Zoosyst. Evol. 100 (2) 2024, 603-623 | DOI 10.3897/zse.100.118912 yee BERLIN On the occurrence of the deep-sea barnacle Tetrachaelasma southwardi Newman & Ross, 1971 (Cirripedia, Balanomorpha, Bathylasmatidae) in the Mar del Plata Submarine Canyon, Argentina: supplementary description and taxonomic remarks on the genus Ignacio L. Chiesa!, Emanuel Pereira**, Daniel Roccatagliata* 1 Centro Austral de Investigaciones Cientificas (CADIC-CONICET), CP 9410, Ushuaia, Tierra del Fuego, Argentina 2 Instituto de Biodiversidad y Biologia Experimental y Aplicada (IBBEA), Universidad de Buenos Aires-CONICET, CP 1428, Buenos Aires, Argentina 3 Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Biodiversidad y Biologia Experimental (DBBE), CP 1428, Buenos Aires, Argentina https://zoobank. org/07F55091-6F 85-44BB-BBC5-09BE2F0C733A Corresponding author: Daniel Roccatagliata (daniel.roccatagliata@gmail.com) Academic editor: Luiz F. Andrade # Received 16 January 2024 # Accepted 2 April 2024 Published 17 May 2024 Abstract Tetrachaelasma southwardi Newman & Ross, 1971, a bathylasmatine balanomorph, has been recorded from the Mar del Plata Sub- marine Canyon (ca. 38°S, off the coast of Argentina), at two stations located at significantly different depths (1950 m and 2934 m). A total of 29 specimens, complete or damaged but with soft parts intact, were collected. This unusually large number of well-preserved specimens allows us to add supplementary descriptions and document intraspecific morphological variations. The differences be- tween 7. southwardi and T. tasmanicum Buckeridge, 1999, the second species of this genus, are re-evaluated. A map and an updated list including all the records of the genus 7etrachaelasma Newman & Ross, 1971 are provided. The distribution of the genus Tertra- chaelasma in the Southern Ocean is discussed. Furthermore, a single specimen of another bathylasmatine balanomorph, which was assigned to the genus Bathylasma Newman & Ross, 1971, was also obtained at the 1950 m station herein studied. This is the first record of the genus Bathylasma from the South-West Atlantic. This specimen has one 7? southwardi attached to it, marking the first time that members of these two genera have been found living together. Key Words Bathylasma sp., distribution, South-West Atlantic, 7etrachaelasma southwardi, T. tasmanicum Introduction On the genera Tetrachaelasma and Bathylasma The genus Zetrachaelasma contains only two species, which inhabit much greater depths than any other Bala- nomorpha, up to 3600 m (Newman and Ross 1971, 1976; Buckeridge 2010; Table 1). Newman and Ross (1971) erected this genus for the reception of 7? southwardi, a new species that these authors described based on seven complete specimens taken by the RV “Eltanin” at a sin- gle station in the Central South Pacific (2304-2328 m depth). Newman and Ross (op. cit.) also listed among the material studied loose plates taken by the RV “Eltanin” at three other localities, 1.e., off southern Chile (1190-1263 m depth), off Malvinas/Falkland Is. (1720-1739 m depth), and at the Sars Bank in the Drake Passage (1207-1591 m depth). Furthermore, Newman and Ross (1976) reported extensive accumulations of loose plates of 7etrachaelasma sp. on the flanks of a seamount off Madagascar at compa- rable depths (~1800 m). In addition, the RV “Atlantis IT” obtained about 70 disarticulated plates of 7 cf. southwardi Copyright Chiesa, |.L. et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 604 Chiesa, I.L. et al.: Tetrachaelasma southwardi from the Mar del Plata Submarine Canyon at the Mid-Atlantic Ridge (ca. 41°S) in 1980 (see the SIO- BIC website in the References section). More recently, Buckeridge (1999) described the second species of the genus, 7? tasmanicum, based on a single incomplete spec- imen (comprising the carina, left carinolateral, scuta, and terga, plus body parts) and numerous loose plates collected at the South Tasman Rise (2030-3600 m depth) by the RV “Rig Seismic”. All these records, and a few others retrieved from the website, are listed in Table 1 and charted in Fig. 9. An unusually high number of 7: southwardi specimens, complete or damaged but all with soft parts intact, were re- cently collected at two stations from considerably different depths (1950 m and 2934 m). This material was taken from the Mar del Plata Submarine Canyon (ca. 38°S, off the coast of Argentina) during the Talud Continental I and II expedi- tions performed by the RV “Puerto Deseado”. Based on this material, supplementary descriptions of 7’ southwardi are presented, and intraspecific morphological variations are documented. In addition, the differences between 7? south- wardi and T: tasmanicum are discussed. Furthermore, a sec- ond bathylasmatine balanomorph was collected at the 1950 m depth station, which was assigned to the genus Bathy- lasma Newman & Ross, 1971. This genus encompassed