Wednesday 15 July 2020

Squalus shiraii: A new species of Dogfish from Japan.

Dogfish Sharks of the genus Squalus are commercially-important taxa within the world Fish trade, in which individuals are caught through direct or indirect fisheries and traded for consumption of meat, fins and liver oil as primary products. Although highly exploited, the landing reports, observer and logbooks data and/or fisheries surveys have constantly pointed out that accurate identification of species is scarce. Fisheries management and conservation of Dogfish thus still outstanding as species-specific catch and landing statistics, population threats and trends are not yet acknowledged. Japan is one of the world’s leading Shark fishing countries, whose estimated annual shark landing data for between 1992 and 2000 was 19 600–28 700 tonnes. Over 78 Elasmobranch species currently evaluated as threatened (Vulnerable; Critically Endangered; or Endangered) in the Red List of Threatened Species of the International Union for Conservation of Nature are traded in the Japanese Shark fin markets, highlighting it as a priority country for Elasmobranch conservation, even though Shark catches have decreased drastically. Squalus and other Squalid Sharks have been exploited in Japan as by-catch fisheries of Pacific Salmon gillnets, Tuna longliners and Squid driftnets, but fishery statistics related to Squalus stocks are unreported or deficient and usually inserted into the category of 'other species'.

In a paper published in the journal Zoosystematics and Evolution on 10 June 2020, Sarah Viana of the South African Institute for Aquatic Biodiversity and the Departamento de Zoologia at the Universidade de São Paulo, and Marcelo de Carvalho, also of the Departamento de Zoologia at the Universidade de São Paulo,describe a new species of Dogfish from the Okinawa Trench, Japan.

Five of 36 valid Squalus species are often recognised in Japan, Squalus blainvillei, Squalus suckleyi, Squalus mitsukurii, Squalus japonicus, and Squalus brevirostris. Two additional nominal species, Squalus acutirostris and Squalus wakiyae, originally described from Japan, are considered junior synonyms of Squalus mitsukurii and Squalus acanthias, respectively. A sixth species, Squalus formosus, a Taiwanese endemic, is likely to inhabit Japanese waters as well, but requires confirmation. These species are found on lower continental shelves, upper continental slopes and, more rarely, on insular slopes and seamounts of the North Pacific and Eastern Indian Oceans between Russia, Japan, China, South Korea, Taiwan and the Philippines. These species are all now classified as Data Deficient nder the terms of the  International Union for the Conservation of Nature’s Red List of  Threatened Species, except for Squalus suckleyi which is of Least Concern. A key criterion to be inserted into the first category is unreliability of species identification and, thus, species-specific data are inaccurate or doubtful and may not be incorporated into the assessments.

Systematic approaches of the last decade support that Squalus mitsukurii comprises a species complex that also includes Squalus blainvillei, a species originally described from the Mediterranean Sea. Two additional species, previously misidentified with Squalus mitsukurii, were also more recently recognised in the North Pacific Ocean, Squalus hawaiiensis and Squalus boretzi. Records of Squalus japonicus and Squalus brevirostris are exclusive to the Indo-Pacific region, with uncertainties regarding their occurrences in Northern Australia and Indonesia due to morphological similarities with Squalus megalops and Squalus nasutus.

Taxonomic confusions within this Linnaean group are ordinary because of fair original descriptions, indistinct indistinct morphological diagnostic characters and exaggerated availability of synonyms. Species delimitation using DNA barcoding of mitochondrial genes alone has shown to be ineffective on the genus when non-designation of species identifications in the DNA reference libraries persists as collection-based data are not incorporated within the taxonomic investigations. Comparative examination of over 150 specimens of Squalus from the North Pacific Ocean, particularly from Japan, support the recognition of a novel species that has not been included in the latest molecular genetic and morphological taxonomic analyses of the genus. Viana and de Carvalho's study describes a new species of Squalus, based on morphological characters.

The new species is named Squalus shiraii, in honour of Shigeru Shirai, an ichthyologist at the Tokyo University of Agriculture, for his valuable contributions to the systematics of Squaliformes. The species is described from sixteen specimens collected from depths of between 310 and 390 in the shallow waters of the upper continental slope off Southern Japan between 1978 and 1997.

Map of the North-western Pacific Ocean, showing the known geographical distribution of Squalus shiraii. Viana & de Carvalho (2020).

