Claus Hedegaard1 and Hans-Rudolf Wenk2

RESEARCH NOTES 133

REFERENCES

KNUDSEN, J. 1991. Proc 3rd Int. Mar. Biol. Work-shop: the Marine Flora & Fauna of Albany, W.Australia, 641-660. Western Australian Museum,Perth.

2. LAWS, H.M. 1971. Vetiger, 13:115-121.3. BARASH, A.L. & DANIN, Z. 1986. Spixiana, 9:117-

141.4. MATSUNAGA, N. 1964. Venus,23:149-157.5. CERNOHORSKY, W.0.1968. Veliger, 10: 275-280.

J. Moll. Stud. (1998), 64,133-136 © The Malacological Society of London 1998

Microstructure and texture patterns of mollusc shells

Claus Hedegaard1 and Hans-Rudolf Wenk2

'Institute of Biology, Department of Ecology and Genetics, University of Aarhus, 8000 Aarhus C, Denmark;Museum of Paleontology, and, Department of Integrative Biology r University of California, Berkeley,

California 94720, USA ([email protected])2Department of Geology & Geophysics, University of California, Berkeley, California 94720, USA

(wenk@seismo. Berkeley. Edu)

Mollusc shells are polycrystalline composites ofcalcium carbonate and proteins and glycoproteins,shielding the animals from the environment. Theirstrength and toughness are crucial for survival of theindividuals. These properties depend on micro-structural characteristics, phase (calcite or aragonite),grain morphology and aggregation1, and on crystal-lite orientation (texture)2. In this report we use quanti-tative x-ray texture analysis and scanning electronmicroscopy to investigate the relationship betweenmorphology of some aragonitic shell structures andthe preferred crystallographic orientation. We find thatthere is no simple relationship between morphologyand crystallography, and that patterns of preferredorientation vary substantially. This has importantimplications for our understanding of conchiferanevolution, and the mechanical properties of shells.

The nature of the organic material determineswhether the calcium carbonate is deposited as trigo-nal calcite or orthorhombic aragonite, as well as themorphology of the structural elements34. The crystalstructure of the calcium carbonate is induced by theorganic material5-6. Investigations of gastropod shellstructures usually emphasize the morphology7-8-9, orthe crystallographic properties of local elements10"12.

Texture analysis is an increasingly importantmethod to determine the orientation distribution ofcrystals in composite materials and relating them toanisotropic properties13. Texture may develop duringdeformation (e.g., rolling and wire drawing of metals,tectonic deformation of rocks) or during growth (e.g.,epitaxial films on various substrates). Conventionallytexture information is represented in pole figures,displaying the statistical alignment of lattice planeshkl relative to the sample surface. They are oftenmeasured by x-ray diffraction with a pole figuregoniometer14. The pole figure goniometer brings thesample into different orientations and the recordedintensity (after proper corrections) is proportional tothe number of lattice planes that are in Bragg orien-tation. We used shell fragments a few millimetres insize, parallel to the fairly flat interior surface, cutfrom large (3-15 cm) shells as samples. The x-ray

beam (in our case FeKa) covers a surface of 1-2 mmin diameter and due to absorption, penetrates to adepth of 50-100 microns. Given the size of first-orderelements of approximately 10 microns, we obtain anaverage over 500-1000 elements in a small sector ofthe shell. Orthorhombic aragonite has four reason-ably separated diffraction peaks (221, 102, 111 and012), amenable to pole figure measurements. In eachcase, we measured pole figures on several shell frag-ments to ensure that the results are representative oflarge segments in a single shell and of different indi-viduals of the same species.

