A new deep-sea balanomorph barnacle (Cirripedia: Thoracica: Bathylasmatidae) from Chile

Deep waters of the South Pacific off northern Chile remain poorly studied, particularly in regard to invertebrate faunas. Some recent works include new records on deep-water species, mostly from the bycatch of benthic fisheries concentrated along the continental margin of the country. Among these, a few specimens of an unidentified bathylasmatine balanomorph were collected off Caldera, northern Chile, and they are described here as Bathylasma chilense sp. nov. While this is the second report of a bathylasmatid in the Eastern Pacific Ocean, the first being Tetrachaelasma southwardi Newman & Ross, 1971, it is not only the first but the deepest known (1800–2000 m) species of Bathylasma. Its discovery increases the number of described Bathylasma species to eight, four of which are extant. This is the third deep-water balanomorph cirriped recorded for the region where it may represent an isolate from a West Wind Drift fauna, an immigrant from the western Pacific, or a relict of a once cosmopolitan Paleocene-Eocene fauna now having an amphitropical component.


Introduction
Cirripeds are basically attached, setose-feeding crustaceans frequently encountered in high densities in the intertidal and coastal inshore shallow waters [1]. Most have six free-living planktotrophic naupliar stages and a non-feeding cyprid stage that selects the place to settle and metamorphose into the juvenile. Balanomorph cirripeds, the most species rich group of thoracic barnacles, are members of the suborder Balanomorpha Pilsbry, 1916 (sensu Koči et al. [2]). This group encompasses sessile barnacle species having a symmetrical wall formed of eight, six, or four plates or a whole solid or concrescent shell, and an operculum formed by paired terga and scuta, except in Xenobalanus Steenstrup, 1852, where the operculum has been lost [3,4]. The balanomorphs include a large diversity of species largely found in shallow waters around the world; they are usually gregarious, often found in large communities along the shore where they may represent important components of many coastal ecosystems [5]. a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 the bycatch fauna of commercial fisheries off northern Chile, which have revealed several new species and new records for the south-eastern Pacific [20][21][22][23][24][25][26], we present the first record of the genus Bathylasma Newman & Ross, 1971 in the Eastern Pacific Ocean. It also constitutes the deepest unequivocal record for the genus.

Sample and morphological examination
The specimens of the new species were collected attached to an unidentified zoantharian entangled in a longline from between 1800-2000 m depth, as bycatch from the fishery for the Chilean Seabass, Dissostichus eleginoides, from about 32 km off Caldera (26˚44' S; 71˚07' W), Región de Atacama, northern Chile. As this material was serendipitously collected in the fish bycatch, no permit was necessary for the current research. Cirripeds are not endangered nor protected by local law. The specimens were examined by light and scanning electron microscopy (SEM), and measured with vernier callipers reading to two decimal places. The appendages were mounted on glass slides in glycerine jelly for illustration. For the morphometric analysis, the works of Newman & Ross [3] and Jones [14] were followed. The systematic arrangement follows Newman & Ross [3] and Newman [16], as modified by Koči et al. [2]; the interesting but presently intractable complications introduced by the genetics of Tsang et al. [17] for the present being ignored. A scheme for the wall construction (plan views) and for plate nomenclature in Bathylasmatidae genera is presented in Fig 1.

Deposition of types
The holotype is deposited in the collections of the Museo Nacional de Historia Natural, at Santiago, Chile (MNHNCL CIR-15057), the first paratype is deposited in the Benthic Invertebrate Collection of the Scripps Institution of Oceanography, San Diego, USA (SIO/BIC Cat. No.12199) and the second paratype in the collection of the Museo Paleontológico de Caldera, in Caldera, Chile (MPCCL020518).

