Introduction

The deepwater cardinalfishes of the family Epigonidae Poey 1861 are a group of bathydemersal marine fishes occurring on the continental and island slopes of all tropical and warm temperate oceans (Okamoto et al. 2011; Van der Laan et al. 2014; Nelson et al. 2016), at depths from 100 to more than 1,000 m. The family is currently arranged in the order Perciformes (Nelson et al. 2016). The Epigonidae comprises six valid genera, including Brephostoma Alcock 1889 (a single valid species, the circumtropical Brephostoma carpenteri Alcock 1889), Epigonus Rafinesque 1810 (40 valid species), Florenciella Mead and De Falla 1965 (a single species, Florenciella lugubris Mead and De Falla 1965, in the Indo-West Pacific), Microichthys Rüppell 1852 (3 valid species in the northeastern Atlantic and Mediterranean), Rosenblattia Mead and De Falla 1965 (a single, southern circumglobal species, Rosenblattia robusta Mead and De Falla 1965), and Sphyraenops Gill in Poey 1861 (a single species, Sphyraenops bairdianus Poey 1861) (Fricke et al. 2022b). Microichthys is a genus of small species living pelagic above the continental or insular slope in the northeastern Atlantic and Mediterranean Sea.

The discovery of a new specimen of Microichthys coccoi Rüppell 1852 from the Balearic Islands (Ordines et al. 2018) initiated a revision of the genus, which was subsequently published by Fricke et al. (2020). Microichthys is known from very few specimens and includes the following species: Microichthys atlanticus Fricke et al. 2020 from the Azores; M. coccoi from the Mediterranean Sea; and Microichthys sanzoi Spartà 1950 from Sicily, Mediterranean Sea.

During the ICES coordinated International Blue Whiting Spawning Stock Survey (IBWSS; ICES 2021) off Ireland, a relatively large specimen of Microichthys was collected that turned out to represent an undescribed species. The new species is described and compared with other species in the present paper.

Materials and methods

The holotype of this new species was collected with a 4300M midwater trawl. The circumference of the opening of the net was 860 m; the vertical opening in the range of 30–70 m; the stretched mesh size in the codend 40 mm. Fishing took place with towing speed of 3.5–4.0 knots at 0–537 m depth.

Methods follow Mayer (1974); fin-ray counts follow Fricke (1983). The starting point for length measurements is the middle of the upper lip. The standard length (measured from the middle of the upper lip to the middle of the urohyal/caudal-fin base) is abbreviated SL. The lateral-line scale formula counts the scales on the body + those on the caudal fin. The number of vertebrae and ribs was examined using micro-CT scans. Species classification and nomenclature follow Fricke et al. (2022a). References and journals are cited according to Fricke (2022) and Fricke and Eschmeyer (2022b) . The map was composed using QGIS 2.12.2.

The holotype of the new species is deposited in the Staatliches Museum für Naturkunde in Stuttgart (SMNS). Comparative materials are listed below. Abbreviations of museum collections (see below) follow Fricke and Eschmeyer (2022a); the Institute for Environmental Protection and Research, Laboratory of Milazzo, Italy, is abbreviated ISPRA.

Microichthys grandis sp. nov.

(New English name: Great deepwater cardinalfish)

(Figs. 1, 2, 3, 4; Tables 1, 2)

Fig. 1
figure 1

Microichthys grandis n. sp., SMNS 27380, holotype, 54.5 mm SL, Northeastern Atlantic Ocean, continental slope off southwest coast of Ireland, 324 km west of Dingle. Fresh colouration. Lateral view. (Photograph: Bram Couperus)

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Fig. 2
figure 2

Microichthys grandis n. sp., SMNS 27380, holotype, 54.5 mm SL, Northeastern Atlantic Ocean, continental slope off southwest coast of Ireland, 324 km west of Dingle. Colouration of preserved specimen

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Fig. 3
figure 3

Microichthys grandis n. sp., SMNS 27380, holotype, 54.5 mm SL, Northeastern Atlantic Ocean, continental slope off southwest coast of Ireland, 324 km west of Dingle. Sketch of interorbital region (dorsal view of head); ior interorbital ridge. Scale indicates 5 mm

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Fig. 4
figure 4

Microichthys grandis n. sp., SMNS 27380, holotype, 54.5 mm SL, Northeastern Atlantic Ocean, continental slope off southwest coast of Ireland, 324 km west of Dingle. Sketch of opercular region (lateral view of head); ops opercular spine, pr preopercle. Scale indicates 5 mm

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Table 1 Microichthys grandis n. sp., holotype, northeastern Atlantic Ocean, off Ireland: measurements and proportions
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Table 2 Comparison of selected characters among the species of the genus Microichthys
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Holotype. SMNS 27380, 54.5 mm SL, northeastern Atlantic Ocean, continental slope off southwest coast of Ireland, 324 km west of Dingle, 52°02.74′N, 14°57.70′W, (0–)537 m depth, R/V Tridens, Bram Couperus, International Blue Whiting Spawning Stock Survey, 24 Mar. 2022.