We examined the textures of the nacreous (mother-of-pearl) layers of the gastropods Haliotis cracherodiLeach, 1814, Tectus pyramis (Born, 1778), Turbopetholatus Linnaes, 1758, Perotrochus quoyanus(Fischer & Bernardi, 1856), of the bivalves Atrinamaura (Sowerby, 1835), Pinna nobilis Linnss, 1758,Pinctada margaritifera (Linnaes, 1758), P. maxima(Jameson, 1901), Neotrigonia sp., Lampsilis alatus (Say,1817), and of the cephalopods Nautilus macro-mphalus Sowerby, 1849, and Nautilus pompiliusLinnaes, 1758, and the inner crossed lamellar layers ofthe gastropod Scutellastra tabularis (Krauss, 1848)and of the bivalve Fragum fragum (Linnses, 1758).

Nacreous structures are "aragonitic laminar struc-tures consisting of polygonal to rounded tabletsarranged in broad, regularly formed, parallel sheets"15.Sheet nacre (Figure 1), found in some bivalves andMonoplacophora, is deposited over most of the innersurface of the shell, and the tablets are stacked in abrick wall pattern. Columnar nacre (Figures 2-3),found in some gastropods and cephalopods, isdeposited near the margin of the shell, and the tabletsare stacked in columns with coinciding centres.

Simple crossed lamellar structures (Figures 4-5)have lamellae, composed of thin, mutually parallellaths or rods, which show two non-horizontal dipdirections, alternating in adjacent lamellae7*15.

The recorded pole figures are difficult to interpret.We followed metallurgical practice and calculated thefull three-dimensional orientation distribution (OD)from the pole figures16 for orthorhombic crystal and

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134 RESEARCH NOTES

(100) (010) (110)

3.

5.

Figures 1-5. All the shell structures are from fractures cross-sections of the shells; the exteriors of the shells areat the top and interiors at the bottom of the figure. Scale bars are 20 (j.m. In the corresponding recalculated ODpole figures, (100) is the pole of the a-axis, (010) the pole of the b-axis, (001) the pole of the c-axis, and (110) isthe pole of the twin plane. Pole figures are recorded, so the plane of the shell is in the plane of the figure, andthe margin of the shell parallel to the top of the figure. Logarithmic contour lines (0.5, 0.7, 1 m.r.d.) give poledensities in multiples of a random distribution. A dot pattern marks pole densities below 1 m.r.d. Equal areaprojection onto the surface of the shell. Figure 1. Sheet nacre and corresponding pole figures from Pinctadamargaritifera. Figure 2. Columnar nacre and corresponding pole figures from Nautilus macromphalus. Figure 3.Columnar nacre and corresponding pole figures from Haliotis cracherodi. Figure 4. Simple crossed lamellarstructure and corresponding pole figures from Scutellastra tabiilaris. Figure 5. Simple crossed lamellar structureand corresponding pole figures from Fragum fragum.

triclinic sample symmetry, using the WIMV algo-rithm in the texture package BEARTEX". From theOD any pole figure can be recalculated, including

those which can not be measured directly. It is thefirst application of quantitative texture analysis tosuch low symmetry. Figures 1-5 display recalculated


RESEARCH NOTES 135

pole figures for 100, 010, 001 and 110 which are mostinformative, for nacres from a bivalve, a cephalopod.a gastropod, and crossed lamellar structures from agastropod and a bivalve.

All textures are very strong with OD maxima rang-ing from 175 times that of samples with a randomorientation distribution (m.r.d.) for N. macro-mphalus to 1500 m.r.d. for P. margaritifera. This ismore than ten times stronger than textures in verystrongly deformed metals and similar to epitaxialfilms. Yet these are not single crystals as is obviousfrom Figure 1 and from the scattering width oftexture peaks, which is about 15-20 degrees at halfmaximum for (001) peaks from nacre.

The simplest texture pattern is that of nacre fromP. margaritifera (Figure 1). It contains a single orien-tation component, resembling a single crystal withthe c-axis perpendicular to the surface of the shell,the b-axis in the growth direction, and the a-axis par-allel to the margin. Consequently, the individualtablets in bivalve nacre (Figure 1) are highly alignedsingle crystals, contrary to the view that presentsthem as cyclic twins10".

