Gammarus sezgini, Baytaşoğlu & Aksu & Özbek, 2024

Baytaşoğlu, Hazel, Aksu, İsmail & Özbek, Murat, 2024, Gammarus sezgini sp. nov. (Arthropoda, Amphipoda, Gammaridae), a new amphipod species from the Eastern Black Sea region of Türkiye, Zoosystematics and Evolution 100 (3), pp. 989-1004 : 989-1004

Gammarus sezgini sp. nov.

Figs 2 , 3 , 4 , 5 , 6 , 7

Material examined.

Holotype: Türki̇y • Male; 9.8 mm; Rize Province, Yeşildere stream / Balat stream / Taşlı Stream ; coordinates: 40.9493 ° N, 40.5394 ° E / 41.0227 ° N, 40.7130 ° E / 40.8701 ° N, 40.5859 ° E. Specimens collected by Hazel BAYTAŞOĞLU; 16 October 2019 and 1 September 2020 . Holotypes with paratypes are stored under catalog number RTEÜ-FFR 200001; (GenBank accession numbers: PP 457383, PP 457384 for COI, and PP 456726, PP 456727 for 28 S; PP 457381, PP 457382 for COI, and PP 456724, PP 456725 for 28 S; PP 457385 for COI and PP 456728 for 28 S).

Paratypes: 38 males and 34 females, same data as the holotype.

Genetic material.

RTEÜ-FFR-DNA K 2, K 4, Yeşildere stream, Rize Province, Türkiye, 40.9493 ° N, 40.5394 ° E (GenBank accession numbers: PP 457383, PP 457384 for COI, and PP 456726, PP 456727 for 28 S) - RTEÜ-FFR-DNA K 5, K 8, Balat stream, Rize Province, Türkiye, 41.0227 ° N, 40.7130 ° E (GenBank accession numbers: PP 457381, PP 457382 for COI, and PP 456724, PP 456725 for 28 S) - RTEÜ-FFR-DNA K 9, Taşlı stream, Rize Province, Türkiye, 40.8701 ° N, 40.5859 ° E (GenBank accession numbers: PP 457385 for COI and PP 456728 for 28 S).

Diagnosis.

A medium-large species. The body is yellowish; no dorsal keel or hump; the eyes are well developed; kidney-shaped; the extremities are not elongated; the second antenna bears numerous groups of long setae on the peduncle and flagellar segments; the antennal gland cone is straight and reaches to the distal end of the third peduncular segment; posterior margin of pereopod 3 densely setose; the setae on the posterior edge of pereopod 4 are shorter and fewer in number; the anterior margins of pereopods 5 to 7 bear spines in the male, while they bear long setae along with the spines in females; epimeral plates are pointed; the inner ramus of uropod 3 is slightly longer than 0.8 of the outer one; each telson lobe bears a pair of spines distally and setae longer than the spines.

Description of male holotype.

Head: Rostrum absent, inferior antennal sinus deep, rounded. Eyes kidney-shaped; length is slightly shorter than the diameter of the first peduncular segment of antenna 1 (Fig. 2 ).

Antennae: Antenna 1 (Fig. 4 A ) is half as long as the body length; the length ratio of the peduncular segments is 1: 0.75: 0.38; peduncle segments bear a few groups of minute setae; the length of the setae is much shorter than the segment where they are implanted; the main flagellum with 32 segments; each segment bears a few short setae in distal side; aesthetasc absent; accessory flagellum with five segments. Antenna 2 (Fig. 4 B ) is shorter than antenna 1 (ratio 1: 0.7); the antennal gland cone is straight and reaches the distal end of the third peduncular segment; setation is rich both on peduncular and flagellar segments; peduncular segments 4 and 5 bear many groups of setae; the setae on the ventral part of the peduncle segments are shorter than the diameter of the segment but longer than those on the dorsal part; flagellum consists of 12 segments; flagellar segments are setose and swollen; each segment bears many long setae groups on both dorsal and ventral sides; calceoli absent.

Mouthparts: Upper lip (Fig. 3 B ) with numerous minute setules in the distal part.

Left mandible (Fig. 3 F ) with 4 - toothed incisor, lacinia mobilis with 4 dentitions, molar triturative. The first article of palp without setae; the second one bears 14 setae; the setae become shorter from distal to proximal. The third segment has 33 D-setae, 4–5 E-setae, one group of A-setae, and one group of B-setae. C-setae absent.

Right mandible (Fig. 3 E ) has a 4 - toothed incisor and bifurcate lacinia mobilis.

