Adipicola leticiae, Crespo, 2017

Crespo, Enrique, 2017, First record of the genus Adipicola (Mollusca: Bivalvia: Mytilidae) and description of a new species from the Argentine SW Atlantic Ocean, Zootaxa 4318 (2), pp. 325-338 : 329-332

Adipicola leticiae new species

Figs 2A ̄J, 3ĀK, 4ĀD

Type material. Holotype CNP-Inv 2 229 and twelve paratypes: CNP-Inv 2230, one articulated specimen, CNP-Inv 2231, one articulated specimen, CNP-Inv 2232, one articulated specimen and CNP-Inv 2233, nine articulated specimens, all attached to the skull of a sei whale Balaenoptera borealis .

Type locality. 46° 10´S; 61° 50´W, off San Jorge Gulf , Argentina, from a depth of 104 m ( Fig. 1 ). The collection date was July 21, 2010.

Etymology. This species is dedicated to Leticia Mercante, the first author´s wife, for her invaluable support during many years.

Diagnosis. The new species is characterized by the combination of the following characters: adult shell of medium size, up to 20 mm in length, elongate, fragile, ventral margin straight, distance from umbos to anterior shell margin about 30% of shell length; larval shell with prodissoconch 1 of about 175 µm and prodissoconch 2 with a size between 425 to 580 µm in length.

Description. Shell, fragile, subequilateral, equivalve, subrectangular, with anterior end rounded, dorsalanterior margin anteriorly inclined, lunule not defined; dorsal-posterior margin straight, posterior end rounded, ventral margin straight; umbones prosogyrate, not in touch, placed at 30% of shell length measured from the anterior margin. Periostracum dark-reddish brown. Ligament extending along the dorsal-posterior margin for about 65% of posterior length. External surface smooth with irregular growth wrinkles, periostracal hair absent. Internally white, porcelaneous, pallial line complete, without sinus; hinge plate without teeth or crenulations below umbo or behind ligament; posterior adductor scar circular, anterior scar oval, slightly smaller; pedal retractors fused with the posterior byssal retractors just above the posterior scar; anterior retractor scars not observed; prodissoconch 1 about 175 µm, prodissoconch 2 size between 425 to 580 µm in length. No direct anatomical observations could be made on the dried remains.

Morphological comparisons. The conchological features are compared with other Bathymodiolinae species that were or still are placed in Adipicola . Among these, A. osseocola Dell, 1987 , has a more sharply pointed anterior shell margin and its ventral margin is softly arched rather than straight ( Figs 5A ̄B).

Adipicola pelagica (Forbes in Woodward, 1854) has a slightly arched ventral margin and a less forward-placed umbo (see B/L ratio) than A. leticiae n. sp.; it also attains a larger size of up to 36 mm in the holotype ( Figs 5 C̄F).

The taxonomic position of A. projecta ( Verco, 1908) is contentious. Originally described as Modiola Lamarck, 1801 , Lamprell & Healy (1998) included it in Idas, Dell (1987) placed it in Adipicola , and Huber (2010) suggested a new genus for this species. Type material was not found in the cabinets of SAMA. The original description and illustration by Verco (1908, p. 195, pl.13, figs. 12̄13) suggest an Adipicola species distinguishable from our new species by a less elongate shell. Although the ventral border is nearly straight and almost parallel with the dorsal margin, both the shell length and ligament groove are shorter than in A. leticiae n. sp.

“ Adipicola ” longissima Thiele & Jaeckel, 1931, is a regionally rather restricted species ( Pante et al. 2012) that is predominantly attached to Nypa fruticans nuts, in localities where palm trees grow ( Philippines, Indonesia and Solomon Islands mainly). Genetically, it clusters with “ Adipicola ” simpsoni Marshall, 1900. Therefore, Thubaut et al. (2013b) proposed a new generic name, “Nypamodiolus” for the two species. Unfortunately, the introduction of the genus name does not fulfil the requirements of the ICZN code (especially Art. 13) and is unavailable, therefore.

The types of “ A. ” longissima are smaller than the present species and possess a pellucid shell with a sharper, anteriorly more elongated and overall arched shell outline ( Figs 5 ĪJ). “ Adipicola ” simpsoni is characterized by a more elliptical, posteriorly expanded shell ( Figs 6A ̄D) in contrast to the more subrectangular shape of our species. This is also reflected by different H/L and B/L ratios ( Table 2).

The holotype of Gigantidas crypta ( Dall, Bartsch & Rehder, 1938) , previously placed in Adipicola by Dell (1987), is only about 4 mm long and has a slightly elevated and much forward-placed umbo. The shell outline is oval with a smaller posterior than anterior end and a slightly concave ventral margin. Much larger specimens of up to 52.9 mm were reported from Japan ( Habe 1977; Habe in Koyama et al. 1981; Kurozumi 2000). According to genetic studies, the species must be placed in the genus Gigantidas Cosel & Marshall, 2003 , related to G. gladius Cosel & Marshall, 2003 and the “ Bathymodiolus ” childressi group ( Thubaut et al. 2013a).

