Sea urchins are a class of creatures belonging to the phylum Echinodermata, found in nearly all oceans around the world.
In terms of appearance, mature sea urchins have a diameter ranging from 3 to 10 cm and a thickness of about 3 to 4 cm. True to their name, the entire surface of this creature is covered with sharp spines. If anyone happens to be pricked by this defensive layer, they will immediately experience pain in the affected area. However, this injury typically does not significantly impact overall health.
Sea urchins are found in nearly all oceans around the world.
One could compare sea urchins to a hollow spiky ball, as the amount of flesh inside is very small compared to the overall shell mass. This flesh is yellow or orange and adheres along the shell in a star-like pattern with 5 to 8 points.
In addition to their peculiar shape, the reproduction and development of this animal is also quite unique, as they possess the remarkable ability to “invert from the inside out to regenerate.”
Specifically, during the breeding season (Spring), male sea urchins release a massive amount of sperm into the ocean, while female sea urchins do the same with their eggs. However, only a small fraction of these eggs and sperm manage to meet and fertilize in the vast sea.
After fertilization, the eggs develop into sea urchin embryos. From the moment they hatch, sea urchin larvae can swim, and as they grow a bit more, they take on a larval form resembling an umbrella. Interestingly, larval sea urchins are completely blind and deaf, yet they continue to swim for weeks and even months in this state, surviving by consuming algae.
At a certain point, the larvae must find a surface to settle on.
During their journey across the ocean, a mature sea urchin, which is now at a microscopic size, gradually forms and develops within the larva. At a certain point, the larva must find a surface to settle on before the sea urchin inside becomes too large. As they approach maturity, ocean currents carry the larvae close to the shore, right near the areas where waves crash and create white foam. The disturbances caused by the waves, combined with the enticing scent of seaweed, signal to the larvae that the time has come.
At this moment, the larvae will stop swimming and drop freely to the ocean floor. The next development is truly the strangest part of the entire growth process. From within the larva, tube feet begin to emerge. These pioneering feet will pull the entire mature sea urchin inside out, just like how we would turn a sock inside out. At this point, the sea urchin has officially transitioned to its adult form.
Discover the unique reproduction and growth process of sea urchins in the video below: