Radular morphology
The radula of Lepidochitona cinerea is docogloss with seven teeth per row: the central tooth (Ct) is median, flanked by one small lateral tooth (Lt I), one dominant lateral tooth (Lt II), and one marginal tooth (Mt) on every side (Fig. 1a). The central tooth is small and slender with a small spoon-like cusp. The lateral tooth I is of rather similar shape as the central tooth but exceed it in size. The lateral tooth II is the largest and most prominent tooth on the radula, its cusps is of paw-like shape possessing three denticles. The marginal teeth are slender, their styli are of a roundish shape and their cusps have a spoon-like morphology.
Radular colour
Overall, the radula has a yellowish colour gradually shifting into orange from the radular sac to the working zone, while remaining somewhat transparent (Fig. 2a). The most prominent colour change during maturation can be observed in the lateral teeth II. In the radular sac they are of a rather uniformly distributed white-yellow colour. Within three rows, starting in tooth row 8, their cusps change colour from pale yellow to a deeper yellow. From row 10 on, cusps become red and turn black and no transparency is left after row 17. The shift in colour from the black cusps to the yellow colour of the stylus in abrupt and not gradual (Figs. 1g and 2a, Additional file 1: Fig. 7a). This area was termed previously as junction zone [see e.g. 50, 51].
The iron dominated lateral II tooth cusps of the treated specimens are similar in colour to the native ones; however, the styli and bases of the lateral teeth II as well as the centrals, lateral teeth I, marginals are rather whitish (Fig. 1h, Additional file 1: Fig. 7b, Additional file 1: Fig. 5a).
Tooth failure during breaking stress experiments
Certain mechanical behaviours are observed during breaking stress experiments (see Additional file 1: Fig. 3 and Additional file 1: Table 9): some teeth break close to the membrane at their basis or their styli, some are completely crushed, break underneath their junction zone or at their junction zone. In some cases, it is impossible to break teeth due to bending or twisting.
Native and treated radulae exhibit similar mechanical behaviours: wet marginal teeth, lateral teeth I, and central teeth bent in most experiments and thus do not fail; especially the younger teeth of the radular sac and mineralization zone. If breaking occurs, teeth either break at their basis or stylus (Additional file 1: Fig. 2b and c). Wet lateral teeth II of the working and mineralization zone fail underneath their junction zone. In the radular sac, these teeth are, however, capable of bending (in 36% of cases). Dry marginal teeth fail in all cases at their basis, dry lateral teeth I, and central teeth are always completely crushed. Dry lateral teeth II of the working and mineralization zones always fail at their junction zone (Additional file 1: Fig. 2b and c). In the radular sac, these teeth fail in 24 cases at their junction zone and are crushed in 23 cases.
Variability between the radulae, sorted to each treatment group (native or treated)
Elements
In all radulae of Lepidochitona cinerea (native and treated) we detect Na, K, Si, Mg, P, S, Cl, Ca, and Fe. These elements are found in all three zones of every individual. Proportions of every element in every radula are non-normally distributed (when the data from all teeth is pooled together). Kruskal–Wallis test detects no significant difference between the radulae of either the native or the treated group in the proportions of the individual element; only for Si we detect significant differences between radulae from the native group (see Additional file 1: Table 1).
Young’s modulus and hardness
In all individual radulae of Lepidochitona cinerea (native and treated), the data on E and H are normally distributed (when the data of all teeth is pooled together). ANOVA detects significant differences between the individuals of the native group for E and H and between the individuals of the treated group for H (see Additional file 1: Table 1).
Breaking force and stress
Breaking force and stress of every individual radula are normally distributed (when the data from all teeth is pooled together). T test detects no significant differences for both parameters between the individuals of the following groups: native dry, native wet, treated dry, treated wet (see Additional file 1: Table 1).
Parameters of the whole radulae (data from all zones is pooled together)
Elements
Proportions of all elements (Ae) are non-normal distributed in native and treated group. The native radulae possess more elements and Wilcoxon test detects significant difference between native and treated group for proportions of Ae (see Additional file 1: Table 2).
For the native radulae, we determine that Fe occurs with the highest proportion, followed by Ca, P, Na, Mg, S, Cl, K, and finally Si (see Additional file 1: Table 2). In the treated radulae, Fe is also present with the highest proportion, followed by Ca, Cl, P, K, S, Mg, Na, and finally Si.
