Monomitopus ainonaka, Girard & Carter & Johnson, 2023

Girard, Matthew G., Carter, H. Jacque & Johnson, G. David, 2023, New species of Monomitopus (Ophidiidae) from Hawai‘i, with the description of a larval coiling behavior, Zootaxa 5330 (2), pp. 265-279 : 268-273

Monomitopus ainonaka sp. nov.

English name: Ai’s cusk urn:lsid:zoobank.org:act:F9B58F1C-89E3-4A1C-8D4C-D00BFC93CBE6

Holotype. USNM 267749 ; 135 mm SL ( Figs. 1A–B , 2 ; Table 2 ).

Collection information: 16 July 1972; 768–796 m depth. Station data from NOAAS Townsend Cromwell lists collection as North of Maui , Hawaiʻi, 21º 09’ N 156º 11’ W ( Fig. 2A ). Hand-written label found with the specimen lists collection at 21° 07’ N 156° 12’ W, but we think this is inaccurate based on the station information from preceding and subsequent trawls of cruse 59 .

Counts: 105 dorsal-fin rays, 92 anal-fin rays, 35 pectoral-fin rays, 8 caudal-fin rays, 1 pelvic-fin ray, 12 precaudal vertebrae, 62 total vertebrae, 8 branchiostegals (6 epihyal, 2 ceratohyal), 5 rudimentary gill rakers on hypobranchial 1, 17 elongate gill rakers on ceratobranchial 1 (including angle), 4 club-like gill rakers on epibranchial 1, anteriormost anal-fin ray ventral to 18 th dorsal-fin ray and 14 th vertebra, anteriormost dorsal-fin ray above fourth vertebra.

Measurements as % SL: 22.5 head length, 4.3 snout length, 11.4 upper jaw, 5.2 horizontal eye diameter, 36.0 preanal distance, 22.3 predorsal distance, 17.0 distance from base of pelvic-fin ray to vent, 16.2 body depth at level of vent, 12.1 pelvic-fin ray length, 6.2 interorbital width, 12.9 postorbital length.

Diagnosis. A species of Monomitopus with weak, flap-like spines on preopercle, downward-sloped dorsal profile between eye and origin of the dorsal fin as adult. It can be further differentiated from species of Monomitopus in the M. nigripinnis group occurring in the Pacific (i.e., M. garmani [Celebes to New Caledonia] and M. malispinosus [Eastern Central Pacific]) by greater numbers of dorsal-fin rays (105 vs. 94–103 in M. garmani ; 96–99 in M. malispinosus ), anal-fin rays (92 vs. 76–87 in M. garmani ; 81–84 in M. malispinosus ), pectoral-fin rays (35 vs. 28–32 in M. garmani ; 26–29 in M. malispinosus ) and smaller ratio of eye diameter relative to head length (4.4 vs. 4.6–5.6 in M. garmani ; 4.6–4.9 in M. malispinosus ; see Table 2 ).

Adult description. Head compressed, dorsal margin sloped downward to snout. Nostrils widely separated, anterior with tube-like flap, posterior close to eye. Eye longer than deep, about four times into head length, film of tissue covering eye. Infraorbital sensory canal not evident. Broad flap of tissue covering dorsal half of maxilla. Mouth large, slightly oblique; lower jaw inferior. Distal end of maxilla dorsoventrally expanded with concave posterior margin. Supramaxilla present. Premaxilla and dentary with small, numerous, villiform teeth all subequal in size. Intraoral teeth similar in size to oral teeth. Palatine tooth patch elongate, rounded anteriorly, tapering posteriorly; vomerine tooth patch with anterior bulbous patch, tapering to lateral arms. Dentition interrupted across upper and lower jaw symphyses. Eight branchiostegals (full complement), 2 on ceratohyal. One elongate basibranchial 3 tooth plate posterior to basihyal, anteriorly broad and tapering posteriorly, overlapping basibranchials 1 and 2. Tooth plates on epibranchials 2, 3, and 4. Pseudobranch short and rudimentary, with two filaments. Preopercle with four weak, flap-like spines; easily bent if probed. Opercle strongly bent at acute angle with posterior spine piercing skin. Opercular spine narrowly ovoid in cross-section. Opercular tissue connected dorsally to body via broad flap of skin. Head scaleless, body with deciduous scales. Many lateral-line scales missing, but canal appears to end near level of anus, as in other species of Monomitopus (see Carter & Cohen 1985). Pectoral fin shallow, all fin rays subequal. Pelvic girdle minute, inserts immediately behind cleithral symphysis. Single pelvic-fin ray thin with broad base. Gut moderate in size. Dorsal- and anal-fin rays approximately subequal in length to each other and along each fin. Caudal-fin rays elongate with truncate posterior margin. Body pale brown, speckled with melanophores; opercular tissue dark.

