Plesiocleidochasma clandestinum

Yang, Ho Jin, Seo, Ji Eun & Gordon, Dennis P., 2018, Sixteen new generic records of Korean Bryozoa from southern coastal waters and Jeju Island, East China Sea: evidence of tropical affinities, Zootaxa 4422 (4), pp. 493-518: 512-513

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Plesiocleidochasma clandestinum

n. sp.

Plesiocleidochasma clandestinum  n. sp.

( Figs 37–39View FIGURES 37–39)

Etymology. Latin clandestinus, secret, concealed, hidden, alluding to the ooecium, which is difficult to see in mature parts of the colony.

Material examined. Holotype: NIBRIV0000805895, one of two colonies found at Munseom Island, Seogwipo , Jeju Island, 24 December 2016, 20 m, on oyster shell, collected by Ho Jin Yang. 

Description. Colony encrusting, multiserial, unilaminar, very pale orange, 5 mm across. Autozooids more or less subhexagonal to subpentagonal, boundaries seen clearly only near growing edge. Frontal shield porcellanous, initially smooth, becoming pimpled with small scattered points, a suboral elevation present in many zooids, with 4–5 small areolar-septular pores near margin, these persisting in older zooids, with thin circular rim around each pore. Orifice somewhat cleithridiate, longer than wide, anter with strongly sloping shoulders proximally, leading to drop-shaped sinus delimited by stout downcurved condyles. Ephemeral short oral spines 2–4, only in zooids at colony margin.

Avicularia single, not on all zooids, borne not quite midlaterally, directed mostly laterally, rostrum triangular with slightly irregular triangular foramen owing to crenulated edges, crossbar well developed, with small semicircular opesial foramen and short, steeply descending smooth cryptocyst.

Ooecium inconspicuous, immersed, forming low bulge in proximal end of distal zooid; proximofrontal area of ooecium develops last, leaving small oval foramen in frontal surface; lines marking fronts or boundaries of thin secondary calcification from adjacent zooids can cross ooecial surface, confusing its outline.

Thin pore-chambers seen in developing zooids. Ancestrula not seen.

Measurements. ZL 243–375 (300) µm; ZW 140–248 (195) µm; OrL 105–114 (109) µm; OrW 53–72 (63) µm; AvL 51–72 (61) µm; AvW 33–43 (37) µm; OoL 64–72 (67) µm; OoW 62–120 (94) µm.

Remarks. Eleven described Recent species can with some confidence be included in Plesiocleidochasma ( Bock 2017)  . All in which the ooecium has been described have a labellum (very short in Plesiocleidochasma septemspinosa Tilbrook, 2006  ), which is lacking in P. clandestinum  n. sp. Three of these species have an ooecial foramen, but it is unlike that in P. clandestinum  n. sp.; it is transient in Plesiocleidochasma mediterraneum Chimenz Gusso & Soule, 2003  and a transverse slit in Plesiocleidochasma porcellaniforme ( Soule, Soule & Chaney, 1991b)  . Plesiocleidochasma immersum ( Soule, Soule & Chaney, 1991b)  and two species in which the ooecium is not known differ from P. clandestinum  n. sp. in the characters of the frontal shield and avicularia. Overall, P. clandestinum  n. sp. is notable for its inconspicuous ooecia that lack a labellum and have a tiny ooecial foramen.

The ooecium of P. clandestinum  n. sp. is more like that in species of Metacleidochasma Soule, Soule & Chaney, 1991b  , but Metacleidochasma  species have a beaded (denticulate) distal oral rim. Dakaria coarctata Androsova, 1958  from the Japan Sea is depicted as having an orifice and other characters suggestive of Plesiocleidochasma  , but she notes the interzooidal boundaries as being distinct and the ooecium as large, almost half the length of the zooid and very elevated, which is not the case in P. clandestinum  n. sp.; D. coarctata  also has a small tubercle in the middle of the frontal shield.

Distribution. Korea: Seogwipo, Jeju Island, 20 m depth.