Bysone brevicaudatus, 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 : 441-444

Bysone brevicaudatus sp. nov.

( Figs. 292 , 293 )

Type material. Holotype ♀ (dissected and mounted on a slide, MNHN-IU-2014-21355 ) from Stolonica multitestis Monniot C., Monniot F., Griffiths & Schleyer, 2001 (MNHN-IT-2008-8032 = MNHN A1/RIT/25), south of Matemo, Ibo I., Mozambique, Monniot coll., 17 November 1995.

Etymology. The specific name is derived from the Latin brev (=short) and caud (=tail) and refers to the relatively stout caudal rami.

Descriptionoffemale. Body ( Fig. 292A ) stout, curved ventrally. Prosome 1.40 mm long: dorsal cephalic shield clearly defined; first to fourth pedigerous somites discernible by constrictions; second and third pedigerous somites each with trace of tergite. Third and fourth pedigerous somites forming brood pouch. Free urosome ( Fig. 292B ) 5-segmented: genital somite well defined ventrally from preceding somite, but dorsally confluent. Anal somite about 1.5 times longer than preceding abdominal somite. Caudalramus ( Fig. 292C ) nearly rectangular, about 2.7 times longerthan wide (148×54 μm) and aslongas anal somite: armedwith 6 smallsetae (outer proximal, ventral, and 4 distal); all setae small, less than half of width of ramus at base; outer proximal and ventral setae located at 38 and 62% of ramus length, respectively.

Rostrum ( Fig. 292D ) 95×118 μm, indistinctly articulated at base tapering distally towards rounded apex. Antennule ( Fig. 292E ) 270 μmlong, 6-segmented; armatureformula 3, 14, 7+aesthetasc, 3+aesthetasc, 2+aesthetasc, and 9+2 aesthetascs; setae naked and short; distal seta on fourth and fifth segment annulated at base. Antenna ( Fig. 292F ) 4-segmented; coxa, basis, and first endopodal segment unarmed; compound distal endopodal segment about 3.1 times longer than wide (75×24 μm) and 1.3 times longerthan firstendopodal segment: armed with 1 subdistal and 3 distal setae, plus strongly curved terminal claw, about 0.65 times as long as segment.

Labrum ( Fig. 292G ) with concave posterior margin, pointed posterolateral corners, and broad posteromedian lobe bearing 2 denticles at apex. Mandible ( Fig. 292H ) withbroadcoxalgnathobasebearing 5 teethand 1 spinulelike subsidiary tooth distally at base of distalmost tooth: basis with 1 setaon medial margin; exopod as long as endopod, armed with 2 long proximal and 2 short distal setae: endopodarmedwith 2 and 5 setaeon firstand second segments, respectively; middle seta on second segment markedly longer than other 4 setae. Maxillule ( Fig. 292I ) with 9 setae on arthrite, 1 thin seta on coxal endite, 2 unequal setaeon epipodite, 3 unequal setae onmedial margin of basis, 3 setae distally on exopod and 2 setae distally on endopod. Maxilla ( Fig. 293A ) 5-segmented; syncoxa with 8 setae arranged as 3, 1, 2, and 2; basis with robust claw plus 1 seta; endopod with 1, 1, and 3 setae on first to third segments, respectively. Maxilliped ( Fig. 293B ) unsegmented, armed with 8 medial setae and 1 long outer seta.

Leg 1 ( Fig. 293C ) small, with 2-segmented rami; second segments of both rami compound, with traces of articulation; inner coxal seta pinnate; outer seta on basis much larger than that of legs 2–4; first exopodal segment lacking inner seta. Legs 2–4 each with 3-segmented exopod and 2-segmented endopod ( Fig. 293D, E ); both rami elongate; endopod extending to distal border of second exopodal segment; second endopodal segment about 3 times longer than first in legs 2 and 3, about 4 times longer in leg 4. Inner coxal seta present in legs 2–4, but that of leg 4 minute. Outer and distal setae on exopod of leg 2-4 short, naked, and bluntly tipped. Inner setae on exopod and all setae on endopod of leg 4 small and naked. Armature formula for legs 1–4 as follows:

	Coxa	Basis	Exopod	Endopod
Leg 1	0-1	1-0	I-0; IV, I, 3	0-1; 1, 2, 4
Legs 2 & 3	0-1	1-0	1-1; 1-1; 3, 1, 5	0-1; 1, 2, 5
Leg 4	0-1	1-0	1-1; 1-1; 2, 1, 5	0-1; 1, 2, 4

Leg 5 ( Fig. 293F ) represented by small ventrodistal lobe bearing 2 small exopodal setae distally; protopodal seta lacking.

Male. Unknown.

Remarks. The genus Bysone has comprised only the type species, B. operculatus since it was originally established based on material collected in Madagascar ( Stock & Humes, 1970). Bysone brevicaudatus sp. nov. shares important character states with the type species including: a 6-segmented antennule, 4 setae on the mandibular exopod, 2 and 5 setae, respectively, on the first and second segments of mandibular endopod, a 2- segmented exopod of leg 1, a similar form of maxilliped, and a similar form of the mandibular gnathobase which bears a spinule-like distal subsidiary denticle and has no proximal seta. These similarities are sufficient to confirm the placement of the new species in Bysone . The two species can be distinguished by reference to the maxilla, which has a stong claw plus a seta on the basis and the endopod is 3-segmented in the new species. In contrast, B. operculatus has the maxillary basis armed with only 2 setae and the endopod is 2-segmented. These differences in the maxilla are significant.

There are other differences between these two species: the maxilliped is armed with 9 (8+1) setae in B. brevicaudatus sp. nov. (vs. 5+1 setaein B. operculatus ), the maxillulary arthrite is armed with 9 setae in B. brevicaudatus sp. nov. (vs. 5 setaein B. operculatus ), the basis of leg 1 bears an outer seta (vs. seta absent in B. operculatus ), the second endopodal segment of leg 1 is armed with 7 setae (vs. 6 setae), and leg 5 is short, lobate (vs. digitiform in B. operculatus ). These differences are sufficient to support the establishment of the new species.

Figures

FIGURE 292. Bysone brevicaudatus sp. nov., female. A, habitus, right; B, urosome, dorsal; C, right caudal ramus, dorsal; D, rostrum; E, antennule; F, antenna; G, labrum; H, mandible; I, maxillule. Scale bars: A, 0.2 mm; B, 0.1 mm; C–I, 0.05 mm.

FIGURE 293. Bysone brevicaudatus sp. nov., female. A, maxilla; B, maxilliped; C, leg 1; D, leg 2; E, leg 4; F, leg 5. Scale bars: A–C, 0.05 mm; D, E, 0.1 mm; F, 0.02 mm.