Halicyclops pumilus, Chang & Lee, 2012

Chang, Cheon Young & Lee, Jimin, 2012, Two new species of Halicyclops (Copepoda, Cyclopoida) from the estuarine interstitial waters in South Korea *, Zootaxa 3368 (1), pp. 197-210 : 205-209

Halicyclops pumilus sp. nov.

( Figs 5 –7)

Type locality. Estuary of Seomjin River , Gwangyang, South Korea, 34°59′03.85″N 127°46′17.62″E .

Type material. Holotype, intact female in alcohol (NIBRINV0000245093). Paratypes: two dissected females (DB20038, 20039). All collected at the type locality, 17 October 1995, leg. C.Y. Chang & J.M. Lee.

Additional material examined. Three females from Jakyakdo Island, Incheon, South Korea, 37°29′15.35″N 126°37′18.82″E, 30 May 1988 , leg C.Y. Chang. Four females from Sumunpo Beach , Boseong, South Korea, 34°37′52.20″N 127°01′40.56″E, 21 October 1995, leg. C.Y. Chang & J.M. Lee.

Etymology. The specific name comes from the Latin pumilus , meaning dwarf, which refers to the diminutive habitus of the species.

Description. Female. Body ( Fig. 5A ) very small, 387.7 µm long (m = 383.6, N = 8), excluding caudal setae; maximum width at posterior end of cephalothorax, 136.9 µm; L/W about 2.83. Body a little flattened dorsoventrally. Prosome comprising cephalothorax incorporating first pedigerous somite and 3 free pedigerous somites; posterior margins of prosomites serrated; outer distal corners of prosomites weakly expanded. Cephalothorax bell-shaped, L/W about 1.06; ovoid integumental window (or depression) present; 20–24 sensilla scattered on dorsal and lateral surfaces. Rostral expansion rarely visible from dorsal view. Genital double-somite ( Fig. 5B ) a little longer than wide; lateral sides not expanded or swollen, lacking spinous processes. Mid-dorsal hyaline fringe of preanal somite strongly protruded, with 6 acute teeth along posterior margin, far extending over anal operculum.

Fu ( Fig. 5B ) elongate, about 1.7–1.9 times (m = 1.78, N = 8) longer than wide; divergent posteriorly. Lateral caudal seta lying about halfway and a little dorsally. Inner caudal seta minute. Outer caudal seta 1.76 times longer than ramus. Dorsal caudal seta arising from distal end of longitudinal protuberance, 3.15 times longer than ramus, and 1.78 times longer than outer caudal seta. Small cuticular tube present outside the longitudinal protuberance, with a pore on its tip. Both inner and outer terminal caudal setae with breaking planes; proximal third bare, second third pinnate, distal third plumose.

A1 ( Fig. 6A ) not reaching to halfway of cephalothorax; 6-segmented. Fourth segment 1.74 times as long as wide; last segment 2.60 times longer than wide. Second segment bearing 12 setae, including 1 plumose and 1 bipinnate setae. Third segment shortest, bearing 5 setae including 1 short spiniform seta distally. Aesthetasc each arising from fourth and last segment distally. Seta formula: 8, 12, 5, 6 (+ 1 aesthetasc), 2, 10 (+ 1 aesthetasc).

A2 ( Fig. 6B ) 3-segmented, comprising coxobasis and 2-segmented endopod. Coxobasis unornamented, armed with 2 inner distal setae, lacking outer distal seta representing exopod. First endopodal segment bearing 1 inner seta, with smooth margins lacking spinules. Second endopodal segment 3.1 times as long as wide, about 1.5 times longer than first endopodal segment; ornamented with a few spinules on caudal face proximally; armed with 5 lateral and 7 apical setae.

Mandible ( Fig. 6C ), with well-developed coxal gnathobase, armed with 5 smooth teeth, 4 slender spiniform setae and 1 strong pinnate seta along cutting edge, flanking 1 outer distal pinnate seta. Palp very reduced, represented by 2 naked setae on small protuberance, of which shorter one minute.

Maxillule ( Fig. 6D ), praecoxal arthrite bearing 4 strong spines of which innermost one ornamented with 1–2 accessory setae proximally; 7 elements with various shapes present along inner margin, including 1 strong proximalmost projection. Palp 2-segmented; coxobasis bearing 1 strong spinulose and 2 slender naked setae distally, plus 1 outer pinnate seta representing exopod; distal segment, representing endopod, with 3 long plumose setae.

Maxilla ( Fig. 6E ) 4-segmented, comprising praecoxa, coxa, basis and 1-segmented endopod. Praecoxal endite with 2 setae, one of which pinnate. Coxa with 1 naked seta representing proximal endite; distal endite movable, forming 1 bisetose lobe completely fused with naked seta proximally. Basis forming 2 strong claw-like spines with 1 naked seta between them basally. Endopod carrying 5 elements of 1 naked, 2 spiniform and 2 spinulose setae.