The body of Squalus shiraii is elongate (590–770 mm maximum length in adults), fusiform and robust, arched dorsally from anterior margin of the eye to insertion of first dorsal fin, turning straight from abdomen to caudal fin, it is equally deep from head to abdomen with head height. The body is at its greatest width at head, with the head width being  1.0–1.2 times the trunk width and 1.2–1.5 times the abdomen width. The head is flattened anterior-dorsally, arched posterior-dorsally and elongate, its length, forming 24.1% of the total body length. The snout is conspicuously obtuse at tip and large, with the preorbital length forming 7.4%–8.1% of the total body length. The anterior margin of the nostrils is strongly broad and bi-lobed, its distance to snout tip is1.1–1.3 times its distance to upper labial furrow; the prenarial length is 0.5–0.6 times the preoral length; the internarial space is 0.8–1.1 times the eye length. The eyes are oval and enlarged, the eye length is 1.9–3.4 times the eye height; the anterior margin of the eyes is convex, and the posterior margin slightly notched. The prespiracular length is 1.7–1.7 times the preorbital length and 0.6–0.6 times the prepectoral length. The spiracles are crescent-shaped and large, with a length of 1.0%–1.3% the total length, located laterally behind the eyes. The gill slits are tall and somewhat vertical, placed anteriorly to the pectoral-fin origin; the fifth gill slit height ia 1.1–1.7 times the first gill slit height.

Squalus shiraii: lateral (A)–(C) and ventral (D), (E) views; first (F) and second (G) dorsal fins; pectoral (H) and caudal (I) fins. HUMZ 149389 (holotype), adult male, 590 mm total length (A), (D), (F)–(I); HUMZ 80329 (paratype), adult female, 770 mm total length (B); HUMZ 80330 (paratype), juvenile male, 365 mm total length (C), (E). Scale bars: 50 mm (A)–(E); 20 mm (F)–(I). Viana & de Carvalho (2020).

The preoral length is 1.3–1.5 times the mouth width and 0.4–0.5 times the head length. The upper labial furrow is short, its length is 0.4–0.5 being times eye length, with a thick and small fold; the lower labial furrow is elongate with an inconspicuous fold. The mouth arched and broad, its width is 1.3–1.4 times the prenarial length and 1.5–1.7 times the internarial space. The teeth are similar in both jaws; the upper teeth are smaller than the lower teeth; with oblique cusps, which are thick and small; the mesial cutting edge is slightly convex; the distal heel is rounded; the mesial heel notched; the apron short and heavy. There are two and two to three series of functional teeth in the upper and lower jaws, respectively, 13–14 tooth rows on the upper jaw and 11–12 tooth rows on lower jaw.

Upper (A) and lower (B) teeth of Squalus shiraii. Scale bar is 1 mm. Viana & de Carvalho (2020).

The origin of the first dorsal fin is over a vertical line traced at pectoral-fin insertion. First dorsal fin elongate, its length is 1.4–1.7 times the height of the first dorsal fin; the first dorsal fin is conspicuously tall, its height is 1.5–1.8 times the first dorsal-fin inner margin length; the first dorsal-fin anterior margin is convex, the posterior margin is straight in the upper half and conspicuously concave in the lower half; the first dorsal-fin is apex rounded and evidently slender at the fin web; the first dorsal-fin free rear tip is triangular and the first dorsal-fin inner margin is short, its length 0.5–0.7 times the first dorsal-fin base length. The first dorsal-fin spine is thick with base width of 0.9–0.9% of the total body length and elongate (the first dorsal-fin spine length is 0.4–0.5 times the height of the first dorsal fin), although never reaching first dorsal-fin apex. The interdorsal space is 1.0–1.2 times the prepectoral length and 2.2–2.8 times the dorsal-caudal space. The pre-second dorsal length is 2.0–2.1 times the pre-first dorsal length. The second dorsal fin is elongate, its length is 0.7–0.9 times the length of the first dorsal fin; the second dorsal fin is tall, its height is 1.4–1.8 times the second dorsal-fin inner margin length; the second dorsal-fin anterior margin is convex; the second dorsal-fin posterior margin is strongly concave and falcate; the second dorsal-fin apex is rounded and lobe-like; the second dorsal-fin inner margin is small, its length is 0.5–0.6 times the second dorsal-fin base length. The second dorsal-fin spine is thick with a base width 0.7%–0.8% of the total body length; the second dorsal-fin spine is elongate, its length is 0.7–0.9 times the height of the second dorsal fin and 1.2–1.6 times the length of the first dorsal-fin spine, although not reaching the second dorsal-fin apex.

