Coping with abrasive food: diverging composition of radular teeth in two Porifera-consuming nudibranch species (Mollusca, Gastropoda)

Abstract

Molluscs forage with their radula, a chitinous membrane with teeth. Adaptations to hard or abrasive ingesta were well studied in Polyplacophora and Patellogastropoda, but for other taxa there are large gaps in knowledge. Here, we investigated the nudibranch gastropods Felimare picta and Doris pseudoargus, both of which feed on Porifera. Tooth morphologies were documented by scanning electron microscopy, and mechanical properties were tested by nanoindentation. We found that these parameters are rather similar in both species, indicating that teeth are similar in their function. To study the composition, teeth were visualized using confocal laser scanning microscopy (CLSM), to determine the degree of tanning, and analysed with energy-dispersive X-ray spectroscopy, to test the elemental composition. The emitted autofluorescence signal and the inorganic content differed between the species. This was especially prominent when studying the inner and outer tooth surfaces (leading and trailing edges). In F. picta, we detected high proportions of Si, whereas teeth of D. pseudoargus contained high amounts of Ca, which influenced the autofluorescence signal in CLSM. Employing nanoindentation, we determined high Young's modulus and hardness values for the leading edges of teeth, which relate to the Si and Ca content. This highlights that teeth with a similar morphology and mechanical properties can be mechanically enhanced via different chemical pathways in Nudibranchia.

Keywords: Nudibranchia; biomineralization; elemental composition; feeding; mechanical properties; mollusca.

Figure 1.

SEM images of the radulae in (a–d) Felimare picta and (e–h) Doris pseudoargus. (a) Working and maturation zone. (b,c) Outer working zone. (d) Outer lateral teeth. (e) Whole radula. (f) Working zone. (g) Outer lateral teeth. (h) Inner lateral and medial lateral teeth. iLT, inner lateral teeth; mLT, medial lateral teeth; oLT, outer lateral teeth. Scale bars: (a) 600 µm; (b) 300 µm; (c,d,h) 60 µm; (e) 400 µm; (f,g) 200 µm.

Figure 2.

Left side: Felimare picta, right side: Doris pseudoargus. (a) SEM image of one inner lateral tooth. (b) SEM image of medial lateral teeth. (c) Longitudinal sections from inner laterals (c). Prior to EDX, the radular sample was embedded in epoxy resin and polished. Blue areas highlight the region of the leading edge, where point measurements were performed, yellow ones the trailing edge and red ones the inner tooth structure. (d) Fractured tooth showing the inner tooth structure and the outer tooth layer. (e) Summary of EDX analyses (tooth types pooled together): atomic percentage from leading edge, trailing edge and inner structure. IS, inner structure; LE, leading edge; OL, outer layer; TE, trailing edge. Scale bars: (a,b) 20 µm; (c) 4 µm; (d) 2 µm.

Figure 3.

Doris pseudoargus. (a) Embedded and polished radula with highlighted localities, tested using nanoindentation (left side in (a): tooth with intact outer layer, right side in (a): teeth without outer layer). (b) SEM image of inner lateral teeth with highlighted localities (basis, stylus, tip = denticle), tested on the embedded teeth. (c,d). Representative nanoindentation graphs. The nanoindenter tip was pressed into the embedded and polished tooth. The graphs show the local changes in Young's modulus and hardness from the tooth surface (outer layer) to the inner tooth structure. In some localities, the teeth possessed an outer layer (leading or trailing edge), whereas on other localities, the outer layer was removed by polishing. (c) Young's modulus versus displacement into the surface. (d) Hardness versus displacement into the surface. (e) SEM image of one fractured tooth with highlighted outer layer and inner structure. The red arrow represents the nanoindenter tip and the direction of the indentation. BA, basis; IS, inner tooth structure; LE, leading edge; OL, outer layer; ST, stylus; TE, trailing edge; TI, tip. Scale bars: (a) 200 µm; (b) 20 µm; (e) 500 µm.

Figure 4.

Results from the EDX analysis (inner structure and outer layer are pooled together). Elemental proportions (atomic percentage) of all teeth, pooled together, for Felimare picta (a) and Doris pseudoargus (b) are given. For values, see electronic supplementary material, table S1.

Figure 5.

Results from the EDX analysis (inner structure and outer layer are pooled together). For each species and each tooth type, atomic percentage of analysed elements is given. For values, see electronic supplementary material, table S3.

Figure 6.

Results from the EDX analysis (only the inner tooth structure). For each species, atomic percentage of analysed elements is given. For values, see electronic supplementary material, table S5.

Figure 7.

CLSM images of the working zones of investigated radulae. (a,b) Felimare picta. (a) Whole radula. (b) Magnification of inner lateral teeth. (c,d) Doris pseudoargus. (c) Whole radula. (d) Magnification of medial lateral teeth. BA, basis; iLT, inner lateral teeth; mLT, medial lateral teeth; oLT, outer lateral teeth; ST, stylus; TI, tip = denticle. Scale bars: (a) 600 µm; (b,d) 60 µm; (c) 800 µm.

Figure 8.

Results from nanoindentation: E and H of the inner tooth structure, sorted according to each species, are given. For values, see electronic supplementary material, tables S1 and S5. LT, lateral tooth.

Figure 9.

Results from nanoindentation: E and H of the leading and trailing edges, sorted to each species, are given. For values, see electronic supplementary material, table S5. LT, lateral tooth.

Figure 10.

Relationship between the mechanical properties H and E and the elementary proportions for Felimare picta (left) and Doris pseudoargus (right). Proportions of Ae, Ca and (for F. picta) Si show a close relationship with H and E. Si seems to enhance the mechanical properties in F. picta, and Ca in D. pseudoargus.

Figure 11.

Proposed mechanical behaviour of the teeth during interaction with Porifera. (a) Teeth interact with the sponge resulting in high local pressure at the tip. During this, the leading edge of the tooth experiences tension and the trailing edge compression. The high values of Young's modulus and hardness at the tip reduce local stress and deformation. (b) Afterwards, the more flexible and softer bases and styli probably allow the bending of the teeth to adjust to the Porifera structure, which could reduce overcritical stresses. Then, the radula is probably pulled towards the mouth opening by various muscles, transferring the torn sponge tissue into the pharynx.