Dendrogaster tanabensis Saito & Moritaki & Minakata & Wakabayashi 2024, sp. nov.

Dendrogaster tanabensis Saito & Wakabayashi sp. nov.

[New Japanese name: hime-no-shidamushi]

(Figs. 1D, 3)

Material examined. Holotype: larvigerous female (mantle size 28.6 mm), NSMT-Cr 31584, found beside the host on 14 March 2022 from Henricia sp. (Echinodermata: Asteroidea: Spinulosida: Echinasteridae) (R 36.0 mm, r 10.7 mm), TAMBL-EC 36, off Tanabe, Wakayama Prefecture, southwestern Kii Peninsula, Pacific coast of central Japan, approximately 200–300 m in depth, coll. K. Minakata on 9 March 2022.

No males were found from the holotype female.

Description of holotype female. Carapace (or mantle) markedly branched; surface smooth (Fig. 3A, B). Middle piece elliptical, approximately twice as long as wide. Pair of main branches directed laterally from the base of middle piece, divided into 3 (anterior, central, and posterior) outspreading primary branches. Each anterior primary branch divided dichotomously into secondary branches, these in turn giving off short oppositely placed tertiary branches with terminal protuberances. Central primary branch straight, giving off 4 oppositely placed secondary branches with terminal protuberances on each side. Branching pattern of posterior pair of primary branches similar to that of anterior primary branches.

Antennules 4-segmented and subchelate (Fig. 3C). First segment large, trapezoidal. Second segment trapezoidal, distal margin curved. Third segment with fusion seam crossing proximal-dorsal angle and 2 spiniform setae at distal end of seam. Fourth segment rectangular, with movable terminal claw, large, cylindrical claw guard, and rudimental proximal sensory process; bearing 1 seta at dorsal base of terminal claw (another seta at ventral probably missing), 2 distal setae on claw guard, and 1 seta and 1 aesthetasc on proximal sensory process.

Oral cone formed by labrum ensheathing maxillae. Maxillule and mandibles absent. Maxillae (Fig. 3D) harpoon-like, right and left maxillae fused medially for most of their length but separated at distal end; apex bifid, right distal prong straight and ventral hooks reflected backward, but left of these recurved laterally.

Rudiments of thoracopods absent.

Nauplius (outhonauplius) with bowl-shaped carapace, oval in dorsal view, 0.82× 0.53 mm (n = 20); antennules, antennae, mandibles, labrum, and anal spine present; frontal filaments and medial knob, naupliar eye, maxillules, buds of thoracopods, caudal appendages, and abdominal spines absent. Antennules (Fig. 3E) uniramous with 9 indistinct segments, bearing 3 terminal and 1 distal setae.Antennae (Fig. 3F) biramous; exopod with 12 indistinct segments, bearing 3 terminal setae, and 1 long seta on each of distal 4 segments; endopod with 8 indistinct segments, bearing 3 terminal setae, and 1 long seta on each of 4 segments. Mandibles (Fig. 3G) biramous; exopod with 12 indistinct segments, bearing 3 terminal setae, and 1 long seta on each of 4 segments; endopod with 4 indistinct segments, 3 setae at distal end. Labrum pointed ventrally.

Coloration. In living female specimen, mantle pale pink (Fig. 1D).

Genetic information. K2P distances of the partial COI gene sequences determined from the holotype (NSMT-Cr 31584, GenBank No. PP118259) was 0.229 –0.367 from the other 7 species of Dendrogaster (Table 1).

Ecological note. The host echinasterid sea star, Henricia sp. (Spinulosida: Echinasteridae) infested with Dendrogaster tanabensis sp. nov. in the coelomic cavity was solely housed in a home aquarium for ornamental purposes. Dendrogaster tanabensis sp. nov. was found completely exposed to the water beside the host sea star which dorsal wall of an arm collapsed. A single female parasite infested the coelomic cavity of an arm of the host sea star. The main branch of the female mantle was filled with numerous nauplii; however, no ascothoracid larvae were observed.

Etymology. Named after the type locality, Tanabe, Kii Peninsula, Pacific coast of central Japan.

Remarks. The three-pronged primary branches of the female mantle of Dendrogaster tanabensis sp. nov. link to D. arbusculus and D. okadai (Yosii 1931; Grygier 1982); however, Dendrogaster tanabensis sp. nov. can be easily distinguished from these congeners by the absence of an extra branch opposite to the middle piece (vs. presence in D. arbusculus), and straight, not dichotomous secondary branches of the central primary branches (vs. dichotomous in D. okadai).

Dendrogaster astericola Knipovich, 1890, the type species of this genus, is known from another echinasterid sea star, Henricia sanguinolenta (O.F. Müller, 1776) (Knipovich 1890; Wagin 1950, 1976). However, Dendrogaster tanabensis sp. nov. differs from D. astericola by the large body of females (vs. small, 8–11 mm, in D. astericola), and the well-developed ramification of the female mantle (vs. rudimentary in D. astericola). Three other echinasterid sea stars, Echinaster purpureus (Gray, 1840), E. stereosomus Fisher, 1913b, and E. luzonicus (Gray, 1840), are known hosts of Dendrogaster ludwigi Le Roi, 1905 (or Dendrogaster sp. cf. ludwigi) (Le Roi 1905, 1907; Grygier 1988, 1996; Yoshimoto et al. 2020).