A new species of Metacyclops from a hyporheic habitat in North Vietnam (Crustacea, Copepoda, Cyclopidae)

Abstract A new species of Metacyclops is described from hyporheic waters and small rock depression with leaf litter in North Vietnam, the Tam Đao Mountains). Metacyclops amicitiae sp. n. can be distinguished from its congeners by the unique combination of the following characters: 12-segmented antennule, distal segment of P4 endopodite bearing a single apical spine, and the surface ornamentation of the intercoxal sclerites in P1–P4 (pilose on the distal margin of P1-P4 and spinulose on the caudal surface of P4). The latter character separates the new Metacyclops from its closest relative, Metacyclops ryukyuensis, known only from the Ryukyu Islands (Ishigaki). The genus Metacyclops with the new species described herein is also for the first time recorded from Vietnam. An identification key is provided to the south and east Asian species of the genus.


Material and methods
Specimens fixed in 5% formalin on the spot were transferred to 70% ethanol in the laboratory. Alcohol-preserved specimens were dissected in glycerine under an Olympus zoom stereomicroscope. Slide preparations were sealed with nail-polish. Lightmicroscopy examinations were made with Olympus BX 50 compound microscope with Nomarski optics, the pencil drawings were made with aid of a drawing tube attached to the compound microscope. Ink drawings were scanned and edited with the computer program GIMP and Inkscape. Etymology. The species is dedicated to the historical tradition of sympathy and friendship between the Poles and Hungarians. The species name amicitiae is the dative case of the ancient Latin word "amicitia," a singular noun which means "friendship". The gender is feminine.
Genital double-somite ( Fig. 1A) about 1.21 times broader than long, no transverse ridges or hairs on the somite. Seminal receptacle "T"-shaped. Posterior margin of anal somite bearing continuous row of strong spinules.
Caudal rami (Fig. 1B, C) 2.26 times longer than wide and bearing six setae, medial margin naked. Spinules present at insertion of antero-and posterolateral caudal setae. Inner and outer terminal caudal setae with breaking plane.
Mandible ( Fig. 2C) with reduced palp bearing two long plumose setae and short naked seta. Gnathobase with 8 teeth and dorsal seta-like element.
Leg 4 (Fig. 3C): intercoxal sclerite with row of small spinules on caudal surface. Coxopodite bearing five rows of spinules on caudal surface and robust spinules on lateral margin. Distal segment of P4 endopodite 2.12 times as long as wide, apical spine 0.82 times as long as segment.
Leg 5 (Fig. 1A, D) with one free segment 1.6 times as long as wide, bearing one medial spine and one lateral seta. Long lateral seta inserted on laterodorsal surface of pediger 5.
Leg 6 ( Fig. 1E) represented by small plate located laterodorsally in anterior fouth of genital double-somite, and bearing one seta and two subequal lateral spines; seta ca. 3 times as long as lateral spines.
Caudal rami (Fig. 4A) 2.25 times longer than wide. Relative length of caudal setae from terminal accessory (innermost) to posterolateral (outermost) caudal setae: 1.6, 7.0, 4.2, 1.0. Dorsal caudal seta 0.88 times as long as posterolateral caudal seta. Setulation of caudal setae homonomous. Antennule (Fig. 4B C) 16-segmented and armed as follows: 8 + 3 aesthetascs (and row of spinules ventrally), 4, 2, 2 + aesthetasc, 1, 2, 2, 2, 1 + aeathetasc+ spine, 2, 2, 2, (setation of segments 13-15 could not be verified), [4 + 1 aesthetasc and 7 + 1 aesthetasc] (one element broken one segment 16). Second endopodal segment of antenna with seven setae only (Fig. 4D). Surface ornamentation of antennal coxobasis (Fig.  4D) similar to that in female. Segmentation and setation of swimming legs, and leg 5    as in female. Distal segment of P4 endopodite 1.75 times as long as wide, apical spine 1.16 times as long as segment. Surface ornamentation of P1-P4 intercoxal sclerites similar to that in female. P6 (Fig. 4E) bearing two elements only, lateral seta 1.6 times as long as medial spine. Remarks. The 12-segmented antennule in the female of M. amicitiae sp. n. is a very rare trait among the Old World Metacyclops taxa. The only other species that shows the same segmentation pattern is M. ryukyuensis from Ishigaki Island, Ryukyus, Japan (Fig. 5) (Ishida, 1995). However, the 12-segmented state of the antennule is not unusual among the Middle and South American taxa [M. brauni Herbst, 1962, M. botosaneanui Pesce, 1985, M. hartmani Herbst, 1960, M. laticornis (Lowndes, 1934, M. necessarius (Kiefer, 1926), M. mendocinus venezolanus Kiefer, 1956, M. leptopus (Kiefer, 1927, M. leptopus mucubajiensis Kiefer, 1956, M. mendocinus (Wierzejski, 1892), M. problematicus Dumont, 1973, M. janstocki Herbst, 1990, M. hirsutus Rocha, 1994] (see Herbst 1988Herbst , 1990da Rocha 1994) M. amicitiae sp. n. and M. ryukyuensis not only share several morphological characters (e.g. segmentation and setation of the antennule, mouthparts and leg morphology, relatively long terminal accessory caudal seta and small body size), but they also show similarities in habitat preference as both species seem to be related to benthic and hyporheic habitats. Metacyclops ryukyuensis was found in a detritus sample from a shallow stream with gravel and mud deposit ("The sample was scraped by a small hand net ... from the bottom" -p. 33 in Ishida 1995). The poor information available on the geographic distribution of M. amicitiae sp. n. and M. ryukyuensis makes difficult any inference about the age of their divergence. Nonetheless, it is worth mentioning that Ryukyus Islands, which are presently isolated from Japan, China, and Taiwan by the sea (the terra typica of M. rykyuensis, Ishigaki Island, is located 240 km east of Taiwan), constituted a volcanic arc on the margin of the Asian continent (China) and separated from the Chinese mainland by the opening of Okinawa trough 1.55 million years ago (Osozawa 2013). Hence the geological history would support strong faunal relationships between subtropical Asia and the Ryukyus (see also Bănărescu 1992), and might suggest divergence of the ancestors of these Metacyclops species not earlier than 1.55 million years ago.

Identification key to the South and East Asian species of Metacyclops (females)
[the key is based on information given in the original descriptions, except for M. minutus, in which a description provided by Einsle (1993)