Caymanostella davidalani sp. nov.

Figures 11–13

Alvarado et al. (2022) Table 1, as Belyaevostella sp. Caymanostella sp.

Diagnosis. Body subpentagonal. Thin epidermis.Arms broad.Abactinal plates oval to polygonal shaped with curved margin, tightly overlapping. Abactinal armament spiniform and sparsely distributed. Central disc plates imbricate irregularly. One row of dorsal-lateral plates on each side of a row of carinal plates. Terminal plate rectangular-shaped. Each inferomarginal plate larger than the adjoining superomarginals. Gonopores visible, located in notches at the radial margin of the proximal-most superomarginal plates. Adambulacral spines thorny, narrow and elongated.

Materials Examined. Holotype: SIO-BIC E7101 (prepared for SEM), on wood at Seamount 2, Costa Rica, 8.8003 ° N, 85.1899 ° W, 1837 m depth, June 9, 2017, HOV Alvin dive 4926, collectors Greg Rouse and Josh Sisson [GenBank COI = PP627124; 16S = PP572463; H3= PP658047] . Paratype: MZUCR ECH2403, on wood at Mound 12, Costa Rica, 8.9325 ° N, 84.3074 ° W, 1002 m depth, June 5, 2017, HOV Alvin dive 4922, collectors Jen Le and Chris Roman. Other materials: SIO-BIC E7226 (specimen completely used up for DNA extraction; GenBank COI = PP627128), on deployed wood at Jaco Scar, Costa Rica, 9.1146 ° N, 84.8356 ° W, 1845 m depth, October 18, 2018, HOV Alvin dive 4972, collectors Greg Rouse and Avery Hiley ; SIO-BIC E7238 (specimen completely used up for DNA extraction; GenBank COI = PP627127), SIO-BIC E7242 (specimen completely used up for DNA extraction; GenBank COI = PP627126), on wood trap at Jaco Scar, Costa Rica, 9.1151 ° N, 84.8398 ° W, 1887 m depth, October 22, 2018, HOV Alvin dive 4976, collectors Shana Goffredi and Drew Bewley.

Description. Adult body subpentagonal (Range of adult specimens: R = 4.30–5.19 mm, r = 3.34–4.15 mm, R/r = 1.25–1.28; Holotype R = 5.19 mm, r = 4.15 mm, R/r = 1.25, bent, measurement may not be accurate). Thin epidermis covers abactinal and actinal surface, dissolves easily with bleach. Abactinal plates irregularly oval-shaped with curved edges and imbricate distally. These plates overlap tightly, and outline of plates may be inconspicuous. Central disc plates oval to polygonal with curved edges, irregularly imbricating. Central disc plates covered with elongated and spiniform abactinal spinelets of various lengths (0.08–0.24 mm). Spinelets consist of a smooth and sometimes wide base, stem with longitudinal ridges, and slightly expanded thorny crown (Fig. 13E, F). Other abactinal plates bear abactinal spinelets of similar shape with density ~30/mm 2. Most spines on central disc plates lost. Arms broad. One row of distal-lateral plate on each side of a somewhat discernable row of carinal plates, imbricating with irregular patterns (Fig. 13A). Terminal plate rectangular with pore, marginal edge slightly narrower (Fig. 13A) and bearing abactinal spinelets slightly longer and larger than other abactinal spinelets on the arm (> 0.16 mm). Two fringe spines (0.45 mm) at the margin of each terminal plate. Madreporite not visible on the holotype, but visible in paratype MZUCR ECH2403, appearing as a single slit (Fig. 11E).

One row of superomarginal plates and one row of inferomarginal plates are parallel with each other along each side of the arm. Both rows are imbricating. Inferomarginal plates are more elongated and rectangular, longer than wide. Superomarginals are of similar shape but smaller than inferomarginals. The proximal-most superomarginal plates differ from the other superomarginals in being much larger in size and almost oval-shaped, wider than long. The row of inferomarginals forms a flat and outstanding margin relative to the slightly convex abactinal surface (Fig. 13A). Gonopores are present between the proximal edges of the first and second proximal-most superomarginal plates, each at a notch of the proximal-most superomarginal plate (Figs 11G, 13B). There are 13 or fewer abactinal spines per inferomarginal, similar in length to those on central disc plates (~ 0.13 mm) and those closer to the margin tend to appear longer (Fig. 11F). Distal-most abactinal spinelets on the inferomarginals are longer than other abactinal spinelets on the plate (> 0.17 mm). There are 2–3 club-shaped fringe spines (0.45–0.52 mm) at the margin. Fringe spines are slightly compressed laterally, have more thorns on the abactinal side, and appear wing-shaped in lateral view (Fig. 13G, H).

Ambulacral furrows slightly petaloid and each includes 12 pairs of tube feet. Bar-shaped adambulacral plates (10 pairs) bear adambulacral spines (0.39–0.67 mm) arranging in one or two alternating rows with uniform orientation towards body margin (Figs 11I, 13D), with larger plates having more spines. Each adambulacral plate has one furrow spine with varying orientations, from pointing towards the furrow to towards body margin like adambulacral spines. Adambulacral spines and furrow spines are tapering, thorny, and elongated (Figs 11I, 13C, D, I, J). There are few thorns on the lateral side. Adambulacral spines tightly lie on the adambulacral plates, pointing towards the body margin while obscuring the outline of the plates. The oral side of inferomarginal plates bears spines morphologically identical to adambulacral spines and with similar orientation. Oral opening is distorted, large, and with slightly everting stomach (Fig. 11B). Paired oral plates are at the base of the interradius and form a ridge in-between. Each oral plate has one suboral spine and two marginal spines pointing towards the oral opening, similar to adambulacral spines but wider (Fig. 11H). Actinal chambers are distorted, irregular in shape and contain gonads (Fig. 11H). Actinal chambers are located at the proximal part of the interradius and are bounded by the oral plates and the proximal-most adambulacral plates.

