A new species of the genus Nihonotrypaea Manning & Tamaki, 1998 (Crustacea, Decapoda, Axiidea, Callianassidae) from the South China Sea

Abstract A new species of the genus Nihonotrypaea Manning & Tamaki, 1998, Nihonotrypaea hainanensis sp. n., collected from the South China Sea, is described and illustrated. It is distinguishable from Nihonotrypaea harmandi (Bouvier, 1901), Nihonotrypaea japonica (Ortmann, 1891), Nihonotrypaea thermophila Lin, Komai & Chan, 2007 and Nihonotrypaea makarovi Martin, 2013 by having the elongated carpus of the male and female major cheliped. The new species is distinguishable from Nihonotrypaea petalura (Stimpson, 1860) by the proximolower margin of the carpus of the male major cheliped bearing several small denticles.


Introduction
While working on the taxonomic study of the axiidean fauna (Crustacea, Decapoda) of the China Sea, an undescribed species assignable to the genus Nihonotrypaea Manning & Tamaki, 1998 was found from the intertidal sand flat of Hainan Province, South China Sea. The genus Nihonotrypaea is characterized by the following characters: carapace lacking rostral spine, minute median terminal spinule present or absent; antennular and antennal peduncles subequal in length; third maxilliped lacking exopod, ischium-merus operculiform, distal margin of merus slightly projecting beyond articulation with carpus; chelipeds unequal, both with lobe-like protrusions on lower margin of merus; first pleopod slender, uniramous in both sexes; second pleopod absent in male, slender, biramous in female; third to fifth pleopods with stubby, slightly projecting appendix internae in both sexes (Manning andTamaki 1998, Lin et al. 2007).
Five species are known, all from the northwestern Pacific: N. harmandi (Bouvier, 1901), N. japonica (Ortmann, 1891), N. petalura (Stimpson, 1860), N. thermophila Lin, Komai &Chan, 2007 andN. makarovi Martin, 2013. The status of the genus has been subject to disagreement. Sakai (1999Sakai ( , 2005 treated Nihonotrypaea as a synonym of Callianassa Leach, 1814. Subsequently, Sakai (2011) redefined several genera belonging to the subfamily Callianassinae, and placed Nihonotrypaea under the synonymy of Trypaea Dana, 1852. On the other hand, Nihonotrypaea was recognized as a valid genus by Tudge et al. (2000), Lin et al. (2007) and Martin (2013). Preliminary molecular phylogenetic analyses of the family Callianassidae (Felder and Robles 2009) showed that the representatives of the genus Nihonotrypaea grouped together with those of the genus Neotrypaea in a strongly supported monophyletic clade, encompassing somewhat less supported subclades that do not clearly resolve the status of Nihonotrypaea. Meanwhile, the specific status of N. harmandi has been subject of debate: for example, Tamaki et al. (1997Tamaki et al. ( , 1999, Tamaki and Miyabe (2000), Wardiatno and Tamaki (2001), Tamaki and Harada (2006) and Lin et al. (2007) considered it a valid species. On the other hand, Sakai (1969Sakai ( , 1987Sakai ( , 1999Sakai ( , 2004Sakai ( , 2011 considered it as a junior synonym of Nihonotrypaea japonica or Trypaea japonica. In this study, we provisionally recognize the genus Nihonotrypaea according to the latest literature (Martin 2013), and the higher classification of Callianassidae is not discussed in depth, as it is beyond the scope of this paper. Here we describe and illustrate a new species, referred to Nihonotrypaea from the Chinese seas.

Methods
All specimens examined have been deposited in the Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China (IOCAS). The drawings were made with the aid of drawing tube mounted on a Zeiss Stemi Sv11 compound microscope. The following abbreviation is used throughout the text: CL: carapace length. Description. Rostrum (Fig. 1A, B) broadly triangular, obtuse, directed slightly downwards, reaching proximal 0.2 length of eyestalk in dorsal view. Carapace smooth ( Fig. 1A, B), approximately 0.3 of total body length; dorsal oval well defined, 0.74 as long as carapace. Cervical groove located at posterior quarter; linea thalassinica complete.
