Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan

Simon Weigmann; Ofer Gon; Ruth H. Leeney; Ellen Barrowclift; Per Berggren; Narriman Jiddawi; Andrew J. Temple

doi:10.1371/journal.pone.0228791

Abstract

Recent sampling efforts in Madagascar and Zanzibar, as well as examinations of six-gilled sawsharks in several museum collections provided evidence for a complex of species within Pliotrema warreni Regan. The present manuscript contains a redescription of P. warreni involving the syntypes and additional material, as well as formal descriptions of two new species of Pliotrema Regan. All specimens of both new species were found in the western Indian Ocean. Individuals of the first new species, hereafter referred to as P. kajae sp. nov., were identified originating from Madagascar and the Mascarene Ridge. Specimens of the second new species, hereafter referred to as P. annae sp. nov., were only found off Zanzibar. Pliotrema kajae sp. nov. appears to inhabit upper insular slopes and submarine ridges at depths of 214–320 m, P. annae sp. nov. so far is only known from shallow waters (20–35 m). Both new species differ from P. warreni in a number of characteristics including the known distribution range and fresh coloration. Taxonomical differences include barbels that are situated approximately half way from rostral tip to mouth, with prebarbel length equidistant from barbel origin to symphysis of the upper jaw in P. kajae sp. nov. and P. annae sp. nov. (vs. about two thirds way from rostral tip to mouth, with prebarbel length about twice the distance from barbel origin to symphysis of upper jaw in P. warreni) and rostra that are clearly and slightly constricted between barbel origin and nostrils, respectively (vs. rostrum not constricted). Pliotrema kajae sp. nov. differs from P. annae sp. nov. in a longer snout, more numerous large lateral rostral teeth and upper jaw tooth rows, jaw teeth with (vs. without) sharp basal folds, and coloration, particularly pale to light brown (vs. medium to dark brown) dorsal coloration with (vs. without) two indistinct yellowish stripes. A revised diagnosis of Pliotrema and a key to the species are provided.

Citation: Weigmann S, Gon O, Leeney RH, Barrowclift E, Berggren P, Jiddawi N, et al. (2020) Revision of the sixgill sawsharks, genus Pliotrema (Chondrichthyes, Pristiophoriformes), with descriptions of two new species and a redescription of P. warreni Regan. PLoS ONE 15(3): e0228791. https://doi.org/10.1371/journal.pone.0228791

Editor: Michael Schubert, Laboratoire de Biologie du Développement de Villefranche-sur-Mer, FRANCE

Received: October 12, 2019; Accepted: January 21, 2020; Published: March 18, 2020

Copyright: © 2020 Weigmann et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript.

Funding: The specimens from off Zanzibar were collected as a result of work supported by the Western Indian Ocean Marine Science Association (WIOMSA) (Grant Number MASMA/CP/2014/01) to AJT, NJ and PB. Partial support was also granted by the South African National Research Foundation (NRF) to OG and the Save Our Seas Foundation kindly agreed to pay the publication fees and supported RHL’s fieldwork in Madagascar. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Pristiophoriform sharks (sawsharks) possess a flat, greatly elongated, and saw-like snout, which bears long ventral barbels and closely-set rows of lateral and ventral teeth [1–3]. Further, the anterior-most basiventral cartilages are laterally expanded and have curved, dorsally reflected margins [4,5]. Based on genetic analyses, previous studies have shown that the Pristiophoriformes form a clade with the Squaliformes and Squatiniformes but with the exclusion of the Hexanchiformes [6–8]. However, Naylor et al. [8] highlighted that the interrelationships between the Echinorhinidae, Pristiophoriformes, and Squatiniformes remain unclear due to weak support in the respective datasets. Interestingly, the current proposed interrelationships appear to differ significantly between published studies [9–11], highlighting the need to clarify understanding of the interrelationships of these groups. Regardless of the lack of detailed empirical data, there is morphological support for the interrelationships reported by Naylor et al. [6–8] for the Pristiophoriformes and Squatiniformes by, e.g. [12] based mainly on skeletal and myological features, as well as for the Pristiophoriformes and Squaliformes by, e.g. [3] based on characteristics of the claspers. However, Weigmann et al. [3] pointed out that, despite sharing many characteristics, the claspers of pristiophoriforms and squatiniforms differ greatly. These authors proposed that the similarity in clasper characters between pristiophoriforms and squaliforms could be regarded as a plesiomorphic character, whereas the differences in clasper morphology between pristiophoriforms and squatiniforms could be considered as autapomorphies, and the similarities in body shape and morphology of the first basiventral cartilages of the latter two orders as evidences of interrelationship.

Members of the monotypic genus Pliotrema Regan clearly differ from species of Pristiophorus Müller and Henle in their possession of six (vs. five) pairs of gill slits and serrated (vs. smooth-edged) large lateral rostral teeth [3]. Furthermore, Springer and Bullis [4] reported that the bases of the jaw teeth of species from the genus Pliotrema have 3 to 6 short but distinct ridges, somewhat stronger on the upper jaw teeth, whereas these ridges are indistinct or absent in Pristiophorus species. The only recorded species of Pliotrema, P. warreni Regan, has a known distribution from the southwestern Indian Ocean and southeastern Atlantic, and has been recorded at depths between 26 and 500 m [13,14]. It has been reported to reach a total length (TL) of up to 170 cm, making P. warreni the largest known species of sawshark [14,15]. Pristiophorus species are small to medium-sized, demersal sharks known from continental and insular shelves and upper slopes of tropical and temperate latitudes of all three major oceans, but with the center of distribution in the western Pacific Ocean and only one recently described, poorly known dwarf species in the western Indian Ocean [2,14]. Members of this genus attain maximum sizes of 62–153 cm TL and occur in depths between 0 and 1240 m [14].

