Rhabderemia ginamaensis, Ise & Tajiri & Mizuyama & Fujita, 2025

Ise, Yuji, Tajiri, Rie, Mizuyama, Masaru & Fujita, Yoshihisa, 2025, Submarine cave sponges of the genus Rhabderemia (Demospongiae, Biemnida, Rhabderemiidae) from the Ryukyu Islands, southwestern Japan: rediscovery of R. forcipula and description of a new species, Zootaxa 5569 (2), pp. 365-382 : 367-371

Rhabderemia ginamaensis sp. nov. Ise, Mizuyama & Fujita

Figs. 1–3 ; Table 1

Materials examined. Holotype: NSMT-Po- 2509 , inside of submarine cave “Hedo Dome” , Ginama , Kunigami-son Village , Okinawa Island, the Ryukyu Islands, Japan, 12 m depth, 11 August 2016, coll. by Y. Ise, M. Mizuyama and Y. Fujita, SCUBA.

Description of holotype. External morphology ( Figs. 1A, B ). Very thinly encrusting, covering the substrate ( Fig.1A ), ca. 400 µm thick in the preserved state. Surface with a thin membrane, semi-transparent in life due to the presence of inflated subdermal canals ( Fig.1B ) assembling into oscula forming an astrorhiza that shrinks when fixed. Color pale yellow-green to beige when alive, beige in ethanol.

Skeleton ( Fig. 1C ). Skeletal structure hymedesmioid. Choanosomal larger rhabdostyle erect, singly or in bundles, perpenticularly penetrating to the surface with tips outwardly directed and blunt base embedded on the base. Thickness of sponge tissue less than length of chonosomal rhabdostyle. Ectosomal region unspecialized. Microscleres scattered throughout the sponge tissue.

Spicules ( Figs. 2 , 3 ). Rhabdostyles as megascleres ( Fig. 2 ). Microstyles, spirosigmas and microxeas as micrscleres ( Fig. 3 ). Rhabdostyles, shaft straight, surface usually sparsely spined, smooth around the base. Tip sharply pointed, base usually single spiral or slightly curved ( Fig. 2 ). Size in two categories. Larger rhabdostyles ( Fig. 2A ), 392.2–(436.8)–510.7 µm in length, 10.7–(14.7)–17.8 µm in shaft width. Smaller rhabdostyles in wide size range ( Fig. 2B ), 81.6–(212.1)–325.7 µm in length, 6.7–(12.1)–16.8 µm in shaft width. Rugose microstyles ( Fig. 3A ) curved, covered by fine spines ( Fig. 3B ). Size, 20–30 µm in length, 0.8–1.0 in base width, 0.5–0.8 µm in shaft width. Spirosigmas in two size classes ( Figs. 3C, D ). Larger spirosigmas ( Fig. 3C ), contorted, covered with fine spines. Size, 14.0–18.0 µm in total length, 1.2–1.8 µm in shaft width. Smaller spirosigmas ( Fig. 3D ), contorted, slightly centrotylote, usually rough and covered with tiny spines, except at centrotylote midpoint. Size, 9.0–11.0 µm in total length, 0.4–0.8 µm in shaft width. Microxeas ( Fig. 3E ) straight, sharply pointed at both tips. Size, 40–50 µm in length.

Etymology. The species epithet refers to its type locality: Ginama.

Origin of Japanese name. A new Japanese vernacular name: Ginama-kiseru-kaimen is proposed here. “Ginama” is its type locality. “Kiseru” is a smoking pipe in Japanese, derived from the shape of the rhabdostyle. “Kaimen” is sponge in Japanese. The Japanese names “Kiseru-kaimen-zoku” is proposed for the genus Rhabderemia (“zoku” is genus in Japanese) and “Kiseru-kaimen-ka” for the family Rhabderemiidae (“ka” is family in Japanese).

Ecology. The type specimen was collected from the submarine cave “Hedo Dome” (Ginama, Kunigami-son Village ), located near the northernmost point of Okinawa Island, the Ryukyu Islands , Japan. Its entrance opens at the depth of 15–18 m and extends into the underground for about 53 m ( Osawa & Fujita, 2019). Most of the inner part of the cave is under anchialine habitat, where the species was found, and the type specimen was found covering the stalactite. No other specimens have been found from other submarine caves around Okinawa.

