Habitat conditions of pliocardiines in Kemp Caldera
Living pliocardiine clams were definitively observed for the first time in the Southern Ocean in 2010 at the study sites for the present paper, in two areas and different sites around the resurgent cone in the Kemp Caldera at depths ranging from 1320 to 1487 m (Fig. 1); large quantities of dead shells were also observed. The southern area was characterised by basalt cobbles and rough-edged basalt boulders with no visible sediment-covered areas, and the pliocardiines were living epifaunally on the hard rock (Fig. 2a; Table 1). In 2019, the sites with epifaunal clams were surveyed again but only dead shells were found.
In the northern area of the Kemp Caldera, multiple sites were characterised by sediment-covered areas, in the form of either small areas between basalt cobbles and boulders or wide-ranging sediment coverage. In both 2010 and 2019, pliocardiines were observed at some of these sites either fully buried with the siphons visible, sticking out of the sediment, or half-buried with shell and siphons visible (Fig. 2b,c; Table 1). Temperature measurements taken in 2019 found that one area densely populated by clams had a sediment temperature 4.4 °C above the 0.3 °C ambient temperature, compared to 0.25 °C above ambient in the sediment outside the clam bed. Another measurement found a thermal anomaly 0.3 °C above ambient temperature in the water immediately above a dense aggregation of clams (< 1 m altitude).
The overall allometry of shell length and height followed a consistent pattern across clams living epilithically and infaunally, indicating that there is no overall difference in the change of shape over growth that is characteristic to these habitat types (Fig. 3). Of the three epilithic and five infaunal individuals used for molecular sequencing, partial sequences of COI, 18S, and 28S genes were successfully obtained with the exception of 28S from one infaunal individual. For COI (638 bp), a dominant haplotype was shared across six individuals from both ecotypes, and the other two individuals (one epifaunal and one infaunal) exhibited singleton haplotypes that differed from the dominant haplotype by a single base. The uncorrected p-distances among the haplotypes ranged between 0.0016 and 0.0032. For both 18S (517 bp) and 28S (757 bp), all individuals shared the same haplotype regardless of their ecotype. These results suggest that the two ecotypes are genetically indistinguishable.
Although a range of animal sizes was used in experiments, there was only a weak positive correlation of size and metabolic rate (VO2: μmol O2 h−1) in either group (4 °C: R2 = 0.004, p = 0.78; 10 °C: R2 = 0.39, p = 0.02) insufficient to determine a robust estimate of a metabolic scaling exponent. The mean metabolic rate (VO2) did not significantly differ between the 4 °C and 10 °C treatment groups (Mann–Whitney U test, W = 129, p = 0.99). The mean mass-specific metabolic rate (MO2: μmol O2 h−1 g−1 dry tissue mass) was much higher in the 4 °C group (6.59 μmol O2 h−1 g−1 dry tissue mass) than in the 10 °C group (3.13 μmol O2 h−1 g−1 dry tissue mass); however this difference was attributable to higher metabolic rates in the smallest individuals sampled in the 4 °C group and the rates across the two treatments were not statistically significantly different (W = 115, p = 0.60; Fig. 4).
Animals kept in shipboard aquaria remained active and responsive for several days (video provided in Online Resource 1), but after 2–3 days began to deteriorate, the foot and siphons were less frequently extended and the foot appeared paler in colour. Animals remained alive for up to 5 days, and would respond to handling by rapidly clamping shut and ejecting water from the siphon.
We identified the pliocardiine species from Kemp Caldera and Larson B as conspecifics of a species previously known only from the South Atlantic on the Argentinian continental slope off Buenos Aires in 877–2204 m depth and originally described as Laubiericoncha puertodeseadoi (Signorelli and Pastorino 2015) based on the Argentinian material (Signorelli and Pastorino 2015). This is also supported by similar shell length to height ratios across sizes between the type series and clams collected from the Southern Ocean (Fig. 3). The Southern Ocean records considerably extend the known geographic range of this species. While the depths of the Kemp Caldera records were within the known bathymetric range, additional Larsen B specimens were collected at a shallower depth of 852 m. Morphological comparisons with closely related species indicated that this species should be assigned to Archivesica sensu lato, with more details presented in the following section.
Class BIVALVIA Linnaeus 1758.
Subclass HETERODONTA Neumayr 1884.
Order VENERIDA Gray 1854.
Superfamily GLOSSOIDEA Gray, 1847.
Family VESICOMYIDAE Dall et Simpson, 1901.
Subfamily PLIOCARDIINAE Woodring, 1925.
Archivesica Dall, 1908 sensu lato.
Archivesica s.l. puertodeseadoi (Signorelli et Pastorino, 2015).
Laubiericoncha puertodeseadoi Signorelli and Pastorino 2015: 352, Figs. 1–28.
Archivesica puertodeseadoi – Hansen et al. 2017: 272.
Southwestern Atlantic Ocean, off Buenos Aires province coast (37°54.206′S, 54°2.616′W), 2419.59 m; RV “Puerto Deseado”, station 24, bottom trawl, 14 Aug 2012 (Signorelli and Pastorino 2015).
Off Buenos Aires province coast, Argentina, southwestern Atlantic Ocean in 877–2,204 m depth (Signorelli and Pastorino 2015); Kemp Caldera, South Sandwich Arc in 1320–1487 m depth; Larsen B area, Weddell Sea, 852 m depth.
