The transition from stiff to compliant materials in squid beaks

Abstract

The beak of the Humboldt squid Dosidicus gigas represents one of the hardest and stiffest wholly organic materials known. As it is deeply embedded within the soft buccal envelope, the manner in which impact forces are transmitted between beak and envelope is a matter of considerable scientific interest. Here, we show that the hydrated beak exhibits a large stiffness gradient, spanning two orders of magnitude from the tip to the base. This gradient is correlated with a chemical gradient involving mixtures of chitin, water, and His-rich proteins that contain 3,4-dihydroxyphenyl-L-alanine (dopa) and undergo extensive stabilization by histidyl-dopa cross-link formation. These findings may serve as a foundation for identifying design principles for attaching mechanically mismatched materials in engineering and biological applications.

Figure 1

Beak of the Humboldt squid Dosidicus gigas. (A) Side view of a full upper beak after removal from the buccal mass showing natural pigmentation. (B) Split beak highlighting the relation of the wing to the rostrum. (C) Dissected wing. (D) Beak in (A) after staining for dopa-containing proteins with a catechol-specific reagent. (E) Series of sections representing different shades of the pigmentation gradient.

Figure 2

Chemical and gravimetric analyses of squid beak cutouts. (A) Absorbance spectra of acid hydrolysates. GA, glucosamine; Hylys, hydroxylysine. (B) Compositions of the dominant amino acids in the hydrolysates. (C) Optical image of the whole beak after alkaline peroxidation (top), and high-magnification scanning electron image of the chitin fiber network in the rostrum (bottom). (D) Chitin, protein, and pigment contents ascertained from gravimetric measurements.

Figure 3

Dopa and cross-link derivatives. (A) His-dopa peptide (m/z 335) and His-dopa cross-link (m/z 351) captured by phenylboronate chromatography of a hydrolyzed beak. (B) Prominent ion fragments in acidinsoluble black pigment, obtained by laser desorption/ionization–time-of-flight mass spectrometry.

Figure 4

Mechanical properties of squid beaks. (A) Typical tensile stress/strain curves for hydrated beak specimens. Tests could not be performed on the most heavily tanned material at the rostrum because of its size and shape. (B) Summary of Young’s moduli obtained for dry and hydrated coupons by tensile testing and/or nanoindentation. Nanoindentation tests proved useful for measuring E for all but the two softest states; in untanned and lightly tanned rehydrated samples, the displacements necessary for reliable modulus measurement exceeded the accessible range of the instrument. Tests on peroxidized samples showed no effect of the initial pigmentation level, and thus all data have been combined (far right). Error bars indicate 1 SD. (C) Young’s modulus plotted against both chitin content and combined chitin/protein content in both dry and hydrated coupons. Error bars indicate 1 SD.