Nomenclatural acts
The electronic edition of this article conforms to the requirements of the amended International Code of Zoological Nomenclature, and hence the new names contained herein are available under that Code from the electronic edition of this article. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix "http://zoobank.org/". The LSID for this publication is: urn:lsid:zoobank.org:pub: F7B5CB72-7DFA-48E2-98B1-0388472A5DEB. The electronic edition of this work was published in a journal with an ISSN, and has been archived and is available from the following digital repositories: PubMed Central, LOCKSS. Diagnosis: Adult wall of six relatively thin, calcareous compartmental plates (R-CL1-CL2-C); rostrum compound (not tripartite). Parietes with prominent horizontal growth ridges lined with small setae; chitinous laminae absent. External alar ridges diverging from inferior alar margin; superior alar margin with welting. Basis membranous. Articular  Newman & Ross, 1971. C, plate nomenclature, showing schematic left lateral carinolateral wall plate between adjacent plates, and viewed from within. 1: total height; 2: height below sheath; 3: height of entire sheath; 4: height of sheath crossed by growth lines; 5: width of basal margin; 6: width of ala; 7: inferior alar margin; 8: alar angle; 9: secondary alar increment or welting; 10: superior alar margin; 11: apex; 12: portion of wall overlapping adjacent plate (in this case without radii, a radius being the exterior counterpart of the welting (9) and serving much the same function) (modified from Newman & Ross [3]).
Description: The holotype of Bathylasma chilense sp. nov. is moderately large in size, with a rostro-carinal diameter of 31.2 mm and a carinal height of 13.9 mm (Fig 2A and 2B). The conical wall of parietal plates gently slopes at an angle between 45˚-60˚from the base. The solid parietal plates are chalky white, with slightly irregular growth ridges covered by a thin epidermis with numerous hair-like pale brown chitinous bristles in bundles along the growth lines. The inner surface of the parietes is matte white, porcellaneous and smooth in their lower halves, and the sheath in their upper part is crossed by fine horizontal lines. The rostral plate is broad and convex, with a basal width about equal to that of the carinal plate. The carina is slightly flared; the secondary carinolateral plates are narrow, of 33-50% of the width of the basal margin of primary carinolateral plates. As seen from the exterior, the alae are sharply delimited from the parietal portion of the plate and usually exposed about 66% of the height of the wall, below which point the parietal portions of the plates come into contact. External alar growth lines parallel the inferior alar margin and then diverge abruptly near alar angle. Sheath noted above slightly overhanging the interior of the compartment; crossed horizontally by fine striae occupying 50% the length of plate. Basis membranous, very thin.
Orifice small, ovate-rhomboidal in outline. Opercular plates lodged upright in orifice. Scutum (Fig 2E and 2F) larger and broader than tergum; triangular, elongated, slightly concave in  the middle; length of convex basal margin about 75% length of occludent margin; articular margin curved toward occludent margin; externally growth ridges regular, slightly sinuous and covered with minute but noticeable setae, without radial striations; apex blunt, incurved; internally adductor muscle pit large, shallow, marked whitish; situated about 50% the length of valve; lateral depressor muscle pit barely visible, whitish. Tergum (Fig 2C and 2D) more or less triangular, much wider than scutum, curved and thickened towards articular margin; covered with a yellowish cuticle sculptured by fine growth lines and a few axial lines, articular margin with growth ridges raised to form elevated ridge; internally with 7-8 distinct, marked and prominent muscle attachment crests for depressor muscle crest not projecting beyond basal margin; inner surface smooth. Articular furrow marked, narrow; tergal spur rounded, about 25% of the width of basal margin. Tergal notch large and deep (Fig 3C). The interior margin of the apical parts of scutum and tergum lined with fine dark-brown setae.
