A Rosetta Stone for eastern Pacific Caecidae (Gastropoda: Caenogastropoda) Author Raines, Bret K. text Zootaxa 2020 2020-08-11 4827 1 1 146 journal article 8690 10.11646/zootaxa.4827.1.1 c93b6c76-aaac-4fcd-acca-3dc3e0fd62d4 1175-5326 4402308 2CE528C2-DE07-4155-B37D-BE2C342A3901 Caecum cocoense Sibaja-Cordero, García-Méndez & Troncoso, 2013 Figures 102 A–E, 103A–G, 104A–B, 128D Caecum cocoensis [ sic] Sibaja-Cordero et al. 2013: 144 , figs. 24a–n, holotype MZUCR9740-01 , paratypes MZUCR9741-01 , MZUCR9742-01 , Isla del Coco, Costa Rica . Material examined. In addition to the type material the following lots were examined. SBMNH : SBMNH 619898 , Isla Jaltemba , Nayarit , Mexico , 10–25 m , 3 sh ; SBMNH 620535 , Barra de Navidad , Jalisco , Mexico , 3–45 m , 1 sh . Original diagnosis. “Shell length: 2.10 mm ±0.20; width min: 0.47 mm ± 0.01, and width max: 0.50 mm ± 0.03. The shell is slightly arched and translucent, with some tan colour ( Fig. 24a ). The form of the septum is mammillate, tapering, and constant ( Figs. 24 g–j). There is no mucro present in the specimens collected. The sculpture is formed by rings more widely spaced and well marked distally, becoming near the aperture narrower and less well marked. The shell presented some porosity with minute holes in the rings and marked pits in the inter ring spaces ( Figs. 24 l–n). Moreover, subadult traits are present in some sections of the tube of the specimens, where the ringed sculpture appears to be interrupted by incised lines ( Figs. 24d, e, k, m ), representing a transitional zone between the teleoconch growth stages. The aperture is smooth and underdeveloped, another indication of a subadult stage; and the operculum is brown, flat, and presents a heavy concentric sculpture ( Figs. 24b, c, f ). Finally a hole or deep depression is present in the septum of the specimens ( Figs. 24 g–j).” Sibaja-Cordero et al. (2013: 144) . FIGURE 102. Caecum cocoense . A . Profile view (SBMNH 619898). B . Profile view (SBMNH 619898). C . Ventral view (SBMNH 620535). D . Operculum, exterior surface ( Sibaja-Cordero et al. 2013 ). E . Holotype, MZUCR9740-01. Description. Protoconch and teleoconch I unknown. Teleoconch II short, sculptured with axial growth lines and weak longitudinal striae ( Fig. 103B ), transition to teleoconch III marked by slight axial constriction ( Fig. 103A ). Teleoconch III about four times length of teleoconch II, sculptured with sparse, thin, longitudinal striae ( Fig. 103C ), transition to teleoconch IV marked by suture, followed by axial ribs ( Fig. 103A ). Teleoconch IV sculptured with diffuse orange peel texture and flattened, axial ribs, irregularly spaced ( Figs. 103 D–E), septum with squared rim, slightly concave ( Fig. 103D ), transition to teleoconch V marked by incised axial line ( Fig. 103D ). Teleoconch V (last growth stage) small for genus [Tol 1.64–1.95 mm ], tubular, rather thick, mildly to moderately and regularly arched [Larc: 0.58–0.70 mm ; Arc: 0.18–0.22 mm ], subcylindrical, semi-translucent ( Figs. 102 A–B). Axial sculpture of wide, flattened axial ribs, spacing and intensity rather irregular, becoming obsolete centrally ( Fig. 103G ). Surface sculptured with diffused orange peel texture crossed by sparse, thin, longitudinal striae ( Fig. 103F ). Posterior [Dpe: 0.30–0.32 mm ] with moderately thick edge, rounded shoulder ( Fig. 102C ). Septum mucronate, slightly recessed ( Fig. 102C ). Mucro low, heel-like, positioned on dorsal margin, covered by septum ( Fig. 103G ). Aperture [Da: 0.35–0.40 mm ] thick, with one wide, flattened, axial rib ( Figs. 102C , 103G ). No varix. Lip smooth, weakly developed, with slightly deflected peristome ( Fig. 102C ). Periostracum thin to obsolete, yellowish ( Fig. 102A ). Operculum thin, light brown; exterior surface with 6–7 concentric rings ( Fig. 102D ). Conceptual reconstruction of growth stages shown in Fig. 104A . FIGURE 103. Caecum cocoense shells at different growth stages. A . Teleoconchs II to IV (SBMNH 619898). B . Surface of teleoconch II (SBMNH 619898). C . Surface of teleoconch III (SBMNH 619898). D . Teleoconchs IV to V (MZUCR9740-01). E . Surface of teleoconch IV (SBMNH 619898). F . Surface of teleoconch V (SBMNH 619898). G . Teleoconch V (SBMNH 619898). FIGURE 104. Caecum cocoense . A . Conceptual reconstruction of growth stages. B . Species distribution map. Distribution and Habitat. Originally described from Cocos Island at 9– 14 m . However, one early subadult stage and three adult specimens have also been found on the mainland of Costa Rica and as far north as Nayarit , Mexico . Subtidal, 3– 45 m . Uncommon on coarse sand and gravel substrates. Remarks. The type material of Caecum cocoense is comprised of three late subadult stages ( holotype MZUCR9740-01, Fig. 102 E ). Additional specimens located in the SBMNH collection allowed for a more complete description.