A new genus and species of clingfish from the Rangitāhua Kermadec Islands of New Zealand (Teleostei, Gobiesocidae)

Abstract Flexorincus, new genus and species, is described from 15 specimens (14.0–27.2 mm SL) collected from shallow (0–9 meters) intertidal and sub-tidal waters of the Rangitāhua Kermadec Islands, New Zealand. The new taxon is distinguished from all other members of the Gobiesocidae by a combination of characters, including a heterodont dentition comprising both conical and distinct incisiviform teeth that are laterally compressed with a strongly recurved cusp, an oval-shaped opening between premaxillae, a double adhesive disc with a well-developed articulation between basipterygia and ventral postcleithra, and many reductions in the cephalic lateral line canal system. The new taxon is tentatively placed within the subfamily Diplocrepinae but shares a number of characteristics of the oral jaws and the adhesive disc skeleton with certain members of the Aspasminae and Diademichthyinae.


Introduction
"The discovery of this and several other new genera in recent years makes it necessary to reconsider the characterization and relationships of various subfamilies within the Gobiesocidae" Briggs (1993: 197) AIM Auckland War Memorial Museum, Auckland NMNZ Museum of New Zealand Te Papa Tongarewa, Wellington ROM Royal Ontario Museum, Toronto NMST-P National Museum of Nature and Science, Tsukuba SAIAB South African Institute of Aquatic Biodiversity, Grahamstown TCWC Biodiversity Research and Teaching Collections, Texas A&M University, College Station USNM National Museum of Natural History, Washington D.C.

WAM
Western Australian Museum, Perth.
Head and body measurements reported follow Conway et al. (2014) and are expressed as percent of standard length (SL) or head length (HL). Adhesive disc papillae terminology follows Briggs (1955) and Hutchins (2008). Cephalic lateral line pore terminology follows Shiogaki and Dotsu (1983), except that we also use numbers to refer to individual pores following Conway et al. (2017b), with pores numbered along a particular canal from anterior to posterior or dorsal to ventral (lachrymal canal only). General osteological terminology follows that of Springer and Fraser (1976), except that we use the term anguloarticular instead of articular, anterior ceratohyal instead of ceratohyal, autopalatine instead of palatine, epicentral instead of epipleural (following Gemballa and Britz 1998), pharyngobranchial instead of infrapharyngobranchial, posterior ceratohyal instead of epihyal, and retroarticular instead of angular.
Select specimens were cleared and double stained (C&S) for bone and cartilage investigation using the protocol of Taylor and Van Dyke (1985). Select specimens were reversibly stained using cyanine blue following Saruwatari et al. (1997) to aid examination of adhesive disc papillae and cephalic lateral line canal pores. Specimens or parts thereof were observed and photographed using a ZEISS SteREO Discovery V20 stereomicroscope equipped with a ZEISS Axiocam MRc5 digital camera. Digital images were typically stacked using ZEISS Axiovision software. Computed tomography (CT) scans of select specimens were also obtained at the Karel F. Liem BioImaging Center (Friday Harbor Laboratories, University of Washington) using a Bruker (Billerica, MA) SkyScan 1173 scanner with a 1 mm aluminium filter at 60 kV and 110 μA on a 2240 × 2240 pixel CCD at a resolution of 8.8 μm. Specimens were scanned simultaneously in a 50ml plastic Falcon tube (Corning, NY), in which they were wrapped with cheesecloth moistened with ethanol (70%) to prevent movement during scanning. The resulting CT data were visualised, segmented, and rendered in Horos (www.horosproject.org) and Amira (FEI). The premaxilla and dentary from the right side were removed from select specimens and prepared for scanning electron microscopy (SEM) following the protocol outlined in Conway et al. (2015). Coated specimens were examined using a Tescan Vega3 SB scanning electron microscope. All digital images were processed using Adobe Photoshop and Adobe Illustrator.

