The Shocking Truth: 15+ Things Starfish Eat And Their Bizarre External-Digestion Method

Despite their slow, serene appearance, starfish—or more accurately, sea stars—are ravenous, top-tier predators of the marine world, utilizing a unique and often gruesome feeding technique that sets them apart from nearly every other ocean creature. As of late 2024, the ecological role of these invertebrates remains a critical topic, especially given the impact of Sea Star Wasting Disease, which has highlighted their importance as "keystone predators" that keep entire ecosystems, like kelp forests, healthy by controlling the population of their favorite prey: the sea urchin. The diet of a sea star is surprisingly varied, spanning from shelled mollusks and other invertebrates to microscopic algae and detritus, depending on the specific species and its habitat. Understanding *what* they eat is intrinsically linked to *how* they eat, which involves a bizarre, outside-the-body digestion process that allows them to consume prey much larger than their tiny mouths.

The Starfish Diet: A Comprehensive List of Their Primary Prey

The diet of the over 2,000 known species of sea stars is diverse, but most fall into the category of carnivores or scavengers. Their slow-moving nature means their prey is typically equally slow or sessile (attached to a surface). The following list details the core components of a sea star's meal plan, ranging from their most common targets to specialized food sources.

1. Bivalve Mollusks: The Starfish's Favorite Meal

Bivalves are the most famous prey of many common sea stars, such as the Ochre Sea Star (*Pisaster ochraceus*). These shelled organisms are a primary food source, and the sea star has evolved a powerful technique to overcome their hard defenses.

• Mussels: Especially common in intertidal zones, mussels are a staple for many species.
• Clams: Larger clams are often targeted, with the sea star using its collective arm strength to pry the shells open.
• Oysters: Starfish are notorious for being pests in oyster beds due to their efficient predation.
• Scallops: Some species, like the Pacific Blood Star, will also feed on scallops.

2. Other Shelled and Non-Shelled Invertebrates

Beyond bivalves, the sea star's diet is a who's who of benthic (bottom-dwelling) organisms. The variety ensures they maintain a well-rounded diet high in necessary nutrients.

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• Sea Urchins: A crucial food source, particularly for large species like the Sunflower Sea Star (*Pycnopodia helianthoides*). Research has shown that juvenile sea stars can eat up to 18 baby sea urchins per day, demonstrating their significant ecological role.
• Snails and Limpets (Gastropods): Slow-moving, single-shelled mollusks that are easily captured.
• Barnacles and Chitons: These sessile or slow-moving creatures are scraped off rocks and consumed.
• Polychaete Worms: Segmented marine worms found in the sediment.
• Crabs and Crustacea: Small crabs and other crustaceans are opportunistic targets.
• Sea Anemones and Sea Sponges: The Leather Sea Star, for example, is known to primarily feed on these organisms.

3. Specialized and Opportunistic Diets

Not all sea stars are classic mollusk-eaters. Some have highly specialized diets, while others are generalist scavengers, cleaning up the ocean floor.

• Coral Polyps: The infamous Crown-of-Thorns Sea Star is a specialist predator that feeds almost exclusively on the polyps of hard corals, making it a major concern for coral reef ecosystems when its population explodes.
• Detritus and Algae: Omnivorous species, such as the Bat Star, are essential scavengers. They graze on microalgae, bacteria, and detritus—the decomposed organic matter and leftover food from other animals—helping to keep the marine environment clean.
• Other Sea Stars (Cannibalism): In some cases, sea stars are known to prey on smaller or injured members of their own species or other sea star species.
• Plankton: The larvae of sea stars, as well as some adult species, feed on zooplankton and phytoplankton.

The Bizarre Feeding Mechanism: How Starfish Eat

The most fascinating aspect of the sea star's diet is not *what* they eat, but *how* they manage to consume large, shelled prey without teeth or crushing jaws. This process involves two key adaptations: their powerful tube feet and their ability to evert their stomach.

The Prey Capture: A Tug-of-War with the Tube Feet

When a sea star encounters a bivalve, such as a clam or mussel, it wraps its arms around the shell. The underside of each arm is lined with hundreds of tiny, suction-cupped appendages called tube feet.

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The sea star uses its tube feet to attach firmly to both halves of the bivalve's shell. It then exerts a steady, relentless pull. While the force of a single sea star is not immense, the continuous, prolonged tension eventually fatigues the bivalve's powerful adductor muscles, which hold the shell closed. Once the shell opens even a tiny crack—just 0.1 to 1.0 millimeters—the next stage of the bizarre feast begins.

External Digestion: The Stomach Eversion

Unlike most animals, sea stars do not need to swallow their prey whole. They possess a two-part stomach: the cardiac stomach and the pyloric stomach. The cardiac stomach is the key to their unique feeding method.

Through the tiny gap in the shell, the sea star everts its cardiac stomach—essentially pushing it out of its mouth, which is located on the underside of its central disc. This stomach wraps around the soft, internal body of the prey.

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The sea star then secretes powerful digestive enzymes directly onto the prey's tissues. The digestion process occurs externally, outside of the sea star's body. The liquefied, partially-digested tissue is then absorbed back into the sea star's body as the stomach is retracted. This allows the sea star to consume prey that would be physically impossible to swallow whole.

The Ecological Impact: Starfish as Keystone Predators

The feeding habits of sea stars are not just a biological curiosity; they are vital to the health of coastal marine environments. Certain species, such as the Ochre Sea Star and the Sunflower Sea Star, are classified as keystone predators. This designation means their presence is disproportionately important to maintaining the structure and diversity of their ecosystem.

The classic example of this ecological role is the relationship between the Ochre Sea Star and mussels. Without the sea star preying on mussels, the mussels would quickly outcompete other species, covering all available rock space and dramatically reducing the biodiversity of the intertidal zone. By eating the mussels, the sea star creates patches of open space, allowing other organisms like barnacles, limpets, and algae to thrive.

The recent devastating effects of Sea Star Wasting Disease across the Pacific coast have provided a stark, real-world example of this keystone role. The massive die-off of Sunflower Sea Stars, in particular, led to an explosion in the population of their primary prey, the sea urchin. These unchecked sea urchin populations then overgrazed kelp forests, creating "urchin barrens" and causing widespread, lasting damage to the ecosystem.

Therefore, the simple question of "what do starfish eat" leads to a profound answer: they eat the organisms necessary to regulate a balanced and diverse marine habitat. Their bizarre, external-digestion diet is a fundamental force in maintaining the structure of the world's oceans.