Recognizing incipient epizootic shell disease lesions in the carapace of the American lobster, Homarus americanus

Abstract

Causal factors leading to epizootic shell disease (ESD) lesions in American lobster, Homarus americanus H. Milne-Edwards, 1837, are not well understood. We explore the structural and physiological bases for development of ESD from preclinical stages invisible to unaided eye to early visible stages. We present a lobster shell model, which develops structural functional vulnerability and suggests plausible routes to ESD. Medallions of carapace cuticle were obtained from carapace fixed with protocols to minimize movement of mineral and macromolecular components. Rapid processing of medallions was used to encourage large sample sizes compatible with environmental surveys. One- and two-dimensional analytic maps of polished sections of the cuticle, obtained with an electron microprobe, described the composite mineral and polymeric structures. Micro-Raman spectroscopy was used to identify bond properties of phosphates and carbonates, as well as signatures of organic structures. The frequency and properties of structures identified can be monitored through the lobster molting cycle using a high throughput application of micro-computed tomography (μCT). We observed density differences in the calcite layer, exocuticle, and endocuticle, and the frequency and structure of CaCO3 structures in the endocuticle and membranous layer of carapace cuticle during chosen stages of the molting cycle. The correlative microscopy and μCT of shell structures provides improved understanding of the lobster cuticle structure. Detailed structural differences quantified through development and under different environmental conditions can provide insight into causes and vulnerabilities associated with ESD.