Pleusymtes actiniae, Marin & Sinelnikov & Antokhina, 2022

Marin, Ivan N., Sinelnikov, Sergey Yu. & Antokhina, Tatiana I., 2022, The first Arctic conspicuously coloured Pleusymtes (Crustacea: Amphipoda: Pleustidae) associated with sea anemones in the Barents Sea, European Journal of Taxonomy 819, pp. 166-187 : 168-177

Pleusymtes actiniae sp. nov.

urn:lsid:zoobank.org:act:3053898B-8088-4AFD-84D7-4CCED1F81E97

Figs 1–5

Diagnosis

Eyes with distinct light rim; AI with peduncular article 1 relatively short, not produced distoventrally; Gn I–II weakly dimorphic, GI smaller than GII, propodus (palm) strongly beveled with a well-defined palmar margin, carpus of GnII with blunt distoventral process; EpI–III without strongly produced ventroproximal corners; PpIII–VII with long and simple dactyli, PVII with smooth posterior margin of basis; UI peduncle without distal ecdysial spine; telson entire, suboval; colouration distinctly yellowbrown-white banded.

Etymology

The species is named after its host – actinians, or sea anemones (Cnidaria: Anthozoa: Hexacorallia: Actiniaria ), in the English-speaking version of the common name of these animals. However, in Russianspeaking version, the word ‘ АКТиния ’ (Eng: ‘ actinia ’) (gender feminine) has a more common meaning for these animals.

Type material

Holotype NORTHEAST ATLANTIC – Barents Sea • ♀ (tbl. 8.5 mm); Russian (Murman) coast, Dolgaya Bay ( Guba Dolgaya ); 69°11′07.5″ N, 34°58′02.8″ E; 16–18 m depth; 21 Aug 2019; T. Antokhina leg.; SCUBA, on sea anemones Urticina eques growing on a rock wall; ZMMU Mb-1222 .

Paratypes NORTHEAST ATLANTIC – Barents Sea • 1 ♀ (tbl. 7.5 mm); same collection data as for holotype; ZMMU Mb-1224 • 1 ♂ (tbl. 7.0 mm) (dissected); same collection data as for holotype; ZMMU Mb- 1223 .

Additional material

NORTHEAST ATLANTIC – Barents Sea • 2 ♀♀, 1 ♂; same collection data as for holotype; LEMMI .

Description

BODY ( Fig. 1 ). Slightly inflated, pereonal and pleonal sternites smooth, without any sculpture or dorsal/ middorsal carina.

HEAD ( Fig. 1 ). Slightly longer than pereonites I and II combined, rostrum short, about 0.2 times length of basal peduncular segment of antenna I; eyes large, occupying most of the head, subreniform, with distinct light rim; anterior head lobe prominent, bluntly rounded.

ANTENNA I ( Fig. 2a ). Medium length, nearly 0.8 times body length ( Fig. 1 ); peduncular article 1 about 1.5 times as long as wide; about twice as long as peduncular article 2, covered with simple setae, with small blunt anterodorsal process ( Fig. 2a ); article 2 about as long as wide, with small blunt anterodorsal process; article 3 about 1.3 times as long as wide, equal to article 2; with small blunt anterodorsal process; accessory flagellum reduced; flagellum bearing 30–35 articles, aesthetascs absent.

ANTENNA II ( Fig. 2b ). About 0.7 times as long as antenna I; peduncle with short article 5; article 4 as long as wide, equal to article 3 with blunt anteroventral protrusion ( Fig. 2b ); article 3 about as long as wide; about half the length of article 2; article 2 about 2.5 times as long as wide, slightly longer than article 1, about 2.5 times as long as wide; flagellum with about 25 articles.

MANDIBLE ( Fig. 2d–e ). With strong molar process; incisor process 5-dentate, left lacinia mobilis 5–7-dentate, with 5 robust plumose accessory setae ( Fig. 2e ); palp 3-articulated, article 1 without setae, article 2 with 6 simple D-setae, article 3 equal to article 2, slightly curved, with posterior row of 6–7 simple D-setae, and 6–7 subdistal E-setae.

