Falsanathelges grandiceps, Shinoda & Fujita & Nakano & Shimomura, 2025

Falsanathelges grandiceps sp. nov.

[New Japanese name: Yakko-yadokarinoharayadori]

( Figs.1–6 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Material examined. Holotype. Mature female (BL 7.3 mm) ( SMBL-V0708 ), infesting female Cancellus mayoae (SL 8.0 mm), St-TY-03-15, Ooshima Shin-sone, off the coast of Amami Island, Kagoshima Prefecture, Japan, 28º52.14’N, 129º32.99’E, 163–167 m depth, coll. S. Ohtsuka and M. Shimomura, 27 May 2003. GoogleMaps

Allotype. Mature male (BL 2.7 mm) ( SMBL-V0709 ), data same as holotype. GoogleMaps

Description of holotype female ( Figs. 1–4 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 ). Body length 7.3 mm, maximal width 5.0 mm; head length 1.7 mm, head width 1.4 mm, pleon length 1.7 mm. Body longer than broad, nearly symmetrical. Head, pereomeres 1–7 and pleomeres 1–5 distinctly separated. Segmentation between pleomere 5 and pleotelson indistinct, fused dorsally ( Fig. 2A, B View FIGURE 2 ).

Head ( Fig. 2A View FIGURE 2 ) longer than broad; head length approximately one-fourth of body length: anterior margin convex, slightly pointed in middle; posterior and lateral margins convex. Eyes lacking. Antennule ( Fig. 2C, D View FIGURE 2 ) of three articles: articles 2 and 3 each with some fine setae distally; article 2 with some fine setae laterally. Antenna ( Fig. 2C, D View FIGURE 2 ) of four articles: article 1 with some fine setae medially and laterally; article 2 with some fine setae laterally and distally; articles 3 and 4 each with some fine setae distally. Maxilliped ( Fig. 2E View FIGURE 2 ) with subtriangular anterior lobe, lacking palp: posterior lobe subtriangular, with long thin spur. Barbula broken in dissection.

Pereon ( Fig. 2A View FIGURE 2 ) of seven pereomeres, broadest at pereomeres 3 and 4, tapering anteriorly and posteriorly; pereomeres 1–3 anteriorly concave and posteriorly convex; pereomere 4 anteriorly concave and posteriorly slightly convex; pereomere 5 with more or less straight margins; pereomeres 6 and 7 anteriorly convex and posteriorly concave. Oostegites ( Fig. 2B View FIGURE 2 ) completely enclosing brood pouch. Oostegite 1 ( Fig. 2F–I View FIGURE 2 ) slightly extended overhead: anterior lobe irregularly ovate with edges folded under; posterior lobe with large ovate expansion and smaller; internal ridge with proximal digitations and distal lobe. Oostegite 5 with fringe of setae on posterior margin; each seta of oostegite 5 approximately 0.06 mm in length. All pereopods ( Fig. 3A–H View FIGURE 3 ) with basis and ischia elongated and smooth; carpi with distoventral tuft of setae. Pereopod 1 ( Figs. 2A View FIGURE 2 , 3A View FIGURE 3 ) anterior to head, with carpus bearing simple scales, and few widely scattered short setae on margin; merus with few widely scattered short setae on margin; pereopods 2 and 3 ( Fig. 2A View FIGURE 2 ) parallel to head. Propodus of pereopods 3 and 4 ( Fig. 3D, E View FIGURE 3 ) smaller than pereopods 2–5. Pereopods 2–6 ( Fig. 3B–F View FIGURE 3 ) similar in size and shape. Pereopods 1 and 7 ( Fig. 3A, G, H View FIGURE 3 ) shorter than others.

Pleon ( Fig. 2A View FIGURE 2 ) of six pleomeres, pleomeres 1–4 dorsally distinct; pleomeres 5 and 6 dorsally fused; pleomeres 1–5 each with elongate-ovate biramous pleopods and uniramous lateral plates ( Fig. 4A–E View FIGURE 4 ) arising from common peduncle. Pleotelson ( Fig. 4F View FIGURE 4 ) with biramous uropods; base of uropods without peduncle.

