 0.32 mm, the velum disappears. When the animal reaches about 0.5 mm, the buccal cones, tentacles, hooks and radula have developed, while the wings start to develop from the ventral footparts.


Ecology

This forma feeds on Limacina retroversa and Limacina helicina. Juveniles are found to feed also on Prorocentrum micans. Feeding of this and the preceding form was well studied by Conover and Lalli (1972, 1974). This species assimilates carbon from its prey L. retroversa, with greater than 90% efficiency and nitrogen with nearly 100% efficiency.

Distribution

This forma is found in the N-Atlantic, see the Clione l.l. minuta map.

Types

Clione minuta Pruvot-Fol, 1926: 25, pl. 2, figs. 48-54 bis, pl. 3, fig. 100 (Clione l.l. minuta paralectotype).
Lectotype: MOM dissected and originally figured (alcohol collection) 1) Paralectotype: MOM, 1 spec. dissected 2)
Type locality: 58°50'N 4°50'E l) and 57°30'N 3°50'E 2), Coll.: CPAE, stat. 2799 1) and stat. 2486 2).

Clione limacina limacina minuta