While collecting data on elasmobranchs caught in small-scale fisheries in Madagascar, RHL collected two rostra of Pliotrema. Subsequently, EB, AJT and PB collected a specimen of Pliotrema from off Zanzibar during a 12-month fisheries landings observer program across the coastal regions of Kenya, Zanzibar, and northern Madagascar [16]. The rostra and the specimen from off Zanzibar turned out to represent two undescribed species of Pliotrema. During a search of museum collection, SW further identified a number of complete specimens of the same undescribed species represented by the two rostra, NJ and AJT managed to collect an additional specimen of the other new species from off Zanzibar, which was transported to the UK by PB. The availability of all these specimens enabled formal descriptions of both new species based on morphological, morphometric, and meristic data. The formal descriptions and a redescription of P. warreni based on the syntypes and additional specimens are presented herein; a revised generic diagnosis and a key to the species of Pliotrema are also given.

Materials and methods

Institutional acronyms follow Sabaj [17] except for DMM = Deutsches Meeresmuseum in Stralsund (Germany), ERB = Elasmobranch Research Belgium, Bonheiden (Belgium), RHL = Ruth H. Leeney personal collection, SW = Simon Weigmann personal collection. Specimens were fixed with formalin and stored in 70% ethanol except for dried rostra. Morphometric measurements were taken between perpendicular lines, where relevant, by vernier caliper to one tenth of a millimeter and follow Compagno [18], Yearsley et al. [19] and Weigmann et al. [3]. In measurements involving the ventral origin of the caudal fin, the origin was set anteriorly including the low anterior fin ridge following Kaschner et al. [20], Weigmann et al. [21], Weigmann and Kaschner [22], and Weigmann et al. [23]. Vertebral counts and terminology follow Springer and Garrick [24]. Vertebrae were counted from radiographs, tooth rows from radiographs and directly from specimens. Teeth, rostral teeth, and dermal denticles were examined using a scanning electron microscope (SEM). The map with the catch locations of the examined specimens of all three species of Pliotrema was generated based on the Global Relief Model ETOPO1 by NOAA (the National Oceanic and Atmospheric Administration) [25]. Country borders, lakes, and rivers were visualized by means of the shapefiles supplied by ESRI for the ArcExplorer-Java Edition for Education 2.3.2 (AEJEE). None of the specimens analyzed in this study were collected specifically for the purposes of this study. All specimens examined are preserved in scientific collections. No special permissions were required to obtain specimens as the areas fished were not protected.

Comparative material examined

Pristiophorus cirratus (Latham) (2 specimens).

LACM 42620–20: adult male, 870 mm TL, east of Sydney (33°46’S 151°49’E), collected by C.C Swift and pty on 09 September 1981 with 27 m headrope otter trawl, 421–507 m depth. ZMH 8503: juvenile male, 506 mm TL, RV ‘Southern Surveyor’, Station SS 5/94/95 (Bass Strait: 38°42.4’S 148°16.8’E), collected on 26 August 1994 with bottom trawl, 86–87 m depth.

Pristiophorus japonicus Günther (4 specimens).

Syntype (BMNH 1862.11.1.37): juvenile male, 734 mm TL, off Japan (photographs only). Syntype (BMNH 1867.2.20.1), female, ~1000 mm TL, off Japan (photographs only). Syntype (BMNH 1867.2.20.2), female, ~1200 mm TL, off Japan (photographs only). Syntype (BMNH 1953.8.10.6): juvenile female, ~700 mm TL, off Japan (photographs only).

Pristiophorus lanae Ebert and Wilms (1 specimen).

ZSM 45812: presumably adult female, 900 mm TL, off Culasi, Antique province, Panay Island, Philippines, collected on 10 March 2016 by Alexandra Bagarinar and Wilfredo L. Campos.

Pristiophorus nancyae Ebert and Cailliet (23 specimens).

