Bittium nanum

Bittium nanum (Mayer, 1864)

Plate 2 A 1 -A 2

* Cerithiopsis nana Mayer 1864: 66: 6, fig. 46 (in text fig. 49; lapsus).

Cerithiopsis scabra Olivi — Mayer 1864: 67 [non Murex scaber Olivi, 1792, = Bittium reticulatum (da Costa, 1778)].

Bittium reticulatum (da Costa, 1778)— Dautzenberg 1889: 41.

Bittium reticulatum (da Costa, 1778)— Morton et al. 1998: 64, 88-89, 97.

Bittium cf. latreillii (Payraudeau, 1826) — Martins et al. 2009: 58, figs. 33-34 [non Bittium latreillii (Payraudeau, 1826)]. Bittium nanum (Mayer, 1864) — Moreno 2011: 61, figs. 1-32.

Type material. Single specimen height 5.0 mm, width 2.0 mm, Praia and Prainha (Pleistocene localities), Santa Maria Island, Azores; whereabouts unknown (Beu 2017:165).

Santa Maria material examined. Maximum height 4.9 mm, width 2.0 mm. DBUA-F 1296-2 (1), Malbusca, Santa Maria Island, Azores, Touril Complex, Lower Pliocene.

Original description. “ Obwohl dieses kleine Ding wahrscheinlich quartär ist und also wohl auch lebend vorkömmt, so muss ich es doch als neu ansehen, indem nicht gelungen ist, trotz Nachfragen bei den Herrn Deshayes und Hoernes und gelegentlichem Nachsehen in Reeve, ein benanntes Analogon dafür zu finden. Oberflächlich betrachtet, sieht dieses Schneckchen der Cerithiopis scabra ähnlich; allein bei genauer Prüfung entdeckt man, dass sein Bau bedeutend von dem der genannten Art abweicht. Eine viel grössere Aehnlichkeit besitzt die Species mit Cerithium lignitarum: es ist fast ein Miniaturbild davon; doch fehlt ihr unter Anderem der Columellarzahn. Von C. tubercularis und Verwandten scheidet sie das ganz verschiedene, sehr enge Körnernetz. Es ist nicht zu bezweifeln, dass diese organische Form ausgewachsen und zwerghaft sei; die Menge der gleich grossen Stücke, welche der Tuff von Praia und Prainha, in Gesellschaft von fast lauter kleinen Arten birgt spricht entschieden daf̧r [Although this little thing is probably Quaternary and therefore is probably also extant, I nevertheless have to consider it new, in that it has not been possible to find a named analogue for it, despite inquiries with Mr. Deshayes and Hoernes and occasional look in Reeve. Generally, this snail looks similar to Cerithiopis [sic] scabra; however, close examination reveals that its construction differs significantly from that of the mentioned species. The species has a much greater similarity with Cerithium lignitarum: it is almost a miniature version of it; but it lacks, among other things, the columellar tooth. From C. tubercularis and relatives it is separated in the very different, very narrow granular network. There is no doubt that this organic form is fully grown and dwarf-like; the number of specimens of the same size, from the tuff of Praia and Prainha, in the assemblage with almost all small sizes, speaks decisively for it]” (Mayer 1864: 66).

Latin description. “ Testa minuta, turrito-conica, turgidula, solidula interdum varicosa; anfractibus 7, applanatis, sutura profunda separatis, primis serie 2, ultimis series 4 granulorum transversas exhibentus; granis majuscolis, densis in serie longitudinali recta dispositis; apertura subquadrata, in canalem brevem exenute; labio simplici ” (Mayer 1864: 66).

Revised description. Shell small, turriculate, with tall, moderately slender spire. Protoconch and earliest whorls missing. Five weakly convex teleoconch whorls preserved with periphery at abapical cord, just above suture. Suture weakly impressed. Sculpture on penultimate whorl of about 14 low ribs and five cords forming small, rounded tubercles at sculptural intersection. Last whorl weakly convex, moderately constricted at base, bearing six tuberculose cords above the level of the aperture, single varix placed just above outer lip; base slightly concave, delimited by double smooth peribasal cords bearing five further smooth cords medially. Aperture ovate, outer lip thin, convex, ending in short siphonal canal. Columella slightly thickened, almost rectilinear. Columellar callus narrow, poorly developed.

Discussion. We have interpreted this specimen to represent Cerithiopsis nana Mayer, 1864. The wide apical angle and the strongly tubercular sculpture agree with the rather schematic picture given by Mayer (1864: pl. 6, fig. 46). The presence of a varix on the last whorl and cords over the base suggest that this is a Bittium Gray, 1847, rather than a member of the Cerithiopsidae.

The type material was collected from the deposits of Praia and Prainha which are Pleistocene in age. Mayer (1864: 67) misidentify specimens from the Pliocene locality of Feteirinhas (= Ponta das Salinas) as Cerithiopsis scabra (Olivi, 1792) [= Bittium reticulatum (da Costa, 1778)]. We confirm its presence in the Lower Pliocene assemblages of Santa Maria and add the locality of Malbusca.

Moreno (2011) recognized the species as occurring in the extant Azorean faunas and it is one of the most abundant components of the rocky shore environments (Costa & Ávila 2001). The species was confused by numerous authors with similar congeners such as B. reticulatum (da Costa, 1778) (Mac Andrew 1856; Dautzenberg 1889; Nobre 1924, 1930; Azevedo 1990, 1991; Azevedo & Gofas 1990; Bullock et al. 1990; Morton 1990; Bullock 1995; Morton & Britton 1995; Ávila et al. 1997, 1998, 2000; Morton et al. 1998) or B. latreillii (Payraudeau, 1826) (Martins et al. 2009).

European Bittium species are in some cases difficult to separate in the absence of the protoconch, which is missing in the Azorean fossil material. The specimen at hand is characterized by relatively flat whorls, weakly impressed suture, finely beaded sculpture with six rows of beads on the penultimate and last whorls above the aperture, and the base delimited by a double peribasal cords plus five further weaker smooth cords entirely covering the base. This species is most similar to the extant Bittium latreillii. The Azorean species seems to have a wider apical angle, the number of ribs and cords is similar, but the cords are more closely spaced and develop strong tubercles on all the cords whereas in B. latreillii the beading weakens from the adapical cord downwards. Bittium lacteum (Philippi, 1836) also has strongly beaded cords but it is immediately separated by having fewer cords on the whorls and on the base. Bittium incile Watson, 1897, reported from to Madeira, Selvagens, Canary Islands and the Mediterranean, differs in having only three or occasionally four spiral rows of tubercles (Verduin 1976: 114). Bittium depauperatum Watson, 1897, was described as endemic from Madeira and was later found also in the Canary Islands (Hernández et al. 2011). Bittium reticulatum differs in only having four spiral cords of which the first and second are more closely spaced than the third and fourth and has numerous varices distributed over all the teleoconch whorls. Based on the scant material available, we provisionally accept this taxon as valid.

Mayer (1864: 66) considered the specimen available to him to be Quaternary. A specimen from the DBUA-F collection is labelled as coming from Lower Pliocene strata, and at Malbusca only Pliocene deposits occur. We tentatively include the specimen herein although we remain uncertain if the specimen is Pliocene or Pleistocene, and Bittium nanum may have been endemic in the Azores since at least lower Pliocene times and is still present and an abundant component of rocky shore habitats.

Distribution. Lower Pliocene:?Atlantic, Santa Maria Island, Azores (Bronn in Reiss 1862; Mayer 1864). Present-day: Azores (Moreno 2011; Cordeiro et al. 2015).