Unusual lophophore innervation in ctenostome Flustrellidra hispida (Bryozoa)

Abstract

Since ctenostomes are traditionally regarded as an ancestral clade to some other bryozoan groups, the study of additional species may help to clarify questions on bryozoan evolution and phylogeny. One of these questions is the bryozoan lophophore evolution: whether it occurred through simplification or complication. The morphology and innervation of the ctenostome Flustrellidra hispida (Fabricius, 1780) lophophore have been studied with electron microscopy and immunocytochemistry with confocal laser scanning microscopy. Lophophore nervous system of F. hispida consists of several main nerve elements: cerebral ganglion, circumoral nerve ring, and the outer nerve ring. Serotonin‐like immunoreactive perikarya, which connect with the circumoral nerve ring, bear the cilium that directs to the abfrontal side of the lophophore and extends between tentacle bases. The circumoral nerve ring gives rise to the intertentacular and frontal tentacle nerves. The outer nerve ring gives rise to the abfrontal neurites, which connect to the outer groups of perikarya and contribute to the formation of the abfrontal tentacle nerve. The outer nerve ring has been described before in other bryozoans, but it never contributes to the innervation of tentacles. The presence of the outer nerve ring participating in the innervation of tentacles makes the F. hispida lophophore nervous system particularly similar to the lophophore nervous system of phoronids. This similarity allows to suggest that organization of the F. hispida lophophore nervous system may reflect the ancestral state for all bryozoans. The possible scenario of evolutionary transformation of the lophophore nervous system within bryozoans is suggested.

Homology of the lophophore and its evolution within lophophorates

“…In most of studied bryozoans, the outer nerve ring does not connect to other nerves and does not contribute to tentacle innervation. In Flustrellidra hispida, however, which has large bell-shaped lophophore, the outer nerve ring gives rise to the abfrontal neurites, which contribute to the tentacle abfrontal nerves (Temereva et al, 2022).…”

“…Concerning the evolution of the bryozoan lophophore, two different ideas have also been advanced: the evolution from simple to complex (Schwaha, Wanninger, 2015) or from complex to simple (Temereva, Kosevich, 2016). Both notions are discussed in the literature (Schwaha, 2020;Isaeva et al, 2021;Temereva et al, 2022).…”

“…The bryozoan ancestor presumably had a bell-shaped lophophore, such as in F. hispida (Temereva et al, 2022). This type showed two evolutionary pathways: (i) miniaturization in all myolaemates and (ii) complication in phylactolaemates.…”