Chaetogammarus trichiatus, MARTYNOV, 1932

Copilaș-Ciocianu, Denis, Palatov, Dmitry, Rewicz, Tomasz, Sands, Arthur F., Arbačiauskas, Kęstutis, Haaren, Ton Van, Hebert, Paul D. N., Grabowski, Michał & Marin, Ivan, 2023, A widespread Ponto-Caspian invader with a mistaken identity: integrative taxonomy elucidates the confusing taxonomy of Trichogammarus trichiatus (= Echinogammarus) (Crustacea: Amphipoda), Zoological Journal of the Linnean Society 198, pp. 821-846 : 834

CHAETOGAMMARUS TRICHIATUS MARTYNOV, 1932

( FIGS 8–11 )

Brief redescription: Based on specimen ZMMU Mb-1227, ♂, Russia, Krasnodar Krai, Sochi Urban Okrug, Lazarevsky District, mouth of the Ashe River, in the stream under stones (43°57.376ʹN, 39°15.954ʹE), 13 May 2019, coll. D. Palatov & I. Marin. Relatively large species (≤ 15 mm); females smaller than males. Head with oblique anteroventral lobe with distally produced anterior margin ( Fig. 6B ), eyes large, reniform, well pigmented. Body unpigmented, moderately elongated, generally smooth, non-carinate. Urosomites 1–3 smooth, without elevations; urosomites 1 and 2 with solitary strong median and submedian spines; urosomite 3 with two submedian spines and a pair of median simple setae ( Fig. 6A ). Antenna 1 feebly setose, with small aesthetascs; accessory flagellum eight-segmented ( Figs 7A , 8A ). Antenna 2 is about one-third shorter than antenna 1, massive and deeply setose in males and less setose in females, without calceoli and aesthetascs ( Figs 7B , 8C, D ). Upper lip (labrum) with convex distal part ( Fig. 9A ). Lower lip (labium) usually with mostly reduced inner lobes ( Fig. 9B ). Mandible with well-developed palp and setose basal segment ( Fig. 9C–F ). Maxilla 1 with outer lobe bearing hairbrush-like distal setae, subequal to inner lobe; inner lobe distally expanding with a row of marginal setae ( Fig. 9G ). Maxilla 2 with outer lobe wider than inner lobe, bluntly expanding distally ( Fig. 9I ). Maxilliped with outer and inner plates wide ( Fig. 9J ). Gnathopod 1 is smaller than gnathopod 2, with weak sexual dimorphism, stronger in males; propodus (palm) teardrop shaped, with oblique palmar margin in males and females ( Fig. 8E, F ). Gnathopod 2 is significantly larger in males, rectangularly elongated, with straight palmar margin in both males and females ( Fig. 8H, I ). Basis (article 2) of pereopods 5–7 with feebly marked ventral lobes ( Fig. 10D, F, G ); pereopod 3 with highly developed coiled setation along posterior margin ( Figs 7D , 10A ); basis of pereopod 7 elongated, ~2.5 times as long as wide, without ventral lobe ( Figs 7C , 10G ). Pleopods with two elongated hooks and one or two thick bristles in retinacules ( Fig. 11F, G ). Uropod 3 exopod broad, furnished with clusters of long, distally coiled setae, about four to six times as long as wide, with reduced distal article furnished with numerous long, simple distal setae ( Figs 6D , 11K, J ). Epimeral plates 1–3 with sharply produced inferoposterior corners ( Fig. 11A–C ). Telson deeply cleft into suboval lobes, abruptly tapering distally, bearing clusters of strong, stout setae distally and submedially ( Figs 6A , 11D, E ).

Remarks: Males of C. trichiatus can be distinguished from males of other species of Chaetogammarus by the highly developed coiled setation on pereopod 3 posterior margin and uropod 3 exopod, acutely produced anterolateral head lobes, and telson lobes abruptly tapering distally. Chaetogammarus trichiatus is most similar to C. ischnus , but differs from that species in the following traits: significantly more elongate appendages ( Fig. 5C ); epimeres 2 and 3 with more produced posterodistal corners; presence of coiled setae along the outer margin of uropod 3 exopod ( Fig. 6D ); longer basis of pereopod 7 (length = 3 × width vs. length = 2 × width; Fig. 7C ).

Distribution: Chaetogammarus trichiatus is endemic to lakes, lower (slow current) river stretches and river mouths along the north-eastern Black Sea coast, from Abrau to the Shahe River, and a separate population is known from New Athos ( Fig. 4 ). Found only on substrates with pebbles or large stones and boulders, apparently avoiding sandy and silty substrates.

Type locality: Estuarine part of Khosta River , Sochi area, Russia. Presently absent at its original type locality owing to anthropogenic influence and the urban reconstruction of the lower part of the river .

Figures

Figure 4. Distribution of focal taxa. Small dots represent distribution records from the literature (after CopilaȘ-Ciocianu et al., 2023); large dots represent sequenced localities (black outline, literature data; white outline, present study). Arrows represent the type localities of Chaetogammarus trichiatus (orange) and Spirogammarus major (purple). The question mark represents a doubtful distribution record of Chaetogammarus ischnus. Green dashed lines indicate the native range.

Figure 5. Morphological differentiation among the focal taxa. A, principal components analysis showing variability in morphospace across the first three axes (PC 1–PC 3) for males (top) and females (below). B, dendrogram based on squared Mahalanobis distances. C, boxplots depicting variation in length of selected morphological traits (corrected for body size).

Figure 6. Scanning electron micrographs. A, dorsal side of urosome. B, lateral head lobes. C, gnathopod 2 propodus. D, uropod 3. Scale bars: 0.2 mm.

Figure 7. Scanning electron micrographs. A, antenna 1. B, antenna 2. C, basis of pereopod 7. D, pereopod 3. Scale bars: 0.2 mm.

Figure 8. Chaetogammarus trichiatus, ♂ (A–C, F, G, I) and ♀ (D, E, H), Shahe River, Krasnodar region. A, antenna 1. B, accessory flagellum of antenna 1. C, D, antenna 2. E, F, gnathopod 1. G, distoventral corner of chela of gnathopod 1. H, I, gnathopod 2.

Figure 9. Chaetogammarus trichiatus, ♂, Shahe River, Krasnodar region. A, labrum (upper lip). B, labium (lower lip). C, E, mandible. D, F, incisor process and pars incisiva of mandible. G, left maxilla 1. H, right maxilla 1 distal segment of palp. I, maxilla 2. J, maxilliped.

Figure 10. Chaetogammarus trichiatus, ♂, Shahe River, Krasnodar region. A, pereopod 3. B, pereopod 4. C, dactylus of pereopod 4. D, pereopod 5. E, dactylus of pereopod 5. F, pereopod 6. G, pereopod 7. H, dactylus of pereopod 7.

Figure 11. Chaetogammarus trichiatus, ♂ (A–D, F–J) and ♀ (E, K), Shahe River, Krasnodar region. A–C, epimeral plates 1–3. D, E, telson. F, pleopod 1. G, retinacula of pleopod 1. H, uropod 1. I, uropod 2; J, K, uropod 3.