LABRUM. Without specific features, apical lobes well marked.

LABIUM ( Fig. 2c ). With inner lobes feebly developed; outer lobes well marked, subdistally rounded.

MAXILLA I ( Fig. 3d ). With suboval inner plate bearing 3 plumose setae apically; outer plate elongate, apically with 9 strong pectinate setae; palp 2-articulated, apical margin of article 2 with 5–6 stout spinelike setae and with a row of 6–7 subapical simple setae.

MAXILLA II ( Fig. 3e ). With almost subequal inner and outer plates; inner plate narrowing distally, with group of dense short spine-like setae on apex, fringed with plumose and simple setae; outer plate weakly narrowing distally, with 8 apical long and simple setae.

MAXILLIPED ( Fig. 3f ). With small inner plates, fringed with plumose setae distally; outer plate elongated, rounded apically, inner margin covered with numerous submarginal stiff simple setae of different length; palp 4-articulated; article 4 (dactylus) slender, curved.

GNATHOPOD I ( Fig. 3a ). Coxa deep, anterior margin distally expanded, with posteroventral blunt tooth and with 7–10 small distal setae; basis straight anteromarginally, distal half expanded posteromarginally, with a pair of posterodistal setae; ischium about as long as wide, with several posterodistal setae; merus about as long as ischium, about 1.3 times as long as wide, with a simple long seta in medial part of posterior margin, posterodistal margin with a dense row of numerous long simple setae; carpus about 1.5 times as long as maximal width, equal to propodus, posterior lobe shallow, with several groups of long, simple and plumose setae; propodus ( Fig. 3b ) suboval, about 2.2 times as long as wide, palmar margin smoothly oblique, 1.4 times as long as posterior margin, with several clusters of posterodistal robust setae associated with a few setae; dactylus shorter than palmar margin of propodus, curved, pointed distally, with 3 outer simple setae.

GNATHOPOD II ( Fig. 3c ). Coxa deep, subquadrate, larger than coxa of gnathopod I, with posteroventral blunt tooth and with 7–10 small distal setae; basis straight anteromarginally, distal half expanded posteromarginally, with several long simple posterodistal setae; ischium triangular, about as long as wide, with several posterodistal setae; merus slightly longer than ischium, about 1.5 times as long as wide, with subdistal produced lobe, armed with a dense row of numerous long simple setae; carpus about 1.4 times of maximal width, smaller than propodus, with well-marked posterodistal lobe, armed with several groups of long simple and plumose setae, with long simple and plumose setae in medial part; propodus ( Fig. 3d ) suboval, about 2.2 times as long as wide, palmar margin smoothly oblique, 1.5 times as long as posterior margin, with several clusters of posterodistal robust setae associated with a few setae; dactylus shorter than palmar margin of propodus, curved, pointed distally, with 3 outer simple setae.

PEREOPOD III ( Fig. 4a ). Coxa relatively shallow, rectangular, about 2.5 times as long as wide, anterior and posterior margins smooth, unarmed; basis linear, about 5 times as long as wide; ischium about as long as wide, unarmed; merus about 3 times as long as wide, slightly longer than carpus, with distally produced anterodistal angle; carpus about 2.5 times as long as wide, with several groups of spine-like setae along posterior margin; propodus linear, narrower and 1.8 times as long as carpus, about 5.5 times as long as wide, with several small spines along posterior margin, with a pair of small spines posterodistally, and a tuft of long simple setae anterodistally; dactylus ( Fig. 4b ) long, curved, about 6 times as long as wide, and 0.6–0.7 times as long as propodus, with sharp tip.

PEREOPOD IV ( Fig. 4c ). Generally similar to pereopod III; coxa about as long as wide, subquadrate, rounded distally and with deep excavation posteromarginally; basis linear, about 4 times as long as wide; ischium about as long as wide, unarmed; merus about 2.5 times as long as wide, slightly longer than carpus, with feebly produced anterodistal angle; carpus about 3 times as long as wide, with a group of spine-like setae posterodistally; propodus linear, narrower and 1.7 times as long as carpus, about 6 times as long as wide, with several small spines along posterior margin, with a pair of small spines posterodistally, and a tuft of long simple setae anterodistally; dactylus long, curved, about 6 times as long as wide, and 0.6–0.7 times as long as propodus, with sharp tip.

