Cordesia atlantica, Souza & Passos & Shimabukuro & Sumida, 2021

Souza, Bruno H. M., Passos, Flávio D., Shimabukuro, Maurício & Sumida, Paulo Y. G., 2021, An integrative approach distinguishes three new species of Abyssochrysoidea (Mollusca: Caenogastropoda) associated with organic falls of the deep south-west Atlantic, Zoological Journal of the Linnean Society 191 : -

CORDESIA ATLANTICA SP. NOV.

( FIGS 9, 10)

LSID: zoobank.org:act: AEB78F45-7D70-462E-BE65- D234D850DFD4.

Type locality: Off Rio de Janeiro, Brazil, 3285 m, 25°20’18”S, 39°38’28.3”W, on and around organic falls (whale bones and wood parcels) .

Holotype: ZUEC-GAS 7920 ( Fig. 9A, D), sta. RJ3300 (25°20’18”S, 39°38’28.3”W) off Rio de Janeiro State, Brazil – ‘BioSuOr Project’ coll., 23 May 2015, depth 3285 m. Measurements: shell height: 7.05 mm; shell width: 4.9 mm.

Paratypes: #1 ZUEC-GAS 7921 , sta. same as holotype (total of 17 specimens, two of them in Fig. 9B, E); #2 ZUEC-GAS 7922 , sta. SP3300 (28°01’42.4”S, 43°31’46.8”W) off São Paulo State, May 2015, depth 3358 m (total of five specimens, one of them in Fig. 9F); #3 ZUEC-GAS 7923 , sta. ES3300 (22°50’27.1”S, 38°24’58.8”W) off Espírito Santo State, May 2015, depth 3322 m (six specimens) ; #4 MZUSP 151716 , sta. SP3300 (28°01’42.4”S, 43°31’46.8”W), May 2015, depth 3358 m (five specimens) ; #5 MZUSP 151718 , sta. ES3300 (22°50’27.1”S, 38°24’58.8”W) off Espírito Santo State, May 2015, depth 3322 m (six specimens) ; #6 MNRJ 23583 , sta. ES3300 (22°50’27.1”S, 38°24’58.8”W) off Espírito Santo State, May 2015, depth 3322 m (six specimens) .

Other materials examined: c. 380 specimens stored at ColBIO (Coleção Biológica Prof. Edmundo F. Nonato) at IOUSP. Collected in 22º50.4’S, 38º25.2’W, 3322 m depth (ES-3300); 25º20.4’S, 39º38.4’W, 3285 m depth (RJ-3300); 28º01.8’S, 43º31.8’W, 3358 m depth (SP- 3300), May/ June 2015 .

Etymology: Named after the Atlantic Ocean where this species was found. Used as a feminine adjective.

Diagnosis: Shell thin, short and broad, with teleoconch formed by up to three whorls bearing a uniformly rounded profile, delimited by shallow sulcate suture; surface brownish, sculptured by seven to eight spiral cords at the end of penultimate whorl and nine to ten more well-defined ones on the basal surface of the body whorl; 22 broad axial ribs present in the upper part of body whorl; periostracum thin, translucent, forming small bristles and axial fine lamellae; broad aperture with a narrow and short siphonal channel. Protoconch whitish, with two whorls, with a cancellate sculpture. Radula taenioglossate (formula 2 + 1 + C + 1 + 2), with a central tooth triangular-shaped and a pointed apical part set off, laterals with apical cusp bearing one or two lateral denticles and another strong denticle well below, and marginals with apical and lateral denticles, larger and less abundant in the inner marginal.

