Sea Stars: Slow Movers, Surprising Predators
By Gomen See, Marine Science Team of HKMM
Sea stars are echinoderms, a group of marine invertebrates found in a wide range of habitats, including intertidal zones, coral reefs, and sandy seabeds. They typically have five arms, although some species possess eight or even more arms.
When people think of predators, they often imagine fast hunters such as sharks or orcas. Sea stars, with their slow movements and seemingly gentle appearance, rarely come to mind. Yet many sea stars are surprisingly effective predators which sometimes play an important role in maintaining marine ecosystems.
Despite moving only within a meter per minute, sea stars are skilled hunters. Rather than chasing their prey, they target animals that are even slower or completely stationary. Common preys include mussels, sea urchins, barnacles and sea cucumbers. Using hundreds of tiny tube feet on the underside of their arms, sea stars can slowly grip and manipulate their prey.
One of the most fascinating facts about sea star feeding is how they eat. Many species can partially push their stomach outside their bodies and into the shell of a mussel or clam. They digest the prey externally before absorbing the nutrients, allowing them to consume animals that would otherwise be impossible to swallow whole.
Because some sea stars feed on abundant animals such as mussels, they can have a huge influence on the communities around them. In fact, some sea stars are considered keystone species, meaning have a disproportionately large impact on their community or ecosystem relative to their abundance. A famous example is Pisaster ochraceus, also known as the ochre sea star or purple sea star, found along the Pacific coast of North America. Scientists discovered that this species preys heavily on mussels, preventing them from dominating rocky shore habitats. When the ochre sea star is completely removed, mussels will take over available space, greatly reducing the variety of other marine organisms living there.
This discovery helped establishing the ecological concept of Keystone Species, showing that even a single predator can shape an entire ecosystem. Pisaster ochraceus demonstrates that ecological importance is not determined by size, speed, or abundance. Instead, it is the impact a species has on the environment that matters most.
Marine Ecology Q&A
Q: Do sea stars have eyes?
A: Although sea stars do not have eyes like humans, they have a simple eyespot at the tip of each arm. These eyespots cannot see fine details or colors, but they can detect light and the outlines of objects, helping sea stars distinguish between light and dark, explore their surroundings, and avoid danger.
Q: How do sea stars eat?
A: When a sea star encounters prey, it uses the tube feet on its arms to attach to the prey. It then turns its stomach inside out through its mouth and releases digestive liquid onto the prey. The prey is digested externally, and the nutrients are then absorbed back into the sea star's body.
Q: Can you touch a sea star?
A: It is best to avoid touching sea stars unnecessarily.
Although sea stars may look tough and rigid, excessive handling can cause them stress or even injury. Some species are covered with venomous spines which can cause series injuries that require medical attention.
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Ocean Guardians Series - Embark on an underwater adventure in Hong Kong with Dr. Connie
Are you ready? Season 2 of Ocean Guardians is back with an exciting new chapter!
In this episode, along with the adorable new AI assistant Bobo, Dr. Connie rushes to a Sai Kung beach to rescue a starfish being thrown around as a frisbee by kids! Although starfish have amazing regenerative powers, damaged tube feet can severely impair their ability to move and hunt for food. Follow along with Dr. Connie to discover the story behind this starfish rescue and learn some fun ocean facts!
About Ocean Guardians Series - Embark on an underwater adventure in Hong Kong with Dr. Connie. Follow Dr. Connie and her marine friends as they dive into exciting underwater missions. Along the journey, Dr. Connie interacts with various local sea creatures and uses special magic tools to carry out rescue operations. The story highlights the impact of human activities on the marine environment, inspiring audiences to reflect and take action to protect our precious ocean.
