Amphiblestrum crassispinosum ( Silén, 1954 )

Martino, Emanuela Di, 2023, Scanning electron microscopy study of Lars Silén’s cheilostome bryozoan type specimens in the historical collections of natural history museums in Sweden, Zootaxa 5379 (1), pp. 1-106 : 11-13

Amphiblestrum crassispinosum ( Silén, 1954)

( Fig. 2 ; Table 3)

Rhamphonotus crassispinosus Silén, 1954: 10 , fig. 3.

Material examined. Lectotype (designated here) LUZM 50 a, the largest colony among the syntype material, off Rockingham , south of Garden Island , Western Australia; depth 9–18 m. Leg. Prof. T. Gislén, Australia Expedition 1951–1952, collected 4.1.1952 . Paralectotype: the remaining colony LUZM 50 b.

Description. Colonies encrusting algae, the largest colony 3 × 3.5 mm in size.

Autozooids hexagonal, longer than wide (mean L/ W 1.27), flat, distinct, separated by grooves ( Fig. 2A ). Gymnocyst extensive proximally (120–160 µm), smooth, forming medially a suboral mucro lodging the avicularium and a raised rim at its limit with the adjacent cryptocyst; cryptocyst slightly depressed in respect to the gymnocyst, granular, extended for 50–90 µm.

Opesia bell-shaped with pointed triangular lateral constrictions and slightly concave proximal margin ( Fig. 2B ); a pair of robust, club-shaped spines placed laterally at level with the lateral constrictions, 90–160 µm long and with basal diameter 25–30 µm and tip diameter 35–50 µm, persisting in ovicellate zooids ( Fig. 2C ); a stout, gymnocystal mucro developed suborally and medially, 110–130 µm long and with basal diameter 70–90 µm and tip diameter about 30 µm ( Fig. 2C, D ).

Avicularium adventitious, placed on the suboral mucro, seemingly oval ( Fig. 2D ).

Ovicells squared, prominent, convex frontally and indented distally by the suboral mucro of the next zooid in the row, closed by the operculum ( Fig. 2B–D ); ectooecium smooth, partially calcified, sometimes developing a blunt umbo medially and leaving proximally a semielliptical to bell-shaped fenestra exposing the granular endooecium ( Fig. 2B ).

Remarks. This species, originally described as Ramphonotus , is currently accepted as Amphiblestrum , the main recognized difference between the two genera being the ectooecium, completely calcified in Ramphonotus and partially calcified in Amphiblestrum ( Bishop & Hayward 1989) . However, the boundary between these two genera remains ambiguous. It has been observed, for example, that in colonies of Amphiblestrum , e.g. A. lyrulatum ( Calvet, 1907) , the extent of the ectoooecium calcification can vary, sometimes even appearing uniformly calcified ( López de la Cuadra & García-Gómez 1994; Di Martino et al. 2022, fig. 3).

The ‘prickly’ appearance of this species is typical of taxa encrusting ephemeral, flexible substrates such as algae and seagrass leaves. The presence of thick, long, latero-oral spines and the development of a stout suboral mucro and sometimes of an umbo on the ovicell is likely a protection used to reduce colony damage consequent to friction between fronds ( Di Martino & Rosso 2021).

Genus Cauloramphus Norman, 1903

Figures

FIGURE 2. Amphiblestrum crassispinosum (Silén, 1954). Lectotype (designated here) LUZM 50a, Western Australia. A. General view of the colony encrusting an alga. B. Close-up of the opesia, with the two robust distolateral spines, and the ooecium. C. Tilted view of ovicellate zooids showing the length of the spines and the aviculiferous suboral mucro, broken in some zooids. D. Frontal view of ovicellate zooids showing the suboral aviculiferous mucro of the distal zooid resting on the ooecium. Scale bars: A = 500 µm; B = 150 µm; C, D = 250 µm.