Fe, Ca, Na, K, Si, Mg, P, S, and Cl proportions of native and treated radulae are non-normal distributed. Between treated and native radulae, Wilcoxon test detects no significant difference in Fe and K, but significant difference in Na, Mg, Si, P, Cl, and Ca (see Additional file 1: Table 2).
Young’s modulus and hardness
Both E and H values of the treated and the native group are non-normally distributed. The native radulae have higher E and H values than the treated radulae. Wilcoxon test detects significant difference for E and H between treated and native radulae (see Additional file 1: Table 2).
Breaking force and stress
Breaking force and breaking stress values at different testing condition (dry or wet) are normally distributed. Between dry and wet condition, t test detects significant differences for force and stress. Breaking force and stress are higher for wet teeth than for dry ones (see Additional file 1: Table 2).
Breaking force and stress sorted to the treated dry, treated wet, native dry, or the native wet group are normally distributed. Highest force must be exerted to break teeth of the treated wet radulae, followed by native wet, native dry, and finally treated dry ones. ANOVA detects significant differences between these groups (see Additional file 1: Table 2).
Highest breaking stress is calculated for the treated wet radulae, followed by native wet, treated dry, and finally native dry ones. ANOVA detects significant differences between these groups (see Additional file 1: Table 2).
Parameters of each ontogenetic zone (data from tooth types is pooled together)
Elements
Native. For native radulae, the highest proportion of Ae is found in the working zone, followed by the mineralization zone, and finally radular sac (see Additional file 1: Table 3). The same pattern is detected for Fe, P, Si, and Mg proportions. For Na, K, and Ca, the highest proportions are determined for the working zone, followed by the radular sac, and finally mineralization zone. For S and Cl, the highest proportions are found in the mineralization zone, followed by the radular sac, and finally working zone. Proportions of each element, sorted to each zone (radular sac, mineralization zone, working zone), are non-normally distributed and Kruskal–Wallis test detects significant differences between radular sac, mineralization zone, and working zone (see Additional file 1: Table 3).
Treated. For treated radulae, the highest proportion of Ae is determined for the working zone, followed by the mineralization zone, and finally radular sac (see Additional file 1: Table 3). The same pattern is found for Fe and Ca. For K, the highest proportion is detected in the working zone, followed by the radular sac, and finally mineralization zone. For Cl and S, the highest proportion is found in the mineralization zone, followed by the radular sac, and finally working zone. For Mg, the highest proportion is detected in the working zone, followed by the radular sac, and finally mineralization zone. Na, Si, and P are only found in one zone. Proportions of each element, sorted to each zone (radular sac, mineralization zone, working zone), are non-normally distributed. Kruskal–Wallis test determines no significant difference between proportions of the radular sac, mineralization zone, and working zone for Na, Mg, Si, P, and S (see Additional file 1: Table 3). Significant differences are found for Cl, K, Ca, Fe, and Ae.
Young’s modulus and hardness
For native and treated radulae, the highest E and H are found in the working zone, followed by the mineralization zone, and finally the radular sac (see Additional file 1: Table 3). The E and H for every zone are normally distributed and ANOVA detects significant differences between radular sac, mineralization zone, and working zone of either treated or native radulae (see Additional file 1: Table 3).
Breaking force and stress
Breaking force and stress, sorted to zone (radular sac, mineralization zone, working zone) and condition (treated dry, treated wet, native dry, or the native wet), are non-normally distributed. For every parameter, Kruskal–Wallis test detects significant differences (see Additional file 1: Table 3). The highest forces and the highest stress are calculated for breaking teeth of the working zone, followed by the mineralization zone, and finally the radular sac for every condition (see Additional file 1: Table 3).
