Temereva E.N. 2025. Organization of phoronid larval tentacles, their metamorphic remodeling, and a scenario of the lophophore origin // Invert. Zool. Vol.22. No.1: 178–201 [in English].
Evolutionary Biology Department, Faculty of Biology, Lomonosov Moscow State University, Leninskie Gory 1-12, Moscow 119234 Russia.
Elena Temereva: temereva@mail.ru ORCID https://orcid.org/0000-0001-7791-0553
doi: 10.15298/invertzool.22.1.12
ABSTRACT: Phoronida is the phylum within stem Spiralia comprising marine worm-like animals with biphasic life cycle: adult phoronids live as benthic organisms in soft or hard substrata, whereas larvae (actinotrochs) are planktonic. Both adults and larvae have tentacle apparatus, which supplies the capture of food particles. Organization of phoronid tentacles may shed light on the problem of appearance and evolution of appendages in Bilateria in general. Different methods were used in order to study the morphology, ultrastructure, proliferative activity, and metamorphic remodeling of larval tentacles in different phoronid species. In phoronid larvae, tentacle apparatus differs in tentacle number, shape, diameter, and length. The postoral ciliated band extends along the lateral sides of the tentacles. Its cilia beat from the top to the down, i.e. band works as it does in tornaria larva. Interesting, the preoral ciliated band works as in trochophore; thereby, in actinotrochs, the mechanism of the ciliated bands processing combines features of protostomes and deuterostomes. Larval tentacles exhibit prominent zonality: there are six differently ciliated zones, which are accompanied with the nerve tracts and muscle bundles. Such type of tentacles can be regarded as “highly specialized”. During metamorphosis, larval tentacles undergo great transformation, which occurs in different ways: partial or complete reduction. In newly formed juvenile, tentacles mostly lack prominent zonality and look like “less specialized type”. This type of organization may recapitulate an evolutionary step when the last common bilaterian ancestor had been benthic suspense feeder and had non-specialized or less specialized tentacles. Because all recent lophophorates have highly specialized lophophore, which might be evolved in only water column, I have suggested the origin of the lophophore at pelagic stage of the life cycle of the last common lophophorate ancestor. This scenario should suggest that all lophophorates are descendants of the pelagic juvenile of the last common bilaterian ancestor.
KEY WORDS: lophophore, metamorphosis, larvae, proliferation, coelomic lining, EdU, filter feeding, upstream-collecting mechanism.