Litorogammarus, Marin & Palatov & Copilaș-Ciocianu, 2023

Marin, Ivan, Palatov, Dmitry & Copilaș-Ciocianu, Denis, 2023, The remarkable Ponto-Caspian amphipod diversity of the lower Durso River (SW Caucasus) with the description of Litorogammarus dursi gen. et sp. nov., Zootaxa 5297 (4), pp. 483-517 : 487-491

Litorogammarus gen. nov.

Included species. Litorogammarus mazestiensis ( Marin & Palatov, 2021) comb. nov. (the type species of the genus),

Litorogammarus karadagiensis ( Grintsov, 2009) comb nov., Litorogammarus dursi sp. nov. and an undescribed species from the Caspian Sea (D. CopilaȘ-Ciocianu, pers. comm.).

Etymology. The genus name comes from the habitat of the known species in the upper littoral of the pebble beaches of the northern part of the Black Sea. Litore - (Latin)—beach.

Diagnosis. Body slender, generally smooth, non-carinate. Pleon with dorsal surface of urosomal segments armed with numerous stout spine-like setae arranged in rows. Head with distally rounded or oblique anteroventral lobe, lacking an acute projection. Antenna I with 6-segmented accessory flagellum. Antenna II subequal or shorter than AI, densely covered with long distally curled setae in males, and non-setose in females, without calceoli in both males and females. Upper lip (labrum) with concave or convex distal part. Gnathopod I with teardrop-shaped propodus (palm), with oblique palmar margin in males and mostly rectangular with straight palmar margin in females. Gnathopod II also sexually dimorphic, significantly larger in males, deeply trapezoidal with oblique palmar margin in males and rectangular elongated with straight palmar margin in females. Pereopods III–IV bearing sparse and short setae along the posterior margin of carpal and meral articles. Pereopods V–VII sparsely setose, with basis (article 2) bearing a row of short strong spines along dorsal margins, PV basis with moderately developed ventral lobe, PVI–PVII without a ventral lobe. Pleopods with 2 elongated hooks and 1–2 thick bristles in retinacules. Epimeral plates armed with spines along distal edge, lacking long setae. Uropod III dimorphic in male and females, with outer ramus broad, about 3 and 4 times as long as wide in females and males, respectively, with reduced distal article furnished with numerous long simple distal setae. Inner ramus is 4–5 times shorter than the outer ramus.

Molecular-genetic analysis. Litorogammarus gen.nov. is deeply nested within the Ponto-Caspian gammaroidean ( Gammaridae ) radiation, forming a well-supported separate clade that is apparently sister to a large clade containing mainly fossorial pontogammarids ( Pontogammaridae ) (see Fig. 3 ). The phylogenetic distinctiveness and monophyly of the genus is also supported in a recently published multi-locus phylogenetic analysis (Copilaș-Ciocianu et al., 2023). Each of the four species within the genus are molecularly well separated from each other. The undescribed species from the Caspian Sea is represented by a few damaged specimens and will be described in detail once more material is collected.

Taxonomic remarks. The new genus is mostly phylogenetically related (see Fig. 3 ) to the Ponto-Caspian Chaetogammarus Martynov, 1924 (see Fig. 3 ).

Litorogammarus gen. nov. can be distinguished from Echinogammarus -like gammarid amphipods (including Chaetogammarus ) by the following combination of traits: smooth body lacking dorsal dense setation or armature (spines and carinae), distally rounded anterolateral lobe of the head; AII subequal to AI and densely armed with curled setae in males; the absence of calceoli; well-marked sexual dimorphism in AII and both gnathopods; relatively well-developed ventral lobes on basis of PV and their absence on PVI–VII; poorly setose PIII–VII; distal margin of epimeral plates armed with spines only; telson lobes longer than broad and gradually tapering towards the distal end. A detailed comparison is given in Table 2 .

Habitats and distribution. The three described species of the genus were found in pebble littoral habitats of the northern coastline of the Black Sea. However, the undescribed Caspian species was collected from 8–20 m depth from sandy/detrital substrate (Copilaș-Ciocianu et al. 2022). Some species are very numerous forming dense aggregations living in the upper layer of the pebbles, under stones/boulders, in the estuarine part of the rivers (for example, L. karadagiensis comb. nov. and L. mazestiensis comb. nov. ( Grintsov, 2009; Marin & Palatov, 2021)), while others are living deeply in pebbles (for example, L. dursi sp. nov. (present paper)). The species of the genus are probably somewhat tolerant to the low salinity, but L. mazestiensis comb. nov. and L. dursi sp. nov. have never been found far from the brackish water part of the river estuary, neither in the direction of the sea, nor in fresh water, while L. karadagiensis comb. nov. is presently known only from the beach localities, which are very distant from any coastal rivers, for example Laspi and Karadag Bays. Apparently, the evolution of the genus is associated with pebbles and the exploitation of the maximum possible number of niches in this biotope.

Figures

FIGURE 3. Maximum-likelihood phylogenetic tree based on COI. Number above nodes represent ultrafast bootstrap (UFBS) and Shimodaira-Hasegawa approximate likelihood ratio test (SHaLRT) values. Number below nodes represent posterior probabilities (PP). Strongly supported nodes in all analyses are shown with green circles. Species from Durso River are indicated with red arrows. Taxonomically relevant taxa are shown with various colors. Species belonging to Chaetogammarus are shown with dotted branches.

Tables

TABLE 2. Morphological comparison between Litorogammarus gen. nov. and related genera. The terms “short” and “long” are defined as shorter or longer than the underlying segment, respectively.

Character	Litorogammarus gen. nov.	Chaetogammarus	Spirogammarus	Dinarogammarus	Echinogammarus	Pectenogammarus (= Homoeogammarus )	Iberogammarus	Marinogammarus	Parhomoeogammarus	Relictogammarus
AI setation	Sparse, short/ long	Sparse, short	Sparse, short	Sparse, short	Sparse, short	Sparse, short/long	Sparse, short	Sparse/dense, short/long	Sparse, short	Sparse, short
AI acc. flag. articles	4–8	2–8	Up to 9	Up to 6	5–6	4	4	5–9	5	4
AII setation	Dense, long, curled	Dense, long, straight/curled/ coiled	Dense, long, coiled	Dense/sparse, long straight	Sparse/ dense, short/ moderately long, straight	Dense/sparse, long, straight	Dense, long, straight/curled	Dense, short/ long, straight	Sparse/dense, short	Sparse, short, straight
Calceoli	Absent	Absent	Absent	Absent	Absent	Absent/present	Absent?	Absent/ present	Absent/present	Absent
Head lobes	Straight, rounded	Acute/straight	Straight	Straight	Straight, rounded	Rounded, straight, acute	Straight	Rounded/ straight	Straight/rounded	Rounded
GI vs. GII propodus	Shorter	Shorter	Subequal	Shorter	Shorter	Shorter	Longer	Longer	Shorter/subequal	Longer
PIII setation	Sparse, short	Dense/sparse/ short/long, straight/curly/ coiled	Dense, long, coiled	Dense, moderately long, straight	Dense, long, straight/curled	Dense/sparse short/ long, straight/curled	Moderately dense, long, straight	Absent/ sparse/dense, short/long, straight	Dense, long, straight	Absent/weak

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