Discotrema Briggs, 1976

Fujiwara, Kyoji, Motomura, Hiroyuki, Summers, Adam P. & Conway, Kevin W., 2024, A new generic name for the " Lepadichthys " lineatus complex with a rediagnosis of Discotrema, a senior synonym of Unguitrema, and comments on their phylogenetic relationships (Gobiesocidae: Diademichthyinae), Vertebrate Zoology 74, pp. 279-301 : 279

Discotrema Briggs, 1976

Discotrema Briggs, 1976: 339 (type species: Discotrema crinophilum Briggs, 1976, by original designation, monotypic)

Unguitrema Fricke, 2014: 36 (type species: Unguitrema nigrum Fricke, 2014, by original designation, monotypic)

Type species.

Discotrema crinophilum Briggs, 1976 (Fig. 10E-G )

Revised diagnosis.

A genus of the Gobiesocidae belonging to the Diademichthyinae and distinguished from all other genera by the following unique characters: a well-developed lateral process on pterotic immediately posterior to exit of otic canal (Figs 11A, B , 12 ); upper pharyngeal teeth arranged in a single row along posteroventral edge of pharyngobranchial 3 toothplate, with tooth tips directed posteriorly (Figs 11C , 13E, F ); a deep cavity at center of disc region C (Fig. 14A ); disc papillae heterogeneous, including small, flattened papillae with a roughly hexagonal or circular outline around outer margin of disc and larger, circular papillae with a hardened (potentially keratinized) cap, with a raised, dome-like surface (Fig. 14A ); three paired and one median cluster of small papillae (reminiscent of bunches of grapes) across adhesive disc (Fig. 14A ); ventral postcleithrum divided into an anterior and posterior element, separated via a specialized joint (Figs 11C , 14B, D, E ).

The following characters are also diagnostic, although not unique to the genus (details discussed in "Relationships of Rhinolepadichthys gen. nov. and Discotrema " [mostly same as those of Rhinolepadichthys gen. nov.; see below]): snout tip well extended, distinctly beyond tip of lower jaw (more pronounced in specimens smaller than ca. 25 mm SL) (Fig. 15B, E ); upper lip fused with skin of snout, usually lacking a distinct groove between lip margin and snout, if present, groove very weak, restricted to posterior portion of upper jaw only (Fig. 15A, B, D, E ); anterior part of lower lip without skin lobes and fused with skin of gular area (Fig. 15C, F ); upper-jaw teeth only weakly hooked, with tip of crown blunt or rounded (Figs 11B , 13A, C ); medial edge of premaxilla very weakly concave, forming indistinct, narrow oval opening anteriorly in dorsal view between premaxillae (Fig. 11A ); gill membrane attached to isthmus; gill rakers present only on posterior edge on ceratobranchials 1-3 (absent from anterior edge of ceratobranchial 1-3, and from ceratobranchial 4) (Fig. 13D ); ceratobranchial 5 edentulous (Fig. 13D, G ); preopercle without lateral-line canal and associated pores (PR1-3) (Fig. 15 ); single adhesive disc (Fig. 14A ); anterolateral part of ventral postcleithrum extended anteriorly as a well-developed rod-like process, its tip close to base of pelvic-fin soft ray 4 (Fig. 11C , 14B ).

Description.

General body appearance as in Fig. 10 . Photographs and line drawing of heads, showing cephalic lateral-line canal pores plus snout and lip characters, given in Fig. 15 . Features of osteology shown in Figs 11 - 14 , 16 . Body cylindrical laterally, compressed at caudal peduncle. Anus closer to anal-fin origin than to posterior margin of disc. Head depressed anteriorly, its width wider than body width. Snout tip weakly pointed (lateral view), semi-elliptical (dorsal view); no skin fold (see Hutchins 1984: fig. 2b) across dorsal surface; dorsal profile of snout rounded. Mouth terminal, its posterior tip level with vertical through anterior margin of eye lens. Posterior part of lower lip well separated from skin of gular area, with distinct groove between both structures. Anterior and posterior nostrils close to each other, located dorsolaterally; posterior nostril vertically level with anterior margin of eye; both nostrils with a membranous tube, that of former slightly longer than that of latter. Interorbital region flattened. Upper end of gill opening level with base of 4th-6th pectoral-fin ray (lateral view); lower 5th-8th pectoral-fin ray base attached to disc by membrane. First to third gill arches with 2 rows of gill filaments, fourth arch without filaments (= 3 gills sensu Briggs 1955). Cephalic lateral-line canal pores with 2 nasal (NC), 2 lachrymal (LC) and 2 postorbital (PO) canal pores (Fig. 15 ). NC1 located vertical through anterior nostrils (dorsal view); NC2 slightly before posterior nostrils (dorsal view); LC1 just in front of anterior margin of eye; LC2 posteroventrally below LC1; PO1 just behind posterior margin of orbit; PO2 similar horizontal level with or slightly above PO1. Dorsal and anal fins located posteriorly, their bases relatively short; not connected to caudal fin by membranes. Dorsal- and anal-fin height almost equal, except anteriorly. Pectoral- and caudal-fin margins rounded.

