Boholina reducta, Tran & Chang, 2020

Tran, Duc Luong & Chang, Cheon Young, 2020, First record of the genus Boholina (Copepoda, Calanoida, Pseudocyclopidae) in Vietnam, with description of a new species from an anchialine cave in Tra Ban Island, ZooKeys 977, pp. 1-23 : 1

Boholina reducta sp. nov. Figs 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9

Type material.

Holotype: ♀ (IEBR-COP3480-3481), 933 µm long; Quang Ninh Province, Tra Ban Island, Nha Tro Cave; 20°57'31.0"N, 107°29'12.1"E; 9 May 2018; D.L. Tran leg.; a pool inside the cave; dissected and mounted on two slides in glycerol. Allotype: ♂ (IEBR-COP3482-3483), 812 µm long; same data as for holotype; dissected and mounted on two slides in glycerol. Paratypes: 2 ♂♂ (IEBR-COP3484, 3485) and 5 ♀♀ (IEBR-COP3486-3490); same data as for holotype; dissected and mounted in glycerol.

Additional material.

60 ♂♂ and more than 100 ♀♀, same data as for holotype, preserved in 70% ethanol, IEBR-COP-AED05.2018.13; 5 ♂♂ and 5 ♀♀, same data as for holotype, prepared for SEM examination, retained in the collection of the first author (DLT).

Type locality.

A pool inside the Nha Tro Cave (geographic coordinates of the cave entrance: 20°57'31.0"N, 107°29'12.1"E) in Tra Ban Island, Bai Tu Long Bay, Quang Ninh Province, north Vietnam.

Etymology.

The proposed name refers to reduction of the terminal spine on the distal exopod segment of P5 in the male as well as the proximal outer spine on the distal exopodal segment of P5 in the female, which are the most remarkable characteristics differentiating this new species from all the congeneric species of Boholina .

Diagnosis.

Boholinid form in both sexes. Postero-lateral corners of second and third pedigerous somites rounded, fourth and fifth pedigerous somites completely fused. Rostrum represented by a narrow chitinized projection with pointed tip. Medial lobe of distal segment A2 endopod with nine setae. Mandibular palp basis with three setae; distal segment of endopod with 11 setae; seta on first segment of exopod present. Maxillule exopod with 11 setae and seta on basal exite absent. Second and third segments of maxilliped endopod with three setae each. Terminal spine on exopod of leg 4 modified with row of large spinules on mid-inner margin. Female: Gonopores on double-somite located close together on mid-ventral surface, septum between gonopores reduced to vestige deep inside genital opening. P5 Exp-3 with only one spine on outer margin and four setae on inner margin; distal segment of P5 endopod with one seta on outer margin. Male: Process at antepenultimate segment of right antennule absent. Right P5 exopod 2-segmented; distal segment with three spines, including a vestigial one on outer margin, while terminal and inner spine absent.

Description of adult female.

Total length (without furcal setae) 858-944 µm (mean 892 µm, N = 20). Ratio of prosome to urosome length about 3.1:1 (Fig. 2A, B ). Prosome ovoid in dorsal view, 5-segmented, comprising cephalosome; first pedigerous somite separated from cephalosome; second and third free pedigerous somites with postero-lateral corners rounded; fourth and fifth pedigerous somites completely fused.

Urosome 4-segmented, comprising genital double-somite, two free abdominal somites and anal somite. Genital double-somite slightly asymmetrical, widest about at mid-length; posterior margin ornamented with smooth hyaline membrane dorsally and small dentate hyaline frill ventrally, about as long as wide; paired gonopores equal in size, located close together on mid-ventral surface, the septum between gonopores reduced to vestige, deep inside genital opening; gonoporal plates small, and gonoporal slits large; two pairs of sensilla present (Figs 2D , 9C, D , arrows), one pair positioned adjacent to posterior margin of gonopores and second pair located ventrolaterally near posterior margin of double-somite. Third and fourth abdominal somites cylindrical, subequal in length (Fig. 2D ); third with finely serrated hyaline membrane all around posterior margin, fourth with posterior margin hyaline membrane expanded mid-dorsally to four large spines functioning as pseudoperculum concealing anal opening and mid-ventrally with finely serrated hyaline membrane on posterior margin. Anal somite extremely short, posterior margin smooth, concealed within posterior rim and hyaline membrane of second free abdominal somite.