Archivesica s.l. puertodeseadoi: RRS James Cook cruise 42, ROV Isis, Scotia Sea, South Sandwich Arc, Kemp Caldera, 59°41.677 S, 28°21.086 W, 1320–1487 m, 2010, many specimens with soft parts (BAS, IORAN); RV Polarstern cruise ANT XXIII/8, western Weddell Sea, Larsen B area, 65°26′S, 61°26.5′W, 852 m, 15 January 2007, 2 dry valves and fragments (MARUM).
Laubiericoncha myriamae: DIAPISUB stn PL DS 03/1, Barbados accretionary prism, site Orénoque A, 10°20.27′N, 58°53.73′W, 1730 m, paratype, 1 specimen with soft parts (MNHN 20,551); R/V Ronald H. Brown, ROV Jason II dive 283, Gulf of Mexico, 21˚N, 91˚ W, 2276–2530 m, 2 July 2007, 3 specimens with soft bodies (IORAN); DSV Alvin dive 3917, Gulf of Mexico, 27°14.02 N, 85°36.66 W, 3234 m, 13 October 2003, 50 articulated dry shells (IORAN).
Archivesica s.l. chuni: BIOZAIRE 3 cruise, RV Atalante, REGAB site, West of Congo river mouth, 5°46.89S, 9°44.65E, 3113–3159 m, 2 January 2004, 2 dry valves (IORAN).
Specimens from the Kemp Caldera were consistent with the type material of A. s.l. puertodeseadoi (Signorelli & Pastorino 2015) in the following diagnostic features: oval-elongated valves with slightly tapering posterior margins and 1–2 shallow sulci in dorsal-posterior area, a similar hinge margin with subumbonal pit, the strongest tooth being 1 on the right valve and 2b on the left valve, a pallial line originating from the postero-ventral margin of the anterior adductor scar with nearly triangular pallial sinus and going slightly behind the anterior margin of the posterior adductor scar.
Comparison of the Kemp Caldera material with the type species of Laubiericoncha, Laubiericoncha myriamae Cosel and Olu 2008, based on a paratype specimen as well as additional material, revealed that in spite of the obvious similarity in the general shell outline and the hinge margin configuration, the newly collected specimens differed from L. myriamae in several important characters. These include exhibiting a subumbonal pit in the hinge margin, pallial line lacking in ventral prolongation posteriorly at the end of pallial sinus, and the interlamellar septae in the gills lacking in tubular structure. Furthermore, the inhalant siphon valve only carry short papillae along its margin, in sharp contrast with L. myriamae where the entire surface of valve is covered by conspicuous papillae, a unique feature among known pliocardiines. The inner surface of the inhalant siphon of L. myriamae is also covered by rather large papillae, which were absent in the Southern Ocean clams. On the basis of these differences, we consider that A. s.l. puertodeseadoi and L. myriamae are not congeneric species.
We further compared the Kemp Caldera clams with Archivesica s.l. chuni and Archivesica gigas, the type species of the genus Archivesica s.s.. We found that A. s.l. chuni and A. s.l. puertodeseadoi share morphological features in the abovementioned key characters that differentiate them from L. myriamae, including the same structure of gill interlamellar septae lacking tubes, having similar vulve on the inhalant siphon, and a pallilal line lacking posterior extension. Additionally, both species, A. s.l. chuni and A. s.l. puertodeseadoi, differ from A. gigas by longer syphons and triangular pallial sinuses. According to the molecular results, A. s.l. chuni was nested in a well-defined unnamed subclade, sister to a subclade Archivesica s.s. containing A. gigas (Johnson et al. 2017). So, here for A. s.l. chuni and A. s.l. puertodeseadoi we provisionally use the genus name Archivesica in the wide sense (sensu lato).
In the Kemp Caldera A. s.l. puertodeseadoi was reported in several hard rock and soft sediment habitats (Linse et al. 2019a) which dominant or visually distinct fauna comprised the seastar Paulasterias tyleri, the limpets Cocculina enigmadonta and Lepetodrilus concentricus, the pyconogonids Sericosura bamberi, S. curva, and S. dimorpha, and actinosolid anemones (Mah et al. 2015; Linse et al. 2019b; Chen and Linse 2020).
Notes on Laubiericoncha
Taxonomy of Pliocardiinae is still far from being stabilised, with shell characters used in early taxonomic accounts often failing to provide sufficient discrimination among genera. Including soft anatomical traits has considerably improved the understanding of the taxonomy at the generic level (Cosel and Salas 2001; Krylova and Sahling 2006; Krylova et al. 2014) and recent studies combining soft part morphology and multi-gene molecular analyses have clarified some genus-level characteristics (Johnson et al. 2017). Nevertheless, there are still unresolved problems at both genus- and species-levels due to a high degree of morphological variability, with a number of cryptic and synonymous species revealed in the group by molecular approaches (Audzijonyte et al. 2012; Decker et al. 2012). In these respects, the pliocardiine species from the Kemp Caldera is of particular interest, as it demonstrates a wide range of morphological variability and ecological plasticity.
Laubiericoncha myriamae and A. s.l chuni were initially both placed in Laubiericoncha (Cosel and Olu 2008; Signorelli and Pastorino 2015), but molecular phylogeny indicated that they were divergent at genus level (Decker et al. 2012; Johnson et al. 2017). At present, Laubiericoncha is best considered as a monotypic genus with the only species L. myriamae distributed in the West Atlantic near off Barbados and in the Gulf of Mexico (1170–3234 m).