Setosellina constricta Harmer, 1926

Martino, Emanuela Di & Taylor, Paul D., 2018, Early Pleistocene and Holocene bryozoans from Indonesia, Zootaxa 4419 (1), pp. 1-70 : 12

Setosellina constricta Harmer, 1926

( Figs 19–22 ; Table 7)

Setosellina constricta Harmer, 1926: 264 , pl. 16, fig. 1.

Figured material. RGM.1350548, RGM.1350549, RGM.1350550, Holocene, UPGG 041, off South Sulawesi.

Description. Colony encrusting sand grains, 1–4 mm in diameter, frequently enveloping the substrate. Ancestrula subcircular to elliptical, smaller and with a narrower cryptocyst than budded autozooids, the periancestrular vibracula and daughter zooids arranged in a clockwise spiral pattern. Pore-chamber windows absent. Autozooids distinct, bordered by grooves, oval, elongate (mean L/W = 1.46). Gymnocyst minimal, smooth, convex. Cryptocyst very thin proximally and laterally, disappearing distally, sloping steeply inwards, markedly transversely crenulate. Opesia occupying almost the entire frontal surface, oval, pear-shaped or irregular, narrower distally. Vibracula globular to oval, present distally of each autozooid; opesia occupying almost the entire frontal surface, figure of 8-shaped, constricted in the middle by two small rounded teeth. Ooecia presumed absent. Kenozooids marginal, terminal, budded on the underside of the substrate, irregularly shaped; gymnocyst variably developed, cryptocyst crenulate, forming a depressed shelf with a small, circular or slit-like, central opening. Intramural buds common in both autozooids and vibracula.

N, Number of colonies and number of zooids measured; SD, standard deviation.

Remarks. Two hundred and two colonies of Setosellina constricta were found in our samples, encrusting different types of sand grains, clastic and bioclastic, such as foraminifera, bivalve shells and erect bryozoan fragments. Despite the large number of specimens found, none has ooecia, which are also lacking in the type and non-type material available in the zoological collection of the NHMUK from the Siboga Expedition (NHMUK 1928.3.6.75 and 76), and used here for comparison ( Figs 23–24 ). No ooecia were observed either in the large suite of colonies of S. cf. constricta available from the Miocene (Serravallian) of East Kalimantan ( Di Martino & Taylor 2014). In the genus Setosellina ovicells can be present or absent ( Ostrovsky 2013). Given the large number of specimens observed, we presume that the lack of ooecia in this species is real and not a sampling bias and that very likely S. constricta is an internal brooder. Setosellina constricta is easily distinguished from all other congenerics in the absence of pore chambers, the very thin lateral cryptocyst, and the lack of closing laminae, which usually develop very early in astogeny, in the older, central zooids. The development of marginal, terminal kenozooids on the underside of the encrusted substrate in S. constricta is observed here for the first time. Similar marginal, terminal kenozooids were figured by Canu & Bassler (1929, pl. 7, fig. 4) in Vibracellina crassatina Canu & Bassler, 1929 . At the present day, Setosellina constricta is reported from the Borneo Bank in the Makassar Strait, New Guinea and the Celebes Sea at 0–59 m depth.

Figures

FIGURES 19–22. Setosellina constricta Harmer, 1926, Holocene, UPGG041, off South Sulawesi. 19, 20. RGM.1350548. 19. General view of a colony encrusting a sediment grain. 20. Close-up of part of the colony, including ancestrula (An), periancestrular vibracula and zooids, and intramural buds in some zooids. 21. RGM.1350549, group of zooids and associated vibracula, two of which with intramural buds. 22. RGM.1350550, marginal kenozooids on the underside of the substrate. Scale bars: Fig. 19 = 500 µm; Figs 20–22 = 200 µm.

FIGURES 23, 24. Setosellina constricta Harmer, 1926, Recent, Malay Archipelago. 23. NHMUK 1928.3.6.75 (part of type material), Siboga Expedition Station 77 (59 m depth), Borneo Bank, Makassar Strait, young colony with opercula and setae. 24. NHMUK 1928.3.6.76, Siboga Expedition Station 102 (18 m depth), off Loslos Island, north end of New Guinea, general view of a colony with ancestrula (An) and associated periancestrular vibracula. Scale bars: 200 µm.