|In sea urchins the sexes are separate, although in most cases males and females are externally identical. The life cycle begins with the release of vast numbers of sperm and eggs into the water column. Spawning usually takes place synchronously within populations, probably chemically mediated. Eggs are fertilised in the water column and proceed to develop rapidly into a swimming larva, the echinopluteus. These larvae migrate into the upper surface waters and have long ciliated arms that they use to capture the phytoplankton on which they feed. Their arms are supported by slender calcite rods. After several weeks of living and feeding in the plankton a cluster of cells in the lower left-hand side of the body start to differentiate rapidly. This is the rudiment and it is these cells that give rise to the adult sea urchin. As the rudiment develops, adult features, such as tube-feet, begin to appear, while larval structures, including the arms and even the larval gut and mouth, are resorbed and eventually lost. The newly metamorphosed juvenile sinks to the sea floor to begin life as a benthic adult. Sea urchins which must pass through a feeding larval stage in their life cycle are termed planktotrophs. The opposite condition, where the larva develops directly into the adult form without needing to feed is called lecithotrophy. A schematic life cycle is shown below.
A small number of species have modified their life cycle to omit the planktonic stage (inner cycle in diagram). These lecithotrophs produce a smaller number of much larger yolky eggs which are released into special holding chambers or brood pouches externally. Fertilisation is still external but the eggs and developing larvae remain with the mother. These eggs develop directly into small adults without passing through a pluteus stage and without needing to feed in the surface waters. Lecithotrophic species are today largely confined to polar waters. Because female lecithotrophs produce large eggs and have specialised brood chambers they are often morphologically distinguishable from males of the same species. Sea urchins are primitively planktotrophic and only secondarily do they adopt a lecithotrophic life cycle.