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Peraclis valdiviae (Meisenheimer, 1905c)

Overview

This is a pelagic snail with a left coiled shell, 0.7 cm in height (probably up to 2 cm); it has 4 whorls and a deep suture with radial crests. The spire is strongly depressed. The aperture has an aperture tooth and a small rib; the shell has a spiral pattern of undulating lines. The aperture is wide and oval. The shell surface has a reticulate pattern. The description by Tesch was slightly different as can be seen from this picture Peraclis valdiviae 1. The wings are fused into a swimming plate. This species usually lives at great depths in all oceans.

Taxonomic Description

According to Tesch (1948) the thin calcareous shell closely resembles that of Peraclis bispinosa. The shell is depressed, the body whorl constitutes the greater portion of the shell. A strong keel runs along the suture. Thin crests are found between the suture and keel. The columellar membrane is well developed (Peraclis valdiviae with sculpture). Whole spiral ribs are found on the body, which are stronger near the aperture. In P. bispinosa about seven faint spiral lines are found on the body whorl near the aperture. These lines indicate an affinity between the present species and Peraclis bispinosa. The sculpture of hexagonal or quadrangular reticulation on the first whorls is the same as in Peraclis bispinosa. The Peraclis valdiviae operculum with 5 whorls is characterised by double lines along the spire. Transverse striae are found between these double lines. The Peraclis valdiviae radula formula is (1)-1-1-1-(1). The Peraclis valdiviae median tooth is monocuspoid with numerous lateral denticules. The impression given by Tesch in Peraclis valdiviae line drawing differs from the above description but it was based on badly preserved material.
Shell measurements: height up to 6.2 mm, maximum diameter up to 5.0 mm. These sizes are taken from Tesch (1948: 37) who found smaller specimens (7 mm), as the soft parts were distinctly smaller than previously described (22 mm in size).

Taxonomic comparison with Peraclis philliporum

When the shell is intact and reconstructed (Peraclis valdiviae reconstructed) its height may be 6 mm. The shell has 3.75 whorls; the body whorl is very large. The spire is relatively high, pointing above the body whorl. The calcareous, sinistral shell is covered with a hexagonal calcareous reticulum except for the embryonic, first whorl and the last half of the body whorl. On the last whorl a thickened rib runs parallel to the suture; in between this thick rib and the suture, transverse rims are found and growth lines are visible. Growth lines are distinct over the whole shell but more prominent on the body whorl. As can be concluded from the growth lines near the aperture of the shell, a small anterior spine had been present on the aperture, probably supported by the thickened rib. The wavy, spiral ribs around the body whorl, 9 or 10 in number, become stronger near the aperture; the four central ribs are more prominent than the outer ones. The aperture border is discontinuous, spindle-shaped; a very broad columellar membrane is present forming a spoon-like protrusion on the underside of the aperture which is supported by the columella. In an intact specimen a pointed rostrum is supposed to be present as illustrated (Peraclis valdiviae recon). The operculum with 4.5 whorls is nearly round, very thin and flexible. The first whorls and the last one are coiled in a higher level than the second last. The typical striation of the operculum, described by Tesch (1948), could not be found in the present specimens; only an irregular reticulated pattern is found in the centre of the whorls. The illustrations of the shell given by Tesch (1948) give the impression that the shell is relatively depressed although the spire is high. As can be seen from the illustrations presented here the shell is slender and resembles more Peraclis bispinosa Pelseneer, 1888 and Peraclis philiporum (Gilmer, 1990). The statement of Tesch that the shell is much more depressed than in Peraclis bispinosa seems incorrect as the shell is elongate. The shells of both Peraclis philiporum and Peraclis valdiviae are slender. The spiral ribs on the body whorl separates Peraclis valdiviae from the other species in the genus.
Gilmer (1990) described Procymbulia philiporum as a close relative of Peraclis valdiviae and stated that the genera Peraclis and Procymbulia should be kept separate as the latter is intermediate between the Peraclididae and the Cymbuliidae. All the shell characters that Gilmer (1990) considered to separate Procymbulia from Peraclis can, however, also be found in different Peraclis species. Gilmer (1990) stated "The thin shell could not be removed from the animal... consequently it dissolved after several weeks in the preservative" so that one can conclude that the shell was not properly described as it was observed through the tissues and "pseudoconch" in an aggressive preservative. Therefore the description of shell structure and sculpture are doubtful and are not sufficient to support the separate status of the genus Procymbulia. As a consequence the generic name Peraclis is retained here for the species valdiviae, philiporum and michaelsarsi (Bonnevie, 1913).