PEREOPOD V ( Fig. 4d ). Coxa posterolobate, with rounded anterior margin, posteriorly subquadrate, posterior lobe pulled back and down; basis wide, with straight parallel margins, without posterior wing, anterior margin armed with 4 medial small and stout spines-like or robust setae and 1 small seta anterodistally, posterior margin smooth, unarmed; ischium small, about as long as wide, unarmed; merus about 2 times as long as wide, slightly longer than carpus, with posterodistal angle produced forward and armed with several small spine-like setae, anterior margin armed with several group of small spines accompanied with simple setae; carpus about 2.2 times as long as wide, with a group of spine-like setae along anterior margin and anterodistally; propodus linear, more narrow and 1.3 times as long as carpus, about 6 times as long as wide, with several small spine accompanied with small simple setae along posterior margin, with a pair of small spines posterodistally, and a tuft of long simple setae anterodistally; dactylus ( Fig. 4e ) long, curved, about 6 times as long as wide, and 0.6–0.7 times as long as propodus, with sharp tip.

PEREOPOD VI ( Fig. 4f ). Coxa posterolobate, with rounded anterior margin, posterior lobe pulled back and down, proximally with produced angulated posterior margin; basis with narrow posterior lobe, width 0.4–0.5 times as length, posterior margin convex, with numerous small notches, anterior margin armed with 5–8 small stout spine-like setae; ischium small, about as long as wide, unarmed; merus about 2.2 times as long as wide, slightly longer than carpus, with posterodistal angle produced forward and armed with several small spine-like setae, anterior margin armed with several groups of small spines accompanied with simple setae; carpus about 3.2 times as long as wide, with a group of spine-like setae along anterior and posterior margins; propodus linear, more narrow and 1.3 times as long as carpus, about 6 times as long as wide, with several small spines accompanied with small simple setae along posterior margin, with a pair of small spines posterodistally, and a tuft of long simple setae anterodistally; dactylus long, curved, about 6 times as long as wide, and 0.6–0.7 times as long as propodus, with sharp tip.

PEREOPOD VII ( Fig. 4g ). Coxa suboval; basis with narrow posterior lobe, width 0.4–0.5 times of length, posterior margin convex, with numerous small notches, anterior margin armed with 5–8 small stout spine-like setae; ischium small, about as long as wide, unarmed; merus about 2.2 times as long as wide, equal to carpus, with posterodistal angle produced forward and armed with several small spine-like setae, anterior margin armed with several group of small spines accompanied with simple setae; carpus about 3.2 times as long as wide, with a group of spine-like setae along anterior and posterior margins; propodus linear, narrower and 1.2 times as long as carpus, about 6 times as long as wide, with several small spine accompanied with small simple setae along posterior margin, with a pair of small spines posterodistally, and a tuft of long simple setae anterodistally; dactylus ( Fig. 4h ) long, curved, about 6 times as long as wide, and 0.6–0.7 times as long as propodus, with sharp tip.

COXAL GILLS II–VI. Without specific features.

EPIMERAL PLATES ( Fig. 5a–c ). With anterior, posterior and ventral margins unarmed; epimeral plate I ( Fig. 5a ) nearly triangular, with bluntly pointed ventral margin, unarmed; epimeral plate II ( Fig. 5b ) with slightly produced posteroventral corner; epimeral plate III ( Fig. 5c ) with slightly produced and pointed posteroventral corner.

PLEOPODS ( Fig. 5f ). Normal; peduncle with 2 small coupling hooks and 1 long simple seta in retinacula ( Fig. 5g ); inner ramus slightly shorter than outer ramus.

