ИСТИНА

Evolutionary relationships among member of the Lophophorata remain unclear. Traditionally, the Lophophorata included three phyla: Brachiopoda, Bryozoa or Ectoprocta, and Phoronida. All species in these phyla have a lophophore, which is a tentacle-bearing part of the mesosome. The lophophore is one of the main morphological characters used to suggest the unity of lophophorates. If the Lophophorata are truly monophyletic, we further suggest that the lophophore should have been inherited by all lophophorates from a common ancestor. If this were the case, the presence of common features of lophophore innervation in all lophophorates would help establish the homology of the lophophore and would also support the traditional view that the lophophorates represent a monophyletic group. Using methods of immunocytochemistry and laser confocal microscopy, we have studied the innervation of the lophophore of inarticulate brachiopod Lingula anatina Lamarck, 1801. The lophohpore of L. anatina contains three brachial nerves, which extend along each brachium of the lophophore: the main, accessory, and lower brachial nerves (Figure 1). The main brachial nerve gives rise to the cross neurite bundles, which connect the main and accessory brachial nerves. The accessory brachial nerve gives rise to the frontal, latero-frontal, and latero-abfrontal neurite bundles in tentacles of inner and outer rows. The lower brachial nerve gives rise to the abfrontal neurite bundles of the outer tentacles. The tentacles are innervated by accessory and lower brachial nerves. The accessory nerve gives rise to bundles of thin neurites, which pass between inner tentacles. At the tentacle base, these bundles form thick sites, where neurites subdivide into several groups (Figure 1). The first group consists of the frontal neurites of the inner tentacles. In each inner tentacle, frontal neurites arise from two bundles: left and right (Figure 1). The second group consists of the frontal neurites of the outer tentacles. This group of thick bundles passes from the inner to the outer side of the lophophore and then passes along the deep frontal groove of the outer tentacle. The third group of neurites gives rise to the lateral neurite bundles (Figure 1). They pass between the two rows of tentacles. Here, each lateral neurite bundle has a T-like shape and forms two branches: one branch passes to the latero-frontal side of the outer tentacle, and the other passes to the latero-abfrontal side of the inner tentacle. The latero-frontal bundles of adjacent outer tentacles connect between tentacles and form short nerves, which are located between the two rows of tentacles along the longitudinal axis of the lophophore (Figure 1). In the middle of each short nerve, a group of FMRF-amide-reactive perikarya is located. Each group of perikarya gives rise to the medioabfrontal neurite bundle of the inner tentacles. The lophophore of all lophophorates contains identical nerve elements. Their location with respect to other organs suggests that the main brachial nerve of L. anatina, the dorsal ganglion of phoronids, and the cerebral ganglion of bryozoans are likely homologous. The accessory brachial nerve of L. anatina can be regarded as a homolog of the minor nerve ring of phoronids and the circum-oral nerve ring of bryozoans. In L. anatina and phoronids, the lophophore has an outer nerve. This nerve extends along the abfrontal side of the tentacles at their bases (Figure 2A, B). In L. anatina, this nerve is represented by the lower brachial nerve. In phoronids, the outer lophophoral nerve corresponds to the tentacular nerve ring. Bryozoans lack an outer lophophoral nerve (Figure 2C). The absence of the outer nerve in bryozoans might also be attributed to a reduction due to the small body size. The innervation of tentacles exhibits the following features in all lophophorates: the presence of intertentacular neurite bundles, peculiarities of the innervation of the frontal side of the tentacles, and the presence of the subperitoneal neurites. These similarities correlate in part with the similar mechanism of filter feeding, which is common in the lophophorates and may in part reflect homology between tentacles of the lophophorates. We conclude that innervation of the lophophore and tentacles has a similar ground plan in the lophophorates. Our results support the homology of the lophophore and the monophyly of the Lophophorata. This research was supported in part by several grants. The collection of material was done with support from the Russian Foundation of Basic Research (#14-04-00238), the TEM and 3-D investigations were done with support from the Russian Scientific Fund (#14-04-262), and the processing of the paper was supported by Grants of the President of Russia (#MD-5812.2015.4; # NSH-1801.2014.4).