Metacyclops amicitiae, Kolaczynski, Andrzej, 2015

Kolaczynski, Andrzej, 2015, A new species of Metacyclops from a hyporheic habitat in North Vietnam (Crustacea, Copepoda, Cyclopidae), ZooKeys 522, pp. 141-152 : 142-146

Taxon classification Animalia Cyclopoida Cyclopidae

Metacyclops amicitiae sp. n.

Material examined.

Holotype: female dissected on two slides [MIZ 6/2015/1], Vietnam, Tam Đao, 21°45'N 105°64'E, ca. 930 m above sea level, water seeping into a shallow pit dug in gravel deposit of a creek (no name), leg. M. Hołyńska 02 Apr. 1999. Paratypes: six females on two slides each [MIZ 6/2015/2, MIZ 6/2015/3, MIZ 6/2015/4, MIZ 6/2015/5, MIZ 6/2015/6, MIZ6/2015/7], and two males on one slide each [MIZ 6/2015/8, MIZ 6/2015/9] from the same sample as the holotype; two females on two slides each [MIZ 6/2015/10, MIZ 6/2015/11], Tam Đao, 21°45'N 105°64'E, small rock depression filled with wet leaf litter, leg. M. Hołyńska 01 Apr. 1999.

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.

Female. Length of holotype 635 µm; range and mean of body length 610-660 µm and 634 µm, respectively (n = 9). Length of prosome 413 µm (cephalothorax: 225 µm, prosomite 2: 88 µm, prosomite 3: 56 µm, prosomite 4: 44 µm). Length of urosome 222 µm. Prosome and urosome length ratio approximately 1.86. Antennule is very short. The length of antennule does not exceed the length of cephalothorax.

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.

Relative length of caudal setae from terminal accessory (innermost) to posterolateral (outermost) caudal setae: 1.7 (75 µm), 9.6 (422 µm), 5.5 (242 µm), 1.0 (44 µm). Dorsal caudal seta 36 µm, 0.82 times as long as posterolateral caudal seta. Setulation of caudal setae homonomous.

Antennule (Fig. 1F, G, H) 12-segmented and armed as follows: 8 (and row of spinules ventrally), 4, 6, 2, 1 + spine, 2, 3, 1 + aesthetasc, 1, 2, 2 + aesthetasc, 7 + aesthetasc.

Antenna (Fig. 2A, B) armed with 3, 1, 9, and 7 setae on coxobasis and three-segmented endopodite, respectively. Exopodite seta long, reaching beyond enp3. Coxobasis bearing five groups of spinules on caudal surface (Fig. 2B), longer spinules present on lateral margin near base of segment, and three groups of spinules present on frontal surface (Fig. 2A).

Mandible (Fig. 2C) with reduced palp bearing two long plumose setae and short naked seta. Gnathobase with 8 teeth and dorsal seta-like element.

Maxillule (Fig. 2D, E) comprised of praecoxopodite and palp. Palp (Fig. 2E) two-segmented and bearing seven setae: three setae apically, three setae on lateral lobe (segment), and one seta proximally. Praecoxopodite has three large apical spines fused at their base, one seta on caudal surface next to base of apical spines, and seven elements on medial margin.

Maxilla (Fig. 2F) five-segmented, consisting of praecoxopodite and coxopodite (separated on caudal surface yet fused frontally), basipodite, and two-segmented endopodite. Arthrodial membrane absent between distal endopodal segment and large distal claw-like seta on caudal surface. Setal formula: 2, 3, 2, 2, 3. Short seta (Fig. 2F arrowed) inserted on caudal surface of basipodite, next to base of medial claw-like attenuation of segment.

Maxilliped (Fig. 2G) four-segmented, comprising syncoxopodite, basipodite and two-segmented endopodite. Setal formula 3, 2, 1, 3. Basipodite with thin hairs on frontal surface and one group of spinules caudally near lateral margin.

P1-P4 (Fig. 3) rami two-segmented, spine formula 3-4-4-3 (Table 1).

P1-P4: intercoxal sclerites with fine hairs on distal margin; mediodistal part of basipodite rounded and pilose. Leg 1 (Fig. 3A): coxopodite with laterodistal row of spinules. Two small spinules present next to insertion of medial spine of basipodite; spine reaching beyond distal margin of enp1. Leg 2 (Fig. 3B) coxopodite bearing two rows of spinules on caudal surface and short spinules on lateral margin. Leg 3 differing from leg 2 in size only.

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.

Male. Length of two paratypes 537 and 539 µm. Length of prosome 334 µm (cephalothorax: 182 µm, prosomite 2: 65 µm, prosomite 3: 53 µm, prosomite 4: 34 µm). Length of urosome 203 µm. Prosome and urosome length ratio about 1.64.

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 Metacyclops 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 Metacyclops 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 [ Metacyclops brauni Herbst, 1962, Metacyclops botosaneanui Pesce, 1985, Metacyclops hartmani Herbst, 1960, Metacyclops laticornis (Lowndes, 1934), Metacyclops necessarius (Kiefer, 1926), Metacyclops mendocinus venezolanus Kiefer, 1956, Metacyclops leptopus (Kiefer, 1927), Metacyclops leptopus mucubajiensis Kiefer, 1956, Metacyclops mendocinus (Wierzejski, 1892), Metacyclops problematicus Dumont, 1973, Metacyclops janstocki Herbst, 1990, Metacyclops hirsutus Rocha, 1994] (see Herbst 1988, 1990; da Rocha 1994), but those New World species have two spines on the distal endopodal segment of P4, instead of the single one present in Metacyclops amicitiae and Metacyclops ryukyuensis . Metacyclops amicitiae sp. n. differs from Metacyclops ryukyuensis in the surface ornamentation of the intercoxal sclerites of the swimming legs (sclerites smooth in Metacyclops ryukyuensis vs. sclerites bearing distal hairs in P1-P4, and spinules on the caudal surface of P4 both in female and male of Metacyclops amicitiae ). Other characters that also distinguish Metacyclops amicitiae sp. n. from Metacyclops ryukyuensis are: number of setae on the second endopodal segment of the female antenna (nine in the new species, eight in Metacyclops ryukyuensis ); the size of the posterior sac of the seminal receptacle (long, approaching posterior margin of the genital double-somite in Metacyclops amicitiae sp. n., vs. reaching nearly the middle of the somite in Metacyclops ryukyuensis ); P4 coxopodite with five rows of spinules on the caudal surface, robust spinules near the proximal margin of the segment in Metacyclops amicitiae , vs. four rows of spinules, tiny spinules near the proximal margin of the coxopodite in Metacyclops ryukyuensis ; apical spine of P4 endopodite is 0.82 times as long as distal segment in Metacyclops amicitiae , vs. 0.67 times as long as distal segment in Metacyclops ryukyuensis ; genital double-somite wider than long in Metacyclops amicitiae , vs. as long as wide in Metacyclops ryukyuensis ; presence of spinules at insertion of the anterolateral caudal setae in Metacyclops amicitiae sp. n., vs. absence in Metacyclops ryukyuensis , and finally surface ornamentation of the anal sinus (naked in Metacyclops amicitiae sp. n., with spinules in Metacyclops ryukyuensis ).

Metacyclops amicitiae sp. n. and Metacyclops 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 Metacyclops amicitiae sp. n. and Metacyclops 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 Metacyclops 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.