UROPOD I ( Fig. 5h–i ). Peduncle about 5.5 times as long as wide, with dorsomedial row of 7–9 thin short spines, dorsolateral row of 4–5 thin short spines, and 1 subdistal short spine and 1 dorsolateral thin short spine, without distal ecdysial spine; outer ramus subequal to inner one in length; both outer and inner rami with 4 dorsolateral, 1 subapical spine; distal part of both outer and inner rami armed with specific structures including pointed distal protrusions with a movable thickened spine in the middle, accompanying subapical thickened spine-like setae ( Fig. 5j ).

UROPOD II ( Fig. 5k ). Peduncle about 3 times as long as wide, subequal to outer ramus in length and slightly shorter than inner ramus, with 1 strong basal, 3 medial smaller and 1 short subdistal outer spine; outer ramus about 0.8 times length of endopodite, with 3 outer spines; inner ramus with 3 outer spines; distal part of both outer and inner rami armed with specific structures including pointed distal protrusions with a movable thickened spine in the middle, accompanying subapical thickened spine-like setae.

UROPOD III ( Fig. 5l ). Peduncle about 1.5 times as long as wide, about twice shorter than inner ramus in length, with 1 leaf-shaped subdistal outer spine ( Fig. 5m ); outer ramus about 0.8 times length of inner ramus, with 3 outer spines, including leaf-like most distal spine, and pointed apex; inner ramus with 3 outer simple spines and pointed apex.

TELSON ( Fig. 5d–e ). Suboval, elongate, tapering distally and apically rounded, about 1.5 times as long as wide, with well-developed ventral keel, with a pair of plumose medial marginal setae and a small simple subapical seta on each side.

Taxonomic remarks

Among congeners, the new species is morphologically similar to P. glaber , P. pulchella , P. kariana and P. karstensi , which are probably closely related. These species are rather distinct from other species of the genus by the weakly dimorphic gnathopods, not strongly produced ventroproximal corners of epimeral plates, long and simple dactyli of pereopods III–VII, entire telson and the geographic distribution in cold western Arctic seas. Although these species, especially the last two, are described rather poorly, nevertheless, the new species has enough features to differ from them by the features of antennae, gnathopods, epimeral plates, uropod III and conspicuous colouration.

From P. glaber , a relatively common Arctic-Boreal shallow benthic species (see G.O. Sars 1893: pl. 126; Gurjanova 1951: fig. 450; Bousfield 1973; Berge 2003), the new species can be easily separated by 1) not distinctly produced distoventral margin of peduncular article 1 of antenna I (vs distinctly produced in P. glaber ); 2) not distinctly produced distoventral margin of epimeral plate III (vs distinctly produced pointed posteroventral corner in P. glaber ); 3) smooth posterior margin of basis of pereopod VII (vs serrated in P. glaber ); and 4) distinctly banded yellow-brown-white colouration (vs whitish with reddish brown patches in P. glaber ).

From P. pulchella , known from Greenland and Iceland to the North and East Siberian seas, usually at a depth of 30–40 m, but can be at depths of more than 700–1300 m in the Norwegian Sea (see G.O. Sars 1893: pl. 126; Gurjanova 1951: fig. 451), the new species can be clearly separated by 1) shorter peduncular articles of both antenna I and II; 2) carpus of gnathopod II directed forward, about 1.5 times as long as wide, with blunt distoventral process (vs triangular carpus without distoventral process in P. pulchella ); 3) propodus (palm) of gnathopod I and II strongly beveled with a well-defined palmar margin (vs short sinuous palmar margin in P. pulchella ); and 4) distinctly banded yellow-brown-white colouration (whereas the colouration of P. pulchella is described as “color albidus” (after G.O. Sars 1876) and whitish with small pinky spots (after G.O. Sars 1893).

From P. kariana , known from the Kara Sea at the depth of 165 m (see Stappers 1911: pl. II, 10–20; Gurjanova 1951: fig. 453), the new species can be clearly separated by 1) eyes with distinct light rim (vs without light rim in P. kariana ); 2) carpus of gnathopod II with blunt distoventral process (vs without distoventral process in P. kariana ); and 3) anterior margin of coxal plate I distally expanded (vs coxal plate I with parallel margins in P. kariana ).