four extant and four fossil species (see Araya and Newman 2018). This is the first time that the genus Bathylasma has been recorded from the South-West Atlantic. A brief review of the family Bathylasmatidae Newman & Ross, 1971 Newman and Ross (1971) erected the deep-sea family Bathylasmatidae to include the new genera Bathylasma, Tetrachaelasma, and Aaptolasma (currently a synonym of Hexelasma), as well as the previously known genera Hexelasma Hoek, 1913 and Tessarelasma Withers, 1936. Later, Newman and Ross (1976) grouped Bathylasmati- dae, Tetraclitidae Gruvel, 1903, and Coronulidae Leach, 1817, under the superfamily Balanomorphoidea (new sta- tus, Coronuloidea Leach, 1817; see Newman and Ross (1977)), and divided the bathylasmatids into the subfam- ilies Bathylasmatinae and Hexelasmatinae. In addition, Newman and Ross (1976) suggested that Bathylasmati- dae gave rise to Tetraclitidae (see fig. 5). Foster (1978) stated that “the relationship between Pachylasma and Hexelasma is obvious; they are weakly constructed, deep-sea forms with wide parietal alae and no radial interlocking of the plates.” Accordingly, Foster (1978) placed the bathylasmatids in the family Pachy- lasmatidae Utinomi, 1968, contradicting the proposal by Newman and Ross (1976). Foster’s nomenclatural deci- sion was subsequently followed by Jones (2000, 2012), who included the subfamilies Bathylasmatinae and Hex- elasmatinae in Pachylasmatidae, under the superfamily Pachylasmatoidea Utinomi, 1968. Buckeridge and Newman (2010) revised the classifica- tion of Balanomorpha and grouped Bathylasmatidae and Tetraclitidae under the superfamily Tetraclitoidea Gruvel, zse.pensoft.net 1903. More recently, Chan et al. (2017), in a molecular phylogenetic study for Balanomorpha, proposed that Bathylasmatidae is more closely associated with Tetra- clitoidea than with Pachylasmatoidea. This result is con- sistent with the proposal by Newman and Ross (1976), Buckeridge and Newman (2010), as well as with a mo- lecular phylogenetic study of Tetraclitoidea presented by Tsang et al. (2015). Finally, Chan et al. (2021), in a thor- ough revision of the barnacle classification, placed Bath- ylasmatidae in the superfamily Coronuloidea, together with Tetraclitidae, Austrobalanidae Newman & Ross, 1976, Coronulidae, and Chelonibiidae Pilsbry, 1916. Bathylasmatidae currently encompasses the subfam- ilies Hexelasmatinae (genus Hexelasma) and Bathylas- matinae (genera Bathylasma, Tetrachaelasma, Tessa- relasma, and Mesolasma Foster, 1981). The former subfamily has a wall of six plates with longitudinal chi- tinous laminae and/or strips, whereas the latter subfam- ily has four or six plates and lacks chitinous material. Tessarelasma is only known from a fossil record from India. Tetrachaelasma is the only living genus with four wall plates. A key for the identification of extant genera 1s presented by Araya and Newman (2018). For diagnoses of the subfamilies and genera, see Jones (2000). Description of the study area The Mar del Plata Submarine Canyon is located in the southwestern Atlantic Ocean at around 38°S (Fig. 1). This canyon is not connected to the Argentine continental shelf and has a typical “V” shape (Violante et al. 2010; Bozzano et al. 2017). It begins on the upper continental slope at a depth of ~SO0—1000 m and extends for about 110 km downslope to reach a depth of ~3900 m (Voigt et al. 2013). Oceanographically, this canyon is located in one of the most dynamic and highly variable areas of the world ocean, the Brazil/Malvinas Confluence, which is generated by the encounter of the Brazil and Malvinas/ Falklands currents (Piola and Matano 2001; Matano et al. 2010; Preu et al. 2013). Material and methods Field work Twenty-nine specimens (complete or damaged) and a few loose plates of Tetrachaelasma southwardi were collected from the Mar del Plata Submarine Canyon during the Ta- lud Continental I and III expeditions, carried out by the RV “Puerto Deseado” in 2012 and 2013, respectively. The spec- imens were obtained at two stations, one at 1950 m depth using an epibenthic sledge, similar to the one designed by Hessler and Sanders (1967), and the other at 2934 m depth using a bottom otter trawl (Fig. 1). In addition, one speci- men identified as Bathylasma sp. was also collected at the 1950 m depth station (see Material Examined section). Zoosyst. Evol. 100 (2) 2024, 603-623 605 Table 1. Records of Tetrachaelasma species reported in this study and by previous authors. Abbreviations: CSIRO-MIIC — Common- wealth Scientific and Industrial Research Organization — Marine Invertebrate Image Collection; GBIF — Global Biodiversity Informa- tion Facility, NNUNH — National Museum of Natural History, Smithsonian Institution; SIO-BIC — Scripps Institution of Oceanogra- phy — Benthic Invertebrate Collection. Links to these institutions/organizations in the References section. Note: The catalog numbers for T. southwardi are those published on the NMNH website, not those mentioned in Newman and Ross (1971) and Jones (2000). Species Ships and/ Stations Geographic coordinates or Cruises (Institutions) T. southwardi RV “Eltanin” (SOSC) — Sta. 6 52°10'S, 142°10'W T. southwardi RV “Eltanin” (USARP) Sta. 216 52°53'S, 75-30 W T. southwardi RV “Eltanin” (USARP) Sta. 376 54°03'S, 56°03'W T. southwardi RV “Eltanin” /Cruise T 59°45'S, 68°50'W to 05 59°46'S, 68°50'W T. southwardi RV “Puerto Deseado” Sta. 25 37°51.688'S, / Talud Continental | 54°10.550'W T. southwardi RV “Puerto Deseado” Sta.45 38°1.913'S, 53°39.268'W / Talud Continental Ill T. cf. southwardi RV “Atlantis Il” (WHOI) Dredge 41°14.9'S, 16°36.2'W 06 T. tasmanicum RV “Rig Seismic”/ Sta.D12 = 45°09.0'S to 45°10.2'S, Cruise 147 145°25.1'E to 145°23.8'E T. tasmanicum RV “Rig Seismic”/ Sta.D25 49°04.3'S to 49°04.0'S, Cruise 147 146°16.0'E to 146°17.4'E T. tasmanicum RV “Rig Seismic” / Sta. D41 44°14'S, 149°26'E Cruise 147 T. tasmanicum RV “Rig Seismic”/ Sta.D43 43°54.0'S, 151°19.2'E to Cruise 147 43°54.0'S, 151°17.8'E T. tasmanicum RV “Rig Seismic”/ Sta.D44 44°36.3'S to 44°36.0'S, Cruise 147 147°14.7E to 147°14.8'E T. tasmanicum RV “Rig Seismic”/ Sta.D45 44°39.2'S to 44°39.5'S, Cruise 147 147°26.4'E to 147°26.5'E T. tasmanicum RV “Rig Seismic”/ Sta.D53 45°21.8'S to 45°21.1'S, Cruise 147 146°43.2'E to 146°43.7'E T. tasmanicum RV “Rig Seismic”/ Sta.D57 ~=44°31.7'S to 44°32.8'S, Cruise 147 146°00.4'E to 146°00.6'E T. tasmanicum RV “Thomas G. Sta. J2- 43°48'25.2'S, Thompson” / Cruise 390-008- 150°20'24.0"E 771200801 002 T. tasmanicum RV “Thomas G. Sta. J2- 45°18'01.4'S, Thompson” / Cruise 392-012- 146°07'15.6'E 71200801 001 T. tasmanicum RV “Thomas G. Sta. J2- 43°49'42.2'S, Thompson” / Cruise 390-015 150°30'00.0"E 71200801 T. tasmanicum RV “Thomas G. Sta. J2- 45°22 27.2'S, Thompson” / Cruise 391-011 144°35'34.8'E 771200801 Tetrachaelasma — RV “Argo” / CIRCE Sta. 26°29'S, 46°07'E sp. Expedition DR124 Depths Dates Locations Catalog References and/ (m) numbers or websites 2304- Mar 21, South Pacific Ocean USNM 125305 Newman and Ross 2328 1965 (Holotype) (1971), NMNH USNM 125306 USNM 125307 1190- Sep 16, Off southern Chile USNM 125309 Newman and Ross 1263 1962 (1971), NMNH 1720- Dec 20- Off Malvinas/ USNM 125308 Newman and Ross 1739 21, 1962 Falkland Is. (1971), NMINH 1207- Oct 10, Sars Bank in Drake Weisbord (1965, 1591 1962 Passage 1967) 1950 Aug 15, Mar del Plata MACN-In 44478 — Current study 2012 Submarine Canyon 2934 Sep 05, Mar del Plata MACN4n 44479 — Current study 2013 Submarine Canyon 2175- Jun 20, Mid-Atlantic Ridge, BIC C8156 SIO-BIC 2600 1980 — South Atlantic Ocean 2100- Feb 05, South Tasmania Buckeridge (1999) 3000 1995 2420- Feb 12, South Tasmania CPC 34698- Buckeridge (1999) 3300 1995 34702 2850 Feb 18, South Tasmania - Buckeridge (1999) 1995 2030- Feb 19, South Tasmania Buckeridge (1999) 3600 1995 2250- Feb 22, South Tasmania Buckeridge (1999) 2400 1995 2600- Feb 22, South Tasmania Buckeridge (1999) 2800 1995 2770- Feb 25, South Tasmania - Buckeridge (1999) 3000 1995 2300- Feb 26, South Tasmania CPC 34697 ~—Buckeridge (1999) 2850 1995 (Holotype) 2082 Jan 05, South Tasmania MIIC 02727 CSIRO-MIIC 2009 2213 Jan 11, South Tasmania MIIC 02729 CSIRO-MIIC 2009 1061 Jan 08, South Tasmania NMV J68079 GBIF 2009 3271 Jan 08, South Tasmania NMV J68084. GBIF 2009 1783- Sep 29, South Madagascar BIC C8158 Newman and Ross 1838 1968 (1976), SIO-BIC + Weisbord (1965, 1967) did not report the station number. Newman and Ross (1971) ambiguously mentioned “Sta. 225” and “Sta. 255”, for the material taken in the Sars Bank. All the specimens were fixed on board in 10% sea- water formalin (buffered with sodium borate) and later transferred to 96% ethanol in the laboratory. Laboratory work diameters were measured to the nearest 0.01 mm using a digital calliper. Some specimens were dissected under a stereomicro- scope (Leica MZ8), and appendages were temporarily mounted on slides in glycerin medium. Drawings of the appendages were prepared using a Carl Zeiss (Axioskop) compound microscope equipped with a camera lucida. The shell and opercular plates were disassembled from most of the specimens. When necessary, terga and scu- ta were cleaned by soaking in dilute bleach (sodium hy- pochlorite). Parietes, opercular plates, and rostral-carina Line drawings were rendered in digital format using a Wacom tablet and the Adobe Illustrator program after Coleman (2003). All dissected appendages were finally dismounted from the temporary slides and stored in 96% zse.pensoft.net 606 Chiesa, I.L. et al.: Tetrachaelasma southwardi from the Mar del Plata Submarine Canyon 37°30 37°45) 38°00! 38°15' B 94°30! Mar del Plata Submarine Canyon 54°00! 