Organogenesis in the budding process of the freshwater bryozoan Cristatella mucedo Cuvier, 1798 (bryozoa, phylactolaemata)

Abstract

The phylogenetic position of bryozoans has been disputed for decades, and molecular phylogenetic analyzes have not unequivocally clarified their position within the Bilateria. As probably the most basal bryozoans, Phylactolaemata is the most promising taxon for large-scale phylogenetic comparisons. These comparisons require extending the morphological and developmental data by investigating different phylactolaemate species to identify basal characters and resolve in-group phylogeny. Accordingly, we analyzed the bud development and the organogenesis of the freshwater bryozoan Cristatella mucedo, with special focus on the formation of the digestive tract and differentiation of the coelomic compartments. Most parts of the digestive tract are formed as an outpocketing at the future anal side growing towards the mouth area. The ganglion is formed by an invagination between the anlagen of the mouth and anus. The lophophoral arms develop as paired lateral protrusions into the lumen of the bud and are temporarily connected by a median, thin bridge. All coelomic compartments are confluent during their development and also in the adult. The epistome coelom develops by fusion of two peritoneal infolds between the gut loop and overgrows the ganglion medially. The coelomic ring canal on the oral side develops by two lateral ingrowths and supplies the oral tentacles. On the forked canal, supplying the innermost row of tentacles above the epistome, a bladder-shaped swelling, probably with excretory function, is present in some adults. It remains difficult to draw comparisons to other phyla because only few studies have dealt with budding of potentially related taxa in more detail. Nonetheless, our results show that comparative organogenesis can contribute to phylactolaemate systematics and, when more data are available, possibly to that of other bryozoan classes and bilaterian phyla.

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

“…The latter does not primarily form from an outgrowth of the prospective mouth region as indicated for A. gelatinosa (Oka 1891), but from the prospective anal area which was also considered by Mukai (1982). This confirms that the formation of the gut is similar if not identical in all Phylactolaemata and Bryozoa (Schwaha et al 2011; Schwaha and Wood 2011). …”

“…Consequently, most of our available information on the organogenesis resides with the description of C. mucedo (cf. Schwaha et al 2011). The current investigation shows that the general budding process in the lophopodid L. crystallinus is very similar to other phylactolaemates and in particular organogenesis is similar to C. mucedo .…”

“…Most other descriptions on the polypide organogenesis in Phylactolaemata were conducted on Cristatella mucedo (Braem 1890; Davenport 1890; Schwaha et al 2011) or focus on early bud formation in plumatellids (see Brien 1960; Mukai 1982, summarized in Schwaha et al 2011). Consequently, most of our available information on the organogenesis resides with the description of C. mucedo (cf.…”

“…The current investigation shows that the general budding process in the lophopodid L. crystallinus is very similar to other phylactolaemates and in particular organogenesis is similar to C. mucedo . Buds in all bryozoans develop as two layered invaginations of the body wall that form the outer and inner budding layer (Schwaha et al 2011). The outer derives and corresponds to the peritoneal layer whereas the inner to the epidermis.…”

“…Another feature present in phoronids, brachiopods and phylactolaemate bryozoans is a lip-like fold over the mouth opening, the epistome (e.g., Hyman 1959). In bryozoans a coelomic cavity is present in the epistome that is in open connection to the remaining visceral coelom (Gruhl et al 2009; Schwaha et al 2011). Muscles are also present traversing the coelomic cavity of the epistome or lining its epithelial wall (Gawin et al 2017).…”