Structure of neurocranium (Fig. 12 ), lachrymal (Fig. 12 ), viscerocranium (Fig. 13 ), paired-fin girdles (Figs 14 , 16 ), and caudal skeleton (Fig. 9B ) generally similar to that described above for Rhinolepadichthys gen. nov., except for the following differences. Pterotic with elongate lateral process immediately posterior to exit of otic canal (Figs 11 , 12 ), tightly connected to posterior process on hyomandibular by ligament (Fig. 13B ). Posterovenrally directed processes on hyomandibular somewhat expanded dorsolaterally; ectopterygoid triangular, dorsal edge weakly concave; preopercle relatively well ossified, its ventral edge rounded (Fig. 11C , 13A ). Cartilaginous tip of basihyal elongate, occupying half the length of basihyal; pharyngobranchial 3 toothplate trapezoid; gill rakers present on posterior edge of ceratobranchials 1-3 only; ceratobranchial 5 rod-like (Fig. 11C , 13D-H ). Posterior 3/4th of urohyal well forked in lateral view (Fig. 13I ). Single small accessory ossicle variably present between ventral postcleithra (Fig. 14B, E ; no accessory ossicle in BPBM 40504: Fig. 11C ).

Dorsal-fin rays 7-10. Anal-fin rays 7-9. Pectoral-fin rays 23-27. Based on Discotrema crinophilum (ROM 85350), total number of vertebrae 35, including 18 abdominal and 17 caudal. Ribs 14, associated with vertebrae 3-16. Epicentrals 19 (left side) or 20 (right side), associated with vertebrae 2-21 (right side) or 3-21 (left side). First dorsal-fin pterygiophore inserted between neural spines of vertebrae 21/22. First anal-fin pterygiophore inserted between hemal spines of vertebrae 23/24.

Included species.

The genus contains the following four valid species, Discotrema crinophilum Briggs, 1976, Discotrema monogrammum Craig & Randall, 2008, Discotrema nigrum (Fricke, 2014), comb. nov. (validity tentative, see below), and Discotrema zonatum Craig & Randall, 2008.

Figures

Figure 9. Caudal skeletons (left side, anterior to left) of A Rhinolepadichthys polyastrous (SAIAB 9319, 25.0 mm SL) and B Discotrema crinophilum (ROM 85350, 25.4 mm SL). Abbreviations: AFR, anal-fin rays; DFR, dorsal-fin rays; DRC, cartilaginous distal radial; Ep, epural; HSPU 2, hemal spine of preural centrum 2; LHP, lower hypural plate; NSPU 2, neural spine of preural centrum 2; PhC, parhypural cartilage; PU 2, 3, preural centrum 2, 3; P-MR, proximal-middle radial; UC, ural centrum; UHP, upper hypural plate. White asterisks (*) indicates caudal-fin rays associated with upper and lower hypural plates.

Figure 10. Photographs of preserved holotypes of A - D Unguitrema nigrum (NTUM 10603, 11.1 mm SL); D close-up of surface features of adhesive disc; E - G Discotrema crinophilum (USNM 215329, 22.1 mm SL). A, E dorsal views; B, F lateral views; C, D, G ventral views.

Figure 11. CT scanned anterior skeleton, including cranium, paired-fin girdles, and abdominal region of vertebral column, of Discotrema monogrammum (BPBM 40504, 16.4 mm SL). A dorsal view; B lateral view; C ventral view. Abbreviations: ACh, anterior ceratohyal; Ana, angulo-articular; Apa, autopalatine; Bh, basihyal; Boc, basioccipital; Bp, basipterygium; BR, branchiostegal rays; Cb, ceratobranchials; Cl, cleithrum; CoPr, coronoid process; Cor, coracoid; DHh, dorsal hypohyal; Dn, dentary; DPcl, dorsal postcleithrum; Ect, ectopterygoid; EpC, epicentral; Epoc, epiotic; Exoc, exoccipital; Fr, frontal; Hy, hyomandibular; I, pelvic-fin spine; Ih, interhyal; Iop, interopercle; La, lachrymal; LE, lateral ethmoid; M, mesethmoid; Mx, maxilla; Na, nasal; NS 1, 5, neural spine of vertebral centrum 1, 5; Op, opercle; Pa, parietal; PbTP 3, pharyngobranchial 3 toothplate; PCh, posterior ceratohyal; PecFR, pectoral-fin rays; PecR, pectoral radials; PelR, pelvic-fin soft rays; Pmx, premaxilla; Pop, preopercle; Pro, prootic; Psph, parasphenoid; Pt, posttemporal; Pte, pterotic; Q, quadrate; Ra, retroarticular; Ri, ribs; Sc, scapula; Scl, supracleithrum; Soc, supraoccipital; Sop, subopercle; Sph, sphenotic; Sym, symplectic; Ur, urohyal; V 1, vertebral centrum 1; Vo, vomer; VPcl (a), ventral postcleithrum (anterior); VPcl (p), ventral postcleithrum (posterior).

