Early Fertilization Dynamics in <i>Lytechinus variegatus</i> : Ionic Specificity, Voltage‐Dependent Entry, and Cortical Granule Activation

Sea urchin fertilization is a rapid and coordinated cascade of electrical, ionic, and biochemical events that transform the egg into a developmentally competent zygote. While many of these transitions have been described, several remain obscure, misinterpreted, or lost to time. This review reexamines early fertilization events with emphasis on electrophysiological and calcium-dependent signatures, integrating voltage-clamp and unclamped recordings with cytoskeletal and metabolic remodeling. We delineate a biphasic response-excitation and activation-initiated by sperm-derived conductance and sustained through egg-intrinsic sodium channels. By mapping these transitions onto a unified timeline, we highlight transient but critical events such as the cortical flash, calcium-wave propagation, and actin-mediated cytoskeletal remodeling, restoring mechanistic clarity to the ionic and structural transitions that define successful fertilization. We also revisit voltage-dependent sperm entry and propose a mechanism by which excessive calcium influx may destabilize the fusion interface, potentially resulting in unfusion. Ion substitution experiments further reveal that although Sr and Ba can support sperm entry at negative potentials, they elicit only a diminished early activation current, underscoring calcium's unique specificity in triggering downstream signaling. This framework invites renewed attention to the electrical and ionic foundations of monospermy, egg activation, and zygotic competence, and highlights unresolved questions in how early ionic events drive subsequent structural transitions.