53°30! W Figure 1. A. Study area location (inset); B. Map including the two stations sampled at the Mar del Plata Submarine Canyon. The sea- bed topography of the study area is represented by roughly calculated isobaths. Abbreviation: SWAO — South-West Atlantic Ocean. ethanol, along with other soft body remains, the wall, and opercular plates. Light photographs were taken with a Nikon D7500 digital camera equipped with a Sigma 105 mm f2.8 EX macro lens and the Zerene stacking software v.1.04 (Zerene Systems LLC 2023). For SEM images, the labrum of two specimens was cleaned with 0.5% Triton X-100 nonionic detergent and ultrasonicated. Afterwards, the specimens were dehydrat- ed through a graded ethanol series and later transferred to increasing concentrations of hexamethyldisilazane (HMDS). Specimens in HMDS 100% were allowed to air dry and then mounted on aluminum stubs and coated with gold-palladium. Finally, the material was examined using a Zeiss Gemini SEM 360 microscope. High-resolution images of the parietes, terga, and scu- ta of the holotype of 7Tetrachaelasma southwardi New- man & Ross, 1971, deposited in the National Museum of Natural History (USNM 125305), were used for compar- ison purposes. Table 1 includes records from regular scientific pub- lications as well as records taken from web databases where the name of a trained taxonomist in Cirripedia is responsible for the identification. The two stations at the Mar del Plata Submarine Can- yon (Fig. 1) and all the records of the 7etrachaelasma species around the world (Fig. 9) were charted using the PanMap software v.0.9.6 (Diepenbroek et al. 2002). All specimens studied were deposited in the Inverte- brate Collection of the Museo Argentino de Ciencias Na- turales “Bernardino Rivadavia” (MACN-In). zse.pensoft.net Abbreviations and terminology The following abbreviations are used in the text: R for rostrum, C for carina, CL for carinolateral, S for scutum, and T for tergum. Antenniform cirral articles are defined as those ar- ticles with only one whorl of distal setae; however, if the antenniform cirral article also has lateral filter setae, then the latter are equal to or shorter than the whorl of distal setae. The following terminology is used to describe the scu- to-tergal articulation: ¢ Scutal articular ridge (sar): prominent outgrowth extending along a-a’ that fits into the tergal articular furrow (Fig. 2A, B). ¢ Upper articular furrow (uaf): scutal distal groove that receives the tergal articular ridge (Fig. 2C). ¢ Lower articular furrow (laf): scutal proximal groove that receives the vertical articular ridge of the tergum (Fig. 2C). ¢ Tergal articular ridge (tar): prominent distal out- growth that fits into the upper articular furrow of the scutum (Fig. 2E). ¢ Vertical articular ridge (var): slanted outgrowth of the tergum extending along b-b’. Its wider basal part fits into the lower articular furrow of the scu- tum (Fig. 2D, E). ¢ Tergal articular furrow (taf): broad and deep groove extending along c-c’ that receives the scutum artic- ular ridge (Fig. 2D, E, F). Zoosyst. Evol. 100 (2) 2024, 603-623 Results Superfamily Coronuloidea Leach, 1817 Family Bathylasmatidae Newman & Ross, 1971 Subfamily Bathylasmatinae Newman & Ross, 1976 Genus Tetrachaelasma Newman & Ross, 1971 Diagnosis. Shell conical or columnar, with 4 thick, solid, calcareous wall plates, including compound rostral plate, paired CL, and C (R-CL-C). Parietes covered with numer- ous fine bristles along horizontal growth lines; chitinous laminae absent. External alar growth lines diverge from the inferior alar margin; superior alar margin with narrow, coarse welting. Carina supports large alae that internally contribute to nearly half the total sheath circumference. Radii absent. Basis membranous. Scutum articular ridge distinctly pro- jected beyond the articular margin. Tergum slightly thinner than scutum; articular margin sinusoidal in internal view but smoothly concave in external view; with few depres- sor muscle crests, weak to well developed, extending at the most 4 along basal margin. Rami of cirri II and II antenni- form; intermediate articles of cirrus VI bearing 3 or 4 pairs of major setae. Mandible quadridentoid. Caudal appendages absent. Deep-sea species, Southern Ocean. Type species. 7etrachaelasma southwardi Newman & Ross, 1971. Current species composition. 