Diagnosis. A species of Microichthys with the following combination of characters: dorsal-fin rays VII–I, 8; total pectoral-fin rays 18; total gill rakers 25; pyloric caeca 3; pored lateral-line scales 41 + 2; vomerine teeth 6, in a single row; denticles on posterior margin of preopercle 3, on lower margin 3; nape not humped; bony rim of orbit raised above dorsal profile; ridge present in interorbital region; lateral line gradually running towards midline of body, crossing last dark lateral bar above its centre; oblique dark lateral bars 20, lower counter-bars 12; vertical black bar on caudal peduncle present, together with another black bar below the end of the second dorsal fin; peritoneum silvery; pelvic fin shorter than head length; pelvic fins black.

Description. D VII–I, viii; A II, v+ (posterior part damaged); P1 ii,14,ii; P2 I, 5; C (v),i,14,i,(vi); gill rakers (first arch) 5 + 20 = 25; branchiostegal rays 6; pored lateral-line scales 41 + 2; pyloric caeca 3; vertebrae 10 + 14; first pterygiophore supporting 1 spine. Measurements of the holotype are given in Table 1.

Body slender, laterally compressed, deepest at beginning of second dorsal-fin base; nape not humped. Head large, compressed. Snout short, upper lip slightly pointed, its length 0.7 in interorbital width. Eye large, round, orbital diameter significantly larger than snout and postorbital length; bony rim of orbit raised above dorsal profile; interorbital region with one ridge, as long as half of eye diameter, above the anterior margin of the eyes (Fig. 3). Mouth large, terminal; gape angular; posterior margin of maxilla extending to level of anterior margin of pupil; lower jaw slightly protruding anteriorly when mouth closed; anteriorly projecting teeth or nub-like structure absent on symphysis of lower jaw. Teeth only present anteriorly on jaws. Lower jaw with a single row of small curved teeth. Upper jaw with a single row of small, curved teeth. Vomer with a single, anterior row of 6 relatively large, curved teeth. Opercular spine present, pungent, forming ridge; preopercular margin with 3 small denticles, lower margin with 3 small denticles (Fig. 4).

Origin of first dorsal fin well behind lower end of pectoral-fin base. First and second dorsal fins separated by a gap of double pupil diameter; isolated dorsal-fin spine absent. Origin of anal fin below base of second soft ray of second dorsal fin. Anal fin with two spines, second spine sharp and robust. Uppermost and lowermost two pectoral-fin rays unbranched, the remainder branched; pectoral-fin rays attached in pairs to shoulder girdle. Posterior tip of pectoral-fin rays not reaching vertical line drawn from anus. Upper margin of pectoral-fin base on level of lower edge of orbit. Pelvic fins not reaching to anus when laid back. Caudal fin forked.

Anus located on level of origin of second dorsal fin. Ribs present on last abdominal vertebra. Scales deciduous, small, all ctenoid, covering whole body except area anterior to rim of orbit and surfaces of jaws; scales also present on bases of second dorsal, anal, and caudal fins; lateral line starting immediately behind upper operculum and reaching the caudal peduncle; gently curving along the sides of the body; lateral line gradually running towards midline of body on caudal peduncle; lateral line crossing last dark lateral bar on side of body above centre of bar; series of pored lateral-line scales complete, scales on body ca. 41, pored scales on caudal fin 2. No trace of luminous organ around belly or visceral organ.

Colour when fresh (Fig. 1). Head and body greyish rose, iris posterodorsally dark grey, otherwise silver, postorbital region black, mouth cavity rose, operculum black, peritoneum silvery; scales on sides of body surrounding lateral line with dark grey margins, but lateral line whitish; sides of body with 20 oblique black bars along the myomeres (anterior bars dark grey to silvery), posterior 12 bars continuing to the lower side of the body; lateral bars between interdorsal space and caudal peduncle 12 in upper series; caudal peduncle with a broad black bar; another, narrower, vertical black bar below end of second dorsal fin; fin membranes translucent, dorsal fins distally with black pigment; pelvic fin black, anterior margin of spine silvery.

Colour in preservative (Fig. 2). Head and body whitish, iris posterodorsally dark grey, postorbital region black, otherwise whitish, mouth cavity cream, opercle silvery, peritoneum silvery; scales on sides of body surrounding lateral line with dark grey margins, but lateral line whitish; sides of body with 20 oblique black bars (anterior bars silvery), posterior 12 bars continuing to the lower side of the body; lateral bars between interdorsal space and caudal peduncle 12 in upper series; caudal peduncle with a broad black bar; another, narrower, vertical black bar below end of second dorsal fin; fin membranes translucent, dorsal fins distally with black pigment; pelvic fin black, anterior margin of spine silvery.

