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Clione limacina (Phipps, 1774) subsp. limacina (Phipps, 1774) forma limacina (Phipps, 1774)

Overview

This is a naked pelagic snail, up to 4 cm long, with a mainly transparent body that only shows orange-red colouration 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 and N-Pacific Oceans in the upper water layers where it can occur in mass blooms (Clione l.l. limacina).

Taxonomic Description

The species has a barrel-shaped body, pointed posteriorly. Gills are absent. The visceral mass reaches no further than half the length of the body. There are three pairs of buccal cones of which the dorsal pair is slightly larger than the ventral ones (Clione l.l. limacina buccal). The cones are arranged so that the base of the three left cones touch, as do the bases of the three right cones. The skin has no chromatophores. According to Massy (1920) the integument in the subsp. limacina is thinner than in the subsp. antarctica, but in larger specimens of the first subspecies a rather thicker skin may be found. The footlobes are well developed, the posterior one is moderately long. The radula formula varies between 6-1-6 and 15-1-15 (Clione l.l. limacina radula after P.F., Clione l.l. limacina radula). The median plate usually has a small central cusp and numerous very small denticles. Hook sacs are medium sized with numerous slightly bent hooks, unequal in size (Clione l.l. limacina hooks).
Body length up to 40 mm.

Juveniles

Only the very young larvae have a shell (Clione l.l. limacina protoconch). The aperture is slightly oval. Protoconch II is nearly as long as protoconch I and becomes increasingly wider, it has many growth lines and longitudinal striae. Protoconch I is oval to round but is always slightly pointed posteriorly. There is no clear incision between protoconch I and II. The development from egg to adult is shown in the illustrations (Clione l.l. limacina devel).

Reproduction

Clione limacina limacina forma limacina is a protandric hermaphrodite. Copulation in the present species, in all probability this forma, was described by Boas (1886a). Two copulating specimens are found with the ventral sides turned towards each other with the heads in the same position. The copulation is mutual, the penis of one specimen is put in the sexual aperture of the partner and the latter puts its penis in the aperture of the former. The penis (p in the figure: Clione l.l. limacina penis) can be retracted in a sheet (s) and it has a penial sucker (ps). In posterior view (A) and anterior view (B) dents are seen on the penial sides. The sucker is rather large (C and D), the penis is beset with tubercles (E). During copulation the two penes are crossed. The penis, the thick and short part, is put in the sexual aperture of the partner, the second and thinner branch, probably the one provided with the sucker, is stretched backwards. It is very strange that the sucker bearing branch is turned backwards as it should be used to hold the other animal. This way of copulation indicates that the animals should be functional hermaphrodites and bursa seminis should be present (cf. Tesch, 1950). For a further description of the penis refer to the forma minuta. The present forma in juvenile stage is found at 41°N 72°W in May and along the north coast of Canada larvae are found throughout the winter months.

Ecology

As in all pteropods locomotion is effected by the wings. Sinking is accomplished by the simple retraction of the wings. Active movements are horizontal and vertically upwards. Usually the wings move synchronously, beating from caudo-ventral to dorso-cranial. With regard to the wing attachment, this is the ineffective beat where the effective beat goes the reverse way. The ineffective upward beat has no influence on the movement of the body as the wing tip is brought upwards in a line quite near the body. The effect of the downward movement is in first instance caused by a twist of the wing at the narrowest point, quite near the body, in such a matter that the ventral surface is directed caudally and secondly by the fact that the wing is curved somewhat caudally. In studying movements in the forma minuta, Morton (1958) found that the upward movement has also an effect. The entire wing movement described by Morton is more complicated than in the forma limacina, by the same principle, but it gives somewhat different components. Movements are rather quick compared to less streamlined species like Limacina retroversa. This form feeds exclusively on Limacina (Clione l.l. limacina feeding).
The hyperiid amphipod, Hyperiella dilatata, uses living specimens of the present species as a chemical defence system (McClintock and Janssen, 1990).

Practical importance

Clione limacina limacina forma limacina can occur in high numbers so that it forms an important food source for fish and whales in the N-Atlantic, for Balaena mysticetus it is a principal food source. The species is named "krill" by Norwegian whalers, this is the same word as used for Euphausia superba in Antarctic waters.

Distribution

Found in the N-Atlantic and N-Pacific Oceans, this area is called the "Clio Region" by Gran (1902); he found associated with this forma: Ceratium arcticum and Limacina helicina. See the Clione l.l. limacina map. In the plankton of this "Clio Region" influences were found from Labrador elements, Boreal oceanic, Temperate neritic, Arctic neritic and Boreal neritic elements. The forma limacina is, however, a boreo-oceanic element, when it penetrates shallow waters, as described by Bigelow and Sears (1939) for the area of Cape Cod, reproduction is sometimes not executed. Most authors, however, found juveniles and reproduction to also occur in shallow waters. The different formae represented by this species are explained by the stenotherm character of the populations (cf. Manteufel, 1937). For the forma limacina a temperature range of -1.7° to +6.2°C was found.
The forma minuta is the second representative of this species also found in the N-Pacific Ocean. The forma meridionalis is typical of deeper waters of the southern part of the range of the species. The formae gracilis, elegantissima and filifera are all restricted to the Boreal N-Atlantic. The horizontal and vertical distribution of the formae is too incompletely known to draw conclusions on the taxonomic status of these taxa and on their zoogeography.
In the northern part of the North Sea between 57°N-59°N and 0°W-3°W population fluctuations of apparently the forma minuta were studied by Glover (1957). This forma appeared to be absent or rare in the years 1947-1950, but common in the years 1951-1955. In the latter period the seasonal maximum was found earlier each year, respectively in late August (for 1952), in early August (for 1953), in late July (for 1954) and in late June (for 1955). The first appearance of this forma in the northern North Sea is about three months later than in the Atlantic Ocean for the same years, so it is possible that it migrates from the Atlantic to the North Sea. After the postlarvae appeared, shelled larvae were found. These shelled larvae had in all probability arised from the first post-larval peak in July 1953 and they give rise to the second post-larval peak in August 1953.
The subspecies antarctica is circumpolar and found in the Subantarctic and Antarctic together with Clio pyramidata forma sulcata, Limacina helicina subsp. antarctica, Spongiobrancheae australis and Sagitta gazellae.

Other

Neurosecretion is described for this species (see the References under key word 'neurophysiology').

Types

Types and type locality unknown.

Clione limacina limacina limacina