Eulimnogammarus tchernykhi, Moskalenko & Neretina & Yampolsky, 2020

Moskalenko, Victoria N., Neretina, Tatiana V. & Yampolsky, Lev Y., 2020, To the origin of Lake Baikal endemic gammarid radiations, with description of two new Eulimnogammarus spp., Zootaxa 4766 (3), pp. 457-471 : 465-466

Eulimnogammarus tchernykhi sp. nov.

( Fig. 1B , Fig. 3 )

Holotype: female, body length 8 mm, Baikal , Bolshoy Ushkanii Island, Pescherka bay, 53.852N, 108.660E, depth 15–20 m, dredge on sandy substrate with pebbles, August 2013. 2 paratypes (1 male, 1 female), same location and date. Holotype location: Zoological Museum , Moscow State University, Moscow, Russia. Paratypes: Department of Biological sciences, East Tennessee State Universiry, Johnson City, TN, USA and Baikal Museum, Listvyanka, Russia. See Supplementary materials for holotype museum ID.

Etymology: The species is named after Valery Chernykh who participated in the collection of the holotypes.

Diagnosis: This species can be distinguished from other Eulimnogammarus spp . by 1- or 2-articulate accessory flagellum, lack of calceoli, a single pair of spines only on the last 2 segments of urosome and dense setation of outer ramus and strong spines on peduncle of uropod III.

Description: Description is based on female holotype and two male paratypes. No sexual dimorphism noted other than the presence of brood chamber in females. Body compact, smooth, 6-10 mm, red when alive, with crosshatching on the head segment. Two strong spines on the last two segments of the urosome, bundles of setae on the last segment of pleosome and all segment of urosome, individual setae on the 2 nd segment of pleosome. Eyes kidney-shaped, black. Rostrum very short, rounded, antennal sinus deeply cut ( Fig. 1B ).

Antennae I short (1/3 body length) with the first peduncle article about 1/2 as long as the head segment and the length ratio of peduncle segments 1:0.8:0.6, carrying a few setae on the distal end of each peduncle segment. Flagellum consists of 12–15 articles ( Fig. 3A ). Accessory flagellum 1-articulate (in holotype female and several paratype males, sometimes 2-articulate (in two paratype males), in this case the second article is very short, 1/10 of the length of the terminal setae of the 1 st article. Antennae II ¾ as long as antennae I, peduncle and flagellum segments with groups of long setae on ventral side, flagellum consists of 6 articles. No calceoli found ( Fig. 3B ).

Mouthparts typical for subgenus Philolimnogammarus (sensu Bazikalova 1945) . Basal article of mandible palpus without setae, terminal article slightly shorter than the subterminal one; the comb consists of two rows of spines and is much shorter than the apical setae ( Fig. 3C ). Outer plate of maxillae I carries hooks with 6-7 short dense teeth, inner plate triangle with dense pennate setae ( Fig. 3D ). Maxillae II ( Fig. 3E ) and maxillipedae ( Fig. 3F ) as in any Eulimnogammarus spp .

Gnathopods I with very strong and powerful almond-shaped propodus with 4–5 strong spines on posterior margin and a large blunt spine at the middle of the palmate margin and with with a group of long setae at the base of the dactylus ( Fig. 3G ). Gnathopods II with much weaker, heavily setose subrectangular elongated propodus with 4 transverse rows of long setae on the posterior edge ( Fig. 3H ).

Pereopods short, with strong spines and few setae, coxal plates short, rounded, with very short setae on the posterior edge. Basipodite of pereopod VII with a triangle lobe tempering distally into a very short triangle protrusion not reaching the distal end of the basipodite ( Fig. 3I ). Posterior edge with 5-6 setae one of which, near the distant end, longer than others. Anterior margin with 3 spines, the one on distal-anterior corner of the basipodite stronger and longer than others. Pleopods of typical gammarid morphology, with 2 small (50 microns) retinacles on the basal article, with convex side smooth and 4–5 hooks on the concave side.

Uropodes I and II with spines. Uropod III short, carrying numerous simple setae in bundles and strong spines on both sides of the outer ramus, which is twice as long as the peduncle. Peduncle with 1 weak and 3 strong spines, that reach beyond the tip of the inner ramus, which is short, triangle, ¼ as long as the outer ramus and caries only 4 long terminal setae. ( Fig. 3J ). Telson cleft to the base, carries 1 spine and 5 long setae on each branch ( Fig. 3K ).

Remarks: Molecular phylogeny places this species within the paraphyletic genus Eulimnogammarus , ( Fig. 4 below). Uropod III structure similar to that of E. viridiformes , E. lividus and E. cyanoides , but differs from these species by smaller body size, red coloration, much shorter antennae and lack of calceoli, and only one pair of spines only on the last 2 segments of urosome. Differs from E. obsoletus , E. exiguus and E. viridulus by the lack of distally protruding distant-posterior corner of posterior lobe on basipodite of pereopod VII and much denser setation of outer ramus of uropod III; from E. exiguus , to which it is probably closely related, also by much denser setation of antenna II, shorter inner ramus of uropod III and by presence of strong spines on peduncle of uropod III and on telson. Differs from all other Eulimnogammarus spp . except the previous species by having a 1-articulate accessory flagellum in at least some adults. This species is different from the previous one by very strong gnathopods, inner branch of uropod III shorter than 1/4 of the outer branch, dense setation on antennae II and uropod III and a nearly absent distal lobe on pereopod VII basipodite. The two species are remarkably similar in their red coloration when live, a typical homoplasy common among Baikal amphipods.

Figures

FIGURE 1. Whole body outline of two newly described species. A: Eulimnogammarus etingovae sp. nov. B: Eulimnogammarus tchernykhi sp. nov. Bar = 1 mm.

FIGURE 3. Eulimnogammarus tchernykhi sp. nov. Holotype, female. A: antenna I (inset: accessory flagellum enlarged); B: antenna II; C: left mandible; D: maxilla I; E: maxilla II; F: maxilliped; G: propodus of gnatopod I; H: propodus of gnatopod II; I: basipodit of pereopod VII; J: uropod III; K: telson. Bars = 0.1 mm.

FIGURE 4. Bayesian reconstruction of select Gammarus clades including two Baikal endemic radiations based on nucleotide sequences of 16S, 18S and 28S rRNAs, COI and Histone 3 genes. Size of node diamonds represent posterior probability of each node. Node bars represent 95% highest posterior density interval for each node age, not shown where less wide than the width of node symbols. Here and on Fig. 5 the outgroup branch interrupted to allow better resolution of Gammaridae clade; the inset shows the tree with actual outgroup branch length.