Eyestalks (Fig. 1A, B) moderately long, sub-triangular in dorsal view, lateral margin distinctly sinuous, anterolateral margin forming thin ridge extending to rounded terminal margin, reaching 4/5 length of first article of antennular peduncle; cornea subterminal, disk-shaped, swollen in lateral view, with scattered brown-pigmented spots, corneal width less than half of basal width of eyestalk.
Antennular peduncle (Fig. 1A, B) subequal in length to antennal peduncle; third article 2.4 times as long as second article; outer flagellum and inner flagellum shorter than peduncle. Antennal peduncle with rudimentary scale on article 3; fifth article distinctly shorter than fourth article.
Third maxilliped ( Fig. 2A, B) without exopod; ischium-merus operculiform, with dense setae on ventral margin; ischium broader than long, inner surface with crista dentata consisting of slightly sinuous row of small and sharp denticles; merus broader than long and approximately 0.7 times as long as ischium; carpus cup-shaped, shorter than merus; propodus slender, almost as long as carpus; dactylus moderately slender, digitiform, shorter than propodus. Inner surface of merus to dactylus with blunt longitudinal ridge bearing rows of long setae.
Pereopods 1 unequal and strongly dissimilar. Male major cheliped massive (Fig.  2C). Ischium 3.0 times as long as high; upper margin sinuous, with 1 small denticle proximally; lower margin slightly convex, armed with 4 inconspicuous denticles in distal 0.3. Merus as long as ischium, 1.9 times as long as high; upper margin slightly convex, unarmed; lower margin sinuous, with prominent lobe-like process proximally and 2 minute, inconspicuous denticles subdistally; lobe-like process terminating in acutely, with 7 sharp denticles except for the tip on ventral margin and 1 subterminal denticle distally. Carpus elongated, 1.7 times as long as high, 1.3 times as long as merus; upper margin almost straight; proximolower margin gently convex, with row of several small, blunt denticles. Chela heavy, approximately 1.6 times as long as high; palm subquadrate, approximately 1.1 times as long as high, 0.6 times as long as carpus; fixed finger 0.5 times as long as palm, cutting edge with some inconspicuous denticles in distal 0.3; dactylus slightly curved distally, slightly longer than fixed finger, cutting edge sinuous, entire in the distal 0.2 and proximal 0.3, and armed with row of denticles on middle.
Male minor cheliped (Fig. 2D) with ischium 3.5 times as long as high, unarmed. Merus 0.9 times as long as ischium, about 2.3 times as long as high, upper and lower margin slightly convex, unarmed; outer surface medially swollen. Carpus 2.5 times as long as high, 1.6 times as long as merus, abruptly narrowed at base; upper margin almost straight; proximolower margin convex. Chela gapped between slender dactylus and fixed finger; palm nearly as long as high, 0.5 times as long as carpus; fixed finger tapering distally to acute tip, cutting edge unarmed; dactylus slightly longer as palm, slender, unarmed on concave cutting edge.
Female major cheliped (Fig. 2E) with ischium 3.5 times as long as high, upper margin almost straight; lower margin slightly concave, unarmed. Merus almost as long as ischium, upper margin slightly convex, unarmed; lower margin slightly convex, with small tooth at midlength. Carpus 2.0 times as long as high, 1.4 times as long as merus; upper margin almost straight. Chela similar to that of male but relatively smaller; palm subquadrate, almost as long as high; fixed finger 0.5 times as long as palm, cutting edge with some small denticles on proximal half; dactylus slightly longer than fixed finger, cutting edge sinuate and with some small denticles on middle. Minor cheliped in female similar to that of male, and about 0.8 times as long as major cheliped.
Pereopod 2 (Fig. 3A) chelate. Ischium 2.0 times as long as high; merus about 3.0 times as long as high, upper margin smooth, lower margin protruding and with row of dense long setae; carpus subtriangular, shorter than merus; chela slightly shorter than carpus, with dense setae on lower and upper margins; palm with upper margin slightly convex; dactylus 1.9 times as long as upper margin of palm; carpus and chela fringed with short to long setae along margins.
