Sympodium subtilis sp. n.

Figs. 12, 13, 14A

Material. Japan. Syntypes: SMNHTAU_ Co _38204, Nagasaki, Danjo Islands, south of Arasesaki, Ojima, NW side (32.051168°N, 128.391857°E), 23 September 2018, 10– 20 m, coll. C. McFadden ; Paratypes: SMNHTAU_ Co _38092 Nagasaki, Danjo Islands, Ojima, SE side (32.038886°N, 128.387278°E), 22 September 2018, 10– 20 m, coll. C. McFadden (molecular sample only); SMNHTAU_Co_38103 Nagasaki, Danjo Islands, rocks west of Yorijima (32.024824°N, 128.364289°E), 23 September 2018, 10– 20 m, coll. C. McFadden (molecular sample only); SMNHTAU_ Co _38104, same details .

Description. The syntypes comprise several irregular encrusting colonies with a delicate thin spreading membrane (<0.5 mm) attached to a bivalve shell (Fig. 12A). The largest colony has a surface area of ca. 15 mm 2. In addition, there are a few small colonies measuring 3–5 mm 2. It was not possible to separate a single colony as a holotype without compromising its integrity and therefore syntypes were assigned. The polyps emerge from the membrane 1–5 mm apart and exhibit various degrees of retraction (Fig. 12B). The few expanded polyps are up to 1.5 mm long and their tentacles are approximately 0.5 mm long. The pinnules are either totally or partially contracted, with a single row of 3–5 pinnules on either side of the tentacle.

The sclerites of the syntypes are ellipsoid platelets, abundant throughout the colony and measuring 0.010 –0.012 x 0.013 –0.015 mm in diameter (Fig.13A). They are composed of densely packed calcite rods whose tips provide a uniform granular appearance to the sclerite surface (Fig. 13B).

Color. The ethanol-preserved syntypes are light gray, almost white.

Etymology. The species’ name, subtilis from the Latin meaning delicate, refers to the delicate encrusting membrane of the species.

Variation. The paratype SMNHTAU_Co_38104 is a small colony measuring only a few mm in size, but otherwise resembles the holotype.

Living features. The live colonies feature polyps arranged in groups (Fig. 14A).

Distribution. Japan.

Remarks. The colonies are characterized by a remarkably thin and delicate spreading membrane which is suggested to be diagnostic for this species. The ethanol-preserved syntypes and paratypes were sequenced and subsequently assigned to MOTU68.