Holotype (SAM 34013/MB-F034013): adult male, 616 mm TL, RV ‘Algoa’, Mozambique Scad Survey, Station C00840 014 037 3074 (Mozambique Channel: 22°07’S 35°45’E), collected on 19 June 1994 with bottom trawl, 500 m depth (photographs only). Paratype (SAM 33477/MB-F033477): subadult male, 440 mm TL, same data as holotype (photographs only). Paratype (SAM 33502/MB-F033502): subadult female, 573 mm TL, RV ‘Algoa’, Mozambique Scad Survey, Station C00848 014 045 3179 (off South Mozambique: 25°21’S 34°30’E), collected on 21 June 1994 with bottom trawl, 286 m depth (photographs only). Five paratypes (SAM 33511/MB-F033511): two juvenile males (314 and 358 mm TL), one juvenile female (391 mm TL), two adult females (522 and 550 mm TL), RV ‘Algoa’, Mozambique Scad Survey, Station C00841 014 038 3118 (Mozambique Channel: 23°32’S 35°51’E), collected on 20 June 1994 with bottom trawl, 490 m depth (photographs only). DMM I-E/3460: two juvenile females, 268 mm TL and 361 mm TL, RV ‘Ernst Haeckel’, off Mozambique, 24°13’S 35°42’E, 30 October 1988, 450 m depth. DMM I-E/4506: adult female, 640 mm TL, RV ‘Ernst Haeckel’ Cruise 51, Haul 89/80, off Mozambique, 23°56’S 35°48’E, 15 June 1980 (photographs only). DMM I-E/4817: juvenile male, 351 mm TL, RV ‘Ernst Haeckel’ Cruise 51, Haul 567/80, off Mozambique, 23°56’S 35°48’E, 21 September 1980. DMM I-E/4872: adult male, 555 mm TL, off Mozambique, February to March 1983. DMM I-E/4902: adult female, 700 mm TL, off Mozambique, February to March 1983. ZMH 25963: adult male, 581 mm TL fresh, 578 mm TL 70% ethanol preserved, RV ‘Vityaz’ Cruise 17, Station 2560 (off Socotra Islands: 12°16’6”N 53°08’2”E–12°14’7”N 53°06’2”E), collected on 27 October 1988 with 29 m shrimp trawl, trawl no. 2, on the bottom for 45 min, 375–380 m depth. ZMH 25964: adult male, 547 mm TL fresh, 540 mm TL 70% ethanol preserved, same data as ZMH 25963. ZMH 25965: adult male, 563 mm TL fresh, 558 mm TL 70% ethanol preserved, same data as ZMH 25963. ZMH 25966: juvenile male, 334 mm TL fresh, 328 mm TL 70% ethanol preserved, same data as ZMH 25963. ZMH 25967: juvenile female, 327 mm TL fresh, 324 mm TL 70% ethanol preserved, same data as ZMH 25963. ZMH 25968: adult female, 574 mm TL fresh, 568 mm TL 70% ethanol preserved, RV ‘Vityaz’ Cruise 17, Station 2631 (off South Mozambique: 25°30’4”S 35°08’2”E–25°34’2”S 35°01’5”E), collected on 23 November 1988 with 29 m shrimp trawl, trawl no. 28, on the bottom for 78 min, 500–570 m depth. ZMH 25969: juvenile female, 283 mm TL fresh, 275 mm TL 70% ethanol preserved, RV ‘Vityaz’ Cruise 17, Station 2830 (off Socotra Islands: 12°14’8”N 53°06’2”E–12°17’8”N 53°08’9”E), collected on 16 January 1989 with 29 m shrimp trawl, trawl no. 101, on the bottom for 80 min, 395–420 m depth. ZMH 25970: juvenile female, 374 mm TL fresh, 367 mm TL 70% ethanol preserved, same data as ZMH 25969. ZMMU P 14847: adult female, 621 mm TL, RV ‘Prof. Mesyatsev’ Cruise 5, Station 10 (off Kenya: 02°59’5”S 40°30’E), collected by A.D. Druzhinin on 22 December 1975, 287–300 m depth.

Pristiophorus nudipinnis Günther (1 specimen).

ZMH 8504: juvenile male, 474 mm TL, RV ‘Southern Surveyor’, Station SS 5/94/30 (Bass Strait: 39°00.1’S 146°35.8’E), collected on 24 August 1994 with bottom trawl, 43–44 m depth.

Pristiophorus schroederi Springer and Bullis (4 specimens).

Holotype (USNM 185946): juvenile female, 383 mm TL, RV ‘Combat’, Station 449 (east of Dog Rocks, Cay Sal Bank: 24°05’N 79°46’W), collected on 24 June 1957 with beam trawl, 640 m depth (radiographs only). Two paratypes (USNM 185947): juvenile male, 645 mm TL and female, 805 mm TL, RV ‘Silver Bay’, Station 445 (north of Little Bahama Bank: 28°03’N 78°46’W), collected on 09 June 1958, 914–951 m depth (radiographs only). USNM 202479: subadult male, 766 mm TL, RV ‘Silver Bay’, Station 2458 (Santaren Channel off Cuba: 23°40’N 79°18’W), collected by S. Springer on 05 November 1960 with bottom trawl, 530 m depth.

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:6281D5F4-DC2B-4E6D-B2F9-97734E545220. 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.

Results

Systematic account

Order Pristiophoriformes Berg.

Family Pristiophoridae Bleeker.

Genus Pliotrema Regan.

Type species. Pliotrema warreni Regan by original designation.

Revised generic diagnosis of Pliotrema Regan.

Pristiophoriform sharks are characterized by a flat, greatly elongated, and saw-like snout with long ventral barbels and closely-set rows of lateral teeth and ventral spines, as well as anterior-most basiventral cartilages that are laterally expanded and have curved, dorsally reflected margins. The teeth on the lateral edges of the snout are alternately large and small in juveniles but the number of small teeth in the interspaces between large teeth increases ontogenetically. The individual rostral teeth are fixed to the dermis and not embedded in sockets. The ventral spines are more or less pronounced and decrease in size and partially get lost with age, in some species they are colored black. In embryos, the lateral rostral teeth and ventral rostral spines are folded back beneath the skin. Oral teeth small, with conical cusp on broad base, arranged in several series.

The genus Pliotrema is characterized by the possession of six gill slits and serrated large lateral rostral teeth. Like Pristiophorus, Pliotrema lacks nictitating lower eyelids, has large spiracles situated behind the eyes, two dorsal fins without spines–the first in front of pelvic fins, no anal fin, no precaudal pit, and a caudal fin with subterminal notch and strongly reduced lower lobe. In Pliotrema, barbel origin to anterior nostrils 1.4–2.3 times anterior nostrils to symphysis upper jaw; prenarial length 1.3–1.7 times prebarbel length; preoral length 1.5–2.7 times interdorsal space; pectoral-fin anterior margin 1.2–1.6 times dorsal–caudal space; mouth width 2.7–6.6 times spiracle length. First dorsal fin originates about opposite pectoral-fin free rear tips. Lateral trunk dermal denticles tricuspidate, rather flat and imbricated. Color uniform pale to dark brown dorsally, sometimes with one or two weak to rather pronounced yellowish longitudinal stripes, partially with whitish fin margins (particularly caudal and pectoral fins); ventrally whitish, partially with sparse darker mottling; dorsal rostrum surface with two distinct longitudinal dark stripes, lateral rostral teeth edged dark. Monospondylous centra 52–57; precaudal diplospondylous centra 46–56; total vertebral centra 151–164.

Pliotrema kajae Weigmann, Gon, Leeney & Temple sp. nov.

urn:lsid:zoobank.org:act:D4EF80CA-3448-4015-A96C-279C5A0A7970.

