Starfish belong to a group of animals known as Astroidea, meaning having rays or arms. Although sea stars are simple organisms as they lack brains and other sensory organs, they are the most complex invertebrates known. Contrary to popular belief, sea stars are not fish. Sea stars are symmetric in a way that other animals are not. They are symmetric through every axis, not just the midline.
Sexual Reproduction
Sea stars have two separate and distinct sexes, male and female. When it is time to spawn, the sea stars will congregate in large groups to give themselves a greater chance of success in getting the eggs fertilized. Some species of sea star will release her eggs into the water to be fertilized, and some will carry them on their undersides. The males release their sperm into the water to intermingle with the eggs, fertilizing them.
Development
The eggs will live among the other zoo plankton until they hatch into stage one larvae known as bipinnaria. These larvae will do nothing except eat and grow, where they hatch into first stage larvae. The larvae, known at this stage as bipinnaria will do nothing but eat and grow. After reaching a specific size, the larvae will be considered second stage larvae, known as brachiolaria. In both larval stages, the tiny sea stars will live and grow among the zoo plankton, some eating, some being eaten, until they reach the stage of development known as metamorphosis. The larvae are bilaterally symmetric, meaning they are the same on the right side as they are on the left, which is vastly different from the adult forms of this animal.
Metamorphosis
When the larvae have reached the point of development where they are ready to enter metamorphosis, they drop down from the surface of the ocean to the floor. This is the stage where the sea stars will take on their more familiar radially symmetric shape. Most sea stars have five arms, but some species have more.
Life Expectancy
The average life expectancy for a sea star has been documented as up to eight years in captivity. Sea stars in the wild have varying lifespan, and though scientists do not know much about the life expectancy of sea stars in the wild, they estimate the average lifespan is anywhere from five to 20 years.
Asexual Reproduction
Due to the ability of sea stars to regenerate body parts, the sea star is able to reproduce asexually as well. This is accomplished by a process called fission. The sea star will divide itself into two separate halves and as long as a portion of the central disk is present, the two halves can regenerate the missing body parts and become whole again.
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