Pachypygus gibber ( Thorell, 1859 )

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 : 105-110

Pachypygus gibber ( Thorell, 1859)

( Figs. 68–70 )

Material examined. 14 ♀♀ (MNHN-IU-2018-1781) from Molgulasocialis Alder, 1863 , Wimereux, France; 1 ♀ from Ascidia mentula M̹ller, 1776, Kristineberg, 1962; 13 ♀♀ (MNHN-IU-2018-1782) and dissected 1 ♀, 1 ♂ (figured) from Ciona intestinalis (Linnaeus, 1758) , Thau 1957; 1 ♀, 1 ♂ (MNHN-IU-2018-1783) from Molgula sp., collection data unknown; 1 ♀ (MNHN-IU-2018- 1784) from Ascidia tenue Monniot C., 1983 , St. David, Bermuda; 1 ♀ from Phallusia nigra Savigny, 1816 , Bermuda; 3 ♀♀ (MNHN-IU-2018-1785) from A. tenue, St. David , Bermuda; 1 ♀ (MNHN-IU-2018-1786) from Molgula azorensis Monniot C., 1971 , Azores, Biacores 216; 1 ♀ (dissected), 1 ♂ (MNHN-IU-2018-1787) from M. azorensis . Azores, Biacores cruise, 1971; 2 ♀♀ (MNHN-IU-2018-1788) and 1 dissected ♀ from M. azorensis, Biacores ; 2 ♀♀ (MNHN-IU-2018-1789) from Polycarpa tenera Lacaze-Duthiers & Delage, 1892 , Glenan; 3 ♀♀ (MNHN-IU-2018-1790) from Ascidia conchilega M̹ller, 1776, Kristineberg; 1 ♀ (dissected) from Ecteinascidia turbinata Herdman, 1880 , Ibiza, Balearic Islands, Spain; 1 ♀ (MNHN-IU-2018-1791) and 1 dissected ♀ from Ascidia curvata (Traustedt, 1882) , Bermuda; 2 ♀♀ (MNHN-IU-2018-1792) from Ascidia virginea M̹ller, 1776, Adriatic Sea; 9 ♀♀ (MNHN-IU-2018-1793) from Ciona intestinalis, Bonifacio 1983 ; 6 ♀♀ (MNHN-IU-2018- 1794) from C. intestinalis, Bonifacio ; 2 ♀♀ (MNHN-IU- 2018-1795) from Molgula manhattensis (De Kay, 1843) , Port de Lorient, August 1992; 1 ♀ (MNHN-IU-2017- 1796) from Molgula socialis , Port de Treguier, August 1992; 1 ♀ (MNHN-IU-2018-1797) from Ascidia archaia Sluiter, 1890 , CRCHO 239 Palau Malakai Harbor, depth 1 m; 2 ♀♀, 4 ♂♂ (MNHN-IU-2018-1798) and 1 dissected ♀ from Molgula socialis , Port du Harvre, 19 November 1997; 4♀♀, 4 ♂♂ (MNHN-IU-2018-1799) and dissected 1 ♀, 1 ♂dissected from Ascidia ceratodes (Huntsman, 1912) , California, Lambert coll.; 2 ♀♀, 5 ♂♂ (MNHN-IU-2017-2167) from Molgulasocialis, ORHAGO 14, Stn 14, 10 (47°19’N 02°43’W), depth 34 m. 14 November 2014.

Supplementary description of female. Body ( Fig. 68A ) laterally compressed, comprising inflated prosome, flexed ventrally, and slender urosome. Bodylength of figured specimen 7.27 mm. Prosome 5-segmented, but in fully expanded adult first pedigerous somite concealed by cephalosome in lateral view. Third pedigerous somite markedly expanded. Brood pouch angular or slightly produced at postero-ventral corner. Urosome ( Fig. 68B ) 6-segmented, but fifthpedigerous somite not articulated from brood pouch. All urosomites wider than long, gradually shorter and narrower posteriorly. Third abdominal somite with soft posteromedian process dorsally and paired, sclerotized posterolateral processes ( Fig. 68C, D ); posterior margin oblique in lateral view, with longer dorsal margin and shorter ventral margin ( Fig. 68C ). Anal somite concealed by third abdominal somite in dorsal view ( Fig. 68C, D ), strongly sclerotized ventrally, with paired tubercles ornamented with spinules on ventral surface ( Fig. 68C, E ). Caudal ramus slightly curved ventrally, about 3.7 times as long as wide (545×148 μm), narrowing distally, and armed with 4 claws and 2 setae; terminal claw largest, 106 μm long, other 3 subdistal claws smaller, 42, 30, and 30 μm long; setae small, positioned at 58% and 79% of ramus length.

