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Two Species of the Genus Oncaea (Copepoda, Poecilostomatoida, Oncaeidae)
from the East Sea, Korea
Jin Hee Wi
1, Hae-Lip Suh
2, Han Soeb Yang
3, and Ho Young Soh
1*
1Division of Marine Technology, Chonnam National University, Yeosu 550-749, Korea 2Department of Oceanography, Chonnam National University, Gwangju 500-757, Korea 3Department of Oceanography, Pukyong National University, Busan 608-737, Korea
Received 14 October 2008; Revised 19 November 2008; Accepted 10 December 2008
Abstract − Oncaea venusta Philippi, 1843 has been known as having some different size groups, but recent genetic study for cyt b and ITS 1 (Elvers et al. 2006) suggests that these size groups can be considered as different species. Of these size groups, the largest O. venusta Philippi and the smallest O. venella Farran, 1929 were first described in Korean waters. The latter is easily distinguishable from the former in the following characteristics in addition to its small size: (1) length to width ratio of genital double somite of two genders smaller, and (2) female second pediger bearing inconspicuous dorso-posterior swelling. Oncaea venusta and O. venella co-occur in Korean waters during spring to fall, but their occurrence patterns seasonally differ: the former shows higher density in fall while the latter does in summer.
Key words − Oncaeidae, Oncaea venusta, Oncaea venella, microcopepods, Korean waters
1. Introduction
Pelagic microcopepods of the family Oncaeidae are ubiquitous in the oceans (Malt 1983; Böttger-Schnack et al. 2004.) and have a major importance in marine ecosystems (Paffenhöfer 1993). In particular, the type species of the
marine microcopepod, Oncaea venusta Philippi, 1843
(Poecilostomatoida, Oncaeidae), is widely distributed in the Atlantic, Pacific and Indian Oceans between 65°N and
45°S (Malt 1982). Various size-groups of Oncaea venusta
Philippi, 1843 have been reported from the Indian Ocean and Red Sea (Tanaka 1960; Sewell 1947; Böttger-Schnack et al. 1989; Böttger-Schnack 2001), the Atlantic (Farran
1936; Ferrari 1975; Boxshall 1977) and the North West Pacific (Heron & Bradford 1995; Heron 1977, 2002). Farran (1929) first described two size groups of O. venusta from the Atlantic. Heron (2002) discovered three size-groups in the Atlantic and the North West Pacific Ocean and of them the smallest species was raised to species level as O. venella Farran, 1929. Moreover, recent genetic study (Elvers et al. 2006) shows that size groups of O. venusta can be genetically distinct different species. We also discovered three size-groups in the East Sea and the South Sea of
Korea, and of them first described the largest one, O.
venusta and the smallest one, O. venella in Korean waters.
2. Materials and Methods
Zooplankton were taken in the East Sea and the adjacent waters of the Tsushima current of the South Sea of Korea by using simultaneously a Bongo net (mesh size 150 µm; mouth diameter 60 cm) and a Conical Net (mesh size 200 µm; mouth diameter 45 cm) on May, August, October and November, 2007, respectively. The Bongo net was obliquely towed from 300 m depth to surface and the Conical Net vertically done from bottom to surface. The specimens
were fixed in 99.8% ethylalcohol. Oncaea species were
sorted out from zooplankton specimens. Each appendage of Oncaea species was dissected in CMC-10, mounted on slides and sealed with high-quality nail-varnish. Drawings have been done using an optical microscope (Nikon AFX-II) equipped with a drawing tube. Scale bars are given in µm. Total body length and the ratio of prosome to urosome
*Corresponding author. E-mail: [email protected]
(excluding caudal rami) were calculated along the lateral aspect.
The terminology follows Huys and Boxshall (1991). Abbreviations used in the text and figures are: ae, aesthetasc; P1-P6, first to sixth thoracopods; exp, exopod; enp, endopod; exp(enp)-1(-2,-3), to denote the proximal (middle, distal) segment of a ramus.
3. Description
Order Poecilostomatoida Thorell, 1859 Family Oncaeidae Giesbrecht, 1892
Genus Oncaea Philippi, 1843
Oncaea venusta Philippi, 1843
Synonyms: Oncaea venusta Farran (1929): [as Oncœa
venusta forma typica]: 284-285, Fig. 33, Oncäa venusta Giesbrecht, 1893: 590-604, 755, 756, 774, 789, Plate 2, Fig. 5, Plate 3, Fig. 7, Plate 47, Fig. 2, 5, 13, 19, 39, 44, 48, 50,
54, 58, Oncaea praeclara Humes, 1988: 475-485, Figs. 1a,
b, d, 2a-I, 3a-g (♀ ), 4a-g ( ♂ ). Materials examined
East Sea, 5♀ ♀ and 5 ♂ , ♂ , October 2007.
