Cranosina coronata ( Hincks, 1881 )

Dick, Matthew H. & Grischenko, Andrei V., 2016, Rocky-intertidal cheilostome bryozoans from the vicinity of the Sesoko Biological Station, west-central Okinawa, Japan, Journal of Natural History 51 (3 - 4), pp. 141-266 : 163-164

Cranosina coronata ( Hincks, 1881)

( Figure 6 (e, f))

Membranipora coronata Hincks, 1881, p. 147 , pl. 10, fig. 1.

Setosellina coronata: Harmer 1926, p. 265 , pl. XVI, figs 2–4.

For other synonyms and records, see Harmer (1926) and Tilbrook (2006).

Material examined

NSMT-Te 1065 ( MIN- 3), bleached, on SEM stub; NSMT-Te 1066, five dried specimens, SES site; NSMT-Te 1067, three dried specimens, REEF site.

Measurements

AzL, 0.53–0.72 (0.614 ± 0.050); AzW, 0.39–0.50 (0.430 ± 0.035). OpL, 0.36–0.53 (0.419 ± 0.044); OpW, 0.20–0.28 (0.232 ± 0.025). AvRosL, 0.16–0.25 (0.198 ± 0.023). (All n = 15, 1). Largest colony observed 10 mm across.

Description

Colony forming an irregular, unilaminar, encrusting sheet; off- white in colour. Zooids distinct, hexagonal, separated by thin incision. Opesia large, widest in middle or proximal third. Cryptocyst broad, sloping with coarse granulation organised in irregular rows perpendicular to opesial margin, not extending around orifice; cryptocystal texture completely covers tapering proximal end of zooids. Distal end of zooid raised as a smooth gymnocystal cowl around orifice. Interzooidal avicularium distal to each zooid; rostrum tilted in proximal direction and pointing laterally or distolaterally; wide, prominent hinge denticles, but no complete hinge bar; mandible (not shown) long, filiform, sometimes exceeding width of zooid. Spines and ovicells lacking. Ancestrula not observed.

Remarks

Species in this genus lack ooecia and brood embryos in an internal sac ( Harmer 1926; Ostrovsky et al. 2009; Ostrovsky 2013). Interestingly, apparently as a means of regeneration following the internal destruction of a zooid by predation, many zooids in our specimens have an intramurally budded cystid, with a secondary sloping cryptocyst laid down inside and overlapping the first (with the two cystids delineated by a suture line between the cryptocysts and a gap between the respective smooth distal margins).

Tilbrook (2006, p. 25, pl. 2E) noted that the interzooidal avicularium in this species has the mandible directed proximolaterally, and illustrates this condition in a specimen from the Solomon Islands. In our specimen, the long axis of the rostrum points laterally or, more often, slightly distolaterally, an orientation also figured by Harmer (1926, pl. XVI, fig. 3), Ryland and Hayward (1992, fig. 2(e)) and Liu et al. (2001, pl. 18, fig. 5).

Occurrence

This species occurred at all three sites, but was most abundant at SES ( Table 1). Cranosina coronata is quite common in the Into-West Pacific ( Tilbrook 2006). In Japan, it has been previously reported from the Pleistocene Ryukyu Limestone, northern Nansei islands ( Kataoka 1961) and the Pliocene Shinzato tuff, Shimajiri Formation, Okinawa ( Hayami 1971; Sakagami et al. 1980).