Caudal rami (Fig. 2A, D ) short, about 1.5 times longer than wide, with pointed dorsal process in middle of distal margin; distal inner margin with a row of setules; ventral surface with a small pore near inner distal edge; ornamented six caudal setae; seta I lacking, seta II spiniform, about 1.2 time as long as caudal ramus; setae III-VI plumose, ratio of setae V:IV:VI:III:II as 5.8:4.3:4.1:2.4:1.0; dorsal seta VII short, naked, about 0.5 times as long as seta II.

Rostral filaments absent, rostrum represented by a narrow chitinized projection with pointed tip (Figs 2B, C , 9B , arrows); pair of long sensilla present in proximal part of rostrum.

Antennules (Figs 2A , 3A ) symmetrical, extending to middle area of pedigerous somite 5, 24-segmented with ancestral segments II-IV and XXVII-XXVIII fused, X-XI party fused, other articulations expressed, ventral surface of segment 1 with a row of small oblique spines. Armature formula as follows (s - setae, ae - aesthetasc): segment 1 (ancestral segment I) 1s + 1ae, segment 2 (II-IV) 6s + 1ae, segment 3 (V) 2s + 1ae, segment 4 (VI) 2s, segment 5 (VII) 2s + 1ae, segment 6 (VIII) 2s, segment 7 (IX) 2s + 1ae, segment 8 (X-XI) 3s + 2ae, segment 9 (XII) 1s, segment 10 (XIII) 1s + 1ae, segment 11 (XIV) 1s + 1ae, segment 12 (XV) 1s + 1ae, segment 13 (XVI) 1s + 1ae, segment 14 (XVII) 1s, segment 15 (XVIII) 1s + 1ae, segment 16 (XIX) 1s, segment 17 (XX) 1s, segment 18 (XXI) 1s + 1ae, segment 19 (XXII) 1s, segment 20 (XXIII) 1s, segment 21 (XXIV) 2s, segment 22 (XXV) 2s + 1ae, segment 23 (XXVI) 2s, segment 24 (XXVII-XXVIII) 5s + 1ae.

Antenna (Figs 3B , 8A ) biramous. Coxa and basis separate, coxa small, with a seta. Basis robust with two setae on inner distal corner. Endopod 2-segmented; proximal segment elongated, 2.2 times as long as wide, with two naked setae at 1/3 distal length of inner margin; distal segment with two lobes, medial lobe bearing six setae distally and three setae on inner distal margin, outer lobe with six long setae terminally and a short sub-terminal seta, outer margin ornamented with small serrated process (Fig. 3B , arrow) subdistally on medial margin and adjacent tiny spinules. Exopod 9-segmented, with setal formula of 1, 1, 1, 1, 1, 1, 1, 1, 3.

Mandible (Figs 3C , 8B ) with about eight small teeth on gnathobase plus small distal spinulose seta; ventral-most teeth largest. Mandibular palp biramous; basis robust with three unequal smooth setae on inner margin. Exopod 5-segmented, setal formula 1, 1, 1, 1, 2. Endopod 2-segmented, proximal with four smooth setae at distomedial angle, distal segment with 11 naked setae on distal margin.

Maxillule (Figs 4A , 8C ) with 10 marginal spinulose spines, one naked seta on anterior surface and four stiff setae on posterior surface of praecoxal arthrite. Coxal epipodite with seven plumose setae and two naked setae; coxal endite with four plumose setae. Basis fused to exopod, proximal basal endite with four plumose setae, distal basal endite sub-separated with endopod carrying five plumose setae; basal exite bared. Exopod completely fused to basis bearing 11 plumose setae, posterior surface with a slender oblique row of setules. Endopod with segments 1 and 2 fused, segments 2 and 3 separate, with three plumose, four bare and seven (2 plumose + 5 bare) setae, respectively.

Maxilla (Figs 4B , 8D ) 7-segmented, comprising syncoxa, basis and 5-segmented endopod. Syncoxa with five setae on praecoxal endite and three setae on coxal endite. Basis with two endites, each armed with three apical setae. Endopod 5-segmented; proximal endopodal segment developed, enditic-like; second and third segments partly fused; other segments divided; setal formula 4, 2, 2, 2, 2.

