Lyonsiella tentaculata, Pacheco & Teso & Pastorino, 2024

Pacheco, Leonel I., Teso, Valeria & Pastorino, Guido, 2024, Use of traditional tools and micro-computed tomography for the taxonomy of carnivorous bivalves from the deep waters of Southwestern Atlantic, Zoological Journal of the Linnean Society 202, pp. 1-23 : 16-20

Lyonsiella tentaculata sp. nov.

( Figs 11A, B, D, F, G, 12)

urn:lsid:zoobank.org:act:1443D27C-D033-4721-B12C-

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Type material: Holotype: MACN-In 44327, one individual. Paratypes: MACN-In 44326, two valves.

Type locality: Mar del Plata submarine Canyon (38°01 ʹ 26″S, 53° 51 ʹ 01″W), 2212 m .

Etymology: Ŋe name tentaculata , from the Latin tentaculum, refers to the numerous siphonal tentacles in comparison to other species of the genus.

Distribution

New records: Mar del Plata Submarine Canyon (1395–2212 m) and Burdwood Bank (647 m).

Bathymetry: 647–2212 m. Material examined: MACN-In 44328 (37°49 ʹ 40″S, 54°07 ʹ 56″W, 1395 m) one individual; MACN-In 44326 (54°36 ʹ 14″S, 62°50 ʹ 55″W, 647 m) two valves (paratypes).

Diagnosis

Shell ≤ 14 mm long, ornamented by ≤50 radial lines spaced by radially oriented groups of pyramidal spines. Siphons surrounded

by 25 siphonal tentacles, 12 associated with inhalant siphon and 13 with exhalant.

Description

Shell medium in size (≤ 14 mm L, 11 mm H, 4.6 mm W), fragile, somewhat translucid, oval-trapezoidal, inequilateral. Inequivalve; left valve more rectangular, right valve slightly larger and with dorsal and ventral margins projecting beyond the left valve. Surface covered by ≤50 radial lines spaced by radially oriented groups of four to eight pyramidal spines, usually covered by debris. Umbo prosogyrus, displaced anteriorly. Anterodorsal margin diagonally inclined, lunule weak, anterior to resilium. Anterior margin rounded, slightly extended beyond umbo. Posterodorsal margin somewhat straight, almost parallel to ventral margin; both margins connect with nearly vertical posterior margin through rounded corners. Hinge edentulous, with elongated lithodesma. Interior iridescent, with radial lines corresponding to external striae. Adductor muscle scars slightly marked, anterior more than posterior. Palial line noticeable. Ctenidium composed of a reduced outer demibranch and an inner demibranch with ~50 and ~60 branchial filaments, respectively. Inhalant siphon with 12 inner arborescent tentacles on the edge of the aperture and 10 outer small tentacles surrounding it, plus two small dorsal tentacles. Exhalant siphon surrounded by 13 outer small tentacles (seven larger intercalated by six smaller). Byssus present.

Remarks

Lyonsiella abyssicola is a comparable species; however, it has 16 tentacles surrounding both siphons, whereas L. tentaculata has 25. The number of tentacles is the same both in the largest individual (MACN-In 44327, 14 mm L) and in the smallest individual (MACN-In 44328, 7 mm L), with a similar size to L. abyssicola , hence it can be discarded as a variation attributable to size. Additionally, the new species reaches a larger size and exhibits more radial lines than L. abyssicola , (≤ 14 mm and 50 lines vs. 5 mm and 20 lines) ( Fig. 11A, B). A clear character visible by SEM is the shape of the microscopic spines. Lyonsiella tentaculata has pyramidal and acuminated spines, whereas L abyssicola , Lyonsiella frieli Allen & Turner, 1974 and Allogramma formosa (Jeffreys, 1882) , have thinner spines with stellated termination ( Oliveira and Absalão 2010a) ( Fig. 12C, D).