Description of the allotype male ( Figs. 5 View FIGURE 5 , 6 View FIGURE 6 ). Body length 2.7 mm, maximum width 1.1 mm; head length 0.3 mm, head width 0.7 mm, pleon length 0.8 mm. Head subovate, widest postero-medially, not distinct with pereomere 1 ( Fig. 5A View FIGURE 5 ). Small eyes on posterolateral margins. Antennule ( Fig. 5D View FIGURE 5 ) of three articles: article 1 with some fine setae medially, articles 2 and 3 each with some fine setae distally. Antenna ( Fig. 5E View FIGURE 5 ) of five articles: articles 3–5 distally setose. Pereon widest at pereomere 3 and 4, tapering anteriorly and posteriorly. Pereomeres 1–4 directed laterally with rounded distal margins; pereomeres 5–7 slightly curved postero-laterally. Pereopods 1–6 ( Fig. 6A–F View FIGURE 6 ) similar in size. Pereopod 7 ( Fig. 6G View FIGURE 6 ) smaller than pereopods 1–6. All pereopods with distinct articles; propodus with long setae on dorsal margin and few blunt teeth on ventral margin near to insertion of dactylitip; ventral margin of carpus with distoventral tuft of setae. Pleon ( Fig. 5A–C View FIGURE 5 ) subovate, all pleomeres fused without lateral indication of segmentation; posterior margin rounded, with pair of setal tuft distally, without pleopods.

Remarks. The present species is similar to Falsanathelges mariae Williams & Boyko, 2016 known from Philippines in having biramous uropods. It is distinguished from F. mariae by the following features (those of F. mariae given in parentheses): head longer than wide in female (length and width subequal), approximately one-fourth of body in length in female (one-seven), head anterior margin convex in female (deeply concave); maximum body width on pereomeres 3 and 4 in female (on pereomeres 3–7), pereopods 2–6 similar in size, pereopods 1 and 7 shorter than pereopods 2–6 in female (all pereopods of similar size), all pereopods lacking large lobes on median margin on each basis in female (with large lobes), pleon dorsal segmentation of pleomeres 5 and 6 fused in female (distinct), biramous uropods in female (uniramous), articles 3–5 of antenna with some distal fine setae in male (on article 2–5); pereopods 1–6 of similar sizes and pereopod 7 smaller in male (all pereopods subequal in size), dorsal edge of all propodus with long setae in male (without long setae); pleon subovate in male (tapering posteriorly), without lateral segmentation in male (with variable degrees of lateral indication of segmentation), and with a pair of tubercles on posterior margin in male (without tubercles).

Falsanathelges grandiceps sp. nov. is distinguished from F. muelleri from Malaysia by the following features (those of F. muelleri given in parentheses): head longer than wide in female (as long as wide), head approximately one-fourth of body in length female (approximately one-seven), anterior margin of head convex in female (concave), pereomeres 3 and 4 broadest in female (pereomeres 3–5), setae on posterior margin of oostegite 5 of approximately 0.06 mm in length in female (0.15 mm); pereopods 2–6 similar in size and pereopods 1 and 7 shorter than pereopods 2–6 in female (all pereopods of similar in sizes), ischia of pereopods all without a large lobe in female (with large lobe); dorsal segmentation of pleomeres 1–4 distinct and pleomeres 5 and pleotelson fused dorsally in female (indistinct in all 6 pleomeres), base of uropods without peduncle in female (with peduncle), pereomere 3 wider in male (pereomere 4 wider), pleon subovate, with all pleomeres fused without lateral segmentation in male (pleon tapering posteriorly with lateral indication of segmentation), and pleon posterior margin with a pair of tubercles in male (without a pair of tubercles). The characteristics of all species in the genus have been summarised in Table 1 View TABLE 1 .

This is the second time that a hermit crab of the genus Cancellus has been reported as a host of the family Bopyridae , the first was Claustrathelges macdermotti Williams & Boyko, 2016 ( Williams & Boyko 2016). The present new species is the first member of Falsanathelges from Cancellus .

The species of Falsanathelges may also be distributed further north than Southeast Asia and in waters deeper than the shallow subtidal zone. A detailed record of all species of the genus is given in Table 2 View TABLE 2 .

Etymology. The specific name is the Latin adjective grandiceps , meaning large headed, in recognition of the size of the head being larger than other species of the genus.

The Japanese name comes from the host’s Japanese name, Yakko-yadokari.