Proposed English vernacular name: Kaja’s sixgill sawshark.

Proposed German vernacular name: Kajas Sechskiemer-Sägehai.

Local name: vae vae.

Figs 1–14; Tables 1–2.

Poliotrema warreni (misspelling for Pliotrema): Séret [26]: 1.

Pliotrema warreni: Compagno et al. [27]: 73 (in part), based on Séret [26].

Assuming that this new species does not occur off the southern African continent (South Africa, Mozambique), no records of Pliotrema kajae sp. nov. (as Pliotrema warreni) before Séret [26] have been found. All subsequent records of Pliotrema from off Madagascar are apparently based on Séret [26]. The holotype is deposited in the Muséum national d’Histoire naturelle, Paris (MNHN), 10 paratypes are deposited in the South African Institute for Aquatic Biodiversity (SAIAB), and one paratype in each of the Natural History Museum, London (BMNH), Ruth H. Leeney personal collection (RHL), and Simon Weigmann personal collection (SW), National Museum of Natural History, Smithsonian Institution, Washington D.C. (USNM), and Zoological Museum Hamburg (ZMH).

Holotype MNHN 1987–1266, juvenile female, 560 mm TL, off Tulear (Madagascar), 23°19’58.8” S 43°31’1.2” E, 320 m depth, Dec 1985.

Paratypes (15) SAIAB 84039, gravid female, 1170 mm TL fresh, 1143 mm TL 70% ethanol preserved, RV ‘Dr. Fridtjof Nansen’, Survey 2008407, Station 7, Mascarene Ridge, 16°27.62’S 60°16.84’E, 214–219 m depth, bottom trawl # 22, duration 27.3 minutes, 14 Oct 2008 (taken together with 1 further specimen, which was not retained); SAIAB 84096, adult male, 970 mm TL fresh, 940 mm TL 70% ethanol preserved, RV ‘Dr. Fridtjof Nansen’, Survey 2008407, Station 11, Mascarene Ridge, 15°41.11’S 61°4.54’E, 302–305 m depth, bottom trawl # 22, duration 34.3 minutes, 18 Oct 2008 (taken together with 1 further specimen, which was not retained); SAIAB 189447, 1 gravid female, 3 of 6 mid- to late-term embryos (1 male: 246 mm TL; 2 females, 320 mm TL, 324 mm TL; three embryos of 243 mm TL, 318 mm TL, and 329 mm TL were donated to the ZMH, BMNH and USNM collections, respectively), and 4 early embryos (71+ mm TL with tail broken off, 95 mm TL, 103 mm TL, 110 mm TL), RV ‘Dr. Fridtjof Nansen’, Survey 2009408, Station 24, off western Madagascar, 21°58.79’S 43°8.38’E, 235–239 m depth, trawl, duration 30.1 minutes, 07 Sep 2009; BMNH 2019.1.28.1 (ex SAIAB 189447), male late-term embryo, 318 mm TL, data the same as SAIAB 189447; RHL-Mad-01 (dried rostrum), presumably adult female, estimated TL 1100 mm (TL estimated based on the prebarbel length [160 mm] in comparison to the ratios prebarbel length to TL in other type specimens), taken off southwestern Madagascar by local fishermen; SW 01–2016 (dried rostrum), presumably adult female, estimated TL 1300 mm (prebarbel length 191.1 mm), taken off southwestern Madagascar by local fishermen, USNM 443683 (ex SAIAB 189447), male late-term embryo, 329 mm TL, data the same as SAIAB 189447; ZMH 26360 (ex SAIAB 189447), female mid-term embryo, 243 mm TL, data the same as SAIAB 189447.

Diagnosis.

A large six-gilled sawshark with the following characters: barbel origin to anterior nostrils 1.4–2.3 times anterior nostrils to symphysis upper jaw; prenarial length 1.5–1.7 times prebarbel length; preoral length 2.0–2.7 times interdorsal space; pectoral-fin anterior margin 1.2–1.6 times dorsal–caudal space; mouth width 2.8–6.6 times spiracle length. First dorsal fin originates about opposite pectoral-fin free rear tips. Lateral trunk dermal denticles tricuspidate, rather flat and imbricated. Color pale to light brown dorsally with two thin yellowish longitudinal stripes; uniform white ventrally; fins with rather indistinct white posterior fin margins; dorsal rostrum surface with two distinct longitudinal dark stripes, lateral rostral teeth dark-edged. Monospondylous centra 52–57; precaudal diplospondylous centra 48–56; total vertebral centra 151–164. This new species is distinguished from its two congeners, Pliotrema warreni and the second new species, by a combination of characteristics, including most notably, a rostrum that is clearly constricted between barbel origin and nostrils. Furthermore, P. kajae has sharp folds in both upper and lower jaw teeth, as well as a posteriorly notched, teardrop-shaped dorsal fenestra of the precerebral fossa. Pliotrema kajae is further distinguished from P. warreni by barbels that are situated about half way from rostral tip to mouth, with prebarbel length about equidistant from barbel origin to symphysis of upper jaw (vs. barbels about two thirds way from rostral tip to mouth, with prebarbel length about twice distance from barbel origin to symphysis of upper jaw) and the presence of two indistinct, yellowish longitudinal stripes on the dorsal surface (vs. one pronounced yellowish longitudinal stripe). Pliotrema kajae also clearly differs from the second new species in a generally longer snout, more upper and lower jaw tooth rows, higher total large lateral rostral tooth and ventral rostral spine counts, and a pale to light brown dorsal coloration with two indistinct yellowish stripes, uniform white ventral coloration, and posterior fin margins with narrow white edges (vs. uniform medium to dark brown dorsally without longitudinal stripes, white ventrally but with few indistinct dark blotches on belly, posterior fin margins conspicuously white-edged).

