Pseudoscopelus Lütken, 1892

Melo, Marcelo R. S., 2019, A revision of the genus Pseudoscopelus Lütken (Chiasmodontidae: Acanthomorphata) with descriptions of three new species 2710, Zootaxa 2710 (1), pp. 1-78 : 9-11

Pseudoscopelus Lütken, 1892

Line-bellied swallowers

Pseudoscopelus scriptus Lütken 1892: 284–286 , plate 1, figures 4–5 [Type by monotypy Pseudoscopelus scriptus Lütken ].

Myersiscus Fowler 1934: 362 , figures 112–3 [type by monotypy Myersiscus obtusifrons Fowler ].

Diagnosis. The genus Pseudoscopelus is diagnosed within Chiasmodontidae by two unique characteristics present in all species: the dorsal margin of orbit formed by infraorbital 6 (vs. infraorbital 6 on the posterior dorsolateral border of eye); and the position of the last infraorbital pore located anterior to middle of the pupil (vs. last infraorbital pore posterior to pupil).

Pseudoscopelus can be further distinguished from Chiasmodon by having: the premaxillary teeth arranged in three series, lateral, middle, and mesial (vs. premaxillary teeth in two series); dentary teeth arranged in two series, lateral and mesial, with mesial series in two or more rows (vs. dentary teeth in two series, mesial in a single row); anteriormost premaxillary tooth not very well developed (vs. anteriormost premaxillary tooth a very well-developed canine, see Melo 2009); total vertebra 31–38, precaudal 16–19 (vs. total vertebra 43–48, precaudal 19–23).

Pseudoscopelus can be further distinguished from Kali and Dysalotus by having the cranial bones strongly calcified and compact (vs. bones forming cranium weakly calcified with trabeculae easily distinguishable with the naked eye); nasal strongly calcified, spoon-shaped, located anterolaterally, and covered by thin transparent skin (vs. nasal weakly calcified, circular but not spoon-shaped, located dorsally, and covered by pigmented skin); parietals well developed, of same length or little smaller than supraoccipital (vs. parietals poorly developed, less than half the length of supraoccipital); crest on frontal weakly developed (vs. well developed longitudinal crest on frontal); pre- and postzygapophyses on vertebrae well developed and articulating (vs. pre- and postzygapophyses poorly developed, not contacting); presence of enlarged, elongated neuromasts exposed on surface of head, on supraoccipital, parietals, frontals and pterotic cranial bones, face, snout, lower jaw, and on body as two parallel rows anterior to first dorsal fin (vs. exposed neuromasts on head and body circular and small, present only on sides of head, anterior to preopercle, or as two rows anterior to first dorsal fin; no elongated neuromasts present); the last pored scale of lateral line well separated from the penultimate scale and positioned over the lower lobe of the caudal fin (vs. last pored lateral line scale contiguous with penultimate scale). From Kali , it can be also be distinguished by having the teeth of the premaxilla and dentary in several rows (vs. two rows), and from Dysalotus by lacking prickles on body (vs. prickles present on flank, from mid-body to caudal peduncle).

Description. Largest specimen examined 234.0 mm. Body elongate, laterally compressed; progressively more compressed in caudal region. Greatest body depth at origin of first dorsal fin. Scales absent, except for lateral-line scales, embedded in skin.

Anterior profile of head elongate in lateral view, tip of snout protruding, with gentle curve at level of nasal bone in Pseudoscopelus astronesthidens , P. australis and P. odontoglossum ; snout moderately blunt in P. altipinnis , P. aphos , P. cephalus , P. cordilluminatus , P. lavenbergi , P. parini , P. paxtoni , P. pierbartus , P. sagamianus , and P. scriptus ; strongly curved in P. obtusifrons ; and snout concave in P. bothrorrhinos and P.scutatus . Mouth terminal, oblique, and enlarged; premaxilla and maxilla reaching ventral edge of preopercle. Delicate fold of skin connect anterior contra lateral premaxillae and dentaries; basihyal frequently found hiding under fold of skin.

Orbit circular. Nares distinctly separated. Anterior naris circular, facing lateral in Pseudoscopelus altipinnis , P. aphos , P. astronesthidens , P. australis , P. cephalus , P. cordilluminatus , P. lavenbergi , P. obtusifrons , P. odontoglossum , P. parini , P. paxtoni , P. pierbartus , P. sagamianus , and P. scriptus ; facing anterior in P. bothrorrhinos and P. scutatus . Posterior naris crescent shaped, closer to anterodorsal margin of eye than to anterior naris, lateral in all species. Bones of cranium compact and well calcified.

