Lichomolgidium bipartitum, Kim & Boxshall, 2021

Kim, Il-Hoi & Boxshall, Geoff A., 2021, Copepods associated with Ascidian hosts (Tunicata): Intramolgidae and Lichomolgidae, with descriptions of four new genera and 13 new species, Zootaxa 5013 (1), pp. 1-75 : 24-28

Lichomolgidium bipartitum sp. nov.

( Figs. 15 , 16 )

Type material. Holotype ♀ (MNHN-IU-2014-21597, dissected and mounted on a slide) from myura stolonẚfera ( Heller , 1878) (MNHN-IT-2008-7777); MRAC-ULB expedition (Musée Royal de l'Afrique Centrale et Université Libre de Bruxelles), Inhaca Is., Mozambique (26°03´S, 32°54´E), depth 10-20 m, 07 August 1969.

Etymology. The name of the new species refers to the 2-segmented endopod of leg 4.

Description of female. Body ( Fig. 15A ) moderately broad; body length 1.89 μm; prosome 1.06× 0.67 mm. Urosome ( Fig. 15B ) 5-segmented; fifth pedigerous somite 276 μm wide, ornamented with minute spinules at tip of lateral margin. Genital double-somite 255×269 μm, slightly wider than long; genital apertures located dorsally at midlength of double-somite. Three free abdominal somites 109×185, 95×149, and 127×129 μm, respectively. Anal somite ornamented with fine spinules posteroventrally near base of caudal rami. Caudal ramus ( Fig. 15C ) about 3.4 times longer than wide (182×53 μm) and about 1.4 times longer than anal somite, gradually narrowing distally: armed with 6 naked setae; lateral seta (seta II) reduced (length less than half width of ramus), positioned at 75% of ramus length; 2 mid-terminal setae (setae IV and V) flattened, tape-like, unilaterally fringed with membrane.

Rostrum ( Fig. 15D ) narrow, longer than wide (about 110×75 μm), tapering to rounded apex. Antennule ( Fig. 15E ) 356 μm long, 7-segmented; armature formula 3, 13, 6, 3, 4+aesthetasc, 2+aesthetasc, and 7+aesthetasc; all setae naked; posterodistal seta on fifth segment reduced, setule-like. Antenna ( Fig. 15F ) 4-segmented; armature formula 1, 1, 3, and 4+3 claws; terminal segment about 2.4 times longer than wide (85×35 μm); terminal claws consisting of 1 thicker claw plus 2 slender, annulated claws.

Labrum ( Fig. 15G ) medially incised, distinctly broadened distally, with rounded posterior lobes and membranous fringe along inner margin of lobes. Mandible ( Fig. 15H ) with 2 small lobes outer margin; distal blade short, broad, with about 3 rows of spinules on convex side, and 1 row of spinules along concave inner margin. Maxillule ( Fig. 15I ) armed with 1 lateral and 2 apical crossing setae, plus small papilliform process subapically. Maxilla ( Fig. 15J ) with unarmed syncoxa; basis armed only with spiniform anterior seta; inner and proximal setae (seta I and III) absent; distal lash rather short, pectinate along convex outer margin. Maxilliped ( Fig. 15K, L ) 2-segmented; proximal segment unarmed; distal segment unequally bifurcate and blunt distally, armed with 2 setae on inner side, and ornamented with numerous minute spinules on outer surface.

Legs 1–4 biramous with 2-segmented protopods ( Fig. 16A–D ). Legs 1-3 with 3-segmented rami; leg 4 with 3- segmented exopod and 2-segmented endopod. Outer seta on basis pinnate in legs 1 and 4, but naked in legs 2 and 3. Third exopodal segments of legs 1-4 with bifid terminal process ( Fig. 16A, B, D ). Distal segment of leg 4 endopod with lateral constriction marking original plane of articulation between second and third segments. Terminal spines of leg 4 endopod 47 μm (outer) and 55 μm long (inner). Armature formula for legs 1–4 as follows:

Coxa Basis Exopod Endopod

Leg 1: 0-1 1-0 I-0; I-1; III, I, 4 0-1; 0-1; I, 1, 4

Leg 2: 0-1 1-0 I-0; I-1; III, I, 5 0-1; 0-2; I, II, 3

Leg 3: 0-1 1-0 I-0; I-1; III, I, 5 0-1; 0-2; I, II, 2

Leg 4: 0-1 1-0 I-0; I-1; II, I, 5 0-1; 0, II, 1

Leg 5 ( Fig. 16E ) consisting of dorsolateral seta on fifth pedigerous somite and free exopod; exopodal segment about 1.6 times longer than wide (106×65 μm), inner margin protruding with angular apex; terminal spine 47 μm and terminal seta 98 μm. Leg 6 ( Fig. 16E ) represented by 2 small setae and 1 denticle on genital operculum.