Parameters of each tooth type (data from all zones is pooled together)
Elements
Native. The highest proportions of Ae, Ca, and Si are detected in the lateral teeth II, followed by the marginal teeth, lateral teeth I, and finally central teeth (see Additional file 1: Table 4). For Fe, the highest proportions are found in the lateral teeth II, followed by marginal teeth, central teeth, and finally lateral teeth I. For K, lateral teeth II have the highest proportions, followed by central teeth, marginal teeth, and finally lateral teeth I. Cl is mostly present in lateral teeth II, followed by lateral teeth I, marginal teeth, and finally central teeth. The highest proportions of S are measured in the marginal teeth, followed by lateral teeth II, lateral teeth I, and finally central teeth. Na and P are only detected in the lateral teeth II, here with the highest proportions, and in the marginal teeth. Mg is found only in the lateral teeth II.
Proportions of each element, sorted to each tooth type (central tooth, lateral tooth I, lateral tooth II, marginal tooth), are non-normal distributed and Kruskal–Wallis test detects in most cases significant differences (except for Si, S, and Cl) (see Additional file 1: Table 4).
Treated. For the treated radulae, the highest proportions of Ae, Fe, and Ca are detected in the lateral teeth II, followed by the marginal teeth, central teeth, and finally lateral teeth I (see Additional file 1: Table 4). K is determined in the lateral teeth II with the highest proportion, followed by marginal teeth, and central teeth, but not in the lateral teeth I. S is not detected in the marginal teeth; its highest proportion is found in the lateral teeth I, followed by lateral teeth II, and central teeth. P is not detected in marginals and centrals; its highest proportion is found in lateral teeth I followed by lateral teeth II. Si is only determined in marginal teeth, Mg in central teeth, and Na in lateral teeth II.
Proportions of each element, sorted to each tooth type (central tooth, lateral tooth I, lateral tooth II, marginal tooth), are non-normally distributed and Kruskal–Wallis test detects in most cases no significant differences (except for Mg, Ca, Fe, and Ae) (see Additional file 1: Table 4). Wilcoxon test (see Additional file 1: Table 5) detects significant differences between proportions of Ae, Ca, and S in treated and native radulae, sorted to each tooth type, and no significant differences between Fe and K proportions in treated and native radulae, sorted to each tooth type. For all other elements, the picture is rather puzzling.
Young’s modulus and hardness
For treated and native radulae, E and H values, sorted to each tooth type, are normally distributed. The hardest and stiffest teeth are always the lateral teeth II, followed by marginal teeth, lateral teeth I, and finally central teeth (see Additional file 1: Table 4). ANOVA detects significant differences for E and H between the tooth types within the either treated or native group (see Additional file 1: Table 4). Tukey–Kramer test reveals significant differences between E and H of treated and native radulae, sorted to each tooth type (see Additional file 1: Table 5).
Breaking force and stress
Breaking force and breaking stress, sorted to tooth type and condition (treated dry, treated wet, native dry, or the native wet), are normally distributed. For breaking force, ANOVA detects significant differences. For breaking stress, t test also detects significant differences (see Additional file 1: Table 4).
For each condition (treated dry, treated wet, native dry, or the native wet), the highest forces are needed for breaking lateral teeth II, followed by central teeth, lateral teeth I, and finally marginal teeth. The highest breaking stresses are calculated for lateral teeth II and the lowest one for marginal teeth (see Additional file 1: Table 4).
Tukey–Kramer test reveals significant differences in most cases for breaking force and breaking stress between dry or wet, treated or native condition (see Additional file 1: Table 5).
Parameters of each tooth type (sorted to each ontogenetic zone)
Elements
Elements are non-normal distributed. Regarding Ae, the Wilcoxon method detects significant differences between treated and native marginal teeth, lateral teeth II, lateral teeth I, and central teeth of both working and mineralization zones, but only between treated and native lateral teeth II and marginal teeth of the radular sac (see Additional file 1: Table 8). For Fe and K, no significant differences between treated and native teeth are detected. For Ca, all teeth of both working and mineralization zones are significantly different. Only the central teeth or radular sac are not different. For Cl, significant differences are detected in the mineralization zone for the lateral tooth I, lateral tooth II, and marginal tooth. For S, significant differences are detected for each tooth type of the mineralization zone. For the radular sac, only the lateral tooth I and marginal teeth differ significantly. For the working zone, only the lateral teeth II differ significantly. With regard to P and Mg, lateral teeth II from the working zone and mineralization zone differ significantly. For Si, differences are detected for lateral teeth II of the working zone and for marginal teeth of the mineralization zone. For Na, treated and native lateral teeth II are significantly different in each zone. In the radular sac, marginal teeth differ significantly.