7) southwardi Newman & Ross, 1971 and 7! tasmanicum Buckeridge, 1999. Remarks. Newman and Ross (1971) established the ge- nus 7etrachaelasma (a name that refers to the wall of four plates) to include 7’ southwardi. In addition, they discussed the affinities with Bathylasma and presented a key to sep- arate the five genera of the family Bathylasmatidae. New- man and Ross (1976) placed this genus in the subfamily Bathylasmatinae. Buckeridge (1999) gave a brief diagnosis of this genus and described its second species, 7. tasman- icum. Jones (2000) re-examined the holotype of 7 south- wardi and provided a more complete diagnosis of the ge- nus. However, this author failed to include 7? tasmanicum, a species that had been published the previous year. More recently, Araya and Newman (2018) presented an updated key to separate the extant genera currently in the family. Tetrachaelasma southwardi Newman & Ross, 1971 Figs 2-9, 11 Hexelasma antarcticum Botradaile, 1916. —Weisbord 1965: 1015— 1016 (Sars Bank material); 1967: 51-56, pl. I, figs 7-8, pl. II, figs 7-8 (Sars Bank material). Tetrachaelasma southwardi Newman & Ross, 1971: 152-155, fig. 74, pls. XXVI-XXXI (description). —Buckeridge 1999: 521, 522, 526 (comparison with 7. tasmanicum). —Jones 2000: 150, 237—239 (re- marks on the holotype, tables 24, 25, fig. 50 (distribution map)). Material examined. Zalud Continental I expedition, RV “Puerto Deseado”, Mar del Plata Submarine Canyon, 607 Sta. 25, 37°51.688'S, 54°10.550'W, 1950 m depth, 15 Aug 2012, epibenthic sledge, coll. I. Chiesa; 21 complete or damaged specimens (all with soft body parts intact) and | batch of disarticulated plates, namely: 1 damaged specimen (R missing) [wall and opercular plates disartic- ulated, mouthparts dissected, T and S photos] (MACN-In 44478a); 1 complete specimen [wall and opercular plates disarticulated, mouthparts dissected, T and S photos, la- brum SEM] (MACN-In 44478b); 1 complete specimen [not dissected] (MACN-In 44478c); 1 complete specimen [not dissected] (MACN-In 44478d); 1 complete specimen [not dissected, photos of the habitus] (MACN-In 44478e): 1 complete specimen [wall and opercular plates disartic- ulated; mouthparts dissected; R, C, T, and S photos; cirral counts] (MACN-In 44478f); 1 complete specimen [not dissected, photos of the habitus] (MACN-In 44478g); 4 complete specimens, attached one over the other [not dissected, photos of the habitus] (MACN-In 44478h-k); 1 complete specimen [wall and opercular plates disarticu- lated; mouthparts and cirri drawn; R, C, T, and S photos; cirral counts] (MACN-In 444781); 1 damaged specimen (R and both CL missing) [wall and opercular plates dis- articulated, mouthparts dissected] (MACN-In 44478m); 1 complete specimen [not dissected] (MACN-In 44478n); 1 damaged specimen, with a large number of developing eggs in the mantle cavity (R and left CL missing) [wall and opercular plates disarticulated, mouthparts dissected] (MACN-In 444780); 1 complete specimen [wall and opercular plates disarticulated, mouthparts dissected, photo serpulid epibiont] (MACN-In 44478p); 1 complete specimen [not dissected] (MACN-In 44478q); 1 dam- aged specimen (R missing) [wall plates disarticulated] (MACN-In 44478r); 1 damaged specimen (R missing) [not dissected] (MACN-In 44478s); 1 complete specimen [not dissected] (MACN-In 44478t); 1 complete specimen [not dissected, photo soft octocoral A/cyonium sp. epibi- ont] (MACN-In 44478u); batch of plates: 4 R, 3 C, 3 CL (MACN-In 44478v). Talud Continental IIT expedition, RV “Puerto De- seado”, Mar del Plata Submarine Canyon, Sta. 45, 38°1.913'S, 53°39.268'W, 2934 m depth, 05 Sep 2013, bottom otter trawl, colls. I. Chiesa and A. Martinez; 8 complete or damaged specimens (all with soft body parts intact) and | batch of disarticulated plates, namely: 1 com- plete specimen [wall plates articulated, opercular plates disarticulated; mouthparts dissected; habitus, T and S photos] (MACN-In 44479a); 1 complete specimen [wall and opercular plates disarticulated] (MACN-In 44479b); 1 complete specimen [not dissected, photos of the habitus] (MACN-In 44479c); 1 damaged specimen (R and left CL missing) [wall and opercular plates disarticulated, mouth- parts dissected, T and S photos, cirral counts] (MACN-In 44479d); 1 damaged specimen (R and right CL missing) [wall and opercular plates disarticulated] (MACN-In 44479e);, 1 damaged specimen (R and both CL missing) [wall and opercular plates disarticulated, mouthparts dis- sected] (MACN-In 44479f); 1 damaged specimen (R and 1 CL missing) [wall and opercular plates disarticulated, zse.pensoft.net 608 Chiesa, I.L. et al.: Tetrachaelasma southwardi from the Mar del Plata Submarine Canyon mouthparts dissected] (MACN-In 44479g): 1 damaged specimen (R and both CL missing) [wall and opercular plates disarticulated, mouthparts dissected, T and S pho- tos, labrum SEM, cirral counts] (MACN-In 44479h); batch of plates: 3 R (one of them with a Regioscalpellum epibiont, photo), 3 C, 4 CL (MACN-In 444791). Supplementary description. Newman and Ross (1971) gave a detailed description of 7. southwardi. Jones (2000) re-examined the holotype and summarized the main features of the species in tables 24—25. All the information presented in these tables had already been mentioned by Newman and Ross (1971). Therefore, we only consider the original description of Newman and Ross (1971) for comparison purposes. Size (rostro-carinal diameter): 13.0—47.1 mm (n= 13). Shell conical in young specimens and conical or co- lumnar in older (larger) specimens (Fig. 3). Shell wall covered with yellow cuticle and