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Limacina (Limacina) retroversa (Fleming, 1823) subsp. retroversa (Fleming, 1823) forma retroversa (Fleming, 1823)

Overview

This is a small, shelled, pelagic thecosomatous pteropod with a left-coiled shell, 0.3 cm in diameter. The spire is moderately highly coiled. It has more than 7 colourless or brown-purple transparent whorls. There is no umbilical keel. Transverse striae are present. It is found in colder waters of the N-Atlantic Ocean where it feeds on phytoplankton and it is a mucus feeder (Limacina r. retroversa).

Taxonomic Description

The small, transparent shell is spirally coiled and somewhat more depressed and smaller than in the forma balea. The number of whorls is 6-7. The last whorl is large, occupying 2/5 of the total shell, the suture is clear (Limacina spp.). The shell aperture is not large and its border is irregularly rounded. The umbilicus is narrow but deep. A fine spiral striation, composed of small dots, is found especially on the last whorl. A wing protrusion is present (Limacina r. retroversa soft parts). The Limacina r. retroversa radula formula is 1-1-1 it is composed of about 9 rows. The Limacina r. retroversa operculum is sometimes lost in adults, it is oval spoon-shaped.
Shell measurements: shell height about 2.5 mm, maximum diameter 3 mm, average diameter 2.6 mm.

Morphology and Structure

The salivary glands are minute, the secreted mucus enters the buccal cavity through the ciliated roof. A gizzard with gizzard plates is present. The fifth, smaller plate lies posteriorly near the threshold where the real stomach begins, only forming a flat sheet of thickened cuticle. The gizzard is of an oesophagal origin. The appendages of the stomach are the caecum or style sac, the liver ducts and the intestine. The stomach is lined throughout with fine cilia and only forms an annex to the diverticulum. The style sac (25 µm in diameter) is lined with cuboid squamous epithelium, which do not have glandular activity. There are long cilia in the caecum but a style is absent. The function as style sac is probably of minor importance. The digestive diverticulum is bounded by a pair of closely approximated folds and its cilia beat outwards from the stomach. The intestine is uniformly coated with cilia. The faeces are transported by cilia-beating and peristalsis. The oesophagus has 5-6 folds as in most Thecosomata. The watery mucoid substance in the gizzard, containing yellowish-green droplets and black and brown particles extruded from the liver, flows constantly from the stomach to the digestive diverticulum and vice versa by pumping of the gizzard and stomach. Only at intervals are the particles forcibly ejected into the intestine, while some particles, like diatom skeletons, pass directly into the intestine. No glandular particles are found in the oesophagus and stomach.
The liver has an open communication with the alimentary system. It is composed of tubes with an unicellular wall, set close together so that the basal membranes touch each other. In the digestive gland or liver three types of cells are distinguished, viz.: a) digestive cells (mostly numerous); b) excretory cells of the larger type; c) excretory cells of the smaller type. The digestive cells (a) are subdivided into two types: a' with basal vacuoles and fine granular material staining blue in Azan and a" with superficial vacuoles, larger than a' and colourless in Azan. No fragmentation phagocytes are found, neither are they to be expected. The large type of excretory cells have round nuclei with prominent nucleoli and dark staining chromatin. The second type of excretory cells are small and filled with small, distinctly black staining spherules in the cytoplasm.

Juveniles

The juveniles have a small left coiled shell. (Limacina r. retroversa juvenile).

Reproduction

These animals are protandric hermaphrodites. They are present throughout the year and only in January were ripe eggs absent in the sexual system. The animals are filled with eggs, especially in the summer. The eggs are spawned in planktonic, floating egg ribbons consisting of gelatinous material. The ribbon measures about 2.0 by 0.64 mm, the eggs are about 0.9-0.1 mm by 0.06-0.07 mm, eggs and matrix are colourless. The development of the embryos starts at once and hatching may occur on the second day. The development of the shell at first gives rise to a symmetrical shell 0.05 mm across. On the fourth day the shell, 0.1 mm across, becomes asymmetrical and the velum having the same diameter has developed. An operculum is visible in animals with a shell of 0.13 diameter and a foot pulled out into a ciliated tongue with larger cilia at the top; eyes, mouth and statocysts are visible. The only coloured parts in these animals are the black "anal" gland and the brownish-yellow liver. During growth, with the succession of the seasons, male- and female-products in the gonad changes, so four different stages are distinguished, viz.:

A. Small sized specimens, sexual undifferentiated gonads.
B. Specimens with a maximum diameter between 0.85 and 1.10, pure male gonad
with few or no female products.
C. Larger specimens, hermaphrodite gonads with more than 50% male products
(hermaphrodite males).
D. Larger specimens, hermaphrodite gonads with less than 50% male products
(hermaphrodite females).

Besides the stages normally found the germinal cells, the primary and secondary spermatocytes and sperm, aberrant spermatids were found. The spermatids are represented in this species and also in other Limacinidae, by more stages than in Cavoliniidae. Even the largest specimens were hermaphrodites. Specimens may be functioning both as male and female at the same time, but this is apparently restricted to the gonad, the whole animal would not have been functioning as a male and female at the same time. The ova were always found at the periphery of the gonad. It is evident that the development of Limacina retroversa starts in the winter while a new generation is born in the spring and summer. It is clear that growth is dependent on the season and especially on the temperature of the water. This is best demonstrated by the speed of growth which is faster in the animals that invaded the Gulf of Maine in June than in those that invaded in April and the latter grow faster than those that invaded in December. This is a species without brood protection.

