The genus Tetrachaelasma contains only two species, which inhabit much greater depths than any other Balanomorpha, up to 3600 m (Newman and Ross 1971, 1976; Buckeridge 2010; Table 1). Newman and Ross (1971) erected this genus for the reception of T. southwardi, a new species that these authors described based on seven complete specimens taken by the RV “Eltanin” at a single 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, i.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 Tetrachaelasma sp. on the flanks of a seamount off Madagascar at comparable depths (~1800 m). In addition, the RV “Atlantis II” obtained about 70 disarticulated plates of T. cf. southwardi 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, T. tasmanicum, based on a single incomplete specimen (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 T. southwardi specimens, complete or damaged but all with soft parts intact, were recently 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 III expeditions performed by the RV “Puerto Deseado”. Based on this material, supplementary descriptions of T. southwardi are presented, and intraspecific morphological variations are documented. In addition, the differences between T. southwardi and T. tasmanicum are discussed. Furthermore, a second bathylasmatine balanomorph was collected at the 1950 m depth station, which was assigned to the genus Bathylasma 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.

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 Bathylasmatidae, Tetraclitidae Gruvel, 1903, and Coronulidae Leach, 1817, under the superfamily Balanomorphoidea (new status, Coronuloidea Leach, 1817; see Newman and Ross (1977)), and divided the bathylasmatids into the subfamilies Bathylasmatinae and Hexelasmatinae. In addition, Newman and Ross (1976) suggested that Bathylasmatidae 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 Pachylasmatidae Utinomi, 1968, contradicting the proposal by Newman and Ross (1976). Foster’s nomenclatural decision was subsequently followed by Jones (2000, 2012), who included the subfamilies Bathylasmatinae and Hexelasmatinae in Pachylasmatidae, under the superfamily Pachylasmatoidea Utinomi, 1968.

Buckeridge and Newman (2010) revised the classification of Balanomorpha and grouped Bathylasmatidae and Tetraclitidae under the superfamily Tetraclitoidea Gruvel, 1903. More recently, Chan et al. (2017), in a molecular phylogenetic study for Balanomorpha, proposed that Bathylasmatidae is more closely associated with Tetraclitoidea than with Pachylasmatoidea. This result is consistent with the proposal by Newman and Ross (1976), Buckeridge and Newman (2010), as well as with a molecular phylogenetic study of Tetraclitoidea presented by Tsang et al. (2015). Finally, Chan et al. (2021), in a thorough revision of the barnacle classification, placed Bathylasmatidae in the superfamily Coronuloidea, together with Tetraclitidae, Austrobalanidae Newman & Ross, 1976, Coronulidae, and Chelonibiidae Pilsbry, 1916.

Bathylasmatidae currently encompasses the subfamilies Hexelasmatinae (genus Hexelasma) and Bathylasmatinae (genera Bathylasma, Tetrachaelasma, Tessa­re­lasma, and Mesolasma Foster, 1981). The former subfamily has a wall of six plates with longitudinal chitinous laminae and/or strips, whereas the latter subfamily 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 is presented by Araya and Newman (2018). For diagnoses of the subfamilies and genera, see Jones (2000).

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 ~500–1000 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).

Figure 1. 

A. Study area location (inset); B. Map including the two stations sampled at the Mar del Plata Submarine Canyon. The seabed topography of the study area is represented by roughly calculated isobaths. Abbreviation: SWAO — South-West Atlantic Ocean.

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 Talud Continental I and III expeditions, carried out by the RV “Puerto Deseado” in 2012 and 2013, respectively. The specimens 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 specimen identified as Bathylasma sp. was also collected at the 1950 m depth station (see Material Examined section).

All the specimens were fixed on board in 10% seawater formalin (buffered with sodium borate) and later transferred to 96% ethanol in the laboratory.

The shell and opercular plates were disassembled from most of the specimens. When necessary, terga and scuta were cleaned by soaking in dilute bleach (sodium hypochlorite). Parietes, opercular plates, and rostral-carina diameters were measured to the nearest 0.01 mm using a digital calliper.

