Mesochra bisetosa, Lee & Chang, 2008

Lee, J. M. & Chang, C. Y., 2008, Two canthocamptid copepods of the genera Itunella and Mesochra (Harpacticoida, Canthocamptidae) from brackish waters in South Korea, Journal of Natural History 42 (25 - 26), pp. 1729-1747 : 1739-1746

Mesochra bisetosa sp. nov.

( Figures 7–10 )

Type material

Holotype ♀ (DB20018), allotype „ (DB20019), dissected in lactophenol, mouth of streamlet, Imjado Island , Sinan (35 ° 089290N, 126 ° 089230E), 17 January 2006 (leg. H.W. Lim) . Paratypes: three ♀♀, two „„, collection details same as in holotype, including three undissected (♀, NHM reg. no. 2008. 962; „, NHM reg. no. 2008. 963; ♀, NIBRIV0000100515) and two dissected paratypes (♀, DB20020; „, DB20021).

Additional material examined

Eight ♀♀, three „„, mouth of streamlet, Imjado Island, Sinan, 17 January 2006 (H.W. Lim); five ♀♀, one „, estuary of Maesancheon Stream, Dangjin, 4 March 2007 (C.Y. Chang and J.M. Lee) .

Description

Female. Body ( Figure 7A ) small, cylindrical, 740¡30 Mm (n 59) in length, tapering behind from border of cephalothorax. Rostrum large, broad, protruding anteroventrally, not defined from cephalic shield at base. Prosomites ornamented with hyaline membrane and one or two pairs of sensillae along posterior margin dorsally. Cephalothorax a little longer than sum of next three prosomites, with large integumental depression (nuchal organ). Outer distal margin of each prosomite not strongly protruded. Urosomites armed with spinule row along posterior margin ventrally ( Figure 7C ) and dorsolaterally ( Figure 7A ). Genital double-somite with subcuticular ridge marking faint line of fusion (suture) in dorsolateral and ventral sides. Genital field ( Figure 7C ) with separate genital apertures, ending with vestigial P6, each bearing one plumose seta and two minute conical projections medially; copulatory pore situated medially, posterior to genital apertures. Anterior part of ventral surface of anal somite inconspicuous with internal cuticular thickenings ( Figure 7C ). Anal operculum convex with smooth posterior edge ( Figure 7B ).

Caudal rami cylindrical, 1.3–1.5 times longer than wide, narrowing a little posteriorly, much divergent from each other posteriorly ( Figure 7B ). Inner margin armed generally with two spinule rows at about the proximal quarter and distal third of the inner margin (sometimes with only one row distally); posterior margin with one transverse row of four or five spinules ventrally ( Figure 7C ). Lateral caudal setae (caudal seta I and II) locating at distal quarter of lateral margin of caudal ramus, flanked with three or four spinules. Outer caudal seta (caudal seta III) lacking. Terminal caudal setae (caudal seta IV and V) well developed, pinnate; inner one about 2.3 times longer than outer one. Inner caudal seta (caudal seta VI) slender, naked, a little shorter than caudal ramus. Dorsal caudal seta (caudal seta VII) locating near inner margin of caudal ramus.

A1 short, five-segmented ( Figure 8A ); segment one with one row of spinules proximally; segment 3 with one long aesthetasc distally, its tip nearly reaching to distal end of A1; segment 4 small; segment 5 elongate (resulted from complete fusion of two distal segments), about 4.2 times longer than broad. Setal formula: 1-[1 pinnate], 2-[5 pinnate+4 naked], 3-[8 naked+aesthetasc], 4-[1 naked], 5-[4 pinnate+4 naked+trithek (2 naked+1 aesthetasc)]. A2 ( Figure 8B ) three-segmented, consisting of coxa, allobasis and free endopod; endopod bearing three subapical and four apical elements; exopod two-segmented, with four setae in total. Mandible ( Figure 8C ) armed with well-developed coxal gnathobase bearing several bicuspidate teeth along distal margin and two setae at dorsal corner; palp three-segmented; basis with one inner distal seta; exopod represented by a small segment bearing one seta; endopod well developed with one proximal and three distal setae. Maxillule ( Figure 8D ) with praecoxal arthrite bearing nine distal elements with two surface setae; coxa with cylindrical endite bearing two apical setae; endopod fused to basis, bearing nine setae in total; exopod represented by small protuberance with two setae. Maxilla ( Figure 8E ) armed with two syncoxal endites, each bearing three setiform elements; allobasis forming one strong pectinate claw, flanked by two proximal setae; endopod represented by protuberance bearing four bare setae. Maxilliped ( Figure 8F ) subchelate; syncoxa with one pinnate seta inner distally and group of eight setules outer distally; basis armed with a row of long setules along inner margin and five spinules on outer distal edge; endopod represented by one curved claw, bearing one long seta as accessory armature.

