Myzomolgus cucullatus, 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 : 296-300

Myzomolgus cucullatus n. sp. ( Figs. 7 , 8 )

Material examined. 2$$ from washings of sipunculid worms, northeastern tidal flat in Phuket, Thailand (approximately 8°01′25″N, 98°24′53″E), 13 July 2015. Holotype ($, MABIK CR00247443 ). Dissected paratype ($) retained in the collection of the junior author.

Female. Body ( Fig. 7A ) dorsoventrally flattened, consisting of broad prosome and small urosome. Body length 1.07 mm. Prosome 795 × 448 μm, occupying 74% of body length. Prosomal somites with well-developed epimera. Cephalothorax consisting of completely fused cephalosome and first abdominal somite, lacking dorsal suture line. Tergite of fourth pedigerous somite hood-like, slightly narrower but distinctly longer than those of second and third pedigerous somites. Urosome ( Fig. 7B, C ) 5-segmented. Fifth pedigerous somite 132 μm wide, wider than genital double-somite, with 3 faint membranes on each side of dorsal surface and 4 transverse rows of spinules (including 2 middle rows consisting of minute spinules) on ventral surface ( Fig. 7C ). Genital double-somite hexagonal, tapering laterally, with lateral apex at 40% region of double-somite length, small transparent lobe at lateral apex, and several membranes on dorsal surface; genital apertures positioning ventrally ( Fig. 7C ). Three free abdominal somites 34 × 66, 23 × 62, and 15 × 54 μm, respectively. Anal somite ( Fig. 8A ) obscurely articulated dorsally from second free abdominal somite, with several spinules at outer distal corners. Caudal ramus ( Fig. 8A ) 1.60 times longer than wide (32 × 20 μm), armed with 7 naked setae, and ornamented with row of spinules subdistally; seta V the longest, 427 μm long; seta IV the second longest 136 μm long.

Rostrum ( Fig. 7D ) extremely broad, much broader than long, unornamented, with slightly convex posterior margin. Antennule ( Fig. 7E ) slender, 298 μm long, 6-seg- mented; armature formula 4, 9, 8, 4 + aesthetasc, 2 + aesthetasc, and 7 + aesthetasc; setae thin, 5 of them pinnate (1 on fourth, 1 on fifth, and 3 on sixth segments); aesthetascs extremely thin, confusable with setae; first segment lacking any process. Antenna ( Fig. 7F ) consisting of coxobasis and 3-segmented endopod; spinulose coxobasis and setulose first endopodal segment unarmed; second endopodal segment armed with 1 large pecunculate suck- er and 2 setae (1 proximal and 1 distal); third endopodal segment 27 × 15 μm, armed with 6 setae (2 smaller ones weakly pinnate, 4 larger ones naked).

Labrum missing (easily destroyed). Mandible ( Fig. 7G ) with membranous flap on ventral side, distally armed with 4 elements: 1 stout ventral spine bearing 2 large setules and 3 longer, spinulose dorsal spines (or spiniform setae). Paragnath ( Fig. 8B ) as small, spinulose digitiform lobe. Maxillule ( Fig. 8C ) as short lobe bearing 4 naked setae apically. Maxilla ( Fig. 8D ) 2-segmented; proximal segment (syncoxa) large, tapering medially, with 1 row of spinules along posterior surface and numerous minute spinules on anterior side of ventral surface; distal segment (basis; Fig. 8E ) small, tapering distally, tipped with 1 spine, and bearing 2 small setae subdistally. Maxilliped absent.

Legs 1-4 with 3-segmented rami ( Fig. 8F- H ). Coxa lacking inner seta in legs 1-4, but with few small spinules on outer margin. Intercoxal plate fringed with spinules along distal margin. Basis broadened, with spinules on distal margin. Inner distal spine on basis of leg 1 small, straight, with spinules on both margins. Rami of legs 1-4 spinulose along outer margin. Endopods slightly longer than exopods. Leg 3 same as leg 2 in segmentation and armature condition. Endopod of leg 4 incompletely articulated between second and third segments. Armature formula for legs 1-4 as follows:

Coxa Basis Exopod Endopod

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

Leg 5 ( Fig. 8I ) relatively large, consisting of protopod and 1-segmented exopod; protopod distinctly articulated from somite, bearing 1 pinnate seta at outer distal corner and 1 row of spinules on outer side; exopod nearly fusiform, 2.21 times longer than wide (93 × 42 μm) extending over distal margin of genital double-somite, armed with 3 spines and 1 seta, and ornamented with 3 transverse rows of spinules; lengths of spines and seta 55, 51, 45 (seta), and 55 μm, from outer proximal to distal. Leg 6 ( Fig. 7H ) represented by 3 small, spinule-like setae in genital aperture.

Male. Unknown.

Etymology. The specific name is derived from the Latin cucullus (= a hood), alluding to the expanded, hood-like dorsal tergite of the fourth pedigerous somite.

Remarks. The proximal two segments of the antenna (the basis and the first endopodal segment) of M. cucullatus n. sp. are unarmed. This feature of M. cucullatus n. sp. is shared with three congeners ( M. stupendus , M. sipunculensis , and M. spatulatus n. sp.). Myzomolgus spatulatus n. sp. may be excluded from a futher comparison with M. cucullatus n. sp., because the first antennular segment in M. spatulatus n. sp. bears a spatulate process, legs 1-3 bear an inner coxal seta (this seta is absent in M. stupendus , M. sipunculensis , and M. cucullatus n. sp.), and the third endopodal segment of legs 1-3 is armed with five armature elements (not six as in the three latter species).

The caudal ramus of M. stupendus is six times longer than wide (Bocuet and Stock, 1957), much longer than that of M. cucullatus n. sp. (1.60 times longer than wide). The rostrm is absent in M. sipunculensis , whereas it is well-developed in M. cucullatus n. sp. Threfore, M. cucullatus n. sp. can easily be differentiated from M. stupendus and M. sipunculensis . Most of all, M. cucullatus n. sp. may be distinguished from all congeners by its three outstanding features, the hood-like, broad tergite of the fourth pedigerous somite, the presence of a small lobe at lateral apice of the genital double-somite, and the ventrally positioned female genital apertures.

Major morphological differences between seven known species of Myzomolgus are summarized in Table 1.

Figures

Fig. 7. Myzomolgus cucullatus n. sp., female. A, habitus, dorsal; B, urosome, dorsal; C, urosome, ventral; D, rostral area, ventral; E, antennule; F, antenna; G, mandible; H, right genital area, ventral. Scale bars: A, 0.1 mm; B-E, 0.05mm; F-H, 0.02 mm.

Fig. 8. Myzomolgus cucullatus n. sp., female. A, anal somite and caudal rami, dorsal; B, paragnath; C, maxillule; D, maxilla; E, distal segment of maxilla; F, leg 1; G, leg 2; H, leg 4; I, leg 5. Scale bars:A, D, E, 0.02 mm; B, C, 0.01mm; F-I, 0.05 mm.