Tenuipedium, 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 : 44-45

Tenuipedium gen. nov.

Type species. Macrobrachium palaemonoides Holthuis, 1950 , by present designation and monotypy.

Diagnosis. Small-sized prawn of subcylindrical body form. Rostrum well developed, compressed; dorsal carina armed with 6–9 teeth, at least 1 tooth completely postorbital; ventral carina armed with 6–8 teeth; plumose setae present between teeth; lateral carina well developed. Carapace glabrous; antennal and hepatic spines present; branchiostegal groove present, running below hepatic spine, from antennal carapace margin to some distance behind hepatic 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. Mouth parts similar to those of Macrobrachium ; mandible with 3-segmented palp. Second pereiopod very slender, as wide as first pereiopod, similar in shape, equal in size, maximum 2.6 times as long as carapace length. Carpus slightly tapered. All segments glabrous, covered with few scattered short stiff setae. Pereiopods equally developed in both sexes. Third, fourth and fifth pereiopods with few scattered short stiff setae, otherwise glabrous; third pereiopod shorter than second pereiopod, fourth pereiopod longer than second pereiopod, fifth pereiopod distinctly longer than fourth pereiopod; third and fourth pereiopods with several ventral spines distributed along length of propodus but without distal paired spines; fifth pereiopod with 6 ventral spines distributed along length of propodus, transverse rows of setae present at outer margin of distoventral propodus. T5 with 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 or masculina, endopod smaller than exopod, kidney-shaped, exopod long, slender; second pleopod with appendix interna and masculina in male specimen, appendix masculina slightly longer than appendix interna, exopod and endopod long, 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 spines longer than outer pair, with 1 pair of long plumose subventral setae which are shorter than inner pair of spines. Uropods glabrous; exopod with mobile mesial spine distinctly longer than distolateral tooth.

Etymology. The genus name Tenuipedium is derived from the Latin words tenuis for thin and pedis for foot, alluding to the thin and slender pereiopods. Gender is neuter.

Remarks. Tenuipedium gen. nov., is established here for Macrobrachium palaemonoides Holthuis, 1950 . The relatively longer posteroventral branchiostegal groove behind and below the hepatic spine of M. palaemonoides almost led Chace & Bruce (1993) to assign this species to Palaemon . However, the position of the branchiostegal groove, and the presence of a hepatic spine convinced them to retain it in Macrobrachium , albeit with considerable reservation.

Macrobrachium palaemonoides however, has a set of several unique morphological characters that excludes it from Macrobrachium s. str. and its related genera. Therefore, Tenuipedium gen. nov. is established for this species. The new genus can be distinguished from Macrobrachium by its long branchiostegal groove running below the hepatic spine, from the antennal carapace margin to some distance behind the hepatic spine (shorter, running from base of hepatic spine to antennal carapace margin in Macrobrachium ); absence of T8 anterior lobes (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 diameter of the second pereiopod being as wide as the first pereiopod (distinctly wider in Macrobrachium ); the second pereiopod being equally developed in both adult males and females (larger and more developed in fully developed males in Macrobrachium ); the fourth and 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 have a bilobed posterolateral margin (unilobed in Macrobrachium ); the fifth abdominal pleurite possess a bilobed posterolateral margin (unilobed in Macrobrachium ); the ventroposterior margin of telson has only one pair of plumose setae (at least eight pairs in Macrobrachium ); and adult females reach a larger size than adult males (adult females smaller than adult males in Macrobrachium ).

Distribution. Simeulue Island, off Sumatra, Indian Ocean.