Arachnochium, Wowor, Daisy & Ng, Peter K. L., 2010

Wowor, Daisy & Ng, Peter K. L., 2010, On two new genera of Asian prawns previously assigned to Macrobrachium (Crustacea: Decapoda: Caridea: Palaemonidae) *, Zootaxa 2372, pp. 37-52 : 38-39

Arachnochium gen. nov.

Type species. Palaemon mirabilis Kemp, 1917 , by present designation.

Diagnosis. Small-sized prawn of subcylindrical body form. Rostrum well developed, compressed; dorsal carina armed with 14–17 teeth, 4 or 5 teeth completely postorbital; ventral carina armed with at least 1 tooth; plumose setae present between teeth; lateral carina well developed. Carapace with antennal and hepatic spines; branchiostegal groove present, running from base of hepatic spine to antennal carapace margin below antennal spine. Antennule with 3-segmented peduncle, ventromedial tooth on basal segment small; distolateral angle of basal segment well developed; 2 flagella present, upper flagellum biramous with rami fused proximally. Antenna with basicerite unarmed, 1 flagellum present; scaphocerite well developed, distal lamina clearly extending beyond distolateral tooth. Ocular beak well developed. Epistome bilobed. Mouthparts similar to those of Macrobrachium ; mandible with 3-segmented palp. Second pereiopod very slender, about 1.6 times as wide as first pereiopod, similar in shape, equal in size, maximum length twice as long as carapace. Carpus slightly tapered. All segments glabrous. Pereiopods equally developed in both sexes. Third, fourth and fifth pereiopods glabrous; third pereiopod shorter than second pereipod, fourth pereiopod longer than second pereiopod, fifth pereiopod distinctly longer than fourth pereiopod; third and fourth pereiopods with 5 or 6 ventral spines distributed along length of propodus but without distal paired spines; fifth pereiopod with 8 ventral spines along length of propodus, transverse rows of setae present at outer margin of distoventral propodus. T5 without transverse plate; male T8 without anterior lobes, with median process posteriorly. Fifth segment of abdominal pleura with bilobed posterolateral margin. First pleopod without appendix interna and masculina, endopod smaller than exopod, kidney-shaped, exopod long, slender; second pleopod with appendix interna and masculina in males, appendix masculina slightly longer than appendix interna, exopod and endopod long and slender; third, fourth and fifth pleopods with appendix interna only, exopod and endopod long, slender. Inter-uropodal sclerite not developed, preanal carina absent. Telson elongated, slender, glabrous, 2 pairs of dorsal spines present; median projection of posterior margin shorter than inner pair of posterior spines, inner pair of spines longer than outer pair, with most 2 pairs of long plumose subventral setae, longer than inner pair of spines. Uropods glabrous; exopod with mobile mesial spine, distinctly longer than distolateral tooth.

Etymology. The genus name Arachnochium is an arbitrary combination of the Greek words arachne for spider and brachion for leg, alluding to the unusually long pereiopods, like those of a spider. Gender is neuter.

Remarks. Palaemon mirabilis was first placed in Macrobrachium Bate, 1868 , by Suvatti (1937) and has been retained there despite doubts expressed ( Chace & Bruce 1993). It is herein transferred to Arachnochium gen. nov.

Kemp (1917) believed that his Palaemon mirabilis was more closely related to species in the genus Leander (= currently Palaemon Weber ) than to those of Palaemon (= currently Macrobrachium Bate ). However, the presence of a prominent hepatic spine led him to retain this species in Palaemon . After examining specimens from Thailand, Chace & Bruce (1993) concluded that Palaemon mirabilis does not belong to Macrobrachium , but did not belong to Palaemon either, because of the presence of a hepatic spine.

They commented that this species should be assigned to a new genus, but deferred from doing so because they believed they did not have male specimens. In the present study, numerous male specimens were available, and all agreed very well with the diagnostic characters of the females. Interestingly, among the USNM material examined by Chace & Bruce (1993), there was in fact a male specimen which had been incorrectly sexed (see material examined).

Arachnochium can be distinguished from Macrobrachium s. str. by: the absence of a T5 transverse plate (present in Macrobrachium ); absence of anterior lobes on the male T8 (present in Macrobrachium ); the second pereiopod being very slender and glabrous (moderately slender to robust and covered with spines, spinules and scales in Macrobrachium ); the second pereiopod being at most twice as long as the carapace (at least 2.5 times in Macrobrachium ); the second pereiopod being equally developed in both sexes (larger and more developed in fully developed males in Macrobrachium ); the fourth and the fifth pereiopods being longer than the second pereiopod (shorter in Macrobrachium ); the fifth pereiopod being distinctly longer than the fourth pereiopod (same length in Macrobrachium ); the propodus of the third and the fourth pereiopods are without distal paired spines (with distal paired spines in Macrobrachium ); the fifth abdominal pleurite possess a bilobed posterolateral margin (unilobed in Macrobrachium ); the ventroposterior margin of the telson has at most two pairs of plumose setae (at least eight pairs in Macrobrachium ), and the adult females reaching a larger size than adult males (adult females usually distinctly smaller than adult males in Macrobrachium ). In addition to the absence of T8 anterior lobes, the bases of the fifth pereiopods of male Arachnochium are also widely separated compared to the condition in Macrobrachium species. The form of the T8, however, is typical of female Macrobrachium . Therefore, one can easily confuse male specimens of Arachnochium and females of Macrobrachium . Only the presence of a genital flap at the base of the fifth pereiopod and an appendix masculina on the endopod of the second pleopod can ascertain the male sex of the specimen.

An allied species, Macrobrachium kulsiense from the Kulsi River of Brahmaputra drainage, India, has a very high rostral basal crest which a maximum depth distinctly more than the maximum dorsoventral diameter of the cornea and the adult females are also larger than the adult males. Although specimens were not examined for the present study, the description by Jayachandran et al. (2007), especially with regard to the two key generic characters mentioned above, clearly indicates that this species also belongs to Arachnochium as presently defined. Unlike A. mirabile , A. kulsiense comb. nov., has only a few large eggs, a longer rostrum with the tip reaching the end of the scaphocerite and fewer dorsal rostral teeth (see also Jayachandran et al. 2007; Klotz, pers. comm.).

Species included. Macrobrachium kulsiense Jayachandran, Lal Mohan & Raji, 2007 ; Palaemon mirabilis Kemp, 1917 .