Placotrochides minuta, Cairns, 2004

Cairns, S. D., 2004, The Azooxanthellate Scleractinia (Coelenterata: Anthozoa) of Australia, Records of the Australian Museum 56, pp. 259-329 : 305-307

Placotrochides minuta n.sp.

Figs. 10E–H

Records/Types. Holotype: Franklin 03/99/D11, AM G16747 . Paratypes: Franklin 03/99/D10, 8, USNM 1008853 ; Franklin 03/99/D11, 31: 29 (including SEM 1022 ), USNM 1008854 and 2, ZMUZ; Franklin 03/99/D12, 8, USNM 1008855 ; Franklin 03/99/D13, 1, USNM 1008856 ; Cidaris I 43–2, 4, MTQ G56406 ; Karubar 7 (Banda Sea), 9, USNM 1008857 . Type Locality: 20°14.49'S 151°47.53'E ( Marion Plateau , Queensland), 342 m.

Description. Corallum (anthocyathus) compressedcylindrical, having rounded thecal faces and edges, the latter almost parallel, diverging at an edge angle of 10–11°. Largest corallum (Cidaris I 43–2) 4.48× 3.14 mm in CD and 10.3 mm in height; the holotype measures 3.67× 2.08 mm in CD and 3.23 mm in height, having a basal scar of 3.22× 2.04 mm. Calice elliptical and usually symmetric, with a GCD:LCD of 1.43–2.00, but in some cases (about 20%) the curvature of one side of the calice has a slightly different radius, leading to an asymmetry of the calicular perimeter. Basal scar elliptical, projecting downward in a V-shape, 3.1– 4.0 mm in greater diameter, and having a GSD:LSD of 1.58– 1.94. The basal scar clearly reveals all 24 septa of the incipient anthocyathus ( Fig. 10H). Basal scar usually not much smaller than calice, sometimes the same or even larger because many coralla undergo a retrenchment of growth 2–3 mm above the scar resulting in a slight reduction of the corallum diameter, above which it gradually expands again. Theca smooth and porcellaneous, covered with closelyspaced, chevron-shaped growth lines that peak at the longitudinal insertion lines corresponding to the S1–2. Flat costae 0.28–0.30 mm wide, separated by very thin (7–8 µm) intercostal striae. Theca white. Anthocaulus unknown.

Septa usually hexamerally arranged in 4 cycles, the fourth cycle never complete, the maximum number of septa observed being 38 arranged: S1–2>>S3>S4. As mentioned above, in some cases the calice is asymmetric, which leads to a slightly different length to each side of the calice and thus a different number of septa on each side; this occasionally also disrupts the hexameral nature of development, resulting in 11, 13, or 14 primary septa (S1– 2). Nonetheless, coralla with a GCD less than 3.45 mm invariably have only 24 septa, those between 3.45 and 3.8 mm in GCD have 1–12 additional S4, usually added in pairs in the end half-systems, but not always in pairs or in those systems; and above a GCD of 3.8 mm most coralla have 32 septa. S1–2 non-exsert, rather narrow, and have very sinuous, smooth axial edges that solidly fuse to the columella lower in the fossa. Faces of S1–2 bear tall (up to 0.11 mm) slender spines but no crests. S3 about one-quarter width of an S1, having laciniate axial edges or simply consist of a disconnected series of tall spines, When present, S4 rudimentary. Fossa of moderate depth, containing an elongate, non-discrete columella composed of a loose fusion of trabeculae.

Remarks. Among the four species in the genus, P. minuta is most similar to P. scaphula , agreeing in almost every character listed in Table 6. Placotrochides minuta differs primarily in size, and in having correspondingly fewer septa at the same CD. Placotrochides minuta is not interpreted as an early ontogenetic stage of P. scaphula because, although the calicular diameter and number of septa usually increases with age, the size of the basal scar remains constant, and is thus probably a better differentiating character for this genus and others transversely dividing genera such as Truncatoflabellum than is calicular diameter. Placotrochides minimus also differs in having highly sinuous septal axial edges.

Etymology. minutus, Latin for “small”, an allusion to the small size of the corallum in relation to congenerics.

Distribution. Banda Sea; off northeastern Queensland; 282– 458 m.