Systematics
Flexor gen. n. http://zoobank.org/23E32F56-CAF3-4B3D-938A-0BF6A590C070 Diagnosis. A genus of the Gobiesocidae differing from all other genera by a combination of characters, including: head and anteriormost part of body similar in width; a relatively elongate body with a small, double adhesive disc located beneath anteriormost part of body; an oval-shaped gap between premaxillae formed by a semicircular indentation along medial edge of premaxilla; premaxilla with a single row of teeth, comprising 2-3 peg-like, conical teeth anteriorly at, and adjacent to, symphysis and 10-12 strongly laterally compressed, incisiviform teeth with strongly recurved cusp, along outer margin of bone; lower jaw with a single row of 14-16 small, conical teeth with sharply pointed and slightly recurved tip; posterior tip of basipterygium expanded and articulating with anteromedial edge of ventral postcleithrum via a shallow concave facet; mandibular portion of preoperculo-mandibular lateral line canal absent; lachrymal canal with two pores; upper and lower lip simple, uniform in thickness along jaw margin.
Etymology. New Latin, anatomical term for muscles, from the Latin flexus, past participle of flectere, to bend. In reference to the great flexibility of clingfishes, many of which have the ability to bend the body so that the tail end comes to lie close to the head.  Table 1. Head relatively small (less than one third of body length), slightly dorso-ventrally compressed. Body moderately elongate, circular in cross-section anteriorly, becoming increasingly laterally compressed posteriorly. Widest point of body immediately behind head; as wide as widest point of head. Body width tapering gradually posteriorly. Body depth relatively uniform anterior to dorsal and anal fins; shallowest along caudal peduncle. Eye large, positioned on dorsolateral surface of head; orbit barely visible in ventral view. Centre of eye closer to tip of snout than to posterior margin of operculum. Snout of moderate length, broad, anterior margin rounded (Figs 1-3). Anterior nostril a small tubular opening ( Figure 3). Posterior nostril surrounded by a low, fleshy rim; situated along anterodorsal margin of orbit ( Figure 3). Gill membranes free from isthmus.
Mouth terminal, small; posterior tip of upper jaw reaching imaginary vertical line through anterior nostril when mouth closed. Upper and lower lip narrow; upper lip uniform in thickness along length of jaw; lower lip thicker along lateral margins of lower jaw, narrower at jaw symphysis ( Figure 3). Upper jaw slightly wider and longer than lower jaw, creating a narrow gap between teeth of upper and lower jaws when jaws closed ( Figure 5). Premaxilla with a single row of teeth, comprising 2-3 peg-like, conical teeth anteriorly at and adjacent to symphysis, and 10-12 strongly laterally compressed incisiviform teeth, each with a single strongly recurved cusp, along outer margin of bone ( Figure 7A-C). Dentary with a single row of 14-16 small, conical teeth with sharply pointed and slightly recurved tip ( Figure 7D). Pharyngeal jaws comprising patch of 4-6 small conical teeth with slightly recurved tips on pharyngobranchial toothplate 3 and row of 3-5 small conical teeth with slightly recurved tips along ceratobranchial 5 ( Figure 8D). 5-6 small triangular gill rakers located along anterior and posterior edge of ceratobranchials 2-3 and anterior edge of ceratobranchial 4; one or two tiny gill rakers located along anterior edge of ceratobranchial 1 ( Figure 8D). Paired rows of gill filaments (holobranch) on gill arches I-III (three gill filaments of Briggs 1955). Basihyal an elongate rod, widest posteriorly at point of articulation with dorsal hypohyals; anterior edge tipped with cartilage ( Figure 8C). Six brachiostegal rays ( Figure 8A); anteriormost ray sepa-  rate from hyoid bar; second ray articulating medially with hyoid bar along anterior ceratohyal; posterior rays articulating with hyoid bar laterally, including two along posteriormost part of anterior ceratohyal, one straddling junction between anterior and posterior ceratohyals, and posteriormost along anteriormost part of posterior ceratohyal ( Figure 8B). Anteriormost branchiostegal ray shorter than posterior rays. Three posteriormost branchiostegal rays similar in width and length, approximately twice as long and thick as anteriormost ray. Intervening rays intermediate in width and length ( Figure 8B). Cephalic lateral-line system with 2 pores in nasal canal; 2 pores in postorbital canal; 2 pores in lachrymal canal; 3 pores in preopercular canal ( Figure 3). Mandibular canal absent. Canal pores minute; typically flush with surface of skin and difficult to locate. Supraorbital canals (including nasal canal plus anteriormost region of postorbital canal of Shiogaki and Dotsu 1983) connected across midline via epiphyseal commissure ( Figure 6A). Superficial neuromasts on surface of head not arranged in obvious rows ( Figure 3); each surrounded by a shallow groove.
