Helicoidal architecture of fish eggshell.

Previous publications show arced patterns in electron micrographs of either microfibrils or canals in sectioned fish eggshells, but these have been misinterpreted. We show here that such patterns in the inner layer of cod (Gadus morrhua), plaice (Pleuronectes platessa) and trout (Salmo gairdneri) eggs arise from a helicoidal structure. This consists of a laminate of protein microfibrils, with the direction of ply processing like the steps of a spiral staircase and with the same sense as a left-handed corkscrew. Mechanically, this is an ideal way to strengthen a spherical shell, to resist deforming forces equally from any direction. Radial canals which traverse this layer are forced into flattened and twisted ribbons. Both the helicoidal microfibrillar structure and the canal shape in fish eggshells show remarkable convergent evolution with similar structures in insect cuticles. Trout eggs were resistant to deforming forces as high as 380,000 N/m2. Fish eggshells, like those of many other organisms, are mechanically well designed.