Lernanthropus hexodonis Kovaleva, 2025

Lernanthropus hexodonis Kovaleva , sp. nov.

( Figs 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 )

Lernanthropus nemipteri View in CoL : Kazachenko et al. 2018: 35; 2020:18; Hà et al. 2020: 212-214. Misidentification.

Type material. Holotype ♀ from Nemipterus hexodon ( Nemipteridae ), Vietnam, Nha Trang ( South China Sea), 13.X.2017, 1 ♀ in 1 of 5 examined fishes, gill lobes, collected by V.N. Kazachenko, [ EATB].

Diagnosis. Female of the new species easily differs from those of other species by the following characters: setules on the antennule are thick, in the form of cylinder ( Fig. 2D,E View FIGURE 2 ); smaller lobe maxillule tipped with 2 spiniform elements ( Fig. 3B,C View FIGURE 3 ). Male is unknown.

Description of female holotype. Body relatively strong and thick ( Figs 1A–F View FIGURE 1 , 2A–C View FIGURE 2 ). Female body comprising cephalothorax, trunk and small urosome completely concealed beneath dorsal trunk plate. Body length 3.03 mm (from anterior rim of head to end of dorsal trunk plate). Cephalothorax fused with first thoracic segment, separated from trunk by a distinct partition. Cephalothorax 0.75x 0.83 mm. From top of cephalothorax, on anterior margin, there are two conical projections and a small hard shield. From base, hard shield descends, forming margins and a second hard shield. Lateral margins of cephalothorax form rounded, ventrally directed lobes. Antennule is located between conical projections and hard shield ( Fig. 2A,E View FIGURE 2 ). On dorsal side, second and third thoracic segments of trunk are separated dorsally into two distinct segments ( Fig. 2C View FIGURE 2 ). Posterior part (fourth pedigerous somite) covered by dorsal trunk plate. Dorsal trunk plate entirely concealing urosome and 4–5 legs in dorsal view. Urosome comprising fifth pedigerous somite, genital complex and abdomen, all fused. Trunk without dorsal trunk plate 0.68х 0.62 mm and width with protopodites 3 legs 2.37 mm. Dorsal trunk plate abruptly constricted at two-thirds of its distance from base, and further on narrows to end in a distinctly bilobed broad apex. Protopodites 3rd legs are formed on sides of front part of trunk (second and third pedigerous somites), very similar to plates, placed obliquely and projecting slightly forwards and considerably backwards ( Figs 1 A–C,E View FIGURE 1 , 2A,C View FIGURE 2 ). Dorsal trunk plate and protopodites of 3rd legs rigid, ornamented with spinules in dorsally. Dorsal trunk plate 1.60х 1.12 mm. Protopodites of 3rd legs 1.25 (0.83) mm. Fifth thoracic segment is fused with genital complex. Abdomen bearing paired caudal rami. Genital complex and abdomen 1-segmented.Abdomen 0.07х 0.12 mm. Paired caudal rami elongate, narrowing progressively distally; indistinctly 3-segmented; densely ornamented with setae ( Fig. 3K View FIGURE 3 ). Paired caudal rami 0.10х 0.03mm. First segment armed with two bristles on proximal part; second segment armed with two bristles of unequal length; distal end of third segment armed with two bristles. Egg sac not seen.

