Wollastonia, Mattia, Willy De, Neiber, Marco T. & Groh, Klaus, 2018

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 : 23-26

Wollastonia gen. n.

Remarks.

The synonymy is the same as for the genus Hystricella (see above).

Type species.

Helix [ Helicella ] turricula R. T. Lowe, 1831 - herewith by original designation.

Included taxa.

Wollastonia turricula (R. T. Lowe, 1831), comb. n., W. vermetiformis (R. T. Lowe, 1855), comb. n., W. ripkeni De Mattia & Groh, sp. n., W. falknerorum Groh, Neiber & De Mattia, sp. n., W. leacockiana (Wollaston, 1878), comb. n., W. beckmanni De Mattia & Groh, sp. n., W. jessicae jessicae De Mattia, Neiber & Groh, sp. n., W. jessicae monticola De Mattia, Neiber & Groh, ssp. n., W. klausgrohi De Mattia & Neiber, sp. n., W. oxytropis (R. T. Lowe, 1831), comb. n., W. subcarinulata (Wollaston, 1878), comb. n., W. inexpectata De Mattia & Groh, sp. n.

Description of the genus.

Shell. The shell is dextral and hairless. Its shape can be very variable, from conical, elongated and scalariform with deep sutures to rather flattened, with very shallow sutures. The protoconch is from whitish to completely dark brown with 1.5 to 2.5 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 4.3 to 7.0 rapidly increasing whorls. It is usually dark brown with brick-red and/or dark violet shades in colour. The dark areas of the shells are mottled with more or less light brown to whitish areas, usually placed longitudinally and slightly slanting. In some species, the lighter areas tend to be more evident along the keel. No band pattern is visible along the upper whorls. On the lower part of the last whorl two dark, more or less broad bands may be present. In some specimens, the two bands merge together forming a single wide, dark band. Sometimes the bands are interrupted by yellowish to whitish sections. The area around the umbilicus is usually the lightest in colour. Some species are usually covered entirely with soil or sand grains which probably serve as a camouflage.

The spire is very variable in height, ranging from flattened, almost discoidal to slender and remarkably conical in shape. The number and the extent of the keels are variable from species to species. Some species have one or two distinct keels starting already from the second whorl of the teleoconch, while others have a single, more or less strong keel only along the last whorl. Some species have a lower, distinct principal keel somewhat protruding the body whorl, with an upper secondary and much less evident keel. The whorls can be either convex or flat. In some species, the whorls form a “shoulder”, giving the whorls and angular contour. The sutures are usually deep, even if in some species they may also be rather shallow.

The external surface has from very fine to strong, clearly visible, irregularly spaced, growth lines. Irregularly disposed tubercles are found all over the teleoconch. The dimensions and the arrangement of these tubercles varies considerably from species to species, from large and scattered tubercles to a very dense pattern of small papilla-like tubercles all over the shell. In the species with larger tubercles, these are somewhat denser along the keels of the penultimate and last whorls, giving to the keel(s) the appearance of (a) rough chord(s). The last whorl is usually large, descending near the aperture with a contribution ranging from 40% to 60% of the total shell height. The umbilicus is open but very narrow, either concentric or eccentric, and measures approximately 10% of the maximum shell diameter. The aperture is elliptical with a faint thickening along the columellar portion of the aperture. Sometimes this thickening can also extend as far the parietal side of the aperture. The peristome is continuous, slightly to evidently reflected, with the columellar margin somewhat thicker and more reflected.

The shells of the species belonging to Wollastonia gen. n. show a remarkable plasticity, with regard to dimensions, shape and ornamentation. This variability undoubtedly exceeds that of the genus Hystricella . Thus, for each taxon, the shell’s features will be described in detail.