Some specimens were dissected under a stereomicroscope (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. 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% 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 dehydrated 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 scuta of the holotype of Tetrachaelasma southwardi Newman & Ross, 1971, deposited in the National Museum of Natural History (USNM 125305), were used for comparison purposes.

Table 1 includes records from regular scientific publications 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 Canyon (Fig. 1) and all the records of the Tetrachaelasma 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 Invertebrate Collection of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN-In).

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 articles 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.

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; laf – lower articular furrow; sar – scutum articular ridge; taf – tergal articular furrow; tar – tergal articular ridge; uaf – upper articular furrow; var – vertical articular ridge.

The following terminology is used to describe the scuto-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 outgrowth 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 scutum (Fig. 2D, E).
• Tergal articular furrow (taf) : broad and deep groove extending along c-c’ that receives the scutum articular ridge (Fig. 2D, E, F).

Tetrachaelasma encompasses only two species: T. southwardi Newman & Ross, 1971, recorded from just a few, widely apart, stations in the Southern Ocean, and T. tasmanicum Buckeridge, 1999, restricted to South Tasmania (Table 1, Fig. 9).

Buckeridge (1999) stated that T. southwardi and T. tasmanicum are two closely related species, distinguished mainly by (1) the tergal articulation (sinusoidal vs. smoothly concave), (2) the ornamentation of the crest of the labrum (smooth vs. with teeth), and (3) the tergal spur (almost contiguous with articular margin vs. removed from articular margin as much as ⅓ its width).

(1) Newman and Ross (1971) stated that the articular margin of the tergum of T. southwardi is more or less sinusoidal, a character also used to define the genus. Conversely, Buckeridge (1999) stated that the tergum of T. tasmanicum has a smoothly concave articular margin (as opposed to sinusoidal).

The term “sinusoidal” was coined by Newman and Ross (1971) to describe the S-shaped aspect of the tergal articular margin. In our specimens, the sinusoidal margin is only fully visible in the internal view and results from the combination of the concave tergal articular ridge (tar) and the convex vertical articular ridge (var) (see Fig. 2F, red line). By contrast, in external view, the articular margin is smoothly concave and runs parallel to the outer edge of the external furrow (Fig. 2E, yellow line). Thus, the tergal articular margin is sinusoidal or smoothly concave, depending on the view (internal or external).

Newman and Ross (1971: pl. XXVI) presented images of the tergum of the holotype of T. southwardi. In these images, the articular margin of the tergum looks sinusoidal in both external (fig. E) and internal (fig. G) views. However, in the holotype, the articular margin is eroded in the middle part, and the external furrow is eroded distally. This condition was confirmed after examining high-resolution images of the holotype, which is deposited in the NMNH, Smithsonian Institution (see Material and Methods). In conclusion, the S-shaped aspect of the articular margin of the tergum of the holotype, in external view, is due to it being worn in the middle part.

Based on this new information, it appears that the tergal articular margin of both T. southwardi (including the type material and our specimens) and T. tasmanicum do not differ from each other, i.e., the articular margin is sinusoidal in the internal view and smoothly concave in the external view for both species. Thus, the S-shaped tergal articular margin is an important character to define the genus, but it seems not to be a reliable character to separate its two current species.

(2) The labrum of the specimens herein examined has short serrate setae on the crest and small teeth contiguous to them on its inner surface, i.e., teeth are neither on the top of the crest nor projecting beyond the crest. Newman and Ross (1971) stated that the crest of the labrum of T. southwardi is smooth. Although neither setae nor teeth are shown in the central part of the crest by these authors in fig. 74H, setae are shown on the lateral part of the crest in fig. 74G. It remains to be confirmed whether the central setae are actually absent in the holotype of T. southwardi. One possibility is that the holotype´s setae were worn, detached, or covered with biofilm (see embedded top setae in Fig. 7B), thus preventing their observation under a light microscope. In contrast, Buckeridge (1999) stated that T. tasmanicum has small teeth on the crest of the labrum. However, fig. 4D presented by this author is not detailed enough, and there is a possibility that the teeth are actually setae. It is worth noting that high magnification is necessary to distinguish small teeth from short serrate setae. Thus, the type material of these two species should be re-examined, preferably under SEM, to clarify this matter.

