Thomas, F.I.M.; Bolton, T.F.: Mechanical forces imposed on echinoid eggs during spawning: mitigation of force by fibrous networks within egg extracellular layers

Eggs of echinoids are exposed to mechanical forces during spawning. The magnitude of these forces depends on the properties of the eggs and the relative dimensions of the egg and gonopores. Recent evidence suggests that mechanical forces have the propensity to damage eggs, thereby reducing their fertilizability, or to destroy them. Jelly coats that surround echinoderm eggs have a composite structure of irregularly arranged fiberous networks embedded in a matrix. These fibrous networks have characteristics that are similar to engineered and biological materials that are known to reduce the transmission of mechanical forces to other structures. Experimental evidence shows that these coats protect eggs from shear stresses and reduce the strain imposed on eggs under a compressive force. Given the similarity of the jelly coats structure to other materials, three possible mechanisms for the action of the jelly coat may be identified. First, the apparent viscosity of the eggs of echinoids declines as the rate at which they are sheared during spawning increases. This reduction in viscosity with increasing shear rate (shear thinning) may in part be due to the structure of the jelly coats and their resultant non-newtonian nature. Second, experimental evidence indicates that the jelly coat preferentially deforms under a compressive load reducing the transmission of that load to the egg. Thirdly, the jelly (but not the fibers in the coating) may deform in a nearly viscous manner. In this case, the fibers may serve to provide an elastic “backbone” to the layer, and remain in place to stiffen the outer layer of the egg. The composite structure of the jelly coat and the morphology of the fibrous network are likely to be critical to all these mechanisms.