Euclichthys McCulloch, 1926

Last, Peter R. & Pogonoski, John J., 2020, Revision of the fish family Euclichthyidae (Pisces: Gadiformes) with the description of two new species from the Western Pacific, Zootaxa 4758 (2), pp. 231-256 : 234-236

Genus Euclichthys McCulloch

Euclichthys McCulloch, 1926: 174 (masculine; type species Euclichthys polynemus McCulloch by original designation).

Diagnosis. Monotypic, as for definition of family.

General features. D1 1, 11–15, D2 67–88; A 1, 78–102; P 19–23; V 6; C 30–45; primary gill rakers 4–6 + 12–19 = 17–25, 0–1 rudiments on upper limb, 0–3 rudiments on lower limb; vertebrae 14–17 abdominal + 49–56 caudal, total 64–73.

Body strongly compressed, tapering in width and depth beyond anus; slender, greatest depth 4.5–6 in SL. Head relatively small, compressed, length 4.4–6.4 and width between preopercles 9–13 times in SL, respectively; snout rounded anteriorly, horizontal length subequal or shorter than eye diameter; head partly scaled with shallow scale pockets (usually damaged), bones fine and skin thin; scales on isthmus usually intact, smaller than those of upper head; deep sensory canal extending from snout, around suborbit and posterior eye, additional canals associated with preopercles, outer part of lower jaw and extending posteriorly along lateral margin of nape from posterior eye. Mouth moderate in size (maxilla length 1.7–1.9 in head length), subterminal, directed upward; upper jaw terminating more or less beneath posterior margin of eye. Teeth in both jaws small, villiform, similar in size and shape, in multiple irregular rows; rows not joined at symphysis of upper jaw. Nostrils small, located near anterior margin of orbit and slightly above horizontal through middle of eye; anterior aperture suboval, not tubular nor elevated into a broad flap, smaller than posterior aperture. Eye relatively large, diameter 3.1–4.0 in head length, orbit subcircular with slightly raised margin; interorbital space very slightly convex, narrow, 1.1–1.6 in eye diameter. Opercular bones delicate; gill openings united ventrally below centre of orbit. Gill rakers long, slender, flattened slightly with fine lingual denticles. Innermost edge of first arch, and 2nd–4th arches with much shorter, apically thickened rakers set with small denticles.

Otolith size medium (length to at least 7.7 mm), oval (terminology following Furlani et al. 2007); dorsal margin sinuate to almost entire; ventral margin weakly lobed, sinuate or almost entire; sulcus homosulcoid or weakly heterosulcoid, openings ostial or osteo-caudal; colliculi roughly equal in size; collum broad (slightly narrower to slightly more than half height of cauda), pseudocolliculum sometimes distinct, in ventral part of collum; crista superior weak to well developed, usually prominent, dorsal area depression usually evident, crista inferior similarly developed; rostrum short and broad to barely discernible, antirostrum poorly developed; excisura narrow to broad, notch small to rudimentary; pseudoexcisura prominent, narrow to absent; ventral area groove present, or absent (sometimes very weak).

Dorsal fins barely separated; first dorsal fin taller and much shorter based than second dorsal fin; unpaired fins partly enveloped in a sheath of skin. First dorsal fin originating above pectoral-fin base; first ray very short and spinous, rays 2–5 longest. Second dorsal-fin contour tallest anteriorly then tapering in height to form a shallow notch posteriorly (some penultimate rays of fin slightly longer than those anteriorly). Most of unpaired fins covered by thin fleshy sheaths; fin rays unconnected by membranes distal to sheath. Pelvic fins jugular, inserted ventrally slightly forward of vertical through posterior margin of preopercle; longest ray reaching to or beyond anus. Pectoral fins relatively short, tips not reaching origin of anal fin; proximal half of pectoral fin not encased in a fleshy sheath. Anal-fin rays of anterior part shorter than longest rays of first dorsal fin; longest posterior rays of second dorsal fin exceeding longest rays of posterior part of anal fin. Anus and origin of anal fin closely adjacent. Caudal peduncle short, slender, strongly compressed, usually less than half length of orbit diameter.

Caudal fin asymmetrical, mid-lower branched rays longest; ventral procurrent and unbranched rays more numerous and extending more anteriorly than those of dorsal part of fin; fin covered by very thin sheath of skin; upper procurrent + unbranched rays 5–12, upper branched (segmented) rays 9–13, lower branched (segmented) rays 5–8, lower procurrent + unbranched rays 8–17. Caudal skeleton consisting of two fused hypurals associated with the first preural centrum (and first ural centrum) and fused hypurals 3–5 associated with a second ural centrum; uroneural present; two epurals; parahypural present; 1–2 neural spines and 1–2 hemal spines on preural centrum 2; X and Y accessory caudal bones present or absent, inter- and intraspecifically variable (i.e. absent in two species, mostly present in third species); anteriormost upper procurrent ray originating near tips of neural spines of pre-ural vertebrae 6–10; anteriormost lower procurrent ray originating near tips of hemal spines of pre-ural vertebrae 7–12.

Body covered with small, highly deciduous cycloid scales with delicate scale pockets; scales absent from jaws, fin sheaths and over branchiostegal membranes; lateral line pores indeterminate.

Upper body whitish to silvery, areas of dark luminous tissue ventrally on head, belly and around anus; pelvic-fin rays dark; margins of dorsal fin, caudal fin and parts of anal fin sometimes black, remainder of fins pale to translucent; scale pocket membranes demarcated when intact.

Species. Euclichthys polynemus McCulloch, 1926 and two new species, E. microdorsalis sp. nov. and E. robertsi sp. nov.