Morphology and Structure

This species is usually collected without its shell and has been regarded as a missing link between the Peraclididae and Cymbuliidae. It seems appropriate to give a brief note on its anatomy.
The thick, muscular wings are fused into a swimming disc and have an operculum underneath. The swimming plate grows large in colder regions, about 22 mm in diameter and small in warmer areas, about 7 mm. The wings are chocolate-brown or purple-brown in older specimens. Meisenheimer (1905a) described and figured a stub-like process, resembling the tactile organs in Desmopterus, on each side of the blunt median projection at the ventral border of the swimming disc. This pair of processes was also observed by Massy (1932) in some of her specimens, but Tesch (1946, 1948) and Hubendick (1951) failed to detect them. The absence of these organs is probably dependent on contraction or on age of the specimen in question.
The muscle fibers in the swimming plate run parallel to the margin on the dorsal surface leaving a narrow raphe from the proboscis to the medio-ventral lobe. Two transverse bundles of muscle fibres are found at the ventral side of the swimming plate. The posterior footlobe forms the conspicuous ventral fold of the proboscis on the dorsal side of the swimming plate. The lateral footlobes, with this ventral fold, form a true funnel used for collecting food. In the centre of the funnel the mouth with the lateral lips is situated. Each jaw shows, like in other Peraclis species, six parallel series of minute plates finely denticulate at the frontal border. The radula with the formula (1)-1-1-1-(1) is strongly developed. The salivary glands are extremely tiny, Tesch (1948) did not find these but Meisenheimer (1905a) described them correctly.
The mantle nearly encircles the complete body. A voluminous mantle cavity and pallial gland are found at the right isde; this consists of one cell type, but is divided by transverse folds. At the ventral side of the pallial cavity a ctenidium directed to the right side, is attached to the medio-dorsal line. At the base of the ctenidium the heart-kidney system is found.
The gizzard has two pairs of gizzard plates and a triangular accessory plate at the ventral side and is situated anterior to the stomach. From the right side a caecum enters the posterior portion of the stomach. The anus is formed by a siphon-like structure in the medio-ventral line of the body and not at the right side. Tesch (1948) stated that he course of the intestine is variable, he believes the difference between type A and B he described is age dependent. Differences in feeding may, however, also account for these differences in intestinal structure. The gonad fills the apical coils of the visceral mass and the accessory sexual gland lies at the upper tip of the gizzard. The penis, only found in younger specimens, consists of a coiled pouch near the medial line of the neck region and possesses several blind sacs; one of which has a triangular stylet. An essential character of the central nervous system is the symmetry of the visceral ganglia, the separate abdominal ganglion and the delicate posterior, or second pedal commissure. This discussion of the anatomy clearly shows that the species belongs to this genus.

Juveniles

The juveniles have a small left coiled shell. A special description is not available.

Reproduction

This species is a protandric hermaphrodite.

Ecology

This species is phytophagous and mesopelagic to bathypelagic Temperature preference is 7.9°C.

Distribution

The range of this species is difficult to determine due to considerable lack of data. The records from the Gulf of Panama are isolated in the Pacific; records from the Tasman Sea and the Indo-Malayan archipelago are the only known Pacific records. In the N and S-Indian Ocean and in the entire Atlantic Ocean records are present. The discontinuity of its distribution is strange and it is expected that in the future more localities will become known. From the depth range it is evident that this species occurs at shallower depth in colder areas than in warmer areas. Peraclis valdiviae shows a very pronounced subtropical submergence at 40-45°N and 40-45°S, see the Peraclis valdiviae map.

Types

Procymbulia valdiviae Meisenheimer, 1905: 14, pl. I figs. 6-7.
Type not present in HMEB u.s.
Type locality: 34°14'S 80°31'E. Coll.: CVDE, stat. 169.

Peraclis valdiviae