Reproductive biology

All echinoids have separate sexes, which cannot be distinguished by their external appearance. Only Antarctic urchins show a tendency to brood their young. The sexual dimorphism of these brooding heart urchins is generally easy to see. The "marsupial" is pronounced in females; present but not conspicuous in males. Generally, females are broadcast spawners, releasing millions of eggs into the water column to be fertilized by male sperm. Sea urchin gonads are connected to small openings in the body wall called gonopores, through which eggs and sperm are released. The resultant larvae undergo a multistage planktonic phase before settlement and metamorphosis. Most have a characteristic larvae development, passing through a free-swimming stage called echino-pluteus. Larvae are bilaterally symmetric and show no signs of pentaradiate symmetry, a characteristic of sea urchins. Metamorphosis gives rise to an adult body form either before or after settling on the sea floor. The duration between planktonic development and settlement is species- and geographically dependent.

Very few echinoids are brooders. The heart urchin, Aba-tus cordatus, broods its young to increase survival in freezing temperatures and food-deprived environments. These species omit a planktonic stage (lecithotrophs), a strategy to produce a small number of offspring that are larger in size.

Unlike sea stars, urchins are less likely to regenerate, although they will grow lost spines, tube feet, and repair holes in test.

A sand dollar (Echinarachnius parma) on its side for feeding. (Photo by ©Andrew J. Martinez/Photo Researchers, Inc. Reproduced by permission.)

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