Mechanical forces imposed on echinoid eggs during spawning: Mitigation of forces by fibrous networks within egg extracellular layers

Many echinoderms spawn large numbers of eggs in rapid spawning bouts direct ly into the water column. During spawning, the eggs pass from the gonad alo ng a narrow oviduct and through a rigid gonopore before reaching the water. As the eggs traverse this pathway, they are exposed to shear stress induce d by the development of a velocity gradient within the fluid formed by the eggs. In some species, the diameter of the eggs is larger than the diameter of the gonopore, In these cases, the eggs also experience strain resulting from compression of the egg as it passes through the relatively small gono pore, The magnitude of shear stress experienced by eggs differs among speci es depending upon the viscosity of the eggs. The degree of strain experienc ed by eggs differs among species depending upon the relative sizes of the e gg and gonopore and also changes within species as they grow. Recent eviden ce suggests that these forces have the propensity to damage eggs, thereby r educing their fertilizability, or to destroy them. Experimental evidence sh ows that the jelly coat around the eggs can protect them from shear stresse s and reduce the strain imposed on them under a compressive force. Echinode rm eggs are surrounded by a jelly coat that has a composite structure of ir regularly arranged fibrous networks embedded in a jelly matrix. These fibro us networks have characteristics that are similar to those of engineered an d biological materials that are known to reduce the transmission of mechani cal forces to other structures. Given this similarity to other materials, t hree possible mechanisms for the action of the jelly coat may be identified . First, the apparent viscosity of the eggs of echinoids declines as the ra te at which they are sheared during spawning increases. This reduction in v iscosity with increasing shear rate (shear-thinning) may, in part, be due t o the structure of the jelly coat and its resultant non-Newtonian nature. S econd, experimental evidence indicates that the jelly coat preferentially d eforms under a compressive load, reducing the transmission of that load to the egg. Third, 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 la yer of the egg. The composite structure of the jelly coat and the morpholog y of the fibrous network are likely to be critical to all these mechanisms.