Pectoral fin well developed; very elongated, reaching or slightly passing level of anus in Pseudoscopelus cordilluminatus , P. lavenbergi , P. obtusifrons , P. pierbartus , P. sagamianus and P. scriptus ; short, not reaching level of anus in P. altipinnis , P. aphos , P. astronesthidens , P. australis , P. bothrorrhinos , P. odontoglossum , P. parini , P. paxtoni and P. scutatus ; unknown in P. cephalus . Pelvic-fin origin slightly posterior to level of pectoral-fin base. Dorsal fins two, first dorsal-fin insertion slightly posterior to level of pelvic-fin insertion; second dorsal-fin origin at level of anal-fin origin. Anal fin opposite to second dorsal fin. Caudal fin forked. True spines (sensu Johnson and Patterson 1993) present in first dorsal-fin rays, and first pelvic-fin ray. Lateral line complete, with pores opening between each scale; last scale of lateral line separate from anterior, in lower lobe of caudal fin, with two pores. Sensory system of as described and illustrated by Melo et al. (2007).

Gill arches four; pseudobranchia present. Proximal half of first epibranchial connected to internal part of opercle; ceratobranchial and hypobranchial of first arch free. Epibranchials of second, third, and fourth arches connected distally to each other and to the infrapharyngobranchials; ceratobranchial of second arch free. Hypobranchial and basibranchial of second, third, and fourth arches connected medially. Ceratobranchial of third arch connected to fourth arch; ceratobranchial of fourth arch connected medially to body wall at anterior half; gill slit present between fourth ceratobranchial and body. Fifth ceratobranchial connected to fourth ceratobranchial.

Basihyal, elongated, free anteriorly, with posterior and ventral connection to first basibranchial; first basibranchial small, triangular, connected to basihyal and second basibranchial; second basibranchial elongated; third basibranchial extremely elongated, extending posteriorly beyond fourth basibranchial, posterior half cartilaginous and ventrally curved; fourth basibranchial small, diamond-shaped, cartilaginous. Hypohyals with dorsal and ventral elements, attached to first basibranchial; first hypobranchial attached immediately posterior to basihyal, between joint of first and second basibranchials; second hypobranchial connected to joint between second and third basibranchials; third hypobranchial proximally connected to anterior edge of fourth basibranchial, distal tip of third hypobranchial ventrally pointed; fourth hypobranchial absent or fused to ceratobranchial, fourth ceratobranchial connected to posterior edge of fourth basibranchial.

Dentition. Enlarged teeth on premaxilla, dentary and palatine. General premaxillary and dentary teeth arrangement illustrated in Figure 3 . Premaxilla moderately wide, widest point in premaxillary body, 10–20 % in premaxillary length. All teeth with small crown without flange.

Premaxillary teeth on head, neck, body and caudal process. Lateral series of teeth in single longitudinal row, along lateral edge of premaxillary head, neck, body and caudal process; teeth type 4, conical, slightly curved; in Pseudoscopelus obtusifrons teeth hook-like and flared outwards in premaxillary head; in P.altipinnis , P. paxtoni , P. astronesthidens , P. australis and P. odontoglossum , three to 12 posteriormost teeth type 1, conical, straight and thicker. Canine and fang on ventral shelf of premaxillary head. Canine, single tooth, type 1, curved. Fang, single tooth, type 4, straight. Middle and mesial series on ventral shelf of premaxillary body. Middle series in single, longitudinal, regular row of teeth in P. cordilluminatus , P. obtusifrons and P. pierbartus ; in two, longitudinal, irregular rows of teeth in P. aphos , P. bothrorrhinos , P. cephalus , P. parini ,, P. paxtoni , P. sagamianus , P. scutatus and P. scriptus ; in three longitudinal, irregular rows of teeth in P. altipinnis , P. astronesthidens , P. australis , P. lavenbergi and P. odontoglossum ; teeth type 1, conical, needle-like in all species, except P. obtusifrons ; in P. obtusifrons teeth type 1, conical, curved. Mesial series in single, longitudinal, regular row of teeth in P. bothrorrhinos , P. scutatus , P. obtusifrons and P. pierbartus ; in transverse rows of two to five teeth in P. altipinnis , P. astronesthidens , P. australis , P. aphos , P. cephalus , P. cordilluminatus , P.lavenbergi , P. odontoglossum , P. parini , P. paxtoni , P. pierbartus , P. sagamianus and P. scriptus ; teeth type 4, needle-like, slightly curved in all species except P. obtusifrons ; in P. obtusifrons , teeth type 4, conical, strongly curved.

Dentary teeth in lateral and mesial series. Lateral series along lateral shelf of dentary, in single, longitudinal row of teeth, extending from symphysis to posterior tip; teeth type 4, conical, slightly curved; in Pseudoscopelus obtusifrons teeth hook-like and flared outwards in anterior curvature of dentary. Mesial series on dorsal shelf of dentary; in single, longitudinal row of teeth in P. scutatus , P. obtusifrons and P. pierbartus ; in transverse rows of two to seven teeth in P. altipinnis , P. astronesthidens , P. australis , P. aphos , P. bothrorrhinos , P. cephalus , P. cordilluminatus , P. lavenbergi , P. odontoglossum , P. parini , P. paxtoni , P. sagamianus and P. scriptus ; teeth type 4, needle-like, slightly curved in all species except P. obtusifrons ; in P. obtusifrons , teeth type 4, conical, strongly curved.

