Pseudohalmyrapseudes alexeitiunovi, Marin & Palatov & Thịnh, 2024

Marin, Ivan, Palatov, Dmitry & Th ịnh, Nguy ễn Văn, 2024, An unusual species of the genus Pseudohalmyrapseudes Larsen & Hansknecht, 2004 (Tanaidacea: Parapseudidae) from the mangrove swamps of the C ần Gi ờ Biosphere Reserve, South Vietnam, Zootaxa 5433 (3), pp. 373-388 : 376-387

Pseudohalmyrapseudes alexeitiunovi sp. nov.

( Figs 2–8 )

Material examined. Holotype, ovigerous ♀, bl. 4.0 mm, ZMMU Mh-7— VIETNAM, Ho Chi Minh District, Cần Gi ờ Mangrove Biosphere Reserve , 10°27’29.7”N 106°53’30.1”E, the shallow-water lagoon in the cut part of deep channel connected to the estuary, deep muddy bottom, collected from the upper 1 cm upper layer of mud/ silt with hand net, medium tide level, coll. D. Palatov & I. Marin, 25.09.2022.

Allotype, adult ♂, bl. 4.5 mm, ZMMU Mh-8—same location and date as for holotype.

Paratypes, 1 non-ovigerous ♀ (bl. 3.8 mm), 1 ovigerous ♀ (bl. 4.0 mm), 1 sub-adult ♂ (bl. 4.2 mm) and 1 adult ♂ (bl. 4.4 mm), ZMMU Mh-9—same location and date as for holotype .

Additional material: 6 adult ♂♂, 5 sub-adult ♂♂, 5 non-ovigerous ♀♀ and 3 non-ovigerous ♀♀, LEMMI — same location and date as for holotype .

Diagnosis. Pleotelson slightly longer than wide, blunt posteriorly, about 0.6 times the length of all pleonites combined. Antennule inner rami with three articles in female and four articles in male. Antenna article I with blunt and smooth distal lobe. Male cheliped with relatively small blunt ventral tooth on carpus, fixed finger triangular in shape, and dactylus with distinct basal tooth. Pereopod IV dactylus slightly longer than dactyli of other pereopods. Pereopod VI without plumose ventral setae on merus, carpus and propodus. Uropod with short exopodite consisting of three articles; endopodite with 19 articles, about 0.8 length of pleon.

Description. Based on holotype ♀. Body ( Fig. 2 ) dorsoventrally flattened, about seven times as long as wide. Cephalothorax about 0.25 of total body length, about as long as wide; posterior region with dorsal groove; eyes welldefined, ommatidia brightly white; rostrum triangular, with sharp tip. Pereon is approximately 0.52 times as long as total body length; pereonite I broadest, slightly narrow than cephalothorax, about 0.37 times as long as broad; pereonite II about 0.72 times as long as pereonite I, 0.6 times as long as broad, with two anterolateral and a single posterolateral simple setae; pereonite III slightly longer than pereonite II, about 0.65 times as long as broad, with two small anterolateral and a single posterolateral simple setae; pereonite IV slightly longer than pereonite III, 0.65 times as long as broad, with two small anterolateral and a single posterolateral simple setae; pereonite V slightly longer then pereonite IV, with two small anterolateral and a single posterolateral simple setae; pereonite VI about 0.85 times as long as pereonite V, about 0.52 times as long as broad, anterolateral margin with three small simple setae, posterolateral margin with two strong plumose setae ( Fig. 7 f ); pereonites I–V each without hyposphenia, pereonite VI with genital cone. Pleon about 0.3 of total body length, pleonites I–V free, wider than long, with triangular epimera, densely covered with dorsolateral and lateral plumose setae; length of pleonites I–V gradually increases towards pleotelson and measures 1: 1.03: 1.20: 1.22: 1.57, respectively (as in Fig. 8 k ). Pleotelson ( Fig. 7 a, b ) slightly longer than wide, blunt posteriorly, densely covered with plumose setae, along lateral margin; dorsal side with several long simple setae.