Right maxilla 1 (Fig. 3 H, H ’) is asymmetric to the left; it has 16 plumose setae along the inner margin of the inner lobe. The outer lobe bears 11 distal stout serrate spines and some tiny setules on the inner margin. Palp of the outer lobe with no setae in the first segment and six stout spines (one of them lost) and three setae (two of them robust) on the distal part of the second segment, in addition to a marginal seta along the outer margin. The second article of left palp is elongated and bears 10 spines, five simple setae on its distal part, and no setae along the outer margin (Fig. 3 G, G ’).

Lower lip (Fig. 3 A ) has no inner lobe and bears numerous small simple setae along the distal margins of both lobes.

Maxilla 2 (Fig. 3 C ) has 20–30 simple setae in the distal part of the outer lobe and a few tiny hairs along the outer margin. The inner lobe also has 10–15 simple setae in the distal part in addition to 14–15 (two of them lost) plumose setae located in a diagonal row along the inner margin. There are also a few tiny hairs on the proximal part of the inner margin of the lobe.

Maxilliped (Fig. 3 D ) inner plate has three tooth-like spines and a spine in the distal part and the distal corner, respectively. Additionally, there are 10 plumose setae along the inner margin of the lobe. Outer plate armed with 5–6 serrate stout setae in the distal part and 12 spines along its inner margin.

Coxal plates: Coxal plate 1 (Fig. 4 C ) is rectangular, the distal part slightly widened, the ventral margin slightly convex, and bears four antero-distal setae and two postero-distal setae. Coxal plate 2 (Fig. 4 D ) is in the shape of an elongated rectangle; distal part narrower than the proximal; the ventral margin is highly convex; anterodistal part with five setae; and postero-distal part with one seta. Coxal plate 3 (Fig. 5 A ) is similar to coxal plate 2 in shape, with four and two setae in the antero- and postero-distal ends, respectively. The ventral edge of the coxal plate 4 (Fig. 5 B ) is slightly convex and bears three and eight setae along the anteroventral and posterior margins, respectively. Coxal plate 5 (Fig. 5 C ) bilobate and has one and five setae in the anterior and posterior lobes, respectively. Coxal plate 6 (Fig. 5 D ) bilobate and has one seta in the posterior lobe. Coxal plate 7 (Fig. 5 E ) is characterized by the presence of four setae on the postero-ventral margin.

Gnathopods: Basal segment of gnathopod 1 (Fig. 4 C, C ’) bears many long setae along both margins; the length of the setae can be longer than twice the diameter of the segment. Ischium bears a group of setae in postero-ventral corner. Merus is in diamond shape and bears some setae along its disto-posterior margin. Carpus is triangular and bears two groups of setae along the anterior margin, in addition to many setae groups on both ventral and posterior sides. Propodus pyriform, the length / width ratio is 1: 0.60; anterior margin with four groups of setae; medial palmar spine is present; posterodistal corner armed with two strong spines in addition to some small spines; posterior margin bears 4–5 groups of setae. Dactylus reaches the postero-distal corner and bears a simple seta along the outer margin in addition to a small setule around the distal part of the inner margin.

The basis and ischium of gnathopod 2 (Fig. 4 D, D ’) are similar to those of gnathopod 1 but have denser setae. Merus and carpus are more setose than those of gnathopod 1. Carpus is triangular, densely setose along the posterior margin, in addition to two groups of setae along the anterior margin. Propodus is densely setose and has a sub-rectangular shape; the length / width ratio is 1: 0.6; anterior margin bears six groups of setae; posterior margin with many groups of setae; medial palmar spine is present; the postero-distal corner is armed with six strong spines. Dactylus reaches the postero-distal corner and bears a simple seta along the outer margin in addition to a small setule around the distal part of the inner margin.

Pereopods: Anterior and posterior margins of the basal segment of pereopod 3 bear long setae; the setae along the posterior margin are longer than those in the anterior margin; posterior margins of the merus, carpus, and propodus bear long setae; the setae can be more than three times the diameter of the segment where they are implanted. Dactylus slim, a minute plumose seta occurs on the outer margin; the inner margin with two small setules (Fig. 5 A ).

The basal segment of pereopod 4 (Fig. 5 B ) has a similar setation to that of pereopod 3. Ischium, merus, carpus, and propodus have groups of setae along their posterior margins, but they are much shorter and less than those in pereopod 3. The length of the setae can be as long as (or slightly longer) than the diameter of the segment where they are implanted. Dactylus slim, a minute plumose seta, occurs on the outer margin; the inner margin with two small setules.

