Organization of the lophophore in the deep-sea brachiopod Pelagodiscus atlanticus and evolution of the lophophore in the Brachiozoa
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Molecular data indicate that brachiopods and phoronids form a clade Brachiozoa. In both groups, the lophophore consists of a brachial axis that bears a row of tentacles and does take different forms. Pelagodiscus atlanticus is a brachiopod with an unusual lophophore combining primitive (horseshoe-shaped brachial axis) and advanced (brachial axis forms two arms that are raised freely into the mantle cavity) features. The organization of the lophophore of P. atlanticus was studied by histological and MicroCT methods. Lophophoral arms of P. atlanticus are directed posteriorly. Each arm is formed by a looped brachial axis. As in the lateral arm of a zygolophe in terebratulids, the two parts of the looped brachial axis run in a common extracellular matrix. Apparently, P. atlanticus demonstrates a distinct path in the evolution of the brachiopod lophophore. Although the lophophore of P. atlanticus is a result of the paedomorphic morphology of this brachiopod, a lophophore of similar shape could be an initial step for the development of the lophophore of other discinids. We suggest based on morphological analyses of phoronid and brachiopod lophophores that a crescent-shaped taxolophe with one coelomic canal and a single row of tentacles may represent an initial stage in the evolution of the lophophores in the Brachiozoa. In brachiopods, the brachial axis apparently became more complex with a double row of tentacles and large and small coelomic canals. In phoronids, the brachial axis preserved a simple organization that occurs in the taxolophe and the trocholophe of brachiopods. During the evolution of the phoronid lophophore, the brachial axis obtained a shape of horseshoe with two loops, in which two parts of the looped brachial axis were fused. Such a structure resembles the zygolophe of brachiopods.
KeywordsPhoronida Brachiopoda Bryozoa Lophophore Tentacle Brachial axis
The authors are very grateful to Alexandrea Petrunina and Alexei Chernyshev, who collected the animals during the German-Russian expedition Kurambio II in 2016. The micro-CT research was performed at the Laboratory of Natural Resources, Geological faculty of Lomonosov Moscow State University.
This study was supported from the Russian Science Foundation (#14–50-00034 – MicroCT study). Microscopical studies were done with the support from the Russian Science Foundation (#18-14-00082). The processing of the paper is supported by a grant of the Russian Foundation for Basic Researches (#17-04-00586).
Compliance with ethical standards
The use of brachiopods in the laboratory does not raise any ethical issues, and therefore approval from regional and local research ethics committees was not required. The field sampling did not involve endangered or protected species. In accordance with local guidelines, permission for collection of material was not required.
Conflict of interest
The authors declare that they have no conflict of interest.
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