Caecum spiculum species novum

Figures 120 A–E, 121A–H, 122A–B, 126Q

Fartulum sp. 1— Kaiser & Bryce, 2001: 18, pl. 20, figs. 1–1a.

Type material. Holotype, SBMNH 637742; paratypes, SBMNH 637740 (3 figured); SBMNH 637743 (3 figured, 3 unfigured).

Additional material examined. SBMNH: SBMNH 637741, Isla de Malpelo, Colombia, 12–30 m, 2 sh ; SBMNH 637738, Isla de Malpelo, Colombia, 12–24 m, 3 sh ; SBMNH 637739, Isla de Malpelo, Colombia, 17–30 m, 3 sh .

Type locality. Isla de Malpelo, Colombia.

Diagnosis. Teleoconch subcylindrical, with slight increase in diameter from posterior end to aperture. Surface semi-translucent, sculptured with fine, sinuous, longitudinal striae. Septum mucronate, with pronounced finger-like, dart-shaped mucro. Aperture simple, without a varix. Periostracum thin, translucent tan.

Description. Protoconch and teleoconchs I, II, III unknown. Teleoconch IV sculptured with short, sinuous, conspicuous, dense, longitudinal striae (Fig. 121F), transition to teleoconch V marked by slight incised axial line, immediately followed by increase in diameter (Figs. 121A, D). Teleoconch V (last growth stage) small for genus [Tol 1.51–1.60 mm], tubular, mildly to moderately and regularly arched [Larc: 0.59–0.65 mm; Arc: 0.14–0.16 mm], subcylindrical with slight increase in diameter from posterior to aperture, semi-translucent (Fig. 120 A–B). Surface sculptured with fine, sinuous, dense, longitudinal striae (Figs. 121 G–H). Striae continuous and finer than previous stage (Fig. 121G). Posterior [Dpe: 0.25–0.26 mm], with rounded shoulder (Fig. 120D). Septum mucronate, inflated (Fig. 120D). Mucro finger-like, dart-shaped, conical, positioned on dorsal margin (Figs. 121E, H). Aperture [Da: 0.32–0.34 mm] simple (Fig. 120D). No varix. Lip smooth (Fig. 120D). Periostracum thin, translucent tan (Fig. 120A). Operculum unknown. Conceptual reconstruction of growth stages shown in Fig. 122A.

Etymology. The name is derived from the septum’s mucro, which has dart-like shape.

Distribution and Habitat. Currently only known from type locality, Isla de Malpelo, Colombia. Subtidal, 12– 30 m. Uncommon on rock and dead coral substrates.

Remarks. Caecum spiculum, with its pronounced dart-shaped mucro and ribless teleoconch is unique among eastern Pacific caecids. In fact, there are only two species from the western Atlantic that are even remotely comparable, Caecum lineicinctum de Folin, 1880 and Caecum trindadense Lima, Santos & Absalão, 2013. While C. trindadense has somewhat similar longitudinal striae, the apertural end has a well-defined swelling, and the mucro is strongly recessed and blunt. Caecum lineicinctum also has a blunt recessed mucro, and the teleoconch has axial growth rings that increase in strength toward the apertural end, forming a varix.