Antennule ( Fig. 3 a ) about 1.1 times as long as cephalothorax; article I short, about 3.5 times as long as wide; article II subcylindrical, about three times as long as wide, with four simple setae on outer margin and five long simple setae, several minute setae and plumose setae on inner margin; article III subcylindrical, about 0.4 times as long as article II, about 1.7 times as long as wide, with three outer and one inner distal simple setae; article IV about 0.5 times as long as article III, with one long simple setae distally; outer flagellum with eight articles bearing long simple setae, distal article with single aesthetasc; inner flagellum with three articles, bearing long simple setae.

Antenna ( Fig. 3 d ) with article I bearing blunt and smooth distal lobe, with three short plumose setae on inner margin; article II oblong, about 1.7 times as long as wide, with a small single seta on inner margin; squama elongated, with three distal simple setae; article III about 1.1 times as long as wide, with four long inner setae; article IV subequal than article III in length, with a single outer and a single inner long simple setae, accompanied by small plumose setae each; article V with five inner and two outer long simple setae, accompanied several small plumose setae; flagella with four articles bearing long simple setae.

Labrum ( Fig. 3 f ) bipartite, slightly convex distal margin with setules.

Labium ( Fig. 3 g ) with lobe bearing outer serration; palp setulate, with two distal simple spiniform setae, lateral margins covered with numerous setules.

Epignath ( Fig. 3 h ): triangular, with setulate spiniform terminal seta.

Mandibles ( Fig. 4 a, c ) with molar process well developed, bearing distal row of denticles. Left mandible ( Fig. 4 c ) incisor with four teeth; setiferous lobe with five multifurcate setae and two simple setae; lacinia mobilis with three blunt teeth ( Fig. 4 d ). Right mandible ( Fig. 4 a, b ) incisor with four teeth; setiferous lobe with five multifurcate setae and two simple setae ( Fig. 4 b ); palp three-articled; article I with three simple setae ventrally; article II with 17 ventral simple setae; article III with 11 ventral simple setae.

Maxillule ( Fig. 4 e ) palp two-articled, bearing four distal simple setae; outer endite with seven distal spiniform setae, two small subdistal setae, with outer and inner clumps of setules; inner endite curving inward, with four distal setulate setae, one outer simple seta, inner clump of simple setae, and well-marked outer process.

Maxilla ( Fig. 4 f ) with inner lobe of fixed endite bearing seven setulate setae and one simple seta; outer lobe of fixed endite with five simple and two pinnate setae, one setulate seta and three multifurcate spiniform setae; inner lobe of movable endite with 15 simple setae and two comb-shaped setae; outer lobe of movable endite with two subdistal and three distal comb-shaped setae.

Maxilliped ( Figs 3 g ; 7 f ) basis with five long simple setae, mostly trapezoidal, about 0.9 times longer than wide; endite with two simple and eight spiniform setae on distal side, with outer side is fine-serrated and saturated and with inner side with six setulate setae and five coupling hooks ( Fig. 7 f ); palp four-articled, article I short and wide, with one spiniform seta on outer margin and one simple seta on inner margin, article II about 2.5 times longer than article I, with one spiniform seta distally on outer margin, with rows of 22 short and six long simple setae along entire length of inner margin, article III oval, about 1.2 times longer than wide, with 12 long simple setae along inner margin; article IV about 1.6 times smaller than article III, oval, with three subdistal simple setae and four distal pinnate spiniform setae.