From P. karstensi , known from the coastal waters of Spitzbergen ( Svalbard) (see Barnard, 1959; Macnaughton et al. 2007: figs 7–10), the new species can be clearly separated by 1) larger eyes with distinct light rim (vs without light rim in P. karstensi ); 2) more elongated suboval telson; and 3) distinctly banded yellow-brown-white colouration (vs translucent reddish in P. karstensi ).

Morphologically similar species from other areas, such as Pleusymtes glabroides ( Dunbar, 1954) , another subarctic species described from Ungava Bay in Canada ( Dunbar 1954), and Pleusymtes subglaber ( Barnard & Given, 1960) , known from southern California ( Barnard & Given 1960; Blake et al. 1995), can be separated from the new species by the sinuous posterior margins and well-developed posteroventral teeth of epimeral plates II–III and the presence of a distinctly produced distoventral margin of peduncular article 1 of antenna I in P. glabroides (that is rather similar to P. glaber ) (see Dunbar 1954; Barnard & Given 1960).

Measurements

The largest collected ♀ has tbl. 8.5 mm; the largest collected ♂ has tbl. 8.5 mm.

Living colouration

Conspicuously striped species; antennas I–II, dorsal part of body and telson are yellow or light green; lateral median margin of coxal plates and entire epimeral plates are brown; ventral margin of coxal plates and appendages are white; eyes are a gold colour ( Fig. 6 ).

Ecology

Amphipods were collected from both the stalk and from the tentacles of the sea anemone Urticina eques (Gosse, 1858) ( Anthozoa: Actiniaria : Actiniidae ), which were found growing on a steep underwater rock wall at a depth of about 16–18 meters, where, apparently, there is a weak but constant flow of water. Clusters of amphipods, up to two dozen individuals, were observed on some of the studied sea anemones. It is worth noting that amphipods were observed only on sea anemones living on this rock wall, whereas they were not found on other sea anemone specimens within this bay, although quite a diversity of sea anemones (including Metridium spp. ) were examined during the survey. Previously, this species was not observed from benthic samples taken in this bay and other places along Murman coast of the Barents Sea.

Distribution

The new species is currently known only from the type locality, Dolgaya Bay in the Barents Sea (69°11′07.5″ N, 34°58′02.8″ E), but we are confident that this species is widespread along the coast of the Barents Sea and the surrounding area, at least in the cold-water Arctic.

Figures

Fig. 1. General view (habitus) of Pleusymtes actiniae sp. nov., holotype, ♀, from Dolgay Bay, Barents Sea (ZMMU Mb-1222).

Fig. 2. Pleusymtes actiniae sp. nov., paratype, ♀ (ZMMU Mb-1224). a. Antenna I. b. Antenna II. c. Labium (lower lip). d. Left mandible. e. Left lacinia mobilis. f. Maxilla I. g. Maxilla II. h. Maxilliped.

Fig. 3. Pleusymtes actiniae sp. nov., paratype, ♂ (ZMMU Mb-1223). a. Gnathopod I. b. Distoventral palmar margin of propodus (chela) of GnI. c. Gnathopod II. d. Distoventral palmar margin of propodus (chela) of GnII.

Fig. 4. Pleusymtes actiniae sp. nov., paratype, ♀ (ZMMU Mb-1224). a. Pereopod III. b. Dactylus of PIII. c. Pereopod IV. d. Pereopod V. e. Dactylus of PV. f. Pereopod VI. g. Pereopod VII. h. Dactylus of PVII.

Fig. 5. Pleusymtes actiniae sp. nov. a–d, f–m. Paratype, ♀ (ZMMU Mb-1224). e. Paratype, ♂ (ZMMU Mb-1223). a. Epimeral plate I. b. Epimeral plate II. c. Epimeral plate III. d–e. Telson. f. Pleopod II. g. Retinacula of pleopod I. h–i. Uropod I. j. Same, distal part of outer ramus. k. Uropod II. l. Uropod III. m. Distodorsal angular spine of peduncle of uropod III.

Fig. 6. Photo in situ of the sea anemone Urticina eques (Gosse, 1858) with associated amphipods Pleusymtes actiniae sp. nov. from Dolgaya Bay, Barents Sea.