Diceratus unidentatus, 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 : 503-506

Diceratus unidentatus gen. et sp. nov.

( Figs. 334 , 335 )

Typematerial. Holotype (intact ♀, MNHN-IU-2014- 21383 ) anddissectedparatype (♀, figured) from Eudistoma hepaticum (Van Name, 1921) (MNHN-IT-2008-4067 = MNHNA 3/ EUD /18), westof Îleta Cochons, Guadeloupe, depth 1–5 m, 01 January 1981.

Etymology. The specific name is a combination of the Latin uni (=one) and dentat (=toothed), referring to the single tooth on the coxal gnathobase of the mandible.

Descriptionoffemale. Body ( Fig. 334A ) stout, strongly curved ventrally: body length 1.12 mm. Dorsal cephalic shield expanded ventrolaterally, with 2 pairs of horn-likeprocesses posterolaterally ( Fig. 334B ); ventral process smaller than dorsal. Metasome 4-segmented: fourth pedigerous somite inflated, as long as anterior part of prosome. Free urosome recurved ventrally, 5- segmented. Anal somite and caudal rami densely covered with ornamentation of thick setules. Caudal rami ( Fig. 334C ) originating close to each other, elongate, about 3.5 times longer than wide (80×23 μm): armed with 6 setae, but setae hardly discernible from setules; 2 proximal setae positioned at 36 and 50% of ramus length.

Rostrum ( Fig. 334D ) large, longerthanwide (86×55 μm), gently tapering in proximal two-thirds and steeply tapering in distal third towards blunt apex, ornamented with many surface setules (or spinules). Antennule ( Fig. 334E ) stronglytapering, 7-segmented; armature formula 2, 16, 7, 3+aesthetasc, 3+aesthetasc, 1, and 10+2 aesthetascs; setae naked, crowded, and relatively short. Antenna ( Fig. 334F ) stout, 3-segmented; coxa and basis unarmed; endopod 41×15 μm, 0.67 times as long as basis, armed with 1 seta in middle and 3 setae distally plus weakly curved terminal claw, less than half length of endopod.

Labrum ( Fig. 334G ) strongly tapering, with soft, setulose distal part. Mandible ( Fig. 334H ) with coxal gnathobaseforkedintostrongdistaltoothwidelyseparated from narrow proximal branch provided with pectinate distal margin: basiswith 1 setaonmedial margin: exopod with 4 or 5 setae (outermost seta present or absent, if present, only half as long as other 4 setae): endopod with 1 and 7 setae on first and second segments, respectively; setae on second segment unequal, outer margin seta very small, spinule-like, distalmost seta (fifth seta) much longerthan others. Maxillule ( Fig. 334I ) with 6 setaeon arthrite, 1 on epipodite, 2 on basis, 4 onexopodand 3 (middle seta longest) on endopod; coxal endite absent. Maxilla ( Fig. 334J ) 4-segmented; syncoxawith 3, 2, and 2 setae on first to third endites, respectively; basis with 2 small setae and large claw ornamented with large spinules along distal half of concave margin; endopod small with 1 and 3 setae on first and second segments, respectively. Maxilliped ( Fig. 335A ) small, unsegmented, armedwith 2 setae distally, ornamented with spinules mainly on outer surface.

Leg 1 ( Fig. 335B ) with 3-segmented exopod and 2- segmented endopod; second endopodal segment armed with only 5 setae; inner and distal setae on third exopodal segment naked, all other setaeon both rami feebly pinnate. Legs 2–4 ( Fig. 335 C-E) with 3-segmented rami; endopods shorter than exopods. Inner coxal seta absentin legs 1–4. Outer seta on basis of legs 1–4 naked and uniformly sized. Basis of leg 1 with broad inner distal seta. Outer spine on first exopodal segment of leg 1 arched, large, 36 μm long, extending to base of second outer armature element of third segment. All setae on rami of legs 2–4 naked and bluntly tipped. Armature formula for legs 1–4 as follows:

Leg 5 ( Fig. 335F ) positioned posteroventrally on somite, represented by outer digitiform protopodal process tipped with seta and inner, pointed exopodal process bearing smallseta on outer margin.

Male. Unknown.

Remarks. The differences between the type species Diceratus unidentatus gen. et sp. nov. and D. bispinosus (Ooishi, 1998) comb. nov. include: (1) the apex of the rostrum is blunt in the type species (vs. acutely pointed in D. bispinosus ); (2) the caudalramus is elongate, about 3.5 timeslongerthan wide in Diceratus unidentatus gen. et sp. nov. (vs. at most 3.0 timeslongerthan wide in D. bispinosus , measured from the original illustration); (3) the coxal gnathobase of the mandible bears only 1 major tooth (vs. 1 major and 1 or 2 subsidiary teeth in D. bispinosus ); (4) the maxillular arthrite bears 6 setae (vs. 4 or 5 setae in D. bispinosus ); (5) the endopod of leg 1 is 2-segmented (vs. 3-segmented in D. bispinosus ); and (6) the third endopodal segment of leg 2 is armed with 4 setae (vs. 3 setaein D. bispinosus ).

Figures

FIGURE 334. Diceratus unidentatus gen. et sp. nov., female. A, habitus, right; B, cephalic horns; C, caudal rami, dorsal; D, rostrum; E, antennule; F, antenna; G, labrum; H, mandible; I, maxillule; J, maxilla. Scale bars: A, 0.1 mm; B, 0.05 mm; C–I, 0.02 mm.

FIGURE 335. Diceratus unidentatus gen. et sp. nov., female. A, maxilliped; B, leg 1; C, leg 2; D, leg 3; E, leg 4; F, leg 5. Scale bars: 0.02 mm.