Dendrogaster danni Saito & Wakabayashi, 2024

Dendrogaster danni Saito & Wakabayashi sp. nov.

[New Japanese name: Dann-no-shidamushi]

( Figs. 1B, C View FIGURE 1 , 2 View FIGURE 2 )

Material examined. Holotype: larvigerous female (mantle size 66.4 mm), NSMT-Cr 31582 , removed on 18 December 2020 from Neoferdina japonica Oguro & Misaki, 1986 (Echinodermata: Asteroidea: Valvatida : Goniasteridae ) (R 61.0 mm, r 14.2 mm), TAMBL-EC 35 ( Fig. 1A View FIGURE 1 ), off Minabe, Wakayama Prefecture , southwestern Kii Peninsula, Pacific coast of central Japan, approximately 100 m in depth, coll. I. Dann on 13 December 2020.

No males were found from the holotype female.

Description of holotype female. Carapace (or mantle) markedly branched, ramified three-dimensionally; surface smooth ( Fig. 2A, B View FIGURE 2 ). Middle piece elliptical, approximately 2.3 times as long as wide, but more elongated when alive ( Figs 1B View FIGURE 1 , 2C View FIGURE 2 ). Pair of main branches directed laterally from base of middle piece, dividing into 2 (anterior and posterior) outspread primary branches. Each anterior primary branch dividing dichotomously into secondary branches and dividing dichotomously into tertiary branches, secondary and tertiary branches in turn giving off alternately placed branches on each side with terminal protuberances. Posterior primary branches dividing dichotomously into secondary branches, secondary branches in turn giving off alternately placed tertiary branches on each side with terminal protuberances.

Antennules 4-segmented and subchelate ( Fig. 2D View FIGURE 2 ). First segment missing. Second segment rectangular. Third segment elliptical, 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 rudimentary proximal sensory process; bearing a pair of setae at dorsal and ventral base of terminal claw, 3 distal setae on claw guard, and 1 seta and 1 small aesthetasc on proximal sensory process.

Oral cone formed by labrum ensheathing maxillae. Maxillule and mandibles absent. Maxillae ( Fig. 2E View FIGURE 2 ) harpoon-like, right and left members of pair fused medially for most of their length but separated at distal end; apex bifid, with straight distal prong and ventral hooks reflected backward.

Rudiments of thoracopods absent.

Nauplius (orthonauplius) ( Fig. 2F View FIGURE 2 ) with bowl-shaped carapace, oval in dorsal view, 0.78× 0.49 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. 2G View FIGURE 2 ) uniramous with 4 indistinct segments, bearing 3 terminal and 2 distal setae. Antennae ( Fig. 2H View FIGURE 2 ) biramous; exopod with 12 indistinct segments, bearing 2 terminal setae, and 1 long seta on each of distal 6 segments; endopod with 7 indistinct segments, 3 setae at distal end. Mandibles ( Fig. 2I View FIGURE 2 ) biramous; exopod with 10 indistinct segments, bearing 2 terminal setae, and 1 long seta on each of distal 4 segments; endopod with 4 indistinct segments, bearing 1 seta at distal end, and 1 seta at distal segment. Labrum pointed ventrally.

Coloration. In living female specimen, mantle white-to-cream yellow ( Fig. 1C View FIGURE 1 ).

Genetic information. K2P distances of the partial COI gene sequences determined from the holotype (NSMTCr 31582, GenBand No. PP118258) was 0.149 –0.375 from the other 7 species of Dendrogaster ( Table 1 View TABLE 1 ).

Ecological note. The host sea star N. japonica ( Valvatida : Goniasteridae ), infested with Dendrogaster danni sp. nov. in its coelomic cavity, was housed in an aquarium for ornamental purposes. The dorsal body wall of the host sea star collapsed when the parasite emerged from the host. At that time, Dendrogaster danni sp. nov. was exposed to the water ( Fig. 1B View FIGURE 1 ). A single female parasite infested the coelomic cavity at the base of an arm of the host sea star. The main branch of the female mantle was filled with numerous nauplii, but no ascothoracid larvae were observed.

Etymology. The specific name is the Latin genitive danni selected to honour the collector of the type material, Mr. Ichinosuke Dann, an amateur echinodermologist. He was an elementary school student when he collected the specimen.

Remarks. The ramification pattern of the female mantle of Dendrogaster danni sp. nov., dividing dichotomously the main branch into primary, secondary, and tertiary branches is shared with three other congeners: Dendrogaster dichotoma Wagin, 1950 from Crossaster papposus ( Linnaeus, 1767) ( Solasteridae ), D. punctata Grygier, 1982 from Poraniopsis inflata ( Fisher, 1906) ( Poraniidae ), and D. adhaerens from Certonardoa semiregularis ( Müller & Troschel, 1842) ( Ophidiasteridae ) ( Wagin 1950; Grygier 1982; Yoshimoto et al. 2020). Dendrogaster danni sp. nov. can be clearly distinguished from these species by quaternary branches arising off alternately on each side of anterior tertiary branches. Namely, quaternary branches arising bifurcations or irregular in D. punctata and D. adhaerens , and dividing dichotomously in D. dichotoma .

In this study, we report the discovery of a new species of Dendrogaster that infects a Japanese goniasterid sea star. Recently, four goniasterid species from Japan ( Lithosoma japonica Hayashi, 1952 , Mediaster arcuatus ( Sladen, 1889) , M. brachiatus Goto, 1914 , and Nymphaster euryplax Fisher, 1913 ) have been reported as hosts for three species of Dendrogaster ( D. komatsuae Saito, Wakabayashi, & Moritaki, 2020 , D. tobasuii , and D. nagasakimaruae Saito, Wakabayashi, & Moritaki, 2020 ) ( Saito et al. 2020). In addition, two species of goniasterid sea stars from North American waters are known hosts of Dendrogaster : Hippasteria californica Fisher, 1905 infected with D. arbusculus Fisher, 1911 (q.v., Grygier 1982) and H. phrygiana ( Parelius, 1768) infected with Dendrogaster sp. ( Hamel & Mercier 1994, 1999).