Innervation of the lophophore suggests that the phoronid Phoronis ovalis is a link between phoronids and bryozoans

Elena N Temereva

doi:10.1038/s41598-017-14590-8

Abstract

The validity of the Lophophorata as a monophyletic group remains controversial. New data on the innervation of the lophophore, which is a unique feature of the lophophorates, may help clarify the status of the Lophophorata and provide new information on the early evolution of the group. In this paper, the organization of the nervous system of the lophophore is described in adults of the minute phoronid Phoronis ovalis. The lophophore nervous system includes a dorsal ganglion, a tentacular nerve ring, an inner ganglion, an inner nerve ring, and six nerves in each tentacle. The inner ganglion and inner nerve ring, which is associated with sensory cells, are described for the first time in adult phoronids. The general plan of the nervous system of the lophophore and tentacles is similar in P. ovalis and bryozoans. These new results suggest the presence of two nerve centers and two nerve rings in the last common ancestor of phoronids and bryozoans. During evolution, bryozoans may have lost the outer nerve center and outer nerve ring, whereas phoronids may have lost the inner nerve center and inner nerve ring. These morphological results evidence the lophophorates are monophyletic.

Highlights

Tentacular apparatuses are known in many different diploblastic and bilaterian animals
The general morphology of the lophophore is similar among different lophophorates, it differs in detail between and within phyla[3,14,15]
In one scheme, which is characteristic of all brachiopods, new tentacles arise at the tip of each lophophore arm

Summary

Introduction

Tentacular apparatuses are known in many different diploblastic and bilaterian animals. The lophophore is specific tentacular apparatus, in which tentacles surround the mouth but not the anus Based on their possession of a lophophore, three phyla of invertebrates have been traditionally regarded as lophophorates: phoronids, brachiopods, and bryozoans (=ectoprocts)[1,2,3,4]. Bryozoans have a lot in common with phoronids Both groups have a similar bauplan: the dorsal side of adult animals is very short but the ventral side is very long; the digestive tract is U-shaped: mouth and anus are placed closely each other[13]. There is tendency for the inner lophophoral nerve ring to be weakly developed This tendency was recently described in two brachiopod species, which belong to two different classes, but have the same type of the lophophore[19]. Most species of bryozoans have only the inner lophophoral nerve ring; the outer lophophoral nerve ring has been described only in Amathia gracilis[6]

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Conclusion