Macrobrachium chainatense, Saengphan & Panijpan & Senapin & Laosinchai & Ruenwongsa & Suksomnit & Phiwsaiya, 2019

Saengphan, Nukul, Panijpan, Bhinyo, Senapin, Saengchan, Laosinchai, Parames, Ruenwongsa, Pintip, Suksomnit, Auaree & Phiwsaiya, Kornsunee, 2019, Macrobrachium chainatense sp. nov. (Decapoda: Palaemonidae): a freshwater prawn from Thailand based on morphology and molecular phylogeny, Zootaxa 4664 (2), pp. 274-284 : 277-283

Macrobrachium chainatense , sp. nov.

Figs. (2–6)

Material examined. Holotype: male (CL 5.9 mm), 10 October 2010, Thammamun subdistrict, Mueang Chai Nat district, Chai Nat Province (15°13′20.8″ N, 100°6′5.7″ E) . Paratypes: 9 males (CL 3.8–6.1 mm) and 6 females (CL 6.0– 8.3 mm), 10 October 2014 ; 14 males (CL 4.0– 6.3 mm) and 7 females (CL 5.8–7.6 mm), 6 April 2015, same location as holotype .

Largest male: total length 22.0 mm, carapace length 6.3 mm, smaller than fully grown female (fig. 2).

Largest ovigerous female: total length 29.6 mm, carapace length 8.3 mm.

Description. ( Fig. 3A ) Rostrum: straight, elevating over postorbital margins bending downward gradually to straight tip, extending beyond or as long as antennular peduncle (but not reaching end of scaphocerite), length about one-half of carapace; upper part having 7–9 teeth, 2–3 teeth (majority having 2) behind postorbital margin; lower part of rostrum having 1–2 teeth; distance between first posterior tooth and second one being longer than distance between succeeding teeth.

Antennal spine: well developed with pointed end and situated below orbital angle (suborbital angle); hepatic spine of similar size to that of antennal spine and located slightly behind and lower than antennal spine; paired hepatic spines situated between upper first and second teeth of rostrum on carapace.

Carapace (fig. 4): smooth; thoracic sternite of fourth segment without median process; sixth abdominal somite 1.8 times as long as fifth; (fig. 3B) upper telson smooth, 1.24 times length of sixth segment with 2 dorsal paired spines; tail tapering toward posterior pointed end (median spine), each side flanked by 2 spines, outer lateral spine and inner intermediate spine; several pairs of plumose setae found between two flanking spines; lateral spines having similar size to dorsal spines; first, second, and third abdominal sternites having transverse ridges; first and second sternites having median teeth, indistinct or absent on third sternite; preanal region unarmed.

Eyes (fig. 3C): well developed with cornea diameter longer than stalk.

Basal segment of antennular peduncle: broad, stylocerite distinctly pointed, reaching to about one-third of basal segment; anterolateral tooth reaching to about middle of second segment; second segment as long as third segment.

Scaphocerite (fig. 3D): length 3.17 times width, outer lateral side straight.

Epistome (fig. 3E): bilobed by a deep depression.

Mandibular palp (fig. 3F): slender with three segments, incisor process robust.

Maxillular palp (fig. 3G): bilobed, upper lobe slender without apical and subapical hairless setae, lower lobe stout with distal blunt knob bearing a short sinuous apical seta.

Maxillar palp (fig. 3H): simple (without setae proximally), basal endite deeply bilobed, scaphognathite normal.

First maxilliped (fig. 3I): with setose palp, basal and coxal endites distinct, flagellum of exopod with numerous plumose setae distally, epipod bilobed.

Second maxilliped (fig. 3J): with normal endopod, flagellum with numerous setae distally, epipod simple, with well-developed podobranch.

Third maxilliped (fig. 3K): with robust endopod, exopod with numerous plumose setae distally, reaching beyond distal margin of ischiomerus.

First pereiopod (fig. 3L): slender, reaching beyond scaphocerite by entire distal margin of palm, equal in length, similar in form; palm as long as finger, carpus 2.0 times as long as chela, merus shorter than carpus; male second pereiopod equal in length, similar in form.

Second pereiopod (fig. 3M): reaching beyond scaphocerite by two-thirds of carpus to one-fifth of palm length, shorter than total length; palm, carpus, menus, ischium smooth; each movable finger bending upwards, 0.7 times palm length (fig. 5); 3–5 small teeth with blunt ends in proximal half of cutting edge of each movable finger (figs. 6); each fixed finger having acute end; proximal one-half of cutting edge of immovable finger armed with 3–5 small teeth; both fingers curving inwards, tips crossing and not gaping when fingers closed; palm cylindrical, length 4.7 times breath; carpus shorter than chela, longer than palm.