Non-type larval specimen. USNM 454563 ; 14.4 mm SL; preflexion; 11 November 2021; off Kona , Hawaiʻi; 0–18 m over a depth 1500 m ( Figs. 1C–D , 2A , 4A ) .

Counts: 100+ dorsal-fin rays (posteriormost rays undifferentiated), 82+ anal-fin rays (posteriormost rays undifferentiated), 35 pectoral-fin rays, caudal-fin rays undifferentiated, 1 pelvic-fin ray, 12 precaudal myomeres, 55+ total myomeres (posteriormost myomeres indiscernible), anteriormost anal-fin ray ventral to 17 th dorsal-fin ray, anteriormost dorsal-fin ray above fourth myomere.

Larval description. Head large, deeper than long, body tapering posteriorly to narrow caudal fin. Eye small, deeper than wide, ovoid. Maxilla and premaxilla at oblique upturned angle. Premaxilla and dentary with small, distantly spaced teeth. Distal end of maxilla dorsoventrally expanded, posterior margin convex. Posterior tip of premaxilla nearly reaching posterior margin of maxilla. Supramaxilla present. Large rostral cartilage attached to ascending process of premaxilla. Nostrils in close proximity, anterior to eye. Symphysis of dentary with slight ventral expansion. Opercular series without spines. Body and head scaleless. Dorsal fin inserting in line with pectoral-fin base. Pectoral fin large, fan like, with broad base. Pectoral-fin membrane between each ray entire. Pelvic-fin ray minute, inserting immediately behind cleithral symphysis. Gut moderate, internal to body and lacks exterilium morphology of some other larval ophidiids ( Fraser & Smith, 1974; Fahay & Nielsen, 2003; Okiyama, 2014). Internal gut loops not evident. Anus just anterior to anal fin with no external extension. Dorsal, anal, and caudal fins confluent. Dorsal- and anal-fin rays approximately subequal in length to each other and along each fin. Larval caudal fin not fully formed. Melanophores few and minute on head and body, distributed randomly across larva.

Following description based on in-situ image of larval specimen. Specimen speckled with melanophores across head and body, distributed evenly across the body, denser above braincase. Dorsal and anal fins banded, with dorsalfin-ray pattern: three-to-four rays pigmented, three-to-four rays lacking pigment, etc.; anal-fin-ray pattern, four-tofive rays pigmented, three-to-four rays lacking pigment, etc.

Genotypic data. The barcode sequence from the larva ([OR263969]) was compared to sequences from six of the 14 species of Monomitopus and was ≥6.5% divergent (see Table 1 for BOLD and GenBank accession numbers). Our phylogenetic analysis resulted in a single optimal topology (ln L = -2418.422) with M. ainonaka in a clade of Pacific species of Monomitopus , sister to M. kumae . Despite the moderate branch support, all tree searches resulted in the same topology shown in Fig. 2B but with different branch lengths.

Distribution. Both larval and adult specimens have been collected from waters surrounding the Hawaiian Islands, and this is the only confirmed locality for this species. Unidentified individuals of Monomitopus have been recorded from Wake Island as well as Howland and Baker Islands at depths of 1000–3000 meters by NOAA Ocean Exploration (2016, 2017), but these individuals were not collected. We have examined both still and video images of these individuals, and they appear more similar to Monomitopus garmani or M. microlepis based on fin-ray counts (i.e., ~102–103 dorsal-fin rays, ~89–90 anal-fin rays, ~30 pectoral-fin rays) and horizontal eye diameter in head length (~5.1–5.5; see Table 2 ) than to the new species described from Hawai’i. Additional sampling from these regions is needed to understand the full distributions of species in this genus.

Etymology. Named for Ai Nonaka (USNM) for her interest in ophidiid larvae and dedication to the discovery, identification, and curation of larval fishes.

Figures

FIGURE 1. Adult and larva of Monomitopus ainonaka sp. nov. from the Hawaiian Islands. A) USNM 267749 holotype. B) Radiograph of holotype. C) Larva, USNM 454563, captured by A. and N. Deloach offshore of Kona, Hawaiʻi, 11 November 2021. D) Close-up of USNM 454563 head.

