Morimotobathynella koreana Ji, Camacho & Min, 2024

Morimotobathynella koreana Ji, Camacho & Min, 2024 sp. nov.

Figs 1 View Figure 1 , 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5 , 6 Korean name: 한국모리모토옛새우 (신칭 View Figure 6 )

Type locality.

Hongcheon-gun   GoogleMaps (37°41'15.82"N, 127°41'0.53"E), South Korea; collected by G.-S. Min, C.-W. Lee, and H.-M. Yang on 4 March 2016.

Type materials.

Holotype: male ( NNIBRIV 136387 ), dissected on six slides GoogleMaps . Allotype: female ( NNIBRIV 136388 ), dissected on five slides GoogleMaps . Paratypes: 3 females ( NNIBRIV 136389 –136391) GoogleMaps .

Diagnosis.

Antennule and antenna 7 - segmented; antennule much longer than antenna. Antenna exopod without median seta. Mandible: mandibular palp with three articles; pars molaris with two lobes bearing small denticles distally, lacking a prominent terminal tooth. Endopod of thoracopods I – VII 4 - segmented; coxa of thoracopod VII with a strong plumose seta on thoracopod I; sexually dimorphic in thoracopod VIII of males and females. Male thoracopod VIII: massive protopod with penial region forming an “ atrium, ” inner and outer lamellae curving towards the center; basipod with a rounded crest and one distal seta; exopod elongated with a robust apex and elliptical transversal section; endopod absent. Female thoracopod VIII: coxa with a small protrusion with setules, a very large and well-developed epipod exceeding the basipod length, and an exopod with two equal long setae, lacking an endopod. Uropod: sympod with five spines; endopod with long and strong terminal setae; exopod with four setae, two barbed terminally. Furcal rami with the first spine nearly twice as long as the others.

Description.

Adult male. Total body (Fig. 1 View Figure 1 ) 0.9 mm in length. Cylindrical and elongate body with a similar diameter on thoracic and abdominal articles. Head longer than wide. Pleotelson with one small dorsal seta on each side.

Antennule (Fig. 2 A View Figure 2 ) 7 - segmented; the first three articles equal in length to the last four combined; the first article is the longest and the fourth is the smallest; small rectangular inner flagellum; the third article with five smooth setae; the sixth and seventh articles with three aesthetascs of different sizes; antennule much longer than antenna.

Antenna (Fig. 2 B View Figure 2 ) 7 - segmented; without a medial seta on exopod; the first, fourth, and sixth articles similar in length; the second and third articles similar in length and the shortest; fifth article small, measuring just over half length of first article; the last article is the longest, 1 / 3 longer than the first one; setal formula: 0 + 0 / 2 + 0 / 2 + 0 / 2 + 0 / 0 + 0 / 1 + 2 / 4.

Labrum (Fig. 2 C View Figure 2 ). The distal smooth free edge, with central irregular protuberances.

Paragnaths (Fig. 2 D View Figure 2 ) almost rectangular; having setulation on the distal half.

Mandible (Fig. 3 A View Figure 3 ). Palp with three articles, the third article with two claws of different length, the first and second articles rectangular and robust, and the third article small and almost square; masticatory part: incisor process (pars incisiva) with two teeth; accessory incisor process (processus incisivus accessorius) with one tooth and one tiny spine; molar part (pars molaris) with two lobes having small denticles.

Maxillule (Fig. 3 B View Figure 3 ). The proximal endite with four setae, all setulose; the distal endite having five teeth and the distal one with three denticles; three plumose setae of different length on the outer margin.

Maxilla (Fig. 3 C View Figure 3 ) 4 - segmented; setal formula 6, 4, 6, and 4.

Thoracopods I – VII (Figs 3 D, E View Figure 3 , 4 A – D View Figure 4 , 5 A View Figure 5 ). Well developed; thoracopod I – III (Figs 3 D, E View Figure 3 , 4 A View Figure 4 ) progressively longer; thoracopod IV – VI (Fig. 4 B – D View Figure 4 ) of similar length; thoracopod VII (Fig. 5 A View Figure 5 ) longer than the others; thoracopod I – VII with epipod a little longer than half the basipod; coxa with long strong plumose seta on thoracopod I; rectangular basipod with two smooth setae on thoracopods I – IV, with only one seta on thoracopod V – VII.

Exopods of thoracopods I – VII (Figs 3 D, E View Figure 3 , 4 A – D View Figure 4 , 5 A View Figure 5 ) 1 - segmented with barbed setae (two terminal, one dorsal and two ventral) and shorter than endopods of thoracopods I – VII; as long as the first two articles combined in thoracopods I – III and VI – VII, reaching the middle of the third endopodal article in thoracopods IV and V.