Colouration. In alcohol, head and body background colour typically uniformly pale cream to yellow (Figure 3). Holotype (Figure 1) has retained pinkish purple colour of live individuals and is an exception. Fins hyaline.
In life (Figure 2), body uniformly pinkish purple to grey, with diffuse, pale markings ranging from bars to irregular blotches. Head pinkish purple to grey, with diffuse, pale areas around nostrils and tip of snout. Iris red. Fins transparent.
Distribution and habitat. Known to date only from intertidal and subtidal waters of the Kermadec Islands (Figure 11), including Raoul Island (type locality) and L'Esperance Rock. The majority of available specimens were collected from rock pools and from shallower subtidal areas (down to 9 meters) over rock and coral rubble substrates using ichthyocides (Stewart 2015). However, a single specimen of the new species has been observed (and photographed) at 28 meters in depth ( Figure 2).
Etymology. Incus is the Latin word for anvil, in reference to the anvil-like outline of Raoul Island, the largest island in the Kermadec archipelago and type locality of the new species. A noun in apposition.
Gut content. Hard and irregular shaped items ranging in size from 50-300 μm are scattered throughout the stomach of the CT scanned individual ( Figure 12A). Smaller elements have smooth surfaces and could not be identified. Several of the larger elements appear to exhibit a porous (potentially stereomic) surface and are tentatively identified as echinoderm remains. Hard elements inside the stomach of the single C&S individual survived the clearing and staining process and could be dissected and photographed ( Figure 12B, C). These elements are tentatively identified as either stereomic ossicles from the terminal disc of an echinoid ( Figure 12B) or ossicles from the body of a holothuroid ( Figure 12C) suggesting that the new species consumes echinoderms or parts of echinoderms.

Discussion
The Kermadec Islands are a group of tiny remote islands almost 1,000 km from New Zealand, about half way between Tonga and Auckland, and lie between 29°15'S and 31°21'S. The Kermadec-Tonga arc is the longest submarine arc on the planet, running 2,500 km along the boundary of the Pacific and Australian Plates. It is a region of high geothermal activity with 83% of investigated volcanic centres venting (de Ronde et al. 2007). Directly to the east of the ridge lies the Kermadec Trench, the fourth deepest in the world which reaches to ca 10,000 meters deep at the deepest point. The Kermadec Islands constitute a series of four small emergent islands (Raoul, Macauley, Curtis, and Nugent) and two large rocks (L'Esperance and Havre). The largest of these islands is Raoul Island (2,040 ha, rising to 520 m asl) and the smallest L'Esperance Rock (4.8 ha, 70 m als) (Trnski and de Lange 2015). Havre Rock only just breaks the surface at low tide. The total land area of these islands and rocks is ca. 33 km 2 (Mortimer and Campbell 2014). The cones have had a highly dynamic recent history with explosive emergence and collapse (Lloyd et al. 1996). In spite of geothermal activity and remote- ness, the Kermadec Ridge and adjacent trench is inhabited by over 2,000 taxa (Duffy and Ahyong 2015) including 397 species of fishes (Te Papa unpublished records), of which nine are known to be endemic to the shelf (0-200 m depth). These include Anarchias supremus, Microbrotula punicea, Lepidotrigla robinsi, Hypoplectrodes sp. n., Enneapterygius kermadecensis, Eviota kermadecensis, Arnoglossus sp. n., Lophonectes sp. n., and the new clingfish species described herein (Roberts et al. 2015; Duff and Ahyong 2015; Te Papa unpublished records).
