Tsuranarthrura Kakui & Tomioka 2018, gen. nov.

Genus Tsuranarthrura gen. nov.

Type species. Tsuranarthrura shinsei sp. nov., by original designation.

Diagnosis. Body wall not heavily calcified. Pereonite 6 and pleon fused. Antenna with five articles; articles 1 and 2 naked. Labrum conical, not laterally compressed. Maxillipedal endite not tapering distally, with ventrosubdistal seta. Maxilliped-palp article 2 with three inner setae; article 3 with three long and one short inner setae. Chelipedal merus with ventral seta. Cheliped propodus with ventral seta but without outer crenulation, folds or ridges. Pereopods 2 and 3 merus naked. Pereopods 4 and 5 basis cylindrical (not swollen).

Etymology. The genus name (feminine) combines the Japanese word tsuranaru (‘continue without cutting’) with anarthrura (a genus name in the Anarthruridae), referring to the fused segment consisting of pereonite 6 and the pleon.

Remarks. In Anarthruridae, several authors have described antennal article 1 as being fused to the cephalothorax (e.g., Błażewicz-Paszkowycz et al. 2013; Larsen 2013). In their species, the antenna seems to be five-articulate. This is also the case with our specimens: we observed only five antennal articles in individuals of Tsuranarthrura gen. nov., even by SEM. Although the most proximal article we observed seems to correspond to article 2 in other paratanaoids (e.g., Larsen 2003a, fig. 2D as Tanaellidae; Bird 2011, fig. 18F as Paratanaidae; Bird 2012, fig. 4D as Heterotanoididae; Kakui and Angsupanich 2012, fig. 5B as Nototanaidae; Kakui and Shimada 2017, fig. 7E as Tanaopsidae), we designate it herein as ‘article 1’. This is because we followed Larsen’s (2003a) terminology and there is the possibility that the most proximal article could be equivalent to fused articles 1 and 2 in 6-articulate paratanaoids.

Our specimens of Tsuranarthrura gen. nov. lack the pereopod 6. This is similar to the condition in Coalecerotanais, the sole paratanaoid genus having a fused segment composed of the pereonite 6 and the pleon, and lacking the pereopod 6. The early manca stage in tanaidaceans generally lacks the pereopod 6 (cf. Larsen 2003a), and Larsen (2003b) and Błażewicz-Paszkowycz et al. (2011) proposed that Coalecerotanais adults may lack this pereopod through neoteny. As we could not judge whether our specimens were in the manca or postmanca stage, we did not include this character state in the diagnosis of Tsuranarthrura gen. nov.

The confamilial genera Tsuranarthrura gen. nov., Anarthrura, Anarthrurella, and Crenicarpus all have the labrum not compressed laterally, and the fifth and fourth antennal articles from distal end naked (Bird 2004; Drumm and Bird 2016). Tsuranarthrura gen. nov. differs from the other three in having pereonite 6 fused with the pleon, the chelipedal merus with a ventral seta, the maxillipedal endite with a ventrosubdistal seta, and the merus of pereopods 2 and 3 without setae (Table 1).