numerous fine bristles. Growth lines all along the plates are equi- distant from each other (Fig. 3A—E). However, basally growth lines are narrowly spaced in a columnar specimen (Fig. 3G) as well as in some isolated plates (Fig. 11E), all of them collected at 2934 m depth. Bristles are as long as, or longer than, the distance between growth lines. Figure 2. Tetrachaelasma southwardi Newman & Ross, 1971. Nomenclature used for the scuto-tergal articulation (the specimen shown in Fig. 4J—L is taken as a model). Scutum: A. Exterior view; B. Lateral view; C. Interior view. Tergum: D. Lateral view; E. Exterior view, smoothly concave articular margin painted in yellow; F. Internal view, sinusoidal articular margin painted in red. Abbreviations: a-a’ — distal and basal ends of the scutum articular ridge; b-b’ — distal and basal ends of the vertical articular ridge; c-c’ — distal and basal ends of the tergal articular furrow; /af— lower articular furrow; sar — scutum articular ridge; taf— tergal artic- ular furrow; tar — tergal articular ridge; waf— upper articular furrow; var — vertical articular ridge. zse.pensoft.net Zoosyst. Evol. 100 (2) 2024, 603-623 609 Figure 3. Zetrachaelasma southwardi Newman & Ross, 1971. Specimen (MACN-In 44478g): A. Rostral view; B. Carinal view; C, D. Left and right carinolateral views, respectively; E. Top view. Specimen (MACN-In 44479a, columnar) with a small specimen (MACN-In 44479b) attached to its rostrum: F. Left carinolateral view; G. Carinal view; H. Top view. Scale bars: 10 mm. zse.pensoft.net 610 Chiesa, I.L. et al.: Tetrachaelasma southwardi from the Mar del Plata Submarine Canyon Figure 4. 7etrachaelasma southwardi Newman & Ross, 1971. Intraspecific variation of opercular plates among the material collect- ed at 1950 m depth. Specimen (MACN-In 44478f): A, D. Left tergum in external and internal views, respectively; B, C. Left scu- tum in external and internal views, respectively. Specimen (MACN-In 44478a): E, H. Right scutum in external and internal views, respectively; KF, G. Right tergum in external and internal views, respectively. Specimen (MACN-In 44478b): I, L. Right scutum in external and internal views, respectively; J, K. Right tergum in external and internal views, respectively. Specimen (MACN-In 444781): M, N. Left and right terga in external view; O, P. Left tergum and scutum in external view. Scale bars: 10 mm. zse.pensoft.net Zoosyst. Evol. 100 (2) 2024, 603-623 611 Figure 5. Tetrachaelasma southwardi Newman & Ross, 1971. Specimen (MACN-In 44478f): A, B. Rostrum in external and in- ternal views, respectively; C. Carina in lateral view. Specimen (MACN-In 444781): D, E. Rostrum in external and internal views, respectively; F. Carina in lateral view. Abbreviations: aw — alar welting; rs — rostral sheath. Scale bars: 10 mm. Rostrum (Figs 3, 5A, B, D, E), 1.4-2.7 times (n = 13) wider than carina, only slightly bowed transversely, shape variable. In eight of the 13 specimens measured, the rostrum is the widest plate. In contrast, in five speci- mens, one of the CL plates (exceptionally both) is slight- ly wider than the rostrum. The sheath is flanked by very broad articular areas receiving alae of CL plates and oc- cupies 1/2 to 1/3 of the height of the rostrum in nine out of the 11 specimens dissected (Fig. SE); in the remaining two specimens, it occupies almost 2/3 of the height of the plate (Fig. 5B). Carinolaterals (Fig. 3C, E, F), as mentioned by New- man and Ross (1971). Carina (Figs 3, 5C, F) is the highest and narrowest of the wall plates. Shape variable. As mentioned by New- man and Ross (1971), it supports large alae that internally contribute to nearly half the total circumference of the sheath. Externally, alar growth lines similar to those of CL plates. Scutum (Figs 4, 6), as mentioned by Newman and Ross (1971), except for the articular ridge (sar), which varies from prominent to moderately projected beyond the articular margin (compare Fig. 4B, C, E, H, with Fig. 41, L). Note: The exposure of the sar depends on the shape (straight or twisted) of the scutum and on the angle of inclination at which the scutum is positioned. Adductor muscle pit shallow; boundaries weakly defined, i.e., not limited above and laterally by a distinct line (Figs 4C, H, L, 6D, I, M). External surface worn smooth at the apex in large specimens (Figs 4E, 6H, L). Some external growth lines may be slightly inflected close to the occludent mar- gin, occasionally forming a weak apico-basal ridge (see Fig. 6E). Note: This apico-basal ridge is also present in T. tasmanicum (see Buckeridge 1999). Tergum (Figs 4, 6) fully agrees with Newman and Ross’s 1971 description, except for: the separation of the tergal spur from the articular margin, measured at base, varies from almost imperceptible to as much as 0.48 of zse.pensoft.net 612 Chiesa, I.L. et al.: Tetrachaelasma southwardi from the Mar del Plata Submarine Canyon Figure 6. 