Variation. The limited material of C. davidalani sp. nov. does not allow for a detailed analysis of morphological variations across individuals and ontogeny. Paratype MZUCR ECH2403 (R = 4.3 mm, r = 3.34 mm, R/r = 1.28) is smaller, yet has a more conspicuous single-slit madreporite (Fig. 11E) and gonopores (Fig. 11C) than the holotype. Gonopores are present between proximal edges of the first and second proximal-most superomarginal plates, and are slightly protruding, with a narrow slit at the center. The abactinal spinelets are sparse, and inferomarginal plates do not form a flat and outstanding margin, but instead form a convex abactinal surface with the other plates. Differences in size and shape between inferomarginals and superomarginals are less conspicuous in MZUCR ECH2403. Gonad clusters are less developed, which indicates an early life stage compared to the holotype (Fig. 11D). Three early-stage juveniles (R = 0.31–0.68 mm; r = 0.28–0.55 mm; R/r = 1.10–1.24) (identities all confirmed by DNA) are more pentagonal than stellate (average R/r = 1.18) (Fig. 12A–F). The body of the juveniles is thin and semi-transparent. Abactinal armaments are few and short. Pores are visible at the arm tip (Fig. 12A, C–F). Ambulacral furrows are wide and tube feet can be transparent in live specimens (Fig. 13D). Tube feet of the smallest specimen (SIO-BIC E7238, Fig. 12E, F) form almost a circle outlining the body margin. All juveniles were used up for DNA extraction.

Distribution. Known only from the Pacific margin of Costa Rica, 1002–1887 m.

Remarks. The body shape of the Caymanostella davidalani sp. nov. holotype is not as stellate as in C. scrippscognaticausa sp. nov. of similar radii, yet more stellate than those of other species of Caymanostella . The epidermis appears thinner than that in C. scrippscognaticausa sp. nov. and more easily dissolved with bleach. Abactinal plates are thinner and overlap more tightly than those of C. scrippscognaticausa sp. nov., so that the surface appears more even. Terminal plates tend to be more elongated (Fig.13A) than the square-shaped terminal plates of C. scrippscognaticausa sp. nov. (Fig. 8A). Comparing fully mature specimens, the arm tip of C. davidalani sp. nov. appears wider than that of C. scrippscognaticausa sp. nov. Because of the thin epidermis, thorns of adambulacral spines are more conspicuous in C. davidalani sp. nov. (Fig. 13C) than those in C. scrippscognaticausa sp. nov. (Fig. 8C). Adambulacral spines are generally longer and narrower in C. davidalani sp. nov. compared to those in C. scrippscognaticausa sp. nov. (Table 6). Juveniles and small paratypes of C. scrippscognaticausa sp. nov. (SMF 6939, Fig. 6; SIO-BIC E11221B, Fig. 9G, H) and C. davidalani sp. nov. (MZUCR ECH2403, Fig. 11C, D) are morphologically similar. The spiniform abactinal armament is like that of C. laguardai but less dense. The abactinal spinelets are like those of C. scrippscognaticausa sp. nov. (Table 6). The shape and pattern of the central disc plates are different from those in C. admiranda and C. madagascarensis (Table 6). The position of the gonopores is like those of C. scrippscognaticausa sp. nov. and C. spinimarginata (Table 6). Inferomarginals of C. davidalani sp. nov. are not as elongated as those of Belyaevostella (Rowe 1989; Fujita et al. 1994). Maximum uncorrected intraspecific COI distance of C. davidalani was 1.0% (Table 3). C. davidalani sp. nov. was recovered as sister to the clade of C. scrippscognaticausa sp. nov. and C. laguardai sp. nov., and the three species formed a well-supported clade sister to C. cf. spinimarginata (Fig. 3). The minimum uncorrected divergence between C. davidalani sp. nov. and C. scrippscognaticausa sp. nov. was 10.5%, while the uncorrected 16S distance between C. laguardai and C. davidalani sp. nov. was 8.6% (Tables 3, 4).

A sub-adult paratype of C. davidalani sp. nov. (MZUCR ECH2403) was collected from the same piece of wood harboring several paratypes of C. scrippscognaticausa sp. nov. (Dive 4922, Mound 12, June 5, 2017, Fig. 2B), showing sympatry of the two species. Two juveniles (SIO-BIC E7238, E7242) were on the same wood trap (Dive 4976, deployed May 29, 2017, and recovered October 22, 2018) where the holotype of the ribbon worm Alvinonemertes christianeae Sagorny, von Döhren, Rouse & Tilic, 2022 was collected (Sagorny et al. 2022).

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Etymology. Caymanostella davidalani sp. nov. is named for David Alan Lewis, in appreciation of his support of marine biodiversity research and the Scripps Institution of Oceanography Benthic Invertebrate Collection.