Halicyclops ariakensis, Ueda & Nagai, 2009

Ueda, Hiroshi & Nagai, Hidefumi, 2009, Three new species of the brackish-water copepod Halicyclops (Crustacea, Cyclopoida) from Ariake Bay, Japan, Journal of Natural History 43 (5 - 6), pp. 287-307 : 300-304

Halicyclops ariakensis sp. nov.

( Figures 6 , 7 )

Type material

One female holotype ( NSMT Cr-18241) dissected and mounted on three glass slides and six undissected female paratypes ( NSMT Cr-18242) preserved in alcohol; collected from the Kashima-gawa River estuary on 14 March 2002 .

Type locality

Kashima-gawa River estuary (33 ° 069370 N, 130 ° 049130 E) in Ariake Bay, Kyushu, Japan. The salinity was 2 psu .

Etymology

The specific epithet refers to the locality, to which the species is considered endemic.

Description

Female. Body ( Figure 6A ) length 0.50–0.66 mm (holotype 0.60 mm). Prosome L/ W 1.4 –1.5, widest at distal one-third of cephalosome, and 1.5–1.9 times longer than urosome. Forehead round in dorsal view. Genital double-somite ( Figure 6B ) L/ W 0.9 –1.0, without lateral protuberance. Genital double-somite and subsequent two urosomites ( Figure 6B,C ) with finely serrate distal frill; middorsal part of frill of fourth urosomite more coarse and extending nearly to end of anal somite on medium line.

Caudal rami ( Figure 6D ) L/ W 1.4 –1.6; proximal dorsolateral seta as long as ramus; medialmost terminal seta 0.4 times as long as ramus; dorsodistal seta about 2.5 times longer than ramus; medial median terminal seta 1.6 times longer than urosome and about 2.3 times longer than lateral median terminal seta. Ornamentation of middle terminal setae ( Figure 6E ) similar to H. uncus , but with denser ornamentation of certain features, particularly more numerous spinules on proximal half of setae.

Antennule ( Figure 6F ) with setal formula: I58, II513, III55 +spine, IV55+ae, V52 , VI510 +ae; first segment with row of spinules ventrally; fourth segment L/ W 2.6 .

Antenna ( Figure 6G ) coxobasis two rows of fine and large spinules on medial and anterior surfaces, respectively; Enp2 L/W about 2.8, 1.6 times longer than Enp1.

Mandible ( Figure 6H ) and maxillule ( Figure 6I, J ) as in H. uncus sp. nov.

Maxilla ( Figure 6K ) basis with claw and spine armed with long teeth on both sides; spines on endopod with similar spines.

Maxilliped ( Figure 7A ) endopod L/ W 1.8; distolateral two setae more slender than others.

P1–P4 ( Figure 7 B–E) Exp3 with spine and seta formula as follows: III+1/1/3, III/ I+1/4, III/I+1/4, II/I+1/4. P1 basis with medial spine reaching base of Enp3; exopod with no naked spines; proximal spine on Exp3 slightly shorter than distal two and segment; Proximalmost medial setae of P2–P3 Enp3 spiniform at distal two-thirds. P4 Enp3 ( Figure 7E ) L/ W 1.5; medial apical spine 1.5 times longer than segment and 1.7 times longer than lateral apical spine; medial setae spiniform at distal three-quarters.

P5 exopod ( Figure 7F ) L/ W 1.5; length ratio of three spines (from lateral to medial) to segment 1.1:0.9:1.0; seta 1.4 times longer than segment.

Other diagnostic features as in H. continentalis .

