Abstract
The radula, a chitinous membrane with embedded tooth rows, is the molluscan autapomorphy for feeding. The morphologies, arrangements and mechanical properties of teeth can vary between taxa, which is usually interpreted as adaptation to food. In previous studies, we proposed about trophic and other functional specialisations in taenioglossan radulae from species of African paludomid gastropods. These were based on the analysis of shape, material properties, force-resistance, and the mechanical behaviour of teeth, when interacting with an obstacle. The latter was previously simulated for one species (Spekia zonata) by the finite-element-analysis (FEA) and, for more species, observed in experiments. In the here presented work we test the previous hypotheses by applying the FEA on 3D modelled radulae, with incorporated material properties, from three additional paludomid species. These species forage either on algae attached to rocks (Lavigeria grandis), covering sand (Cleopatra johnstoni), or attached to plant surface and covering sand (Bridouxia grandidieriana). Since the analysed radulae vary greatly in their general size (e.g. width) and size of teeth between species, we additionally aimed at relating the simulated stress and strain distributions with the tooth sizes by altering the force/volume. For this purpose, we also included S. zonata again in the present study. Our FEA results show that smaller radulae are more affected by stress and strain than larger ones, when each tooth is loaded with the same force. However, the results are not fully in congruence with results from the previous breaking stress experiments, indicating that besides the parameter size, more mechanisms leading to reduced stress/strain must be present in radulae.
Highlights
The radula is the molluscan autapomorphy for food gathering and processing
The models (Fig. 2) were created by studying the radulae from adult gastropod specimens, that had been either collected by Heinz Büscher in Lake Tanganyika at Zambia, Northern Province, Mibwebwe, in 2019 (L. grandis [Smith, 1881]); at Zambia, Northern Providence, Cape Kachese, in 2016 (B. grandidieriana [Bourguignat, 1885]); by Frank Riedel at Burundi and by Heinz Büscher at Zambia, Kalambo Falls Lodge, in 2017 (S. zonata [Woodward, 1859]) or by Anthony Wilson in Zambia, Lake Mweru, Nchelenge, in 2000 (C. johnstoni Smith, 1893). These gastropods were preserved in 70% EtOH and either inventoried at the Zoological Museum Hamburg (ZMH; B. grandidieriana: ZMH 119367/999; L. grandis: ZMH 154657/999; S. zonata: ZMH 150008/999) or the Museum für Naturkunde Berlin (ZMB; C. johnstoni: ZMB 220.102; S. zonata: ZMB 220.144)
Lavigeria grandis and Spekia zonata forage on algae attached to rocks, Cleopatra johnstoni forages on algae that cover sand or mud, and Bridouxia grandidieriana on algae from various surfaces: sand, mud, plant surface
Summary
The radula is the molluscan autapomorphy for food gathering and processing. It consists of a chitinous membrane with embedded rows of teeth, which interact with the preferred food and the substrate the food is attached to (feeding substrate). The Paludomidae from Lake Tanganyika and surrounding water bodies are representatives of a species flock, that is potentially the result of an adaptive radiation accompanied by trophic specialisation (for hypotheses on paludomid evolution see e.g.19,41,59–68) They possess taenioglossan radulae (seven teeth per row: one central, two laterals, two inner and two outer marginals) that show a great interspecific morphological diversity (see e.g.19,41,68,69). Bodies with defined material properties can be tested under the action of outer forces, resulting in the visualization of the deformation and distribution of stress and strain This method was previously employed on various biological objects as a very useful approach in ecomorphological analyses of food processing structures (e.g.70–82)
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