7etrachaelasma southwardi Newman & Ross, 1971. Intraspecific variation of opercular plates among the material collect- ed at 2934 m depth. Specimen (MACN-In 44479h): A, D. Right scutum in external and internal views, respectively; B, C. Right ter- gum in external and internal views, respectively; E. Left scutum in external view, slightly slanted to make the apico-basal ridge more visible; F. Joined right tergum and scutum in internal view. Specimen (MACN-In 44479d): G, J. Left tergum in external and internal views, respectively; H, I. Left scutum in external and internal views, respectively. Specimen (MACN-In 44479a): K, N. Left tergum in external and internal views, respectively; L, M. Left scutum in external and internal views, respectively. Abbreviations: abr — apico-basal ridge; in — indentation (worn) on articular margin. Scale bars: 10 mm. spur width (n = 12; compare Fig. 4A, F, J with Fig. 6G, K). Internal surface with 2—7 depressor muscle crests, weak to well developed, extending at the most 4 along the basal margin (n = 13; compare Fig. 4G with Fig. 4K). The following information not reported by New- man and Ross (1971) is added: in six of the specimens zse.pensoft.net obtained at 1950 m depth, the basal margin of the ter- gum is 1.2 times the length of the occludent margin (Fig. 4A, D, F, G) [exception: basal and occludent margins are equal in length in the largest specimen dissected, Fig. 4J, K]. In contrast, in six of the specimens obtained at 2934 m depth, the basal margin is equal to or slightly shorter Zoosyst. Evol. 100 (2) 2024, 603-623 than the occludent margin (Fig. 6B, C, F, G, J, K, N) [ex- ception: basal margin longer than the occludent margin in two very small specimens]. The apex of the tergum is worn smooth only in the largest specimens (Figs 4F, J, 6G, K). In a few specimens, the articular margin of both terga is eroded near distal end, resulting in two rounded indentations (Fig. 6K). In addition, Newman and Ross (1971) stated, “Terga... with articular margin thrown into sinusoidal curve;” This character is herein described in more detail: Sinusoidal articular margin only fully visible in the internal view of tergum (Fig. 2F, red line); the amplitude of the sinusoi- dal curve increases as the basal part of the var develops (compare Fig. 4A, D with Fig. 4F, G). On the other hand, the articular margin is smoothly concave in the external view of tergum, running parallel to the outer edge of the external furrow (Fig. 2E, yellow line; see also Figs 4A, F, J, 6G). In some specimens, the basal part of the var is more developed in one tergum than in the opposite one (see Fig. 4M, N). The crest of the labrum (Fig. 7) is slightly concave, with many small serrate setae and a few small teeth just below it; in contrast, Newman and Ross (1971) reported that the crest is smooth. Interior surface of the labrum with a dense bundle of downwardly pointed setae on either side. Palps as described by Newman and Ross (1971). Mandibles (Fig. 8A, B) agree with the description pre- sented by Newman and Ross (1971) and the photograph of the holotype USNM 125305 (left mandible?) available on the website of the National Museum of Natural Histo- ry, Smithsonian Institution (link to the NMNH website in the References section). First maxillae slightly differ from Newman and Ross’s (1971) description, 1.e., lower lobe rounded (Fig. 8D) or somewhat straighter (Fig. 8C). Lower cutting margin with about 18—22 strong setae and 5-17 short—some of them pectinate—setae, just above the inferior angle (n = 3). Second maxillae (Fig. 8E) slightly bilobed (not bi- lobed after Newman and Ross (1971)). Cirri (Fig. 8F—I), as mentioned by Newman and Ross (1971). The cirral formula is provided in Table 2. Cirrus II: rami subequal in length, anterior ramus with the larg- est numbers (up to 8) of antenniform articles. Cirrus III: posterior ramus slightly longer than anterior one, carrying the largest number (up to 30) of antenniform articles. Penis (Fig. 8J), as mentioned by Newman and Ross (1971). A more detailed description is provided: 3 or 4 times longer than the pedicel of cirrus VI, finely annulat- ed along all its length (annuli more evident on proximal two thirds); distal third with small setae lateral and distal- ly. Basidorsal point absent. Caudal appendages absent. Settlement and epibionts. Of the 21 specimens of Tetrachaelasma southwardi collected at Sta. 25 (1950 m depth), 16 were complete. Most of these specimens were detached from the substrate (Fig. 11C) or attached to single rocks (Fig. 11A). One complete specimen was 613 Table 2. Tetrachaelasma southwardi Newman & Ross, 1971. Cirral formula of four specimens (two collected at 1950 m depth, two at 2934 m depth) from the Mar del Plata Subma- rine Canyon. Articles not fully separated (partially fused) were counted as single ones. The numbers of antenniform articles of