The role of jelly coats in sea urchin egg fertilization: A combined experimental and numerical investigation.

Abstract

The transparent, extracellular layers surrounding the eggs of the sea urchin <italic>Arbacia punctulata</italic> account for approximately 3--11% of maternal energy in egg production, and thus likely play an important role in successful fertilization of this free-spawning organism. In this research, we have proposed an indirect approach to investigate the protection role of the jelly coat. We utilized shear flow experiment to determine critical flow rates and critical shear stresses that resulted in jelly coat detachment, and to determine deformations induced by the gradient of the velocity of the fluid flow. Our experimental results showed that jelly coats became detached at a shear stress of 22Pa; egg without jelly coat was destructed with shear stress of 24Pa. The egg and jelly layer exhibited nonlinear behavior under external loads. Shear flow produced stresses on the surface of the egg or jelly layer, which satisfied the universal deformation condition due to traction, under assumptions of an isotropic, homogeneous material. With the data from the shear flow experiment, material properties of the egg and jelly coat were found using an inverse finite element method (IFEM) approach, involving a fluid-solid interaction algorithm. The hyperelastic coefficient of the egg was found to be in the range of 110--160 Pa. Some jelly coats had a shear modulus close to that of the egg, while some had 48--64 Pa. With these material properties, simulations were done to mimic the situation that larger egg with or without jelly coat passed through the smaller oviduct during spawning. The results showed that existence of the jelly coat dramatically reduced the maximum Von Mises stress, for compressive loads. Future work will focus on determining the relative importance of the jelly coat's protective role, and its other possible functions in sperm attraction. In this work, experiments using a microfluidic device were initiated to measure the motility coefficient, and the effect of the chemoattractant on the sperm movement can be investigated based on the analysis of convection-diffusion system. By coupling these with more detailed experiments with various dilutions of jelly coats and water, we aim to compare the possible mechanical role of jelly layers, with some of its other, putative functions in a complete way.