Adult female (Figs. 1-2)
Body length in lateral view: 960-1200 µm (average 1080 µm, n = 5 individuals). Exoskeleton heavily chitinitzed, covered with numerous granules. Prosome 1.8 times length of urosome, excluding caudal rami, 1.4 times urosome length including caudal rami. Pleural areas of P4-bearing somite with rounded posterolateral corners. Proportional lengths (%) of urosomites 11.9: 59.5: 8.3: 8.3: 11.9. Proportional lengths (%) of urosomites and caudal rami 9.52: 47.6: 6.7: 6.7: 9.52: 20.0. Genital double-somite 1.35 times as long as maximum width (in dorsal aspect) and 1.76 times as long as postgenital somites united; maximum width measured at anterior two-thirds, lateral margins of genital double-somite rounded, posterior part tapering gradually. Anal somite 1.3 times wider than length; about half length of caudal rami. Anterior margin of anal opening (vestigial anal opening) with line of minute denticles. Caudal rami about 3 times as long as wide, with 6 elements: antero-and posterolateral setae (II, III) long, spiniform antero-and unipinnate along medial margin; outer terminal seta (IV) long, and
bipinnate; inner terminal seta (V) longest and bipinnate; terminal accessory seta (VI) more than 2/3 length of seta (VII) about same length as posterolateral seta (III) and less than half the length of terminal accessory seta, plumose and bi-articulate at base.
Antennule 6-segmented (Fig. 1C). Armature formula: 1-[3], 2-[8], 3-[5], 4-[3+ae], 5-[2+ae], 6-[6+(1+ae)].
Antenna 3-segmented, distinctly reflexed (Fig. 1D): coxobasis with bipinnate seta at inner distal corner; endopodal segments unequal in length; first one longer, subtriangular, anteriodistally with patch of spinules, with row of denticles along inner margin; second one distinctly shorter; posterior surface growing patch of spinules; with 4 lateral setae consisting of 2 naked setae (I, IV) and 2 unipinnate setae (II, III); 7 distal setae forming 5 long setae (A-D) and 2 naked setae (G-F).
Mandible (Fig. 1E): gnathobase with 5 elements, indicated by small letters in Fig. 3E; ventral element (a) shorter than ventral blade, with long, fine setules along dorsal side; ventral blade (b) strong and extensive spiniform, with row of setules on posterior part; dorsal blade (c) strong and broad, with 3 dentiform processes along distal margin; 2 dorsal elements setiform, of which ventral one (d) shorter, flat and densely setose, dorsalmost one (e) longer and multipinnate.
Maxillule (Fig. 1F) single segmented, weakly bilobed: inner lobe (= praecoxal arthrite) with 3 elements and outer lobe with 4 elements: outermost one long spiniform having row of spinules.
Maxilla (Fig. 1G) 2-segmented: syncoxa unarmed; allobasis produced distally into slightly curved claw bearing 2 rows of very strong spinules along medial margin; outer margin with strong seta extending almost to tip of allobasal claw, ornamented with few minute spinules distally; inner margin with slender pinnate seta and strong spine.
Maxilliped (Fig. 1H) 4-segmented, composed of syncoxa, basis, and 2-segmented endopod: syncoxa unarmed; basis extensive and robust, with 2 spiniform spinulose elements nearly equal in length and patches of long setules on inner margin; first endopodal segment without ornamentation; second one with spinulose claw along concave margin, naked seta on outer proximal margin and unipectinate spine joined to inner margin.
Swimming legs (Fig. 2A-D) biramus, intercoxal sclerites well developed; outer margin of exopodal segments with well developed serrated hyaline lamella, inner margin of
Fig. 1. Oncaea venusta, female (East Sea). (A) Habitus, dorsal view, (B) Habitus, lateral view, (C) Antennule, (D) Antenna, (E) Mandible, (F) Maxillule, (G) Maxilla, (H) Maxilliped, (I) P5, (J) P6.
proximal exopodal segment with long setules, terminal spines shorter than distal exopodal segment; outer margin of endopodal segments with long setules, inner seta of proximal endopodal segments slightly swollen at base. Spines and setal formula of P1 to P4 as follows:
P1 (Fig. 2A): inner basal seta spiniform and minutely pinnate, length of exopod and endopod nearly equal; enp-3 with arrowhead-shaped process on distalmost inner seta. Distal margin of P2-P3 enp-3 protruded into conical projection. Outer subdistal spine of P2-P4 enp-3 nearly equal in length compared to outer distal spine of P2-P4 enp-3, always shorter than terminal spine. Outer distal spine of P4 enp-3 about 3/4 length of terminal spine of P4 enp-3 (Fig. 2B-D).