Huber (2010) excluded Prashad’s Modiolus dubius from the genus Idas and placed it within Adipicola due to the absence of periostracal hairs and the non-glossy interior aspect. Our examination of the holotype and paratype (ZMA.MOLL.135265, ZMA.MOLL.135270) ( Figs 5 ḠH) suggests otherwise. The holotype, from Rotti Island, Indonesia, shows a rhomboidal shell outline with conspicuous periostracal hairs over posterior portions of the shell. In addition, it shows a well-developed concentric sculpture and elongate ligament from the beak to virtually the end of the dorsal posterior margin. These morphological features are more characteristic of Idas .

Species Material Length (mm) Height (mm) B (mm) B/L H/L

Adipicola leticiae Holotype 20 7 5 23,9 33,7 Paratype 1 18 6 4 24,2 35,1 Paratype 2 17 6 4 25,2 36,0 Paratype 3 16 5 5 29,2 31,1 Paratype 4 16 6 5 29,0 35,8 Paratype 5 13 5 4 27,7 37,0 Paratype 6 9 4 2 22,8 39,3 Paratype 7 11 4 3 26,8 40,6 Paratype 8 10 4 3 24,0 38,9 Paratype 9 9 4 3 28,3 41,2 Paratype 10 9 3 2 27,1 38,5 Paratype 11 8 3 2 25,5 39,3 Paratype 12 9 4 2 21,1 42,5

A. osseocola * Holotype 24 9 6 24,6 37,3 Paratype 1 28 9 8 28,1 32,7 Paratype 2 24 9 6 26,4 36,2 Paratype 3 19 8 5 24,3 39,7

A. pelagica * Syntype 1 36 17 12 32,0 46,1 Syntype 2 32 14 10 31,0 44,9

“ A. ” simpsoni* Syntype 1 23 10 4 19,6 46,2 Syntype 2 21 10 3 15,0 45,9 Syntype 3 17 7 3 16,4 42,7 North Sea 20 9 4 21,4 44,4 Shetlands 19 8 4 19,4 42,5 Shetlands 18 8 4 19,7 43,8

T. pacifica * Holotype 27 11 6 22,6 42,6 Japan 150 m 26 12 8 32,4 45,3 Japan 150 m 15 7 4 25,0 48,0 Dell (1987) suggested that the types of A. iwaotakii ( Habe, 1958) could belong to larger specimens of A. crypta . However, the genetic analysis by Thubaut et al. (2013b) places this species within Gigantidas and close to the Bathymodiolus “childressi group”.

Terua arcuatilis Dell, 1995 , differs from A. leticiae n. sp. by having an arched rather than straight ventral margin, a longer ligament and a larger and more elongate shell ( Figs 6 ĒF). Terua pacifica ( Dall, Bartsch & Rehder, 1938) ( Figs 6 ḠH) differs from the new species by having an arched ventral margin and a less elongate shell. Its collection sites in the Pacific off Oahu, Hawaii and southern Japan ( Dell 1987; Habe 1977) also suggests that these are different species.

Figures

FIGURE 1. Map showing the type locality (X) of Adipicola leticiae new species.

FIGURE 2. A ̅ J. Adipicola leticiae new species. A – D. Holotype, CNP-Inv 2229. E ̅ J. Paratype CNP-Inv 2233. E ̅ F. internal view of Paratype 2233. G – H. Prodissoconch 2 of paratype 2233. I – J. Detail of anterior end of ligament of Paratype 2233. Scale bars: A – F = 1 cm; G – H = 400 µm; I – J = 1 mm.

FIGURE 5. A – B. Adipicola osseocola, Holotype NMNZ M. 75026 (copyright Te Papa CC-BY-N-ND license). C ̅ F. Adipicola pelagica, Syntypes NHMUK 1849.12.10.67 ̅ 68. G ̅ H. Idas dubius, Holotype ZMA. MOLL 135265, right valve. I ̅ J. “ Adipicola ” longissima, Syntype ZMB 77803. Scale bars: A – B, I – J = 5 mm; C – F = 1 cm; G – H = 2 mm.

FIGURE 6. A ̅ D. “ Adipicola ” simpsoni, Syntypes NHMUK 1900.10.16.19 ̅ 21. E – F. Terua arcuatilis, Holotype NMNZ M. 0 89788 (copyright Te Papa CC-BY-N-ND license). G ̅ H. Terua pacifica, Holotype USNM 173020 (copyright National Museum of Natural History, Smithsonian Institution). Scale bars: A ̅ H = 1 cm.