FIGURE 2. A) Map of collection localities for M. ainonaka sp. nov. Star indicates locality of holotype (USNM 267749). Circle indicates locality of larva (USNM 454563). B) Phylogeny of Monomitopus spp. Bootstrap values>50% listed. Silhouettes indicate developmental stage of the genetic voucher.

FIGURE 4. Blackwater photos of larval Monomitopus spp. in Ianniello’s coil.A) M. ainonaka sp. nov. USNM 454563, captured and photographed by A. Deloach, N. Deloach, and S. Kovacs off Kona, Hawaiʻi, 11 Nov 2021. B) and C) M. agassizii, USNM 465352, captured and photographed by R. Collins, A. Deloach, and N. Deloach off West Palm Beach, Florida, 7 May 2021. D) M. sp. off Anilao, Philippines, ~10 m depth over 150 m, 5 Dec 2019. Photo © S. Kovacs. E) M. cf. magnus off West Palm Beach, Florida, ~15 m depth over 220 m, 25 Feb 2021. Photo © S. Kovacs. F–H) M. agassizii, USNM 465385, transitioning from dextral coil, to uncoiling, to sinistral coil, captured and photographed by D. Devers off West Palm Beach, Florida, 24 Mar 2021. I–P) Uncaptured M. agassizii, repeatedly transitioning coil handedness over three-minute interval (see text for time intervals), photographed by L. Ianniello off West Palm Beach, Florida, 9–12 m depth over 220 m, 2 Mar 2021. Photos © L. Ianniello.

Tables

TABLE 2. Counts and measurements for species of Monomitopus. Values in parentheses indicate counts or measurements from holotype. Values in brackets indicate counts or measurements from syntype. In-table citations for holotype counts: 1Nielsen, 1971; 2Garman, 1899; 3Alcock, 1889.

Species of Monomitopus	Dorsal-fin rays	Anal-fin rays Pectoral-fin rays		Caudal-fin rays	Precaudal vertebrae	Total vertebrae	Elongate gill rakers on lower arm of first gill arch (including angle)	Horizontal eye diameter in head length
M. ainonaka sp. nov.	(105)	(92)	35 (35)	(8)	12 (12)	(62)	(17)	(4.4)
M. agassizii	99–107	83–89	31–34	7–8	12–14	61–64	17–19	4.0–4.2
M. americanum 1	100–108 (102)	82–89 (83)	28–32 (29–30)	8 (8)	13–14 (14)	59–63 (62)	16–17 (16)	3.8
M. conjugator	(90) [96]	(72) [76]	(28) [34]	[8]	[13]	[59]	(12) [14]	4.3–4.7 [4.3]
M. garmani	92–103 (96)	76–87 (80)	28–32 (31)	7–8 (8)	13–14 (13)	57–62 (59)	(22)	4.6–5.6 (5.3)
M. kumae	100–105 (105)	85–88 (88)	29–33 (31)	(8)	12–14 (14)	55–64 (62)	(18–19)	4.6–6.3 (6.3)
M. longiceps	(97)	(82)	(28)	(9)	(12)	(57)	(22)	(4.9)
M. magnus	104–109 (109)	85–92 (92)	27–31 (29)	8 (8)	15 (15)	61–65 (65)	10–11 (10)	4.6–5.3 (5.0)
M. malispinosus 2	96–99 (99)	82 (82)	26–29 (26)	8 (8)	12	58	18 (18)	4.6–4.9 (4.8)
M. metriostoma	95–103	79–87	27–32	8	12–13	58–63	17–18	3.6–4.2
M. microlepis	97–102 (98)	81–84 (82)	29–31 (28)	8 (8)	14 (14)	58–61 (60)	(17)	4.2–5.6 (5.6)
M. nigripinnis 3	91–95 (95)	76–85 (85)	27–30 (28)	8	12–13	54	17–18	3.9–4.8
M. pallidus	87–96 (93)	71–77 (76)	26–28 (28)	7–8 (7)	13 (13)	53–57 (55)	(16)	3.4–4.0 (3.7)
M. torvus 2	98–111 (109)	83–95	28–33 [33]	8	12–15 [13]	58	17–18 (18) [17]	4.0–4.6
M. vitiazi 1	93–98 (93)	78–84 (78)	28–33 (28–29)	8 (8)	13 (13)	56–58 (56)	13–14 (13)	4.4–4.9 (4.9)