Three new species of Myzomolgus (Copepoda, Cyclopoida, Catiniidae) associated with sipunculan worms from a tidal flat in Phuket, Thailand Author Kim, Jae-Sang Hong and Il-Hoi ihkim@gwnu.ac.kr text Journal of Species Research 2021 10 3 287 300 journal article 10.12651/JSR.2021.10.3.287 2713-8615 13140263 Myzomolgus spatulatus n. sp. ( Figs. 4-6 ) Material examined. 2$$, 1♂ from washings of sipunculid worms, northeastern tidal flat in Phuket , Thailand (approximately 8°01′25″N , 98°24′53″E ), 13 July 2015 . Holotype ($, MABIK CR00247442 ). Dissected paratypes (1$, 1♂ ) are retained in the collection of the junior author. Female. Body ( Fig. 4A ) narrow, dorsoventrally flattened. Body length 0.69 mm. Prosome 400 × 193 μm, with nearly parallel lateral margins. Cephalothorax narrowing anteriorly, consisting of completely fused cephalosome and first pedigerous somite, 190 × 184 μm, slightly narrower than second pedigerous somite. Prosomal somites (cephalothorax and second to fouth pedigerous somites) bearing broad membranous flange along posterodorsal margin. Urosome ( Fig. 4B, C ) 5-segmented. Fifth pedigerous somite large, wider than genital double-somite, ornamented with 1 oblique row of spinules on lateral margins, 2 horizontal rows of spinules on ventral surface, and 1 sclerotization band ventrally ( Fig. 4C ). Genital double-somite wider than long (61 × 74 μm), gradually narrowing posteriorly, with 2 horizontal rows of spinules on ventral surface and 1 sclerotization band proximally on ventral surface. Genital apertures not seen in dorsal and ventral views of urosome, probably positioning laterally. Three free abdominal somites 46 × 45, 34 × 33, and 32 × 35 μm, respectively. Anal somite with deep postero- median incision and longitudinal row of several spinules on each side of ventral surface ( Fig. 4C ). Caudal rami divergent, rectangular, 4.55 times longer than wide (50 × 11 μm), armed with 7 setae; seta II positioning dorsally near middle of ramus length; seta VII weakly pinnate, other 6 setae naked; seta IV the longest, 130 μm long, feused at base with second longest seta V (57 μm long). Rostrum ( Fig. 4D ) absent. Ventral rostral area bearing tapering sclerotized region. Antennule ( Fig. 4E ) 6-segmented; armature formula 4, 14, 9, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; all setae naked; first segment with massive, spatulate process on ventral surface; second segment with 1 cusp on proximal fourth of anterior margin. Antenna ( Fig. 5A ) strongly recurved, consisting of coxobasis and 3-segmented endopod; coxobasis the largest segment, unarmed, but ornamented with several rows of setules on surfaces; first endopodal segment unarmed, with several setules on inner side; second endopodal segment armed with 1 large, pedunculated sucker, 1 simple seta, and 1 flattened seta bearing spinules distally; third endopodal segment with 6 setae (1 pinnate and 5 naked, including stiffened distal 4 of latters) and ornamented with 1 patch of setules at outer distal corner. Fig. 4. Myzomolgus spatulatus n. sp. , female. A, habitus, dorsal; B, urosome, dorsal; C, urosome, ventral; D, cephalic region, ventral; E, antennule; F, mandible; G, maxillule. Scale bars: A, 0.1 mm; B, C, 0.05 mm; D, E, 0.02 mm; F, G, 0.01 mm. Fig. 5. Myzomolgus spatulatus n. sp. , female. A, antenna; B, maxilla; C, leg 1; D, leg 2; E, leg 4. Scale bars: A, C- E, 0.02 mm; B, 0.01 mm. Labrum missing. Mandible ( Fig. 4F ) with protrusion subdistally on dorsal side and distally armed with 4 elements: 2 shorter ventral spines, 1 broader, bilaterally spinulose element, and 1 setiform, unilaterally spinulose element. Maxillule ( Fig. 4G ) as small lobe bearing 2 small and 1 longer, simple setae. Maxilla ( Fig. 5B ) 2-segmented; proximal segment (syncoxa) large, unarmed, but ornamented with numerous large spinules on posterior surface; distal segment (basis) rudimentary, tipped with 2 small setae. Maxilliped absent. Fig. 6. Myzomolgus spatulatus n. sp. , male. A, habitus, dorsal; B, urosome, ventral; C, antennule; D, labrum; E, maxilla; F, maxilliped. Scale bars: A, 0.1 mm; B, 0.02 mm; C- F, 0.01 mm. Legs 1-4 with 3-segmented rami ( Fig. 5C- E ). Leg 3 same as leg 2 in form and armature. Coxae ornament- ed with spinules on outer side; inner coxal seta present in legs 1-3, but absent in leg 4. Intercoxal plate with spinules along distal margin. Basis with spinules on out- er and distal margins; outer seta pinnate in