Maxilliped (Figs 4C , 8E ) well developed, 8-segmented with syncoxa, basis and free 6-segmented endopod. Syncoxa comprising praecoxa and coxa, completely fused; praecoxa with three endites; proximal and middle praecoxal endites each bearing one plumose seta, distal praecoxal endite with two plumose setae; coxa with one endite armed with one long plumose seta and two short, naked setae; oblique rows of spinules situated on posterior proximal of middle praecoxal endite and coxal endite. Basis elongated, armed with three plumose setae, inserted at distal 1/3 of medial margin and carrying rows of setules along medial margin and posterior face of segment. Endopod 6-segmented, with setal formula 2, 3, 3, 3, 3 + 1, 4.

P1-P4 (Fig. 5A-D ) biramous, with 3-segmented rami. Intercoxal sclerites of P1-P4 naked on both frontal and caudal surfaces. First to second endopodal segments of P1-P4 with pointed process on distolateral corners. Articulations between endopodal and exopodal segments ornamented with rows of tiny spinules. Armature of P1-P5 as in Table 1 .

Basis of P1 with distally pointed digitiform process on anterior; inner basal seta crooked, bilaterally spinulate, reaching to distal end of second endopodal segment; second exopodal segment with conspicuous spinulate process distally in outer distal corner of segment. Outer proximal spine on third exopodal segment of P1 flagellate, other outer spines on P2-P4 with serrate marginal membrane(s) as figured. Terminal spine on exopod of P1 with naked outer margin and plumose internally; on P2 and P3 with serrate membrane externally and plumose internally; that on P4 modified with row of large spinules on mid-inner margin and armed slender spinules on outer margin and distal part of inner margin (Figs 5D , 9A ).

P5 (Figs 5E, F , 8F ) biramous, with 3-segmented exopod and 2-segmented endopod, intercoxal sclerite smooth and unornamented. Basis small, 1.4 times as wide as long, with acute process on posterior surface near base of exopod. Exopod longer than endopod: tip of endopod only reaching to proximal inner seta on third exopodal segment. Distal endopodal segment 2.4 times as long as wide, armed with three inner, two apical and one outer naked setae. First and second exopodal segments each ornamented with a small pore on anterior surface at origin of outer spine. Distal exopodal segment 2.1 times as long as wide, bearing lateral spine (about 38-41 µm), subapical and apical spines of same length (about 51-54 µm); inner margin with four naked setae.

Description of adult male.

Body smaller than female, 756-825 µm long (mean 799 µm, N = 20). Ratio of prosome to urosome length about 2.7:1 (Fig. 6A, B ). Prosome 5-segmented as in female: cephalosome completely separated from first pedigerous somite; second and third pedigerous somites with rounded ventroposterior corners; fourth and fifth pedigerous somites fused, with only marked articulation dorsolaterally; posterior corners of fifth pedigerous somites rounded in lateral view.

Urosome 5-segmented (Fig. 6A-C ), comprising genital somite, three free abdominal somites and anal somite. Genital somite slightly asymmetrical, distal part of right margin protuberant, slightly more expanded than left margin (Fig. 6C ); both lateral margins smooth; four sensilla along dorsoposterior margin; posterior margin with finely serrated hyaline membrane. Second to fourth free abdominal somites cylindrical, subequal in size; second and third somites with finely serrated hyaline membrane on posterior margin; fourth with posterior margin hyaline membrane expanded mid-dorsally to four large spines functioning as pseudoperculum concealing anal opening and mid-ventrally with finely serrated hyaline membrane on posterior margin. Anal somite short, ring-like, with posterior margin smooth.

Caudal rami symmetrical, 1.5-1.6 times as long as wide (mean 1.57, N = 10), bearing distal spinous process dorsally and row of small setules on distal inner margin; ventral surface with a small pore near inner distal edge; ornamented six caudal setae, included to caudal setae II-VII and absent seta I.

Antennules shorter than in female, asymmetrical. Left antennule non-geniculate, 24-segmented and extending to middle area of pedigerous somite 5, armature segments as in female. Right antennule (Fig. 7A ) geniculate, 22-segmented, extending to middle of last pedigerous somite; segments 13-18 broadened; segments 17-18 with knife-like projection on inner margins; armature formula as follows (s - setae, ae - aesthetasc): segment 1 (ancestral segment I) 1s + 1ae, segment 2 (II-IV) 6s + 1ae; segment 3 (V) 2s + 1ae; segment 4 (VI) 2s; segment 5 (VII) 2s + 1ae; segment 6 (VIII) 2s; segment 7 (IX) 2s + 1ae; segment 8 (X) 1s + 1ae; segment 9 (XI) 1s + 1ae; segment 10 (XII) 1s; segment 11 (XIII) 1s + 1ae; segment 12 (XIV) 1s + 1ae; segment 13 (XV) 1s + 1ae; segment 14 (XVI) 1s + 1ae; segment 15 (XVII) 1s + 1ae; segment 16 (XVIII) 1s; segment 17 (XIX) 1s; segment 18 (XX) 1s; segment 19 (XXI-XXIII) 2s + 1ae; segment 20 (XXIV-XXV) 4s + 1ae; segment 21 (XXVI) 2s; segment 22 (XXVII-XXVIII) 5s + 1ae.

Antennae, mouthparts and P1-P4 as in female.

P5 (Figs 7B, C , 9E, F ) strongly asymmetrical, biramous; coxae and intercoxal sclerite fused to form common base, without armed elements on anterior and posterior surface. Right P5: basis about as long as wide, with slender outer basal seta located on posterior surfaces; exopod 2-segmented, first segment with long bilaterally serrate outer spine (39-42 µm), distal segment large in base and tapering on the tip, slightly curved inward, inner margin smooth; outer margin armed with three spines, proximal serrated spine (48-51 µm), middle serrated spine (33-38 µm) and distal short spine vestige (9-11 µm), terminal spine absent; endopod forming an elongate lobe, about 3.8 times as long as wide, armed with two slightly sigmoid spines, apical spine 11-13 µm long and inner spine 8-9 µm, subdistally. Left P5: basis robust, about 1.08 times as wide as long, with slender outer basal seta located on posterior surface; exopod 3-segmented, first segment with a long serrate outer spine (40-42 µm), second segment modified, bearing strongly reflexed spine (35-38 µm) on outer margin; third segment highly transformed bearing multiple short processes and one long, naked modified seta; endopod unarmed, forming an elongate rounded lobe, about 3.2 times as long as wide.

Variability.

One female paratype (IEBR-COP3488) and three females among ten additional specimens examined showed asymmetrical P5 Exp-3, with a distal inner seta on right leg (Fig. 5E , arrows) while lacking on left one (Fig. 5F , arrow).

Remarks.

The new species agrees well with the generic diagnosis of Boholina given by Fosshagen and Iliffe (1989), Boxshall and Jaume (2012), Moon and Soh (2014), and Boonyanusith et al. (2020): fourth and fifth pedigerous somites completely fused; urosome 4-segmented in the female and 5-segmented in the male with very short anal somite, telescoped within the preceding free abdominal somites in both sexes; genital openings paired, located ventromedially or ventrolaterally of genital double-somite; caudal rami produced into a pointed dorsal process in the middle of the distal margin; female antennule 24-segmented, with segments 8 and 9 partly fused or completely separated; P1 with 3-segmented endopod, each segment with a pointed outer distal corner, distal segment without any outer seta; P4 with slightly modified distomedial spine on the distal segment of the exopod; P5 with 2-segmented endopod in the female; and in the male P5 with a complex grasping organ and a highly modified exopod, reduced 1-segmented endopod on both sides.

Among six congeneric species currently recognized in Boholina , B. reducta sp. nov. shares the paired gonopores located either side of the ventral midline with B. ganghwaensis , B. parapurgata and B. purgata , and shares rounded postero-lateral corners of the second and third free pedigerous somites with B. munaensis , B. crassicephala and B. laorsriae . The new species is similar to B. laorsriae by the medial lobe of the distal segment of the antennary endopod having nine setae (while other congeners have eight setae). Boholina reducta sp. nov. resembles B. munaensis in bearing the single seta on the outer margin of the female P5 Endp-2 (against two in the other congeners). The new species is also similar to B. ganghwaensis in having the distal segment of the mandibular palp endopod with 11 setae (versus ten setae in the other congeners) (Table 2 ).