Body. The head and neck are usually dark grey to grey. The sides and the posterior upper section of the foot are whitish. In some species, the pigmented ommatophoral retractor muscles are visible through the skin of the back of the cephalic area. The foot is white and the sole is 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 of lateral to the dorsal lobes varies from specimen to specimen, also in the same population. In some specimens one of these lobes (either lateral or dorsal) may be totally missing. The walls of the pallial cavity are colourless, without any stripes or spots. A strong pulmonary vein is visible. The jaw is odontognathous and very variable in shape, from almost straight to markedly arched. There are many transversal, smooth ridges, ranging from eight to 25 in number. The right ommatophoral retractor is independent from both penis and vagina.

Genitalia. The general arrangement of the genitalia is semi-diaulic monotrematic. A convoluted to almost straight hermaphroditic duct arises from a multi-lobated gonad. The albumen gland is long and thin and connected to a variably long sperm-oviduct consisting of a prostatic and a uterine portion. The prostatic part extends into a thin vas deferens, roughly as long as the sperm-oviduct, and terminating in the penial complex. The distal portion of the uterine part extends into the free oviduct and turns into a vagina along its course at the level of the duct of the bursa copulatrix. The free oviduct can be as long as the vagina or also three to four times longer. The duct of the bursa copulatrix is usually wide, approximately as long as the penis and uniform in diameter. It extends into a variable, oval to roundish bursa copulatrix. In some species, the transition area between the duct and the bursa itself is not very distinctly delimited; the duct more or less abruptly widens and transforms into the bursa. The spermatophore is unknown. One tuft of digitiform glands arises from the proximal part of the vagina. The glands have usually two or three, approximately equally long and very rarely bifurcated branches. A short and thin vaginal appendix arises from the vagina’s wall, immediately distal of the glandular tuft. Very smooth, rather wide, and little elevated, irregularly 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 and wide or long and thin. Its internal walls can either be smooth or with large and soft pleats running longitudinally as far the genital orifice. The penial complex consists of a flagellum, an epiphallus (which extends from the insertion of the vas deferens to the penial retractor muscle) and a penis that inserts into the atrium. The penial flagellum is short, remarkably cylindrical and with a blunt apex. It is usually as long as the epiphallus. In one species, W. oxytropis , the flagellum is very short ( ¼ of the epiphallus) but remarkably pointed. The internal walls of the flagellum can be either completely smooth or, in one species, can have a digitiform, pointed papilla originating from the proximal end of the flagellum and orientated toward the penial papilla. The epiphallus is usually short to moderately long. Its internal walls have a variable number of longitudinal pleats that can be more or less developed and elevated. The retractor muscle is large, strong and is of a variable 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. Sometimes, a thin sheath consisting of connective tissue envelops the distal penis, causing a partial and longitudinal minor compression. The inner walls of the penis are smooth or with irregular 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. The penial papilla is usually small, reaching maximally ⅛ of the total penial length and is conical to subcylindrical in shape. It has smooth external walls with the opening emerging apically. The penial papilla channel is thin and narrow. The inner lumen of the penial papilla is occupied by a spongy and sturdy tissue, which directly connects with the walls of the epiphallus. The longitudinal section of the penial papilla shows that its walls are the continuation of the penial walls that abruptly bend inwards.

Jaw and radula. As for the genus Hystricella , no notable variability was found among the species of the genus Wollastonia gen. n. and the two genera share almost an identical jaw-radular apparatus. The jaw is odontognathous, arched and with rough wrinkles (irregular ribs). The radula ribbon is typical helicoid, it is elongated but not very slender. A central tooth is 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. Their shapes rather change gradually from the first laterals towards the marginals. Laterals are bicuspid, with rhomboid or triangular and pointed endocones. The ectocones are much smaller, pointed, and triangular. The endocones of the central and 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 occasionally might have three cusps as well. The cusps of the marginals are gradually decreasing in size; therefore, the outermost marginals appear serrated. The jaw is variable in shape: from almost straight to markedly arched. There are many transversal and smooth ridges, ranging from 13 to 20. For Wollastonia jaws and radulae, see Fig. 98.

Distribution.

