Physical forces experienced by echinoid eggs in the oviduct during spawning: comparison of the geminate pair Echinometra vanbrunti and Echinometra lucunter

Abstract During spawning, eggs of echinoids are released from the gonad and traverse a narrow oviduct–gonopore complex before being released into the water surrounding the spawning female. As the eggs traverse the oviduct–gonopore complex, a velocity gradient develops within the fluid formed by the eggs. This velocity gradient imposes a shear stress on the eggs that ranges from a maximal value at the walls of the oviduct–gonopore complex and declines linearly to zero at the center of the complex. In addition to shear stress, the eggs of some echinoids also experience strain. Strain is imposed on the eggs in situations where the diameter of the egg is greater than the diameter of the oviduct–gonopore complex. Both shear stress and strain have the potential to damage echinoid eggs during spawning and are likely selective pressures acting on the physical properties of protective extracellular layers, which surround the eggs of these organisms. The amounts of shear stress and strain experienced by eggs of a geminate pair of echinoids, Echinometra vanbrunti and E. lucunter , are calculated. Calculations of shear stress based on the mean spawning rates and dimensions of the oviduct–gonopore complexes of specimens collected during this study ranged up to 30 Pa for E. vanbrunti and up to 17 Pa for E. lucunter . To obtain conservative estimates of shear stress, these calculations were repeated incorporating a spawning rate that was reduced by one order of magnitude below the mean rate measured for both species. These calculations yielded shear stresses that ranged up to 13 and 6 Pa for E. vanbrunti and E. lucunter , respectively. The diameter of the oviduct–gonopore complex of echinoids increases linearly with the diameter of the tests of the adults. Because the diameter of the oviduct–gonopore complex influences the amount of shear stress experienced by eggs of echinoids, shear stress calculations were repeated incorporating the full range of potential diameters of the oviduct–gonopore complexes from data in the literature. Using the mean spawning rates obtained from each species, the amount of shear stress experienced by eggs ranged up to 32 Pa for E. vanbrunti and up to 398 Pa for E. lucunter . When the mean spawning rates were reduced by one order of magnitude, the amount of shear stress experienced by E. vanbrunti eggs ranged up to 14 Pa and up to 140 Pa for the eggs of E. lucunter . The eggs of E. vanbrunti do not experience strain during spawning because the diameter of the its eggs is smaller than the diameter of its oviduct–gonopore complex at all stages of adult growth. In contrast, the diameter of the eggs of E. lucunter is larger than the diameter of the oviduct–gonopore complex at small adult sizes and the eggs of this species experience true strains ranging up to 1.2. These results indicate that there are substantial differences in the amounts of physical forces that the eggs of echinoids experience during spawning both within species at different stages of adult growth and between species. Different amounts of shear stress and strain experienced by eggs among echinoid species may select for differences in the mechanical properties of protective extracellular layers that surround them.