However, B. reducta sp. nov. is distinguished from all six congeners by the unique characteristics as follows (see Table 2 ): (1) a pair of gonopores are located close together on the mid-ventral surface of the genital double somites, and the septum between gonopores is only visible in the inner part of the genital opening. In Boholina , there are three species ( B. ganghwaensis , B. parapurgata and B. purgata ) with gonopores located on either side of the ventral midline on genital double somites. However, the separation between gonopores is clearly visible in ventral view of the genital double somites ( Fosshagen and Iliffe 1989; Boxshall and Jaume 2012; Moon and Soh 2014). In other species of Boholina , the gonopores are widely separated ( Fosshagen and Iliffe 1989, Boonyanusith et al. 2020), (2) the rostrum has a narrow finger-like process with pointed tip. The shape of rostrum of new species is unique in the Pseudocyclopidae , (3) the basis of mandibular palp has three setae in the new species, while there are four setae in all the species of Boholina , (4) there is no outer seta on basal exite of maxillule and there are 11 setae on exopod of maxillule, while there are only ten setae in other species of Boholina , (5) the second and third endopodal segments of maxilliped have three setae each, (6) the distomedial spine of P4 Exp-3 is modified with a row of spinules inserted in the middle of inner margin of the spine, (7) the female P5 Exp-3 has only one spine on outer margin, and the proximal outer spine is missing. In Boholina , the outer margin of female P5 Exp-3 generally has two spines, (8) female P5 Exp-3 has four setae on the inner margin, while there are three setae both rami in the other species of Boholina ( Fosshagen and Iliffe 1989; Boxshall and Jaume 2012; Moon and Soh 2014 and Boonyanusith et al. 2020), (9) right P5 exopod has two segments in the male, and (10) the distal segment of right P5 exopod in male has only two strong spines and one short vestigial spine on outer margin and the terminal spine of the segment, which are unique in the genus. This is the first record of Boholina and Pseudocyclopidae from Vietnam waters. An updated key to the seven valid species of Boholina is provided.

Figures

Figure 2. Boholina reducta sp. nov., holotype female. A habitus, dorsal view B habitus, lateral view (arrow indicating tip of rostrum) C rostrum, ventral view (arrow indicating tip of rostrum) D urosome, ventral view (arrow indicating sensilla near gonopores). Scale bars: 100 µm.

Figure 3. Boholina reducta sp. nov., holotype female. A antennule B antenna (arrow indicating small serrated process) C mandible. Scale bars: 100 µm.

Figure 4. Boholina reducta sp. nov., holotype female. A maxillula B maxilla C maxilliped. Scale bar: 100 µm.

Figure 5. Boholina reducta sp. nov. A - E holotype female, F paratype female. A P 1, posterior B P 2, posterior C P 3, posterior D P 4, posterior E P 5, posterior (arrows indicating the presence of distal inner setae) F distal segment exopod of left P 5 (arrow indicating a trace of the distal inner seta). Scale bar: 100 µm.

Figure 6. Boholina reducta sp. nov., allotype male. A habitus, dorsal view B habitus, lateral view C urosome, dorsal view. Scale bars: 100 µm.

Figure 7. Boholina reducta sp. nov., allotype male. A rostrum and antennule, ventral view B P 5, posterior C P 5, anterior. Scale bars: 100 µm.

Figure 8. Boholina reducta sp. nov., SEM micrographs, female. A distal segment of antennary endopod B mandibular palp C maxillula D maxilla E maxilliped F P 5, posterior.

Figure 9. Boholina reducta sp. nov., SEM micrographs A-D female E-F male A distal spines of P 4 Exp- 3 B rostrum, ventral view (arrow indicating tip of rostrum) C genital double-somite, ventral view (arrow indicating sensilla near gonopores) D gonopores, latero-ventral view (arrow indicating sensilla near gonopores) E P 5, posterior F P 5, anterior. Abbreviations: gp, gonoporal plates; gs, gonoporal slits.