Heptacarpus elatus, Komai, 2025

Heptacarpus elatus sp. nov.

[New Japanese name: Kobu-toge-tsuno-mo-ebi]

( Figs. 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 )

Heptacarpus geniculatus View in CoL .— Minemizu 2000: 94, unnumbered (upper) fig. Not Heptacarpus geniculatus ( Stimpson, 1860) View in CoL .

Material examined. Holotype. CBM-ZC 16855 , female (cl 6.4 mm; DNA voucher 0248), off Shimoda Port, Izu Peninsula , 34°39.12’N, 138°56.99’E, 42–46 m, sand with algae, 25 October 2017, RV Tsukuba II, dredge, coll. T. Komai. GoogleMaps

Paratype. CBM-ZC 16856 , 1 female (cl 4.9 mm), same data as holotype GoogleMaps .

Description. Holotype female. Body ( Figs. 1 View FIGURE 1 , 2A, D View FIGURE 2 ) slenderly built.

Rostrum ( Fig. 2A, B View FIGURE 2 ) straight, directed forward, generally styliform, reaching distal margin of antennal scaphocerite, 1.4 times carapace length; dorsal margin armed with 4 teeth including 3 on rostrum proper and 1 on carapace, posteriormost tooth arising from 0.1 length of carapace, distal 0.6 of dorsal margin unarmed; ventral blade relatively shallow, deepest just proximally to midlength; ventral margin with 7 teeth; teeth subequal in size except for smaller distalmost tooth; lateral carina blunt. Carapace ( Fig. 2A–C View FIGURE 2 ) with postorbital rostral ridge low, extending to anterior 0.2 of carapace length; dorsal margin in lateral view almost straight; suborbital lobe rounded, but not constricted at base, falling slightly short of small antennal tooth; pterygostomial angle unarmed, broadly rounded.

Pleon ( Fig. 2D View FIGURE 2 ) strongly gibbous. Second somite with faint transverse groove on tergite. Dorsal surface of third tergite prominently produced in posterior part, forming roundly triangular boss; posterodorsal margin of tergite produced into well marked, roundly triangular lobe ( Fig. 2E, F View FIGURE 2 ). Pleura of anterior four somites broadly rounded, fifth pleuron unarmed at the posteroventral angle; posterolateral margin of fifth pleuron faintly sinuous. Sixth somite slightly narrowing in lateral view, about twice as long as fifth somite, about two times longer than high. Telson ( Fig. 2G, H View FIGURE 2 ) about 1.2 length of sixth somite, 4.4 times longer than wide, armed with 4 pairs of dorsolateral spiniform setae; posterior margin with minute sharp median tooth and 2 pairs of unequal spiniform setae.

Eyestalk ( Fig. 2A, C View FIGURE 2 ) generally subpyriform; cornea slightly wider and shorter than remaining part of eyestalk; ocellus distinct, showing as black spot; maximal diameter of cornea 0.14 of carapace length.

Antennular peduncle ( Fig. 2A, C View FIGURE 2 ) not reaching midlength of antennal scaphocerite. First article unarmed on dorsodistal margin; stylocerite reaching dorsodistal margin of first article, acuminate, mesial margin convex or sinuous, closely in touch with first article, leaving keyhole-like hiatus proximally. Second article about 0.30 length of first article, with small spine at dorsolateral distal angle. Third article short, with small spine on dorsodistal margin. Lateral flagellum with thickened aesthetasc-bearing portion 0.3 of carapace length.

Antenna ( Fig. 2A, C, I View FIGURE 2 ) with basicerite bearing moderately large ventrolateral distal tooth. Carpocerite short, reaching 0.2 length of scaphocerite. Scaphocerite 1.2 times as long as carapace and 6.3 times longer than wide, widest point at proximal 0.2; lateral margin gently concave; distal lamella rounded, strongly produced, far exceeding distolateral tooth.

Third maxilliped( Fig.3A,B View FIGURE 3 )moderately stout,short, reaching distal end of antennal carpocerite.Antepenultimate article depressed in proximal part, unarmed on distolateral margin. Ultimate article about 1.9 length of carpus (= penultimate article), tapering distally, with 8 darkly pigmented, stout spiniform setae circumscribing distal portion. Exopod absent. Terminally hooked, strap-like epipod present.

Pereopods short; no epipods present.

First pereopod ( Fig. 3C, D View FIGURE 3 ) moderately stout, not reaching distal end of antennal carpocerite. Merus 1.4 times as long as carpus, with minute denticle proximo-dorsally. Carpus widened distally. Chela 1.50 times as long as carpus 3.6 times longer than wide; palm subcylindrical, 2.6 times as long as wide; fixed finger terminating in single corneous unguis; dactylus gently curved, about half-length of palm, terminating in 2 darkly pigmented, strong corneous ungues.

Second pereopods ( Fig. 3E View FIGURE 3 ) equal, overreaching distal end of carpocerite by length of chela. Merus subequal in length to ischium, with few stiff setae on ventral margin proximally. Carpus divided in 7 segments, third segment longest. Chela 0.3 times as long as carpus; dactylus shorter than palm.

