Vortsepneva E.V.1*, Mikhlina A.L.2, Kantor Yu.I.3 2025. The evolution of the radula in molluscs: what was first, flexoglossy or stereoglossy? // Invert. Zool. Vol.22. No.1: 202–218 [in English].
1 Invertebrate Zoology Department, Lomonosov Moscow State University, Leninskie Gory 1-12, Moscow 119234 Russia.
2 White Sea Biological Station, Lomonosov Moscow State University, Leninskie Gory 1-12, Moscow 119234 Russia.
3 Laboratory of Marine Invertebrate Morphology and Ecology, A.N. Severtsov Institute of Ecology and Evolution RAS, Leninsky Prospekt 33, Moscow 119071 Russia.
Elena Vortsepneva: vortcepneva@gmail.com ORCID https://orcid.org/0000-0002-8163-2733
Anna Mikhlina: mikhleanna@gmail.com ORCID https://orcid.org/0000-0002-9231-0545
Yuri Kantor: kantor.yuri1956@gmail.com ORCID https://orcid.org/0000-0002-3209-4940
* Corresponding author
doi: 10.15298/invertzool.22.1.13
ABSTRACT: Two fundamental types of radulae in Mollusca have been recognized: stereoglossate and flexoglossate. The former is characterized by a fixed connection between the teeth and the radular membrane, a membrane that is not folded longitudinally, and parallel feeding tracks where the teeth act as a rasp. This type of radula has been considered present in Monoplacophora, Polyplacophora, Scaphopoda, and one gastropod subclass, the Patellogastropoda, and is therefore thought to represent the ancestral state of radulae in Gastropoda.
In contrast, the flexoglossate radula is characterized by a mobile connection between the teeth and the radular membrane, longitudinal folding of the membrane within the radular sac, and the mobility of rotating marginal teeth, which follow different trajectories during feeding. The flexoglossate radula is typical of all Gastropoda except Patellogastropoda.
A detailed examination of the radular apparatus of ten gastropod species from different subclasses, including Patellogastropoda, as well as three Polyplacophora species, has allowed us to reevaluate the stereoglossate condition in gastropods and polyplacophorans. We demonstrate that the radular membrane is folded longitudinally in the radular sac in all the species studied. Among Patellogastropoda, this was most obvious in Scutellastra longicosta, which has a broader and more complete radular transverse row (13 teeth), though traces of folding were observed even in Testudinalia testudinalis, which has only 4 teeth in a transverse row. Similarly, the radular membrane was folded longitudinally in the radular sac of chitons, which were previously thought to have a stereoglossate radular type.
Our data allowed us to re-evaluate the established views on the evolution of radula. Most likely, the plesiomorphic state of molluscan radula is a flexible flexoglossate condition. We conclude that the stereoglossate condition in Patellogastropoda is not a distinct radular type but rather a secondary modification of an originally flexoglossate radula. This modification is related to the reduction of teeth in the transverse row, retaining only the central portion of the radula, consisting of two pairs of lateral teeth.
KEY WORDS: Gastropoda, Polyplacophora, docoglossate, taeniolgossate, rhipidoglossate, radula.