Rhynchothorax arenigenus, Lee & Park & Park, 2025

Lee, Damin, Park, Jin-Ho & Park, Taeseo, 2025, Description of new sand-dwelling Rhynchothorax species (Pycnogonida, Rhynchothoracidae) from Korean waters with note on novel morphological traits, Zoosystematics and Evolution 101 (4), pp. 2055-2064 : 2055-2064

publication ID	https://doi.org/10.3897/zse.101.167593
publication LSID	lsid:zoobank.org:pub:FE55872D-977C-400D-ACB1-81C0172D12FB
DOI	https://doi.org/10.5281/zenodo.17468232
persistent identifier	https://treatment.plazi.org/id/46D086CE-5BE0-5E48-AC9C-004AB8765B3D
treatment provided by	Zoosystematics and Evolution by Pensoft
scientific name	Rhynchothorax arenigenus
status	sp. nov.

Treatment

Rhynchothorax arenigenus sp. nov.

Material examined.

Holotype. Republic of Korea • 1 ♂; subtidal zone off Sunrise Peak (Seongsan Ilchulbong) , Seogwipo-si, Jeju-do; 33°27'14.2"N, 126°56'59.2"E; depth 26.7 m; 24 Oct. 2023; DM Lee leg.; sand sieving, 24 ° C, SCUBA diving; NIBRIV 0000927835 GoogleMaps . Paratype. Republic of Korea • 1 ♀; same data for holotype; NIBRIV 0000912890 GoogleMaps • 1 ♀; same data as for holotype; NIBRIV 0000927836 GoogleMaps • 1 ♂; same data as for holotype; NIBRIV 0000927837 GoogleMaps • 1 ♀; same data for holotype; HNIBRIV 23076 GoogleMaps .

Comparative material.

Rhynchothorax philopsammum Hedgpeth, 1951 . Holotype. USA • 1 ♀; California, Tomales Bay Bluff, Marin County; 27 Jan. 1949; R. J. Menzies leg.; USNM 91245 About USNM . Paratype. USA • 2 ♀♀; same data for holotype; USNM 91246 About USNM . Other material. French Polynesia • 5 ♀♀; Reef at Vaitape , Bora Bora, Society Islands; depth 0.5–1 m; 27 Feb. – 06 Mar. 1988; SMF 1080 About SMF . Colombia • 2 ♂♂, 1 juv.; Magdalena, Punta de Betin , Santa Marta; depth 0–0.5 m; 31 Dec. 1985; under stones and sandy bottom; SMF 1538 About SMF .

Description.

Male. Trunk fully segmented, tapering posteriorly, finely granulated, without dorsal tubercles (Figs 2 View Figure 2 , 3 View Figure 3 ). Cephalic segment with pair of projections at anterodorsal margin, pair of low tubercles present at middle of neck, cylindrical lateral extension bearing small ectal tubercle (Figs 2 A View Figure 2 , 3 A, B View Figure 3 ). Lateral processes shorter than diameter, separated by half of diameter; small dorsal seta present on center of first lateral process and on dorsodistal margin of other lateral processes (Fig. 2 A, B View Figure 2 ). First lateral processes with two posterior tubercles; upper tubercle small, present at proximal; lower one large, present at distal. Second lateral processes having smaller anterior tubercle and two similar posterior tubercles. Third lateral processes bearing anterior tubercle and smaller posterior tubercle. Fourth lateral processes with small anterior tubercle. Ocular tubercle absent.

Proboscis spindle-shaped, having low dorsal protuberance at 2 / 3 from base, with three pairs of dorsolateral swellings, bearing pair of furrows on ventral surface (Figs 2 A, B View Figure 2 , 3 A View Figure 3 , 4 C View Figure 4 ); mouth triradiate.

Abdomen cylindrical, tapering distally, directing horizontally, with nodulous dorsal surface, not reaching distal margin of second coxa (Fig. 2 A, B View Figure 2 ).

Chelifores absent.

Palps five-articled, granular (Fig. 2 C View Figure 2 ). First article very short. Second article cylindrical, longest, five times as long as diameter, with seta on dorsomedian and dorsodistal margin. Third article cylindrical, 1.5 times length of diameter, without seta. Fourth article distally widened, 3.7 times length of proximal diameter, bearing large dorsal tubercle at 2 / 3 from base, with setae on dorsal, ventral and distal surface; dorsal tubercle 1.5 times as high as proximal diameter of fourth article, armed with three setae. Fifth article bulged proximally on inner side and dorsally, armed with many setae.

Ovigers 10 - articled, granular (Fig. 2 D View Figure 2 ). First article short, attached to pectoralis major-like basement (Fig. 4 C View Figure 4 ). Second article cylindrical. Third article swollen at inner surface. Fourth article tapering at proximal, longest, with setae on outer and distal surface. Fifth article curved, with short setae on outer surface. Sixth article cylindrical, second-longest, with short setae on outer-distal surface. Seventh article distally widened, as long as third article, bearing four tufts of spines on inner- and latero-distal surface (Fig. 3 C View Figure 3 ). Eighth article bearing four tufts of spines on inner and lateral surface. Ninth article bearing three tufts of spines on inner surface, with seta on lateral surface. Terminal article curved, bearing two tufts of spines on lateral and inner surface, having simple spine and serrated lamina on distal margin, with outer seta. Terminal claw curved, with flattened inner surface (Fig. 3 D View Figure 3 ).

