Scoliosoma haplomerosum, Kim & Boxshall, 2020

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 : 560-563

Scoliosoma haplomerosum gen. et sp. nov.

( Figs. 375 , 376 )

Type material. Holotype (♀, urosome partially damaged, MNHN-IU-2014-21413 ) , paratypes (1 damaged ♀ with maxillae dissected and 5 intact copepodids, MNHN-IU-2014-21414), and dissected paratypes (1 damaged ♀ and 1 copepodid, figured) from galls on the surface of brachial wall of Polycarpa cryptocarpa Sluiter, 1885 (MNHN-IT-2008-6441 = MNHN S1 POL.B 454); 6 copepodids from fibrillar networks on outer surface of one of galls, CRRF OCDN 7521-K, near Port Vila, Vanuatu (17°45.41 Ś, 168°16.90 É), depth 12 m, 16 November 2000.

Etymology. The specific name is derived from the Greek haplo (=single) and mer (=apart), referring to the unsegmented endopod of the maxilla in the new species.

Descriptionoffemale. Body ( Fig. 375A ) strongly curved ventrally, with weak exoskeleton; body length 2.12 mm. Prosome tapering anteriorly, divisible by 3 contrictionsinto cephalosome, firstand secondpedigerous somites; lateral margins of dorsal cephalic shield extended ventrally and concealing entire mouthparts in lateral view. Brood pouch formed by fused third and fourth pedigerous somites. Urosome slightly shorter than prosome, 6-segmented but first urosomite not articulated from prosome. Genitalsomite (second urosomite) short. Posterior part of anal somite ornamented with minute spinules. Caudal rami ( Fig. 375B ) small, 1.27 times longerthanwide (42×33 μm), rounded distally, covered with minute spinules; armed with 3 smallsetae (outer lateral, medioventral, and distal).

Rostrum ( Fig. 375C ) semicircular, rounded apically, wider than long, and covered with minute spinules. Antennule ( Fig. 375D ) shortand broad, leaf-like, 122×102 μm, unsegmented but with trace of articulation on posterior side; armed with several small setae plus minute surface spinules distally. Antenna ( Fig. 375E ) stout, 4-segmented; coxa and basis unarmed, each much wider than long; first endopodal segment also unarmed, as long as wide, its outer margin much shorter than inner margin; second endopodal segment about twice as long as wide; armed with 4 small setae plus small terminal claw, about half as long as segment.

	Coxa	Basis	Exopod	Endopod
Leg 1	0-0	1-0	1-0; 3, 1, 5	1, 2, 4
Leg 2	0-0	1-0	1-0; 4, 1, 5	1, 2, 4
Leg 3	0-0	1-0	1-0; 4, 1, 5	1, 2, 3
Leg 4	0-0	1-0	1-0; 4, 1, 5	0, 2, 3
Legs 5 and 6 each represented by pair of small setae.

Labrum ( Fig. 375F ) shortand broad, unornamented, with convex posterior margin. Mandible ( Fig. 375G ) with broad coxalgnathobase bearing along medial margin, strong distal tooth, uneven serrate middle region, and pointed proximal corner: palp (fused basis, exopod and endopod) bilobed distally; short outer lobe (exopod) with 4 naked setae; longer inner lobe (distal part of basis + unsegmented endopod) with 6 setae, proximal seta (representing medial seta on basis), second proximal seta (originally medial seta of first endopodal segment), and 4 setae from original second endopodal segment; first 2 proximal setae bearing few spinules on margins, other setae naked. Maxillule ( Fig. 375H ) not articulated, distally bilobed; outer lobe (palp, comprising fused basis, exopod and endopod) bearing 1 outer proximal and 4 distal setae, 1 distal seta weakly pinnate; inner lobe (precoxal arthrite) larger than outer lobe, armed with 5 pinnate setae on medial margin. Maxilla ( Fig. 375I ) 3-segmented; syncoxa with 2 endites bearing 2 setae on first endite and 1 large seta on second; basis with robust claw ornamented with serrate membrane in distal third; unsegmented endopod with 2 largeand 2 small setae, all naked. Maxilliped ( Fig. 375J ) as distally narrowing lobe bearing 2 weakly pinnate setaeon apex.