P5 (Fig. 1I) with small plumose seta growing from lateral surface of somite, and small free exopod without ornamentation. Exopod slightly longer than wide, bearing 2 naked setae. P6 (Fig. 1J) expressed as operculum around each genital aperture with spine.
Adult male (Fig. 3)
Body length in lateral view: 780-880 µm (average 830 µm, n = 5 individuals). Sexual dimorphism in antennule, antenna, maxilliped, genital segmentation and ornamentation, endopod of P1 to P3, P5, P6, and in caudal rami. Prosome 1.6 times length of urosome excluding caudal rami, 1.3 times of urosome length including caudal rami. Caudal rami about 2.3 times longer than wide, shorter than in female. Caudal setae with proportional lengths as in female except for seta VI, which is about 2/3 the length of seta IV and 2.0 times length of caudal rami. Surface of genital flaps and ventral surface of anal segment ornamented with minute spinules.
Antennule (Fig. 3C) 4-segmented; distal segment corres-ponding to fused segments 4-6 of female. Armature formula; 1-[3], 2-[8], 3-[4], 4-[11+2ae+(1+ae)].
Antenna (Fig. 3D): coxobasis having naked and short seta at innerdistal corner; distal endopodal segment with seta III more stout than in female, seta IV spiniform and curved,
both elements shorter than in female.
Maxilliped (Fig. 3E) 3-segmented, composed of syncoxa, basis and 1-segmented endopod: syncoxa unarmed; basis bearing 2 small naked setae, oval-shaped in proximal half, ornamented with rows of short flat spinules along inner margin and short spinules of graduated length along palmar margin; endopod streched out into long curved claw, concave margin unornamented, with short, unipectinate spine fused to inner proximal corner of claw, tip of claw with minute hyaline apex.
Swimming legs (Figs. 3F-I) 1-4 with armature and ornamentation as in female; sexual dimorphism in length ratio of outer subdistal spine and outer distal spine on P2 enp-3: outer subdistal spine elongated.
P5 (Fig. 3J) exopod connected from somite, length to width ratio approximately same; armature as in female.
P6 (Fig. 3B) expressed as posterolateral flap closing off genital aperture on either side; covered by minute denticles. Remarks
Oncaea venusta Phiplippi, 1843 is easily distinguished from O. venella Farran, 1929 by the following morphological characteristics: 1) larger body lengths of 960 to 1200 µm in female and of 780 to 880 µm in male; 2) wider prosome and urosome. The prosomes of both genders are more corpulent than O. venella. Elvers et al. (2006) indicate that the large species O. venusta is a distinct clade or lineage by phylogenetic analysis obtained from sequences data of two DNA
markers (cyt b and ITS1). Oncaea praecalara expressed by
Humes (1988) represented main characters as O. venusta,
and then Böttger-Schnack reexamined the specimens from the National Museum of Natural History, Smithsonian
Institute, and confirmed them as O. venusta. Oncaea
venusta, taxonomically first described from Korean waters and abundant from May to November in the South Sea of Korea.
Oncaea venella Farran, 1929
Synonyms: Oncaea venusta venella Farran, 1929 (p. 210,
284, fig. F), Oncaea venusta Farran (1929), 284-285, Fig. 33
(female only) [as Oncæa venusta forma venella], Oncaea
veusta forma venella Farran, 1929. Materials examined
East Sea, 5♀ ♀ and 5 ♂ , ♂ , October 2007.
Leg Coxa Basis Exopod Endopod
P1 0-0 1-I 1-0; I-1; III, I, 4 0-1; 0-1; I, 5 P2 0-0 1-0 1-0; I-1; III, I, 5 0-1; 0-2; I, II, 3 P3 0-0 1-0 1-0; I-1; II, I, 5 0-1; 0-2; I, II, 2 P4 0-0 1-0 1-0; I-1; II, I, 5 0-1; 0-2; I, II, 1
Fig. 3. Oncaea venusta, male (East Sea). (A) Habitus, dorsal view, (B) Habitus, lateral view, (C) Antennule, (D) Antenna, (E) Maxilliped, (F) P1, (G) P2, (H) P3, (I) P4, (J) P5.