Remarks. The presence of rhabdostyles, rugose microstyles, and spirosigmas best placed our specimen in the genus Rhabderemia . To date, thirty species have been recorded for this genus ( de Voogd et al., 2024). Of these, our specimen resembles R. acanthostyla Thomas, 1968 (sensu Van Soest & Hooper, 1993) and R. forcipula in the presence of rhabdostyles, microstyles and two types of spirosigmas (for morphological variation of the sigmoid microscleres, see remarks on R. forcipula in this paper).

Rhabderemia acanthostyla was originally described from the Gulf of Mannar, the Indian Ocean ( Thomas, 1968), and later found in Indonesia and Vietnam ( Van Soest & Hooper, 1993). Van Soest & Hooper (1993) attributed their materials to the species based on the presence of “two sizes” of rhabdostyles. However, their interpretation may be incorrect because the rhabdostyles of R. acanthostyla did not occur in two sizes but in two morphological types: rare smooth rhabdostyles and abundant spiny rhabdostyles (“acanthorhabdostyles” in Thomas, 1968). Furthermore, Thomas (1968) illustrated distinctive curved oxeote rhabdostyles of both types of rhabdostyles, which are not mentioned by Van Soest & Hooper (1993). This latter study has reported also two sizes of “contorted sigmata” and microstyles, not mentioned by Thomas (1968), which they considered to have been overlooked by Thomas (1968). As Van Soest & Hooper (1993) did not observe the type specimen, the records of R. acanthostyla from Indonesia and Vietnam by Van Soest & Hooper (1993) are possible different species from those of the Indian Ocean.

Although R. acanthostyla differs completely from R. ginamaensis sp. nov. by the absence of microxeas, the specimens identified as R. acanthostyla by Van Soest & Hooper (1993) resemble R. ginamaensis sp. nov. by the presence of two sizes of rhabdostyles, two types of spirosigmas, and the sizes of microstyles and spirosigmas ( Table 1 ). However, R. acanthostyla sensu Van Soest & Hooper (1993) lacks microxeas and the dimensions of the rhabdostyles are much smaller than those of R. ginamaensis sp. ( Table 1 ). The color in life is an additional distinguishing feature: lighter or darker brown in Van Soest & Hooper (1993) vs. pale yellow-green to beige in R. ginamaensis sp. nov. which may reflect the difference in habitat that R. acanthostyla sensu Van Soest & Hooper (1993) grows in the open reef at 1–10 m depth, whereas R. ginamaensis sp. nov. lives in the inner part of an anchialine submarine cave in total darkness with a low salinity of about 28 ppt ( Osawa & Fujita, 2019).

Rhabderemia forcipula , redescribed in this study, is also comparable and considered a close species to R. ginamaensis sp. nov. based on its spicule composition, taking into the account the possible morphological variation of larger sigmoid microscleres. However, the external morphology is completely different: massive to erect in R. forcipula and thinly encrusting in R. ginamaensis sp. nov. The sizes of the individual spicule types, rugose microstyles and spirosigmas, are much larger in R. forcipula . In addition, R. forcipula lacks microxeas.

Figures

FIGURE 1. Habit and skeletal structure of Rhabderemiaginamaensis sp. nov., holotype (NSMT-Po-2509).A, habit in life shows thin encrusting specimen on a stalactite. B, precise view of the surface shows semitransparent subdermal canals (arrowheads). C, perpendicular section of skeleton shows hymedesmioid structure. Scale bars: A, 10 mm; B, 3 mm; C, 100 µm.

FIGURE 2. Megascleres of Rhabderemia ginamaensis sp. nov., holotype (NSMT-Po-2509).A, larger rhabdostyles. B, smaller rhabdostyles. Scale bar: 50 µm.

FIGURE 3. Miscroscleres of Rhabderemia ginamaensis sp. nov., holotype (NSMT-Po-2509). Scale bars: A, microstyles. B, magnified view of base of microstyles, showing rough surface. C, larger spirosigma. D, smaller spirosigma. E, microxea. Scale bars: A, 5 µm; B, 1 µm; C, D, 2 µm; E, 10 µm.