Bicornucythere misumiensis, Nakamura & Tsukagoshi, 2022

Nakamura, Daisuke & Tsukagoshi, Akira, 2022, Morphological types of male copulatory organs of Bicornucythere bisanensis (Ostracoda, Crustacea) and the description of a new Bicornucythere species, Zootaxa 5134 (4), pp. 569-587 : 579-584

Bicornucythere misumiensis sp. nov.

( Figs 8–12 )

Type series. All specimens were collected from type locality on 28 November 2019. Holotype: adult male (SUM-CO-2501), right valve length 0.71 mm, height 0.32 mm, left valve length 0.76 mm, height 0.36 mm, appendages mounted on slide and valves preserved on SEM stub. Paratypes: 10 adult males (SUM-CO-2502 and 2503, 2505– 2509, 2511, 2515 and 2516) and 5 adult females (SUM-CO-2504, 2510, 2512–2514). Depth about 5 m. Salinity ca. 30 ‰.

Etymology. After name of the type locality, Misumi-cho (western Japan).

Diagnosis. Carapace length is shorter in females than in males. Carapace reticulation slightly weak at anterior half in female. Second podomere of male right fifth limb divided into 3 parts (IIa, IIb, IIc). IIa and IIb completely fused, and only with remaining segment boundary. IIb and IIc with faint borderline remaining between podomeres. In male copulatory organ, right distal lobe with triangular outline and sharp linearly tip, left one with hump-shaped tip.

Description. Carapace ( Figs 10 , 11G–J , 12 ). Carapace lateral outline groundnut-like in male with greatest height at one-fourths from anterior margin, and sub-rectangular in female. Anterior margin obliquely rounded. Ventral margin concave at middle. In dorsal view, outline sunflower-seed like. In anterior view, outline sub-pentagonal, height of left valve slightly higher than right valve and left one overlapping right along dorsal margin. Five processes ( Fig. 10I ) along anterior ventral margin: some of them bifurcated at distal ends. One strong spine on posterior ventral margin of each valve. Surface fully covered by reticulation in male; vein-like ridges of reticulation with tendency of horizontal lines in posterior half. Reticulation weak at anterior half in female. Surface covered with more than 100 pore systems, classified as corolla-like pore with no seta ( Fig. 11G, H ), simple pore with broom-shape seta ( Fig. 11I ), and simple pore with simple seta ( Fig. 11J ). Four adductor muscle scars, but often a mutation to five is seen. Heart-shaped frontal scar located in front of adductor scars ( Fig. 10J , black arrow). One elliptic mandibular scar in front of lowest adductor scar ( Fig. 10J , gray arrow). Hingement amphidont-type; composed of 1 prominent fan-shaped tooth and 1 large socket present in anterior element, 1 prominent fan-shaped tooth in posterior element, and many sub-ovate sockets in median element in right valve ( Fig. 10K ). Complementary in left valve.

Antennula ( Fig. 9A ). Six articulated podomeres, length ratio among them from proximal to distal 36: 12: 10: 15: 23: 32. First podomere with numerous setulae along ventral margin. Second podomere with 1 very long annulated seta on posterior-distal corner, setulae along anterior and posterior margins. Third podomere with 1 medium to long seta on anterior-distal corner. Fourth podomere with 3 medium to long setae on anterior margin. Fifth podomere with 3 medium to long setae on distal end. Sixth podomere with 3 medium to long setae and 1 aesthetasc on distal end.

Antenna ( Fig. 9B ). Four articulated podomeres, length ratio among them from proximal to distal 9: 4: 9: 2. First podomere with long segmented exopodite in male and reduced segmented exopodite in female. Second podomere with 1 very long annulated seta on posterior-distal end. Third podomere with 2 long setae on ledge of anterior margin, 1 long and 1 short setae on posterior distal end, 1 long and 1 medium-length stout seta, and 1 short aesthetasc seta on ledge of posterior margin. Fourth podomere with 1 large claw-like seta, and 2 medium to long setae on distal end.

Mandibula ( Fig. 9C ). Consisting of five podomeres. First podomere (coxa: C1) with masticatory part of 1 stout and several fine denticles. Basis with two short annulated setae on anterior margin and 5-branched exopodite. First podomere of endopodite with two long plumose setae on anterior distal corner. Second podomere with 1 long seta and 1 plumose seta on anterior distal corner, 3 long and 2 medium to long setae on posterior dorsal corner and 1 medium-length plumose seta on posterior-proximal margin. Third podomere small and armed with 1 medium to long and 2 short setae on distal end.

