Immergentia cheongpodensis, Seo & Chae & Winston & Zágoršek & Gordon, 2018

Seo, Ji-Eun, Chae, Hyun Sook, Winston, Judith E., Zágoršek, Kamil & Gordon, Dennis P., 2018, Korean ctenostome bryozoans-observations on living colonies, new records, five new species, and an updated checklist, Zootaxa 4486 (3), pp. 251-283 : 264-269

Immergentia cheongpodensis n. sp.

( Figs 9A, B, D , 10 , 11 )

Immergentia [sp.] Seo et al. 2016: 9.

Etymology. The species is named for the location where it was found, Cheongpodae Beach, Yellow Sea.

Material examined. Holotype: MBRMKH7, Cheongpodae, Taean Coast National Park, Korean west coast, 36.6334° N, 126.2997° E, intertidal, 16 June 2017 . Paratype: MBRBKP7, same data as holotype .

Description. Colony boring in shells of dead mollusks. Impression of narrow branching stolonate connections and elongate zooid openings seen at surface of eroded shells ( Figs 9A, B , 10 , 11 ). In more-translucent, freshly bored and less-eroded shells, the delicate branching pattern of submerged colonies is visible, especially if shells are wet. Colony extent hard to determine but stolons can trail within or over much of dead shell surface. Zooids have elongate teardrop- to sausage-shaped opening, centered along midline of branch at shell surface ( Fig. 9A, B ). Resin casts of colonies examined by SEM show that, below this opening, buried zooids develop first as bulge along very narrow (10 µm or less) stolonal branch ( Fig. 10A, B ). Mature zooids c. 150 µm long, 50 µm wide, widest at orificial end, non-pedunculate, with narrow, round-tipped basal end. Zooid excavation orientated at oblique angle, some even curving upward toward shell surface. Stoloniform processes branch from abanal and anal ends of zooid tubes and may come to shell surface where they are visible as tiny pores. Casts of structures resembling gonozooids also found; these are expanded, more-triangular than autozooids, with two basal ends, one shallower than the other ( Fig. 10E, F ).

Remarks. Gonozooids have not previously been reported for Immergentia . According to Pohowsky (1978) they are known only in Penetrantia , but the SEM illustrations here show that the gonozooids are connected only to Immergentia cheongpodensis n. sp. and its narrow stolons, rather than to the broader stolons of Penetrantia colonies. Perhaps gonozooids are ephemeral or seasonally produced.

Immergentia species are the smallest of boring ctenostomes, and they are seldom recorded in surveys as it takes careful study of numerous empty mollusk shells under a stereo microscope to find them. Their colonies may be intermingled with the colonies of larger boring taxa such as Penetrantia on a shell, making it hard to separate the traces of the different species. The traces of the present material at the shell surface, as well as zooidal morphology (non-pedunculate) and branching are typical of Immergentiidae , which is monogeneric.

Pohowsky (1978) discussed and illustrated several living and fossil species, three of which occur in the Pacific north of the Equator. Immergentia californica Silén, 1946 differs from Immergentia cheongpodensis n. sp. in having much longer zooids (c. 350 µm) that are pointed proximally, not rounded; Immergentia philippinensis Soule, 1950 also has a proximal end and a surface trace differing from that in I. cheongpodensis n. sp. Zooids of Immergentia angulata Soule & Soule, 1969 from Hawaii are 180–200 mm long, bent sideways, and pointed proximally.

Distribution. Korea: in dead mollusk shells, particularly oysters and clams, from the low-intertidal zone at Cheongpodae beach, west coast of Korea.

Figures

FIGURE 9. Light micrographs of various uncataloged shell-borings of Immergentia cheongpodensis n. sp. and Penetrantia taeanata n. sp. from Cheongpodae. A. Immergentia, frontal view of tear-drop-shaped excavations on bivalve shell. B. Highermagnification view of Immergentia excavations on same shell. C. Penetrantia, characteristic ‘railroad-track’ arrangement of seemingly paired openings either side of stolon traces in a densely bored shell. D. Borings of both species in the same shell, scattered, smaller openings of Immergentia colonies are visible in the upper less-bored half of the image. E. More-magnified Penetrantia colonies showing zooid openings and stolon traces. F. Translucent shell back-lit to show presence of Penetrantia zooid chambers beneath the openings at the shell surface. Scale bars: A = 2 mm; B = 1 mm; C = 4 mm; D = 0.20 mm.

FIGURE 10. Resin casts of shell-boring colonies of Immergentia cheongpodensis n. sp. from Cheongpodae. A. Partial and complete zooid and stolon casts in a shell also heavily bored by filamentous endolithic organisms, note new zooid buds in area under the scale bar. B. Another part of colony, zooid buds under the “B” in upper right of image. C. Autozooid and stolons with branch. D. Zooid and stolon trifurcation. E. swollen gonozooid in partial profile. F. Another gonozooid. Scale bars: A, B = 0.20 mm; C–F = 0.10 mm.

FIGURE 11. Resin casts of Immergentia cheongpodensis n. sp. and Penetrantia taeanata n. sp. growing together at Cheongpodae. A. Resin cast showing mostly Penetrantia stolons and zooids, with those of Immergentia more apparent in the lower right quadrant. B. Close-up of crossing stolons, those of Penetrantia thicker and closer to the shell surface, with a broken zooid clearly with a peduncle, whereas the stolon of Immergentia runs across (beneath, in life orientation) a trifurcation in the Penetrantia colony. Scale bars: A = 0.50 mm; B = 0.10 mm.