legs 1-3, but naked in leg 4. Inner distal spine on basis of leg 1 broad, spinulose along outer margin. Endopods slightly longer than exopods; both rami spinulose along outer margin. Third endopodal segment of leg 4 elongate, as long as proximal 2 segments together. Armature formula for legs 1-4 as follows: Coxa Basis Exopod Endopod Leg 1 0-1 1-I I-0; I-1; I+ 2, 1, 3 0-1; 0-1; 1, 1, 3 Legs 2 & 3 0-1 1-0 I-0; I-1; I+ 1, 1, 4 0-1; 0-1; 1, 1, 3 Leg 4 0-0 1-0 I-0; I-1; I+ 1, 1, 4 0-1; 0-1; 1, I, II Leg 5 ( Fig. 4B, C ) consisting of short protopod and 1-segmented, free exopod; protopod fused with somite, with discernible outer margin but lacking inner margin, armed with 1 naked seta at posterolateral corner; exopodal segment 2.65 times longer than wide (45 × 17 μm), gradually narrowing distally, armed with 4 naked setae (2 outer and 2 distal), and ornamented with 3 transvese rows of spinules. Leg 6 invisible. Male. Body ( Fig. 6A ) similar to that of female. Body length 485 μm. Urosome ( Fig. 6B ) 6-segmented, gradually narrowing posteriorly. Fifth pedigerous somite broader than genital somite, 73 μm wide, with 2 rows of spinules on ventral surface. Genital somite 33 × 57 μm, with 3 rows of spinules, 1 row on anterior ventral surface and 2 rows each on genital operculum. Four abdominal somites 28 × 46, 25 × 39, 18 × 32, and 18 × 28 μm, respectively. First abdominal somite with 3 rows of spinules, 1 row on ventral surface and 2 rows each ventrolaterally. Caudal ramus 2.5 times longer than wide (25 × 10 μm), armed as in female, and ornamented with 4 spinules, 2 distally and 2 subdistally. Rostrum as in female. Antennule ( Fig. 6C ) with 2 additional setae each on first and third segments; thus, armature formula 5, 14, 10, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; one of setae on first segment pinnate, positioning on ventral process. Antennae missing (damaged). Labrum elongate longitudinally; posterior part semicircular, with patch of spinules on its ventral surface. Mandible and maxillule as in female. Maxilla ( Fig. 6E ) similar to that female, but with 2 additional rows of spinules on ventral surface and fewer spinules on posterior surface. Maxilliped ( Fig. 6F ) consisting of 3 segments and terminal claw; first segment (syncoxa) with narrowed proximal part, armed with 1 naked seta on subdistal inner margin; second segment (basis) armed with 2 unequal naked setae subdistally and ornamented with 3 rows of spinules, each row on inner, anterior, and posterior margins; small third segment (endopod) unarmed; terminal claw as long as basis, weakly curved, with 3 setae proximally, 1 cusp near middle of inner margin, and 1 setule subdistally on outer margin. Legs 1-4 as in female. Leg 5 ( Fig. 6B ) also similar to that of female. Leg 6 ( Fig. 6B ) represented by 1 naked seta on distal apex of genital operculum. Etymology. The specific name spatulatus is derived from the Latin spatula (= a spoon), alluding to the spatulate process on the first segment of the antennule. Remarks. The armature conditions of swimming legs of Myzomolgus spatulatus n. sp. are unique within the genus. In the new species the inner coxal seta of swimming legs is present in legs 1-3. This armature condition of M. spatulatus n. sp. differs from that of M. orientalis where only legs 1 and 2 bear the inner coxal seta, that of M. Leptocercosus n. sp. where all of legs 1-4 bear the inner coxal seta, and those of other four species where no swimming legs bear the inner coxal seta. The third endopodal segments of legs 1-3 of M. spatulatus n. sp. are armed each with 5 setae, which is an another unique armature condition, because the numbers of armature elements (setae or spines) on the same segments of legs 1-3 are four in M. tenuipes and six in the remaining five congeners. It is notable that M. spatulatus n. sp. is similar in body form (with narrow, fusiform) to M. tenuis . However, in addition to the differences in leg armatures, there are various differences, such as the absence of the rostrum (as in M. sipunculensis , but well-developed in M. tenuis and other congeners), the posession of the longer caudal rami (4.55 times as long as wide, against to 1.46 times in M. tenuipes ), the possession of the 14 setae on the second antennular segment (cf. 11 setae in M. tenuipes ), and the absence of a seta on the basis of the antenna (cf. the seta present in M. tenuipes , M. orientalis , and M. leptocercosus n. sp. ).