Eurytemora caspica tethysiana, Sukhikh & Zavarzin & Alekseev, 2023

Sukhikh, Natalia, Zavarzin, Denis & Alekseev, Victor, 2023, Origin and taxonomic position of Far Eastern island populations of Eurytemora caspica tethysiana subsp. nov., Zoologischer Anzeiger 306, pp. 119-137 : 125-127

publication ID	https://doi.org/ 10.1016/j.jcz.2023.07.004
persistent identifier	https://treatment.plazi.org/id/1C438784-FFE1-8503-2F5C-F97F49505EA4
treatment provided by	Felipe
scientific name	Eurytemora caspica tethysiana
status	subsp. nov.

Treatment

Eurytemora caspica tethysiana subsp. nov ( Tables 2 – 6,

Figs. 6 – 13 View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig ).

Etymology. The subspecies was named after the Tethys Sea, which probably was the initial area inhabited by Eurytemora caspica .

Type material. Holotype, ZIN RN 55208 View Materials , a female dissected on 1 slide; from the Lake Tunaicha , Sakhalin Isl., Russia (46 ◦ 45′N 143 ◦ 12′E), collected by D. Zavarzin, 01 July 2002. GoogleMaps

Paratypes, ZIN RN 55209–55213 View Materials a male and females from a few lakes on the Sakhalin Isl., Russia and from the Ohnuma Lake , Japan detailed given in Table 6 .

Description. Female ( Fig. 6 View Fig а). Body transparent, yellowish brown coloured. Full body length without caudal setae 1370 μm, with caudal setae 1573 μm; cephalosome 207 μm and 4 free thoracic somites 1/2/3/ 4 = 120/118/109/103 μm. Urosome without caudal setae 530 μm, genital double-somite 165 μm, caudal rami 380 μm.

Cephalothorax as long as wide, with maximum width close to middle, frontal part of cephalothorax oval. Last thoracic somite with 1–2 small sensilla on wing-like outgrowths of lateral margin.

Genital double-somite ( Fig. 2b View Fig and 6 a, b View Fig ) symmetrical with transverse constriction in the central part, 1.2 times as wide as long, due to wing-like outgrowths in the anterior part of the somite, with two relatively long spines on both sides, with seminal receptacle.

Caudal rami ( Figs. 2b View Fig and 6 a, b View Fig ) divergent, 5.4 times as long as wide, with long hair-setae on dorsal outer and inner sides, as well as on last abdominal somite. Caudal setae with segment like division. Length proportions of terminal setae, beginning from outermost caudal seta: 1/ 1.3/1.0/1.1. Length proportions of dorsal and lateral setae to outermost seta 0.3 and 0.8 times, respectively.

Antennulae and antennae ( Fig. 7a and b View Fig ) as well as mouthparts ( Fig. 7c, d, e, f View Fig ), basically look the same in all the representatives of other congeners of Eurytemora , and, in particular similar to E. с aspica с aspica from the Caspian Sea.

Antennules ( Fig. 7 a View Fig ) of 24 segments, reaching end of fifth free thoracic somite, setation of segments beginning from first segment: 4/4/ 3/1/3/1/4/1/3/1/2/2/2/2/2/3/2/2/3/1/1/2/2/7.

Antenna ( Fig. 7 b View Fig ) biramous, of 2-segmented protopod, 7-segmented exopod and 3-segmented endopod. First exopod segment with one seta, second segment with 3 setae, 4–6 segments with 1 seta each, 7 segments with 4 setae. First endopodal segment with 2 setae, second with 8 setae and third with 7 setae at distal end.

Mandible ( Fig. 7 c View Fig ) composed of coxa with gnatobasis, one of its tooth (outermost) significantly larger than others. Coxa in middle with biramous mandibular palp, basis with 4 setae, 5-segmented exopod and 2-segmented endopod. Distal segment of exopod with 2 sub-equal setae, other segments bearing single long seta each. Distal segment of endopod with 6 long and 3 short setae, basal segment of endopod with 4 long setae.

Maxillula ( Fig. 7 d View Fig ) biramous and composed of precoxa with medial arthrite bearing 8 strong clawlike spines, 7 relatively long setae; coxa with elongated endite bearing 6 long setae and outer outgrowth with 7 very strong sub-equal in length setae and 2 thin setae. Basis composed of basal endite, 1-segmented exopod with 9 long subequal setae and 4- segmented endopod bearing 5–4–4–7 (distally) long setae.

Maxilla ( Fig. 7 e View Fig ) uniramous, composed of precoxa with 2 endites; distal endite bearing 2 long and one short setae, first endite with 4 long setae, coxa with two endites bearing 2 long and 1 short seta each and 5- segmented endopod including basal endite with 2 long, 2 short setae and 4 short segments bearing 1 or 2 long distal setae.

Maxilliped ( Fig. 7 f View Fig ) uniramous and 8-segmented, composed of long coxa with 3 hill-shaped endites bearing 2–2–3 setae; basis more wide in middle part with 3 long setae and with complex of long hairs; endite with 2 long setae followed by 5-segmented endopod armed with 2–2–2–2–4(distally) setae.

Swimming legs P1–4 ( Fig. 8a, b, c, d View Fig .) consisting of coxa and basis bearing 3- segmented exopod and 1- (P1) or 2-segmented endopod (P2–4). Swimming legs look the same in all the representatives of this genus and has formula for spine (Arabic) and seta (Roman) for distal exopod segments in P1–4 as follows: 3IV–3V–3V–3V. Length of P4 distal segment apical spine more than 1.2 times the length of the distal segment of P4.