Last three pereiopods: slender, similar in form; propodus, carpus, merus, covered with spinules and setae.

Third pereiopod (fig. 3N): not reaching beyond scaphocerite, propodus 3.15 times as long as dactylus with about 6 movable spines on posterolateral margin.

Fifth pereiopod: somewhat slenderer than third, reaching distal margin of scaphocerite (end of dactylus and end of scaphocerite terminating in equidistance).

First pleopod of male: endopod not reaching distal half of exopod, inner margin concave, outer margin slightly convex.

Appendix musculina of male: longer, stouter than appendix interna with numerous stiff setae.

Uropodal diaeresis (fig. 3O): with inner movable spine longer than outer angle.

Ovigerous females: with eyed eggs 1.25×1.68 – 1.58× 2.15 mm in diameter (n=10).

Etymology. The species name is derived from the name of the province where the prawns were caught.

Molecular phylogeny. DNA sequences: COI 668 base pairs (bp); majority of 18S 1446 bp; consensus alignment of 18S 1453 bp.

Phylogenetic relationship (fig. 7): all sequences of new species form monophyletic clade (clade 1), posterior probability 1; clade 1 and M. niphanae Shokita & Takeda, 1989 form monophyletic clade (clade 2), posterior probability 1; clade 2 and M. dienbienphuense Dang & Nguyen, 1972 forms monophyletic clade (clade 3), posterior probability 1; M. suphanense Saengphan et al. (2018) , and M. sintangense ( De Man, 1898) forms monophyletic clade (clade 4), posterior probability 1, clades 3 and 4 forms monophyletic clade (clade 5), posterior probability 0.95; M. lanchesteri and M. rosenbergii ( De Man, 1879) forms monophyletic clade (clade 6), posterior probability 1; clades 5 and 6 are sisters, posterior probability 1.

Remarks. Morphology. Fully grown wild and bred Macrobrachium chainatense sp. nov. can be morphologically distinguished from closely related Macrobrachium species such as M. thai , M. niphanae , M. dienbienphuense and M. yui Holthuis, 1950 by having smaller body size, larger egg size, and less teeth on the cutting edges. M. chainatense was different from M. thai (collected) found mostly in Northeast Thailand in that: 1) the number of teeth on lower part of rostrum: 1–2 versus 3–4 in M. thai ; 2) carpus of the second pereiopod relatively longer: longer than palm versus distinctively shorter than palm in M. thai ; 3) body size smaller: the largest male 22.0 mm versus 50.0 mm in M. thai ; 4) second pereiopod shorter: shorter than body length versus distinctively longer than body length in M. thai ; 5) fingers of the second pereiopod: cutting edges with a row of 3–5 small teeth versus 18–20 in M. thai ; 6) eggs larger: 1.25×1.68 – 1.58× 2.15 mm versus 0.98×1.35 – 1.22× 1.63 mm in M. thai .

M. chainatense was distinguishable from M. niphanae in that: 1) number of teeth on lower part of rostrum: 1–2 versus 2–4 in M. niphanae ; 2) rostrum: beyond or as long as antennal peduncle vs barely reaching anterior end of antennal scale in M. niphanae ; 3) body size smaller; 4) second pereiopod: shorter than body length versus almost equal in length and slightly longer than body length in M. niphanae ; 5) eggs larger: 1.25×1.68 – 1.58× 2.15 mm versus 1.0×1.4 – 1.1× 1.5 mm in M. niphanae ( Hanamura et al., 2011) ; 6) fingers of the second pereiopod: cutting edges with a row of 3–5 small teeth versus 18–20 in M. niphanae ; 7) no velvety setae on any parts of the second pereiopod versus ischium, merus, and carpus covered with short velvety satae in M. niphanae .