Description of the holotype.

Values of the paratypes are presented in parentheses, more complex differences between holotype and paratypes are described separately. Where relevant, ratios are based on horizontal measurements unless otherwise stated. Morphometric measurements and meristics are given in Table 1.

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Table 1. Pliotrema kajae sp. nov., morphometrics and meristics.

Individual values for the juvenile holotype (MNHN 1987–1266), gravid female paratype SAIAB 84039, and adult male paratype SAIAB 84096, as well as ranges for six embryonic paratypes and means for the holotype and eight paratypes. Proportional values are expressed as percentages of total length (TL) 70% ethanol preserved except for minimum, maximum, and mean of TL in mm.

https://doi.org/10.1371/journal.pone.0228791.t001

External morphology. Body firm and slender, depressed forward of gills, abdomen subcircular in cross-section, tail subtriangular in cross-section, deepest at abdomen; not tapering gradually and evenly beyond pectoral fins; snout flattened, greatly extended, saw-like; abdomen elongate, horizontal head length 0.6 (0.6–0.7) times snout–anterior vent length, pectoral–pelvic space 16.1 (13.5–18.5)% TL; pelvic–caudal space 2.7 (2.5–3.0) times pelvic-fin length; tail flattened ventrally, elongate, snout–anterior vent length 1.5 (1.5–1.6) times anterior vent–caudal tip length; caudal peduncle short, dorsal–caudal space 7.7 (6.7–9.1)% TL, caudal peduncle height 3.5 (2.8–4.0) times in dorsal–caudal space and width 1.0 (0.8–1.6) times in height; ventrolateral keels well developed, extending from slightly behind level of free rear tip of pelvic fins (from about level to slightly behind level) to beyond origin of ventral lobe of caudal fin, converging strongly near their posterior extremity; no precaudal pit; no median predorsal, postdorsal or preventral caudal grooves (Figs 1 and 2).

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Fig 1. Pliotrema kajae sp. nov., holotype, MNHN 1987–1266, juvenile female, 560 mm TL, preserved.

a lateral, b dorsal, and c ventral views. Scale bar: 5 cm.

https://doi.org/10.1371/journal.pone.0228791.g001

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Fig 2. Pliotrema kajae sp. nov. paratypes, preserved.

a,b,c paratype SAIAB 84096, adult male, 940 mm TL, in a lateral, b dorsal, and c ventral views; d paratype SAIAB 84039, gravid female, 1143 mm TL, in lateral view. Scale bars: 5 cm.

https://doi.org/10.1371/journal.pone.0228791.g002

Head narrow, subtriangular and deepest at sixth gill slit, strongly depressed above eyes, head width 6.7 (6.8–8.7)% TL, 1.3 (0.9–1.9) times head height. Snout forming a very elongate, blade-like rostrum. Rostrum triangular in dorsal view; constricted between barbel origin and nostrils, sides of rostrum nearly straight from tip to barbel origin but concave in posterior part from barbel origin to origin of orbit; tip narrowly rounded; rostrum extending laterally below eyes as a well-defined suborbital ridge along ventrolateral edge of head, terminating somewhat behind level of posterior edge of spiracle (Fig 3).

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Fig 3. Pliotrema kajae sp. nov., holotype, MNHN 1987–1266, juvenile female, 560 mm TL, preserved.

Head in a lateral, b dorsal, and c ventral views. Scale bar: 5 cm.

https://doi.org/10.1371/journal.pone.0228791.g003

A slender, filamentous, dorsoventrally flattened barbel originating on the ventrolateral margin about half way from rostral tip to mouth on each side, with prebarbel length 1.0 (1.0–1.1) times distance from barbel origin to symphysis of upper jaw, 51.1 (49.4–52.9)% of preoral length and 15.6 (14.8–16.2)% TL. Barbel length 1.6 (0.8–2.3) times in prebarbel length and 1.5 (0.8–2.1) times in length from barbel origin to symphysis of upper jaw. Preorbital length, horizontally 6.0 (4.8–6.0) times mouth width, 19.7 (16.4–31.5) times spiracle length, 2.9 (2.7–3.6) times first dorsal-fin length, 4.4 (3.4–4.9) times rostral width at anterior nostrils; extremely narrow in lateral view; preoral length 30.6 (28.6–31.3)% TL, 4.6 (3.4–4.4) times head width, 5.1 (4.0–5.4) times rostral width at anterior nostrils, 7.2 (6.0–8.2) times rostral width at origin of barbels, 2.0 (1.9–2.0) times prebarbel length, 1.2 (1.2–1.3) times prenarial length, and 2.3 (2.0–2.7) times interdorsal space (Fig 3).