P1 ( Figure 7D ), endopod equal in length to exopod; inner distal seta of basis stout, spiniform, a little longer than enp 1; enp 1 not elongated, with one long seta inner distally; enp 2 and enp 3 a little longer than enp 1; enp 3 with one spine and two setae apically or subapically; exp 2 not elongated, with one inner seta; exp 3 with two outer spines and two apical setae.

P2–P4, endopods two-segmented, exopods three-segmented. P2 ( Figure 9A ), enp 1 with inner seta; enp 2 elongated with setules along inner margin and spinules along outer margin, armed with one outer spine, one apical and one inner distal setae (outer one about twice as long as inner one); exp 3 bearing three outer spines, two apical setae and one inner proximal seta. P3 ( Figure 9B ), enp 1 with inner seta; enp 2 bearing one outer spine, one apical and one inner distal setae. P4 ( Figure 9C ), enp 1 small, without inner seta; enp 2 bearing one outer spine, one apical and two inner distal setae with two spinules on outer margin.

Seta/spine armature of P1–P4 as follows (Arabic numerals represent setae, while Roman numerals indicate spines):

P1 basis I-I exopod I-0; I-1; II,1,1 endopod 0-1; 0-1; I,1,1

P2 basis I-0 exopod I-0; I-1; III,2,1 endopod 0-1; I,1,1

P3 basis 1-0 exopod I-0; I-1; III,2,2 endopod 0-1; I,1,1

P4 basis 1-0 exopod I-0; I-1; III,2,2 endopod 0-0; I,1,2

P5 ( Figure 9D ) baseoendopod and exopod confluent proximally, each bearing four (excluding lateral seta) and five pinnate spiniform setae, respectively; both inner and outer margins of inner lobe of baseoendopod with spinule row, its tip slightly exceeding exopod; exopod triangular, slightly longer than wide, with longest apical setae flanking one inner and three outer setae.

Male. Body cylindrical, 680¡20 Mm (n 54) in length. General appearance and seta/ spine arrangement similar to those of females. Anal somite with one transverse spinule row at anterior part of ventral surface ( Figure 7E ); internal cuticular thickening faint or unseen. Caudal rami sometimes showing asymmetrical spinule ornamentation on inner face ( Figure 7E ).

A1 ( Figure 9E ) subchirocerate, eight-segmented; segment 1 with three setule rows and one distal seta; segment 2 with nine naked setae; segment 3 incompletely fused, with two pinnate setae and seven naked setae; segment 4 bulbous, bearing seven naked and two short pinnate setae with one long naked seta and one aesthetasc distally; segment 5 narrow, with one pinnate and two naked setae anterodistally; segment 6 with two naked setae; segment 7 elongate and slightly curved with three cusps on anterior margin; segment 8 curved to claw-shape apically, with six naked setae and two basally fused elements (one aesthetasc and one naked seta) distally.

P5 ( Figure 9F ), both sides connected basally without distinct intercoxal sclerite; baseoendopod triangular, bearing three spiniform setae with a few spinules on inner and outer edges; exopod pyriform, with six setae in total, consisting of three outer spiniform setae, one longest pinnate apical seta, two inner setae. P6 ( Figure 9G ) represented by one small triangular plate bearing three setae with two spinules on inner edge.

Variability

Three of 13 females and two of four males examined show the variation at the inner margin of the caudal rami, that is, armed with only one spinule row distally (cf. Figure 10C ), compared with the normal character state of two spinule rows at about the proximal quarter and distal third of the inner margin (cf. Figure 7C ). Two females and one male show the asymmetrical arrangement of the spinulation, that is, two spinule rows on one side and three rows on the other side (cf. Figure 7E ), or two rows on one side but only one row on the other side.