Endopods thoracopods I – VII (Figs 3 D, E View Figure 3 , 4 A – D View Figure 4 , 5 A View Figure 5 ) 4 - segmented; the first two articles similar in length in thoracopod I; second article longer than the first article, equal in length to the third article in thoracopods II and VII; the second article very long in thoracopods VI and VII; the fourth article small in all thoracopods. Setal formula of endopods (the number of setae on basipod in brackets):

Thoracopod I: (2) 2 + 0 / 2 + 1 / 2 + 0 / 3

Thoracopods II, III: (2) 2 + 0 / 2 + 1 / 2 + 0 / 3

Thoracopod IV: (2) 2 + 0 / 2 + 1 / 2 + 0 / 3

Thoracopod V: (1) 2 + 0 / 1 + 1 / 1 + 0 / 3

Thoracopods VI, VII: (1) 0 + 0 / 0 + 1 / 0 + 0 / 2 (1)

Thoracopod VIII (Fig. 5 B View Figure 5 ) with a massive protopod; penial region with the distal prolongation similar in size to the three other formations: an anterior lobe and two formations (inner and outer) resembling lamellae rather than lobes, with the distal part curved towards the median axis of the appendix, forming an “ atrium ”; basipod with a rounded crest with a small seta at the base; robust exopod with elliptic transversal section and four setae; endopod absent.

Pleopods (Fig. 5 D View Figure 5 ) 2 - segmented; the first article with very long smooth seta; the second article with five setae: four smooth setae and one barbed seta of different lengths.

Uropods (Fig. 6 A View Figure 6 ). Sympod 50 % longer than wide and as long as the endopod, with four equal distal spines; endopod almost 50 % longer than exopod, with three strong spines, the distal one being two times longer than second which is longer than the first, and one fourth terminal spine thinner than the other three, and on distal end there are a long and strong special seta, and two plumose setae located dorso-laterally; exopod with four setae, two terminal barbed, of different length, and two short medial setae.

Pleotelson (Fig. 6 B View Figure 6 ) with one barbed dorsal seta on each side near the base of furca, each extending beyond furcal rami

Furcal rami (Fig. 6 B View Figure 6 ) almost square, bearing five spines; the first spine almost twice as long as the fourth and slightly longer than the second, which itself a bit longer than the third; the fourth spine about the same length as the dorsal spine.

Adult female. The female is similar to the male in all its features except for thoracopod VIII.

Thoracopod VIII (Fig. 5 C View Figure 5 ). Coxa with a small protrusion with setules; having a very well-developed epipod exceeding the length of the basipod, reaches the distal end of the exopod; with only one ramus, the exopod with two equal long setae, absent endopod.

Morphological remarks.

Morphological comparisons of the new species with the three subfamilies within Bathynellidae , as well as comparisons between the two species of the genus Morimotobathynella , are listed in the tables (Suppl. material 2: table S 1, Table 1 View Table 1 ). This section presents a detailed examination of the morphological characteristics of the appendages of these two Morimotobathynella species.

The size proportions of antennule articles differ between the two species. In the new species, the first three articles are approximately the same length as the last four articles, with the first being the longest and the fourth being the shortest. In M. miurai , the combined lengths of the first three articles were greater than those of the last four articles. In addition, the second and third articles are twice as long as the fourth and fifth articles, which are the shortest and equally long, respectively.

In the new species, the length of antennule is greater than that of antenna, whereas in M. miurai , antennule is shorter than antenna.

In both species, the longest article is the last article, followed by the first, fourth, and sixth articles of the same length. The second, third, and fifth articles are the shortest and very similar in length in the new species. In contrast, in M. miurai , the fifth article is twice as long as the second and third articles, which are similar in size and are the smallest.

In the new species, the pars molaris of the mandible does not have a terminal tooth larger than the denticles on the two lobes, as observed in M. miurai .

The new species has dense setules on the outer margin of the maxillule, which are absent in M. miurai .

The setal formula of the thoracopods differs between the two species (see Discussion and Table 1 View Table 1 ). However, we cannot compare the proportions of the articles because the thoracopods were not illustrated in the original description of M. miurai .

Thoracopod VIII in males differs significantly between the two species; M. miurai has an endopod with two setae, that are absent in the new species.

In thoracopod VIII of female, M. miurai has a large epipod that extends beyond the exopod, whereas in the new species, the epipod does not exceed the length of the exopod. Morimotobathynella miurai has two setae on the basipod, that are absent in the new species.

In the uropod, M. miurai has four spines on the sympod that are similar in pairs, with the two distal spines being longer than the two proximal spines. In the new species, all four spines on the sympod are similar in length and shorter than those of the type species of the genus. Additionally, in M. miurai , the exopod and endopod are equal in length, whereas in the new species, the endopod is twice as long as the exopod.

In the furcal rami, the longest spine in M. miurai is the second, which has nearly the same length as the third, while the first and fourth spines are similar in size and slightly smaller, and the dorsal spine is very small. In the new species, the longest spine is the first, with the spines gradually decreasing in length; the fourth is one-third the length of the first and similar in size to the dorsal spine.

Etymology.

The specific epithet “ koreana ” is derived from South Korea, the country where the new species was discovered.