The nacre from N. macromphalus also has crystal-lographic c-axes perpendicular to the shell and a-axesparallel to the margin (b-axes in the growth direc-tion), but in addition there are two other componentsat 60 degrees (Figure 2). They can be explained astwinning on (110), the common twin law in aragonite.The 100,010 and 110 pole figures display a character-istic pseudo-hexagonal pattern. From texture analysiswe can not determine whether crystallites areactually twinned, or platelets are arranged merely ina twin-like pattern.

The nacre from Haliotis cracherodi also has c-axesperpendicular to the shell, but a- and b-axes spinfreely around the c-axis, resulting in an axial fibertexture (Figure 3).

The crossed lamellar structures of S. tabularis(Figure 4) and F. fragum (Figure 5) display morecomplicated textures. There are two c-axes maximainclined about 20-251 to axes maxima correspond tothe two dip directions of the lamellae, i.e. basalplanes (001) are parallel to the lamellae. In additionthere are two (110) twin orientations with a-axes andb-axes oriented accordingly. TEM investigationshave previously determined that microstructuraltwins exist in crossed lamellar molluscs12, and opticstudies78' have demonstrated different orientationsof c-axes in adjacent lamellae. The orientation pat-tern of S. tabularis is more highly ordered than thatof F. fragum.

The results in Figures 1-5 are representative of theshell structures in the molluscan classes. All bivalvenacres give textures as seen in Figure 1, both cepha-lopod nacres the pattern of Figure 2, and all gastro-pod nacres the pattern of Figure 3. Species within theclasses show different intensities of the variouspeaks, but no qualitative differences.

The crystallographic orientation pattern in molluscshells is a dominant feature. They are very distinct andhighly diverse. Textures may correlate with micro-structures (such as for crossed-lamellar structures)

or similar microstructures may have very differenttextures (such as for nacres).

We analysed several samples from different partsof the she'll of the same individual, and found thattextures do not vary between sites, with the im-portant caveat that the axes are not parallel in theentire shell, but oriented relative to the margin andsurface of the shell. In other words, the b-axes of P.margaritifera nacre are perpendicular to the margin,pointing away from the centre of the shell in a radiat-ing pattern.

The results force us to reconsider some tenets ofshell properties and of molluscan evolution. Inferredfrom striations in etched nacre tablets. Mutvei"1"suggested individual tablets are cyclic twins, similarto the pseudo-hexagonal (110) trillings of aragonite.or complex polysynthetic twins. This is obviously notthe case for bivalve nacre, that shows virtually notrace of twins. Mutvei's" "more soluble" and "lesssoluble" sectors may be growth sectors, crystallo-graphically nonequivalent. time-synchronous sectorswith different chemical composition and solubil-ity18", rather than individual crystals. We agree withTaylor, Kennedy & Hall9 that bivalve nacre tabletsare single crystals. We identify this as a general pat-tern within each shell and also in different bivalvespecies. Furthermore, we establish tablets are stronglyaligned to each other.

In gastropods we find no evidence for a close latticerelationship between adjacent tablets, whereas RamaSwamy20 found a slightly preferred orientation.

In these and other samples, we have observed onegeneral feature of texture patterns in mollusc shells.The crystallographic c-axis is always oriented largelytowards the interior of the shell, the a-axis usuallyparallel or subparallel to the margin, and the b-axisconsequently parallel to the direction of growth.Interestingly, the direction parallel to the a-axis inaragonite has by far the highest stiffness coefficient2

and textures with this direction parallel to the shellsurface may be mechanically advantageous. Studiesof the mechanical strength of shells emphasize themorphological microstructure1-2122, but it seems thatfor mechanically very anisotropic crystals the prop-erty anisotropy is important for a shell's functionalityand ought to be included in mechanical models.

