Vaticinaria gulicola Valentich-Scott, MacIntosh & Goto, 2026, sp. nov.

Vaticinaria gulicola sp. nov. Figs 3 A – H, 4 A – C, 5 A – D Axinodon cf. bornianus – MacIntosh et al. 2018 Description. Shell. Exterior: Shell thin, slightly translucent, porcellaneous; very inflated, especially medially; equivalve to slightly inequivalve with left valve overlapping right in some; umbones low, broad, weakly prosogyrate; prodissoconch 240–350 µm, with commarginal striae (Fig. 4 C); inequilateral, anterior much longer, about two-thirds of total length; subquadrate to subovate; anterior broadly rounded; posterior end obliquely truncate; lunule broad, demarcated by moderately strong ridge, moderately deep, with medial ridge; escutcheon narrow, shallow, long, weakly demarcated; sculpture of irregular commarginal striae; periostracum translucent to light yellow to light brown, adherent, silky, thin but moderately thick on ends, extending over entire shell, with fine irregular widely-spaced wrinkles on the anterior and posterior slopes. Interior: Ligament internal, elongate, in narrow oblique shallow resilifer; hinge plate narrow, edentulous, with fine granules below and anterior of umbones; right valve hinge plate slightly inserts into shallow depression in left valve; adductor muscle scars ventral of midline, small, ovate, subequal; pallial line broad, irregular, ragged. Maximum dimensions 19.1 mm length, 13.4 mm height, 11.4 mm width. Gross anatomy. (Fig. 5 A, B) Adductor muscles small, subovate, subequal. Ctenidium large, with single inner demibranch; filaments thin, widely-spaced, slightly flattened, not heavily modified; each demibranch with approximately 80 to 120 filaments, depending on length of specimen. Labial palps small, thin and narrow with frilly margins. Foot moderately small and moderately broad, with prominent muscular toe; deep byssal groove running most of the length of foot; many specimens with a bundle of byssal threads. Mantle unfused for most of length, with short fusion at posterior end ventral to the posterior adductor muscle forming one small muscular siphonal opening below the anus; mantle edge thick, strongly wrinkled posteriorly and medially, strongly papillate anteriorly (Fig. 5 A); reflected mantle not observed over the exterior of the shell. Anus terminating into enclosed posterior superbranchial chamber, slightly dorsal to siphonal opening. Large elongate area of digestive diverticula above anterior adductor muscle. Type locality. Australia, Great Australian Bight, 36°4.4'S, 131°43.34'E, 4608 m. Type material. Holotype (Fig. 3 A, B, E, F) • SAMA D 77456, length 17.8 mm, height 13.1 mm, width of 2 valves 9.6 mm, preserved in 95 % EtOH; paratype (Fig. 5 A, B) • SAMA D 77457, length 15.7 mm, height 11.9 mm, width of 2 valves 8.9 mm, preserved in 95 % EtOH, from type locality; paratypes • SAMA D 43228, 10 unmeasured specimens, preserved in 95 % EtOH, from type locality, some specimens opened and dissected and dissociated from shells; paratypes • SBMNH 719798, length 17.2 mm, height 11.6 mm; length 15.1 mm, height 10.7 mm, 2 specimens in 95 % EtOH, from type locality; paratype (Fig. 3 C, D, G, H) • SAMA D 77458, length 15.8 mm, height 12.1 mm, dry shell specimen, from type locality; paratypes • SAMA D 77459, length 15.9 mm, height 11.1 mm; length 15.1 mm, height 11.9 mm, 2 specimens in 95 % EtOH, from type locality; paratype • NMV F 312855, length 19.1 mm, height 13.4 mm, preserved in 95 % EtOH, 35°48'53"S, 132°01'14"E, 3992 m; paratype • NMV F 312856, length 16.4 mm, height 11.4 mm, preserved in 95 % EtOH, 35°48'40.19"S, 131°42'36"E, 4741 m; paratype • NMV F 312857, length 16.6 mm, height 12.7 mm, preserved in 95 % EtOH, 35°42'57"S, 131°39'16"E, 4022 m; paratypes • NMV F 240623, 4 unmeasured intact specimens in 95 % EtOH, 35°48'52"S, 131°42'10"E, 4618 m. Etymology. The specific epithet is a noun in apposition formed from Latin gula, “throat” and - cola, “inhabitant of”, referring to the position of the bivalve in the esophagus of the host urchin. Distribution. Great Australian Bight from 3992–5081 m (Suppl. material 3). Habitat. Found in deep water on soft sediment in the Great Australian Bight. Commensal relationship. Six specimens were found within the esophagus of an extremely fragile urchin retrieved by trawls and only in fragments (Fig. 5 C, D). The esophagus still had portions of the urchin test attached to one end, which initially helped to identify the host as an echinoid. A COI sequence was obtained from a portion of the esophagus / intestine (NMV F 311894) and found to have a 98.9 % match with the irregular echinoid Carnarechinus clypeatus (Nicolas Mongiardino Koch, personal communication, 14 March 2024). Additional occurrences. We have identified in the literature several references to bivalves found in deep-sea urchins that we can now confidently refer to the genus Vaticinaria. In 1976, Cruise 16 of the research vessel Dmitriy Mendeleev surveyed waters around Australia, New Zealand, and in the Southern Ocean (Malyutina 