Antennule ( Fig. 2D,E View FIGURE 2 ) distinctly 7-segmented; some segments with irregular cuticular thickening; setal formula: 0, thick in form of a cylinder 1+ 2, thick in form of a cylinder 2+1, 2, thick in form of a cylinder 1, 4+thick in form of a cylinder 2, thick in form of a cylinder 3+1. Parabasal flagellum ( Fig. 2G View FIGURE 2 ) with swollen base and slender, short, slightly curved distal part; located at base of conical processes between antennule and antenna ( Fig. 2F,G View FIGURE 2 ). Antenna ( Fig. 2I View FIGURE 2 ) massive, 2-segmented; first segment armed with papilliform element proximally on medial surface, second segment armed with small process. Postantennal proces s ( Fig. 3A View FIGURE 3 ) is situated just posterior to antenna, towards lateral margin of anteroventral cephalic region, horseshoe shape without ornamentation. Mandible stylet like, armed with 8 marginal teeth distally ( Fig. 2H View FIGURE 2 ). Maxillule ( Fig. 3B,C View FIGURE 3 ) bilobate; smaller outer lobe (palp) tipped with unequal 2 setal elements; larger inner lobe (praecoxa) bearing 3 unequal hirsute elements apically. Maxilla ( Fig. 3D,E View FIGURE 3 ) 2- segmented, comprising proximal syncoxa (lacertus) and distal basis (brachium); basis bearing prominent scattered sharp teeth, ornamented with patch of spinules distally and bearing bifid element plus process originating adjacent to terminal claw; claw ornamented with sharp denticles along both edges. Maxilliped ( Fig. 3F View FIGURE 3 ) 3-segmented, proximal segment decorated with fine spinules, robust with small myxal process; claw strongly curved. Last segment is short and unarmed. Leg 1 with protopodal part fused to somite; members of leg pair joined by slender intercoxal sclerite ( Fig. 3G View FIGURE 3 ). Each leg biramous; rami 1-segmented; basipod and both rami carry prominent scattered sharp teeth. Exopod broad, is roughly rectangular and its oblique distal border carries five subequal bilaterally barbed teeth. Endopod pear-shaped, bearing single terminal seta. On inner margin of basipodite there is a tubercle with a spine. Leg 2 ( Fig. 3H View FIGURE 3 ) biramous with 1-segmented rami; mounted on inflated subspheri cal prominence derived from incorporated protopod and armed with outer seta; lacking intercoxal sclerite; exopod represented by rod-like process bearing 2 conical spines; both rami ornamented with spinules. Leg 3 ( Figs 1D View FIGURE 1 , 2B View FIGURE 2 ) biramous, are modified into large U-shaped laminae by complete fusion of rami, comprising a large lateral, fleshy lamella (exopod) and inner lobes are linguiform a small, blunt process (endopod) fused along medial margin with other member of same pair. Leg 4 ( Fig. 3I View FIGURE 3 ) biramous with both rami forming elongate flattened processes, proximal part broader but tapering in distal half to narrower distal part; lobes of similar length but outer (exopodal) lobe slightly longer than inner (endopodal). Leg 5 represented by small lobe bearing single seta on apex ( Fig. 3J View FIGURE 3 ). Male. Unknown.

Etymology. The name of the new species refers to the species name of its only known host.

Remarks. The new species most closely resembles Lernanthropus caudatus Wilson, 1922 , L. longilamina Pearse, 1951 , L. kazachenkoi sp. nov., L. manicatus Wilson, 1935 , L. marginatus sp. nov., L. nemipteri , Lernanthropus pagelli Krøyer, 1863 , and L. scribae Krøyer, 1863 . These species are similar in external body structure, with the sides of the second and third thoracic segments forming protopodites of 3rd leg that resemble plates, wings, or lateral margins that extend into long posterolateral processes.

Differs from L. caudatus by shape of dorsal trunk plate; mainly in the structure of antennule; antenna; maxillule; maxilla; leg 2. L. caudatus lacks a parabasal flagellum, whereas L. hexodonis sp. nov. has one. L. longilamina differs from L. hexodonis sp. nov. in the shape of the cephalothorax. In L. longilamina the antennule is 6-segmented, and in the new species – 7-segmented. Second segment of antennae of L. longilamina have two small process in the middle, while L. hexodonis sp. nov. has one. L. longilamina exopodite leg 2 ornamented with spinules with one conical spine, and one long setae on endopodite, whereas the new species bears two conical spines on distal end of exopodite, endopodite unarmed. Exopodite of leg 4 in L. longilamina bears abearing a short spine at distal end, in L. hexodonis sp. nov. spine are absent.

In L. hexodonis sp. nov. the second and third thoracic segments of the trunk are separated on the dorsal side, whereas in L. kazachenkoi sp. nov. and L. marginatus sp. nov. they are fused. Differs from L. kazachenkoi sp. nov. in the structure of the antennule. Postantennal process in horseshoe shape in L. hexodonis sp. nov. and in L. kazachenkoi sp. nov. two discs are crescent shaped and connected. In L. hexodonis sp. nov. antenna 2-segmented, second segment armed with small process, whereas in L. kazachenkoi sp. nov. 4-segmented, second segment armed with 4 spines, distal part with circular ridges of different size. The most readily observable difference between L. hexodonis sp. nov. and L. kazachenkoi sp. nov. is structure of maxillule: in L. hexodonis sp. nov. comprising small inner lobe tipped with two spiniform elements, whereas L. kazachenkoi sp. nov. has one. The species L. hexodonis sp. nov. and L. kazachenkoi sp. nov. can be distinguished by the structure of the maxilla: in L. hexodonis sp. nov. it bears a bifid element plus a process originating adjacent to the terminal claw, whereas L. kazachenkoi sp. nov. it is ornamented with a slender process; the first segment of maxillipede decorated with fine spinules in L. hexodonis sp. nov. and in L. kazachenkoi sp. nov. unornamented; in L. hexodonis sp. nov. the exopodite of the 2nd leg is armed at the distal end with 2 conical spines, whereas in L. kazachenkoi sp. nov. four conical spines are present; paired caudal rami of L. hexodonis sp. nov. are densely ornamented with setae, whereas those of L. kazachenkoi sp. nov. are unornamented; leg 5 single seta on apex in L. hexodonis sp. nov., in L. kazachenkoi sp. nov. seta are no bristles.