Etymology

The proposed specific name, bisetosa , alludes to the ‘‘two setae on distal endopodal segments of legs 2 and 3’’, one of the characteristics differentiating it from the related species.

Ecology

The present species was collected from the mouths of streams discharging into the Yellow Sea, on the western coast of Korea. The bottom type in these localities is muddy sand. This species co-occurred with various brackish-water copepods: Sinodiaptomus tenellus , Pseudodiaptomus inopinus (Calanoida) , Nitokra koreanus ,

Tachidius parvus , Schizopera neglecta , Limnocletodes behningi , Onychocamptus vitiospinulosa (Harpacticoida) , and Halicyclops japonicus (Cyclopoida) .

Remarks

This species is apparently allied with two Far East Asian congeners, M. quadrispinosa recorded from brackish waters or freshwaters on the southern coasts of China ( Shen and Tai 1965) and M. hinumaensis from a brackish-water lake in Ibaraki, northeastern Honshu, Japan ( Kikuchi 1972). All three species are basically from brackish water, and do not show sexual dimorphism in P1–P4. Furthermore, they share short P1 enp 1, three outer spines on exp 3 of P2–P3, and five-segmented A1 with last segment resulting from the complete fusion of two distal segments.

However, M. bisetosa sp. nov. differs from the two related species by having only two setae on P2–P3 enp 3, as indicated in the specific name. Generally this species has one or two additional spinule row(s) along the inner face of the caudal ramus, compared with the other two species that have only one spinule row distally. Moreover, this species does not show the conspicuous internal cuticular thickening at the anterior part of the anal somite, which was mentioned as a ‘‘hyaline frill’’ by Kikuchi (1972) in the description of M. hinumaensis . Mesochra bisetosa is also distinguished from M. quadrispinosa by the number of setae on the P5 exopod in the female (five setae in M. bisetosa , four in M. quadrispinosa ). The three species seem to have diversified recently in the Far East, and apparently form a species group that is clearly discernible from other congeners of the genus Mesochra .

A key to the species of the genus Mesochra from East Asia

1. P2–P3 exp 3 with two outer spines............................ 2

- P2–P3 exp 3 with three outer spines........................... 3

2. Female P5 exopod with four setae/spines...... M. suifunensis Borutsky

- Female P5 exopod with five setae/spines......... M. prowazeki Douwe

3. P1 enp 1 elongate, longer than or as long as exopod; sexually dimorphic in P3–P4 endopods....................................... 4

- P1 enp 1 much shorter than exopod; P3–P4 endopods similar in both sexes................................................. 5

4. Anal operculum lacking spinules on posterior border. M. rapiens (Schmeil)

- Anal operculum with spinules on posterior border... M. alaskana Wilson

5. P2–P3 enp 2 bearing two setae................. M. bisetosa sp. nov.

- P2–P3 enp 2 bearing three setae.............................. 6

6. P5 exopod in female bearing five setae; anal somite with well-developed cuticular thickening along anterior margin..... M. hinumaensis Kikuchi

- P5 exopod in female bearing four setae; anal somite without conspicuous cuticular thickening along anterior margin... M. quadrispinosa Shen and Tai

Figures

Figure 7. Mesochra bisetosa sp. nov. (A–D) Female: (A) habitus, dorsal; (B) right side of anal somite and caudal ramus, dorsal; (C) urosome, ventral; (D) P1. (E) Male: anal somite and caudal rami, ventral (left side) and dorsal (right side). Scale bars: 100 Mm (A); 50 Mm (B–E).

Figure 8. Mesochra bisetosa sp. nov., female. (A) A1; (B) A2; (C) mandible; (D) maxillule; (E) maxilla; (F) maxilliped. Scale bars: 50 Mm.

Figure 9. Mesochra bisetosa sp. nov. (A–D) Female P2–P5. (E–G) Male: (E) A1, ventral; (F) P5; (G) P6. Scale bars: 50 Mm.

Figure 10. Mesochra bisetosa sp. nov., female, scanning electron micrographs. (A) Habitus, ventral; (B) mouthparts; (C) anal somite and caudal rami, ventral. Scale bars: 100 Mm (A); 20Mm (B,C).