(3) The distance that separates the tergal spur from the articular margin is quite variable in our material. Thus, this character does not appear to be useful to separate T. southwardi from T. tasmanicum.

One character not mentioned in previous descriptions is the shape of the basal margin of the scutum. This appears to be gently convex in the holotype of T. southwardi (see figs F and H in pl. XXVI, in Newman and Ross (1971)), and also in our material (Figs 4, 6). In contrast, in T. tasmanicum, the basal margin of the scutum is strongly flexed, with an obtuse angle of about 120 degrees (see figs 1C, 2C, D in Buckeridge (1999)).

In addition, neither Newman and Ross (1971) nor Buckeridge (1999) reported the ratio between the basal and occludent margins of the tergum. The basal/occludent ratio of the holotype of T. southwardi was calculated from high-resolution images provided by the curator of the NMNH (see Material and Methods). These images correspond to figs E and G (pl. XXVI) presented by Newman and Ross (1971). In addition, the basal/occludent ratio of T. tasmanicum was calculated based on figs 2C and D (Buckeridge 1999). These measurements result in a basal margin longer than the occludent margin for the holotype of T. southwardi and similar basal and occludent lengths for the holotype of T. tasmanicum. However, this comparison is based on indirect evidence obtained from the holotypes. The examination of additional specimens from both type localities (Central South Pacific and South Tasmania) is required to confirm whether or not these two species differ in their basal/occludent ratio of the tergum.

The cuticular growth lines are related to the molting cycle, but how these lines depend on parameters such as food availability, temperature, and other environmental factors is unknown for deep-sea barnacles (Yusa et al. 2018). In a large columnar specimen (MACN-In 44479a) collected at 2934 m depth and in a few isolated plates (MACN-In 44479i) from the same station, most basal horizontal growth lines are narrowly spaced (Figs 3F, G, 11E). Newman and Ross (1971) did not have columnar specimens, but the holotype, a medium-sized (carina: ca. 2.9 mm) conical individual, shows a few narrowly spaced horizontal growth lines basally (see figs A–C in pl. XXVI, in Newman and Ross (1971)). It is worth noting that the holotype of T. tasmanicum, a large columnar specimen, has narrowly spaced horizontal growth lines. However, these closely spaced lines are not confined to the basal part of the parietes but extend along their entire surface (figs 1A, B, in Buckeridge (1999)). This narrowly spaced pattern was also observed in additional columnar specimens of T. tasmanicum collected recently (see the CSIRO-MIIC website in the References section CSIRO-MIIC 2024).

In conclusion, Tetrachaelasma southwardi and T. tasmanicum are suspected to be cryptic species that require additional morphological and molecular studies to be distinguished from each other.

Tetrachaelasma southwardi was not common among the material collected during the Talud Continental I, II, and III expeditions. This species was found in only two out of the 46 stations taken between 1000 m and 3447 m depth during these three surveys. No images were captured or quantitative data recorded at any of the 46 stations sampled, preventing an assessment of the abundance of T. southwardi. However, finding 29 specimens in these two samples suggests that this deep-sea barnacle could be relatively abundant in some benthic assemblages in the Mar del Plata Submarine Canyon. In support of this, a deep-sea barnacle tentatively identified as T. tasmanicum was reported at high density (32.1 ind/m²) off southern Tasmania at 2171 m depth (Thresher et al. 2014).