2004). Mironov (1978) examined and commented on GAB specimens of Carnarechinus clypeatus, finding “ the intestine directly at the mouth opening forms a folded expansion in which bivalves are found. Up to nine specimens were found in an expansion. These molluscs were from 1.3 to 15 mm long. ” Based on sketches of these specimens (Elena Krylova, personal communication, May 2024) and their location in the GAB, we believe the examined mollusks to be Vaticinaria gulicola sp. nov. In addition to these findings, the research vessel Akademik Kurchatov surveyed the deep South Atlantic in 1985–1986 (Vinogradova et al. 1990). Describing the echinoid fauna from this survey, Mironov (1993) commented on collected specimens of Carnarechinus clypeatus as follows: [A] small amount of silt and commensal bivalve molluscs were found in the preserved part of the intestine, according to Krylova’s personal communication, belonging to the family Montacutidae [...] Bivalve molluscs were found in a strongly dilated part of the intestine, located in close proximity to the mouth opening. It is characteristic that the found mollusks form a continuous age series: the length of the shells of eight specimens gradually increases from 2.5 to 15 mm. Previously, I suggested (Mironov, 1978) that bivalves serve as prey for sea urchins. However, the good preservation of all mollusks, their confinement to a specific area of the intestine and the presence of age series show that these animals live safely inside sea urchins. They probably trap food particles from the water current created by the sea urchin. (Translation from Russian to English by A. N. Mironov, personal communication, April 2024.) Given their location in the southern Atlantic, we believe the specimens Mironov also described in 1993 are Vaticinaria moseleyi. Also, when Oliver (2012) reviewed echinoid-associated bivalves, he noted that most of them live externally on their host but also mentioned “ internal commensals in intestines close to the peristome of Carnarechinus clypeatus ” from a personal communication from Mironov, which we believe to refer to the same records of Vaticinaria as Mironov earlier described. Surprisingly, our own examination of the syntype of Carnarechinus clypeatus (NHMUK 1956.10. 18.39) revealed multiple specimens of Vaticinaria moseleyi in the specimen’s esophagus. These were collected in the South Atlantic by the 1872–1876 Challenger Expedition (station 133), near the type locality of V. moseleyi. That they had remained hidden for 150 years is testament to their cryptic endosymbiosis. Remarks. Although the host urchin, Carnarechinus clypeatus, is found across the Southern Ocean (Mironov 1978), we have noted subtle but distinct differences between Atlantic, Australian, and Pacific Vaticinaria species. Thus, the central South Pacific Vaticinaria borniana (Dall, 1908) is less inflated and has a narrower hinge plate without granules when compared to Vaticinaria gulicola sp. nov. And when compared to the new species, the central South Atlantic Vaticinaria moseleyi (Smith, 1885) is also less inflated, with a much longer anterior end. Only specimens of V. gulicola sp. nov. from off the southern coast of Australia had material suitable for molecular analyses, but we hope future fresh samples from the Atlantic and Pacific will allow us to confirm these distinct species. We have not been able to examine the shell of the type of the Philippine Cryptodon luzonicus, but we agree with Huber (2015) that this species might belong in Axinodon (Thyasiridae). Pelseneer (1888) made detailed anatomical descriptions of Vaticinaria moseleyi from the Challenger Expedition material. These are very close to our observations of V. gulicola sp. nov., including the single small “ anal aperture ” (= siphon) and large “ tuberculated and papillary ” anterior mantle. Pelseneer described a large gonad dorsal to the anterior adductor muscle, but our examination of V. gulicola sp. nov., suggests this is more likely a large area of digestive diverticula. The thick, highly papillated anterior mantle of Vaticinaria gulicola sp. nov. is striking (Fig. 5 A). This interesting morphology was first reported in Vaticinaria moseleyi by Pelseneer (1888), who did not speculate on its function but dismissed the idea that it may serve as “ mantle gills. ” Many galeommatoideans have a modified pattern of water flow, whereby water is taken in through an anterior mantle aperture and then passed out through a ventral or posterior siphon (Popham 1940). Many of these species have enlarged anterior mantles, producing a “ siphon ” or, to some degree, reflecting over the shell. We have seen no evidence of mantle reflection in Vaticinaria, and the anterior mantle is dense and tough, even taking preservation into account. We suggest the papillated anterior mantle in Vaticinaria functions as an anterior inhalant aperture, with water passing out the posterior exhalant siphon. The papillae may serve to screen out sediment, as the host urchin’s gut is ful