Cheliped ( Fig. 5 a ) exopod two-articled, distal article with three plumose setae; the remaining parts differ significantly in size and shape between males and females. Cheliped in female ( Fig. 5 a ) coxa triangular, expanding ventrally; basis swollen distally, about 2.2 times as long as maximal width, with a pair of ventromedial and pair of distodorsal simple setae; merus rectangular, about 2.5 times as long as maximal width, with five simple lateral setae and six ventrodistal spiniform setae; carpus slightly compressed laterally, slightly expanding distally, about four times as long as wide, with a row of strong simple setae along ventral margin and several rows of setules on lateral surface; propodus expanding distally, about two times shorter than carpus, with a row of ventral setae and tuft of distodorsal setae, with a stout fixed finger which about as long as wide, bearing row of spinules along distal convex part of cutting margin, with strong distal spiniform seta; dactylus equal lightly longer than fixed finger, sabre-shaped, curved downward, with smooth cutting margin, with sharp tip.

Pereopod I ( Fig. 6 a ) of swimming-type; exopod three-articulated with five plumose setae on distal article; coxa rectangular and oblong, wider than long; basis about 2.5 times as long a wide, with smooth margins, several simple ventral setae and tuft of simple setae distoventrally; ischium short, three times wider than long, with two long setae distoventrally; merus slightly expanding distally, equal in length to carpus, covered with numerous simple setae, with one distoventral spiniform seta; carpus bluntly expanding distodorsally, with numerous long simple setae on both margins and lateral surface, with one long distodorsally and two stout ventral spiniform setae; propodus oblong, slightly expanding distodorsally, equal to carpus, with numerous long setae on both margins, with two long stout distodorsally and two stouter ventral spiniform setae, with several strong setae on lateral margin; dactylus short, slender, spiniform, about twice longer than adjacent spiniform setae, with some ventral blunt denticles and sharp tip.

Pereopods II ( Fig. 6 b ) and III ( Fig. 6 c ) are mostly similar; coxal plate wider than long; basis about 3.3 times as long as wide, subequal to combined length of ischium, merus and carpus, with several simple setae along ventral margin and tuft of simple setae distoventrally; ischium small, wider than long, with one distoventral seta; merus expanding distally, slightly shorter than carpus, with slender distoventral spiniform seta, one small distodorsal setae and numerous long setae along ventral margin; carpus subrectangular, expanding distodorsally, with two strong spiniform setae on ventral margin and four slender distodorsal spiniform setae, with numerous setae along ventral margin and several shorter strong setae on lateral surface; propodus oblong, slightly expanding distodorsally, with two ventral spiniform setae and seven long distodorsal slender setae; dactylus long, spiniform, curved distally, slightly longer than adjacent spiniform setae, about 0.7 times shorter than propodus, with sharp tip.

Pereopod IV ( Fig. 6 d ) with coxal plate suboval, wider than long, with bluntly convex ventral margin; basis swollen, about 2.3 times as long as wide, with one small ventral seta and one distoventral long seta; ischium small, slightly wider than long, with 1 distoventral seta; merus triangular, about 1.5 times as long as wide, about half as long as carpus, with several fine distoventral setae; carpus subrectangular, about 2.5 times as long as wide, with one strong distoventral spiniform seta and row of long setae along ventral margin, with seven robust spiniform seta-like distodorsal setae; propodus shorter and slender than carpus, expanding distodorsally, with subdistal row of short spiniform setae and five spiniform setae distodorsally; dactylus robust, about twice shorter that propodus, spiniform, curved downward, with sharp tip.

Pereopod V ( Fig. 6 e ) generally similar to pereopod IV; coxal plate suboval, wider than long; basis swollen, about 2.5 times as long as wide, with one small ventral seta and one distoventral long seta; ischium subquadrate, slightly wider than long, with one distoventral seta; merus subrectangular, about 1.5 times as long as wide, with several slender distoventral setae; carpus subrectangular, about 2.5 times as long as wide, with row of strong ventral and distoventral spiniform setae and accompanying slender setae; propodus slender, about three times as long as wide, slightly shorter and slender than carpus, with one strong distoventral spiniform seta and several slender ventral setae, with six spiniform setae distodorsally; dactylus slender, about 1.5 times longer that neighbouring spiniform setae, about 1.5 times shorter that propodus, spiniform, curved downward, with sharp tip.