Large lateral rostral teeth of prenarial portion of rostrum variable in length, curved, rather stout, serrated, longest near barbel origin and near apex of rostrum posterior to anteriormost two teeth; longest tooth immediately anterior to barbels only slightly shorter than spiracle length, length 1.1 (0.9–2.3)% TL and 0.8 (0.9–2.6) times first complete interspace anterior to barbels, width 0.2 (0.2–0.4)% TL; anteriormost two large rostral teeth on each side of the rostrum very close to snout tip, without interstitial tooth between or anterior to them; large teeth shortest near nostrils, longest rostral tooth posterior to nostrils 0.6 (0.2–0.3)% TL; large teeth absent behind nostrils but interstitial-like teeth present, short to very short and closely set, partially directed almost ventrally, particularly near mouth. Interspaces between large rostral teeth rather regularly sized, about as long as adjacent teeth, with 2–4 (1–4) smaller, variable interstitial teeth. Rostral tooth counts mostly symmetrical between left and right hand sides; left side with 21 (22–31) large teeth, right side with 21 (22–31); anterior to barbels left side with 13 (12–14) large rostral teeth, right side with 13 (13–14), posterior to barbels left side with 8 (9–17) large rostral teeth, right side with 8 (9–17); anterior to nostrils left side with 23 (~19–~24) ventral spines, right side with 23 (~19–~23), anterior to barbel origin left side with 13 (~11–~14) ventral spines, right side with 12 (~12–~13); one enlarged ventral spine, distinctly larger than the other ventral spines, present just in front of each nostril. Large rostral teeth (Fig 4a and 4b) with elongated crown and oval-shaped base, slightly bent to the rear and flattened towards the apex, forming anterior and posterior cutting edges at front and rear, the latter serrated by barbed hooks. Crown base with numerous short longitudinal ridges forming a pronounced transversal crest. Both, anterior and posterior faces of the root are curved outwards from the junction of crown and root towards the base of the root. The basal face shows a deep v-shaped median groove that is antero-posteriorly directed and has an oval-shaped cavity in the center. Interstitial rostral teeth (Fig 4c–4h) with blade-shaped crown and without serration (large interstitial rostral teeth serrated and similar to large lateral rostral teeth in all specimens larger than the holotype). Crown of ventral spines (Fig 4i and 4j) elongated cone-shaped with a pronounced transversal basal ridge, root with roundish and pedestal-like base. The basal face has a large and deep roundish foramen in the center.

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Fig 4. Pliotrema kajae sp. nov., paratype, RHL-Mad-01, presumably adult female, estimated TL 1100 mm, SEM images of rostral teeth.

a,b large lateral rostral tooth in a total and b close-up views; c–h small interstitial lateral rostral teeth in c–f total and g–h close-up views; i,j ventral rostral spine in i total and j close-up views. Scale bars: a,b,d,f 1 mm, c,e 200 μm, g–i 100 μm, j 20 μm.

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Eyes lateral on head, large, oval, length 3.3 (2.8–5.2)% TL; skeletal interorbital space 0.9 (0.7–1.0) times eye length, 9.3 (6.1–9.5) times in horizontal preorbital length; posterior eye notches and suborbital grooves present. Spiracles moderately large, length 1.4 (0.8–1.6)% TL and 0.4 (0.2–0.6) times eye length, left spiracle with 12 (12–15) folds, right one with 13 (12–15); spiracles strongly crescentic, oblique, directed posteroventrally from top to bottom, located just posterior to posterior eye notch, separated by a narrow but deep vertical groove along posterior margin of orbit, shorter than eye; upper edge below level of top of eye. Gill slits small, upright, weakly pleated, lateral on head, close to ventral surface, extending slightly onto ventral surface, subequal in length, sixth slit arches around pectoral-fin origin. Mouth large, strongly inferior, broadly arched, symphysis about level with posterior edge of eye, width 4.5 (4.4–5.4)% TL and 1.5 (1.3–1.8) times in head width; upper labial furrows absent, lower furrows short, 0.4 (0.4–0.5)% TL; corner of mouth partly concealed by lateral muscles of jaw (Fig 5). Teeth unicuspidate, in well-defined series, bases oval and flattened with short but pronounced, narrow median cusp near middle of jaw, no lateral cusps (cusps similar in paratypes except for adult male SAIAB 84096, which has distinctly longer cusps); cusps diminishing in height towards jaw angles, indistinct near jaw corners; about 4–5 series of functional teeth (Figs 6 and 7). Median cusp with labial face slightly convex and with both mesial and distal cutting edges weakly bent mesially and distally in occlusal view, respectively. The mesial and distal crown base parts somewhat curve apically. A pronounced and broad, irregularly shaped apron overlaps the junction of crown and root, building a notch at the junction with both mesial and distal crown base parts. Basal ornamentation, striae or reticulations absent, but sharp folds present in both upper and lower jaw teeth. The lingual face of the cusp is strongly convex, a well-developed uvula is present at the central crown base. The mesial/distal latero-lingual crown faces curve strongly towards the apex of the crown, partially forming a sharp notch with the uvula (Fig 6k and 6l). The root is anaulacorhizid and slightly arched without lobation. The outer surface of the root shows up to five large basal foramina, which are mostly oval-shaped. The inner face of the root shows up to six well-developed foramina along the crown-root junction at each side of the uvula. The basal face of the root is flat, partly showing some outer foramina.

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Fig 5. Pliotrema kajae sp. nov., holotype, MNHN 1987–1266, juvenile female, 560 mm TL, preserved, mouth-nasal region.

Scale bar: 1 cm.

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Fig 6. Pliotrema kajae sp. nov., paratype, SAIAB 84096, adult male, 940 mm TL, SEM images of oral teeth.

a–c upper anterolateral tooth in a oblique-labial and b,c occlusal views; d–f upper posterolateral tooth in d oblique-labial and e,f occlusal views; g–i lower anterolateral tooth in g oblique-labial and h,i occlusal views; j–l lower posterolateral tooth in j oblique-labial and k,l occlusal views. Scale bars: 200 μm.

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Fig 7. Pliotrema kajae sp. nov., paratype, SAIAB 84039, gravid female, 1143 mm TL, SEM images of oral teeth.

a–c upper anterolateral tooth in a oblique-labial and b,c occlusal views; d–f upper posterolateral tooth in d,e oblique-labial and f occlusal views; g–i lower anterolateral tooth in g oblique-labial and h,i occlusal views; j–l lower posterolateral tooth in j oblique-labial and k,l occlusal views. Scale bars: 200 μm.