Palatine teeth type 4, conical, slightly curved, in single, longitudinal row in Pseudoscopelus altipinnis , P. australis , P. aphos , P. bothrorrhinos , P. cephalus , P. cordilluminatus , P. lavenbergi , P. obtusifrons , P. parini , P. paxtoni , P. pierbartus , P. sagamianus , P. scriptus , and P. scutatus . Palatine teeth type 4, needle-like, straight, in two rows anteriorly and single row posteriorly in P. astronesthidens and P. odontoglossum ; posterior teeth ticker, type 1, similar in shape and size to posterior teeth on caudal process of premaxilla.

Teeth on infrapharyngobranchials and fifth ceratobranchial conical, curved, type 4 in all species of Pseudoscopelus . Teeth on second basibranchial small, type 1, conical, in V -shaped or single, irregular row, absent in basihyal in P. altipinnis , P. aphos , P. bothrorrhinos , P. cephalus , P. cordilluminatus , P. lavenbergi , P. obtusifrons , P. parini , P. paxtoni , P. pierbartus , P. sagamianus , P. scriptus , and P. scutatus . Teeth on basihyal and second basibranchial enlarged, type 4, needle-like, in two rows in P. astronesthidens , P. australis and three rows in P. odontoglossum . Teeth absent on other basibranchials.

Gill rakers on first and second hypobranchials and ceratobranchials; absent on other elements of branchial apparatus; absent on first ceratobranchial in Pseudoscopelus australis , and second ceratobranchial in P. australis and P. odontoglossum ; rakers teeth-like, type 1, conical.

Luminescent organs. General groups of photophores summarized in Figure 1 . Specific pattern detailed in species account. Present as discrete photophores on head and body in Pseudoscopelus altipinnis , P. astronesthidens , P. australis , P. bothrorrhinos , P. cephalus , P. cordilluminatus , P. lavenbergi , P. obtusifrons , P. odontoglossum , P. paxtoni , P. pierbartus , P. sagamianus , P. scriptus and P. scutatus ; present as white line on body and head in P. parini ; absent in P. aphos .

Color. Recently collected specimens with body and head uniformly black or dark brown, except for small white area on top of head, over epiphyseal branch. Pigmentation of mouth and fins detailed in species account. Preserved specimens often faded light brown, or bleached white.

Distribution. The genus Pseudoscopelus is distributed in the Atlantic, Indian, Pacific, and Southern Oceans, including polar zones from 60º N to 60º S. It is absent from the enclosed seas, such as the Baltic, Mediterranean, Black, and Red Seas, and some almost enclosed seas, such as the Persian Gulf and Gulf of Oman.

Remarks. Among chiasmodontids, photophores are exclusive to the genus Pseudoscopelus , but are absent in P. aphos and vestigial in P. parini . For the remaining species, the arrangement of photophores groups is diagnostic to the species group level and even for some species. It is not clear so far whether the lack of photophores in certain species is plesiomorphic or a reversal.

Figures

FIGURE 1. Schematic drawing showing all the photophore groups present in the genus Pseudoscopelus. Photophores on head: anteromaxillary photophores (amf); anteropreopercular photophores (apf); dorsonasal photophores (dnf); interorbital photophores (inof 1, 2); longitudinal preopercular photophores (lpf); maxillary photophores (mxf); opercular photophores (opf); posteromandibular photophores (pmf); postorbital photophores (pof); posteropreopercular photophores (ppf); ventro-nasal photophores (vnf). Photophores on body: isthmus photophores (if); lateral pelvic-fin photophores (lvf); pectoral-fin axillary photophores (paf); pectoral-fin photophores (pf); precaudal-fin photophores (prcf); prepelvicfin photophores (prvf); postpelvic-fin photophores (ptvf); random trunk photophores (rtf); anal-fin photophores (saf); supracaudal-fin photophores (scf); suprapectoral-fin photophores (spf); suprapelvic-fin photophores (svf); transverse pelvic-fin photophores (trf); pelvic-fin axillary photophores (vaf); pelvic-fin photophores (vf).

FIGURE 3. Schematic drawing of premaxilla (A) and dentary (B): left side with teeth illustrated and right side with site for tooth attachment represented by circles with diameter proportional to the tooth base width. The teeth filled with red represent the lateral series; with blue, middle series; with yellow, mesial series; with green, fang; and with white, canine; dashed lines represent the limits of premaxillary regions; C stands for canine, and F, for fang.