Acesta cryptadelphe Gagnon, Kenchington, Port, Anstey & Murillo, 2015, sp. nov.

Acesta cryptadelphe sp. nov.

Figs. 2, 3 C–D, 6, 14–16

Material examined. Collection data for each sample set is shown in Table 2. The holotype and three paratypes are from The Gully, off Nova Scotia; nine paratypes are from Beothuk Knoll, off northeast Newfoundland (southwest Flemish Cap), including the tissue from two specimens separated from their respective shell; and two paratypes are from the main basin of Bay d’Espoir, southern Newfoundland (Gagnon & Haedrich 2003).

Table 3 summarizes the catalogue/accession information and basic morphological data for each specimen included in the type series. All specimens are deposited in the Mollusc Collection of the Canadian Museum of Nature; five of these were used to extract partial COI gene sequence datasets, which are available on GenBank.

Description. Holotype (CMNML 097156): Complete specimen, left valve preserved dry (Fig. 14 A), right valve (posterior margin damaged) and tissue preserved in 70% ethanol; nearly equivalved (see byssal notch below), thin, ovate with narrow hinge region (hinge length to shell width ratio = 0.43; Tab. 3), exterior light beige in colour, with whitish radial ribs in median portion of shell; anterior and posterior radial ribs most prominent, attenuating toward median region, becoming broader and flattened; at least 8–9 major growth lines irregularly interrupting radial ribs. Poorly developed anterior auricle, dorsally delineating lunule; posterior auricle extending from umbo and representing about 83% of hinge length (Tab. 3). Lunule length 3.5 cm, representing 41% of shell length, with weak inward curve. Byssal notch indistinct on left valve with lunule inner margin nearly straight, slightly gapping on right valve, particularly toward the dorsal region of the lunule. Shell moderately inflated with highest inflation point at first third of shell length.

Shell interior translucent, glossy, whitish, with weak but demarcated muscle scars in the dorsoposterior region of the shell; pallial line weak. Hinge plate with deep, oblique, V-shape ligament pit extending from shell beak to mid-ventral edge of hinge plate. Hinge line delineating interior edge of hinge plate on either sides of ligament pit, nearly linear; ligament pit extending only slightly ventrally below hinge line; hinge length approx. 32% of shell length; hinge plate approx. 0.5 cm high from beak to hinge line (about 5.2 % of shell length). Umbonal cavity present behind hinge plate.

Soft tissue: mantle margin uniform and unfused, without siphon, bordered by short (contracted) tentacles. Foot small with broad, distal sole with deep byssal groove; three byssal threads attached anterodorsally to base of foot. Mouth bordered above and below by short palps, forming wide “lips”; palps not connected to gills. In living animal, soft tissue bright orange in colour, with long tentacles extending from gaping mantle margin (Fig. 2).

Character variations within type series: Tables 3 and 6 and Figure 7 present some of the morphometric variations between specimens. Shell thin, almost transparent in smallest specimen (CMNML 0 97162, Fig. 15), whitish to beige, translucent in some intermediate specimens (CMNML 0 97158, Fig. 3 D; CMNML 0 97157.1, Fig.14 B) and thick, opaque with patchy whitish, beige to light brown exterior and white interior in oldest specimens (e.g., CMNML 0 92958, CMNML 097161.1); highest valve inflation point closer to middle of shell length in larger specimens (valve inflation to shell length ratio ranges from 0.15 to 0.26; Fig. 7 E), shell width to length ratio ranges from 0.74 to 0.84. Lunule of variable length relative to shell length with ratios ranging from 0.22 to 0.58 (Fig. 7 C). Byssal notch usually represented by slight gapping of right valve, occasionally on both valves (e.g., CMNML 097157.1). Hinge length to shell length ratio ranges from 0.24 to 0.32; hinge plate height to shell length ratio varies from 0.031 in smallest specimens (e.g., Figs. 14 B, 15B) to 0.102 in older specimens (e.g., Fig. 16 B, D); hinge length to shell-width-ratio ranges from 0.30 to 0.43. Prominence of radial ribs on exterior of valves variable, faint in some specimens (e.g., CMNML 0 97160.2, CMNML 097161.1).

The two specimens with well-preserved larval shell from The Gully and Beothuk Knoll (Tab. 5; Fig. 6) display a dome-shaped, subcircular prodissoconch (P1), with a uniform glossy surface; shell type 2D (or 2C?) (Malchus & Sartori 2013). Their lengths vary between 241 and 261 µm. A prodissoconch 2 region could not be clearly identified by light microscopy.

Distribution. Only known from the material examined, off Nova Scotia, in The Gully, from 619 to 1241 m, and around Beothuk Knoll, southwest Flemish Cap, from 710 to 888 m, and from Newfoundland in Bay d’Espoir’s Main Basin, from approx. 400 to 785 m. See Figure 1 for map.

Etymology. From the terms “crypto”, meaning hidden or concealed, and “adelphe”, meaning sister or sibling, referring to the very similar shell morphology between this northwest Atlantic species and Acesta excavata in the northeast Atlantic.

Remarks. There is no DNA information available for the specimens collected by Gagnon & Haedrich (2003) in Bay d’Espoir, Newfoundland. However, according to our shell shape analysis, the specimens cluster with the Beothuk Knoll and The Gully specimens, which plot marginally outside the Acesta excavata cluster suggesting that they share close genetic affinities. The specimens from the Azores also cluster along with the Newfoundland and Nova Scotia specimens in the shell shape analysis, and may also represent A. cryptadelphe sp. nov. Further genetic work on Azorean specimens is required to test that hypothesis.

The molecular data indicate that the Acesta species, including A. cryptadelphe sp. nov., each form a monophyletic clade (Figs. 12, 13). The specimens from Newfoundland (Beothuk Knoll) and Nova Scotia (The Gully) have a low genetic divergence (average 0.012) comparable to that of other Acesta species (range 0.003 to 0.012), and with divergence from other Acesta species an order of magnitude greater; ranging from 0.103 (A. mori) to 0.166 (A. sphoni).

Habitat and species association. In the northwest Atlantic (Fig. 1), all specimens were found associated with rocky substrates below 400 m water depth, either on isolated outcrops, under overhangs or on rock walls. The first two specimens of giant file clams found in Bay d’Espoir, Newfoundland, were associated with a near-vertical underwater cliff and isolated outcrops at depths ranging from 400 to 800 m (Gagnon & Haedrich 2003). A manned diving excursion with a submersible along two vertical transects of this fjord’s rock walls (Goblin Head; Haedrich & Gagnon 1991) revealed large numbers of Acesta specimens but none of the typical cold-water coral association (i.e., Paragorgia arborea, Primnoa resedaeformis, Lophelia pertusa and Madrepora oculata (Linnaeus), Desmophyllum Ehrenberg. In The Gully, Acesta specimens occur on steep cliffs and other rock surfaces, in association with Desmophyllum dianthus (Esper) (Fig. 2 B), Paramuricea Koelliker, and occasionally in the vicinity of Primnoa resedaeformis (Gunnerus) (Kenchington et al. 2014). Around Beothuk Knoll, Murillo et al. (2011) reported Paragorgia arborea and Desmophyllum dianthus whereas Primnoa resedaeformis and Anthothela grandiflora (M. Sars) are present within Flemish Pass, just north of Beothuk Knoll.