Antropora erecta Silén, 1941

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 : 25-27

Antropora erecta Silén, 1941

( Fig. 10 ; Table 11)

Antropora erecta Silén, 1941: 44 , figs 56, 57.

Material examined. Holotype by original designation UPSZTY 2455 , Bonin Islands (Ogasawara), Takino Ura, Japan; depth 45–60 m. Leg. Prof. S. Bock 1914.

Description. The holotype colony Y-shaped and tube-like, probably due to having encrusted an organic substrate such as a seagrass stem, multiserial, unilaminar.

Autozooids pentagonal, hexagonal or rhomboidal with rounded, slightly raised distal margin ( Fig. 10A ), longer than wide (mean L/ W 1.68), distinct, separated by shallow furrows, quincuncially arranged; zooidal shape and arrangement becoming irregular at colony bifurcations, likely owing to the type of substratum encrusted ( Fig. 10D ). Gymnocyst minimal, sometimes visible proximally; cryptocyst extensive, occupying half of the frontal surface, sloping laterally, depressed and flat centrally, granular with granules 2–10 µm in diameter ( Fig. 10B ).

Opesia rounded triangular, occupying the distal half of the frontal surface (mean OpL/ZL 0.49); operculum transversely D-shaped, about 100 µm in length ( Fig. 10A, B ).

Avicularia interzooidal, one or two placed distolateral to each autozooid, teardrop-shaped, rostrum triangular with acute tip directed distally, raised ( Fig. 10B, C ); mandible same shape as the rostrum articulated on two small condyles. Vicarious avicularia not observed.

Ovicells endozooidal; ooecium formed by the distal zooid, exposed frontal area extremely reduced, cap-like, smooth surfaced ( Fig. 10C ).

Remarks. Although Silén (1941) acknowledged that this species/specimen grew around an ephemeral substrate that shaped it as cylindrical and hollow, leaving organic traces on the zooidal underside, he described it as erect. From the study of the holotype, the tube-like form of the colony points to the incrustation of an organic stem that modelled the shape of the colony, as observed in other encrusting species ( Di Martino & Taylor 2014a, fig. 4E, F), which however never developed erect branches independently from the substrate. The erect colony form was the main character distinguishing A. erectirostra Tilbrook, 1998 from Ceylon from this species. Both species have small interzooidal avicularia with pointed triangular and raised rostra directed distally. Antropora erectirostra has also rare vicarious avicularia but given their uncommonness, their absence in A. erecta would not exclude their conspecificity. However, another distinguish character is the surface of the ooecium, smooth in A. erecta and granular in A. erectirostra (see Tilbrook 1998, fig. 3C). Given their small size, ooecia were not observed/described by Silén (1941) but they are indeed present in several zooids of the holotype (some indicated by arrows in Fig. 10A ).

Comparing the mean values of size measurements between the two species, lengths are slightly smaller in A. erecta (ZL 0.45 vs 0.50 mm; OpL 0.22 vs 0.25 mm; AvL 0.10 vs 0.13 mm in A. erecta and A. erectirostra , respectively), while widths are slightly larger (ZW 0.27 vs 0.24 mm; OpW 0.20 vs 0.16 mm in A. erecta and A. erectirostra , respectively).

These morphological and morphometric differences, along with their separate geographic origins, lend strong support to the validity of both species.

Figures

FIGURE 10. Antropora erecta Silén, 1941. Holotype UPSZTY 2455, Bonin Islands, Japan.A. Group of zooids, some ovicellate (arrowed), and interzooidal avicularia. B. Close-up of two zooids, one ovicellate, and interzooidal avicularia. C. Close-up of an ovicell and two interzooidal avicularia. D. Irregularly shaped zooids at colony bifurcation. Scale bars: A, D = 200 µm; B = 120 µm; C = 50 µm.