Rostrum ( Fig. 68G ) small, stronglytapering, withrow of minute spinules along lateral margins. Antennule ( Fig. 68H ) 8-segmented; armatureformula 3, 16, 7+aesthetasc, 5, 3+aesthetasc, 2, 2+aesthetasc, and 7+aesthetasc; first and second segments expanded, much wider than distal segments; all setae small and naked. Antenna ( Fig. 68I ) consisting of short coxa, allobasis and 1-segmented free endopod; allobasis with 1 small seta representing exopod on outer side and rudimentary suture on inner side; free endopodal segment distinctly shorter than basis, 2.1 times as long as wide (136×65 μm) and slightly narrowing distally; armedwith 8 smallsetae (arranged as 1, 3, 1, and 3) plus terminal claw more than half length of segment.

	Coxa	Basis	Exopod	Endopod
Leg 1	0-1	1-I	I-1; I-1; III, I, 4	0-1; 0-1; 1, 2, 3
Leg 2	0-1	1-0	I-1; I-1; III, I, 5	0-1; 0-2; 1, 2, 3
			(or III, I, 0)
Leg 3	0-1	1-0	I-1; I-0; III, I, 0	0-1; 0-2; 1, 2, 3
Leg 4	0-1	1-0	I-0; I-0; III, I, 0	0-1; 0-2; 1, 2, 2

Labrum ( Fig. 68J ) with setulose, semicircular posterolateral lobes, concave posterior margin, and small, spinulose posteromedial lobe. Mandible ( Fig. 69B ) with 5 teeth on coxal gnathobase; basis with 1 seta on subdistal medial margin, patches of setules on medial and posteroventral margins, and blunt protuberance proximally on outer margin; exopod short, unsegmented, with 5 setae (2 distalsetae longerthan other 3); endopod 2-segmented, incompletely articulated from basis, with 4 and 10 setae on first and second segments, respectively, secondand thirdouterdistalsetae of secondsegment much smaller than outermost seta. Paragnath ( Fig. 69A ) with 1 apical and 1 medial subapical denticles, and setulose medial margin. Maxillule ( Fig. 69C ) with 10 setaeon arthrite, 1 on coxal endite, 2 on epipodite, and 4 setae on medial margin of basis (second proximal seta larger than other 3); exopod distinctly shorter than wide, with 4 setae distally; endopod 2-segmented with 4 smallsetae on medial margin of first segment and 3 larger setae on smallsecond segment. Maxilla ( Fig. 69D ) 5-segmented; syncoxawith 4 (all prominent), 1, 2, and 3 setaeonfirst to fourth endites, respectively; basiswith slender, smooth claw plus 2 setae; endopod slender with 1, 1, and 4 setae on first to third segments, respectively. Maxilliped ( Fig. 69E ) 3-segmented, armedwith 9, 1, and 4 setaeonfirst to third segments, respectively; articulation incomplete between first 2 segments; first segment ornamented with several rows of minute spinules on both surfaces.

Legs 1–4 ( Figs. 69F, G , 70A, B ) with 3-segmented rami; outerspinesonexopods strong, well-developed. Inner seta on coxa large in legs 1 and 2, but rudimentary inlegs 3 and 4, thatofleg 4 spiniform. Outersetaonbasis of all swimming legs small and naked. Inner distal spine on basis of leg 1 straight, 120 μm long, longer than first endopodal segment. Basis and first 2 endopodal segments of leg 1 with sensillae on anterior (ventral) surface. Inner seta on first exopodal segment present in legs 1–3 but lacking in leg 4. Second and third exopodal segment of leg 2 with or without vestigial setae on inner margin but these setae lacking in legs 3 and 4, with several spinules instead. Third exopodal segment of legs 2–4 about 2.5 times longer than wide. Armature formula for legs 1–4 as follows:

Leg 5 ( Fig. 69H ) consisting of protopod and exopod. Protopod fused with somite, armed with small outer distal seta and ornamented with several spinules on inner distal margin. Free exopodal segment 2.85 times longer than wide (285×100 μm), not reaching posteriormargin of genital somite, armedwith 1 small spine (45 μm long) and 1 small, thin seta (76 μm long); ornamented with several groups of minute spinules along medial surface.

