Curtitoma contraria, Bonfitto & Morassi, 2012

Bonfitto, A. & Morassi, M., 2012, A new sinistral turriform gastropod (Conoidea: Mangeliidae) from Taiwan, Zootaxa 3415 (1), pp. 63-68 : 64-66

Curtitoma contraria sp. nov.

Type material: Holotype NMNS ( NMNS-6899-001 ) ( Figs. 1A–B, 1D–H ), paratype 1 NMNS ( NMNS-6899-002 ) ( Fig. 1C ), paratype 2 ZRC ( ZRC.MOL.3177), paratype 3 MZB ( MZB 49750) ( Figs. 1 I-M), paratype 4 and 5 MNHN ( MNHN IM-2010-22263).

Type locality: off Suao, NE coast of Taiwan 24° 30' 348'' N, 122° 04' 35'' E, 397–417 m.

Material examined: NE coast of Taiwan: off Suao , 24° 30' 348'' N, 122° 04' 35'' E, 397–417 m [Fishing boat “Rih- Jheng 101” stn CP 269] (six shells: holotype and 5 paratypes) .

Description: Shell sinistral, solid with ovate-cylindrical shape (b/l 0.50–0.54; a/l 0.47–0.51 based on adult shells; n=3). Teleoconch consisting of 2.5 whorls. Whorls angularly shouldered above mid-whorl from beginning of teleoconch. Suture distinct but not channeled, bordered by a spiral cord. Sutural ramp shallowly concave sculptured by coarse anal sinus growth lines. Axial sculpture of narrow, angularly rounded, slightly opisthocline ribs, with much broader interspaces, extending from suture to suture, becoming prosocline, arcuate and much weaker on sutural ramp. There are 14–17 axial ribs on penultimate and last whorls (on latter part of last whorl, two to three axial ribs may be replaced by coarse growth lines). Axial ribs crossed by narrower, widely-spaced spiral cords, producing small angular nodules at points of intersection, particularly prominent on shoulder cord. There are three spiral cords on penultimate whorl ( Fig. 1F ) and four on last whorl, the strongest on shoulder angle. On last whorl, the interspace between the spiral cords may bear one secondary cord ( Fig. 1J ). Base of last whorl sculptured by 10–12 spiral cords. Under SEM, the surface between spiral cords is covered by microscopic, spirally aligned pustules ( Figs. 1G–H ). Aperture somewhat oblanceolate to subrectangular, with its greatest width at posterior third, moderately strongly bent at siphonal canal. Columella rather short, twisted. Inner lip with a relatively thick callus (adult shells) and a small but distinct pseudumbilicus between its abapical end and siphonal canal ( Fig. 1E ). Siphonal canal short, broad and shallow. Outer lip thin with distinct anal sinus ( Fig. 1D ), its deepest point at middle of sutural ramp. Protoconch domed of 1.5 whorls; first whorl depressed, covered with spiral threads rendered irregularly rugulose by even finer, indistinct axial threads; last half whorl with thin axial riblets crossed by spiral cords gradually merging into the post-nuclear sculpture ( Figs. 1K–M ). Diameter: 0.57–0.63 mm (termination ill-defined). Shell glossy yellowish-white (when fresh).

Dimensions: Holotype: 3.6 mm x 1.9 mm, aperture height 1.7 mm; paratype 1 (adult): 3.6 mm x 1.8 mm, aperture height 1.8 mm; paratype 2 (adult): 3.3 mm x 1.8 mm, aperture height 1.7 mm; paratype 3 (juvenile) 2.7 mm x 1.5 mm, aperture height 1.5 mm paratype 4 (juvenile) 2.6 mm x 1.4 mm, aperture height 1.4 mm; paratype 5 (juvenile) 2.3 mm x 1.36 mm, aperture height 1.3 mm.

