When is a fish not really a fish?
When it’s a starfish! Starfish is the common name for a group of animals called sea stars. Sea stars are actually part of the phylum Echinoderm and are related to sea urchins, brittle stars and sea cucumbers – they are not fish at all! Fish are vertebrates; sea stars are invertebrates. Fish are typically covered with scales; sea stars are typically covered with spines. Fish are found living in the sea and freshwater; sea stars live only in marine environments.
|Order:||Brisingida, Forcipulatida, Paxillosida, Notomyotida, Spinulosida, Valvatida, Velatid|
Characteristics of sea stars
There are an estimated 2,000 species of sea stars living throughout the world’s oceans. In the New Zealand region, there are at least 184 species. Adult sea stars are always benthic, which means they live on or near the seabed – however, their habitats vary from intertidal rock pools to the deep seafloor. Sea stars commonly have 5 arms, but there are species with many more, including the New Zealand sea star that has 11 arms.
Sea stars are a diverse group of animals, but most of them share the following characteristics:
- Hard plates under their skin instead of a backbone.
- Spines or spicules covering the top (or dorsal) surface.
- Hundreds of tube feet, which help feeding and movement.
- A mouth that is located in the centre of their bottom side (the ventral surface). When a sea star feeds, it is able to push the lower part of its stomach out through its mouth. The stomach then covers the prey and digestion begins.
- Structures on the ends of their arms that help them to detect changes in light.
- A good sense of smell.
- The ability to regenerate a lost limb. Some sea stars can even regenerate a new animal from one limb.
- A complex network of nerves, called a nerve net, that serves the same purpose as the central nervous system in humans.
- The ability to reproduce by releasing a free-swimming larval stage, as well as through fission or asexual reproduction. This means that the sea star can physically divide itself into 2 to reproduce.
- Chemical defences under their skin to deter predators.
Sea stars and the marine food web
Fish prey upon very small, juvenile sea stars. Adult sea stars avoid predation with their strong chemical defences and sharp spines, but a few predators, such as crayfish, are not deterred and may eat sea stars.
Most sea stars are active predators feeding on almost anything they come across, including mussels, clams and oysters. In New Zealand, they play an important role in keeping the numbers of other organisms down. Sea stars are often referred to as a keystone species, as their feeding has an effect on the whole ecosystem. If numbers of starfish are reduced, this can have dramatic effects for the whole food web.
Visit the Te Ara website to learn more about sea stars and other echinoderms.
Most sea stars have eyes on the tips of their arms. Learn more about these ‘eye spots’ in this blog by Ed Yong.
To extend learners, look inside a sea star to see the inner workings of an animal very different from us in this animation, Sea star body plan.