the cirri II and III are given in parentheses. The cirri I-IV of the specimen (MACN-In 44781) are illustrated in Fig. 8. Specimen Cirralramus | Il lll V Vv Vi (Depth) MACN-In 444781 Left anterior 17) 22(4) + 25(4) 30 32 38 (1950 m) Left posterior 16 24(4) 42(30) 33 36 40 Right anterior 15 Right posterior 15 25 (4) Left anterior 15° 20) Left posterior 14 Right anterior 16 23 (8) Right posterior 14 22 (4) MACN-In 44479d Left anterior 16 24 (5) 2230 333.36 38 36125) Paly 5360536 Zoiks) B2F S30 SZ SOK) e290 ©3025 24(3) 27 29 31 ref OI tle dale (a ay eo) SOAs. 33 MACN-In 44478f (1950 m) (2934 m) Left posterior 14 24(3) 40(3) 33 39 42 Right anterior 13 23(7) 32(5) 33 39 33 Right posterior 15 25(6) 39(26) 35 38 37 MACN-In 44479h_ Left anterior VA ©2042) SIS) e290 e335 34 (2934 m) Left posterior 12 19(2) 46(39) 30 32 34 Right anterior 13 23(3) 28(8) 29 34 34 Right posterior 13 24 (3) 24, 30 33 34 + Broken (terminal articles missing). attached to an isolated CL plate, which has its external surface covered with yellow cuticle and numerous fine bristles and, therefore, most likely belongs to a living specimen of 7’ southwardi that was disarticulated during dredging. In addition, four complete specimens had set- tled one over the other (Fig. 11D). Of the eight 7’ southwardi obtained at Sta. 45 (2934 m depth), three were complete. Of these, one small speci- men is attached to the R plate of a second one (Fig. 3F, H). The third specimen has a rounded mark on its R and right CL plates, evidence that another specimen had been living on them (Fig. 11F). Several associations were observed among the materi- als studied. Two specimens of 7’ southwardi from Sta. 25 (1950 m depth) carried epibionts: one has a soft octocoral Alcyonium sp. on its R and left CL plates (Fig. 11C), and the other has tubes of serpulid polychaete worms on its scuta (Fig. 11B). Station 45 (2934 m depth) includes a pilose scalpellid barnacle attached to an isolated R plate (Fig. 11E). As this plate was covered with cuticles and numerous fine bristles, it likely belongs to a living spec- imen (of 7. southwardi?) that was disarticulated during dredging. This scalpellid fits well with the diagnosis of the genus Regioscalpellum proposed by Gale (2016); however, this author pointed out that the classification of the Scalpellidae is clearly provisional. In addition, one specimen of 7? southwardi, also from Sta. 25, is attached to a specimen of the genus Bathylasma (additional infor- mation in the Bathylasma sp. section). Distribution. 7etrachaelasma southwardi was previ- ously recorded in the Southern Ocean—from both the zse.pensoft.net 614 Chiesa, I.L. et al.: Tetrachaelasma southwardi from the Mar del Plata Submarine Canyon Figure 7. 7etrachaelasma southwardi Newman & Ross, 1971. Labrum SEM photographs (palps removed). Specimen (MACN-In 44478b): A. General aspect from above; B, C. Details of the crest; serrate setae in green, teeth in red. Specimen (MACN-In 44479h): D. General aspect from above; E—-H. Details of the crest. Scale bars: 300 um (A, D); 10 um (B, C, E—-H). Pacific and the Atlantic—and is now reported from the Mar del Plata Submarine Canyon. Depths range: 1190- 2934 m. All the records are listed in Table 1 and mapped in Fig. 9. Remarks. The supplementary description presented above 1s based on 29 specimens and a few loose plates collected in the Mar del Plata Submarine Canyon at two localities with significantly different depths (1950 m and 2934 m, Fig. 1). All these specimens have been assigned to Tetrachaelasma southwardi Newman & Ross, 1971. zse.pensoft.net However, it should be noted that these specimens differ from the original description of 7? southwardi as follows (characters mentioned in the original description are in- cluded in parentheses): (1) the adductor muscle pit of the scutum, which is weakly developed (pit deep, bounded above and lateral by a distinct line); (2) the crest of the labrum with abundant setae (without setae); and (3) the second maxilla, which is slightly bilobed (not bilobed). (1) In regard to the adductor muscle pit, we had the opportunity to examine images of the scutum of the Zoosyst. Evol. 100 (2) 2024, 603-623 615 Figure 8. Tetrachaelasma southwardi Newman & Ross, 1971. Specimen (MACN-In 444781): A, B. Left and right mandibles, re- spectively, inferior angles enlarged; C, D. Right and left first maxillae, respectively; E. Second maxilla, only some setae drawn, all of them serrulate (see detail); F—I. Left cirri I-IV, respectively; the first antenniform article is indicated with an asterisk; J. penis, only a short section of annuli is drawn. Abbreviations: a — anterior ramus; p — posterior ramus. Scale bars: 1 mm (A—E); 0.5 mm (F—J). zse.pensoft.net 616 Chiesa, I.L. et al.: Tetrachaelasma southwardi from the Mar del Plata Submarine Canyon Atlantic Ocean Pacific Ocean _’@MdPSC Tetrachaelasma ” southward | a ¢ 4 = 420° 90° ~——sté«