Native. Whole element content is always highest in lateral tooth II in every zone (see Additional file 1: Table 6). In the radular sac, it is followed by marginal tooth, central tooth, and finally lateral tooth I. In the mineralization and working zones, it is followed by the central tooth, lateral tooth I, and finally marginal tooth.
When analyzing the individual elements, Fe is present in the highest proportions, however, only in lateral tooth II (Additional file 1: Fig. 1). Here proportions increase during ontogeny. For all other teeth, Fe is detected in very small proportions and the distribution does not seem to follow a clear pattern. In the central teeth, the proportions increase during ontogeny. In the lateral tooth I, they decrease from the radular sac to the working zone. For the marginal teeth, Fe proportions first increase from the radular sac to the mineralization zone, but then decrease to the working zone.
The second most abundant element is Ca. In every zone, the lateral tooth II always contains the highest proportions of Ca (Additional file 1: Fig. 1). In the radular sac, it is followed by the marginal tooth, central tooth, and finally lateral tooth I. In the mineralization and working zones it is followed by the central tooth, lateral tooth I, and finally marginal tooth. For the central tooth, lateral tooth I, and marginal tooth, Ca proportions increased during ontogeny. For the lateral tooth II, Ca proportions first increase from the radular sac to the mineralization zone, but then decrease to the working zone.
However, for many elements, especially those that are rarely detected as trace elements, (Na, Si, K, Mg, P, S, Cl), the elemental distributions are rather puzzling and no clear gradients during ontogeny can be detected for the individual tooth types (Additional file 1: Fig. 1; here Na, Mg, Si, P, S, Cl, and K are summarized as Te ‘trace elements ‘). Mg is only detected in the lateral tooth II, its proportions slightly increases during ontogeny. P is detected in the lateral tooth II and marginal tooth, increasing during ontogeny. S and Cl proportions first increase from the radular sac to the mineralization zone, but then decrease in the working zone in each tooth type.
Treated. The highest proportions of Ae are found in the lateral tooth II, in every zone (see Additional file 1: Table 6). In every zone, the content decreases in the series central tooth—> marginal tooth—> lateral tooth I.
Fe is highly present only in the lateral tooth II (Additional file 1: Fig. 1). Highest Fe proportions are detected in the working zone, followed by the mineralization zone, and finally the radular sac. For all other teeth, Fe is detected in very small proportions and its distribution does not seem to follow a clear pattern. However, for central teeth, proportions increase during ontogeny. For the lateral tooth I, the amount of Fe decreases from the radular sac to the working zone. For the marginal teeth, Fe proportions first increase from the radular sac to the mineralization zone, but then decrease to the working zone.
Ca is less abundant than in native radulae (Additional file 1: Fig. 1). However, in each zone, the lateral tooth II contains the highest proportions of Ca. The distribution pattern differs however from the native radulae. In the radular sac and working zone, it is followed by the central tooth, marginal tooth, and finally lateral tooth I. In the mineralization zone, it is followed by the marginal tooth, central tooth, and finally lateral tooth I. For the central tooth, lateral tooth II, and marginal tooth, the highest Ca proportions are detected in the working zone, followed by mineralization zone, and finally radular sac. For the lateral tooth I, Ca is only detected in the mineralization zone.
Na, Mg, Si, P, S, Cl, K are only very occasionally detected in treated radulae (Additional file 1: Fig. 1, here Na, Mg, Si, P, S, Cl, and K are summarized as Te, ‘trace elements ‘).
Young’s modulus and hardness
Both E and H data are normally distributed. The Tukey–Kramer method detects significant differences between treated and native marginal teeth, lateral teeth II, lateral teeth I, and central teeth of the working zone (see Additional file 1: Table 8). For the mineralization zone, significant differences are detected between native and treated lateral teeth II, lateral teeth I, and marginal teeth. For the radular sac, significant differences are detected between native and treated lateral teeth II, and marginal teeth.
Native. Each tooth type becomes stiffer and harder during ontogeny (see Additional file 1: Fig. 1 and Additional file 1: Table 6). In each zone, the lateral tooth II is always the hardest and stiffest one. In the working zone, it is followed by the lateral tooth I, central tooth, and finally marginal tooth. In the mineralization zone and radular sac, it is followed by marginal tooth, lateral tooth I, and finally central tooth.