The prepectoral length is 2.8–2.9 times the preorbital length. Pectoral fins have anterior and inner margins which are conspicuously convex; tje pectoral-fin posterior margin is markedly concave; the pectoral-fin apex and free rear tips are rounded and lobe-like; the pectoral-fin apex conspicuously transcends the horizontal line traced at pectoral-fin free rear tip; the pectoral fins are markedly falcate and broad with pectoral-fin posterior margins 0.4–1.0 times the trunk height; the pectoral-fin anterior margin is 1.7–2.0 times the pectoral-fin inner margin length and 1.4–3.2 times the pectoral-fin posterior margin length. The prepelvic length is 2.0–2.2 times the prepectoral length. The pelvic fins are located slightly nearer to the first dorsal fin than the second dorsal fin. The pelvic fins have margins which are straight and an apex which is rounded; the pelvic-fin free rear tips are rounded to slightly pointed, thin and lobe-like. Adult males have claspers which somewhat transcend the pelvic fin; the clasper inner length is 0.3–1.2 times the pelvic-fin inner margin length; the clasper groove is medial-dorsally and vertical, profound and elongate; the apopyle is broad, located anteriorly in the clasper groove; the hypopyle is broad, and located anterior to the rhipidion; the rhipidion is blade-like, thin and small, placed at the medial-distal end of the clasper.

The pelvic-caudal space is 1.1–1.6 times the pectoral-pelvic space. The caudal keel is evident laterally in the caudal peduncle from the second dorsal fin free rear tip to the caudal-fin origin. The caudal fin has conspicuously thin upper and lower caudal lobes; the caudal fork is concave with its width corresponding to 5.7%–6.4% of the total body length; the dorsal caudal margin is straight and markedly elongate, its length is 0.9–1.0 times the head length and 1.9–2.1 times the length of the preventral caudal margin; the upper postventral margin is straight on its upper half and slightly convex on its lower half; the lower postventral margin is convex; the preventral caudal margin is convex and short, its length is 2.1–2.3 times the length of the pelvic-fin inner margin; the dorsal caudal tip is rounded; the ventral caudal tip is markedly pointed.

The dermal denticles are lanceolate and imbricate, and broad at the crown base (conspicuously broad in females); the length of the dermal denticles is larger than their width (although it is about equal to their width in females); the denticles have a median cusp which is slightly pointed, the lateral cusps are inconspicuous; the median and lateral ridges are thick; the median and lateral ridge is bifurcated anteriorly; the anterior furrow is conspicuously profound and oval.

Scanning electron microscopy of the dermal denticles of Squalus shiraii, HUMZ 149389 (holotype), adult male, 590 mm total length. Scale bars: 200 μm (A), 50μm (B).

The body is brown dorsally, light brown laterally, although whitish latero-ventrally from the pelvic fin to the caudal fin and pale ventrally. The dorsal fins are brownish, somewhat light brown at dorsal fin base; the first dorsal-fin inner margin is white; the second dorsal-fin posterior margin is fairly white at its lower half; the second dorsal-fin apex us somewhat dark brown. The dorsal-fin spines brownish anterior-laterally and white at the tip. The pectoral fins are dark brown, whitish ventrally at pectoral-fin base; the pectoral-fin posterior margin is fairly white (not uniform); the pectoral-fin apex is broadly white. The pelvic fins are brown and whitish at the pelvic-fin base; the pelvic-fin anterior and posterior margins are slightly white. The caudal fin is dark brown at the upper and lower caudal lobes, whitish over  the vertebral column; the postventral caudal margins is white, except at the caudal fork where it is dark brown; the dorsal and ventral caudal tips are broadly white; the preventral caudal margin is white; a dark caudal stripe and bar inconspicuous; a black upper caudal blotch is somewhat evident in adults. Juveniles have a body which is light brown dorsally, pale lateral and ventrally; the first and second dorsal-fin posterior margins are slightly white at the lower half; the dorsal-fin apex is blackish to dark brown at the tip; the pectoral fins are light brown to reddish-brown with the pectoral-fin posterior margins and apex broadly white; the pelvic fins are light brown to reddish-brown; caudal fin dark brown with the postventral caudal margins broadly white, except at the caudal fork; dorsal and ventral caudal tips are broadly white; a conspicuous upper black caudal blotch is present in the midline of the dorsal caudal margin; lower caudal lobe has an oblique black marking at the origin of the lower caudal lobe; and a small black caudal bar at the caudal fork.

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