Pierrella plicata, Grischenko & Gordon & Melnik, 2018

Grischenko, Andrei V., Gordon, Dennis P. & Melnik, Viacheslav P., 2018, Bryozoa (Cyclostomata and Ctenostomata) from polymetallic nodules in the Russian exploration area, Clarion - Clipperton Fracture Zone, eastern Pacific Ocean-taxon novelty and implications of mining, Zootaxa 4484 (1), pp. 1-91 : 74-81

Pierrella plicata n. sp.

( Figs 49 , 50 )

Material examined. Holotype: ZIRAS 1/50727, colony encrusting tubular arenaceous foraminiferan, YMG R.V. Yuzhmorgeologiya cruise BIE 2000, Stn 1, 31 May 2000, 12.91697° N, 128.58775° W, 4850 m. Paratype: ZIRAS 2/ 50728, colony encrusting tubular arenaceous foraminiferan, YMG R.V. Gelendzhik cruise GLD4–11, Stn 230, 4 May 2012, 12.65038° N, 133.29995° W, 4809 m. Additional material: YMG4–07, Stn 139. Total specimens examined six.

Etymology. Latin, plicatus, folded, alluding to the form of the closed orifice.

Description. Colony encrusting, uniserial, sparsely branching laterally ( Figs 49A–E , 50A–D ). Zooids large, transparent, variably claviform, either with elongate-oval dilatation and long filiform cauda of same length ( Figs 49A , 50A, H ), or pyriform to subrounded with truncate or no cauda ( Figs 49B–D , 50C, D, E, G ), in part depending on crowding of zooids on substratum, or intermediate between these morphologies ( Figs 49E , 50B, F ). Lateral branching sporadic, on one side only, at right angle to widest part of dilatation. Aperture elevated, comprising stiff radiating pleats forming stellate pattern when polypide retracted ( Figs 49F, G , 50I –L ); 8–11 pleats seen, each appearing as erect ridge; stellate pattern symmetrical or ridges may appear somewhat longer on proximal side of orificial mound in some zooids ( Fig. 50I, J ). Zooid wall wholly transparent.

Vestibule tapering distad, as long as polypide that lies proximal to it; parieto-diaphragmatic musculature comprising small narrow bundles of fibers at each proximolateral corner. Vestibule 0.084 mm long; retracted tentacle crown 0.096–0.141 mm long. Tentacle number not determined by sectioning; from lateral view of retracted polypide it is likely that tentacle number is 12 or fewer. Anchor point of polypide retractor muscles on proximal or proximolateral wall. No evidence of brown bodies.

Ancestrular zooid not determined.

Measurements (mm). Holotype, ZIRAS 1/50727 ( Figs 49B–E , 50B–H, K, L ): ZL 0.900–1.350 (1.180 ± 0.134); ZDL 0.577–0.825 (0.675 ± 0.078); ZDW 0.402–0.575 (0.475 ± 0.051); CaL 0.325–0.775 (0.505 ± 0.141); OrD 0.058–0.107 (0.083 ± 0.016); OrH 0.044–0.069 (0.055 ± 0.011) (n = 5).

Remarks. We include this new species in the otherwise monotypic genus Pierrella , previously known only as a fossil taxon from the Cretaceous (lower Campanian to lower Maastrichtian) of Wyoming and South Dakota, USA. The type species, P. larsoni , was preserved on inner shell surfaces of empty body chambers of ammonites by a process interpreted as lithoimmuration.

The overall characters of P. larsoni accord with living members of Arachnidiidae in terms of colony form (adnate, uniserial, branching) and zooid shape (clavate/caudate), but the genus was able to be distinguished by its distinctive pleats (folds), eight in all, forming the orifice in the retracted state. This arrangement is unknown in any other arachnidioid, in which the orifice, when closed, has been described as a mamilla or papilla, or is somewhat elevated and squared (d'Hondt 1983). Wilson & Taylor (2013) referred to the pleats in P. larsoni as "setigerous, or pleated, collars", but it remains to be seen if the stiff orificial folds in Pierrella are homologous with the setigerous apparatus of the introvert in vesicularioid ctenostomes. For example, in the ctenostomes studied by McKinney & Dewel (2002), all collars were "composed of membranes that appeared to have their basal attachment at the atrial sphincter (diaphragm)"; these authors distinguished four forms of collar.

Notwithstanding this uncertainty, which has no bearing on the relatedness of Pierrella plicata n. sp. to P. larsoni , the similarity of these two species is compelling. They accord in every important respect—colony form, zooid shape, zooid size, and the overlapping number of stiff orificial pleats (eight in P. larsoni , 8–11 in P. plicata ). The only differences concern budding locus (somewhat more distolateral in P. larsoni ) and budding pattern (morevariable branch angles in P. larsoni ), but these differences can be intrageneric. In both species, zooid length and shape vary, becoming longer and more clavate in distal colony parts. In P. plicata , a squatter zooid shape occurs where zooids are more crowded. An apparent colony origin was found in P. larsoni , giving evidence of a nearcircular ancestrula with two distolateral buds and a proximal bud. An ancestrula was not seen in P. plicata ; its discovery would give more information concerning relatedness.

The long temporal gap of c. 74 million years need not preclude these two species being congeneric. A perusal of the Bryozoa Home Page shows that there are quite a number of Mesozoic genera, from three orders, still present in modern seas, e.g. Arachnidium , Arachnoidella , Buskia (Ctenostomata) , Nellia , Onychocella , Poricellaria , Celleporella (Cheilostomata) , Microeciella , Plagioecia , Mesenteripora , Crisia (Cyclostomata) (see also Voigt 1985).

Pierrella plicata n. sp. encrusts tubular arenaceous foraminiferans resembling Rhabdammina , which live at the sediment surface. The largest colonies comprised fewer than ten zooids. The proportionately very long vestibule in zooids examined in transparency and stained with Rose Bengal give evidence that, when the polypide retracts, the vestibule fills with fine sediment particles. These would be emptied from the vestibule upon the next polypide eversion.

Distribution. Recorded from three stations within coordinates 12.51953– 12.91697° N, 128.58775– 134.60008° W, at depth range 4808–4850 m.

Figures

FIGURE 49. Pierrella plicata n. sp. A, paratype, ZIRAS 2/50728; B–E, holotype, ZIRAS 1/50727; F, G, specimen YMG4–07, Stn 139. Specimens stained in Rose Bengal. A–E, zooids on arenaceous foraminiferan tubes, varying from linear and caudate to squat and more crowded; F, G, stained zooids seen in transmitted light. Abbreviations: at, alimentary tract; d, diaphragm; dmtd, dilator muscles and tendon of diaphragm; ofcp, orifice with folded cuticular pleats; pm, parietal muscles; r, rectum; rmtc, retractor muscles of tentacle crown; rt, retracted tentacles; sc, stomach caecum; t, tentacles; v, vestibule (filled with sediment). Scale bars: A–E, 500 µm; F, G, 50 µm.

FIGURE 50. Pierrella plicata n. sp. A, I, J, paratype, ZIRAS 2/50728; B–H, K, L, holotype, ZIRAS 1/50727. A, H, narrow, long-caudate zooids; C–E, G, squat zooids; B, F, zooids of intermediate shape; I–L, stiffly pleated orificial folds. Scale bars: A–D, 250 µm; E–H, 100 µm; I–L, 25 µm.