The new species differs from L. manicatus in several characters: dorsal trunk plate ends in a distinctly bilobed broad apex and in L. manicatus without bilobed apex; exopod 1st leg armed with five barbed teeth, L. manicatus has six spines; exopodite of 2nd leg armed at distal end with 2 conical spines, whereas in L. manicatus four small, and endopodite with one stout spine. The 4th legs of L. hexodonis sp. nov. barely reaches the constriction of the dorsal trunk plate, whereas in L. manicatus the protruding exopods are considerably longer than the endopods and protrude behind the dorsal trunk plate for a third of their length, while only the extreme tips of the endopods are visible on the dorsal trunk plate.

Shape of the cephalothorax allows to distinguish L. hexodonis sp. nov. from L. marginatus sp. nov. Antennules of L. hexodonis sp. nov. are 7-segmented and have a thin and a thick cylindrical setal, whereas those of L. marginatus sp. nov. are 8-segmented and all thin. Maxillule in L. hexodonis sp. nov. has a small inner lobe with two spiniform elements, whereas L. marginatus has one. Exopodite of the maxillule of L. hexodonis sp. nov. has no small spines, unlike L. marginatus sp. nov., which is covered with them. Endopodite of the 1st leg of L. hexodonis sp. nov. is armed with a long, curved terminal seta, whereas this seta is absent in L. marginatus sp. nov. In L. hexodonis sp. nov. the exopodite of the 2nd leg is armed at the distal end with 2 conical spines, whereas in L. marginatus sp. nov. there are five.

The new species can be readily distinguished from L. nemipteri by the shape of the cephalothorax. There is a marked difference between L. hexodonis sp. nov. and L. nemipteri in the shape of the cephalothorax, and in the structure of the antennule. in L. hexodonis sp. nov. the antennule is 7-segmented, with 9 thick cylindrical bristles and 10 thin bristles, and in L. manicatus it is 8-segmented, are missing their thick cylindrical bristles. L. nemipteri differs from the new species by maxillule has a small inner lobe with one a spiniform element. Third maxilliped segment of L. hexodonis sp. nov. is short and unarmed, and in L. nemipteri carries a prominent blunt accessory process. Endopodite of the 1st leg in the new species is armed with a long curved terminal seta, whereas that of L. nemipteri has a short spine. In L. hexodonis sp. nov. exopodite 2nd leg at distal end carries two chitinous spines, one of which is twice as long as other, whereas those of L. nemipteri exopod with five small spines. 5th leg in L. hexodonis sp. nov. representing a short cylindrical fleshy process with a long seta, in L. nemipteri unarmed.

The new species can be distinguished from L. pagelli by the shape of the cephalothorax: cephalothorax with a small notch on the sides, which lacks in L. hexodonis sp. nov. L. pagelli has a pair of rather large, ovoid projections between the maxilla and maxilliped, which are armed with a pair of small spines, whereas L. hexodonis sp. nov., L. kazachenkoi sp. nov., L. marginatus sp. nov. do not have such projections. During the study of the family Lernanthropidae , I did not come across such growths, perhaps Krøyer was describing postantennal process, which are present in our new species. 4th legs of L. hexodonis sp. nov. reach the constriction of the dorsal trunk plate, branching without spines at the distal ends, and in L. pagelli they reach the end of the dorsal trunk plate, the outer branch at the distal end has a small spike. 5th leg in L. hexodonis sp. nov has a long seta at the distal end, in L. pagelli unarmed.

The new species differs from L. scribae differs by the following: 1) L. scribae it has a small notch on the sides of the cephalothorax, while L. hexodonis sp. nov. has no; 2) in L. scribae posterior edges of the dorsal trunk plate without bilobed apex, and in the new species dorsal trunk plate is sharply narrowed at two thirds of the distance from the base, ending in a distinct bilobed apex.

Pillai (1985) stated that the lateral plates (elytra) formed on the sides of the second and third thoracic segments of the trunk, arranged obliquely, directed anteriorly and considerably posteriorly, are enlarged protopodites of the 3rd legs. I agree with this opinion. In L. kazachenkoi sp. nov. and L. marginatus sp. nov., the protopodites of the 3rd legs are folded along the body, but sometimes they come out at right angle to the body axis at the same level as the dorsal surface in the species L. hexodonis sp. nov. ( Fig. 1 A,D View FIGURE 1 ). The same configuration of protopodites of the 3rd legs was described by Wilson (1935). The dorsal plate is also similar to that in species from the genus Aethon .