Pereopod VI ( Fig. 6 f ) with coxal plate rectangular, slightly wider than long; basis about 3.5 times as long as wide, with ventral and dorsal marginal rows of long plumose setae; ischium short, about as long as wide, with two simple distoventral setae; merus short, about as long as wide, with convex dorsal margin, with six plumose setae along dorsal margin and several simple setae along ventral margin; carpus oblong, about 2.5 times as long as wide, about 1.8 times longer than merus and 1.4 longer than propodus, with row of dorsal plumose setae, ventral margin with row of fine long simple setae; propodus about 2.5 times as long as wide, with one distodorsal spiniform seta and five shorter distodorsal setae, ventral margin with row of numerous short spiniform setae, and three longer distoventral spiniform setae; dactylus slender, equal to propodus in length, spiniform, slightly curved distally, with sharp tip.

Pleopods ( Fig. 7 c ) biramous, all similar in shape and size; basal article as long as wide, with three inner plumose setae; exopod about 0.6 length of endopod, widening distally; endopod oval, covered with plumose setae.

Uropod ( Fig. 7 d ) basal article and endopod combined length 1.5 times longer than pleon; basal article with three distal simple setae, a single long plumose distal seta and two inner distal plumose setae; exopod three-articled, about 0.25 times as long as endopod, article III with four apical simple setae; endopod with 19 articles, with serially repeated articles bearing simple setae and plumose sensory setae.

Male. Allotype ♂ about 1.3 times larger than holotype ♀ ( Fig. 2 ). Body ( Fig. 8 a, b ) dorsoventrally flattened, about eight times as long as wide. Cephalothorax ( Fig. 8 c, b ) about 0.2 of total body length, about as long as wide; rostrum triangular, with blunt tip ( Fig. 8c ); last pereonite with well-marked penis ( Figs 7 e ; 8 e ). Pleotelson ( Fig. 8 k, l ) slightly longer than wide, blunt posteriorly, covered with plumose setae. Antennule ( Figs 3 b, c ; 8 b, c ) significantly longer than in females; with about three times as long as cephalothorax (including flagella); article I short, about as long as wide; article II subcylindrical, long, about four times as long as wide, with eight outer and five inner simple setae, with several minute simple setae and plumose sensory setae on inner and outer margins; article III subcylindrical, about 0.4 times as long as article II, about two times as long as wide, with five outer and one inner subdistal simple setae; article IV about 0.4–0.5 times as long as article III, with one long simple setae distally; outer flagellum with 13 articles bearing long simple setae; penultimate article with two aesthetascs; inner flagellum with four articles, bearing long simple setae. Antenna ( Fig. 3 e ) mostly similar to females, with stouter articles; article I with bluntly produced triangular distal lobe; flagella with five articles bearing long simple setae. Cheliped ( Figs 5 b, c ; 9 f) significantly stronger and robust than in females, with coxal plate subrectangular; basis greatly swollen distally, about two times as long as maximal width, with pair of ventral and pair of distoventral simple setae; merus rectangular, about 2.5 times as long as maximal width, with five distoventral strong setae and two setules along lateral surface; carpus rectangular, slightly compressed from sides, about four times as long as wide, with well-marked blunt median ventral tooth, with row of strong simple setae along ventral margin and several rows of setules on lateral surface; propodus expanding distally, about 1.5 times shorter than carpus, with row of ventral setae and tuft of distodorsal setae, with a stout fixed finger ( Fig. 8 i ), which about as long as wide, bearing row of spinules along distal convex part of cutting margin, with string distal spiniform seta; dactylus ( Figs 5 d ; 8 g, h ) equal lightly longer than fixed finger, stout at basal part, sabre-shaped, curved downward, with well-marked basal blunt tooth in larger males, cutting margin with row of small setules, with sharp tip.

Coloration. The body, antenna, and appendages of alive specimens ( Fig. 2 ) are milky white, with bright white spots on dorsal lateral edges of the pereonites, dorsolateral margins of pleotelson are whitish; omission of the pleonal segments is yellow-brownish; anterodorsal part of cephalothorax brownish; eyes brightly white. The coloration remains whitish after fixation of the specimens.

