Starfish, also known as sea stars, are marine invertebrates found in diverse ocean environments, from shallow intertidal zones to deep abyssal plains. Mussels are bivalve mollusks common in these same coastal habitats. Their interaction, particularly starfish preying on mussels, showcases a unique feeding strategy and highlights intricate connections within marine ecosystems.
The Starfish’s Unique Feeding Method
Starfish employ a specialized feeding method to consume prey. They use hundreds of powerful tube feet, located on their underside, to firmly grip and pry open mussel shells. These tube feet, equipped with suction disks, exert steady, continuous pressure, eventually tiring the mussel’s adductor muscle and creating a tiny gap, sometimes as small as 0.1 millimeters, between the shells.
Once a small opening is achieved, the starfish everts its cardiac stomach through its mouth and into the mussel’s shell. Digestive enzymes are then secreted directly onto the mussel’s soft tissues, initiating external digestion. This process transforms the mussel’s body into a “gastric soup” or liquefied form. The partially digested material is then absorbed back into the starfish’s body, moving into the pyloric stomach and associated digestive glands in each arm.
Mussels as a Primary Food Source
Mussels constitute a significant portion of a starfish’s diet due to their abundance and accessibility in many marine environments. They are commonly found in dense beds within intertidal and subtidal zones, making them a readily available food source. Mussels provide nutritional value that supports the starfish’s metabolic needs.
Certain starfish species are known for their predation on mussels. For example, the ochre sea star (Pisaster ochraceus) and the common sea star (Asterias rubens) frequently target mussels like Mytilus californianus and Mytilus trossulus. Studies have shown that Asterias rubens may exhibit preferences, feeding more readily on Mytilus trossulus and smaller mussels. The intensive predation by starfish can significantly influence the distribution of mussel beds, often controlling their lower limits in certain areas.
Diverse Diets and Ecological Impact
While mussels are a staple, starfish diets are diverse, including other invertebrates. Some species also scavenge on carrion or function as detritivores, consuming decomposing organic matter. This varied diet highlights their adaptable feeding strategies within marine ecosystems.
Starfish are recognized as “keystone species” in many habitats, meaning their presence has a significant effect on the structure and diversity of the ecosystem. The concept was first introduced by Robert Paine in 1966, based on his observations of Pisaster ochraceus. By preying on competitively dominant species like mussels, starfish prevent these organisms from monopolizing space and outcompeting other marine life. This predatory control helps maintain a higher level of biodiversity. Without starfish predation, mussel populations can expand unchecked, reducing overall species diversity and creating an ecosystem imbalance.
Other Invertebrates in Starfish Diet
- Barnacles
- Limpets
- Snails
- Clams
- Oysters
- Sea urchins
- Crustaceans like crabs and shrimp