Gregarinidra incrustans (Silén, 1941)

(Fig. 18; Table 17)

Spiralaria incrustans Silén, 1941: 59, fig. 70.

Material examined. Holotype by original designation UPSZTY 2473, Yodomi, Sagami, Japan; depth 100–135 m; from a sandy sea-bottom, encrusting a worm tube with Arthropoma sp. and the base of an erect phidoloporid. Leg. Prof. S. Bock 1914.

Description. Colony encrusting, multiserial, unilaminar (Fig. 18A).

Autozooids oval to elongate oval, longer than wide (mean L/ W 1.65), distinct, separated by shallow grooves (Fig. 18A, B), arranged in longitudinal rows alternating with interzooidal avicularia or irregularly; autozooid shape becoming irregular at the meeting boundary between different lobes of the same colony. Gymnocyst smooth, minimal, in some zooids more visible proximally; cryptocyst finely granular, forming a narrow rim encircling the opesia.

Opesia oval, occupying most of the frontal surface (mean OpL/ZL 0.86), surrounded by 8–14 flattened to cylindrical, often bifurcated spines, 10–35 µm wide, 70–135 µm long (Fig. 18A, B); the distalmost pair of spines more erect, the remaining pairs overarching the frontal membrane, not reaching the zooidal midline; opercular region transversely D-shaped.

Avicularia interzooidal, placed distally to most autozooids, rostrum opesial face teardrop-shaped (Fig. 18B, C); rostrum acutely triangular, 70–125 µm long, raised, directed obliquely distally to either side; mandible same shape as the rostrum with the tip downcurved; crossbar complete.

Ovicells endozooidal, immersed in the proximal half of the avicularium cystid (Fig. 18C); ooecium smooth, cap-like.

Remarks. This species, described as Spiralaria by Silén (1941), was assigned to the new flustrid genus Hippoflustra by Moyano (1972, p. 83) and later transferred to Gregarinidra following Gordon (1984, p. 26). Gordon (1984) noticed that Hippoflustra could be regarded as a junior synonym of Gregarinidra given that, based on its definition, it would have included exactly the same suite of species.

Berning et al. (2021) indicated the genus Gregarinidra as a junior synonym of Hincksina Norman, 1903, the sole feature distinguishing these two genera being the morphology of the avicularian rostrum, acute in Gregarinidra (as in G. incrustans), rounded in Hincksina. Berning et al. (2021) pointed out that the morphology of the rostrum is usually not regarded as important at generic rank, and also observed that flustrid fossil species with acute avicularia have been assigned to Hincksina (not Gregarinidra). Additionally, some species of Hincksina [e.g. H. alice (Jullien in Jullien & Calvet, 1903)] have avicularian rostra with intermediate morphologies. Examination of the type species of Gregarinidra, G. gregaria (Heller, 1867), as well as some support from molecular sequencing is needed to confirm this suggested synonymy.

Genus Sarsiflustra Jullien, 1903