Sesir parvipes Stock, 1967

Kim, Il-Hoi & Boxshall, Geoff A., 2020, Untold diversity: the astonishing species richness of the Notodelphyidae (Copepoda: Cyclopoida), a family of symbiotic copepods associated with ascidians (Tunicata), Megataxa 4 (1), pp. 1-6 : 409-410

Sesir parvipes Stock, 1967

( Figs. 271 , 272 )

Material examined. 41 ♀♀ (MNHN-IU-2018-1880) and dissected 2 ♀♀, 1 ♂from Pyura gangelion (Savigny, 1816) , Djibouti, 13 October 1996.

Descriptionoffemale. Body ( Fig. 271A ) elongate, cylindrical, curved ventrally, bodylength 1.75 mm. Prosome 1.39 mm in length; cephalosome small, 4 pedigerous somites discernible by weak constrictions, increasing in length from first to fourth. Second to fourth pedigeroussomites forming slender brood pouch. Free urosome ( Fig. 271B ) cylindrical, unsegmented, gradually narrowingposteriorly, about 330×100 μm, excluding caudal rami. Caudalramus ( Fig. 271C ) tapering distally, about 3.7 timeslongerthan wide (115×31 μm), incompletely articulated from abdomen: armed with 2 proximal and 4 distal setae; distal setae not articulated at base, longest seta 64 μm long, inner seta rudimentary; 2 proximal setae positioned at 24 and 29% of ramus length.

Rostrum ( Fig. 271D ) articulated at base, plate-like, 73×67 μm, gradually broadening to maximum width at 60% of rostral length, then strongly tapering towards blunt apex. Antennule ( Fig. 271E ) tapering, 163 μm long, 8-segmented; articulations between distal 3 segments incomplete; armatureformula 3, 16, 9+aesthetasc, 6, 3+aesthetasc, 2, 2+aesthetasc, and 7+aesthetasc; all setae naked. Antenna 4-segmented; coxashortand unarmed; basis 40×17 μm, with vestigial seta at outer distal corner (representing exopod); first endopodal segment 28×22 μm, with seta subdistally on inner margin; compound distal endopodal segment armed with 8 setae (arranged as 1, 3, 2, and 2) and ornamented with 2 groups of spinules on outer margin; slender terminal claw 30 μm long, half aslongas segment.

Labrum weak, damaged during dissection. Mandible ( Fig. 271F ) with slender, elongate coxal gnathobase bearing 6 acutely pointed teeth only: basis with tuft of setules proximally and 1 setasubdistally on medial margin; exopod armed with 5 large setae (proximalmost seta slightly shorter than distal 4); endopod incompletely articulated from basis, 2-segmented, armedwith 4 and 9 setae on first and second segments, respectively; larger distal setaeon second segment naked. Maxillule ( Fig. 271G ) with 6 setae on arthrite; coxal endite and epipodite absent; basis with 1 seta mediodistally; exopod and endopod lobate, not articulated from basis, each armed with 3 setae distally. Maxilla ( Fig. 271H ) 5-segmented, strongly flexed between syncoxa and basis; syncoxa with 5 setae arranged as 2, 1, 1, and 1; basis with 2 setae; endopod with 1, 1, and 3 setae on first to third segments, respectively; seta on first and second endopodal segments and one setaon thirdsegment large and ornamented thick setules along convex margin. Maxilliped ( Fig. 272A ) unsegmented but with small, lobate apical region, armed with 8 medial and 2 unequal apical setae; 8 medial setae grouped as 4 and 4; one seta from each group shorter and broader than others.

Legs 1–3 with 3-segmented rami, leg 4 with 3- segmented exopod and 2-segmented endopod. Basis, exopods and endopods heavily ornamented with rows of spinules along outer margin and on anterodistal surface. Legs 1–4 lacking inner setaon coxa ( Fig. 272B, C ). Outer seta on basis large and pinnate in leg 1, small in legs 2 and 3, and absent in leg 4 ( Fig. 272C ). Inner distal spine on basis of leg 1 proximally pinnate and distally spinulose, slightly shorter than first endopodal segment. All setae on leg 4 naked. Armature formula for legs 1–4 as follows:

	Coxa	Basis	Exopod	Endopod
Leg 1	0-0	1-I	I-1; I-1; II, 2, 3	0-1; 0-1; 1, 2, 2
Leg 2	0-0	1-0	1-1; 1-1; 3, 1, 4	0-1; 0-1; 1, 2, 3
Leg 3	0-0	1-0	1-1; 1-1; 2, 1, 4	0-1; 0-1; 1, 2, 3
Leg 4	0-0	0-0	1-1; 1-1; 2, 1, 3	0-0; 0, 2, 1

Leg 5 ( Fig. 271B , 272D ) rudimentary, consistingof unarmed protopod fused with somite plus oval exopodal segment, about 1.7 timeslongerthan wide (31×18 μm): armedwith 1 seta (13 μmlong) distally.

Description of male. Body ( Fig. 272E ) small, ventrally curved, narrowingposteriorly. Bodylength 0.61 μm. Cephalic shield much wider than pedigerous somites. Urosome ( Fig. 272F ) tapering, incompletely 6-segmented with fifth pedigerous somite obscurely defined from fourth. Genitalsomite 59×98 μm; 4 abdominalsomites 63×78, 48×59, 43×44, and 12×33 μm, respectively. Cuticle of genital and first and second abdominal somites each with conspicuous transverse band of sclerotization proximally. Anal somite ( Fig. 272G ) very short, with transverse sclerotization band in middle. Caudal ramus ( Fig. 272G ) short, but longerthan anal somite and 1.5 timeslongerthan wide (18×12 μm): armed with 1 claw and 4 setae; claw 22 μm long; largest seta 57 μm long, 3.2 times longer than caudal ramus.

Rostrum, antennule, antenna, mouthparts, and legs 1–4 asin female.

Leg 5 ( Fig. 272F ) 2-segmented: protopod very short, unarmed but ornamented with row of minute spinules ventrodistally; free exopodal segment, 19×9 μm, subrectangular, armed distally with 1 seta, and ornamented with spinules on inner margin. Leg 6 ( Fig. 272F ) represented by smallseta on apex of genital operculum.

Remarks. Stock (1967) described this species based on two females collected in the Dahlak Archipelago in the Red Sea. Our specimens were collected from the coast of Djibouti, close to the type locality. They exhibit a few differences from the original description, as follows: (1) the body is not clearly segmented, and this is undoubtedly because the newly-observed specimens are fully grown, expanded females; (2) the secondendopodal segment of the mandible is armed with 9 setae (not 8 as in the original description); (3) the maxillular arthrite is armed with 6 setae (not 7); (4) the protopod of leg 4 is not broadened as in the original description. These differences probably reflect inaccuracies in the original description. The important morphological features of this species, such as the characteristic form of the maxillule, maxilla and coxal gnathobase of mandible, and the leg armature are all consistent with the original description.

Figures

FIGURE 271. Sesir parvipes Stock, 1967, female. A, habitus, right; B, urosome, ventral; C, right caudal ramus, ventral; D, rostrum; E, antennule; F, mandible; G, maxillule; H, maxilla. Scale bars: A, 0.2 mm; B, 0.1 mm; C, 0.05 mm; D–H, 0.02 mm.

FIGURE 272. Sesir parvipes Stock, 1967, female. A, maxilliped; B, leg 1; C, leg 4; D, leg 5. Male: E, habitus, right; F, urosome, ventral; G, anal somite and caudal rami, dorsal. Scale bars: A–D, G, 0.02 mm; E, 0.1 mm; F, 0.05 mm.