Taxonomic remarks. The newly discovered Pseudohalmyrapseudes alexeitiunovi sp. nov. shares morphological features of both Halmyrapseudes and Pseudohalmyrapseudes , with mouthpart features (e.g., well-marked outer serration in lobe of labium, shape and setation of maxillula, specific form and setation of outer lobe of maxilla) and the presence of relatively small blunt ventral tooth on carpus of cheliped in males similar to those of Halmyrapseudes . The inner antennal ramus with three articles in females and four articles in males, the shape of fixed finger and the presence of basal process tooth on dactylus of cheliped in males, as well the absence of plumose ventral setae on merus, carpus and propodus of pereopod VI, resemble such features in the genus Pseudohalmyrapseudes , for example in P. aquadulcis and P. mussauensis (see Larsen & Hansknecht 2004). According to short inner antennal ramus and specific cheliped, the new species is referred to Pseudohalmyrapseudes . At the same time, the combination of the abovementioned morphological features, such as short pleotelson, which is about 0.6 times of the length of all combined pleonites, four- and three-articulated antennular inner ramus in males and females, respectively, specific chelipeds with only a distinct median ventral tooth on the carpus and a process on dactylus, the absence of plumose ventral setae on merus, carpus and propodus on pereopod VI, the relatively short endopodite of the uropods, which is about 0.8 times of pleon, as well as the ecology and area of distribution (see below), makes P. alexeitiunovi an easily recognizable species within Pseudohalmyrapseudes .

Etymology. The new species is named after one of the co-supervisors of the program for the study of the Mekong Delta and mangrove communities in South Vietnam, Dr. Alexei V. Tiunov.

Ecology. The newly described species is the first in the genus Pseudohalmyrapseudes to be described from marine mangrove habitats, as well as the first species from the Asia region. The highest accumulations of the species were observed in the upper 1 cm layer of alluvial silt/mud in the shallow water lagoon located cut part of the channel (see Fig. 1 ), but also inhabits the muddy bottom area along the entire length of the channel, although not reaching the estuary part of the bay. The measured salinity of tidal seawater in the studied lagoon varies from 30 to 33 ‰ (ppt) during high and medium tides ( Fig. 1 e ). During the lowest tide, the exposed bottom mud/silt can become flooded with a large amount of rain freshwater (see Fig. 1 f ).

The sex ratio is approximately three females to one male in the studied population; also, most of the individuals we collected were sexually mature adult or subadult specimens both in males (n=97) and females (n=345); about 50% of collected males were represented by subadult males, easily separated by chelipeds (see Fig. 5 b, c ), and about 10% of collected females (n=27) have eggs at the earliest stage of development.

The abundant but small-sized inhabitants (and hence overall biomass) of the lagoon are the tanaids Pseudohalmyrapseudes alexeitiunovi , and Ctenapseudes sp. (see Marin et al. in press), reaching the average density values for 124±17 inds/m 2 and 1,323±189 inds/m 2 (n=23) respectively, the burrowing shrimps Wolffogebia cangioensis Kolevatov & Marin, 2022 ( Decapoda : Gebiidea: Upogebiidae ) (635±68 inds/m 2) and Alpheus sp. ( Decapoda : Caridea: Alpheidae ) (possibly two different species); there are also one undescribed tanaidacean species, several species of amphipod crustaceans (Crustacea: Amphipoda ) (preliminary identified as Melita sp. ( Melitidae ), Victoriopisa sp. (Eriopsidae), and Grandidierella sp. ( Aoridae )), polychaetes (Polychaeta) and burrowing anemones (Hexacoralia), which were unidentified and their density values not calculated. These species actually form the basis of this community, representing the majority of the abundance and biomass of benthic fauna.

