Hystricella R. T. Lowe, 1855

Mattia, Willy De, Neiber, Marco T. & Groh, Klaus, 2018, Revision of the genus-group Hystricella R. T. Lowe, 1855 from Porto Santo (Madeira Archipelago), with descriptions of new recent and fossil taxa (Gastropoda, Helicoidea, Geomitridae), ZooKeys 732, pp. 1-125 : 5-9

Hystricella R. T. Lowe, 1855

Discula auct., partim [non R. T. Lowe, 1852].

Helix (Ochthephila) sensu Albers (1850) and Albers (1854), partim.

Helix (Octephila) , Paiva (1867) [incorrect subsequent spelling Ochthephila Albers, 1850], partim.

Geomitra ( Actinella (Hystricella) ), Pilsbry (1893-1895).

Geomitra sensu Bank et al. (2002).

Type species, by original designation in R. T. Lowe (1855).

Helix bicarinata G. B. Sowerby I, 1824 = Hystricella bicarinata (G. B. Sowerby I, 1824).

Lowe (1855: 186-187) describes the new genus Hystricella in the section § 24, basing his description upon shell features only, as follows:

(Typ. H. bicarinata , Sow.). T. perf. v. angustae umbil. conuloidea v. trochiformis, aliquando turrito-pyramidata acute v. distincte 1-2-carinata solidiuscula aspero-granulata v. echinulata subfasciata. Anfr. 6-9 lente crescentes planiusculi, ult. valde carinato subtus planato, antice deflexo. Umbil. parvus anguste cylindricus v. subspiralis constrictus. Apert. circularis circinata labris connexis; perist. continuum solutum expanso-reflexiusculum tenue acutum.

Lowe (1855: 186-187) cites for the genus (under section § 24) the following species:

H. bicarinata G. B. Sowerby I, 1824

H. echinulata R. T. Lowe, 1831

H. oxytropis R. T. Lowe, 1831

H. turricula R. T. Lowe, 1831

H. vermetiformis R. T. Lowe, 1855

Hystricella mustelina Reeve, 1854 [under authorship R. T. Lowe, 1855], currently considered as a subspecies of Discula (Discula) cheiranthicola Reeve, 1854 [even though the description was prepared by R. T. Lowe in 1854 the work was only published in 1855 and therefore the earlier publication of that name in Reeve in 1854 has priority].

Hystricella cheiranticola R. T. Lowe, 1831 and its morphae (later put in the mono-specific subgenus [as section] Turritella by Wollaston 1878 [non Lamarck 1799]).

Later, H. mustelina (currently Discula (Discula) cheiranthicola mustelina (Reeve, 1854)) and H. cheiranticola (currently Discula (Discula) cheiranthicola cheiranthicola (R. T. Lowe, 1831)) were assigned to the genus Discula R. T. Lowe, 1852 ( Mandahl-Barth 1950).

Description of the genus.

Shell. The shell is dextral, hairless and it is usually conical and scalariform. The protoconch is whitish to dark brown, with 1.3 to 2.2 whorls. It is almost smooth along the first whorl and shows fine radial striae and extremely small, scattered tubercles along its remaining portion. The teleoconch has from 4.2 to 5.0 rapidly increasing whorls. It is usually dark brown with brick red and/or dark violet colour shades. The dark areas of the shell are mottled with more or less light brown to whitish areas, usually placed longitudinally and slightly slanting. No band pattern is visible along the upper whorls. On the lower part of the last whorl two well-defined, dark bands are visible that can differ in width. The area around the umbilicus is usually the lightest in colour. The spire is slightly variable in height, ranging in shape from compressed to somewhat more elevated. Along the last and the penultimate whorls one to two evident keels are present. The external upper surface has very fine but clearly visible, irregularly spaced growth lines. Along the last whorls the growth lines usually disappear along the lateral area and reappear on the lower part. Irregularly disposed tubercles are found all over the teleoconch. The dimensions of the tubercles tend to increase slightly from the first toward the last whorl, the density remaining, however, approximately the same. The tubercles are somewhat denser along the keels of the penultimate and last whorls, letting the keel/keels appear like a rough chord. On the lower part of the last whorl the tubercles are usually bigger and slightly less dense than in the remaining parts of the shell. The last whorl is large, with a contribution of 60% of the total shell height and descending towards the aperture. The umbilicus is open but very narrow, concentric and has a diameter of approximately 10% of the maximum shell diameter. The aperture is elliptical with a faint thickening along the inner side of the last whorl. The peristome is continuous, slightly reflected with the columellar margin somewhat thicker and more reflected.

Body. Head and neck are grey to pale grey, slightly transparent. The sides and posterior upper section of the foot are whitish. The sole of the foot is whitish and longitudinally divided into three areas. The central area is smooth whereas the two lateral areas are equipped with bands of muscles roughly arranged in a chevron pattern. The mantle border is grey to dark grey with five more or less developed lobes. The ratio among lateral and dorsal lobes varies from specimen to specimen also in the same population. In some specimens, particular lobes (regardless if lateral or dorsal) may be completely missing. The walls of the pallial cavity are colourless, without any stripes or spots. A strong pulmonary vein is visible.

