The nervous system of the most complex lophophore provides new insights into the evolution of Brachiopoda

Unusual body division and epithelium structure in unusual phoronid Phoronis embryolabi

Phoronida is a small phylum of benthic marine invertebrates that can occur in large numbers globally. The study of phoronid morphology and anatomy is important for understanding phoronid biology and the function of benthic communities dominated by phoronids. Because all phoronids are tube-living animals, the study of the morphology and ultrastructure of the body wall is an important step toward understanding the processes of the tube formation, growth, and renovation. This study used epoxy histology, scanning and transmission electron microscopy to describe the body regionalization and ultrastructure of the body wall epithelium of the unusual Phoronis embryolabi, which lives as a commensal in burrows of digging shrimps. The trunk of P. embryolabi consists of 8 zones, which are clearly distinguishable in living individuals. These zones are as follows: long head region, median sphincter with its three different parts (waist, upper and lower), muscular region, reproductive region, zone 7, and ampulla. Such body division can correlate with specificity of life style of P. embryolabi. The ultrastructure of the epithelium of all zones differ from each other in thickness, set and abundance of gland cells, structure of the extracellular matrix that underlies the epithelium, and abundance of neurites. The capacity and distribution of glandular cells correlate with tube formation and remodelling. Bacteria of two different types are described along body wall of all parts of the trunk; reciprocally advantageous phoronid-bacteria interaction is suggested. Our data suggest that P. embryolabi is able to build the tube at the anterior end rather than at the posterior end, as previously suggested for other phoronid species. At the same time, the certain mechanism of phoronid tube growth and remodelling is still unknown for phoronids as well as for many other tube-living invertebrates.

Unusual lophophore innervation in ctenostome Flustrellidra hispida (Bryozoa)

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.

Tentacle muscles in brachiopods: Ultrastructure and relation to peculiarities of life style

Although the morphology of the brachiopod tentacle organ, the lophophore, is diverse, the organization of tentacles has traditionally been thought to be similar among brachiopods. We report here, however, that the structure of the tentacle muscles differs among brachiopod species representing three subphyla: Lingula anatina (Linguliformea: Linguloidea), Pelagodiscus atlanticus (Linguliformea: Discinoidea), Novocrania anomala (Craniiformea), and Coptothyris grayi (Rhynchonelliformea). Although the tentacle muscles in all four species are formed by myoepithelial cells with thick myofilaments of different diameters, three types of tentacle organization were detected. The tentacles of the first type occur in P. atlanticus, C. grayi, and in all rhynchonelliforms studied before. These tentacles have a well-developed frontal muscle and a small abfrontal muscle, which may reflect the ancestral organization of tentacles of all brachiopods. This type of tentacle has presumably been modified in other brachiopods due to changes in life style. Tentacles of the second type occur in the burrowing species L. anatina and are characterized by the presence of equally developed smooth frontal and abfrontal muscles. Tentacles of the third type occur in N. anomala and are characterized by the presence of only well-developed frontal muscles; the abfrontal muscles are reduced due to the specific position of tentacles during filtration and to the presence of numerous peritoneal neurites on the abfrontal side of the tentacles. Tentacles of the first type are also present in phoronids and bryozoans, and may be ancestral for all lophophorates.

First Modern Data on the Lophophore Nervous System in Adult Novocrania anomala and a Current Assessment of Brachiopod Phylogeny

Simple Summary

The nervous system of Novocrania anomala adults is described for the first time. A table containing data on the lophophore innervation in species from three brachiopod subphyla is presented. A comparative analysis suggests a close relationship between the Craniiformea and the Rhynchonelliformea, and thereby supports the “Calciata” hypothesis of brachiopod phylogeny.

Abstract

Although the lophophore is regarded as the main synapomorphy of all lophophorates, the evolution of the lophophore in certain groups of lophophorates remains unclear. To date, the innervation of the lophophore has been studied with modern methods only for three brachiopod species belonging to two subphyla: Linguliformea and Rhynchonelliformea. In the third subphylum, the Craniiformea, there are data for juveniles but not for adults. In the current research, the innervation of the lophophore in Novocrania anomala adults was studied by immunocytochemistry and confocal laser scanning microscopy. In the spiral lophophore of adults of the craniiform N. anomala, each arm is innervated by six brachial nerves: main, additional main, accessory, second accessory, additional lower, and lower brachial nerves. Compared with other brachiopod species, this complex innervation of the lophophore correlates with the presence of many lophophoral muscles. The general anatomy of the lophophore nervous system and the peculiarities of the organization of the subenteric ganglion of the craniiform N. anomala have a lot in common with those of rhynchonelliforms but not with those of linguliforms. These findings are consistent with the “Calciata” hypothesis of the brachiopod phylogeny and are inconsistent with the inference that the Craniiformea and Linguliformea are closely related.