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Nostrils small, widely separated, subcircular; nostril width 0.8 (0.7–1.0)% TL, 4.5 (3.8–5.6) times in internarial width, 5.7 (4.8–7.0) times in mouth width, 7.7 (6.4–9.6) times in width of rostrum at nostrils; located distinctly forward of level of anterior margin of eye; distance from anterior nostrils to symphysis of upper jaw 1.3 (1.2–1.5) times internarial space, distance from barbel origin to anterior nostrils 10.4 (8.5–10.5)% TL. Anterior nasal flaps well developed, leaf-like, extended ventrally beyond nostrils; incurrent and excurrent apertures surrounded by pronounced marginal lobes; no nasoral or circumnarial grooves; no dermal lobes (Fig 5).

Lateral trunk dermal denticles densely set and overlapping, with flat, tricuspidate crowns (Fig 8). The lateral cusps are rather weakly pronounced but situated quite far anteriorly so that the median cusp is not much longer than the lateral cusps. The median ridge is strongly pronounced and reaches the tip of the median cusp. The lateral ridges are less pronounced and do not reach the tips of the lateral cusps. The surface of the denticles is only weakly structured by reticulations very close to base. No sexual dimorphism detectable in the morphology of the trunk dermal denticles. Dermal denticles on rostrum fan-shaped, with an obtusely angled, weakly pronounced median cusp and no lateral cusps but with 6–7 strongly pronounced ridges (Fig 9). The surface of the rostral dermal denticles is only weakly structured by reticulations very close to base (Fig 9a–9c).

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Fig 8. Pliotrema kajae sp. nov., SEM images of lateral trunk dermal denticles in apical views.

a,b paratype, SAIAB 84096, adult male, 940 mm TL, c,d paratype, SAIAB 84039, gravid female, 1143 mm TL. Scale bars: a,c 500 μm, b 100 μm, d 200 μm.

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Fig 9. Pliotrema kajae sp. nov., paratype, RHL-Mad-01, presumably adult female, estimated TL 1100 mm, SEM images of single rostral dermal denticles.

a,b apical views, c lateral view, d–f basal d total and e,f close-up views. Scale bars: a,b,d 100 μm, c 50 μm, e 10 μm, f 3 μm.

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Pectoral fins large, anterior margin weakly convex, 11.4 (10.3–12.2)% TL and 1.5 (1.4–2.0) times inner margin; apex narrowly rounded; posterior margin weakly concave, directed across horizontal axis at about origin of first dorsal fin; inner margin convex and strongly notched basally; free rear tip angular (Figs 3 and 10a). Pelvic fins moderately large, anterior margin almost straight to slightly convex, 6.5 (5.3–6.7)% TL, 1.6 (1.6–1.9) times in first dorsal-fin anterior margin, and 1.5 (1.3–1.7) times in second dorsal-fin anterior margin; apex narrowly rounded; posterior margin concave; inner margin weakly convex and slightly notched basally; free rear tip broadly rounded; origin distinctly posterior to level free tip of first dorsal fin and well forward of level second dorsal fin origin (Fig 10a).

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Fig 10. Pliotrema kajae sp. nov., holotype, MNHN 1987–1266, juvenile female, 560 mm TL, preserved, lateral views of fins.

a pectoral, first dorsal, and pelvic fins, b second dorsal and caudal fins. Scale bar: 5 cm. Note ventral precaudal ridge in b.

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First dorsal fin broad, semifalcate, anterior margin slightly convex; apex narrowly rounded; posterior margin slanting posteroventrally, slightly convex distally, strongly concave in basal three quarters; inner margin straight, free rear tip narrowly pointed; origin about opposite pectoral-fin free rear tips; insertion and free rear tip clearly anterior to level pelvic-fin origins (Fig 10a). Second dorsal fin somewhat smaller than first but of similar shape, anterior margin weakly convex, apex very narrowly rounded; posterior margin weakly convex distally, strongly concave near basal three quarters; inner margin straight, free rear tip narrowly pointed; origin clearly behind level pelvic insertions; interdorsal space 1.4 (1.3–1.7) times first dorsal-fin length, 1.7 (1.3–1.9) times dorsal–caudal space; second dorsal-fin inner margin 1.0 (0.7–1.2) times subterminal caudal-fin margin (Fig 10b).

Caudal fin short, dorsal margin slightly convex, length 18.8 (17.1–19.9)% TL, 1.1 (0.9–1.3) times in pelvic–caudal space and 5.1 (3.9–7.9) times terminal caudal margin; lower post-ventral lobe absent, upper post-ventral margin slightly convex; terminal lobe well developed, caudal terminal margin slightly concave, apices angular (Fig 10b). Ventral origin of caudal fin situated anteriorly due to low anterior fin ridge (Fig 10b).

Clasper morphology: Fig 11 provides photographs of the claspers of adult male SAIAB 84096. The claspers of adult males extend posteriorly to clearly posterior to level of pelvic-fin free rear tips. Clasper shaft flattened rod-shaped. Glans broad and flattened, with long, straight and thorn-like spur. Due to the fragile condition of the specimen, it was not possible to open one of the claspers for further examination.

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Fig 11. Pliotrema kajae sp. nov., paratype, SAIAB 84096, adult male, 940 mm TL, preserved, claspers.

a left clasper in dorsal view, b right and left claspers in ventral view.

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Cranium: five anterior-most basiventral cartilages laterally expanded, with curved, dorsally reflected margins. Chondrocranium and cranial nerves highly modified to accomodate the elongated rostrum. Foramen magnum surrounded by crescent-shaped occipital condyles. Dorsal fenestra of the precerebral fossa teardrop-shaped, with posterior notch (Fig 12a).

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Fig 12. Radiographs of the crania of the three species of Pliotrema in dorsal views.

a P. kajae sp. nov., holotype, MNHN 1987–1266, juvenile female, 560 mm TL (image merged from two radiographs), b P. annae sp. nov., holotype, ZMH 26361, presumably adult female, 981 mm TL, c P. warreni, DMM I-E/4946, female, 785 mm TL.