Posterior margins of the basal segments of pereopods 5 to 7 (Fig. 5 C – E ) are more or less convex and bear many short setae, anterior margins with 3–7 small spines, and no setae present on the inner surfaces of the basal segments; no spine exists in the postero-ventral corner of the basal segment of pereopod 7. Pereopod 7 bears no setae along the anterior margins of merus and carpus, while pereopods 5 and 6 have a few setae accompanying spines along with the mentioned segments. Propodus of pereopods 5 to 7 with 2–3 groups of long setae groups along their outer margins in addition to 5–6 groups of small spines along their inner margins. Dactylus slim, a minute plumose seta, occurs on the outer margin; the inner margin with two small setules.

Epimeral plates: They are slightly pointed. Epimeral plate 1 (Fig. 6 C ) bears 10 long setae along the anteroventral margin, and the postero-ventral corner is angular. Epimeral plate 2 (Fig. 6 B ) bears five setae in the anteroventral corner; the ventral margin is armed with two spines; the posterior margin with 4–5 setules; the postero-ventral corner is pointed. Epimeral plate 3 (Fig. 6 A ) is pointed; the anteroventral corner bears two setae; the ventral margin is armed with three spines in addition to a seta; the posterior margin bears 5–6 setules.

Urosomites: Not elevated (Fig. 2 ). Each segment bears a median and two dorsolateral groups of armaments; each of them consists of 1–2 spines and 3–4 accompanying setae.

Uropods: Uropod 1 (Fig. 6 E ) has a spine in the outer margin of the base; inner margins bear 4 + 5 spines; the peduncle is longer than rami; the length ratio is about 1: 0.7. Peduncle with a spine in the outer margin of the proximal part in addition to three spines along the inner margin and three spines in the distal part. The inner ramus is slightly longer than the outer ramus and bears three spines along their inferior margin in addition to 4–5 distal spines. The outer ramus has two and three spines along the inferior and outer margins in addition to 4–5 distal spines, respectively.

Uropod 2 (Fig. 6 D ) is smaller than the first one; the length ratio is about 1: 0.6; the peduncle segment is slightly longer than the rami and bears 2 + 1 spines along the inner margin and the distal part, respectively. The outer margin is bare. The length and armaments of both rami are similar to each other; they bear two spines along their inner margins in addition to 4–5 longer spines on their distal tips.

Uropod 3 (Fig. 6 F ) is setose and bears both simple and plumose setae. The peduncle segment is much shorter than the outer ramus, and the length ratio is about 1: 0.48. The outer ramus is two articulated and densely setose along both margins; the outer margin bears three groups of spines accompanied by groups of long simple setae; the inner margin with plumose setae; the second article is well developed and longer than the surrounding distal spines. The inner ramus is about 0.78 times the length of the outer ramus. It bears one spine in the proximal part of the outer margin in addition to groups of simple and plumose setae; the inner margin bears both simple and plumose setae.

Telson: Telson (Fig. 6 G ) lobes cleft; each lobe bears two spines and 5–6 simple setae in their distal parts. The setae are twice as long as the spines. There are 3–4 groups of short setae on the dorsal surface of the lobes in addition to two plumose setules. The length / width ratio of each lobe is about 1: 0.5.

Description of females.

Smaller than males. Except for the sexual dimorphism indicated for the genus Gammarus , females do not show obvious differences from males. At first glance, the morphological differences between the female allotype and the male holotype can be listed as follows: More setose antenna 2, less setose and small gnathopod 2, more setose pereopod 4; more setose anterior margins of pereopods 5 to 7 (Figs 7 , 8 ).

Variability.

Some of the paratypes are immature. The eyes are kidney-shaped, or elongated, and oval. The number of flagellar segments in antenna 1 varies between 26 and 29. Similarly, there are 10–11 flagellar segments in antenna 2.

Etymology.

The species epithet is derived from the name of our dear friend Prof. Dr. Murat Sezgin (R. I. P.), who made valuable contributions to the marine amphipod species in Türkiye.

Results of molecular data analyses

We tested the new species with molecular methods as well as morphological characters. For this, firstly, the COI (573 bp.) and 28 S (911 bp.) genes of the new species from type and paratypes were amplified and sequenced. The obtained sequences were deposited in Genbank with the corresponding accession numbers: PP 457381 – PP 457385 for COI and PP 456724 – PP 456728 for 28 S. For molecular comparison, sequences of topotype samples of valid congeners of the new species or otherwise correct sequences of valid species were downloaded from GenBank (see Table 1 ) and used in all analyses.