P5 ( Fig. 8e View Fig ) uniramous and 4-segmented, narrow coxa bearing 1- segmented basipod with one barely visible sensilla on outer edge. Often sensilla is absent (or does not seen). First exopod segment with big inner outgrowth and 2 spines. Big inner outgrowth has segment-like division from basis. Largest spine on the second exopod segment (distal) also has clear seen segment-like divisions.

Distal segment with long apical seta and short lateral spine about 1/3 of apical seta. Tiny spine between spines on the distal segment about 10% of short spine length. Long apical spine has segment like division, which divides the spine into 2 or 3 parts.

Eggs are packed in 1 sac.

Male ( Fig. 9 View Fig ). Body length 1226 μm, with caudal setae 1493.

Cephalothorax 1.5 times as wide as long with maximum width close to caudal end, anterior part of cephalothorax round-shaped. Last thoracic somite without wings and spine on lateral margin. Abdomen 5- segmented.

Caudal rami without spines, with setae on inner sides. Length 8.5 times longer than width. Terminal setae-ratio beginning from outermost caudal seta: 1/1.4/1.5/1.2.

Antennules, antenna, mandible, maxillula, maxilla, maxilliped ( Fig. 10 a, b, c, d, e, f View Fig ) are similar in all the representatives of this genus and, in particular, in female of described subspecies and E. с aspica from the Caspian Sea.

Right Antennule ( Fig. 11 a View Fig ) 21-segmented, 7–12 segments with strong spines, strongest spine (3-4 times as long as any other spine) at segment 12; 17–19 segments with denticulate plates at inner edges, distal segment with several sensitive papillae. Other segments with regular setae as in Fig. 2 View Fig . Left antennule 24 segmented armed totally as in female (see Fig. 2 View Fig ).

The structure of the swimming legs P1–4 ( Fig. 11 a, b, c, d View Fig ) is typical for Eurytemora species and the same as in females. Formula for spine (Arabic) and seta (Roman) for distal exopod segments in P1–4 as follows: 3IV–3V–2V–3V. Some setae in swimming legs with not clear seen segment-like divisions.

Legs P5 ( Fig. 11e View Fig ): right leg with basipodal segment cylindrical in shape and with small hill on inner side pointed with long spine, distal bent segment with 2 short spines in the middle part. Shape of the bent segment closer to E. carolleeae , than E. caspica caspica .

Left leg basipod like in E. carolleeae , cylindrical, about 1.54 times as long as wide, next exopodal segment with two long spines in middle part, distal segment with strong long spine in middle, and hook in end similar to E. carolleeae .

Remarks. In our opinion, Eurytemora caspica tethysiana subsp. nov. like Eurytemora caspica caspica can be discriminated from two closelyrelated species E. affinis and E. carolleeae by a unique combination of characters that to some extent can be found in both other species but never coincide together in the same species. These features in both sexes include: in E. caspica tethysiana subsp. nov. as in E. affinis and E. caspica caspica mandible without a large outside orientated tooth and a big gap separating it from the other teeth as in E. carolleeae ( Figs. 7f View Fig and 10f View Fig ). Also, E. caspica caspica adults, like E. carolleeae , have segment-like division setae on swimming legs and on caudal rami, but the division is not so clear as in E. carolleeae ( Figs. 6b View Fig and 9 View Fig ).

At the same time, E. caspica tethysiana subsp. nov. females similarly to E. carolleeae and E. caspica caspica are equipped with outgrowths of the genital double-somite, but the outgrowths are more than in E. caspica caspica and have a wing-like shape; ratio W1/W2, as in E. carolleeae , is about 1,5 ( Fig. 2a and b View Fig ).

Another difference of E. caspica tethysiana subsp. nov. from E. caspica caspica is the shape of the P5 distal segment, which is longer in E. caspica tethysiana subsp. nov. than in E. caspica caspica . P5 Lseg/Wseg is usually less than 1, 5 in E. caspica caspica and more than 1, 5 in E. caspica tethysiana subsp. nov.

In E. caspica tethysiana subsp. nov., females like E. carolleeae and E. caspica caspica have P5 with a small, tiny spine on the second exopodal segment placed between two distal spines ( Alekseev and Souissi 2011). The length of this tiny spine is less than the width of the nearest spines, or about 10% of the short distal spine length, which definitely separates both species from E. affinis ( Table 5).

In males of E. caspica tethysiana subsp. nov., the caudal rami are naked on both dorsal and ventral sides, as in E. caspica caspica and E. carolleeae , in E. affinis males, caudal rami always have the sets of spines on the dorsal surface (sometimes only a few).

The left P5 of E. caspica tethysiana subsp. nov., has an exopod of more or less cylindrical shape, like E. carolleeae ( Fig. 11e View Fig ), whereas E. caspica caspica and E. affinis have exopods of more triangular shape with a length/width proportion of about 1 ( Table 3).

The male of E. caspica tethysiana subsp. nov., unlike the E. caspica caspica has a right P5 with a special-shaped distal segment with a W1/ W2 proportion of about 4, which is closer to both E. affinis and E. carolleeae than to E. caspica caspica ( Table 3). E. caspica tethysiana subsp. nov. is also different from E. caspica caspica in its L abdomen/L caudal rami ratio, which is equal or more than 1 for E. caspica tethysiana subsp. nov. and equal or less than 1 for E. caspica caspica .

E. caspica tethysiana subsp. nov. differs from E. affinis by its strong and long spine 12 on A1, ratios 12spine/9spine are more than 2.7 and less than 2.7, respectively.

The PCA showed maximal weight of P5 LSP/SP1, furcal L/W indices for females ( Fig. 12 View Fig ) and P5 W1/W2, furcal L/W indices for males ( Fig. 13 View Fig ) of E. caspica subspecies.