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Skeletal meristics (from radiographs): monospondylous trunk vertebral centra: 57 (52–56); diplospondylous precaudal centra: 49 (48–56); total precaudal centra: 106 (101–110); caudal centra: 54 (47–54); total centra: 160 (151–164).

Coloration. Fresh, prior to preservation (paratypes SAIAB 84039 and SAIAB 84096; Fig 13): ground color pale (SAIAB 84096) to light brown (SAIAB 84039) dorsally with two thin yellowish longitudinal stripes (hardly detectable in paratype SAIAB 84096); uniform white ventrally; fins translucent dusky, upper post-ventral caudal-fin and pelvic-fin posterior margins narrowly edged white, weak white edges also present at posterior margins of pectoral and dorsal fins, as well as terminal caudal-fin margin; rostrum translucent dusky, dark edged and with two distinct longitudinal stripes dorsally; lateral rostral teeth dark-edged; ventrolateral keels white. Color in preservative (holotype and paratypes; Figs 1 and 2): coloration similar to fresh coloration but specimen SAIAB 84096 with formerly pale dorsal coloration somewhat darker dorsally, ventral coloration uniform yellowish instead of white as usual, ventrolateral keels also yellowish; dark edging of rostrum and lateral rostral teeth, as well as longitudinal dorsal rostral stripes still conspicuous.

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Fig 13. Pliotrema kajae sp. nov., fresh images of paratypes.

a,b paratype SAIAB 84096, adult male, 970 mm TL fresh, c,d paratype SAIAB 84039, gravid female, 1170 mm TL fresh. The photographs were taken and kindly provided by Oddgeir Berg Alvheim, Institute of Marine Research, Bergen, Norway.

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Size.

A large sawshark species reaching about 1430 mm TL based on photographs of an adult female kindly provided by Blue Ventures along with photographs of an adult male of 1020 mm TL. The female was caught on 19 July 2015 and landed at Andranombala, Madagascar, whilst the male was caught on 22 September 2015 and landed at Andavadoaka, Madagascar. Both specimens were not preserved. The male paratype SAIAB 84096 is adult at 970 mm TL fresh, 940 mm TL preserved, the female paratype SAIAB 84039 is gravid at 1170 mm TL fresh, 1143 mm TL preserved, containing about six eggs (based on radiographs). The size at birth is estimated at around 350 mm TL based on the four near-term embryos of 318 to 329 mm TL.

Distribution.

Known from off Madagascar and the Mascarene Ridge in depths from 214 to 320 m (Fig 14). The depth range of 425–500 m, given for the holotype of Pliotrema kajae sp. nov. by Séret [26] and Compagno et al. [27] is erroneous. Pliotrema kajae sp. nov. is apparently the only species of the genus occurring in this area.

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Fig 14. Map of the southwestern Indian Ocean depicting the catch locations of the examined specimens of all three species of Pliotrema.

Holotype (white star) and paratypes (white circles) of Pliotrema kajae sp. nov., holotype (gray star) and paratype (gray circle) of Pliotrema annae sp. nov., and intact (black diamond) and dissected (black triangle) syntypes, as well as other specimens (black squares) of Pliotrema warreni.

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Etymology.

The new species is named after Kaja Magdalena Weigmann, the daughter of the first author, who had her first contact with chondrichthyan taxonomy when observing with great interest the examination of Pliotrema specimens for the present study. The name “Kaja” also has the Frisian meaning “warrior”, referring to the saw-like rostrum.

Remarks.

There are several morphometric differences between the embryos and the larger type specimens of Pliotrema kajae sp. nov., which might be of ontogenetic nature, most notably, the barbel length. The differences are demonstrated in Table 2. Further ontogenetic differences can be found in the morphology of the lateral interstitial rostral teeth. In specimens larger than the juvenile holotype, larger interstitial teeth are serrated, similar to the large lateral rostral teeth. In smaller specimens up to at least 560 mm TL all interstitial teeth are unserrated.

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Table 2. Pliotrema kajae sp. nov., morphometric (in % TL) and meristic differences between embryonic (n = 6) and larger (n = 3) type specimens (most pronounced differences marked in bold).

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Pliotrema annae Weigmann, Gon, Leeney & Temple sp. nov.

urn:lsid:zoobank.org:act:DE91D0E9-3AED-41FC-8CB6-09A53E50EEBD.

Proposed English vernacular name: Anna’s sixgill sawshark.

Proposed German vernacular name: Annas Sechskiemer-Sägehai.

Local name: Papa Unguja.

Figs 15–25; Table 3.

Pliotrema warreni: Gubanov [28]: 221 (in part)?

The holotype and paratype are deposited in the Zoological Museum Hamburg (ZMH).

Holotype ZMH 26361, presumably adult female, 981 mm TL fresh, caught on or near Kobela Reef (~6°29’35”S 39°22’21”E) in Menai Bay, Unguja Island, Zanzibar, in a demersal longline at ~20–25 m (i.e. set depth of the gear; water depth unknown but probably only 2–5 m deeper than this), haul time ~8 am, time of catch unknown but likely during hours of darkness, date 07 March 2019.

Paratype ZMH 26362, presumably adult female, 980 mm TL fresh, 950 mm TL 70% ethanol preserved, caught off Zanzibar in a longline at ~25–35 m depth during hours of darkness, landed on 24 Feb 2017 in Kizimkazi-Dimbani, Zanzibar (two further specimens were landed at the same place but not retained on 23 Jan 2017: gravid female, ~980 mm TL, with six eggs and on 25 Jan 2017: female with saw cut off, ~580 mm to beginning of saw; both these specimens were also caught in a longline at ~25–35 m depth during hours of darkness). Measurements taken from the fresh photographs of the not retained gravid female show that this specimen can be assigned to P. annae sp. nov. based on the main morphometric characteristics, particularly the generally shorter snout.