Discussion: Following Bogdanov’s (1989) key to the Oenopotinae genera, the new species is tentatively assigned to Curtitoma on the basis of its distinct anal sinus and protoconch sculpture consisting of numerous fine spiral threads. Apart from being left-handed, Curtitoma contraria is further distinguished from all other oenopotine species by possessing a shallow but distinct pseudumbilicus. Among species currently assigned to the genus Curtitoma Bartsch, 1941 (see WoRMS: http://www.marinespecies.org/index.php; Appeltans et al., 2011), Curtitoma contraria sp. nov. resembles Curtitoma hebes ( Verrill, 1880) , described from off Newport, Rhode Island ( Verrill, 1880) but also reported from Japan Sea ( Bogdanov & Ito, 1992; Gul'bin, 2004), in shape and presence of two spiral cords more prominent than others (at and just below shoulder respectively) bearing small nodules ( Fig. 1N , from http://www.femorale.com/ shellphotos/photos12/25561.jpg). Curtitoma contraria differs distinctly from C. hebes in possessing much fewer and stronger axial ribs and fewer, narrower and more elevated spiral cords. Furthermore, the protoconch sculpture of Curtitoma contraria is finer than that of C. hebes . Bela incisula Verrill, 1882 (= Curtitoma hecuba Bartsch, 1941 ), type species of the genus Curtitoma , resembles Curtitoma contraria in shape but otherwise differences in ornamentation are obvious for meaningful comparison. The shell of Curtitoma contraria is very small and we have considered the possibility that the available material is actually represented by juveniles. However, while smaller specimens (less than 3 mm) have a thin inner lip, larger shells (up to 3.6 mm in length) have a distinctly thickened inner lip, suggesting that the former are juveniles while the latter are adults. The small dimensions seem therefore to represent a specific feature.

The vast majority of the approximately 70,000 living gastropod species are dextral. In particular, Van Batenburg and Gittenberger (1996) stated that the proportion is ‘far more than 90%’ while Asami (1993) estimated that the proportion of dextral species of living gastropod species is more than 99%. Although many typically dextral gastropod species have been known to produce aberrantly sinistral individuals ( Rolán-Alvarez and Rolán, 1995; Lee & Frank: http:// www.jaxshells.org/reverse.htm), left-handedness as a species trait is more frequent in land and freshwater pulmonates ( Vermeij, 1975; Asami, 1993; Pierce, 1996; Palmer, 1996). It appears to be less common in marine Gastropoda ( Vermeij, 2002). Within turriform gastropods, cases of sinistral taxa are represented by the genus Antiplanes Dall, 1902 , from northwestern Pacific, in the family Pseudomelatomidae Morrison, 1965 ; the genus Scaevatula Gofas, 1990 (from Gulf of Guinea); and the Eocene Clavatula aralica ( Luković, 1924) from western Kazakhstan (Tshegan Formation) ( Amitrov, 1973; Luković, 1924), in the Clavatulidae Gray, 1853 . Additional taxa include the West African “ Asthenotoma ” sinistralis ( Petit de la Saussaye, 1839), the Brazilian Borsonia brasiliana Tippett, 1983 and the late Eocene southeastern American Sinistrella americana ( Aldrich, 1895) , all belonging to the family Borsoniidae A. Bellardi, 1875 . The European Eocene Eopleurotoma perversa ( Philippi, 1847) , and the proposed subspecies E. perversa sinistralis ( Luković, 1924) from western Kazakhstan (Tshegan Formation) ( Amitrov, 1973; Luković, 1924), are the sole sinistral taxa within the family Turridae H. Adams & A. Adams, 1853 . These few species are the only known sinistral species among more than 11,500 described species of turriform Gastropoda ( Tucker, 2004). The new species is of particular interest because it is the first sinistral species in the family Mangeliidae P. Fischer, 1883 .