Labrum with shallow median depression marked by low, rounded teeth. Mandibular palps oblong, with fine setae along their inner edge (Fig 5E). Mandible quadridentoid, first tooth the largest, separated from second and third, inferior angle dentate, inferior angle with five or so short spines; left mandible with two strong spines between first and second tooth (Fig 5A and  5B). Maxillule setose; two long, stout setae above and followed by 3-4 pairs of smaller setae; straight, stepped cutting margin below with four or five pairs of larger setae; inferior angle small, with three pairs of smaller setae (Fig 5C and 5D). Cirrus I with unequal rami; anterior ramus slightly longer than posterior; both rami with segments setose. Articles wider than high; the anterior surface of each segment is produced in both rami and the projections bear a tuft of spines at their summits. Cirrus II longer than cirrus I; rami unequal, anterior ramus slightly shorter than posterior ramus; all segment setose, articles wider than high and slightly protuberant along the anterior margins. Cirrus III longer than cirrus II; rami unequal; anterior ramus slightly shorter and thinner than posterior ramus, which is much wider; all segment with setae, neither ramus is antenniform. Segments becoming oblong distally, wider than high; slightly protuberant along the anterior margins. Posterior ramus with 3-4 pairs of long setae on anterior faces of distal segments, posterior segments densely setose; anterior ramus with 3-4 pairs of long setae on anterior faces of distal segments, proximal segments with 4 pairs. Cirrus IV to VI similar, longer than cirrus III; rami subequal, segments oblong, with 3-4 pairs of long setae on anterior faces (Fig 4A-4L). Cirral formula is provided in Table 1 as follows: Habitat: The specimens were found attached to stem and branches of an unidentified arborescent parazoanthid (Fig 3A), among the incidental (bycatch) fauna entangled in a longline off Caldera (26˚44' S; 71˚07' W) at about 1800-2000 m depth, January 24, 2015, by the fishing vessel Rocio III (Caldera, Chile).
Etymology: Named for where discovered: Chile, the first Eastern Pacific representative of the genus.
Remarks: Bathylasma chilense sp. nov. is rather variable in its outline, the three specimens ranging from ovate to triangular when viewed from above, with differences in curvature involving the carina and rostrum. We assume this variation is related to the habitat, living in the branches of a parazoanthid. The carinolateral plates are much reduced in width in the two paratypes. The wall plates are weakly articulated as they are in B. corolliforme (Hoek, 1883) collected from near Heard and MacDonald Islands [27]. Of interest is that the soft parts are a uniform reddish-brown to amber color. While this differs from the other species of the genus as well as most other species in the family, Tetrachaelasma tasmanicum Buckeridge, 1999 was reported as having similar reddish-brown soft tissues [28]. No complemental males-recorded as occurring in the exposed apically articular furrow of the tergum by Foster [29] and Dayton et al. [30] for B. alearum and B. corolliforme-were observed in the present specimens. Bathylasma Newman & Ross, 1971 currently contains three extant deepwater species having spatially isolated (allopatric) distributions: 1) Bathylasma hirsutum (Hoek, 1883) in the Northeast Atlantic, ranging from the Faroe Islands south to the Azores at 944-1829 m of depth [12,31], 2) Bathylasma corolliforme (Hoek, 1883) from the circum-Antarctic at depths of 50-1464 m [3], and 3) Bathylasma alearum (Foster, 1978) from New Zealand and eastern Australia northwards to Vanuatu at depths between 414-1750 m [12]. The genus is also represented by four fossil species, Bathylasma aucklandicum (Hector, 1888) from the Oligo-Pliocene of New Zealand, Bathylasma costatum Buckeridge, 1985 from the Lower Miocene of Victoria, Australia [19], Bathylasma corrugatum Zullo & Baum, 1979 from Upper Eocene of North Carolina, USA [18], and Bathylasma rangatira  from the Lower Palaeocene to Lower Eocene of the Chatham Islands [13,14,32]. The new species, Bathylasma chilense from northern Chile, is compared to other extant species of the genus Bathylasma in Table 2, which forms the basis for the key below. Therefore only a brief comparison is given here; it differs from its recent congeners as follows: 1) from B. alearum of New Zealand in having a much smaller carinal plate and a much wider tergum with a more elongated and curved spur; 2) from the circum-Antarctic