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 (3 - 4), pp. 141-266 : 161-163

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 .