Other comparable Lyonsiella species include Lyonsiella subquadrata (Jeffreys, 1882) , distinguished by a wider, more prolonged, and ovate anterior margin ( Fig. 11E); Lyonsiella perplexa Allen & Turner, 1974 , with an angular anterior margin and a lower number of tentacles (~15); Lyonsiella fragilis , with a much more horizontal anterodorsal margin, a more prolonged anterior margin, and fewer radial lines (8–18); and Lyonsiella pipoca Oliveira & Absalão, 2010 , with short and wider spines oriented irregularly, not radially, and with a few isolated delicate ‘projections’.

D I SC USSI O N

Seven species of Cardiomya , Cetoconcha , and Lyonsiella , including two new to science, are here recognized as valid for the Southern Southwestern Atlantic. Four other species from these genera are known in the area, i.e. L. abyssicola , L. perplexa , Cetoconcha sarsii , and Cetoconcha bulla (Dall, 1881) . Lyonsiella abyssicola and L. perplexa were recorded in the Uruguayan Continental Slope by Allen (2008) and Allen and Turner (1974), respectively; nevertheless, as with Cetoconcha sarsii , the lots located either contained specimens damaged beyond recognition or the material is not sufficiently well preserved for a proper redescription. Scarabino et al. (2016) recorded Cetoconcha bulla with doubt on the Uruguayan continental slope based on three specimens recorded by Allen (2008) as Thracia nitida Verrill & Bush, 1898 , a junior synonym of Cetoconcha bulla . However, the lot housed at the MCZ collection (MCZ 375493), containing only one of the three mentioned specimens, differs significantly from the original description, in that it lacks radial lines or granules. Ŋerefore, the record of Cetoconcha bulla in the area of study is uncertain.

Ŋe distribution of Cardiomya fragilissima is expanded northwards. Together with six Cuspidaria species ( Pacheco et al. 2022) and Cetoconcha spinulosa ( Machado et al. 2019) , all of which are Antarctic/sub-Antarctic species, it has reached 37°S. Ŋese eight species have been found in various, sometimes distant, areas of the Southern Ocean, probably owing to the wide dispersion of larvae produced by the Antarctic Circumpolar Current ( Güller et al. 2020).

We provide the first micro-CT scans for Cardiomya cleryana and Cardiomya fragilissima . We agree with most conclusions and observations produced on Cardiomya cleryana by Machado et al. (2016) through more traditional techniques. Both species have a typical Cardiomya body plan already seen in other studies ( Allen and Morgan 1981, Morton 2015, Machado et al. 2016), without any noticeable anatomical distinction between them. Furthermore, the lateral septal muscle, septal pore distribution, siphonal tentacles, and labial palps are identical to those found in Cuspidaria species with body paưern ‘A’ according to Pacheco et al. (2022). Ŋe only clear anatomical difference between species with Cardiomya and Cuspidaria paưern ‘A’ is the presence of an extra lateral septal muscle in the former. Ŋe shells of the genera within Cuspidariidae exhibit marked differences, but a closer revision considering these anatomical characters should be performed.

We found the same anatomical characters of Cetoconcha spinulosa as provided by Machado et al. (2019). However, the presence of ‘visceral muscles’ is documented for the first time in both this species and the genus. Furthermore, although already reported in other Poromyoidea species, the presence of internal siphonal tentacles and a larger number (15) of external siphonal tentacles are recorded for the first time in Cetoconcha spinulosa . In addition, the number of siphonal tentacles exhibits some variation among the Poromyoidea, ranging between 13 and 15, but Cetoconcha gigas displays an unusual number of 19. A more in-depth examination of this species is required to assess its proximity to other Cetoconcha species and its place within Poromyoidea.

