Eisothistos tiomanensis, Chew, Melvin, Rahim, Azman bin Abdul & binti Mohd Yusof, Nurul Yuziana, 2018

Chew, Melvin, Rahim, Azman bin Abdul & binti Mohd Yusof, Nurul Yuziana, 2018, A new species of Eisothistos (Isopoda, Cymothoida) and first molecular data on six species of Anthuroidea from the Peninsular Malaysia, Zoosystematics and Evolution 94 (1), pp. 73-81 : 75-78

Eisothistos tiomanensis sp. n. Figure 3, 5

Holotype.

Adult male, UKMMZ-1559, Labas, Pulau Tioman, Pahang, Malaysia, 2°53'13.71"N; 104°3'54.65"E (DMS), C. Melvin, 18 April 2014, coral rubble, ~15 m.

Description.

Holotype, adult male.

Total body length 2.2 mm (tip of rostrum to base of telson), approximately 9.9 times as long as greatest width. Cephalon with minute rostrum; eyes grossly enlarged, containing many ommatidia laterally. Pereonites 1-7 progressively shorter, pereonites 2-3 longest, each 1.3 times as long as pereonite 1, pereonites 4-5 each the same length as pereonite 1, pereonite 6 about 0.7 times length of pereonite 5, pereonite 7 half as long as pereonite 5, pereonites 4-6 anteriorly narrow, subdistally widest, pereonite 7 anteriorly narrow, widest posteriorly. Pleonites 1-5 progressively shorter with pleonites 1-2 of similar length, pleonite 3 about 0.8 times as long as pleonite 2, pleonites 4 about 0.7 times as long as pleonite 3, pleonite 5 shortest half as long as pleonite 4, posterior margin of pleonite 6 with a medial cleft.

Antenna 1 peduncle article 1 subquadrate, articles 2-3 progressively shorter; flagellum of 9 articles with short proximal article bearing numerous aesthetascs, articles 2-9 progressively narrower, articles 2-6 each bearing 2 aesthetascs and 1 seta, articles 7-8 each bearing 1 aesthetasc distally, terminal article with 3 apical setae.

Antenna 2 peduncle articles equal, articles 1-2 each bearing 2 setae, article 3 with 1 seta, article 4 elongate, 2.4 times as long as wide, article 5 longest, twice as long as article 4; flagellum of 6 articles, progressively smaller, articles 1-4 each with 1 distal aesthetasc, article 5 with 2 distal setae, terminal article with 4 apical setae.

Mouthparts reduced.

Pereopod 1 basis 3.3 times as long as greatest width, anterior margin with 1 medial seta, posterior margin with 1 subdistal seta and fine setae; ischium 3.2 times as long as greatest width, posterior margin subdistally with 1 seta and fine setae; merus anterior margin convex distally, bearing 2 setae, posterior margin with fine setae; carpus anterior margin half the length of posterior margin, posterior margin with 1 seta, with 4 submarginal spinules; propodus 4.4 times as long as greatest width, palm weakly concave with 21 submarginal spinules, anterior margin with 1 distal seta, palmar margin with 1 posterodistal robust seta; unguis half as long as propodus.

Pereopod 2 basis 3.7 times as long as greatest width, posterior margin with 1 subdistal seta and fine setae; ischium 3.1 times as long as greatest width, anterior margin with 2 setae, posterior margin with 2 setae and fine setae; merus anterior margin convex, bearing 1 seta, posterior margin with fine setae; carpus anterior margin half the length of posterior margin, posterior margin with 1 seta and fine setae; propodus 3.9 times as long as greatest width, anterior margin with 2 setae, palmar margin with 1 medial robust seta and 1 posterodistal robust seta, fine setae and subdistal palmar comb; unguis half the length of propodus.

Pereopod 3 basis 3.3 times as long as greatest width, anterior margin with 2 setae and fine setae, posterior margin with 1 seta and fine setae; ischium 3.2 times as long as greatest width, anterior margin with 2 setae and fine setae, posterior margin with 1 seta and fine setae; merus anterior distal margin convex bearing 2 setae, posterior margin with 1 seta, carpus anterior margin one third length of posterior margin, posterior margin with 1 seta; propodus 3.6 times as long as greatest width, anterior margin with 2 setae, palmar margin with fine setae, 2 setae, subdistal palmar comb, 1 medial robust seta and 1 posterodistal robust seta; unguis half as long as propodus.

Pereopod 4 basis 3.9 times as long as greatest width, anterior margin with 2 setae and fine setae, posterior margin with 1 seta and fine setae; ischium 3 times as long as greatest width, anterior margin with 1 seta, posterior margin with 2 setae; merus anterior margin convex, bearing 1 seta, posterior margin with 1 seta, carpus linear twice as long as wide anterior margin with 2 setae, posterior margin with setae comb, 1 seta and 1 robust seta; propodus 4.5 times as long as greatest width, anterior margin with 3 setae, palmar margin bordered with setae comb, 1 medial robust seta and 1 posterodistal robust seta; unguis 0.8 times as long as propodus.

Pereopod 5 basis 4.1 times as long as greatest width, anterior margin with 1 seta, posterior margin with 1 seta and fine setae; ischium 4.1 times as long as greatest width, anterior margin with 1 seta, posterior margin with 3 setae and fine setae; merus anterior margin subdistally convex, bearing 2 setae, posterior margin with fine setae; carpus linear 2.4 times as long as greatest width, anterior margin with 1 seta, posterior margin with setae comb, 1 distal seta and 1 distal robust seta; propodus 4.6 times as long as greatest width, anterior margin with 1 distal seta, palmar margin bordered with setae comb, 1 medial robust seta and 1 posterodistal robust seta; unguis 0.8 times the length of propodus.