Pereopod 3 (Fig. 3B) simple, moderately slender. Ischium slender, approximately 2.0 times as long as high; merus approximately 2.7 times as long as high; carpus subtriangular, shorter and broader than merus, broadest subdistally, approximately 1.9 times as long as high; propodus subrectangular, lower margin roundly swollen, upper margin slightly convex and 0.6 length of carpus, with numerous tufts of setae on lateral surface and row of thick setae along upper and lower margin, no distinct heel delimited; dactylus subtriangular, upper and lower margin convex, outer surface densely setose, terminating in corneous tip.
Pereopod 5 (Fig. 3D) slender, semichelate, all articles unarmed. Ischium rectangular; merus about 2.5 as long as ischium; carpus about 0.6 length of merus, upper margin swollen; propodus about 1.3 times as long as carpus, lower distal corner projecting to form a chela with dactylus, lateral surface beset distally with dense setae; dactylus hooked toward external side of fixed finger, tips of dactylus and fixed finger obtuse.
Pleomeres smooth dorsally. Pleomere 1 narrowing anteriorly in dorsal view; dorsal tergite fused with the lateral pleurites; pleuron weakly developed but with clearly defined ventral margin. Pleomere 2 distinctly longer than other pleomeres, with posterolateral margin of pleuron slightly expanded, bearing lateral row of plumose setae. Pleomere 3-5 with pleura each having tuft of moderately long plumose setae. Pleomere 6 ( Fig. 1C) subquadrate in dorsal view, very slightly narrowed posteriorly; lateral margin smooth, without conspicuous notch. Telson (Fig. 1C) trapezoidal, almost as long as wide and 0.8 times as long as pleomere 6; dorsal surface convex with a short transverse row of setae medially; lateral margin unarmed; posterior margin gently convex, with small median spine.
Remarks. Nihonotrypaea hainanensis sp. n. is the sixth species assigned to the genus. The new species is closely related to N. harmandi (Bouvier, 1901), N. japonica (Ortmann, 1891) and N. petalura (Stimpson, 1860) in the antennular peduncle being subequal in the length to the antennal peduncle, whereas N. thermophila Lin, Komai &Chan, 2007 andN. makarovi Martin, 2013 have the antennular peduncle being slightly to distinctly shorter than the antennal peduncle. Nihonotrypaea hainanensis sp. n. is distinguishable from N. harmandi (Bouvier, 1901), N. japonica (Ortmann, 1891), N. thermophila Lin, Komai &Chan, 2007 andN. makarovi Martin, 2013 by having a relatively long carpus (approximately 1.7 times as long as high in male and 2.0 times as long as high in female) of the major cheliped (versus carpus subquadrate, in male, approximately 1.1 times as long as high in the later four species; in female approximately 1.5 time as long as high in N. makarovi, 1.1 times as long as high in N. harmandi, N. japonica and N. thermophila). Sakai (1969) discussed the variation of the major cheliped in N. petalura: some specimen have the oblong carpus (approximately 1.5-1.8 times as long as high), which is similar to Nihonotrypaea hainanensis sp. n., but the new species can be readily distinguished from N. petalura by the proximolower margin of the carpus of the major cheliped bearing several small denticles (versus smooth in N. petalura).
It is worth mentioning that considerable similarities were also found between species assigned to Nihonotrypaea and those of Biffarius Manning & Felder, 1991, especially regarding operculiform ischium-mems on the third maxilliped. The new species is also closely related to B. ceramicus (Fulton & Grant, 1906) and B. melissae Poore, 2008 in having a broadly triangular rostrum and the antennular peduncle being subequal in length to the antennal peduncle. It is, however, distinguishable from B. melissae Poore, 2008 by having the telson almost as long as wide (telson about 0.8 times as long as wide), and can be distinguished from B. ceramicus (Fulton & Grant, 1906) by having a relatively long carpus of the male major cheliped (approximately 0.8 times as long as high).
Etymology. The species name is based on the type locality, Hainan Province of China. Distribution and habitat. Presently only known from the type locality. Found in the intertidal zone in sand.