Maxillula ( Fig. 9D , 11F ). Thin branchial plate (exopodite) bearing 14 plumose long setae and 1 short segmented seta. Basal podomere bearing 1 palp (endopodite), 3 masticatory endites and numerous setulae on anterior distal end. First podomere of endopodite with 2 medium to long setae and 1 medium-length seta on distal end. Second podomere of endopodite with 2 medium-length setae on distal end. Each endite with several short setae.

Right fifth limb (in male) ( Fig. 9E ). Consisting of 2 articulated podomeres, length ratio among them from proximal to distal 5: 7. First podomere with 1 annulated seta on middle of anterior margin, 2 annulated setae on anteriordistal end, numerous setulae along posterior margin and reduced exopodite on posterior-proximal margin. Second podomere divided into 3 parts (IIa, IIb, IIc). IIa and IIb completely fused, only with remaining segment boundary on posterior margin at one-third from posterior distal end ( Fig. 9E 1 ). Faint segment boundary between IIb and IIc ( Fig. 9E 2 ). IIa of second podomere with 2 fine setae on middle of anterior margin and 1 annulated seta on anterior distal end. IIc of second podomere bearing 1 stout terminal claw on distal end.

Right fifth limb (in female) ( Fig. 9F ). Four articulated podomeres, length ratio among them from proximal to distal 11: 6: 3: 4. First podomere with 2 annulated setae on anterior-proximal margin, 2 annulated setae on distal end and reduced exopodite on posterior-proximal margin. Second podomere with numerous setulae along anterior margin and 1 medium-length plumose seta on anterior distal end. Third podomere with acicular process on posterior distal corner. Fourth podomere with process on anterior distal corner and 1 terminal claw on distal end.

Left fifth limb ( Fig. 9G ). Four articulated podomeres, length ratio among them from proximal to distal 11: 6: 4: 5. First podomere with 2 annulated setae on middle of anterior margin, 2 annulated setae on anterior distal end and reduced exopodite on posterior-proximal margin. Second podomere with numerous setulae along anterior margin and 1 medium to long plumose seta on anterior distal end. Third podomere with acicular process on anterior distal corner. Fourth podomere with acicular process on anterior distal corner and 1 terminal claw on distal end.

Sixth limb ( Fig. 9H ). Four articulated podomeres, length ratio among them from proximal to distal 11: 7: 6: 5. First podomere with 1 annulated seta on anterior margin at one-thirds from proximal end, 1 stout annulated seta on anterior margin at two-thirds from proximal end and 1 annulated seta on distal end. Second podomere with 2 fine setae on middle of anterior margin, 1 medium to long plumose seta on anterior distal end and numerous setulae along distal margin. Third podomere with numerous setulae on distal end. Fourth podomere with numerous setulae and 1 terminal claw on distal end.

Seventh limb ( Fig. 9I ). Four articulated podomeres, length ratio among them from proximal to distal 12: 11: 5: 7. First podomere with 1 long annulated seta on anterior margin at three-fourths from proximal end, 1 medium to long annulated seta on distal end and 1 annulated short seta on posterior-proximal end. Second podomere with 2 fine setae on anterior margin at one-fourths from proximal end, 1 annulated medium to long seta on anterior distal end and numerous setulae along distal margin. Third podomere with numerous setulae on distal end. Fourth podomere with numerous setulae and 1 very long almost straight terminal claw with serrations on distal half.

Brush-shaped organ (in male) ( Fig. 9J ). Consisting of paired symmetrical lobes. Both brunches bearing about 6 long thin and 1 to 2 short thin setae.

Male copulatory organ ( Fig. 8 , 11C, D, E ). Sub-elliptic capsule with long, annulated, arched structure. Distal lobe well-developed. Right distal lobe triangular outline with sharp linearly tip. Left distal lobe also triangular outline but hump-shaped tip. Coiled copulatory duct with thick base and arched. Two clasping apparatus (ca1, ca2) different in shape; proximal-most one (ca1) crescentic shaped, second one (ca2) two-forked shaped with beaked tip ( Fig. 11D ).