Remarks. Family interrelationships of the Euclichthyidae have not been fully resolved with the group comprising a monotypic genus, Euclichthys ( Svetovidov 1969; Cohen 1984 ; Paulin 1983; Roberts & Paulin 1997). In erecting his new family Cohen (1984) wrote: “ Euclichthys (Fig. 137), represented by a single South Australian and New Zealand species, was incorrectly placed in Moridae but removed by Svetovidov (1969), who pointed out some similarities to Macrouridae . Euclichthys cannot be placed in any currently recognized family. It has a free first neural spine, which may indicate an origin prior to Palaeogadus , lacks an otophysic connection, has four hypurals nearly fused to two, and in two specimens has only one of the X–Y bones. As in morids, which are more specialized than macrourids and could not have given rise to them, Euclichthys has an asymmetrical, rather reduced caudal fin. Perhaps this curious fish is a modern representative of a macrourid progenitor.” These characters were subsequently used by Roberts & Paulin (1997) for a diagnosis of the family.

The first neural spine of many gadiforms is closely adpressed to the occipital crest ( Cohen 1984 ) and this condition was observed in material of Euclichthys examined for this study. In some gadiforms, such as Muraenolepis , the first neural spine is free and Cohen (1984) suggested that this condition applied to Euclichthys . However, this character seems variable and needs further evaluation.

The caudal-fin skeleton of Euclichthys has been illustrated and photographed by Paulin (1983: fig. 5C) and Markle (1989: fig. 16), respectively and our findings are in general agreement with the literature. Cohen (1984) reported 4 hypural bones in Euclichthys supporting 6 caudal-fin rays. According to Paulin (1983), in Euclichthys polynemus , hypurals 1 and 2 are fused at their base and hypurals 3, 4 and 5 are partially fused into a single plate. From our radiographs, and a condition consistent for all three species, the hypurals are fused to form two distinct plates consistent with Paulin (but the original 5 hypural elements are barely detectable) and these plates support 6–7 branched fin rays. However, the main discrepancies exist around the accessory caudal-fin elements (X and Y bones). X and Y bones have been reported as being both present and absent in gadiform fishes ( Grande et al. 2013), and these characters are variable within Euclichthys . Neither of these bones were found in 76 specimens of two species (22 E. microdorsalis and 54 E. polynemus ), and all except one of 29 specimens of E. robertsi had an X bone and 20 of 28 specimens a Y bone. A specimen examined by Fahey & Markle (1984: 280, Table 76), labelled Euclichthys sp. with an X but no Y bone, was most likely a specimen of E. robertsi captured east of Newcastle, NSW and sourced from the Australian Museum (possibly AMS I.18838-038 figured by Markle, 1989).

Fahay & Markle (1984) provided a table of meristic characters for the gadiforms, including a single specimen of Euclichthys . All of their data, apart from the pelvic-fin count (given as 5 rays), fall within the ranges observed in this study. Counts of the number of pelvic-fin rays has been confused in the literature ( Roberts & Paulin, 1997) ranging from 4–6 rays and 0–3 branches. In our material, the pelvic fin was consistently formed of 6 long, filamentous rays with the uppermost 3 rays fused basal for a third to half of their length forming a thickened ray; the third ray in this fused uppermost part of the fin can be difficult to see even using a microscope if the ray is broken distally and may lead to counts less than 6. The structure of the pelvic fin is unique within the gadiforms. Asymmetry of a reduced caudal-fin is characteristic of this group and is achieved by having the longest caudal-fin rays associated with the hypurals and parahypural, and the anteriormost part of the fin extending further forward ventrally than dorsally.

The otoliths of extant Euclichthys species are typically oval and of medium size with a single, well-developed sulcus groove, and in shape resembling a generalised gadiform (sensu Endo, 2002) and fossil otoliths from the Eocene and Miocene figured by Schwarzhans (2019): E. lawsoni Nolf & Rundle in Nolf, 2013 , E. noveazeelandiae ( Frost, 1924) and E. eocenicus Schwarzhans, 1980 . From the small number of samples examined, extant Euclich- thys exhibit slight intraspecific variability in shape but interspecific differences were also observed. Grenfell (1984) examined some 100 otoliths of E. noveazeelandiae from sediments of the early Miocene in New Zealand and found within subadults of the same age there was no apparent variation; juveniles tended to be less elongate with smaller colliculi, and adults had a less sculptured dorsal margin. Within the extant species, Euclichthys polynemus has a more broadly oval otolith (ratio otolith length to otolith height <1.7 vs 1.75–1.9) with a more entire margin than its extant congeners, and the ostium is only slightly larger than the cauda (cauda larger than the ostium in E. microdorsalis ). The sulcus opening of E. polynemus appears to be ostial (ostio-caudal in the other species) and a ventral groove is evident in all three replicates (vs. absent). Euclichthys robertsi is similar to E. microdorsalis in shape, but is more slender, has a narrower collum, more elongate ostium and cauda, and usually a more prominent pseudoexcisura.

While substantial detail is provided below for two important anatomical characters in gadiform fishes, the caudal skeleton and otoliths, the main objective of the present study was to elucidate alpha-taxonomic issues in the group rather than focus on phylogenetic aspects. Hence, some other characters considered to be of evolutionary significance and worthy of further investigation, such as the lack of an otophysic connection and the anatomy of the cranial muscle adductor arcus palatine, were not examined in this study. Similarly, in the absence of fresh tissues, molecular data for the two new taxa has not been obtained. This should be an important consideration for future studies of the group.