The genus Wollastonia is endemic to the island of Porto Santo (Madeiran Archipelago, Portugal) and some surrounding islets (Fig. 97). Along the eastern part of the main island, the genus is restricted to the southern mountainous part. It is patchily distributed from the hilly area east of Vila Baleira towards the east to Portela, Zimbreiro and along the highest southern peaks of Porto Santo, namely Pico do Maçarico and Pico do Baixo as far as Ponta da Galé. It is also present along the eastern coast area of Pico do Concelho. Along the western part of Porto Santo, Wollastonia is found on Pico de Ana Ferreira. With regard to the surrounding offshore islet, Wollastonia is present on Ilhéu de Cima and Ilhéu de Cenouras at the eastern and Ilhéu de Ferro at the western end of the island. Subfossil representatives of the genus are found mainly in the mud and aeolinite deposits along the southeastern (Vale do Touro, Ponta do Passo, Barbinha, Calhau da Serra de Fora) and northern (Fonte da Areia) coasts. Wollastonia may be older than Hystricella as suggested by the somewhat deeper divergences in the molecular trees (Fig. 3).

Ecology.

Representatives of the genus Wollastonia are commonly found under volcanic rocks scattered on grassland in open fields and more or less steep mountain slopes. Specimens aestivate on the lower surfaces of the rocks, frequently forming clusters of individuals attached to one another.

Etymology.

Named to honour the late British malacologist and entomologist Thomas Vernon Wollaston for his indispensable contributions to the taxonomy and nomenclature of the terrestrial snails of the mid-Atlantic islands.

Taxonomic remarks.

The genus Wollastonia gen. n. is separated here from Hystricella both upon molecular and morphological features. Moreover, the analysis of the distribution of its species supports this view.

The phylogenetic analyses recovered three clades, in which morphologically mostly similar species were grouped together and that were supported in at least two of the analyses. The clade including recent species hitherto assigned to Hystricella (except H. bicarinata and H. echinulata ), supported in the BI and MP analyses (Figs 3, 5), are here regarded as representing a new genus, Wollastonia gen. n., because representatives of the morphologically distinct Callina species are interspersed between Wollastonia gen. n. and Hystricella in the strict sense receiving significant/meaningful support in the BI and MP analyses of the cox1 data alone and the concatenated mitochondrial and nuclear data. This clade includes the recent species W. leacockiana comb. n., W. turricula comb. n., and W. oxytropis comb. n. as well as three additional recent taxa that will be formally described below: W. klausgrohi sp. n., W. jessicae sp. n., and W. jessicae monticola ssp. n. (Fig. 5). The relationships of most of the taxa within Wollastonia gen. n. could, however, not be well resolved (Fig. 5) on the basis of the phylogenetic analyses of the cox1 data.

Shell size and shape are remarkably more variable in the genus Wollastonia as compared to Hystricella . Most of the Wollastonia gen. n. species have the shell surface covered with small and very dense tubercles that can be compared to a very fine granulation. The typical Hystricella 's “spiny” contour (Lat.: hystrix = spiny, deriving form Greek ὕστριξ ( hústrix = porcupine) is replaced by a smoother one.

The genital morphology of the two genera shows no substantial differences. Nevertheless, some Wollastonia gen. n. species present a higher degree of complexity with regard to the inner ornamentation of the atrium and the flagellum. Structures such as more or less developed pleats, fleshy pads and/or fringes are found in Wollastonia gen. n. that are never present in Hystricella .

The distribution ranges of Hystricella and Wollastonia gen. n. are separated and not considerably overlapping. Wollastonia colonises the southern and southwestern parts of Porto Santo, whereas Hystricella is currently found exclusively in the central and northern parts.

Most of the morphological characters that clearly distinguish Hystricella from the other native Geomitridae of Porto Santo are also valid for Wollastonia . gen. n., e.g. the continuous and detached peristome and the shape of the penial flagellum that is always short and with a remarkably blunt apex (except in W. oxytropis stat. n.). For remarks on the differentiating features, see the respective section in the re-description of the genus Hystricella above.