The obtained results of the composition of the stable isotope (δ 13 C/δ 15 N) showed that Pseudohalmyrapseudes alexeitiunovi (−25.99±0.39 for δ 13 C and 5.44±0.21 for δ 15 N) is a primary consumer (C1), probably feeding on a primary organic matter, most likely the leaves of the local mangrove Rhizophora apiculata (−31.1 ± 2.0 for δ 13 C and 4.8±0.1 for δ 15 N). A detailed study of the trophic structure of this community will be presented later (Marin, Kolevatov et al. in prep.).

Deep areas of mangrove forests and swamps are a reservoir for the accumulation of alluvial deposits. Deep dark grey or black marsh silty substrates are formed here, which are characterized by low oxygen levels and the presence of hydrogen sulphide in deep layers, mainly due to the activity of anaerobic sulphur-reducing bacteria (e.g., Desuifovibrio sp.) ( Ng & Sivasothi 2002; Hossain & Nuruddin 2016). A detailed study of the crustacean biodiversity in the central parts of the Cần Giờ Mangrove Biosphere Reserve revealed several of these specific infaunal complexes associated with muddy habitats of deep mangrove swamps (see Fig. 1 ) (e.g., Marin 2021a, b; Kolevatov & Marin 2022; Marin & Palatov 2022). Moreover, the upper 1 cm level of such substrates is well washed and has sufficient oxygen levels compared to deeper inhabited by a number of specialized small burrowing animals, including tanaidaceans and amphipods.

Distribution. The species is known only from the type locality, the Cần Giờ Mangrove Biosphere Reserve, South Vietnam.

Figures

FIGURE 1. The map of the studied area and the habitat of Pseudohalmyrapseudes alexeitiunovi sp. nov., in the mangrove swamps of the Cần Giớ Biosphere Reserve in South Vietnam: a–c—general map of the location of the study area in different scale; d—general view of the lagoon in in the cut part of the deep mangrove channel during the high level of the tide; e—same lagoon during sampling at the medium level of tide; f—same lagoon during the lowest level of the tide. Photo credits: e, f—from Tien Tran Van.

FIGURE 2. Pseudohalmyrapseudes alexeitiunovi sp. nov., coloration of live specimens, dorsal and lateral views of adult ♂♂ (above) and ♀ (below) habitus.

FIGURE 3. Pseudohalmyrapseudes alexeitiunovi sp. nov., ♀ (a, d, f–h) and ♂ (b, c, e): a–c—antennule; d, e—antenna; f— labrum; g—labium; h—epignath.

FIGURE 4. Pseudohalmyrapseudes alexeitiunovi sp. nov., ♀: a—right mandible; b—same, incisor process; c—left mandible; d—same, incisor process; e—maxillula; f—maxille; g—maxilliped; h—endite of maxilliped.

FIGURE 5. Pseudohalmyrapseudes alexeitiunovi sp. nov., chelipeds, ♀ (a), sub-adult ♂ (b) and adult ♂ (c, d).

FIGURE 6. Pseudohalmyrapseudes alexeitiunovi sp. nov., ♀: a—pereopod I; b—pereopod II; c—pereopod III; d—pereopod IV; e—pereopod V; f—pereopod VI.

FIGURE 7. Pseudohalmyrapseudes alexeitiunovi sp. nov., ♀ (a–d, f) and ♂ (e), LEMMI: a, b—pleotelson, different views; c—pleopod I; d—uropod; e—penis; f—setation of pleonites; g—pereon, dorsal view.

FIGURE 8. Pseudohalmyrapseudes alexeitiunovi sp. nov., ♂, LEMMI, SEM photographs of adult ♂, LEMMI: a—habitus, lateral view; b—habitus dorsal view; c—anterior margin of cephalothorax and antenna; d—antennal flagella; e—penis; f— cheliped; g—dactylus of cheliped; h—ventral margin of dactylus of cheliped; i—distal part of fixed finger of cheliped; k—pleon (pleonites and pleotelson), dorsal view; l—pleotelson, dorsal view.