Radula

The radula is the organ for mechanical food processing and an important autapomorphy of Mollusca. Its chitinous membrane, embedding small radular teeth, is moved by the set of muscles resulting in an interaction with the ingesta, tearing it and collecting loosened particles. Radulae and their teeth can be quite distinct in their morphology and had been of high research interest, but only a few studies have examined the basic functional principles of this organ, the movement and motion during feeding action. Here, the radular motion of 20 representative species, belonging to four major gastropod lineages (Vetigastropoda, Neritimorpha, Caenogastropoda and Heterobranchia) and Polyplacophora, were recorded and classified. Comparisons of the video footage with the scanning electron microscope (SEM) images of the radula resulted in the recognition of functional tooth rows and the correct position of the teeth during feeding. We identified six different types of radular movements, including rotations and bending of the radula itself. In each movement type, different structures act as counter bearings enabling the animals to grab and tear food.

https://journals.biologists.com/bio/article/9/10/bio055699/222829/Not-just-scratching-the-surface-distinct-radular

https://pmc.ncbi.nlm.nih.gov/articles/PMC7595699/

The radular motions of the 20 analysed species were categorised, resulting in six radular motion types, which were split into three phases (Figs 4–7). The first phase is characterised by the protruding of the radula. In the second phase, food particles are sheared, crushed and collected. The last phase comprises the retraction of the organ into the oral cavity. As already described by Hickman (1984), the teeth of different transverse/ontogenetic rows act in concert and assemble to functional rows (functional rows are highlighted in grey in Figs 4–6 and Supplementary data; in Fig. 4C teeth of one transverse/ontogenetic row are highlighted with black boxes).

Radular motion pattern I was observed in L. cinereal (Polyplacophora). In phase I, the radula is protruded to the mouth opening, simultaneously the LTs are unfolded to the sides like a fan (Fig. 4A). This curved flexion is reinforced in phase II and reversed in phase III when the radula is retracted. The rotation of the teeth, especially of the DTs, allows them to grab larger food items (counter bearing I).

The second pattern (II) was found in C. corona and V. turrita (Neritimorpha) and is characterised by a bending or folding of the radula running between the tooth rows (Fig. 4D). In phase I the anterior radular part (Fig. 4D, purple colour) is protruded while being moved in anterior–dorsal direction, followed by a curling or rolling motion in posterior–ventral direction, enabling the shearing of ingesta during phase II. In phase III the anterior part is first retracted into the mouth opening, but since the radula is still bent this retraction allows the posterior part (Fig. 4D, pink colour) to gather the particles and transport them into the oral tube (counter bearing II). All teeth run almost parallel to each other during all phases.

The third pattern (III) was observed in Caenogastropod species, such as S. macilenta, S. torulosa, T. cancellata, B. herculea, F. ater, M. annulus, L. littorea and T. naticoides, and Vetigastropod species, such as R. conus and A. calcar, (Fig. 5A) and is similar to motion pattern II (Fig. 4D). However, here, a sharp bending running along the tooth rows was observed, and additionally, phase II is characterised by an abrupt tearing motion in posterior direction. Subsequently the bent anterior and posterior radular parts converge, allowing the tearing and grabbing of large food items (counter bearing III). This motion pattern, in contrast to pattern II, is also characterised by a presumably lateral tension of the radular membrane resulting in MTs sticking out to the sides.

Motion pattern IV was observed in M. cornuarietis (Caenogastropoda) and is, like pattern III, characterised by a sharp bent radula (counter bearing IV). However, in the phase II of the pattern IV, the radula additionally shakes and vibrates in the lateral direction (Fig. 6A).

The fifth motion pattern (V) was found in the Heterobranch species, such as C. aspersum, T. villosulus, L. stagnalis, P. corneus and P. duryi (Fig. 7A). After the protruding of the radula in phase I, the lateral edges of the radula are pulled caudally until the whole structure is formed into a concave shape like a spoon. While moving into anterior direction, the ingesta is loosened and collected, until the organ reaches its counter bearing, the jaw. Both structures together enable the ability to pull and tear large food items (counter bearing V).

Motion pattern VI was only detected in Heterobranch S. marmorata (Fig. 6D) and is quite unique. Here, the radula is bent along the CTs in anterior–posterior direction; the inner part of the radula is pulled caudally until a u-shape is achieved. Both lateral wings are flapped, gripping and holding the ingesta (counter bearing VI) until the organ with the food is retracted into the oral cavity.