Not just scratching the surface: Distinct radular motion patterns in Mollusca

Abstract

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 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, Heterobranchia) and Polyplacophora, were recorded and classified. Comparisons of the video footage with the 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.

Feeding Experiments on Vittina Turrita (Mollusca, Gastropoda, Neritidae) Reveal Tooth Contact Areas and Bent Radular Shape During Foraging

“…Such bending behavior had been previously described for 'Archaeogastropoda', which bent and unfold their radula at their mouth opening to build a broad area for grazing, followed by a folding in the opposite direction, to obtain a smaller structure that can be stored in the head [47]. Also Neritidae can bend their radulae in such a way that the lateral teeth become closer to the substrate than the medial ones [16,36]. Here, not the center of the radula, but rather its lateral areas will be mechanically stressed [see also 40].…”

“…For feeding experiments, 40 individuals of Vittina turrita (Gastropoda: Neritidae) were obtained from online pet shop (garnelio.de; there labeled as Neritina turrita). This species was chosen for our experimentation, because (1) it possesses a heterodont radula with very distinct tooth morphologies and sizes, (2) animals are easy to obtain, shelter, and feed, (3) and videos of radular motion are available [16].…”

“…As the quantity, shape, arrangement, material properties, and chemical composition of teeth can be highly distinct within one individual radula and between molluscan taxa, hypotheses about distinct tooth functionalities, additionally to hypotheses about their adaptations to distinct ingesta-types, are at hand. Past approaches on radular functional principles involve the observation and documentation of foraging behavior [11][12][13][14][15][16] or the analysis of radular feeding tracks [2, 11-12, 15, 17-24]. However, as the radular motion is highly complex, the teeth are rather small, and feeding tracks are di cult to interpret, the precise function and the interplay between different radular structures (the membrane, the tooth rows, the odontophoral cartilages, the buccal mass musculature) still await further structural and especially experimental investigation.…”

Radular Force Performance of Stylommatophoran Gastropods (Mollusca) with Distinct Body Masses Reflect Diverging Feeding Patterns

“…For force measurements we have chosen ve stylommatophoran species (Gastropoda: Heterobranchia), because all possess a similar radular type (isodont), have a similar radular foraging motion [for details on motion and radular type see e.g. 39,[89][90][91][92][93][94][95], and represent distinct body mass classes (Fig. 1): immature (one juvenile stage) and mature Lissachatina fulica (Bowdich, 1822), mature Cepaea nemoralis (Linnaeus, 1758), mature Cepaea hortensis (Müller, 1774), mature Helix pomatia Linnaeus, 1758, and mature Arion vulgaris Moquin-Tandon, 1855.…”

“…The sometimes highly complex radular motion [see e.g. [34][35][36][37][38][39] brings the tooth cusps in contact with the ingesta leading to the tearing, cutting, and gathering of food.…”