Treated. Each tooth type is the stiffest and hardest in the working zone, followed by the mineralization zone, and finally radular sac (see Additional file 1: Fig. 1 and Additional file 1: Table 6). In each zone, the lateral tooth II is always the hardest and stiffest one. It is followed by the central tooth, lateral tooth I, and marginal tooth in the working zone; by the marginal tooth, lateral tooth I, and central tooth in the mineralization zone and radular sac.
Breaking force and stress
Sorted to each condition (treated dry, treated wet, native dry, or the native wet), breaking force and breaking stress are non-normally distributed. In most cases, significant differences between the treated and native tooth types in every zone are detected (see Additional file 1: Table 8).
Native. Tested under dry condition, the highest force (see Fig. 3c and Additional file 1: Table 7) must be exerted to break the lateral teeth II, followed by central teeth, lateral teeth I, and finally marginal teeth in each zone. Tested under wet condition, the same pattern is found for teeth of the mineralization and working zones. For the wet radular sac, the lateral teeth II are, however, followed by the lateral teeth I, central teeth, and finally marginal teeth. The breaking stress does not fully relate to this pattern (see Additional file 1: Fig. 2a and Additional file 1: Table 7). For every dry radular zone, the highest breaking stress is calculated for the marginal teeth and the lowest one for the lateral teeth II. Tested under wet condition, the highest stress is found for the lateral teeth II and the lowest one for the marginal teeth for each zone.
Treated. Tested under dry condition, the highest force must be exerted to break lateral teeth II, followed again by central teeth, lateral teeth I, and finally marginal teeth of each zone (see Fig. 3c and Additional file 1: Table 7). Tested under wet condition, the same pattern is found for teeth of the radular sac and mineralization zone. For the wet working zone, the lateral teeth II are followed by lateral teeth I, central teeth, and finally marginal teeth. The breaking stress does not fully relate to this pattern (Additional file 1: Fig. 2a). For each zone, the highest breaking stress is calculated for the marginal teeth and the lowest one for the lateral teeth II. Tested under wet condition, the highest stress is found for lateral teeth II and the lowest one for the marginal teeth for each zone.
Ontogenetic changes of each tooth type (focus on gradients within each tooth)
Elements
Overall, no gradients within teeth are determined for Na, Mg, Si, P, S, Cl, and K, since these elements are only occasionally detected in native and treated radulae (see Fig. 4, here these elements are summarized as Te ‘trace elements ‘, and Additional file 1: Table 10). We therefore focus on Ca, Fe, and Ae. Sorted to the tooth type, tooth area (basis, stylus, cusp), and zone, these elements are normally distributed.
Native. For Ca, no clear gradients are detected within central teeth and lateral tooth I (see Fig. 4 and Additional file 1: Table 10). However, the lateral tooth II and marginal tooth exhibit gradients. For the lateral tooth II, gradients are already present in the radular sac (here the basis contains the highest proportion of Ca, followed by the cusp, and finally the stylus). In the mineralization and working zone, the cusps contain most Ca, followed by the stylus, and finally the basis. For the marginal teeth, gradients are detected in the mineralization and working zones. In both zones, the stylus contains the most Ca, followed by the cusp, and finally the basis. For Fe, a gradient is detected in only lateral teeth II (see Fig. 4); the cusps contain the highest Fe proportions, followed by the stylus and finally by the basis in each zone.
Treated. With the one exception (lateral tooth II of the working zone), all teeth are homogenous in their Ca distribution (see Fig. 4 and Additional file 1: Table 10). For Fe, the same pattern as in native radulae is detected (see Fig. 4).
Young’s modulus and hardness
Sorted to the tooth type, tooth area (basis, stylus, cusp), and zone, E and H are normally distributed. Within each tooth type, with some exceptions in the radular sac, t test (for the central and lateral tooth I) or ANOVA (for the lateral tooth II and marginal tooth) detects significant differences between basis, stylus, and cusps for treated and native radulae (see Additional file 1: Table 10).