Distribution. Northeastern Atlantic Ocean: off southwest coast of Ireland (Fig. 5). This new species was collected with a midwater trawl at 0–537 m depth, probably in the lower part of that depth range.

Fig. 5
figure 5

Geographical distribution of the species of Microichthys in the northeastern Atlantic and Mediterranean Sea. Microichthys grandis n. sp. (A), M. atlanticus (B), M. coccoi (C), M. sanzoi (D)

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Etymology. This new species, ‘grandis’, means large; it is named after its unusually large body size, compared with other species of the genus. The name is an adjective, the ending masculine when in Microichthys.

Comparisons. Microichthys grandis n. sp. resembles in colouration M. sanzoi from the Mediterranean, but in some counts are more similar to M. atlanticus from the Azores; however, it is distinguished from other species in the genus by 18 pectoral-fin rays (versus 16 in M. coccoi, 20 in M. sanzoi), a total of 6 vomerine teeth in 1 row (versus 12 teeth in 3 rows in M. coccoi, 3 teeth in 1 row in M. sanzoi), a total of 25 gill rakers (versus 20–23 in M. atlanticus, 28 in M. coccoi), a total of 41 + 2 lateral-line scales (versus 43–44 + 2 in M. coccoi, 46 + 2 in M. sanzoi), lateral line gradually running towards midline of body on caudal peduncle (versus abruptly bent towards midline in M. coccoi), lateral line crossing last dark lateral bar on side of body above centre (versus crossing below centre in M. coccoi), bony rim of orbit raised above dorsal profile (versus not raised above dorsal profile in M. atlanticus and M. coccoi), ridge in interorbital region present (versus absent in M. atlanticus and M. sanzoi), a total of 20 dark lateral bars (versus 14–15 in M. sanzoi), dark lateral bars between interdorsal space and caudal peduncle 12 (versus 10 in M. sanzoi), lower counter-bars 12 (versus 7 in M. atlanticus, 8 in M. coccoi, 12 in M. sanzoi), vertical bar across caudal peduncle present (versus absent in M. atlanticus and M. coccoi), peritoneum silvery (versus black in M. coccoi, pale in M. atlanticus), pelvic fin shorter than head length (versus longer in M. sanzoi), pelvic fin black (versus pale in M. atlanticus and M. coccoi). Microichthys grandis n. sp. is the only species of the genus with a black bar blow the end of the second dorsal fin. The species of the genus are compared in Table 2. A revised key to the species of Microichthys is provided below (modified from Fricke et al. 2020).

Discussion

Fraser (1972) and Mayer (1974) were uncertain about the status of Microichthys, as they had not examined any specimens, and considered the genus as a possible postlarval form and junior synonym of Epigonus. In a previous study (Fricke et al. 2020), male and female specimens could be distinguished, the latter characterized by ripe ovaries, and it was concluded that the larger specimens of 16–40 mm SL were adults, and did not exceed about 40 mm SL as a maximum size. The larvae and postlarvae of Microichthys coccoi were examined and illustrated by Gonzales (1946, table 1, figs. 1–7). The present specimen of Microichthys grandis n. sp. even has a length of 54.5 mm SL, which significantly exceeds previous findings. The sex could not be determined in this specimen, as the belly was damaged.

The characters of the four species of Microichthys are compared in Table 2. Although the new species superficially looks like a large specimen of M. sanzoi, it is clearly distinguished from that species by several morphological characters which are certainly independent of a morphological change with growth, if for instance the pectoral-fin ray number (which has no known variation in any known species of Microichthys) would increase with growth, as the smallest species (M. coccoi) has the lowest number, then M. grandis n. sp. should have a higher number than M. sanzoi (but in fact it was a lower number). The species of Microichthys are distinguished by the number of pectoral-fin rays (16 in M. coccoi, 18 in M. atlanticus and M. grandis n. sp., 20 in M. sanzoi), the total number of vomerine teeth (3 in M. sanzoi, 6 in M. atlanticus and M. grandis n. sp., 12 in M. coccoi), and the colour of the peritoneum in preserved specimens (pale in M. atlanticus, silver in M. grandis n. sp. and M. sanzoi, black in M. coccoi), the pelvic-fin length (shorter than head in M. atlanticus, M. coccoi and M. grandis n. sp., longer in M. sanzoi), as well as a number of other characters. Melanism of the gut or peritoneum in deep fishes apparently is a strategy to reduce the transmittance of the bioluminescence of ingested preys that otherwise would make the fish visible to possible predators (Eastman and DeVries 1997; Fishelson et al. 1997). The black peritoneum of M. coccoi may indicate that this species often feeds on bioluminescent resources that apparently would not be as important in the diet of the three other species of Microichthys.