Furca ( Fig. 9K ). Consisting of paired symmetrical setulous setae.

General morphology of soft parts in Fig. 11A, B .

Dimensions. See Table 4 .

Occurrence. Type locality is mud bottom of Konoura Fishing Port , Misumi-cho, Kumamoto Prefecture, Japan (32°37'4.06"N 130°32'1.08"E) .

Remarks. This new species is different from Bicornucythere bisanensis ( Okubo, 1975) with a smaller and more elongated carapace. The carapace surface reticulation is shallower in the frontal area than that in B. bisanensis . The well-developed asymmetrical distal lobe of the male copulatory organ is species-specific in the new species. The carapace of B. concentrica Yamaguchi et al., 2015, which is a fossil species, shows a more ovate lateral outline, and the vein-like ridges show a concentric pattern.

Figures

FIGURE 1. Map showing the locations of previous studies on Bicornucythere bisanensis from the seas around Japan. Locality numbers (1–23) and letters (X, Y) correspond to the list of reports in Table 1. The white map was taken from CraftMAP (2006).

FIGURE 2. Map showing the locations evaluated in this study.A, Aburatsubo Cove. B, Ushimado. C, Misumi-cho. Misumi-cho is the type locality for Bicornucythere misumiensis sp. nov. and the description of each of these Types and/or species are linked to the type of male copulatory organ and their classification as either B. bisanensis or the new species in this study. Based on a map from CraftMAP (2006).

FIGURE 8. Male copulatory organs of Bicornucythere misumiensis sp. nov. (SUM-CO-2501, holotype). ca, clasping apparatus; cd, copulatory duct; dl, distal lobe. Scale: 100 μm.

FIGURE 9. Appendages of Bicornucythere misumiensis sp. nov. A, antennule. B, antenna. C, mandibula. D, maxillula. E, male right 5th limb. E1, segment boundary in male right 5th limb. E2, faint segment boundary in male right 5th limb. F, female right 5th limb. G, left 5th limb. H, 6th limb. I, 7th limb. J, brush-shaped organ. K, left furca.A, C, J, K SUM-CO-2501, male holotype; B, E, G–I, L, M, SUM-CO-2502, male paratype; D, SUM-CO-2503, paratype; B, F, SUM-CO-2504, female paratype. Scale: 100 μm for A-K, 125 μm for E1, E2.

FIGURE 10. Carapace of Bicornucythere misumiensis sp. nov. A and B, male left and right valves in the external lateral view, respectively (SUM-CO-2501, holotype). C and D, male left and right valves in the internal lateral view, respectively (SUM- CO-2505, paratype). E and F, female left and right valves in the external lateral view, respectively (SUM-CO-2504, paratype). G, male dorsal view (SUM-CO-2506, paratype). H, male anterior view (SUM-CO-2507, paratype). I, process of left valve (SUM-CO-2508, paratype). J, muscle scars of right valve including adductor, frontal (black arrow), and mandibula scars (gray arrow) (SUM-CO-2509, paratype). K, hingement on male right valve (SUM-CO-2509, paratype). L and M, teeth and socket of hingement on male right valve (SUM-CO-2509, paratype). L, anterior element of right hingement. M, posterior element of right hingement. White arrows indicate anterior. Scale: 100 μm for A–H, K, 20 μm for I, J, L, M.

FIGURE 11. Exposed soft parts and pore systems of Bicornucythere misumiensis sp. nov. A and B, soft parts of female and male, respectively. C–E, male left copulatory organ (ventral view). C, whole image; D, extension of the distal lobe; E, tip of the distal lobe. F, maxillula of male. G–J, pore systems. G and H, corolla-like pore (reported by Okada, 1982 as a Ben-type pore). I, broom-shape seta (black arrow). J, simple seta.A, SUM-CO-2510, paratype; B–F, SUM-CO-2511, paratype; G–J, SUM-CO- 2508, paratype. Arrows indicate anterior (A and B). Scale: 100 μm for A and B, 50 μm for C, 30 μm for F, 20 μm for D, 10 μm for I, 2 μm for E and J, 1 μm for G and H.

FIGURE 12. Bicornucythere misumiensis sp. nov. Internal lateral views of male carapace of left (A) and right (B) valves, respectively (SUM-CO-2505, paratype). Arrows indicate anterior. Scale: 100 µm.