Remarks

Halicyclops ariakensis sp. nov. is similar to the following seven species in having the 3.4.4.3 spine formula on the P1–P4 Exp3, serrate urosomal hyaline frills, of which the mid-dorsal part of the fourth urosomite extends beyond the anal operculum, and no conspicuous lateral protuberances on the genital double-somite: H. denticulatus Kiefer, 1960 , H. gauldi Pleşa, 1961 , H. reunionensis Bozic, 1964 , H. clarkei Herbst, 1982 , H. laminifer Herbst, 1982 , H. bowmani Rocha and Iliffe, 1993 and H. lindbergi Rocha, 1995 . These species are distinguishable from the new species first by the shape of the prosome, which is the widest at about the end of the cephalosome as in most other congeners ( Kiefer 1960; Bozic 1964; Herbst 1982; Rocha and Iliffe 1993; Rocha 1995) whereas the new species is widest at the distal one-third of the cephalosome. Other features distinct from the new species are as follows: the genital double-somite is longer in H. bowmani [L/ W 1.2 ( Rocha and Iliffe 1993)]; the middorsal hyaline frill of the fourth urosomite is shorter in H. clarkei [not extending to end of the anal somite on the medium line ( Herbst 1982)] and rectangular in H. laminifer ( Herbst 1982) ; the caudal ramus is shorter in H. laminifer [L/ W 1.1 ( Herbst 1982)] and longer in H. denticulatus [at least 2.0 ( Kiefer 1960)] and H. reunionensis [2.0 ( Bozic 1964)]; the spiniform distomedial seta of the P4 Enp3 is curved and has strong teeth on only the medial margin in H. lindbergi ( Rocha 1995) ; lateral two spines of P5 are much shorter than the segment in H. gauldi and H. reunionensis according to the figures in Pleşa (1961) and Bozic (1964), respectively.

Among hitherto known species from Japan, H. fosteri illustrated by Ishida (2002) from Miyazaki, Kyushu, is most similar to the new species in having the serrate hyaline frills on posterior margins of the second to fourth urosomites, no lateral protuberance on the genital double-somite, and the similar L/W of the caudal ramus. The specimen described by Ishida (2002), however, is neither identical to H. fosteri nor the new species. The H. fosteri s. str. described by Wilson (1958) is characterized by the P4 having two spines and a slender seta between the spines on the Exp3 and with no medial setae on the Enp 3 in females. Halicyclops fosteri described by Ishida (2002) has two medial setae on the P4 Enp3 as in most species of the genus. As for the Exp3, Ishida (2002) did not provide information of the specimen and noted that significant variations of spines and setae on the P4 Exp3 were observed in the population of the type locality. However, the statement in Ishida (2002) is not a correct citation of Wilson’s description, in which there was no description of variation in the P1–P4 from the type locality (Louisiana), with the exception of differences in the spine numbers on the P1–P3 (not P4) Exp3 between two female specimens collected from Texas and between Louisiana and Texas. Ishida (2002) synonymized Halicyclops sp. with the 3.4.4.3 spine formula on the P1–P4 Exp3 collected from Hokkaido ( Ishida 1984) as H. fosteri . This specimen again is neither H. fosteri s. str. nor the present new species because of its obvious lateral protuberance on the genital double-somite. The differences between H. fosteri described by Ishida (2002) and the new species are seen in the following morphologies of the former, which are important in classification of Halicyclops species: the mid-dorsal hyaline frill on the fourth urosomite is not conspicuously extended, which he pointed out as a distinctive feature of the species by an arrow in a figure, and the spines on the P5 are much shorter than the segment.

The new species was collected from the Kashima-gawa, Kase-gawa and Shiotagawa Rivers located in the innermost part of Ariake Bay. The salinity range in which the species occurred was between 2 and 10 psu. The three new species of the present study co-occurred in the 2-psu water of the Kashima-gawa River. It is considered that H. ariakensis sp. nov. is endemic to Ariake Bay, because there have been no records identifiable with this species in either Japan or continental waters.

Figures

Figure 6. Halicyclops ariakensis new species, female. (A) Habitus, dorsal; (B) urosome, dorsal; (C) urosome with right caudal ramus, ventral; (D) right caudal ramus, dorsal; (E) middle caudal setae; (F) antennule, ventral; (G) antenna, anterior; (H) mandible, anterior; (I) maxillule, medial; (J) palp of maxillule; (K) maxilla, anterior. A, non-type from Kashimagawa River; B–D, F–K, holotype; E, non-type from Shiota-gawa River.

Figure 7. Halicyclops ariakensis new species, female. (A) Maxilliped, posterior; (B) P1, anterior; (C) P2, anterior; (D) P3, anterior; (E) P4, anterior; (F) exopod of P5, anterior. A, C, F, holotype; B, D, E, non-type (single specimen) from Koshima-gawa River.