Morphology and ontogeny of Lophopus crystallinus lophophore support the epistome as ancestral character of phylactolaemate bryozoans

Abstract

Phylactolaemate bryozoans are the sister-group to all remaining bryozoan taxa. Consequently, their study is essential to reveal and analyze ancestral traits of Phylactolaemata and Bryozoa in general. They are the only bryozoans to possess an epistome which traditionally has been regarded as shared with phoronids and brachiopods. Contrary to older observations, an epistome was recently reported to be missing in the early branching phylactolaemate Lophopus crystallinus. In this study, the ontogeny of the lophophoral base and also its three-dimensional structure in adult specimens was reinvestigated to assess whether an epistome is never formed during ontogeny and absent in adult specimens. The results show that organogenesis during the budding process in this species is similar to other, previously investigated, species. The epistome anlage in L. crystallinus forms in early buds from the outer budding layer which penetrates the two shanks of the u-shaped gut. This ingression of the epithelium further proceeds distally and starts to wrap over the forming ganglion. The adult epistome is a rather short, but present bulge above the cerebral ganglion with prominent muscle bundles traversing its cavity. Distally it is arched by the forked canal that in L. crystallinus has a particularly thick and prominent epithelium in the three median tentacles. This study shows that neither during ontogeny nor in the adult stage an epistome is absent. The epistome is less pronounced than in other phylactolaemates, but otherwise similar in its general structure. Consequently, an epistome can be assumed to be present in the ground pattern of Phylactolaemata.Electronic supplementary materialThe online version of this article (10.1007/s00435-018-0402-2) contains supplementary material, which is available to authorized users.

Body cavities in bryozoans: Functional and phylogenetic implications

“…According to the description by Braem (), the forked canal corresponds to the inner lophophore arc and his drawings (figs 62 and 63) clearly demonstrate that the forked canal opens into the cavity of the lophophore arm. The same meaning of the term ‘forked canal’ was applied in the recent publications (Gruhl et al, ; Schwaha, ; Schwaha et al, ), although the authors pointed out that it opens into the main body cavity. Such interpretation may have been based on the works of Cori (, ), who stated that the forked canal opens into the main body cavity, while his own detailed drawings of fully protruded polypides of C. mucedo clearly show that the forked canal opens into the lophophore cavity at the base of both lophophore arms.…”

“…More recently, Mukai et al () mentioned only an imperfect partition between the lophophoral coelom and the main body cavity, but provided no further details. It is surprising that the merging of the lophophore and main body cavities at the bases of the lophophore arms was not mentioned in the recent publications (Gruhl et al, ; Schwaha, ; Schwaha et al, ). Instead, the cited studies emphasized that the lophophore and trunk coeloms are connected by the forked canal.…”

“…In other studies (Hyman, ; Mukai et al, ; Nielsen, ; Schmidt‐Rhaesa, ), the episome cavity is mistakenly described as confluent with the lophophore, not the trunk cavity, or even entirely cut off from both (Mukai, ). Only recently, works by Gruhl et al (), Schwaha et al (), and Schwaha () corrected this mistake. Needless to say, our findings are in complete agreement with these later publications.…”

“…Such interpretation may have been based on the works of Cori (, ), who stated that the forked canal opens into the main body cavity, while his own detailed drawings of fully protruded polypides of C. mucedo clearly show that the forked canal opens into the lophophore cavity at the base of both lophophore arms. Another possible explanation comes from the fact that all authors (Gruhl et al, ; Schwaha, ; Schwaha et al, ) examined retracted polypides. Naturally, the soft tissues of a retracted polypide are strongly squeezed.…”

“…The body cavity in phylactolaemates comprises coeloms of the epistome, lophopore, and trunk. According to recent publications, the epistomal coelom joins the trunk coelom via a wide opening, and the lophophoral coelom is connected with the main body cavity by a forked canal (Gruhl et al, ; Schwaha, ; Schwaha, Handschuh, Redl, & Walzl, ). Cyclostomes and gymnolaemates, on the other hand, lack an epistome and their body cavity has two parts, residing within the lophophore and the trunk.…”