Myzomolgus leptocercosus, Kim, 2021

Kim, Jae-Sang Hong and Il-Hoi, 2021, Three new species of Myzomolgus (Copepoda, Cyclopoida, Catiniidae) associated with sipunculan worms from a tidal flat in Phuket, Thailand, Journal of Species Research 10 (3), pp. 287-300 : 288-292

publication ID	https://doi.org/ 10.12651/JSR.2021.10.3.287
persistent identifier	https://treatment.plazi.org/id/038A87E9-FFDD-ED6E-5FE9-D703FBA9A3AB
treatment provided by	Felipe
scientific name	Myzomolgus leptocercosus
status	sp. nov.

Treatment

Myzomolgus leptocercosus View in CoL n. sp. ( Figs. 1-3 View Fig View Fig View Fig )

Material examined. 27$$, 7♂♂ from washings of unidentified sipunculid worms, northeastern tidal flat in Phuket, Thailand (approximately 8°01′25″N, 98°24′53″ E), 13 July 2015. Holotype ($, MABIK CR00247440 ) and paratypes (24$$, 5♂♂, MABIK CR00247441 ). Dissected paratypes (2$$, 2♂♂) are retained in the collection of the junior author. GoogleMaps

Female. Body ( Fig. 1A View Fig ) dorsoventrally flattened. Body length 0.92 mm. Prosome broad, 4-segmented, oocupying 58% of body length. Cephalothorax distinctly wider than long, consisting of completely fused cephalosome and first pedigerous somite, without dorsal suture line between them. Four prosomal somites with well-developed epimera bearing round corners. Urosome ( Fig. 1B View Fig ) narrow, 5-segmented. Fifth pedigerous somite 140 μm wide, tapering laterally, much wider than genital double-somite, with raised dorsal surface, more than 10 sensilla on dorsal raised region, and numerous minute spinules on ventral surface. Genital double-somite wider than long (60 × 90 μm), with broader anterior and narrower posterior halves, horizontal sclerotization band between genital areas, several sensilla on dorsal surface, 3 oblique membranes on both sides of dorsal surface, and minute spinules on anterior region of ventral surface ( Fig. 1C View Fig ); genital apertures positioning dorsolaterally in middle of double-somite length. Three free abdominal somites 60 × 55, 53 × 45, and 23 × 41 μm, respectively. First and second free abdominal somites each with 3 obique membranes on each side of dorsal surface and 1 or 2 pairs of spinules on lateral surfaces. Anal somite very short, with deep median incision and patch of rather prominent spinules on ventral surface ( Fig. 1C View Fig ). Caudal rami ( Fig. 1B, C View Fig ) elongate, slightly divergent, with 4 spinules on posteroventral margin and numerous minute spinules on ventral surface; each ramus 10.7 times longer than wide (150 × 14 μm), armed with 7 naked setae; proximal lateral seta (seta I) small, positioning 10% region of ramus length; other 6 setae (seta II- VII) positioning distally or subdistally; seta V distinctly longer than caudal ramus, but other setae small, much shorter than ramus. Egg sac ( Fig. 1D View Fig ) flattened, 260 × 158 μm, double eggs-layered.

Rostrum ( Fig. 1E View Fig ) broad; lateral margins parallel along its proximal third, but strongly tapering along distal twothirds, with pointed distal apex. Antennule ( Fig. 1F View Fig ) slender, 6-segmented, flexed between penultimate and terminal segments; armature formula 5, 14, 9, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; first segment with conical process on ventral margin; proximalmost seta of first segment short, plumose; 2 distal segments bearing pinnate setae, 1 on fifth and 4 on sixth segments. Antenna ( Fig. 1G View Fig ) 4-segmented, consisting of basis and 3-segmented endopod; basis the longest segment, armed distally with 1 pinnate seta and ornamented with unequal setules on surfaces; first endopodal segment smooth and unarmed; second endopodal segment with 1 small seta on innter margin and 1 large, peduculate sucker; third endopodal segment with 2 very unequal setae and 4 curved, spiniform setae of unequal lengths; each spiniform seta terminating by pad-like expansion, bearing spinulose inner margin.

Labrum ( Fig. 1H View Fig ) much wider than long, triangular, with 13 teeth along mid-region of posterior margin. Mandible ( Fig. 1I View Fig ) strongly curved, unsegmented, armed distally with 4 elements: 1 naked, conical ventral spine, 2 broad, spinulose, plate-like middle elements, and slender, setiform dorsal element. Maxillule ( Fig. 2A View Fig ) as small lobe bearing 4 naked setae. Maxilla ( Fig. 2B View Fig ) 2-segmented; proximal segment (syncoxa) large, unarmed, but ornamented with numerous spinules on ventral surface and posterior margin; distal segment (basis) small, terminating in slender, spiniform process bearing minutely bifurcate tip, armed with 3 setae (1 simple and 2 unilaterally flanged with membrane) and 1 membranous flap. Maxilliped absent.