Regarding the presence of T. southwardi in southern South America, only loose plates have been reported until now. These plates were collected by the RV “Eltanin” in 1962 and 1965, off the Malvinas/Falkland Is. (1720–1739 m depth), at the Sars Bank in the Drake Passage (1207–1591 m depth), and off southern Chile (1190–1263 m depth) (see Newman and Ross (1971); Table 1, Fig. 9). The current record of T. southwardi at ca. 37°50'S is the most septentrional latitude reported for this species in the South-West Atlantic. This finding, however, is not entirely unexpected, as many other benthic invertebrate species that are distributed in Antarctic and/or sub-Antarctic waters have also been recorded from the Mar del Plata Submarine Canyon (Farias et al. 2015; Olguín et al. 2015; Pastorino 2016, 2019; Pastorino and Sánchez 2016; Roccatagliata and Alberico 2016; Lauretta and Penchaszadeh 2017; Rivadeneira et al. 2017; Maggioni et al. 2018; Bernal et al. 2019, 2021; Lauretta and Martinez 2019; Teso et al. 2019; Pereira et al. 2020; Roccatagliata 2020; Flores et al. 2021; Pertossi et al. 2021; Rumbold et al. 2021; Schejter et al. 2021; Pacheco et al. 2022; Pereira 2022; Sánchez et al. 2023). This distribution pattern can be explained by the presence of the Malvinas/Falklands Current, a branch of the Antarctic Circumpolar Current (ACC) that flows northward along the continental slope of Argentina up to around 38°S (Piola and Gordon 1989; Matano et al. 2010).

Tetrachaelasma species inhabit great depths around the Southern Ocean (Fig. 9). Most of its distribution area is under the influence of the Antarctic Circumpolar Current. The ACC is a large and strong ocean current that encircles Antarctica and connects the major ocean basins—the Pacific, Atlantic, and Indian Oceans (Rintoul et al. 2001). This current flows eastward between 40° and 60°S, and its influence extends to the seafloor down to 4000 m depth (Orsi et al. 1995; Barker and Thomas 2004; Barker et al. 2007; Dueñas et al. 2016).

Circumpolar Subantarctic/Antarctic distributions have been documented for Bathylasma corolliforme (Hoek, l883) as well as for other living and fossil barnacles (Newman and Ross 1971; Buckeridge 2015). This circumpolar distribution has also been reported for many other invertebrates, including stylasterid corals, molluscs, echinoids, sea stars, isopods, and decapods (Leese et al. 2010; Díaz et al. 2011; Pérez-Barros et al. 2014; Farias et al. 2015; Moles et al. 2015; Dambach et al. 2016; Pastorino 2016; Lauretta and Martinez 2019; Güller et al. 2020; Bernal et al. 2021; Pacheco et al. 2022).

Knowledge about the life cycle of deep-sea balanomophs is very limited, and no information is available on the Tetrachaelasma species. A closely related Antarctic deep-sea species, Bathylasma corolliforme, has nauplius larvae that are well adapted to planktonic life (Dayton et al. 1982; Foster 1989). Thus, it can be inferred that the members of Tetrachaelasma have free nauplii as well.

Seamounts are numerous but poorly sampled in the Southern Ocean and may act as stepping stones for species dispersal (Auscavitch and Waller 2017). Due to the patchy distribution of hard substrata in the deep sea, barnacle larvae must travel long distances on inhospitable soft bottoms to reach a suitable habitat for settlement. Therefore, it is expected that the nauplius larvae of T. southwardi have an extended life and are passively transported over long distances by the ACC (and off Argentina by one of its branches, the Malvinas/Falklands Current). A comparable hypothesis was proposed by Buhl-Mortensen and Høeg (2006) for the dispersal of the widespread deep-sea Arcoscalpellum michelottianum (Seguenza, 1876).

Bathylasma hirsutum (Hoek, 1883) and B. corolliforme feed passively, with the cirri simply extending into the current (Southward and Southward 1958; Dayton et al. 1982). Since Bathylasma is a genus closely related to Tetrachaelasma, we can assume that its members are also passive feeders. The presence of medium to strong bottom currents around the Mar del Plata Submarine Canyon (see Steinmann et al. 2020; Bozzano et al. 2021) supports this hypothesis.