Genitalia. The general arrangement of the genitalia is semi-diaulic monotrematic. A convoluted to almost straight first hermaphroditic duct arises from a multi-lobated gonad. The albumen gland is long and moderately thin and connected to an equally long sperm-oviduct consisting of a prostatic and a uterine portion. Distally, the prostatic part extends into a thin vas deferens, twice as long as the sperm-oviduct, terminating in the penial complex. The distal portion of the uterine part inserts into the free oviduct, then transforming into a vagina at the level of the insertion of the duct of the bursa copulatrix. The free oviduct is usually three to four times shorter than the vagina. The duct of the bursa copulatrix is usually wide, approximately as long as the penis and uniform in diameter. This duct terminates in a variable, oval to roundish, small bursa copulatrix. The spermatophore is unknown. One tuft of digitiform glands arises from the proximal part of the vagina. There are usually two or three, equally long and very rarely branched glands. A short and thin vaginal appendix arises from the vagina’s wall, immediately distal of the glandular tuft. Very smooth, little elevated and spaced pleats run longitudinally along the inner surface of the vagina, reaching into the genital atrium as far as the genital orifice. The atrium can be short or moderately long. The penial complex consists of a flagellum, an epiphallus (which extends from insertion of the vas deferens to the penial retractor muscle) and a penis that inserts into the genital atrium. The penial flagellum is short, remarkably cylindrical and with a blunt apex. It is usually twice as long as the epiphallus. Its internal walls can be either completely smooth or ornamented with very small papillae distributed mainly close to the blunt end. The epiphallus is usually extremely short and its internal walls are smooth. The retractor muscle is well developed, strong and is variable in length. The penis lacks a muscular or glandular sheath. It is thick-walled and approximately four times longer than the flagellum. It is usually cylindrical to sometimes slightly swollen in its distal part. The inner walls of the penis are usually smooth or with very smooth, little elevated and spaced pleats which run longitudinally and reach the genital atrium. The section where the penial papilla is located is usually detectable from the outside by virtue of a fine circular swelling corresponding to the origin of the papilla itself. The penial papilla is very variable in size, ranging from 1/6 to 1/2 of the total penial length and is conical to subcylindrical in shape. The inner lumen of the penial papilla is filled with a spongy and sturdy tissue which directly connects with the walls of the epiphallus. It has smooth external walls with the opening emerging apically. The channel of the penial papilla is thin and narrow. The longitudinal section of the penial papilla shows that its walls are the continuation of the penial walls that abruptly bend inward.

Jaw and radula. The jaw and radular apparatus of Hystricella is depicted in Fig. 8. No notable variability was found among the species of the genus. The jaw is odontognathous, almost straight to markedly arched, with 13 to 15 smooth ridges. The radula ribbon is typical helicoid, it is elongated but not very slender. Central tooth present, tricuspid, the main cusp (endocone) is rhomboid, pointed; the ectocones are much smaller than the endocone; they are triangular, pointed. There are 19-20 laterals and marginals which do not distinctly differ from each other, i.e., their shape changes gradually from the first laterals towards the marginals. Laterals are bicuspid, endocones are rhomboid or triangular and pointed. The ectocones are much smaller, pointed, and triangular. The endocones of the central tooth and the first laterals are approximately of the same size. Both, the endocone and the ectocone of the laterals gradually become bifurcated towards the marginal teeth, but the ectocones may occasionally have three cusps as well. The cusps of the marginals are gradually decreasing in size; therefore, the outermost marginals appear nearly serrated.

Distribution.

The genus Hystricella is endemic to the island of Porto Santo (Madeiran Archipelago, Portugal). The genus is restricted to the eastern, mountainous part of the island and occurs there only in the central-northern section of this area (Fig. 7). Species assigned to the genus are commonly found on the slopes of Pico do Castelo, Pico do Facho, Pico de Juliana, Pico da Gandaia, Rocha de Nossa Senhora, the western slopes of Pico do Concelho, and those of the Pico Branco and Terra Chã complex. It is also commonly found at lower elevations of Serra de Dentro, Barranco Branco, and Lombo de Paredes. Recent Hystricella are absent from the southeastern part of the island, i.e., from Vale do Touro, Portela, Pico do Maçarico, and Pico do Baixo. It is likewise not present on the small islets surrounding the main island, namely Ilhéu de Cima, Ilhéu de Baixo, and Ilhéu de Ferro. Subfossil representatives of the genus are found mainly in the mud and aeolinite deposits along the southeastern coast, namely Vale do Touro, Ponta do Passo, Barbinha, and Calhau da Serra de Fora.

Ecology.

