The sexes are separate in sea urchins. At breeding time, normally in the summer, the gonads (containing eggs or sperm) swell considerably within the sea urchins' plated skeletons.
Spawning (releasing of eggs and sperm) takes place on a massive scale, at the same time within members of the same population. These products are released from the sea urchins via the genital plates, at the top of the organisms.
Above: a simplified vertical cross-section through a regular sea urchin, showing the gonads. (M = madreporite; GP = genital pore)
External fertilisation then takes place with new sea urchins starting their lives as swimming larvae (echinopluteus). The swimming larvae, unlike most sea urchin adults, are bilaterally symmetrical (one imaginary plane divides them into two equal halves).
These larvae also have arms, unlike all sea urchin adults, which they use to trap plankton for food. After several weeks, a part of each larva (the rudiment) starts to develop into an adult sea urchin and structures such as the arms are resorbed (dissolved and assimilated).
The newly developed sea urchins — those that have not been eaten by predators — then sink to the seafloor. Most of these will have no front or back end (being radially symmetrical) and will be able to move equally easily in all directions.
Some species of sea urchin produce eggs which are brooded by females and develop directly into adult sea urchins (there is no planktonic stage). In these species, the females can be more easily distinguished from the males as they have specialised brood pouches for their eggs. Species which adopt this life-cycle strategy are usually confined to polar waters.
Read more:
What Are Sea Urchins?
What Are the Main Types of Sea Urchin?
How and What Do Sea Urchins Eat?
How Do Sea Urchins Move?
Where Do Sea Urchins Live?
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