Native. In the radular sac, no gradients within the central tooth are detected (see Fig. 4 and Additional file 1: Table 10). They are detectable for the lateral tooth I and marginal tooth and pronounced in the lateral tooth II. The cusp of the tooth are always the stiffest and hardest areas and the stylus was much softer. For the lateral tooth II and marginal tooth E and H of the tooth basis are also measured. In the marginal teeth, the basis is softer and more flexible than the stylus, but for the lateral teeth II, the stylus is softer than the basis. In the mineralization and working zones, all teeth possess pronounced gradients from the cusp, as the stiffest and hardest part, across the stylus to the basis, as the softest and most flexible part.
Treated. In the radular sac, no gradients within the tooth are detected for the lateral tooth I and the marginal tooth (see Fig. 4 and Additional file 1: Table 10). Gradients are present in the central tooth and are pronounced in the lateral tooth II. Here the cusp is again the stiffest and hardest part and the stylus is much softer. In the lateral teeth II, the stylus is again softer than the basis. In the mineralization and working zones, gradients are similar to those of the native group.
Ontogenetic changes between the individual tooth rows
Elements
Native and treated. Proportions of Fe, Ae, and Te in each tooth part gradually increase (see Additional file 1: Fig. 4, Additional file 1: Fig. 5, Additional file 1: Fig. 6, and Additional file 1: Table 12). Ca proportions in the native lateral teeth II first increase during ontogeny and then decrease after row 25 (see Fig. 2 and Additional file 1: Table 12). The Ca content of the treated lateral teeth II fluctuates highly between tooth rows. Proportions of Ae and the Te are significantly lower in each treated tooth row (see Additional file 1: Figs. 4 and 6, and Additional file 1: Table 12).
Young’s modulus and hardness
Native. E and H values of all central tooth parts, all lateral tooth I parts, lateral tooth II cusp and basis, and all marginal tooth parts rather increase gradually during ontogeny (see Fig. 2, Additional file 1: Table 11, and Additional file 1: Table 12). E and H values of the lateral tooth II stylus increase strongly from row 17 to row 22.
Treated. In the treated radulae, the E and H values of all parts of the central and lateral teeth I, the stylus and bases of the lateral teeth II, and all parts of the marginals do not increase much (see Additional file 1: Fig. 5 and Additional file 1: Table 12). The E and H values of the lateral teeth II cusps seem to be, however, not affected by treatment.
Relationship between elements, mechanical properties, and mechanical behaviour
Data from treated and native radulae pooled together
H and E are highly correlated (r = 0.99; see Additional file 1: Table 13 and Additional file 1: Figs. 8 and 9). H correlates with Fe proportion by a correlation coefficient of 0.76 and with Ca with the one of 0.68. For E, a row-wise correlation with Fe of 0.70 and with Ca of 0.70 is obtained. For Na, Mg, Si, P, S, Cl, and K, low correlation coefficients are determined. When relating the mean values of measured mechanical properties (H and E), element proportions (Ca and Fe) to the mean values of breaking force and breaking stress (see Additional file 1: Table 24 and Additional file 1: Figs. 8 and 9), we detect, that H, E, Fe, Ca, dry breaking force, wet breaking force, and wet breaking stress correlate with each other. Dry breaking stress correlates with wet breaking stress to 0.74 and to Fe with 0.34, but to low correlation coefficients with H, E, Ca, dry breaking stress, and wet breaking stress.
Sorted to treatment
For all native teeth together, H and E highly correlate (r = 0.99; see Additional file 1: Table 14). H correlates with Fe proportion by a correlation coefficient of 0.70 and with Ca with the one of 0.71. For E, correlation factors with Fe and Ca of 0.69 are calculated. For Na, Mg, Si, P, S, Cl, and K, small correlation coefficients are determined. For all treated teeth pooled together, H correlates with E (r = 0.94; see Additional file 1: Table 15). H correlates with the Fe proportion by a correlation coefficient of 0.77, but with Ca only with the one of 0.09. E and Fe correlate with the factor of 0.85. Ca and H are related by a correlation factor of 0.10. For Na, Mg, Si, P, S, Cl, and K, very small correlation coefficients are determined.