species B. corolliforme in having smaller and less flaring parietal plates and smaller carinolaterals, a smaller orifice which is smaller than its base; a less curved scutum with very narrower growth-ridges in the superior half of the valve, and a much shorter tergum, with a distinct spur. Bathylasma hirsutum has a larger carinal margin and a more curved carina and a more marked articular ridge in the tergum than in the new species. One or the other mandible of B. chilense sp. nov. apparently differs from all the other Bathylasma species in having two strong spines between the first and second teeth and in lacking small subsidiary cusps on upper or lower margins of tooth 4. The new species differs from the fossil species B. aucklandicum by the presence of a distinct sheath, in having a larger tergal spur and in lacking the tall, thin and cylindrical shell of this species, which can reach up to 18 cm in height [19]. Bathylasma costatum differs from the new species in having much shorter, broader parietes, without strong vertical ribbing. It also differs from B. corrugatum, the second oldest representative of the genus [18], in lacking the regularly spaced and prominent corrugations of the alae, and from the fossil B. rangatira Buckeridge, 1975, in lacking the chevron-shaped growth striae on the parietes, and its smaller size (B. rangatira having a length up to 54 mm high), much thinner shells, and flat rather than convex scuta [33].  [3,27], on the spines of echinoids of genus Cidaris, and in the demosponge Topsentia novaezelandiae [3]. The presence of sponges, anthozoans and several other species [34][35][36] in the hauls from where the new species was discovered indicates the presence of constant currents which, in addition to fallout from high surface productivity, may help this cold water fauna to thrive. High surface productivity due to upwelling is known to occur in the area [37].

Key to the extant species of Bathylasma
A presumed Bathylasma sp. and two species of Hexelasma from deep waters in the subtropical Southwest Pacific were recently determined to nest in the Tetraclitoidea in the genetic study of Tsang et al. [17], confirming an alliance proposed by Newman & Ross [3] and Newman [16]. However, Bathylasma being under subtropical waters is unprecedented and therefore the specimens were sent us by B. K. K. Chan upon our request. As it turned out, not only were the specimens small for Bathylasma, but a calcareous basis could be seen on a few and therefore we returned them with the suggestion that they more likely represent specimens of Mesolasma, Hexelasma, or perhaps a new genus. It follows that the known range of Bathylasma in the southern hemisphere is apparently from Antarctica to as far north as 15˚S under the divergence of the south-flowing East Australian from the Equatorial Current, and to 27˚S under the Humboldt Current off Chile. The sole northern hemisphere representative of the genus, Bathylasma hirsutum, ranges in the NE Atlantic from the Faroes (62˚N) south to the Azores and east to off the Straits of Gibraltar (35˚N) [3,11]. There are yet no records, fossil or extant, from the North Pacific.
A large proportion of the deep-sea thoracican barnacle species present in the Southeast Pacific are pedunculates (largely the Lepadomorpha and Scalpellomorpha), the only two symmetrical sessile barnacles (Balanomorpha) being Solidobalanus nascanus Zullo, 1964 and Eochionelasmus paquensis Yamaguchi & Newman, 1997 [38, 39]. This situation is similar in the adjacent territories of Antarctica, where of the 32 species found, the only balanomorph barnacle being the deepwater species Bathylasma corolliforme [3,11].
Regarding the origin of the new species herein described, while an Austral West Wind Drift origin may seem most likely, we cannot ignore the Rapanuian or more western Indo-Pacific provinces via the seamounts and guyots of the Nazca and Sala y Gómez Ridge as stepping stones involved in their dispersion. The presence of several barnacle species of clear Indo-Pacific origins on an unnamed guyot in the Nazca Ridge [38] support this assertion. This can be tested by further sampling along the submarine features off northern Chile, coastal areas of which may include the easternmost representatives of some Indo-Pacific fauna [38][39][40][41][42][43][44]. On the other hand, in light of the extant North Atlantic population (B. hirsutum) and the U. Eocene record from North Carolina, the hypothesis that the bathylasmids are bipolar relicts of Tethys cannot be ruled out [43].