Colobomatus mylionus, Fukui, 1965

Moon, Seong Yong, 2019, New records of Colobomatus mylionus Fukui, 1965 and Clavellisa chinensis (Yü, 1933) (Crustacea: Copepoda) parasitic on marine fish of Korea, Journal of Species Research 8 (4), pp. 373-379 : 374-376

Colobomatus mylionus Fukui, 1965

Korea name: Gam-seong-dom-goe-beol-rae úḓậijḏNj (ljḡ) ( Fig. 2 )

Colobomatus mylionus Fukui, 1965: p. 60, fig. 1; Bynes and Cressey 1986, p. 388, figs. 1-15; West, 1992, p. 125, figs. 190-207.

Material examined. Korea: Two females (NIBRIV00002 93066, one vial), coast of Shinji-do (34°19 ʹ 27 ʺ N, 126°53 ʹ 08 ʺ E), Jeollanam-do province, southern Korea, 12 September, 2012, Seong Yong Moon.

Description. Female. Total body length ( Fig. 2A ) 3.19 to 4.16 mm long (n =2) and unsegmented, without divisions between cephalosome and trunk or between trunk and urosome. Maximum width of cephalic region, 398 μm, measured in the oral region. Cephalic region with three frontal processes: paired anterolateral processes ( Fig. 2A ) tapering, 356 μm long, with blunt tip and rosette-like papillae in distal region; small mid-frontal process ( Fig. 2B ) about 164 μm long, with rosette-like papillae apically. All processes have pointed tips and bearing spinose orna- mentation. Trunk with two pairs of lateral processes: anterolateral processes located midway level between legs 1 and 2, about 420 μm long, with tip covered with rosetted papillae; posterolateral process positioned at level of leg 3, about 490 μm larger than the anterolateral one, confusing with trunk at base, with distal end bifurcated and covered with rosette papillae. Posterior part of body strongly nar- rowing behind posterolateral processes. Urosome with paired ventrolateral processes in the genital area; these processes weakly tapering, about 440 μm long, directed posterolaterally, with granule-like ornaments near the tip. Genital aperture located dorsolaterally, without any armature element. Abdominal region recurved dorsally. Caudal ramus ( Fig. 2C ) directed anterodorsally, 332 μm long, about 3.1 times as long as wide, with several scattered set- ules.

Antennule ( Fig. 2D ) unsegmented, about 113 μm long, with narrower distal two-fifths, and armed with 21 naked setae and one aesthetasc. Antenna forming anterior margin of buccal capsule, with posteromedial margin indent- ed ( Fig. 2E, a ). Buccal capsule as small ventral projection; mouth organs invisible. Labrum and mandibles not seen. Maxillule small, bearing a single apical spine ( Fig. 2E , me). Maxilla with basal segment bearing ventral spine- cluster in semi-circular formation, denticulate ausiliary spine and denticulate claw ( Fig. 2E , ma). Labium undi- vided ( Fig 2E , la). Maxilliped finger-like with an apical spine ( Fig. 2E , mp).

Leg 1 ( Fig. 2F ) biramous, located at anterior region trunk, consisting of weakly defined protopod and unsegmented rami; protopod with one outer seta; exopod with five setae; small endopod lobate and unarmed. Leg 2 ( Fig. 2G ) biramous, located at level of anterior processes of trunk and consisting of separated outer seta, exopod bearing four setae, endopod lobated and unarmed. Leg 3 ( Fig. 2H ) small, reduced to turbicle bearing two setae. Legs 4 to 6 absent.

Male. Not collected.

Host. Acanthopagrus schelegelii (Bleeker, 1854)

Distribution. Colobomatus mylionus has been recorded in Japanese waters ( Fukui, 1965), and from Eden, Newcastle, Brisbane, Yellon, and Point Sampson ( Byrnes and Cressey, 1986) in Australian waters ( West, 1992). This is the first record of this species in Korean waters.

Remarks. Colobomatus has the following major features which provide a key for females: (1) the body is highly modified and elongated, comprising cephalosome, fused thoracic somites, abdomen, and caudal rami; (2) the presence of at least two pairs of divergent lateral processes in the thoracic region, arranged in the shape of an “x”; and (3) leg 4 being reduced to a single seta and/or absent ( Fukui, 1965; West, 1992; Boxshall and Halsey, 2004; Pereira et al., 2012; Paschoal et al., 2016). To date, the presence of a single midventral simple cephalic process, two lateral cephalic processes, and processes on the trunk in C. mylionus are shared by another 19 species ( Paschoal et al., 2016, see Table 2). Colobomatus mylionus was origi- nal described and illustrated from a single damaged adult female from the A. schlegeli ( Fukui, 1965). After that, 51 females and two males were collected from the cephalic canal system adjacent to the nasal cavity of A. australis (Günther, 1859) , A. berda (Forsskål, 1775) , and A. latus (Houttuyn, 1782) from off Eden, Newcastle, Brisbane, Yelloon, and Point Sampson ( Byrnes and Cressey, 1986). Three female specimens of C. chinensis were found in- fected from gill rakers of the Japanese grenadier anchovy, Coilia nasus . Korean specimens of C. chinensis can be distinguished from other species of the genus by the following characters: the subcylindrical head is nearly rectangular, the neck cyclindrical and the trunk bunkler.

Careful comparison between our material and the com- plete description and illustration provided by West (1992) revealed some differences: (1) shorter thoracic lobes and caudal rami; (2) the protopod of legs 1 and 2 has one lateral seta; (3) the exopod of leg 1 has five setae; (3) the exopod of leg 2 has four setae; and (4) the leg 3 has two setae. However, our material to be identified with C. mylionus because of the presence of rosette-like papillae on the surface of the lateral processes of the female. The new Korean name of “Gam-seong-dom-goe-beol-rae” proposed for C. mylionus .

Figures

Fig. 2. Colobomatus mylionus Fukui, 1965, adult female from Shinji-do Island, Korea.A. habitus, ventral view. B. small mid-frontal process of cephalosome. C. caudal ramus, proximolateral seta of caudal rami. D. antennule.E. buccal area, showing position of antenna(a), maxillule(me), maxilla(ma), maxilliped(mp), and labium(la). F. leg 1. G. leg 2. H. leg 3. Scale bars:A=500μm; B-H=100μm.