Representatives of the genus Hystricella are commonly found under volcanic rocks scattered on grassland in open fields that are more or less strongly sloping. They have been found under stones in pine woods (Pico do Castelo) or in cracks and crevices of rocky walls (Pico da Gandaia). Disturbed or anthropogenic habitats have also been colonised by representatives of the genus, such as stone walls (Pedregal de Dentro) or terraced areas (southern slopes of Pico do Castelo). Specimens aestivate on the lower surfaces of stones or rocks, frequently forming large clusters of individuals attached to one another, reaching more than 40 to 50 in number. Under a single stone of roughly 60 × 40 cm approximately 200 individuals were counted (southern slopes of Pico do Facho).

Nomenclatural and taxonomic remarks.

Hystricella was considered a subgenus of Helix for a long time (until Wollaston 1878) or was replaced by Ochthephila Beck, 1837 ( Albers 1850) or Octephila Paiva, 1867 ( Paiva 1867). Later it was considered as a subgenus of Discula R. T. Lowe, 1852 and only since 2002 ( Bank et al. 2002) accepted as a distinct genus.

Bank et al. (2002) used the generic name Geomitra Swainson, 1840 despite Herrmannsen’s (1847: 470) selection of the nominal species Helix tiarella Webb & Berthelot, 1833 as the type species of that genus. The decision of Herrmannsen was followed by all subsequent authors (e.g., Wollaston 1878; Mandahl-Barth 1950; Waldén 1983). The reason to contradict that type selection was the fact that it was wrong with respect to the rules of the International Commission on Zoological Nomenclature’s (ICZN) Code of Zoological Nomenclature (hereafter 'the Code’) Art. 67.2.1: Swainson introduced Geomitra describing and depicting tiarella , namely on p. 166 and 332, but he did not mention that name, but instead only bicarinata , which therefore would be the type species by monotypy. In consequence Hystricella R. T. Lowe, 1855 would become a synonym of Geomitra and Geomitra sensu auct. has to be referred to Craspedaria R. T. Lowe, 1852. Nevertheless, this decision was much criticised ( Seddon 2008) and a new study of the Code revealed a solution which permits the stabilisation of the former use of Geomitra ( Groh et al. 2009). Following Art. 70.3 of the Code the combination of the figure of tiarella with the name bicarinata can be interpreted as a misidentification by Swainson and therefore following Art. 70.3.2, the name tiarella becomes available for type selection under the synonymous name Geomitra bicarinata Swainson, 1840. This was already recognised by Pfeiffer (1847: 191) who listed Swainson’s combination as a synonym of Helix tiarella (cf. Groh et al. 2009).

Comparison and comments.

Hystricella shows a number of morphological characters that clearly distinguishes it from the other native geomitrid genera from Porto Santo, in particular with regard to the genus Discula s. lat., into which the Hystricella species were previously placed ( Mandahl-Barth 1950; Waldén 1983). On the basis of the shell, the main distinguishing feature of Hystricella from Discula s. lat. is the continuous and detached peristome. On the contrary, in Discula s. lat. and Callina (Figs 10-12) the peristome is always interrupted, forming only a thickening or a callous along the parietal region but not a real and proper detached lip. Other minor differences can be found in the ornamentation of the shell’s surface. Hystricella always possesses round, somewhat spaced tubercles, whereas Discula s. lat. and Callina shows mainly elongated, sometimes drop-like tubercles arranged in a regular pattern. In Hystricella the size of the tubercles can reach large dimensions in comparison to the overall size of the shell and their size can remarkably vary on the same shell. Actinella s. lat. and Caseolus s. lat. (see Figs 13-16) also possess an interrupted peristome. Most Caseolus species have a more or less granulated shell’s surface and, at first glance, could therefore be confused with Hystricella . However, a closer look reveals the different peristome and the overall different shape, with the whorls always rounded and usually without any prominent keel. Serratorotula (Fig. 17) has an extremely ornamented shell that is easily distinguishable from Hystricella . The shell form of Heterostoma is very different from that of Hystricella and these two genera are therefore easily distinguishable (see Seddon 2008: 125). Some species of the genus Spirorbula sometimes have shells with a continuous peristome with detached lips along the parietal side. The very depressed shell shape and different surface ornamentation of Spirorbula however, readily distinguishes the two genera (Fig. 18). Lemniscia usually possesses a smooth and rather glossy shell without tubercles or papillae (Fig. 19). The shell morphology of Wollastonia gen. n. and its differences with regard to Hystricella will be discussed in the section on Wollastonia gen. n. below.

With regard to genital morphology, Hystricella shows a unique feature that allows the separation of the genus from all native Portosanctan Geomitridae , except from Wollastonia gen. n. The main difference is the shape of the penial flagellum which is always short and has a remarkably blunt apex (Fig. 20). Discula s. lat. and Callina , Serratorotula , and Lemniscia (Figs 21-25) have a pointed, more or less elongated flagellum. Hystricella is easily distinguishable from Caseolus s. lat. and Actinella s. lat. by virtue of the single vaginal appendages instead of the two that are present in these genera (see Figs 26-29). Spirorbula is also easily distinguishable from Hystricella because members of this genus possess two extremely long, and sometimes branched, vaginal appendages and a genital atrium with structures on the inner wall consisting of large and partially fringed folds (see Fig. 30).