The holotype of M. grandis n. sp. has only VI spines in the first dorsal fin, while other known species of the genus have VII spines; as this character is usually very conservative, we hesitate to use the number of dorsal-fin spines as a criterion to distinguish this species. Additional specimens are needed to confirm this character. However, as species of Microichthys are very rare and at the same time extremely difficult to collect, with the total number of specimens known in the genus equalling less than 15, we decided to describe the new species from the only known specimen.

The holotype of this new species of Microichthys has a standard length of 54.5 mm, and is thus the largest species known in the genus; M. atlanticus only reaches 22.1 mm SL, M. coccoi 25.8 mm SL, and M. sanzoi 40.3 mm SL. The relatively large body size may be correlated to the latitude; M. grandis n. sp. also has the northermost occurrence in this genus, at ca. 52°N, while M. atlanticus occurs at ca. 36–38°N, M. coccoi at ca. 36–39°N, and M. sanzoi at ca. 39°N. A record from the Mid-Atlantic Ridge north of the Azores was assigned to M. atlanticus by Fricke et al. (2020). As we could not examine this specimen, and now know there is another species of Microichthys present in the northeastern Atlantic, we consider this record as questionable; material is needed to confirm its identity.

The depth of collection of the new species, 537 m, appears typical for species of the genus Microichthys. Other species are known from depths of 0–591 m, usually in the lower part of that depth range (but in the Strait of Messina, Italy, sometimes specimens are washed on shore). The holotype of Microichthys grandis n. sp. was accompanied by 816 kg of blue whiting, Micromesistius poutassou (Risso 1827) (family Gadidae) mixed with 5% of smaller, mesopelagic fishes, mainly Arctozenus risso (Bonaparte 1840) (family Paralepididae).

Key to species of the genus Microichthys

  1. 1a.

    Nape straight, not humped; vertical black bar across anterior caudal peduncle present; bony rim of orbit raised above dorsal profile; pelvic fin plain black; peritoneum silver; 3 denticles on lower margin of preopercle; lower lateral counter-bars 12 … 2

  2. 1b.

    Nape slightly or strongly humped; vertical black bar across anterior caudal peduncle absent; bony rim of orbit not raised above dorsal profile; pelvic fin pale (sometimes distally grey); peritoneum either pale or black; 1–2 denticles on lower margin of preopercle; lower lateral counter-bars 7–8 … 3

  3. 2a.

    Pelvin fin shorter than head length; ridge in interorbital region present; total pectoral-fin rays 18; total vomerine teeth 6; lateral-line scales 41 + 2; dark lateral bars on side of body 20 … M. grandis n. sp.

  4. 2b.

    Pelvic fin significantly longer than head length; ridge in interorbital region absent; total pectoral-fin rays 20; total vomerine teeth 3; lateral-line scales 46 + 2; dark lateral bars on side of body 14–15 … M. sanzoi

  5. 3a.

    Total pectoral-fin rays 16; total gill rakers 28; vomerine teeth 12, arranged in three rows; peritoneum black; lateral line on caudal peduncle abruptly bent above midline of body; lateral line crossing below centre of last lateral bar … M. coccoi

  6. 3b.

    Total pectoral-fin rays 18; total gill rakers 20–23; vomerine teeth 6, arranged in a single row; peritoneum pale; lateral line on caudal peduncle gradually running towards midline of body; lateral line crossing above centre of last lateral bar … M. atlanticus

Comparative materials Microichthys atlanticus: MCZ 149633 (holotype, 22.1 mm SL), northeastern Atlantic Ocean, off Azores Islands, 113 km east of São Miguel Island; MCZ 149631 (1 paratype, 17.1 mm SL), northeastern Atlantic Ocean, off Azores Islands, 125 km south of São Miguel Island; MCZ 149632 (2 paratypes, 15.0–17.2 mm SL), northeastern Atlantic Ocean, off Azores Islands, 130 km east of São Miguel Island; MCZ 149634 (1 paratype, 15.8 mm SL), northeastern Atlantic Ocean, off Azores Islands, 273 km eastnortheast of São Miguel Island. Microichthys coccoi: SMF 1069 (holotype, 25.8 mm SL), Mediterranean Sea, Italy, Sicily; HUJ 20683 (1, 25.4 mm SL), Mediterranean Sea, Balearic Islands, south of Mallorca. Microichthys sanzoi: ISPRA MIC-SAN-01 (1, 40.3 mm SL), Mediterranean Sea, Italy, Sicily.