Fig. 4. Oncaea venella, female (East Sea). (A) Habitus, dorsal view, (B) Habitus, lateral view, (C) Antennule, (D) Antenna, (E) Mandible, (F) Maxillule, (G) Maxilla, (H) Maxilliped.
Adult female ( Figs. 4-5)
Body length in lateral view: 800-830 µm (average: 815 µm, n = 5 individuals). Body proportions different from O. venusta; prosome 1.8 times length of urosome, excluding caudal rami, 1.5 times urosome length including caudal rami. Second pediger somite forming slight dorso-posterior
swelling (no projection). Proportional lengths of
urosomites similar to O . venusta. Genital double-somite
1.7 times as long as maximum width (in dorsal view); lateral margins of genital double-somite weakly extended at anterior part, posterior part nearly straight. Anal somite width about 1.7 times longer than length. Caudal rami about
3.3 times as long as wide, shorter than in O. venusta, outer seta longer than seta III.
Antennule (Fig. 4C) as in O. venusta.
Antenna (Fig. 4D) as in O. venusta except for minor
differences in the ornamentation: setae II on second endopodal
segment slightly longer than that one of O. venusta.
Fig. 6. Oncaea venella, male (East Sea). (A) Habitus, dorsal view, (B) Habitus, lateral view, (C) Antennule, (D) Antenna, (E) Maxilliped, (F) P1, (G) P2, (H) P3, (I) P4, (J) P5 (K) P6.
Mandible (Fig. 4E), maxillule (Fig. 4F), maxilla (Fig.
4G) and maxilliped (Fig. 4H) as in O. venusta.
Swimming legs 1-4 (Fig. 5A-D) as in O. venusta, except
for P3 and P4 enp-3: outer subdistal spine length shorter
than outer distal spine. P5 and P6 as in O. venusta.
Adult male (Fig. 6)
Body length in lateral view: 560-630 µm (average: 595 µm, n = 5 individuals). Sexual dimorphism in antennule, antenna, maxilliped, endopods of P1 to P6, caudal rami and genital segmentation.
Prosome 1.8 times length of urosome, excluding caudal rami, 1.6 times of urosome length including caudal rami.
Proportional lengths of urosomites similar to O. venusta.
Length of genital double-somite 1.7 times as long as wide. Caudal rami about 3 times longer than wide, shorter than in
female. Caudal setae with proportional lengths as in O.
venusta.
Sexual dimorphism in antennule (Fig. 5C), antenna (Fig. 3F), maxilliped (Fig. 5D) and P6 (Fig. 6K) as in O. venusta. Swimming legs 1-4 (Figs, 6F-I) with armature and ornamentation as in female; distal spine of enp-3 of P4 longer than other two spines compared to that of female. P5 (Fig. 6J) with naked seta on lateral surface of somite, and exopod bearing 2 naked setae different in length : length to width ratio of exopod almost equal.
Remarks
Heron (2002) located the smallest Oncaea venusta forma
venella Farran, 1929, from the Northeastern Pacific among size groups of O. venusta Philippi, 1843 in an independent
species O. venella Farran, 1929, by the following
morphological characteristics: 1) smaller body lengths of 750-880 µm in female and 560-630 µm in male; 2) the ratio of length to width in genital double somite narrower; 3) male P5 with naked seta on lateral surface; 4) second pedigerous somite somewhat swollen in lateral view (no projection). The same characters are seen in specimens
from Korean waters, indicating these speciemens are O.
venella. Recently Elvers et al. (2006) showed that the
smallest one of different size-groups of Oncaea venusta
from the Indo-West Pacific (Böttger-Schnack 2001) was genetically a distinct species and others also might be genetically different. However, the smallest specimens
analyzed by Elvers et al. (2006) somewhat differ from O.
venella Farran, 1929 from the Northeastern Pacific in its
size range and morphological characters. These facts suggest
that they might be different species. In Korean waters O.
venella and O. venusta co-occur in the offshore during spring to fall. However, the former was predominant in the summer, while the latter in the fall. During the spring their abundance was almost same. On the other hand,
Böttger-Schnack (1995) showed that O. venella and O. venusta (as
two forms of O. venusta) tend to be vertically separated.
Acknowledgements
We wish to thank J. I. Park (Chonnam National University) for providing oncaeid copopod materials from the East Sea. We also express our sincere thanks to Prof. W. Lee and Dr. W.D Yoon for their s useful comments. This study was supported by the financial support of National University (Pukyong National University Korea Inter-University Institute of Ocean Science) of the Ministry of Education & Human Resources Development, Korean Government.
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