Third leg granular, sparsely setose (Fig. 2 E View Figure 2 ). First coxa as wide as lateral processes, with anterodistal tubercle on first legs; anterodistal tubercle and posteroproximal nodulous ridge on second legs; anterior and posterior tubercles on third legs; small anterior and posterior tubercles on distal margin of fourth legs (Figs 2 A, B View Figure 2 , 4 D View Figure 4 ). Second coxa elongated, longest among three coxae, with large curved tubercle on posterior surface. Third coxa slightly longer than first coxa, with several setae on ventral surface. Femur longest, with long seta on dorsodistal margin. First tibia as long as second tibia, with long seta on dorsodistal margin. Second tibia cylindrical, with long setae on dorso- and ventro-distal margin; dorsodistal seta longer than that of femur and first tibia. Tarsus short, approximately 1 / 6 length of propodus, with long seta on ventral surface. Propodus slightly curved, as long as tibiae, with several setae on dorsal surface, bearing six sole setae, without heel spine. Claw curved, 1 / 3 length of propodus. Auxiliary claws thin, 0.7–0.8 length of claw. Gonopores present on ventral surface of second coxa on fourth legs (Fig. 3 E View Figure 3 ).

Measurements (mm). Holotype ( NIBRIV 0000927835 ): Trunk length, 0.76; trunk width, 0.40; proboscis, 0.43; abdomen, 0.16. Paratype ( NIBRIV 0000927837 ): Third leg, first coxa, 0.07; second coxa, 0.12; third coxa, 0.09; femur, 0.20; first tibia, 0.18; second tibia, 0.18; tarsus, 0.03; propodus, 0.17; claw, 0.05; auxiliary claw, 0.04.

Etymology.

The species name arenigenus is derived from the Latin “ arena (sand) + - genus (meaning ‘ arising from’ or ‘ born off’) ”, referring to its natural habitat on sandy bottoms. The Korean names for the family, genus, and species are all pronounced “ moraebadageomi ”, emphasizing the association with sandy environments.

Distribution.

This species is only known from the type locality, Jejudo Island, Republic of Korea.

Remarks.

Rhynchothorax arenigenus sp. nov. closely resembles R. nopperabo Matsushita & Kakui, 2024 , R. philopsammum Hedgpeth, 1951 , and R. vallatus Child, 1990 in lacking both ocular tubercle and dorsal tubercles on the trunk and proboscis. However, it can be distinguished from R. nopperabo by the presence of a small seta on each lateral process, two posterior tubercles on the first lateral processes, lateral process intervals about half the diameter, and a small tubercle on both the anterior and posterior margin of the first coxa on the fourth legs, whereas in R. nopperabo , no seta is present on the lateral processes, only a posterior tubercle is present on the first lateral processes, the intervals are narrower, and the first coxa of the fourth legs has only a small posterior tubercle (or none).

Although R. nopperabo was originally described as lacking an ectal tubercle on the lateral extension of the cephalic segment and bearing only an anterodistal tubercle on the first coxa of the second legs and a posterior tubercle on that of the third legs ( Matsushita and Kakui 2024), Dr. Keiichi Kakui kindly re-examined the type specimens of R. nopperabo deposited in the Invertebrate Collection of the Hokkaido University Museum and confirmed the presence of intraspecific variations (personal communication, see Table 2 View Table 2 ).

Compared to R. philopsammum , the ectal tubercle on the cephalic segment of the present species is smaller and less conspicuous, and the lateral process intervals are wider (Fig. 5 View Figure 5 ). The fifth segment of the palp is dorsally bulged but does not reach the height of the dorsal tubercle on the fourth segment, while in R. philopsammum , the fifth segment is strongly curved, reaching the dorsal tubercle on the fourth segment. Rhynchothorax arenigenus sp. nov. also differs from R. vallatus in several traits: it has a shorter fifth palp segment, tufts of oviger spines with a formula of 4-4 - 3, a posteroproximal nodulous ridge on the first coxa of the second legs, distinct anterior and posterior tubercles on the first coxa of the third legs, and relatively longer auxiliary claws. In R. vallatus , by contrast, the fifth palp segment is more curved and reaches the height of the dorsal tubercle on the fourth segment, the strigilis bears simple spines with a formula of 2-1 - 1, posterior swellings or tubercles are absent on the first coxa of the second and third legs, and the auxiliary claws are shorter.

The present species exhibits sexual dimorphism in external morphology (Fig. 4 A, B View Figure 4 ). Female specimens display similar or slightly larger trunk lengths compared to males (females NIBRIV 0000912890 , NIBRIV 0000927836 , HNIBRIV 23076 : 0.83 mm, 0.86 mm, and 0.88 mm, respectively; males NIBRIV 0000927835 and NIBRIV 0000927837 : 0.76 mm and 0.83 mm, respectively). Tubercles or projections, such as those on the lateral extension of the cephalic segment, the posterior surface of the first lateral processes, the first and second coxa of the third legs, are less prominent in females. Female gonopores are large and located on the inner surface of the second coxa of the fourth legs, whereas male gonopores are small and situated ventrally on the same coxa (Fig. 3 F View Figure 3 ).

Mitochondrial cox 1 sequence analysis revealed very low intraspecific genetic distances (0–0.17 %) between male and female specimens, whereas interspecific distances range from 24.45 % (between R. arenigenus sp. nov. and R. sidereus ) to 31.07 % (between R. crenatus and R. sidereus ) (Table 3 View Table 3 ). Considering that male and female specimens were collected from the same locality, that they share key diagnostic morphological characters of the new species, and that they exhibit very low genetic divergence, the morphological differences mentioned above are interpreted as sexual dimorphism.