Figure 12. Neurocranium of Discotrema crinophilum (ROM 85350, 25.4 mm SL). A dorsal view; B lateral view; C ventral view. Abbreviations: Boc, basioccipital; EC, epiphyseal commissure of supraorbital canal; EpC, epicentral; Epoc, epiotic; Exoc, exoccipital; Fr, frontal; La, lachrymal; LE, lateral ethmoid; M, mesethmoid; Na, nasal; NS 1, neural spine of vertebral centrum 1; OC, otic canal; Pa, parietal; Pro, prootic; Psph, parasphenoid; Pte, pterotic; SC, supraorbital canal; Soc, supraoccipital; Sph, sphenotic; V 1, vertebral centrum 1; Vo, vomer.

Figure 13. Viscerocranium of Discotrema crinophilum (ROM 85350, 25.4 mm SL mm SL). A Hyopalatine arch and opercular series (right side) in lateral view (image reversed), outline of preopercle, opercle, and subopercle highlighted by thin black line, posterior process of hyomandibular indicted by black asterisk (*); B articulation between hyomandibular and pterotic (left side) in ventral view, ligament between posterior process of hyomandibular (*) and hyomandibular process of pterotic indicted by double arrow head; C close-up of posterior teeth on premaxilla (left side) in lateral view; D lower gill-arch elements in dorsal view; E upper gill-arch elements (right side) in dorsal view; F close-up of PbTP 3 (right side) in ventral view (image reversed); G close-up of Cb 5 (right side) in dorsal view; H close up of gill rakers on posterior edge of ceratobranchials 1 and 2 (right side) in dorsal view; I hyoid bar (left side) in lateral view and urohyal. Abbreviations: AC, acrodin cap; ACh, anterior ceratohyal; Ana, angulo-articular; Apa, autopalatine; Bb 2 - 4, basibranchial 2 - 4; Bh, basihyal; BR, branchiostegal rays; Cb 1 - 5, ceratobranchial 1 - 5; CoPr, coronoid process; DHh, dorsal hypohyal; Dn, dentary; Eb 1 - 4, epibranchial 1 - 4; Ect, ectopterygoid; GR, gill rakers; Hb 1 - 3, hypobranchial 1 - 3; Hy, hyomandibular; Ih, interhyal; Iop, interopercle; MC, meckel's cartilage; Mx, maxilla; Op, opercle; PbTP 3, pharyngobranchial 3 toothplate; PCh, posterior ceratohyal; Pmx, premaxilla; Pop, preopercle; Pte, pterotic; Q, quadrate; Ra, retroarticular; RT, replacement teeth; Sph, sphenotic; Sop, subopercle; Sym, symplectic; Ur, urohyal.

Figure 14. Surface features of the adhesive disc (A KAUM-I 153217, 29.4 mm SL, weak cyanine stain) and internal supporting skeleton of paired-fin girdles (B - E ROM 85350, 25.4 mm SL) of Discotrema crinophilum. B Pelvic disc supporting skeleton, including elements of pelvic- and pectoral-fin girdle in ventral view (anterior to top of page), outline of pelvic-fin spine and rays highlighted by thin white-dashed line; C close up of pelvic-fin ray 1, showing expanded anterior edge ventral to enlarged hardened papillae (outline show in thin white-dashed line); D ventral postcleithrum and surrounding elements (right side) in dorsal view (anterior to top of page); E dissected ventral postcleithrum (left side) in dorsal view (anterior to top of page). Abbreviations: A-C disc region A-C; AO, accessory ossicle; Bp, basipterygium; Cl, cleithrum; DPcl, dorsal postcleithrum; I, pelvic-fin spine; PcLFi, postcleithral fimbrae; PecFR, pectoral-fin rays; PelR 1 - 4, pelvic-fin soft ray 1 - 4; PRC, pelvic radial cartilage; VPcl (a), ventral postcleithrum (anterior); VPcl (p), ventral postcleithrum (posterior).

Figure 15. Heads of Discotrema crinophilum. A - C KAUM-I. 145201, 25.9 mm SL (cyanine stain); D - F WAM P. 26100 - 009, 1 of 6 specimens, 22.2 mm SL (line drawing). A, D dorsal views; B right side in lateral view (image reversed); E left side in lateral view; C, F ventral views. Abbreviations: AN, anterior nostril; LC 1 - 2, lachrymal canal pores; NC 1 - 2, nasal canal pores; PN, posterior nostril; PO 1 - 2 postorbital canal pores. Black asterisks (*) indicates superficial neuromasts.

Figure 16. Pectoral-fin girdle of Discotrema crinophilum (ROM 85350, 25.4 mm SL). A Right side in lateral view (anterior to left, image reversed); B close-up of elements of pectoral-fin endoskeleton articulating with pectoral-fin rays (left side) in lateral view (anterior to left), dorsal postcleithrum removed. Abbreviations: AO, accessory ossicle; Cl, cleithrum; Cor, coracoid; DPcl, dorsal postcleithrum; PcLFi, postcleithral fimbrae; PecFR, pectoral-fin rays; PecR 1 - 4, pectoral radial 1 - 4; Pt, posttemporal; Sc, scapula; Scl, supracleithrum; VPcl (a), ventral postcleithrum (anterior); VPcl (p), ventral postcleithrum (posterior).