Except for stacking pattern, the differences be-tween nacres are often ignored, and the differencesare treated as variations of a plesiomorphic conchi-feran character. We demonstrate substantial crystal-lographic differences between nacres from thevarious molluscan classes, and also that the proper-ties are constant within each class. Recent analyseshave implicitly23-24 or explicitly25 questioned the plesio-morphic condition of nacre in gastropods. Our resultsdemonstrate, that nacres from the molluscan classesdiffer between but not within the classes. A compre-hensive phylogenetic analysis is needed to testwhether all molluscan nacres are homologous andsymplesiomorphic, or whether the observed texturaldifferences are merely apomorphic variations in aplesiomorphic character.


136 RESEARCH NOTES

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2. SIMMONS, G. & WANG, H. 1971. Single crystalelastic constants and calculated aggregate proper-ties: a handbook, 2d ed. MIT Press, Cambridge,Massachusetts.

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7. B0GGILD, O.B. 1930. K. danske. Vidensk. Selsk,Skr., Nat. vid. Mat. Afd., 9. Rk., II.2:231-325.

8. HEDEGAARD, C. 1990. Shell structures of therecent Archaeogastropoda. Thesis University ofAarhus, Denmark, vols. 1-2 (published by Uni-versity Microfilms).

9. TAYLOR, J.D., KENNEDY, W.J. & HALL, A.1969. Bull. Br. Mus. nat. Hist., Zooi, Suppi, 3:1-125.

10. MUTVEI, H. 1979. Scanning Electron Microscopy,1979.2: 457-462.

11. MUTVEI, H. 1980 in The mechanisms ofbiominer-alization in animals and plants; (M. Omori & N.Watabe eds), 49-56. Tokai University Press.

12. WILMOT, N.V., BARBER, D.J., TAYLOR, J.D. &GRAHAM, A.L. 1992. Phil. Trans. R. Soc, B, 337:21-35.

13. KOCKS, U.F., TOME, C. & WENK, H.R. 1996.Textures and arisotropy. Cambridge UniversityPress, London.

14. WENK, H.R. 1985. Preferred orientation in de-formed metals and rocks: An introduction tomodern texture analysis. Academic Press,Orlando.

15. CARTER, J.G., CLARK II, G.R. 1985. in Mollusks,notes for a short course, organized by D.J. Bottjer,C.S. Hickman and P.D. Ward (T.W. Broad-head ed.) 13: 50-71. University of TennesseeDepartment of Geological Sciences Studies inGeology.

16. BUNGE, H.J. 1962. Mathematische methoden dertexturanalyse, Akademie Verlag, Berlin.

17. WENK, H.R., MATTHIES, S. & DONOVAN, J. 1996.In: Textures of materials (Z. Liang, L. Zue, & Y.Chu, eds.) International Academic Publishers,Beijing.

18. REEDER, R.J. & GRAMS, J.C. 1987. Geoch.Cosmo. Acta, 51:187-194.

19. PAOUETTE, J. & REEDER, R.J. 1990. Geology, 18:1244-1247.

20. SWAMY, S.R. 1935. Proc. Ind. Ac. Sci., 11(4): 345-351, pi. XVII-XVIII.

21. CURREY, J.D. 1990. in Skeletal biomineraliration:Patterns, processes and evolutionary trends (J.G.Carter, ed) . Vol. 1, 11-26. Van Nostrand Rein-hold, New York.

22. JACKSON, A.P., VINCENT, J.F.V. & TURNER, R.M.1988. Proc. R. Soc. Lond. B, 234(1277): 415-440.

23. HASZPRUNAR, G. 1988. J. Moll. Stud., 54: 367-441.

24. PONDER, W.F. & LINDBERG, D.R. 1995. in Originand evolutionary radiation of the Mollusca (J.D.Taylor, ed.), 135-154. Oxford University Press,London.

25. HEDEGAARD, C. 19977. Moll. Stud., 63: 369-377.


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Claus Hedegaard1 and Hans-Rudolf Wenk2