Third to fifth pereopods relatively short, similar in structure, but decreasing in length toward posterior. Third pereopod ( Fig. 3F, G View FIGURE 3 ) overreaching midlength of scaphocerite by length of dactylus; ischium unarmed; merus with 5 spiniform setae on lateral surface ventrally; carpus 0.5 times as long as propodus, unarmed; propodus with spiniform setae, arranged in 2 rows, on flexor margin; dactylus 0.35 times as long as propodus, 3.9 times as long as wide, with 6 accessory spiniform setae on flexor margin, noticeably decreasing in size toward proximal, ultimate spiniform setae subequal in length and contiguous to unguis, making distal part biunguiculate. Fourth pereopod ( Fig. 3H, I View FIGURE 3 ) reaching midlength of scaphocerite by tip of dactylus; merus with 4 spiniform setae on lateral surface ventrally; dactylus 0.3 times as long as propodus, with 5 accessory spiniform setae. Fifth pereopod ( Fig. 3J, K View FIGURE 3 ) reaching distal end of carpocerite; merus with 1 spiniform setae near ventrolateral distal angle; propodus with grooming apparatus consisting of row of short stiff setae along flexor distal margin; dactylus 0.3 times as long as propodus, with 5 accessory spiniform setae

Uropodal protopod ( Fig. 2G View FIGURE 2 ) with distolateral margin rounded, non-acuminate; both rami ( Fig. 2G View FIGURE 2 ) slightly overreaching posterior margin of telson, exopod slightly longer than endopod, with 1 posterolateral spiniform seta just mesial to tiny posterolateral tooth.

Paratype female. Generally similar to holotype. Rostrum 1.5 times as long as carapace, with 5 teeth on dorsal margin, including 1 postrostral tooth, ventral margin with 8 teeth. Third to fifth pereopod meri each armed with 4 spiniform setae; dactyli each with 6 accessory spiniform setae.

Colouration in life. Body and appendages with scattered orange-red dots, more intense on pleon; midline of carapace and pleon white; cornea grey; antennal flagellum translucent with brown rings; propodi of third to fifth pereopods yellowish ( Fig. 1 View FIGURE 1 ).

Distribution. Presently known only from two locations of the Izu Peninsula, Japan, namely off Shimoda and Osezaki (see below); 7–46 m deep.

Remarks. Species in Heptacarpus have been classified into five informal species groups based on the development of strap-like epipods on the third maxilliped 3, and first to third pereopods (e.g., Hayashi 1979, 1993; Butler 1980). Heptacarpus elatus sp. nov. is referred to the species group characterized by the possession of an epipod only on the third maxilliped (no epipods on the first to third pereopods) and appears closest to H. geniculatus in having the strongly geniculate pleon and the narrowly elongate antennal scaphocerite (cf. Komai & Ivanov 2008). Nevertheless, the new species is distinctive in the genus in having a prominent, roundly triangular boss on the posterior part of the third pleomere tergite ( Fig. 2D, F View FIGURE 2 ), of which the posterior margin is produced as a well-marked, rounded median lobe ( Fig. 2E View FIGURE 2 ). In other congeneric species, such a prominent boss is not seen; the posterodorsal margin of the third tergite is evenly convex (cf. Rathbun 1904; Schmitt 1921; Hayashi 1979; Butler 1980; Jensen 1983; Wicksten 1984, 1986; Hayashi & Chiba 1987, 1989; Komai & Ivanov 2008; Komai 2023). The absence of a posteroventral tooth on the fifth pleuron ( Fig. 2D View FIGURE 2 ) immediately distinguishes H. elatus sp. nov. from H. geniculatus , the latter has a sharp posteroventral tooth on the fifth pleuron ( Komai & Ivanov 2008: figs. 10, 13B). Habitat may be different between the two species. Heptacarpus geniculatus often occurs in intertidal to shallow subtidal sea grass beds ( Hayashi & Miyake 1968; Komai & Ivanov 2008; personal observation), but specimens identifiable to the present new species have not been encountered in sea grass beds.

Heptacarpus longirostris ( Kobjakova, 1936) , which was confused with H. geniculatus in earlier literature ( Komai & Ivanov 2008), is also similar to the new species, particularly in the unarmed pleuron of the fifth pleomere, but the differentiation between them is easy: the pleon is only weakly geniculate in H. longirostris , of which the tergite of the third pleomere is evenly convex without a well-marked median lobe on the posterodorsal margin ( Komai & Ivanov 2008: fig. 14, 17C); the rostrum is relatively wider in the lateral view in H. longirostris than in H. elatus sp. nov. (cf. Komai & Ivanov 2008: fig. 15A versus Fig. 2B View FIGURE 2 ); the antennal scaphocerite is proportionally wider in H. longirostris than in H. elatus sp. nov. (3.9–4.5 times as long as wide versus 6.3 times as long); each finger of the chela of the first pereopod have three ungues in H. longirostris (cf. Komai & Ivanov 2008: 16 D), rather than one unguis on the fixed finger and two ungues on the dactylus in H. elatus sp. nov.

The phylogenetic reconstruction ( Fig. 4 View FIGURE 4 ) also suggests that Heptacarpus elatus sp. nov. is close to H. geniculatus with a genetic divergence of 4.0%, which indicates that these two taxa are distinct (e.g., Macpherson & Machordom 2005; Komai & Chen 2024). The pairwise distance between H. elatus sp. nov. and other available congeners was 6.6–16.0%. No genetic data is available for H. longirostris . Notably, five species belonging to the group characterized by the possession of an epipod only on the third maxilliped ( H. elatus sp. nov., H. acuticarinatus , H. geniculatus , H. tridens and H. kincaidi ) were clustered in a clade, although the bootstrap support was still weak.

The specimen identified by Minemizu (2000: 94, unnumbered fig.) as Heptacarpus geniculatus agrees well with the present new species, rather than H. geniculatus , particularly in having a prominent hump-like boss on the third pleomere, and also in the similar living colouration.

Etymology. From the Latin “ elatus ” (= elevated), in reference to the posterior part of the third pleomere tergite strongly elevated into a hump-like boss.