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Organization of the lophophore in the deep-sea brachiopod Pelagodiscus atlanticus and evolution of the lophophore in the Brachiozoa
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.