Sorted to treatment and tooth type
Central tooth, native. H and E highly correlate (r = 1.00; see Additional file 1: Table 16). H and E correlate less with Ca (r = 0.32). All other determined correlation coefficients are very small. H, E, Fe, Ca, dry breaking force, and wet breaking force highly correlate: the highest correlation coefficient (1.00) is detected between H and E and the lowest one (0.97) between Ca and Fe (see Additional file 1: Table 25).
Lateral tooth I, native. H highly correlates with E (r = 1.00; see Additional file 1: Table 17). H and E correlate less with Ca (r = 0.28). All other determined correlation coefficients are very small. H, E, Ca, dry breaking force, and wet breaking force highly correlate with each other (see Additional file 1: Table 26). Fe correlates with all other parameters with relatively low correlation coefficients (0.38–0.49).
Lateral tooth II, native. H and E are highly correlated with each other (r = 0.99; see Additional file 1: Table 18). Both H and E are also highly correlated with Fe (r of H = 0.79 and of E = 0.80) and Ca (r of H or E = 0.73). They are less correlated with P (r of H = 0.29 and of E = 0.30). H, E, Fe, Ca, dry breaking force, and wet breaking force highly correlate; the highest coefficient (1.00) is detected for H and E and the lowest ones between Ca and wet breaking force (0.76) and between Ca and Fe (0.76) (see Additional file 1: Table 27). All other determined correlation coefficients are very small.
Marginal tooth, native. H and E are highly correlated with each other (r = 0.99; see Additional file 1: Table 19). Both H and E are only slightly correlated with Ca (r of H = 0.23 and E = 0.24). H, E, Ca, dry breaking force, wet breaking force, dry breaking stress, and wet breaking stress highly correlate; the highest coefficient (1.00) is detected for H and E and the lowest ones between Ca and wet breaking force (0.72) and between H and wet breaking stress (0.76) (see Additional file 1: Table 28). Fe negatively correlates with each parameter. All other determined correlation coefficients are very small.
Central tooth, treated. H and E are highly correlated with each other (r = 0.99; see Additional file 1: Table 20). H, E, Ca, dry breaking force, and wet breaking force are highly correlated (see Additional file 1: Table 29). Fe negatively correlates with each parameter. All other correlation coefficients are very small.
Lateral tooth I, treated. H and E are highly correlated with each other (r = 0.99; see Additional file 1: Table 21). H, E, dry breaking force, and wet breaking force highly are correlated (see Additional file 1: Table 30). Ca negatively correlates with each parameter: with H, E, dry breaking force, and wet breaking force at low negative coefficients (-0.17 – -0.21) and with Fe with a high negative coefficient (-0.94). Fe correlates with all parameters with a low positive coefficient (0.48–0.52). All other correlation coefficients are very small.
Lateral tooth II, treated. H and E are highly correlated with each other (r = 0.92; see Additional file 1: Table 22). Both E and H are highly correlated with Fe (r of H = 0.82 and of E = 0.94). H, E, Fe, Ca, dry breaking force, and wet breaking force are highly correlated: the highest coefficient (1.00) is detected for H and dry breaking stress and the lowest one between Ca and H (0.86) (see Additional file 1: Table 31). All other correlation coefficients are very small.
Marginal tooth, treated. H and E are highly correlated with each other (r = 0.92; see Additional file 1: Table 23). H, E, Ca, dry breaking force, wet breaking force, dry breaking stress, and wet breaking stress are highly correlated: the highest coefficients (1.00) are detected between wet breaking force and E as well as between Ca and E; the lowest ones between dry breaking force and wet breaking stress (0.81) as well as between wet and dry breaking stress (0.81) (see Additional file 1: Table 32). Fe correlates negatively with each parameter. All other correlation coefficients are very small.
3-way ANOVA reveals that state (native or treated), tooth type, and radular zone all have significant effects on E and H values (p < 0.0001 for each, see Additional file 1: Fig. 10 and Additional file 1: Tables 33–34). 4-ANOVA detects that state (native or treated), tooth type, condition (dry or wet), and radular zone all have the same significant effect on E and H values (p < 0.0001 for each, see Additional file 1: Fig. 11 and Additional file 1: Tables 35–36). By PCA for marginals, centrals, and lateral teeth I no clusters are detected (see Additional file 1: Fig. 12).