The structural and compositional basis for the transparency and physical properties of the Skogsbergia lerneri carapace

Authors: Benjamin Rumney (Cardiff University)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: September 2020

Abstract: Background: Skogsbergia lerneri (Kornicker, 1958) is a species of ostracod which, like all crustaceans, has a protective exoskeleton (or carapace). As well as being used for a protective function, the S. lerneri carapace is also transparent. Understanding how the carapace is transparent while retaining its protective function was the primary aim of this thesis. Results: Ultrastructural analysis showed that the carapace consisted of an epicuticle, exocuticle and an endocuticle split into a calcified, crystalline endocuticle and a laminated endocuticle. Numerous structural adaptations were seen to minimise refractive index changes at the layer intersections. These layers developed along with growth of the ostracod, maintaining similar proportions throughout. Microstructural analysis identified a chitin based second harmonic generated signal from the carapace. Pixel analysis of this signal showed a consistent level of chitin expression (~60-80% of the total carapace) throughout all the developmental stages except instar 4, which showed a lower level of expression. Elemental analysis showed that the calcified, crystalline layer comprised mainly calcium, oxygen and magnesium, leading to the assumption that the structures were calcium carbonate. X-ray absorption near edge structure analysis revealed that the calcium carbonate consisted of entirely amorphous calcium carbonate in the early to middle stages of development and a mix of amorphous and aragonite in the later stages. Unlike most crustaceans, calcite was not seen in any carapaces. Optical testing showed a consistent refractive index across all stages (1.401-1.406) leading to a mean 99.94% transmission of light at the carapace surface in seawater. Spectrophotometric results showed that light transmission increased at longer wavelengths in the younger samples but maintained a consistent level by the adult stage. Conclusions: Through the various structural adaptations of the carapace, in combination with the rare calcium carbonate polymorph distribution and its overall thinness, the S. lerneri is able to maintain its transparency without sacrificing many of its physical properties.

Journal Keywords: Ostracod; Skogsbergia lerneri; transparency; carapace; ultrastructure; microstructure; composition; optical properties; electron microscopy; x-ray analysis; non-linear signal

Subject Areas: Biology and Bio-materials

Instruments: I02-Macromolecular Crystallography , I13-1-Coherence , I13-2-Diamond Manchester Imaging , I18-Microfocus Spectroscopy