Specimens of Flexor incus have been referred to previously as Aspasmogaster sp. (Stewart 2015;. This taxonomic assignment, considered "tentative" by Stewart (2015), was based on preliminary attempts to identify the new species by B. Hutchins in 1980s (A. Stewart pers. obs.). Aspasmogaster is currently represented by four species (viz. A. costata, A. liorhynchus, A. occidentalis and A. tasmaniensis) and restricted to coastal areas of temperate Australia (Hutchins 1984;Hutchins 2008). The new species differs most obviously from Aspasmogaster by features of the oral jaw dentition, including both the arrangement (teeth in both jaws arranged in a single row vs. teeth in both jaws arranged in a broad patch anteriorly, tapering to a single row posteriorly; Figure 13A, C) and the type of teeth present on the premaxilla (Flexor is a heterodont with both conical and incisiviform teeth on the premaxilla [ Figure 7A, B] vs. premaxilla with conical teeth only [ Figure 13A, B]). It can be further distinguished from Aspasmogaster by having an oval opening between the premaxillae formed by a characteristic indentation along the medial edge of each premaxilla (vs. medial edge of premaxilla straight, premaxillae abutting along entire medial edge or separated only by a narrow gap [ Figure 14A]), simple lips, both of which are relatively thin and uniform in thickness along the length of the jaws (vs. lower lip expanded adjacent to the sym-  A Dorsal view B Lateral view (left side) C Ventral view. Abbreviations: ACh, anterior ceratohyal; Ana, anguloarticular; Apa, autopalatine; Boc, basioccipital; Bp, basipterygium; Br, branchiostegal rays; Cb5, ceratobranchial 5; Cl, cleithrum; DHh, dorsal hypohyal; DPcl, dorsal postcleithrum; Ec, epicentral; Ect, ectopterygoid; Epoc, epiotic; Exoc, exoccipital; Fr, frontal; Hy, hyomandibular; I, pelvic-fin spine; Iop, interopercle; La, lachrymal; LE, lateral ethmoid; Na, nasal; NS1, 5, neural spine of vertebral centrum 1, 5; Op, opercle; Pa, parietal; PecR, pectoral radial; PecFR, pectoral-fin ray; PelFR1-4, pelvic-fin ray 1-4; Pop, preopercle; Pro, prootic; Psph, parasphenoid; Pt, posttemporal; Pte, pterotic; Q, quadrate; Ra, retroarticular; Ri, rib; Sc, scapula; Scl, supracleithrum; Soc, supraoccipital; Sop, subopercle; Sph, sphenotic; Ur, urohyal; V1, vertebral centrum 1; Vo, vomer; VPcl, ventral postcleithrum. physis into a prominent fleshy fold in all four species of Aspasmogaster; upper lip also expanded and overlapping anterolateral margin of snout in A. occidentalis ;Hutchins 1984), by features of the adhesive disc, including a lower number of transverse rows of papillae in all disc regions (3-4 rows in region A, 4-5 in region B and 2-3 in region C vs. 5-8 rows in region A, 6-9 in region B and 3-5 in region C ;Briggs 1955;Hutchins 1984), a well-developed articulation between the posterior tip of the basipterygium and the anteromedial edge of the ventral postcleithrum (vs. basipterygium and ventral postcleithrum without contact; Figure 14C, see also Hutchins 1984: figure 5), and by features of the cephalic lateral line canals, including the absence (vs. presence) of the mandibular portion of the preoperculo-mandibular canal, and 2 (vs. 3) openings in the lachrymal canal.
Based on the characters listed in the key to the subfamilies of the Gobiesocidae (Briggs 1955: 10), Flexor would be considered a member of the Diplocrepinae, which in addition to Aspasmogaster is also hypothesised to include Cochleoceps, Diplocrepis, Gastrocyathus, Gastrocymba, Gastroscypthus, Parvicrepis, Pherallodus and Propherallodus (Briggs 1955;Shiogaki and Dotsu 1983;Hardy 1984;. The composition of this subfamily has been questioned previously by Briggs (1993) and we suspect that it is not monophyletic (see below). Regardless, Flexor can be distinguished from all of the aforementioned genera except Pherallodus and Propherallodus by features of the adhesive disc, including the absence (vs. presence) of papillae in region D and by having a well-developed articulation between the posterior tip of the basipterygium and the anteromedial edge of the ventral postcleithrum (vs. basipterygium and ventral postcleithrum without contact or with simple contact). It can be further distinguished from all but Pherallodus by the presence of strongly laterally compressed incisiviform teeth with a strongly recurved cusp, along the outer margin of the premaxilla (vs. simple peg-like conical teeth or strongly recurved conical teeth along the outer margin of the premaxilla), and from Pherallodus by the presence (vs. absence) of the preopercular portion of the preoperculo-mandibular canal, the lower jaw with conical teeth only (vs. lower jaw with both conical and incisiviform teeth), and by a complete field of papillae across the centre of region A and C of the adhesive disc (vs. papillae absent from centre of both region A and C) (Shiogaki and Dotsu 1983).