Legs 1-4 with 3-segmented rami ( Fig. 2C- E View Fig ). Endopods longer than exopods. Leg 3 same as leg 2 in armature and ornamentation. Coxae with setulose outer margin in legs 1-4, several long spinules on vetral surface in legs 2-4, and small inner seta in legs 1-4. Intercoxal plates bearing spinules along distal margin. Basis obliquely elongate, spinulose along distal margin; outer margin setulose in legs 1-3, but spinulose in leg 4. Basis of leg 1 with large, medially cuved inner distal spine bearing spinules along its outer margin. Outer margins of rami of legs 1-4 densely spinulose. Outer spines on rami small and naked. Outer setae on rami small. Armature formula for legs 1-4 as follows:

Coxa Basis Exopod Endopod

Leg 1 0-1 1-I I-0; I-1; I + 2, I, 3 0-1; 0-1; I+ 1, I, 3 Legs 2 & 3 0-1 1-0 I-0; I-1; I + 1, I, 4 0-1; 0-1; I+ 1, I, 3 Leg 4 0-1 1-0 I-0; I-1; I + 1, 1, 6 0-1; 0-1; 1, I, 2

Leg 5 ( Fig. 2F View Fig ) 2-segmented, consisting of protopod and exopod; protopod directed posterolaterally ( Fig. 1B View Fig ), distinctly articulated from somite, with spinules on distal surfaces, armed with 1 long, simple posterolateral seta; exopod directed posteriorly, 1.91 times longer than wide (44 × 23 μm), with round distal margin, armed with 4 simple setae, and ornamented with rows of spinules on outer and distal margins; 4 setae on exopod 30, 25, 33, and 33 μm, respectively, from outer proximal to distal. Leg 6 ( Fig. 2G View Fig ) represented by 2 small setae in genital aperture.

Male. Body ( Fig. 3A View Fig ) similar to that of female. Body length of largest dissected specimen 0.72 mm. Prosome 439 × 276 μm, comprising unsegmented cephalothorax and second to fourth pedigerous somites, gradually narrowing posteriorly. Urosome ( Fig. 3B View Fig ) 6-segmented, consisting of fifth pedigerous somite, genital somite, and 4 abdominal somites. Fifth pedigerous somite 114 μm wide, distinctly wider than genital somite, with scattered spinules on ventral surface. Genital somite much wid- er than long (45 × 98 μm) with oblique row of spinules on ventral surface of each genital operculum. Four absominal somites 34 × 58, 42 × 47, 40 × 40, and 14 × 31 μm, respectively. Proximal 3 abdominal somites lacking spinules on ventral surface. Anal somite small, with spinules on ventral surface, incompletely articulated from third abdominal somite on dorsal surface ( Fig. 3C View Fig ). Caudal ramus ( Fig. 3C View Fig ) 7.08 times longer than wide (85 × 12 μm), armed as in female, but dorsal seta (seta VII) pinnate.

Rostrum as in female. Antennule as in female, except bearing 1 additional seta on third segment at place of dark circle in Fig. 1F View Fig . Anatenna ( Fig. 3D View Fig ) different from that of female in having on second endopodal segment 1 curved, additional setiform element bearing row of 8 minute, pedunculate suckers.

Labrum ( Fig. 3E View Fig ) elongated, with pair of suckers posterolaterally on ventral surface; posterior margin concave, with row of 12 spinules. Mandible, maxillule, and maxilla as in female. Maxilliped ( Fig. 3F View Fig ) consisting of 3 segments and terminal claw; first segment (syncoxa) short, wider than long, with 1 long, naked seta at inner distal corner; second segment (basis) nearly rectangular, with 2 small setae at inner subdistal region and patch of about 10 spinules at inner distal region, and ornamented with 3 rows of minute spinules on distal surfaces; short third segment (endopod) unarmed; terminal claw large, weakly curved, with 3 unequal setae on proximal region, row of minute spinules on distal half of inner margin, and 1 small setule subdistally.

Legs 1-4 shaped and armed as in female. Leg 5 ( Fig. 3B View Fig ) consisting of 1 dorsolateral seta of fifth pedigerous somite and 1-segmented, free exopod; exopodal segment about 3.5 times longer than wide, armed and ornamented as in female. Leg 6 absent ( Fig. 3B View Fig ).

Etymology. The specific name is a combination of the Greek words lepto (= slender) and cerco (= tail), alluding to the slender caudal rami of the new species.

Remarks. Myzomolgus leptocercosus n. sp. closely resembles M. orientalis Kim, 2001 in sharing similar forms of the body, mandible and maxilla, and the elongate caudal rami, five setae (rather than 4 setae as in other congeners) on the first antennular segment, and a seta on the basis of the antenna ( M. tenuis Kim, 2001 also has one seta on the basis, but other four congeners lack the seta). Nevertheless, M. leptocercosus n. sp. and M. orientalis cannot be considered to be a same species, due to the significant differences between them, as follows: (1) the caudal ramus of M. leptocercosus n. sp. are relatively longer than that of M. orientalis , 10.7 times longer than wide, compared to 7.14 times longer than wide in the latter species ( Kim, 2001); (2) the second segment of the antennule is armed with 14 setae in M. leptocercosus n. sp. in contrast to 11 setae in M. orientalis , as recorded by Kim (2001); (3) all swimming legs of M. leptocercosus n. sp. are armed with an inner coxal seta, whereas legs 3 and 4 of M. orientalis lack the inner coxal seta; and (4) the first to third abdominal somites of the male of M. leptocercosus n. sp. are smooth, without any spinular ornamentation, but in M. orientalis the first and second abdominal somites of the male are ornamented with a transverse row of spinules along the posteroventral border.