Supplementarydescriptionofmale.Body( Fig.70C ) narrow, 2.07 mmlong. Prosome 5-segmented. Urosome 6-segmented. Third abdominal somite with posterodorsal process as in female, but lacking posterolateral processes. Anal somite with paired tubercles on ventral surface, ornamented with spinules; processes weaker than in female. Caudal ramus about 2.6 times longer than wide (129×50 μm).

Rostrum asin female. Antennule ( Fig. 70D ) 8- segmented; first and second segments only slightly expanded; non-geniculate; second segment with 3 setal elements modified as conical claws; armature formula 3, 13+3 claws, 8+aesthetasc, 5, 3+aesthetasc, 2,2+aesthetasc, and 7+aesthetasc. Antenna as in female. Labrum and mouthparts as in female.

Leg 1 as in female. Leg 2 with samearmature formula as in female, but all medial setae on exopod prominent and 2 terminal setae on third endopodal segment spiniform. Third exopodal segment of leg 3 with 4 spines and 4 setae. Armature formula for legs 3 and 4 ( Fig. 70E, F ) different from that of female, as follows:

	Coxa	Basis	Exopod	Endopod
Leg 3	0-I	1-0	I-1; I-1; III, I, 4	0-1; 0-2; I, II, 3
Leg 4	0-1	1-0	I-0; I-1; III, I, 5	0-1; 0-2; I, II, II

Leg 5 exopod about 2.5 times longer than wide (83×33 μm). Leg 6 represented by 2 distal setae and 1 smaller, spiniform inner seta on genital operculum.

Remarks. This description of Pachypygus gibber is based on specimens taken from the type host Ciona intestinalis living on the French Mediterranean coast. Specimens from several other samples were dissected for comparison, including: three samples from the European Atlantic, three from the Mediterranean, and one each from California and Bermuda. The diagnostic combination of morphological traits of P. gibber can be summarised from this comparison as follows: (1) the terminal claw on the caudal ramus is distinctly larger (about 3 times longer) than the other 3 smaller claws, and the 2 caudal setae are positioned at about 60% and 80% of the ramus length; (2) the antennule is 8-segmented with 3 setae on the first segment; (3) the antenna has an allobasis armed with 1 small seta (naked or pinnate) on the outer side and the free endopod is 1-segmented (segment about twice as long as wide); (4) the mandibular endopod is armed with 4 and 10 setaeon the first and second segments, respectively; (5) the maxillular endopod is armed with 4 and 3 setae on the first and second segments, respectively; (6) the third exopodal segment of legs 2–4 is about 2.5 times longer than wide, and reduced inner setae can be present or absent on leg 2, but are consistently absent on legs 3 and 4; (7) the free exopodal segment of female leg 5 is about 2.8 timeslongerthan wide.

Illg(1958) listedtheknownhostsandthedistributional range of this species. Subsequently Ooishi (1961) and Kim (2012) recorded its occurrence in Japan and Korea, respectively. Here we add California and the West Indies as additional distributional records. Pachypygus gibber seems to be a cosmopolitan species.

Figures

FIGURE 68. Pachypygus gibber (Thorell, 1859), female. A, habitus, right; B, urosome, ventral; C, distal part of abdomen, right; D, distal part of abdomen, dorsal; E, distal part of abdomen, ventral; F, caudal ramus, lateral; G, rostrum; H, antennule; I, antenna; J, labrum. Scale bars: A, 1 mm; B, 0.5 mm; C–F, H–J, 0.01 mm; G, 0.02 mm.

FIGURE 69. Pachypygus gibber (Thorell, 1859), female. A, paragnath; B, mandible; C, maxillule; D, maxilla; E, maxilliped; F, leg 1; G, leg 2; H, leg 5. Scale bars: 0.1 mm.

FIGURE 70. Pachypygus gibber (Thorell, 1859), female: A, leg 3; B, leg 4. Male: C, habitus, right; D, antennule; E, leg 3; F, leg 4; G, leg 5. Scale bars: A, B, 0.1 mm; C, 0.2 mm; D–G, 0.05 mm.