Vermeij (2002) showed that species with left-handed shells arose independently 19 times among Cenozoic marine gastropods, particularly within the Neogastropoda . A significant portion of these left-handed taxa appeared in polar to mild-temperate regions. Of 14 post-Eocene sinistral clades, eight occur in tropical to subtropical seas with the exception of the continental Indo-West Pacific, tropical and warm-temperate eastern Pacific and continental Caribbean. Vermeij (2002) speculated that the geographic distribution of left-handed taxa may be related to intense predation in tropical shallow-water ecosystems. Contrary to this conclusion, Dietl & Hendricks (2006) demonstrated that shells of the Plio- Pleistocene Conus (also reported as “ Contraconus ”) adversarius Conrad, 1840, a sinistral species, showed lower frequencies of repaired crab predation scars than dextral species suggesting that sinistral species had a survivorship advantage relative to dextral species. Vermeij (2002) also proposed the hypothesis that sinistrality in marine gastropods and non-planktotrophic development are linked. This conclusion is supported by the fact that all reported cases of sinistral neogastropods have a non-pelagic larval stage ( Vermeij, 2002; Hendricks, 2009). It is generally agreed that developmental mode may contribute to intraspecific variation within a species because non-planktotrophic species tend to be split more often into isolated populations that are phenotypically distinct than species with planktotrophic development. Non-planktotrophic development may thus promote the establishment of sinistral clades by increasing the likelihood that reproductively compatible sinistral individuals will develop in the same area and mate with each other ( Vermeij, 1975; Hendricks, 2009). Kohn & Perron (1994) provided evidence that most Indo-Pacific Conus species have a plaktotrophic rather than lecitotrophic development. Thus, Vermeij’s (2002) consideration that no sinistral taxa have previously been reported from the Indo-West Pacific may possibly be explained considering differences in larval development rather than predation induced selection. In accord with this view, Curtitoma contraria sp. nov. has a lecitotrophic development. Gould et al. (1985) demonstrated that the shells of six aberrantly sinistral specimens of the pulmonate Cerion differed from dextral conspecifics in possessing a slight twist in their axis of coiling. The author concluded that reversed coiling “engenders consequences that tend to distort the shell’s form in later stages of growth” ( Gould et al., 1985: p. 1377). In our opinion, the distinctive pseudumbilicus of Curtitoma contraria , rather than the variability in shell form, may provide a further example of pleiotropic effects associated with sinistral coiling.

The possibility that the available material of C. contraria is represented by aberrantly sinistral specimens of a normally dextral species is extremely unlikely. The type material of C. contraria consists of six shells, while findings of aberrant specimens for each normally dextral species is often restricted to single individuals. The unique exception reported in the literature is the record of a population of more than ten sinistral specimens of Conus ventricosus from a shallow water bay on the west coast of Sardinia in 1981 ( Donati et al., 1984). However, in that case sinistral specimens “were in groups of 2 or 3 along with dozens of normal specimens of the same size” (p. 21). In the present case, dextral specimens of Curtitoma contraria have been found neither in sympatry with sinistral individuals nor in any other stations off Taiwan. Even considering the remote hypothesis that we are actually discussing sinistral specimens of a dextral species, proposal of a new taxon would be well supported by the morphological features (protoconch and teleoconch sculpture) distinguishing the new species from all other oenopotine taxa known to us.

Figures

FIGURE 1. Curtitoma contraria sp. n. A–B. Holotype (NMNS-6899-001), scale bar 1 mm. C. paratype 1 (NMNS- 6899-002), scale bar 1 mm. D–H. Holotype. D. Apical view, scale bar 600 µm. E. Pseudumbilicus, scale bar 300 µm. F. Penultimate whorl, scale bar 300 µm. G Surface microsculpture, scale bar 50 µm H. Surface microsculpture, scale bar 30 µm. I–M. Paratype 3 (MZB 49750) I Shell, scale bar 800 µm. J. Detail of penultimate whorl, scale bar 200 µm. K–L. Protoconch, scale bar 200 µm. M. Detail of protoconch, scale bar 60 µm. N. Curtitoma hebes (Verrill, 1880) off Cape Hatteras, North Carolina, North Blake Plateau (USA), 1200 meters, height 8.5 mm (Image reproduced by permission of www.femorale.com)