Crassimarginatella extenuata ( Dick, Tilbrook, and Mawatari, 2006 )

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, pp. 141-266 : 161-164

Crassimarginatella extenuata ( Dick, Tilbrook, and Mawatari, 2006) ( Figure 6 (a – d))

Corbulella extenuata Dick, Tilbrook, and Mawatari, 2006, p. 2207 , Figure 4 (a – d).

Material examined

NSMT-Te 1064 ( SES- 41), bleached, on SEM stub.

Measurements

AzL, 0.53 – 0.65 (0.586 ± 0.047); AzW, 0.35 – 0.49 (0.387 ± 0.048) (n = 7,1). OpL, 0.32 – 0.39 (0.346 ± 0.028); OpW, 0.24 – 0.29 (0.263 ± 0.024) (n = 7, 1).

Description

One small, periancestrular colony observed, containing 25 zooids, with only about seven zooids in zone of astogenetic repetition ( Figure 6 (a)); forming a unilaminar, encrusting sheet; light yellowish-tan. Zooids distinct, delineated by a groove. Gymnocyst smooth, sloping, well exposed proximally, tapering laterally. Opesia oval, widest in middle or proximal third, occupying two-thirds to three-quarters of frontal area; distal margin straight ( Figure 6 (d)). Cryptocyst well developed, coarsely granulated, steeply sloping; widest proximally, tapering laterally, narrowest distally, but complete around straight distal margin of opesia. Zooids distally with small, low, smoothly rounded gymnocystal cap in midline. Six to nine coarse, hollow spines (modal number, 8; n = 7) around opesial margin ( Figure 6 (a, b)), including usually two pairs of orificial spines; spines erect or slightly tilted inward, reaching nearly as long as opesial width. Ancestrula ( Figure 6 (c, d)) of same form as subsequent zooids, but smaller, with three orificial and five opesial spines; connecting with six periancestrular zooids, of which proximal three are larger than distal three. Zooids interconnect ( Figure 6 (b)) via broad pore chamber with around five pores in transverse wall, and two pore chambers in each distolateral wall, each with two to four pores.

Remarks

The generic assignment of this species, originally described from Hawaii ( Dick et al. 2006) as Corbulella extenuata , is problematic (see also remarks for Cr. eremitica above). A presumed vicarious avicularium in the holotype specimen appears to lacks spines and to lack serration on the rostral rim, indicative of Crassimarginatella rather than Corbulella ( Gordon 1984) . The nature of the ovicell is unclear. In our small specimen from Okinawa, the cap-like structure at the distal end of the zooid, which Dick et al. (2006) interpreted as a vestigial ooecium, is present in the ancestrula and periancestrular zooids. The cap in the ancestrula bears a median spine base, whereas in other zooids it bears neither a spine nor a pseudopore, suggesting it is simply a swelling in the distal zooidal wall. It may be that this species produces prominent ovicells, which are simply absent in the two specimens of this species found to date.

Occurrence

We found a single small colony at the SES site. Only two small colonies of this species have been reported, one at Hawaii and one at Okinawa; the currently known distribution is the subtropical, central to western North Pacific .

Genus Cranosina Canu and Bassler, 1933

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).

Figures

Figure 4. (a–e) Thalamoporella karesansui sp. nov.: (a) NSMT-Te 1060 (holotype), autozooids, vicarious avicularium and ovicelled zooids; (b) NHMUK 2016.5.13.7 (paratype), vicarious avicularia and surrounding autozooids; (c) SES-specimen A, basal surface of dried colony, showing basal insertions; (d) NHMUK 2016.5.13.7 (paratype), spicules removed by bleaching, including small C-shaped callipers and medium-sized, slightly angled compasses; asterisks mark sponge spicules; (e) NHMUK 2016.5.13.7 (paratype), interzooidal connections; (f) Thalamoporella stapifera (Levinsen), NSMT-Te 1055, interzooidal connections. a, b, e, f, scanning electron microscopic images of bleached material; c, d, photomicrographs. Scale bars: a, c = 500 µm; b = 250 µm; d = 100 µm; e, f = 150 µm.

Figure 6. (a–d) Corbulella extenuata Dick, Tilbrook, and Mawatari, NSMT-Te 1064: (a) autozooids; (b) oblique view of colony margin showing interzooidal connections and presumed vestigial ooecium at early stage of formation (far top right); (c) ancestrula and periancestrular zooids; (d) same ancestrula (asterisk) as in panel (c) after bleaching, with daughter zooids lost from left side. (e, f) Cranosina coronata (Hincks), NSMT-Te 1065: (e) autozooids (the central three showing regenerative, intramurally budded cystids); (f) autozooids at colony margin (central zooid with intramurally budded cystid). Panels are scanning electron microscopic images of dried (a, c) or bleached (b, d–f) specimens. Scale bars: a = 250 µm; b–d = 300 µm; e, f = 400 µm.