Costaticella gisleni Silén, 1954

(Fig. 31; Table 29)

Costaticella gisleni Silén, 1954: 15, fig. 4.

Material examined. Syntypes LUZM 51, Warnbro beach, Perth, Western Australia; depth 18 m. Leg. Prof. T. Gislén, Australia Expedition 1951–1952, collected 30.11.1951.

Description. Colony erect, branching, jointed, consisting of unizooidal internodes becoming bizooidal at bifurcations (Fig. 31A); joints 35–60 µm in length.

Autozooids rhomboidal, some with pointed distolateral processes (Fig. 31B, C), longer than wide (mean L/ W 1.66), those forming the bizooidal internodes usually shorter and narrower; frontal shield convex with a reduced costal field of 5–8 spines, the distalmost pair forming the proximal margin of the orifice, largest (varying from about 30–40 µm at the mid-line to 50–60 µm distally) and with two lumen pores (about 8 µm in diameter), one at each tip, the other spines narrower (varying from about 5–10 µm at the mid-line to 15–25 µm distally) with a single lumen pore distally (3–6 µm in diameter); lacunae between spines fissure-like; 5–7 intracostal windows (circular, elliptical or drop-shaped), 12–26 µm in maximum diameter, bordering the periphery of the costal field (Fig. 31F, G); a pair of fusiform pore-chamber openings present proximally (95–130 µm long by 15–35 µm wide), one circular to elliptical opening visible laterally at about zooidal mid-length (30–36 µm in diameter), and two pairs of circular openings (18–22 µm in diameter) placed one distolaterally and one distally adjacent to the orifice (Fig. 31C, G).

Orifice bell-shaped with raised distal edge and concave proximal margin, almost as long as wide (mean L/ W 0.97) (Fig. 31D).

Avicularia rhomboidal with triangular rostrum directed distally, placed at the distolateral corners of autozooids, not visible in frontal view, about 55 µm long by 35 µm wide (Fig. 31E, G).

Ovicells not observed.

Remarks. In the description of this species, Silén (1954) refers to “three well developed zoaria” that could not be distinguished in the material studied here. The specimen tube labelled as type contains numerous branches of C. gisleni intertwined with Crisia internodes. In addition, no ovicells were observed although Silén (1954, fig. 4b) described and figured one. These observations suggest that additional syntypes of this species may exist that have not yet been located.

In Australia, the genus Costaticella is represented by five species (Cook et al. 2018), among which C. hastata (Busk, 1852a) is the most similar in general appearance as pointed out first by Silén (1954) and later by Gordon (1989). However, the two species differ in several traits such as the extension of the costal field (occupying almost the entire length of the autozooidal frontal in C. hastata and one-third to half of the frontal shield in C. gisleni), in the number of spines composing the costal field (9–10 in C. hastata, 5–8 in C. gisleni), and seemingly in the characters of the fertile segment which, unfortunately, was not observed in the studied specimens. Both Silén (1954) and Gordon (1989) stated that C. gisleni also differs from C. hastata in having invariably five intracostal windows (4–5 in C. hastata) but this seems incorrect. Although C. gisleni shows at least, and more commonly, five intracostal windows, the number can increase up to seven (Fig. 31F, G).