Maxilliped ( Fig. 6F ) 2-segmented, comprising protopod and completely defined endopod; protopod about 2 times longer than endopod. Protopod with 2 naked proximal setae and 1 distal seta ornamented with 2 setules basally each on protuberance, representing endites; 3–4 setules present on outer distal corner of protopod. Endopod bearing 5 setae in total, comprising 2 inner setae, 1 apical, 2 subapical setae, of which inner and apical setae ornamented with 1 or 2 long setules.

P1–P4 (Fig. 7A–D) biramous, both rami 3-segmented. Spine formula of 3,4,4,3. Seta/spine armature of P1–P4 as follows:

P1 coxa 0-1 basis 1-1 exp I-1; I-1; III,2,3 enp 0-1; 0-1; I,I+1,3

P2 coxa 0-1 basis 1-0 exp I-1; I-1; IV,1,4 enp 0-1; 0-1; I,II,3

P3 coxa 0-1 basis 1-0 exp I-1; I-1; IV,1,4 enp 0-1; 0-1; I,II,3

P4 coxa 0-1 basis 1-0 exp I-1; I-1; III,1,4 enp 0-1; 0-1; I,II,2

P1 (Fig. 7A), intercoxal sclerite with hairs along distal margin of both lateral lobes; medial seta on P1 basis short but stout, not reaching to poseterior end of enp1; outer spines on exp3 become a bit (10–30%) longer distally. P2–P4, intercoxal sclerite bare along distal margin of both lateral lobes; enp2 with single inner seta. P4 (Fig. 7D), enp3 1.50 times longer than wide; both inner setae spiniform, short, nearly as long as enp3, and 23% shorter than inner apical spine; inner apical spine 1.23 times longer than enp3, and about 2 times longer than outer apical spine. P5 ( Fig. 5C ) basis and endopod completely incorporated to fifth pedigerous somite; basal seta inserted on small protuberance arising from dorsolateral corner of somite. Exopod nearly rectangular, 1.18 times longer than wide, with straight inner margin and angular inner distal corner; outer margin round, ornamented with spinule row; spines stumpy, inner apical spine longest, only 52% of exopod in length; apical seta 1.1 times longer than exopod, and 2.0 times longer than inner apical spine. P6 indistinct, represented by small genital operculum armed with 3 elements of 2 minute spinous projections and 1 long naked seta.

Male. Unknown.

Ecology. Specimens were collected by rinsing sandy sediments at two river mouths discharging into the southern coast of Korea, and at a small seaside ditch on the western coast of Korea, where brackish shells such as Asian clams, Corbicula fluminea (Müller, 1774) , and purple olive clams, Nuttallia japonica (Deshayes, 1857) , were abundant.

Remarks. This species belongs to the species group with the character combination of 3,4,4,3 spine formula, absence of lateral processes on genital somite, and well developed hyaline fringe of preanal somite. However, this species is characterized and easily distinguishable from its congeners by single inner seta on P2–P4 enp2 and very short spines on oval-shaped P5 exopod.

The new species is strikingly similar to two interstitial species, H. gauldi PleŞa, 1961 from the Atlantic coast of Ghana, West Africa and H. ytororoma Lotufo & Rocha, 1993 from the Atlantic coast of Brazil in sharing a very small body armed with single inner seta on P2–P4 enp2, elongate Fu (about 2 times longer than wide), multipronged pseudoperculum, short medial spine on basis of P1, and very stumpy spines on nearly quadrate P5 exopod.

Considering zoogeographical distribution and the high endemism rate of the interstitial copepods, the morphological similarities between the two marine species from the South Atlantic and the present new species from estuaries in the Northwest Pacific can be easily presumed to be the result of convergent evolution. However, in particular detail, they have a few morphological discrepancies: (1) H. gauldi and H. ytororoma show a spiniform inner proximalmost seta, deformed as a spatulate spine, each on P2–P3 enp3 (see Lotufo & Rocha 1993 for the character state of H. gauldi ), while both of the setae are normally plumose in H. pumilus ; (2) fourth segment of A1 is much shorter in H. pumilus (L/W is about 1.7–1.8, against more than 2 in H. gauldi and H. ytororoma ); and (3) H. gauldi has only 3 setae on P1 enp3, while H. ytororoma and H. pumilus have 4 setae are normal in H. pumilus . Moreover, it is noteworthy to mention that the middorsal integumental depression of cephalothorax is apparently shown in H. pumilus , while any information on it was not given nor figured in the descriptions of H. gauldi and H. ytororoma ( PleŞa 1961; Lotufo & Rocha 1993).

FIGURE 7. Halicyclops pumilus sp. nov., female. A, P1; B, P2; C, P3; D, P4. Scale: 50 µm.

Figures

FIGURE 5. Halicyclops pumilus sp. nov., female. A, habitus, dorsal; B, urosome, dorsal; C, P5 exopod. Scales: A, B = 100 µm; C = 20 µm.

FIGURE 6. Halicyclops pumilus sp. nov., female. A, A1; B, A2; C, mandible; D, maxillule; E, maxilla; F, maxilliped. Scales: 50 µm.