M. chainatense differed from M. dienbienphuense in that: 1) number of teeth on rostrum: 7–9 (2–3) / 1–2 versus 11–13 (3–5) / 2–3 in M. dienbienphuense ; 2) rostrum: beyond or as long as antennal peduncle vs dorsal margin usually convex and reaching end of antennular peduncle in M. dienbienphuense ; 3) body size smaller; 4) second pereiopod: shorter than body length versus distinctly unequal in length but similar in shape, and slightly longer than body length in M. dienbienphuense ; 5) eggs larger: 1.25×1.68 – 1.58× 2.15 mm versus 0.9×1.35 – 1.0× 1.5 mm in M. dienbienphuense ( Hanamura et al., 2011) ; 6) fingers of the second pereiopod: cutting edges with a row of 3–5 small teeth versus 18–22 in M. dienbienphuense ; 7) no velvety setae on any parts of the second pereiopod versus palm and finger covered with long velvety satae in M. dienbienphuense .

M. chainatense was different from M. yui in that: 1) number of teeth on lower part of rostrum: 7–9 (2–3) / 1–2 versus 8–13 (3–5) / 2–3 in M. yui ; 2) body size smaller; 3) fingers of the second pereiopod: cutting edges with a row of 3–5 small teeth versus 20 in M. yui ; 4) eggs larger 5) movable spine on uropodal diaeresis: longer than outer angle versus present in juveniles, reducing in size with growth, completely absent in adults in M. yui .

Additional differences between M. chainatense and closely related members of the genus Macrobrachium mentioned above are: 1) body color: pale orange when caught in the wild; colorless upon rearing in the laboratory, being similar to other small Macrobrachium prawns; 2) second pereiopod colour: orange-to-brown bands along joints between parts (merus/carpus, carpus/palm, palm/fingers); 3) second pereiopod: fragile, without velvety setae on any parts, fingers shorter and palm; 4) found only in stagnant waters of Chai Nat: the prawns attached themselves to water hyacinths roots and vegetation nearby.

Species of Macrobrachium prawns in continental Southeast Asia are generally widely distributed through river and stream systems. Some of about over 240 Macrobrachium species, e.g. M. lanchesteri , may be found in different habitats separated by long distances. Macrobrachium prawns of different species have been found to cohabit in nature.

We have bred more than 5 generations of M. chainatense from hatchlings to maturity in tanks. The resulting prawns of all generations, from post-hatching to post-larval and mature stages, appear identical. Thus M. chainatense is a pure taxon. Compared with other related species viz M. lanchesteri , M. thai and M. tratense , fully grown males of M. chainatense are much smaller in body size and have a different number of teeth on cutting edges of fingers, clearly indicating that M. chainatense is a distinct new species. Females of M. chainatense also carry fewer but larger eggs than those of other species mentioned here. During ontogenesis, many morphological features undergo a wide range of changes which could be used for taxonomic distinction. Here the development of zoea and juvenile stages to maturity of M. chainatense raised in captivity certainly shows differences from those of M. lanchesteri .

We have also compared features of M. chainatense with other small freshwater prawn species from other regions of the world and found M. chainatense to be unlike those others as well. Developments of early hatchlings and juveniles to mature stages of prawns found in other regions have not been reported in the literature.

Molecular phylogeny. Molecular results also confirm that M. chainatense is a new species: The posterior probability supporting the clade of M. chainatense is 1. In addition, the results show that, among all the Macrobrachium species whose COI and 18S sequences are available, M. niphanae is most closely related to M. chainatense , with M. dienbienphuense as their sister. Both close relatives of M. chainatense are widespread while M. suphanense , found only in some nearby provinces, is more distantly related. Other widespread Macrobrachium species are also distantly related to M. chainatense . So, the geographical proximity and distribution of species are poor indications of their relationships.

Morphological evidence does not always provide a clear indication to species relationship either. Somehow tiny M. lanchesteri is closely related to much larger M. rosenbergii . In addition, with many species yet to be discovered, it might be impractical to reconstruct evolutionary relationships among the known species. Fortunately, we do not have the same problems when working with molecular data: the phylogenetic relationship reconstructed from available sequences of not too distantly related species usually yields a satisfactory result as long as the analysis is carried out properly. Thus, molecular work should be employed as an important supplement to morphological work.

Figures

FIGURE 3. Macrobrachium chainatense sp. nov. (male). (A) Entire animal, lateral view; (B) telson and uropod; (C) carapace, top view; (D) scaphocerite; (E) epistome; (F) mandible; (G) maxillula; (H) maxilla; (I) first maxilliped; (J) second maxilliped; (K) third maxilliped; (L) first pereiopod; (M) second pereiopod; (N) third pereiopod; (O) uropodal diaeresis spine. Scales: (A) 5 mm; (B–N) 1 mm, (O) 0.1 mm.