Ecology

The temperature range is 2-15°C, salinity range 3.0-3.6%S. The phytophagous and epipelagic forma feeds chiefly on dinoflagellates (Prorocentrum micans) and diatoms (Asterionella japonica, Skeletonema mediterraneaum and Chaetoceros constrictus). Special predators are the fishes Cetorhinus maximus; the herring; the mackerel; Clione limacina and rorquals. It constitutes about 2.2% of the yearly food supply of the herring in the North Sea.
Feeding is carried out by cilia transporting food particles trapped in a mucous web towards the mouth. As in all Limacinidae the feeding mechanism is rather primitive when compared to Cavoliniidae and Peraclididae. In the present species the mantle cavity is still the most important part of the feeding apparatus, in spite of the loss of the ctenidium.
The median footlobe, finely and uniformly ciliated at its flat surface, is bordered by the lateral footlobes which have a row of forward beating cilia along their edges. The mouth is found in a small, shallow funnel-shaped depression which is bordered in front by a small V-shaped, light-brown fold. The wings are found at both sides of the lateral footlobes which have a ciliated and a non-ciliated area. A horseshoe-shaped fold borders the mouth which has the azygous tentacle with a terminal papilla in front. This unpaired tentacle is peculiar but common in Limacinidae. Near the inhalant opening of the mantle cavity, which is at the left near the posterior corner of the pallial aperture a brim encircled by dense inward beating cilia is present. Their action is effective over a distance of 5-10 mm. The whole inhalant side of the mantle has cilia. The pallial gland, in the concave side of the mantle cavity, covered mucus for the compacting and sticking of the entering particles. The mucus and food are carried forward as a coherent string of particles along the right side of the pallial gland. The string of food particles runs from the inhalant opening down to the end of the pallial cavity and back to the mouth. Only the lighter particles seem to make the complete course around, while coarse material more directly drops into the food string.
The rejected particles find their way along the underside, as well as, the upperside of the wings and along the balancer and are cast off. The transport from the mouth to the stomach takes 30 minutes and from the stomach to the anus 2 minutes (the time spent in the stomach is not included) as determined when carmine is used as "food".
The salinity range for balea is 31.8°/oo-35.00°/oo with an optimum of 32°/oo, the temperature range is 11°C-18°C, exceptionally 2.6°C.

Distribution

The forma retroversa is typical of the transitional area in the North Atlantic Ocean, for the cold temperate waters between 40° and 70°N. As many authors did not distinguish the formae retroversa and balea it was difficult to determine from the literature if both formae have a distinct range. The forma retroversa is typical for the Gulf Stream and its branches, while the forma balea occurs in more northern waters. In the period 1948-1949 the forma was mostly caught in the northern part of the North Sea and over deep water west of Ireland, while after 1950 a slight, and after 1953 a sharp contrast occurs where larger numbers were only collected over or near the continental shelf. A separate population southeast of Ireland, however, in this period had kept its position over deep water. It may be concluded that certain populations change from an oceanic to a neritic way of life. Currents may transport specimens from an oceanic stock population nearer to the coast. A stock living over deep water may be present from time to time over the shelf, but it does not form a self-supporting population and it probably does not persist for a long time nor does it breed. In the Gulf of Maine the temperature range is 2°C to 16°C. For the forma retroversa the maximum temperature range is normally 15°C. As to the lower temperature limit, vertical migrations seem also to be limited by a 2°C layer, and the forma does not penetrate into this colder layer even when this layer lies near the surface.
The local annual and seasonal fluctuations suggest the presence of stock populations. For Limacina retroversa, the greatest stock is between 55°-62°N and 10°W off the east coast of Greenland, while the species is absent in the East Iceland Current. Another great concentration is found north of the Faroes.
The distribution of the forma balea shows a resemblance with that of Limacina helicina forma helicina. In the W-Atlantic it ranges to about 30°N south, due to the Labrador Current, and it is found as far north as 75°N, due to the Gulf stream in the E-Atlantic. The forma retroversa shows a more symmetrical distribution pattern between 50°N and 65°N. In the W-Atlantic retroversa is found living more northward than balea, in the E-Atlantic more southward. It appears that retroversa is more influenced by currents than balea. The forma balea is an index for boreal water, see the Limacina r. retroversa map.
A stock population of balea is present in the southern part of its range from which all the northern living specimens may originate, the result being a yearly re-population of the northern area by young balea specimens. In favour of this theory is the fact that the greatest abundance in the northern part of the area occurs later in the year than in the southern part. However, purely climatological circumstances may also have an important influence on this gradual change in abundance.
In the Mediterranean Limacina retroversa is found east of the Strait of Gibraltar and this population is composed of migrants from the Atlantic. The scarce records in the Aegean Sea may originate from migrants from the Atlantic or from a local population.
The forma balea is most widely distributed over deep water and shows a great dependence on currents whereas the forma retroversa occupies only a small area in the N-Atlantic, over deep, as well as, over shallow water, and it apparently is less dependent on the current systems.

Geological Record

This form is found in the Pleistocene of the Mediterranean and Adriatic Sea. It disappears from the Adriatic in at the end of the Younger Dryas or Boreal. It is found in the Mid Pliocene of Italy.

Types

Heterofusus retroversus (Fusus retroversus) Fleming, 1823:498,pl. 15, fig. 2.
Types could not be located.
Limacina planospira Rehder, 1942: 108, pl. 23.
Holotype: USNM 535416 (dry collection).
Type locality: 49°36'N 28°54°W, bottom sample, depth 1280 m. Coll.: C. S. PIGGOT sample H-63 core 8.

Limacina retroversa retroversa