Ŋe visceral muscles found anchoring the visceral mass to the shell of Cardiomya cleryana , Cardiomya fragilissima , Cetoconcha spinulosa , and Cetoconcha gigas were also observed in the micro-CT scans (hưps://dataverse.harvard.edu/ dataset.xhtml?persistentId=doi:10.7910/DVN/VTWI8Y) of Cuspidaria glacialis (G. O. Sars, 1878) , Poromya rostrata Rehder, 1943 , Cetoconcha spinulosa , Cetoconcha aff. smithii , and Allogramma formosa (Jeffreys, 1882) ( Machado et al. 2019) . Ŋese muscles are distributed in the Poromyoidea, Cuspidarioidea, and Verticordioidea (at least in Lyonsellidae, because these were not observed in the micro-CT scans of Trigonulina ornata d’Orbigny, 1853 ). We failed to observe them in L. tentaculata , perhaps owing to preservation issues. Ŋe visceral muscles exhibited varying degrees of contact with the anterior pedal retractor muscles. A similar paưern was observed by Machado et al. (2016), who noted that muscle bundles aưached to the gonads merged with the posterior retractor muscle before the bifurcation. Ŋese bundles were also observed in our specimens. Consequently, it appears that the visceral mass possesses both anterior and posterior muscle bundles associated with the pedal retractor muscles. Ŋis arrangement is likely to provide a beưer aưachment in less anchored areas, in contrast to the ventral side, which merges with the dorsal wall of the foot.

Ŋere is a notable difference in the development of the septal musculature between Cuspidariidae and Cetoconchidae . Cuspidariidae have a septum characterized by a conspicuous thickness and the absence of gill filaments. Both anterior and posterior septal muscles are well developed, and the lateral septal muscles have fibres independently aưached or congregating in two distinct aưachment points, as observed in certain Cuspidaria species (Pacheco etal. 2022). Incontrast, Cetoconchidaeexhibits a thinner septum, with gill filaments and a more membranous posterior region. Ŋe anterior and posterior septal muscles are rod-like, and the lateral septal muscles have fibres congregating in a single aưachment point. Ŋese features also characterize the Poromyiidae, but Allen and Morgan (1981) emphasized that Cetoconchidae display a slightly more developed septal musculature, with a more reduced membranous portion. Odhner (1960) suggested a possible function of the membranous part in strengthening the posterior region of the septum. Ŋis hypothesis has not yet been checked; however, a possible correlation between the development of the membranous part and the posterior septal muscles should be studied.

Micro-CT has become a popular technique in recent years ( Ziegler et al. 2018, Ziegler 2019, Martínez-Sanjuán et al. 2022), owing to its ability to provide in-depth anatomical information without destroying specimens. Ŋis advantage addresses the dilemma faced by many deep-sea researchers, because they no longer need to damage samples that are ossen challenging to obtain and are limited in number. In this work, we exploit another advantage: studying specimens previously inspected by other colleagues. Ŋis allowed us to observe visceral muscles in other species not collected by us and compare our material of Cetoconcha spinulosa with that of Machado et al. (2019) without requesting additional material, because the scanned specimens remain in the same state of preservation in which the authors studied them. However, this technique has some disadvantages. It does not allow for the distinction of organs with similar density, mainly owing to the non-specificity of the contrast agents ( Ziegler 2019), which could be challenging for someone unfamiliar with the anatomy of scanned specimens. Additionally, the penetration rate of the phosphotungstic acid is very slow ( Ziegler et al. 2018), requiring a large amount of time for adequate sample preparation. Nonetheless, phosphotungstic acid has not been recorded to cause over-staining ( Ziegler 2019), hence there is no apparent danger of overexposing the sample to it. Furthermore, the function of dimethyl sulphoxide was to increase the cell permeability to the contrast agent ( Machado et al. 2019). Although there have been doubts regarding DNA damage asser exposing samples to X-rays, several studies have found no evidence of it ( Faulweưer et al. 2014; Hall et al. 2015). However, we decided cautiously to preserve the only soss tissue of L. tentaculata , excluding it from the micro-CT scanning.

Some studies have taken an exploratory approach to micro-CT, testing its suitability for different higher taxa ( Ziegler et al. 2018, Ziegler 2019), whereas others have focused on specimen description solely through this technique ( Fernández et al. 2014, Machado et al. 2019, Martínez-Sanjuán et al. 2022). Our study successfully adopts an integrative approach, redescribing three species using both micro-CT and traditional tools, such as stereomicroscopy and SEM.

Ŋis study, together with the work by Pacheco et al. (2022), produces another step in the process of filling the gap in the information regarding the biodiversity in the Southwestern Atlantic deep sea, particularly within a poorly understood group, such as the Septibranchia.

SU P P LE M E N TA RY DATA

Supplementary data is available at Zoological Journal of the Linnean Society online.