Male. Unknown.

Remarks. iẚchomolgẚdẚum bẚpartẚtum sp. nov. differs from i. sardum in the following features: (1) the genital double-somite is wider than long (vs. longer than wide in i. sardum ); (2) the second endopodal segment of the antenna is armed with 3 setae (vs. 4 setae in i. sardum ); (3) the caudal ramus is about 1.4 times longer than the anal somite (vs. more than twice as long as the anal somite in i. sardum ); and (4) the endopod of leg 4 is 2-segmented (vs. 3-segmented in i. sardum ).

iẚchomolgẚdẚum tupuhẚae is easily separated from other species of iẚchomolgẚdẚum in having 4 setae on the first antennular segment (cf. 3 in other species), and 2 similar setae, rather than 1 spine and 1 seta, on the exopod of leg 5 ( Jones, 1975).

The endopod of leg 4 of i. bẚpartẚtum sp. nov. is 2-segmented although the plane of fusion of the second and third segments is readily visible, and its armature formula is 0-1; 0, II, 1. These features are shared by several genera in the family Lichomolgidae , including: Ascẚdẚoxynus, Aster ẚcola Rosoll, 1888, maclabẚus Kossmann, 1877, Stellẚcola Kossmann 1877, and Synstellẚcola Humes & Stock, 1972. Nevertheless, the new species is accommodated in iẚchomolgẚdẚum because it shares other typical character states of this genus, such as the possession of 3 (1 thick and 2 slender) terminal claws on the antenna, a short blade on the mandible, the 2-segmented maxilliped with a blunt distal tip on the second segment, the lack of an inner seta (seta I) on the distal segment (basis) of the maxilla, and the inflated exopod of female leg 4.

Genus Lichomolgus Thorell, 1859

Remarks. The genus iẚchomolgus currently contains 30 known species associated mainly with ascidian and bivalve hosts. The segmentation of the antenna is an important character in the taxonomy of lichomolgoid copepods. According to Humes & Stock (1973), the antenna of iẚchomolgus is 4-segmented, although in i. chamarum Humes, 1968 the articulation between the third and fourth segments is obscure. During our comparisons between iẚchomolgus species, we concluded that six species of this genus have been inappropriately placed in this genus, namely: i. arcanus Humes and Cressey, 1958, i. asaphẚdẚs Humes, 1959, i. chamarum, i. hẚppopẚ Humes, 1976, i. spondylẚ Yamaguti, 1936, and i. uncus Jones, 1976. The original descriptions of these six species all refer to the antenna as 4-segmented but the published illustrations in these descriptions all show the antenna as 3-segmented. The terminal segment of the antenna often has a suture-like transverse line subdistally, and we consider this was probably interpreted as a true segmental boundary in these descriptions.

iẚchomolgus ẚnflatus Tanaka, 1961 was also reported as having a 4-segmented antenna in the original description ( Tanaka, 1961). The figure of the female antenna of this species is difficult to interpret, but the male antenna was figured as clearly 3-segmented. Considering that no sexual dimorphism has been reported for the segmentation of the antenna in the Lichomolgidae , we consider that the female antenna should also be regarded as 3-segmented in this species. All seven of the above species are associates of bivalves and it seems likely that a new genus should be established to accommodate these species.

Figures

FIGURE 15. iẚchomolgẚdẚum bẚpartẚtum sp. nov., female. A, habitus, dorsal; B, urosome, dorsal; C, left caudal ramus, dorsal; D, rostrum; E, antennule; F, antenna; G, labrum; H, mandible; I, maxillule; J, maxilla; K, maxilliped; L, distal segment of maxilliped. Scale bars: A, 0.2 mm; B, 0.1 mm; C–G, 0.05 mm; H–L, 0.02 mm.

FIGURE 16. iẚchomolgẚdẚum bẚpartẚtum sp. nov., female. A, leg 1; B, leg 2; C, endopod of leg 3; D, leg 4; E, left leg 5 and genital aperture, dorsal. All scale bars: 0.05 mm.