Strobiligera gaesona

Strobiligera gaesona (Dall, 1927)

Figs 2K, 28–29

BRAZIL – Ceará State: [2, 1 d] specs; Canopus Bank; 02°14´25″ S, 38°22´50″ W; 240–260 m depth; 2005; MZSP 53685 • [1, d] spec. same data as for preceding; MZSP 53697.

EXTERNAL MORPHOLOGY. Body yellowish-white, covered by minute white dots in the posterior whorl adjacent to head-foot.

OPERCULUM. Yellowish, elliptical, moderately thin but rigid, semi-transparent, 3.25 distinct whorls, nucleus slightly eccentric, dislocated ~32% from center towards margin; diameter of operculum does not significantly exceed diameter of opercular pouch.

JAW. Outer side with robust scales, rhombus-shaped, rectangular or rectangular-lanceolate, covered by micro-pores up to 380 nm in diameter; scales of outer side 14.9–21.2 µm long, 7.9–9.5 µm wide, ratio length/width 1.6–2.4 (rhombus-shaped), 10.3–11.3 µm long, 4.4–4.8 µm wide, ratio length/width 2.3–2.4 (rectangular), 11.8–17.6 µm long, 4.2–6.4 µm wide, ratio length/width 2.7–3.3 (rectangular/lanceolate).

RADULA. Formula not discernible, without clear differentiation of central, lateral and marginal teeth, but at least 30 scissor-like teeth per row, frequently overcrowded in its central portion; teeth typically with three cusps, two elongated/curved ones and a smaller marginal cusp positioned sometimes on the left margin, other times on the right margin, with the change of side of the marginal cusp being prone to occur more than once in the same row; the length of marginal cusp greatly varies, often 25–70% of length of main cusps, but it can be reduced until totally disappear or even reach same size of other cusps; rare teeth with two cusps have one cusp longer than the other; rare teeth with four cusps have inner cusps more elongated than outer ones (which reach 43–85% of length of inner cusps), in some cases with an internal diastema between inner cusps; abnormal teeth of three cusps with a much reduced size may be present in a central portion of row; teeth with three cusps typically 2.0– 3.9 µm wide, but abnormally reduced teeth as little as 1.5 µm wide may occur; teeth with two cusps 1.1–1.5 µm wide; teeth with four cusps 2.3–3.5 µm wide.

Even being an upper bathyal species, S. gaesona possesses distinct eyes (Fig. 28A). The elliptical operculum seems to be a common feature of the genus, with the nucleus of S. gaesona being apparently not as eccentric as in S. lubrica (Bouchet & Warén, 1993). The operculum of S. gaesona is yellowish (Fig. 28B), similar to the ‘light tan’ color described for S. lubrica (Bouchet & Warén 1993).

The scissor-like tooth morphology of S. gaesona is peculiar, with undifferentiated teeth and a typical tooth bearing three claw-like cusps, two longer and one external (right or left) smaller cusp, with rare variations of two or four cusps (Fig. 29). This radula is nearly identical to those of S. brychia (Bouchet & Guillemot, 1978) and S. lubrica, although the total number of teeth per row could not be determined in S. gaesona. The radula of S. brychia also shows teeth with two to four cusps, and teeth with three cusps having the smaller cusp positioned to the left or to the right (Bouchet 1985), although the reduced cusp in S. brychia seems to be even shorter than in S. gaesona. The radula of S. lubrica was described as possessing teeth mainly with two claw-like cusps, but outer marginal teeth with a third cusp; however, teeth with two and three cusps (two long, one small cusp) can be found together (Bouchet & Warén 1993: fig. 1284).

The great similarities of operculum and radula between species of Strobiligera with paucispiral (S. gaesona) and multispiral (S. brychia and S. lubrica) protoconchs eliminate any suspicion that both groups could belong to different genera (Fernandes & Pimenta 2014).