Most people can easily recognize an adult sea urchin by its spiny globular shape. However a sea urchin doesn’t always look that way and has to go through a few different stages in its life cycle before it takes on this spiny form. Scientists have studied sea urchin development in detail to gain a better understanding of animal development. Also, in places such as Japan, sea urchin gonads are considered a delicacy. Understanding the sea urchin life cycle is vital to the aquaculture of this animal.
How Sea Urchins Mate
In a big ocean it can be hard to find a mate. Many species increase their odds by having two genders, such as hermaphroditic fish. Not so with sea urchins. Although it can be hard to tell, sea urchins do have distinct genders. However, they don’t reproduce the way humans do. Rather, they practice external fertilization via a process called broadcasting, or spawning, where males and females release their gametes into the water.
Swarthmore College describes the details of how egg and sperm meet in great detail in the article “Brief Description of Sea Urchin Fertilization”. In general, sea urchins increase their odds of creating new life by releasing a very large amount of gametes into the water at the same time. Sperm are then able to swim towards the eggs by sensing the various chemical changes in their environment. John M. Lawrence writes in the book Edible Sea Urchins: Biology and Ecology that the timing of spawning may be related to the lunar cycle or changes in ocean temperature.
Sea Urchins Start Off as Larvae
After fertilization and after many cell divisions zygote becomes larva. According to Swarthmore College the sea urchin is said to be in the pluteus larva stage once gut, mouth, and skeleton have formed. The larva is arrow-shaped with two arms extending around the mouth enabling it to feed in the water column. Larvae may drift in the water column for a few weeks up to a few months. Gregory Wray writes about the dangers marine larvae face in the article “A World Apart” in the March 2001 edition of the Natural History magazine, saying that barely one-fifth of a larval brood will survive two weeks among the plankton. Larvae are susceptible to predators and currents and only a few percent will survive a month in the water column.
Sea Urchin Metamorphosis
Sea urchin larvae stay in the water column eating algae until they are big enough to settle on the seabed and undergo metamorphosis. A sea urchin larva will not settle though until it has found an ideal spot – close to food, other individuals of its species, and far from predators. John M. Lawrence writes that sea urchin larvae use chemosensory cues to find food and aggregate with others of their species. Wray writes that metamorphosis is often rapid. The seabed is an environment so different from the water column that a larva would not last long if it did not change its body. Wray writes that many echinoderms can change from larva form to an adult body form in less than half an hour.
Adult Sea Urchins
After metamorphosis the sea urchin looks like a miniature version of its adult spiny self. It will continue to eat algae and kelp and grow from a juvenile into a mature adult. Depending on the species and availability of nutrients it can take a sea urchin two to six years to reach maturity and be able to reproduce. Adult sea urchins protect themselves from predators via their sharp spines and can slowly move about using hundreds of tube feet on the bottom of their body. Stanford University has an excellent page depicting the adult sea urchin’s anatomy at its “Virtual Urchin” page.
Adulthood in the Sea Urchin Life Cycle
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The Sea Urchin Life Cycle and Aquaculture
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