Pereopod 6 basis 4 times as long as greatest width, anterior margin with 3 setae and fine setae, posterior margin with 1 subdistal seta; ischium 3.8 times as long as greatest width, anterior margin with fine setae, posterior margin with 3 setae and fine setae; merus anterior margin convex, bearing 1 seta, posterior margin with 1 seta; carpus linear 2.6 times as long as greatest width, anterior margin with 1 seta, posterior margin bordered with setae comb, distally with 1 seta and 1 robust seta; propodus 5.3 times as long as greatest width, palmar margin bordered with setae comb, proximally with 1 robust seta, medially with 1 robust seta and posterodistally with 1 robust seta; unguis 0.7 times the length of propodus.

Pereopod 7 basis 4.1 times as long as greatest width, anterior margin with 1 seta, posterior margin with 1 seta; ischium 3.2 times as long as greatest width, anterior margin with 1 seta and fine setae, posterior margin with 3 setae and fine setae; merus slender 3.6 times as long as greatest width with anterior margin slightly convex, bearing 1 seta, posterior margin with 1 seta; carpus linear 3.2 times as long as greatest width, anterior margin with 1 seta, posterior margin with setae comb and 1 robust seta; propodus 7.5 times as long as greatest width, anterior margin with 3 setae, palmar margin bordered with setae comb, 1 medial robust seta and 1 posterodistal robust seta; unguis half as long as propodus.

Pleopod 1 with elongated rami, sympod rectangular; exopod slender 4.3 times as long as greatest width, apex truncated bearing 6 plumose setae; endopod shorter than exopod 0.75 times the length of exopod, apex truncated bearing 5 plumose setae.

Pleopod 2 sympod subrectangular; exopod 3 times as long as wide, apex truncated bearing 7 plumose setae; endopod slender 4.8 times as long as greatest width, apex truncated bearing 5 plumose setae; appendix masculina almost as long as endopod, ending in a somewhat hook-like with a rounded end.

Pleopod 3 not available.

Pleopod 4 sympod subrectangular, inner margin with 1 seta; exopod broadened 1.8 times as long as greatest width, apex truncated bearing 8 plumose setae, medial lateral margin with 1 seta; endopod as long as exopod 1.3 times as long as exopod with apical margin truncated bearing 6 plumose setae.

Pleopod 5 shorter than others with sympod subquadrate; exopod much smaller than endopod half as long as endopod bearing 3 plumose setae; endopod broadend 1.8 times as long as greatest width bearing 2 plumose setae, apical margin medially with two small protuberances.

Uropod sympod with serrated inner margin, inner distal margin raised with an acute apex, outer distal margin raised with 2 acute serrations apically; endopod elongate with serrated margin bearing 13 setae, apically with 1 short robust plumose sensory spine; exopod bipartite with 5 setae proximally, medially with a central elongated spike having 3 subdistal setae and apical margin concave bearing 1 short plumose sensory spike, outer margin with a short slender spike bearing 1 seta apically, inner margin obsolete.

Pleotelson 2.2 times as long as greatest width, narrowest anteriorly, widest near posterior end, dorsal surface with 4 pairs of setae and a middorsal row of 7 obscure denticles, subdistal margin with 2 obscure teeth on each side, posterior margin strongly dentate with 10 teeth and 5 pairs of setae.

Etymology.

This species is named after the type locality, Pulau Tioman, Malaysia.

Molecular data.

n/a.

Remarks.

Only one male specimen is available. The male Eisothistos in general, differs from the female by its grossly enlarged eyes, antenna 1 with short basal article bearing numerous aesthetascs, reduced mouthparts, pleonites 1-3 elongate and pleopods 1-3 with elongated peduncles and rami ( Knight-Jones and Knight-Jones 2002; Poore 2001; Poore and Lew Ton 2002; Wägele 1979). Though they appear distinctive, their tail-fan shape are more conserved within a species. Hence, it is possible to determine the identity of Eisothistos species based on a male specimen. Moreover in this study, the good condition of preserved material and the unique characteristic of its tail-fan, justify a description.

The uropodal exopod form in the Eisothistos can be divided into two types. While most possess the tripartite uropodal exopod shape, several others are described with the uncommon bipartite uropodal exopod shape. Eisothistos tiomanensis sp. n. belongs to the latter group with Eisothistos anomala , E. corinellae , E. macquariensis and E. minutus ( Kensley 1980; Poore and Lew Ton 2002; Sivertsen and Holthuis 1980). It differs in having a unique two slender spine-like structure in the uropodal exopod rather than one central slender spike-like structure along with a proximal lobe as in E. anomala , E. corinellae and E. macquariensis . Eisothistos minutus on the other hand, possesses a form similar to the present species. Sivertsen and Holthuis (1980) described it as "a narrow elongate process on the protopod, it ends in two teeth; the protopod seems to have a second similar process laterally, the nature of this process is not clear". Nevertheless, there is no strong robust seta on the apex of the central spike and a seta on the lateral spike. Additionally, there are no teeth present on the dorsal surface of the pleotelson in E. minutus while there are 7 obscure denticles present in the new Malaysian species.

Eisothistos sp. n. is readily distinguishable from the other Malaysian species, E. besar Müller, 1992 by its bipartite uropodal exopod (tripartite in E. besar ) and the surface of pleotelson with 7 obsure middorsal teeth (4 distinct middorsal teeth in E. besar ).