Sex versus Gender in Sea Urchins and Leeches Two Centuries after Lamarck1816

Bi-parental (sexual) reproduction via the fusion of egg and sperm produced by adult female/male individuals in populations of animals and plants is a key process of Life on Earth. Here, the terms “sex” (fertilization) and “gender” (role of male/female individuals as providers of gametes) are described, based on the concepts of Carolus Linnaeus (1707-1778) and Julius Sachs (1832-1897). In 1816, Jean Lamarck (1744-1829) introduced the purple sea urchin (Paracentrotus lividus) as a new species, and six decades later, Oscar Hertwig (1849-1922) used this model organism to elucidate external fertilization at the sub-cellular level. Moreover, Hertwig referred to “gender” to denote male or female P. lividus-individuals (gonochorists). Sexual reproduction and gender-roles are also outlined with reference to the fish leech (Piscicola geometra), a protandrous hermaphrodite characterized by hypodermic insemination and a gender-ration of 50:50. Finally, gender-issues in vertebrate development and evolution are addressed.