Legs 1–4 ( Figs. 375K, L , 376A, B ) rudimentary, lacking defined protopod. Legs 1–3 bilobed, unsegmented; both rami covered with minute spinules. Inner lobe (endopod) as large as outer lobe (exopod) in leg 1, but much smaller than outer lobe in legs 2 and 3. Leg 4 represented by single lobe (exopod). Legs 1 and 2 with 1 outer proximal seta on outer lobe; this seta absent in legs 3 and 4. Other setae 1, 3, 5, and 7 on exopods of legs 1–4, respectively, and 0, 2, and 3 on endopods of legs 1–3, respectively; all setae rudimentary. Leg 5 absent.

Male. Unknown.

Description of Copepodid V female. Body ( Fig. 376C, D ) not swollen, slightlycurvedventrally, poorly segmented, much smaller than adult (body length of figured specimen 533 μm). Cephalic region widest. Prosome-urosome division obscure. Fifth pedigerous and genital somites ( Fig. 376E ) not articulated from one another.Abdomen 2-segmented in dorsal view, but second abdominal somite with additional suture line on ventral surface. Caudal rami 2.3 times longer than wide (37×16 μm); armedwith 3 smallsetae. Antennule ( Fig. 376F ) much narrower than in adult, 4-segmented; armature formula 13, 11, 3, and 14; setae distinct.Antenna, labrum, mandible, maxillule, maxilla, and maxilliped as in adult female. Legs 1–4 ( Fig. 376 G-J) biramous, each with unsegmented or partially segmented protopod, and welldefined exopod and endopod; exopods incompletely 2- segmented, and endopods unsegmented; setae distinct and naked. Armature formula for legs 1–4 as follows:

Remarks. The galls inhabited by the adult females were flattened and about 1.2 mm in diameter, with irregular margins. Each gall contained a single female and one was found with its anal somite protruding out of the gall. Six copepodids discovered were among fibrillary network on the external surface of one of the galls.

Polycarpa cryptocarpa , the ascidian host of Scoliosoma haplomerosum gen. et sp. nov., is known to harbour several other species of ascidicolous copepods. One of them, Sphaerothylacus polycarpae Sluiter, 1884 , is remarkable as it was first recognized as a cirripede by its original describer ( Sluiter, 1884), but Grygier (1993) later concluded that it was a copepod. The absence of frontolateral horns in the nauplii of S. polycarpae illustrated by Sluiter (1884) indicates that his species cannot be classified as a cirripede, but the presence of transverse sculpturing appearing to represent articulations on the hind body of the nauplii is unusual for copepods. Sphaerothylacus polycarpae is not confusable with Scoliosoma haplomerosum gen. et sp. nov., because it is an almost spherical parasite with a root-like holdfast and a pore on the opposite side of the body from the holdfast.

Another remarkably transformed copepod associate of P. cryptocarpa is Paralobodelphys setipoda Gotto, 1981 , which was described as a new genus and species from the Great Barrier Reef, Australia. The specimens were “contained in a circular area” (Gotto, 1881), suggesting that the species is a gall-inhabitant as is S. haplomerosum gen. et sp. nov. Although this species has a body similar to that of S. haplomerosum gen. et sp. nov, its cephalic appendages and legs are quite different from those of S. haplomerosum gen. et sp. nov. For example, the mandible lacks a coxal gnathobase, the maxilla is only 2-segmented, and the legs have elongate rami in P. setipoda .

Figures

FIGURE 375. Scoliosoma haplomerosum gen. et sp. nov., female.A, habitus, right; B, caudal rami, dorsal; C, rostrum; D, antennule; E, antenna; F, labrum; G, mandible; H, maxillule; I, maxilla; J, maxilliped; K, leg 1; L, leg 2. Scale bars: A, 0.2 mm; B, C, E–L, 0.02 mm; D, 0.05 mm.

FIGURE 376. Scoliosoma haplomerosum gen. et sp. nov., female: A, leg 3; B, leg 4. Copepodid V female: C, habitus, dorsal; D, habitus, left; E, urosome, dorsal; F, antennule; G, leg 1; H, leg 2; I, leg 3; J, leg 4. Scale bars: A, B, F–J, 0.02 mm; C, D, 0.1 mm; E, 0.05 mm.