Shell: Adult shell of medium size, thin, short and broad, width about 70% of height; up to 7.05 mm in height and 4.9 mm in width (measurements of the holotype). Teleoconch formed by up to three whorls of rapid increasing diameter, each one with uniformly rounded profile, delimited by a well-marked, shallow sulcate suture; body whorl about 80% of shell height ( Fig. 9A, B, E, F, I). Umbilicus closed. Surface brownish, sculptured by seven to eight spiral cords at the end of the penultimate whorl and nine to ten a little bit more well-defined ones on the basal surface of the body whorl; in the upper part of body whorl these spiral cords are intersected to right angle by about 22 broad axial ribs. Periostracum thin, translucent, forming small bristles over the spiral sculpture and axial fine lamellae ( Fig. 9C). Opisthocline, broad aperture, about 60% of shell height, with a distinct but narrow and short siphonal channel; outer lip thin, ornamented by the periostracum bristles, with a convex and uniformly rounded profile in larger animals, slightly expanded in lower part ( Fig. 9A, B, E) (in some smaller ones, almost straight in the upper portion and slightly shouldered posteriorly; Fig. 9F); inner lip sinuous. Columella marked by a shallow furrow, curved to the right and forwards anteriorly. Anal canal absent. Apex often eroded, without the protoconch and first teleoconch whorls, sealed with a calcareous plug. Protoconch whitish ( Fig. 9F), with two whorls, with a cancellate sculpture composed by both distinct, welldefined spiral and axial cords ( Figs. 9H, J, K, M), the former ones absent in the upper fourth of each whorl ( Fig. 9M); transition between the proto- and teleoconch well defined. Operculum horny, thin, yellowish brown, paucispiral, with distinct growth lines and nucleus 25% height, width two-thirds the height of the operculum ( Fig. 9N); small relative to the size of the aperture. Specimens with 1.6 mm to 7.2 mm in shell height and 1.1 mm to 4.9 mm in width.

Radula: About 1.8 mm long; narrow, length 12 times as broad in adult specimens ( Fig. 10A). Taenioglossate, formula 2 + 1 + C + 1 + 2 ( Fig. 10B, C). Central tooth with expanded base, triangular-shaped; apical part set off, pointed ( Fig. 10C, D). Lateral tooth robust, with apical cusp bearing one strong central denticle and one or two lateral denticles on outer side ( Figs. 10E, F); another strong denticle is also present well below. Both marginal teeth long, with apical and lateral denticles, larger and less abundant in the inner marginal ( Figs. 10C, G).

Soft parts: Males possess a cylindrical penis, that goes slender on distal side ( Fig. 7D, F). Two large, right, annulated pallial tentacles with almost similar sizes ( Fig. 7D, E). Head-foot of normal size for the group, with a flat and large snout. Large and smooth cylindrical cephalic tentacles without eye-lobes ( Fig. 7D). Ventroapical mouth. Soft parts covered with small sensory cilia.

Remarks: The shell and the radula of C. atlantica are similar to those of C. provannoides , which was described based on three specimens. The holotype of C. provannoides appears to be worn, and its shell looks a little bit narrower ( Warén & Bouchet, 2009: fig. 9E) than that of the new species and the siphonal channel is slightly wider in the latter than in the former. The same arrangement of pallial tentacles is observed on both Cordesia species , but both are of the same size on C. atlantica , while one is much smaller in C. provannoides . The penis is also cylindrical, but it becomes thinner in the new species. More precise comparisons among these species are necessary, this pending on the availability of well-preserved specimens of C. provannoides .

Intraspecific divergences of Cordesia atlantica are lower than 2% for 16S and around 1% for COI. There are no molecular data for C. provannoides to compare with the new species. For COI, C. atlantica is less distant from the genus Abyssochrysos than other genera (13.4% mean distance) and for 16S it is closer to Rubyspira species (13.2% mean distance) (Table 2).

Distribution: Only known from the sites of this study. South-west Atlantic deep-sea, in organic falls from 3285 to 3358 m. Most individuals were collected on the control substrate (rubber mat) (~44%), but they were also abundant on wood (~39%) and, less frequently, on whale bones (17%). It was the first time Cordesia was found on organic falls. The presence of gastropods of different sizes and stages of development indicates a probable recruitment in the South-west Atlantic deep-sea throughout the year. The high number of whorls, axial and spiral sculptures on the larval shell suggests a planktotrophic development, like that of C. provannoides .