Rallocytus, 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 : 64

Rallocytus n. gen.

Type species. Rallocytus ridiculus n. sp.

Etymology. Latin rallus, thin, and Latinized Greek cytus, container, alluding to the cystid of dimorphic zooids inferred to be fertile. Gender masculine.

Diagnosis. Colony erect, pedunculate, autozooidal peristomes radiating around infundibuliform calyx in single irregular whorl; interior surface of peristome lacking spinules or pustules. Dimorphic zooids 1–2 per colony within zooidal whorl, not occupying colony center, inferred to be female, each with reduced opening (presumed ooeciopore); appearing as early as four-zooid stage of colony growth, with reduced opening pertaining to ancestrula. Ancestrula erect, its peristome placed centrally on protoecium.

Remarks. Rallocytus n. gen. is presently monogeneric. In general aspect, the form of the colony resembles that in Calyssopora n. gen., but differs strikingly in that the colony center is not occupied by a capacious incubation chamber. Rather, as evidenced by early colony stages, a dimorphic zooid can occur within a single zooid whorl of four zooids, three of them autozooids; Micro-CT scans of one such colony revealed that the dimorphic zooid, distinguished only by the smaller size of its aperture, is the ancestrular zooid. On the other hand, one of the largest colonies (a paratype) has up to ten peristomes in a single whorl, two of them dimorphic, both with their apertures facing upwards or towards the colony center, rather than outwards like autozooidal peristomial apertures. Since a colony cannot have two ancestrulae, the implication is that the ancestrula, being the oldest still-functional zooid in the colony, is capable of precocious reproduction—assuming that our inference of a reproductive function for the dimorphic zooids is correct. Note that not every ancestrular peristome is dimorphic—we have submature colonies in which it resembles that of ordinary zooids. It is thus highly unlikely that the dimorphic zooids are primary or secondary nanozooids, such as are found in the tubuliporine genus Diplosolen Canu, 1918 and some species of Plagioecia and Favosipora ( Gordon & Taylor 2001) . These small zooids in Diplosolen have very tiny apertures with a single emergent tentacle ( Silén & Harmelin 1974) that seems to have a cleaning function, and occur only in two-dimensionally encrusting colonies, unlike the situation in erect Rallocytus ridiculus n. gen., n. sp. An ancestrula would not be expected to have a nanozooid function, whereas a reproductive function cannot be ruled out.