Bela nebula

Daniele, Scarponi, Landau, Bernard, Janssen, Ronald, Morgenroth, Holly & Bella, Giano Della, 2014, Lectotype designation for Murex nebula Montagu 1803 (Mangeliidae) and its implications for Bela Leach in Gray 1847, Zootaxa 3884 (1), pp. 45-54 : 50-51

Bela nebula Lectotype: implications for Bela and Mangelia

Although the number of species included in the genus Bela has been drastically reduced during the last decades, Bela may still represent several distinct genus-level groups. Doubts on its monophyly have been expressed, in particular concerning Pleurotoma brachystoma Philippi, 1844 and related species (see Mariottini et al. 2008, 2009 for further details). The species illustrated in figures 2A–I and 2M– O give an idea of the broad morphological shell variability seen in the genus Bela , as currently conceived.

The designation of a lectotype for B. nebula allows closer constraint on the usage of Bela Leach in Gray (1847a). The genus Bela should include species with small shells (commonly shell height <15mm), with the protoconch nucleus smooth; if multispiral (i.e., whorls> 2), the protoconch whorls are smooth, except for the last where low, curved axial riblets are overrun by a few rows of obsolete spiral elements forming swollen tubercles at the intersection. Micropustules are sometimes present, especially in proximity to the protoconch-teleoconch junction. The teleoconch is generally fusiform with the spire whorls of slight to medium convexity, hence giving the spire a conical profile. Sutures are commonly bordered by a subsutural, slightly swollen collar. On the teleoconch, the axial sculpture is predominant and extending between the sutures, whereas the numerous spiral elements are subdued and commonly interrupted by dense growth lines. In addition, dense micropustules cover the remaining teleoconch surface. The aperture is lanceolate, elongated, with a thin outer lip smooth inside. The siphonal canal is short and the anal sinus is shallow and placed on the subsutural ramp. Examples of species we consider belonging to Bela are: Ginnania taprurensis Pallary 1904 ; Pleurotoma fuscata Deshayes 1835 sensu Nordsieck (1977) ( Figs. 2 A–C, M); Raphitoma zonata Locard 1891 , R. decussata Locard 1891 and Bela pseudoappeliusi Naldi, Della Bella & Scarponi, 2013 ; we refer to Tucker (2004) and Naldi et al. (2013) for iconographic references.

As mentioned above, Pleurotoma brachystoma Philippi 1844 ( Figs. 2 D–F, N) and allied Pleurotoma scalariforme Brugnone 1862 , Bela nitida Pavia 1976 , B. africana Ardovini 2008 , B. exilis Ardovini 2004 (see Tucker 2004 and Mariottini et al. 2008 for iconographic references), even though often referred to Bela , are dubiously attributed to it (e.g., Mariottini et al. 2008). These species have very small shells (shell height <10mm), with protoconch ornamentation similar to that of B. nebula , except the sculpture on the last protoconch whorl is stronger, the tubercles are pointed at the intersection with the axials, and extend over more of the surface. With respect to Bela nebula , the teleoconch of P. brachystoma and allied species (what we subsequently refer to as the ‘ B. brachystoma group’) is characterized by inflated whorls separated by a deep suture that gives the spire a cyrtoconid-turriculate profile. In addition, the sculpture is more pronounced than in B. nebula and allied species, especially the spiral elements, which are more widely spaced, less numerous and more variable in thickness (i.e., very thin on the subsutural ramp and thicker below the shoulder). The aperture of P. brachystoma is also peculiar, with a sub-rectangular outline and a very thin, almost transparent, outer lip. For anatomical features see Fretter & Graham (1985) and Mariottini et al. (2008).

Additional species within the genus Bela (see Tucker 2004 for references) that share features that are distinct from those of members of the B. nebula group include a set of fossil species: Pleurotoma nevropleura Brugnone 1862 , Raphitoma plicatella Bellardi 1847 and R. hispidula Bellardi 1847 ( Figs. 2 G–I, O). These species have larger shell dimensions in comparison to members of the B. nebula group; however, shell height is commonly less than 30 mm. The protoconch ornamentation is comparable to that of B. brachystoma group, but with weaker sculpture. The teleoconch shape is distinct from that of the previous groups (i.e., B. nebula and P. brachystoma groups). The teleoconch profile is generally biconic, the whorls are depressed to slightly concave above the angular periphery, flat-sided below, resulting in a markedly turriculate spire profile. The suture is linear, bordered by a narrow subsutural collar (like in B. nebula ). The axial sculpture is more pronounced and generally sharper than in the previous groups. The spiral ornamentation is denser and stronger than in the B. nebula group, composed of cords of very irregular strength, usually deeply indented by growth lines, most conspicuously so below the periphery (such as in the P. brachystoma group). The aperture is elongate and narrow to relatively wide; the outer lip is thin and smooth inside, and the anal sinus is on the shoulder slope, relatively wide, but shallow; compared to the previous groups. The siphonal canal is longer than in previous groups.

With this stricter definition proposed for the genus Bela , and the designation of a neotype for the genus Mangelia type species: M. striolata ( Figs. 2 J–L, P) by Spada & Della Bella (2010), differences between the allied (and sometime interchanged) genera become clearer.

In Mangelia , the outer lip is thickened by a varix externally and internally thickened by a callus extending over the outer lip or forming a bulge in the proximity of the anal sinus. The anal sinus is well-developed in most species, C-shaped, and placed subsuturally ( Figs. 2 K–L). The axials tend to be thin (smaller than interspaces), sharp, slightly sinuous with maximum curvature in their upper part. In Bela , the outer lip is thin, lacking an external varix and smooth internally; the axial sculpture is composed of broad, rounded ribs, usually wider than their interspaces. In contrast, in the genus Bela , the anal sinus is shallow, v-shaped and placed on the subsutural ramp. There are also small differences in the protoconch sculpture: on the last protoconch whorl in Mangelia the spiral cordlets are continuous and swollen over the axial riblets ( Fig. 2 P), whereas in Bela the cordlets are weaker and pointed over the axials ( Figs. 1 J–L; 2M). Anatomical features for B. nebula and M. striolata are described respectively in Fretter & Graham (1985) and Spada & Della Bella (2010).

Figures

FIGURE 1. Optical microscope and scanning electron microscope (SEM) photos of Bela nebula (Montagu 1803); scale bars 1 mm and 0.1 mm, respectively. A – C lectotype, # Exem: Moll 4253 / 5; D – F topotype, # NMW 1953.183.02275 / 1; G – K topotype # NMW 1953.183.02276 / 1; L topotype # NMW 1953.183.02281 / 1. Topotype specimens from Torbay, Devon (UK). # shell number in Table 1.

FIGURE 2. Optical microscope photos of teleoconch features; scale bars 1 mm. A – C) Bela fuscata (Deshayes, 1835) sensu Nordsieck (1977), from Punta Ala (Grosseto, Italy). D) Bela brachystoma (Philippi, 1844) from Djerba (Tunisia), and (F – G) from Porto Azzurro (Leghorn, Italy). G – I) † Bela hispidula (Bellardi, 1847) from Formello Quarry (Rome, Italy). J – L) Mangelia striolata Risso, 1826 from Procida Island (Naples, Italy). SEM photos of protoconch features; scale bar 0.1 mm. M) B. fuscata from Punta Pioppeto (Naples, Italy). N) B. brachystoma from Elba Island (Grosseto, Italy). O) † B. hispidula from Ca’ Lametta (Bologna, Italy). P) M. striolata from Djerba (Tunisia). Symbol: † fossil.