Limacina (Thilea) helicoides Jeffreys, 1877
Overview
Limacina (Thilea) helicoides is a typical deep-sea pelagic snail with a large, thick, dark brown, coiled shell. The body is also dark brown. The wings are fleshy. Small juveniles are slightly lighter in colour and may occur at a shallower depth. It is never collected in large numbers. Foraminifera, Radiolaria and other microplankton are its food. This species is ovoviviparous. It lives in the Atlantic, S-Indian and SW-Pacific Oceans. (Limacina helicoides drawing).
Taxonomic Description
Limacina (Thilea) helicoides has a large, chest-nut brown shell, some spiral lines of punctation are found on the first whorl and on the embryonic shell (Limacina helicoides, Limacina helicoides swimming, Limacina helicoides soft parts). Transverse growth lines are found on the last whorl. The shell is depressed with 3-4 whorls of which the last one is swollen and composes a major part of the shell. In most specimens the narrow umbilicus is closed or absent. The inner aperture margin is curved outwards, a trace of a rostrum is seen with a small columellar membrane-like structure, this is more prominent in the Atlantic specimens than in the Pacific ones. The shell is flatly coiled, the Pacific specimens have a higher spire than the Atlantic ones. The suture is clear but not deep. The aperture is oval, higher than wide. The operculum is more rounded than in most Limacinidae, it is composed of 2.5 whorls in a shell with 4 whorls. The operculum is often lost in older specimens. The colour of the soft parts, outside the shell aperture, is dark purple to black. The wings and the small posterior footlobe are developed into a united swimming plate-like structure, which is thick and fleshy. No wing protrusions are present and in the preserved specimens the wings are usually heavily folded. Between the wings a well-developed cephalic lobe is present. The mouth is not proboscis-like. The dorsal tentacles are unequal in size, the right one being much larger than the left one and enveloped in a sheath. The tops of the tentacles have an ivory patch with the eyes which are considered to be an active organ for light perception. The radula is small with the formula 1-1-1 and is composed of the 19 transverse rows (Limacina helicoides teeth).
Shell measurements: height about 11 mm, maximum diameter of Atlantic specimens, 15 mm and Pacific specimens, 11 mm.
Morphology and Structure
Limacina (Thilea) helicoides (see illustration of soft parts) has relatively well developed eyes in the tentacles, the cells in the eye have large nuclei and appear devoid of pigment. Mantle and mantle cavity are very well developed, the deep cavity reaches from the left to the medio-ventral side of the body. The anterior mantle border is membraneous. The balancer, at the right side, is pointed and slender. A striking difference with Peraclis is the absence of a separated abdominal ganglion in the central nervous system and this character is used to separate this species from Peraclis. The balancer at the right side is preceded by a large membraneous lobe which may be considered the gill, but this organ is surely not homologous with a ctenidium. The strongly transversely folded pallial gland is voluminous. The cuboid cells are placed in longitudinal rows. The left and hind margin of the pallial gland are in contact with the kidney. The heart is found posteriorly, connected with the kidney by a long reno-pericardial duct. The oesophagus is rather long, the gizzard shows the normal teeth and circular muscles. The stomach turns sharply to the right. The intestine running on the surface of the liver ends ventrally to the gizzard at the anus. The paired visceral and buccal ganglia are both very compressed, the pallial and cerebral ganglia clearly form separate pairs. In the male stages, with a gonad filled with sperm, an enormous penis is present, which disappears when the male function is fulfilled and eggs start to develop. The gonoduct leads from the anterior side of the gonad to the accessory sexual gland, at the right anterior corner of the gizzard. A swelling in this duct forms the bursa seminis. From the accessory gland opening a seminal groove - or rather a tube- runs along the base of the right wing to the copulatory organ and opens near the sheath of the right tentacle where the penis aperture is also found. The penis is a strongly muscular tube, coiled and ending in a blind sac, no stylets are present.
Juveniles
The juveniles have a small, left coiled shell. As there is brood protection, the young develop completely in the mother (Limacina helicoides 1st apc whorl). The Limacina helicoides protoconch I is covered with pyramidal warts, the protoconch II is smooth and the first whorls of the teleoconch shows spiral lines of small warts.
Reproduction
Limacina (Thilea) helicoides is a protandric hermaphrodite, ovoviviparous species showing sexual dimorphism by the large penis. In the females, eggs and larvae are found in the accessory gland. The coils of the gland in this stage are degenerating. Before the larvae hatch they come into the mantle cavity by tearing off the envelope of the accessory gland. In the mantle cavity the young soon develop into normal small specimens which no longer have larval characters. This species shows brood protection.
Ecology
Limacina (Thilea) helicoides is phytophagous. The temperature range is 2.8-9.39°C, the salinity range is 3.4-3.6%S. The juveniles live at a higher temperature range than the bathypelagic adults.
In the gizzard of the present species the following organisms were found as food: Fragilariopsis antarctica, Thalassiosira, fragments of Coscinodiscus and Chaetoceros This indicates that planktonic organisms up to about 40 µm are caught as food, while other large organisms seem to be rejected as they were found in the surrounding plankton but not in the alimentary system i.e. larger specimens of Chaetoceros with bristles 1000µm long, and large dinoflagellates.
Distribution
The species is ovoviviparous and thus migration and wide distribution by floating eggs or veliger larvae cannot occur. The records west off Italy might be a population that once had contact with the Atlantic population since the Strait of Gibraltar must be in recent times an important barrier. All records so far published for the Mediterranean are dubious, see the Limacina helicoides map. Recently a population in the Indian Ocean is discovered. The specimens found near New Zealand show slight differences with those found in the Atlantic, proving they were first to form a distinctly isolated population.
Types
Limacina helicoides Jeffreys, 1877: 338.
Lectotype: USNM 169400, completely broken (dry collection). Paralectotypes: USNM 9 specimens broken. Coll.: CVAE stat. 12.
Type locality west of Ireland, 1215 fms. and Bay of Biscay, 740-1095 fms.