The pairwise genetic distance amongst Gammarus species based on the COI was calculated to range from 5.24 % ( G. stankokaramani G. Karaman, 1976 - G. salemaai G. Karaman, 1985) to 28.97 % ( G. sezgini sp. nov. - G. roeselii Gervais, 1835 ). The species most closely related to G. sezgini sp. nov. is G. tumaf Özbek, Aksu & Baytaşoğlu, 2023 , with 17.10 %, approximately three times larger than the minimum genetic distance. The pairwise genetic distance amongst Gammarus species based on the 28 S was calculated to range from 0.11 % ( G. halilicae G. Karaman, 1969 - G. pljakici G. Karaman, 1964) to 7.73 % ( G. rambouseki G. Karaman, 1931 - G. stojicevici (S. Karaman, 1929 )). The species most closely related to G. sezgini sp. nov. is G. kesslerianus Martynov, 1931 , with 0.88 %, eight times larger than the minimum genetic distance. All pairwise genetic distance values calculated with the p - distance model based on COI and 28 S genes amongst Gammarus species are given in Suppl. material 1.

Phylogenetic trees constructed with ML, NJ, and BI methods based on the concatenated dataset (28 S + COI) showed similar topologies with a few exceptions and had high bootstrap (ML and NJ BP ≥ 70 %; Fig. 9 ) and posterior probability (BI PP ≥ 0.7; Fig. 9 ) support for a large number of nodes. In the phylogenies constructed according to all three methods, the newly identified species, G. sezgini sp. nov., formed the sister clade of the G. kunti Özbek, Baytaşoğlu & Aksu, 2023 , G. tumaf , and G. baysali Özbek, Yurga & Külköylüoğlu, 2013 ( G. sezgini sp. nov., ( G. kunti , ( G. tumaf + G. baysali ))) and was resolved from it with strong support (ML BP: 78 %, NJ BP: 70 %, and BI PP: 1.0; Fig. 9 ).

The species delimitation analysis we performed according to the ASAP method identified 27 MOTUs for 27 morphologically valid species (Fig. 9 ). The best ASAP score was 3.0 (p = 0.01) at a threshold distance of 0.079053. The analysis identified the species G. stankokaramani and G. salemaai as a single MOTU, while the Bulgarian and Iranian samples of G. komareki were identified as separate MOTUs. The PTP method identified 28 MOTUs for 27 species (Fig. 9 ). p = 0.001, null-model score: 84.937039, best score for single coalescent rate: 95.647569. Similar to ASAP, Bulgarian and Iranian samples of G. komareki formed separate MOTUs, while unlike ASAP, G. stankokaramani and G. salemaai species also formed separate MOTUs. Gammarus sezgini sp. nov. formed a single MOTU independently of other species according to both methods (Fig. 9 ).

Figures

Figure 2. Habitus of the holotype male of Gammarus sezgini sp. nov.

Figure 3. Mouthparts of the holotype male of Gammarus sezgini sp. nov. A. Lower lip; B. Upper lip; C. Maxilla 2; D. Maxilliped; E. Right mandible; F. Left mandible; G. Left maxilla 1; H. Right maxilla 1; G’, H’. Detail of the left and right maxilla 1.

Figure 4. Extremities of the holotype male of Gammarus sezgini sp. nov. A. Antenna 1; B. Antenna 2; C. Gnathopod 1; C’. Palm of Gnathopod 1; D. Gnathopod 2; D’. Gnathopod 2.

Figure 5. Pereopods of the holotype male of Gammarus sezgini sp. nov. A. Pereopod 3; B. Pereopod 4; C. Pereopod 5; D. Pereopod 6; E. Pereopod 7.

Figure 6. Extremities of the holotype male of Gammarus sezgini sp. nov. A. Pleopod 3 and Epimeral plate 3; B. Pleopod 2 and Epimeral plate 2; C. Pleopod 1 and Epimeral plate 1; D. Uropod 2; E. Uropod 1; F. Uropod 3; G. Telson.

Figure 7. Appendages of the allotype female of Gammarus sezgini sp. nov. A. Antenna 1; B. Antenna 2; C. Gnathopod 1; D. Gnathopod 2; E. Pereopod 3; F. Pereopod 4.

Figure 8. Appendages of the allotype female of Gammarus sezgini sp. nov. A. Pereopod 5; B. Pereopod 6; C. Pereopod 7; D. Uropod 3; E. Telson; F. Uropod 1; G. Uropod 2; H. Pleopod 1 and Epimeral Plate 1; I. Pleopod 2 and Epimeral Plate 2; J. Pleopod 3 and Epimeral Plate 3.

Figure 9. Phylogenetic relationships of Gammarus species reconstructed with the ML method based on the concatenated data set (28 S + COI). Since the ML, NJ, and BI methods generally yield similar topologies, only the ML phylogeny is shown. The nodes (ML, NJ, and BI) show the Bayesian posterior probabilities and the bootstrap percentage. For the support values of the nodes, ML ≥ 70 %, NJ ≥ 70 %, and BI ≥ 0.70 are shown. Black bars indicate OTUs. The first column shows morphology-based results, the second column shows ASAP results, and the third column shows PTP results.