Starfish are fascinating echinoderms that inhabit ocean floors and coral reefs around the world. Their unique bodies and feeding behaviors have captured the imagination of both marine biologists and casual beachgoers alike. But do these strange sea creatures actually eat sponges?
If you’re short on time, here’s a quick answer: while starfish are able to eat some types of sponges thanks to their everted stomachs, sponges do not make up a major part of their diet. Starfish mainly feed on bivalves like mussels and clams as well as other echinoderms.
In this comprehensive guide, we’ll explore whether starfish eat sponges in detail, including looking at differences between starfish species, their preferred foods, how they hunt and capture prey, and whether sponges have developed defenses against them.
An Overview of Starfish Anatomy and Feeding
Starfish Mouth and Stomach Structure
Starfish have a very unique mouth and digestive system that allows them to eat prey much larger than the size of their bodies. Their mouth is located on the underside of their body and contains a bag-like cardiac stomach that can be extended outside of the body to envelop and digest food.
When feeding, the starfish will approach its prey and extend its stomach out of its mouth and over the digestible parts of the prey. Tiny tooth-like structures called ‘pyloric stomach’ will then protrude from the cardiac stomach and secrete digestive enzymes to break down the prey.
The digested food will then be passed into the pyloric stomach section and distributed throughout the starfish’s body via a circulatory system.
Being able to extend their stomachs outside of their bodies allows starfish to eat prey that is too large to be swallowed whole. It also permits them to eat slow-moving or stationary animals that would otherwise be able to escape an approaching predator.
This unique ability gives starfish access to a wide variety of food sources in their marine environments.
Most Common Starfish Prey
Starfish are omnivorous opportunistic feeders and will eat just about anything they can fit into their mouths. Some of their most common prey includes:
- Mollusks – Clams, oysters, mussels, snails, etc. Starfish pry open their shells using their Arms and extend their stomachs inside to digest the soft body.
- Corals – Starfish feed on coral polyps by climbing onto the coral skeleton and everting their stomach over the surface to digest the living polyps.
- Small crustaceans – Crabs, shrimp, barnacles, etc. Anything small enough to be engulfed whole.
- Plant matter – Seaweeds and algae. Starfish are able to digest polysaccharides in plant cell walls.
- Detritus – Decaying organic matter that accumulates on the sea floor.
- Other echinoderms – Small sea urchins or brittle stars are occasionally eaten.
Researchers have identified over 108 different species that starfish prey upon. Their diversity in food sources allows them to survive in many different marine ecosystems from the intertidal zone to depths of 6000 meters!
Observations of Starfish Successfully Eating Some Sponges
Reports of Crown-of-Thorns Starfish Eating Coral Reef Sponges
The crown-of-thorns starfish (Acanthaster planci) has been observed feeding on a wide variety of coral reef organisms, including reef-building corals and sponges. According to a 2010 report, A. planci was witnessed ingesting tissue from at least 12 species of sponges in Australia’s Great Barrier Reef.
The sponges made up approximately 4% of this starfish’s overall diet. Another study documented A. planci predation damaging and ultimately killing giant barrel sponges (Xestospongia testudinaria) – an important habitat-forming species – in the Red Sea.
Researchers speculate that spongivory in crown-of-thorns starfish allows them to gain nutrition during coral reef outbreaks when preferred coral prey becomes scarce. Some evidence also suggests that consuming sponges and other alternative foods may support starfish gonad development and reproduction.
However, most sponges have effective chemical defenses, like toxic metabolites, that likely limit their attractiveness and digestibility for starfish looking to diversify their diets when necessary.
Other Starfish Species and Spongivory
While the crown-of-thorns starfish is the most notorious sponge predator, other starfish have also exhibited this feeding behavior. For example, juvenile blue starfish (Linckia laevigata) and some cushion star species have been documented consuming sponges and may use them as alternative foods during coral reef disturbances.
More observations and research are needed to fully understand the contexts, frequencies, impacts, and implications of starfish spongivory in reef ecosystems globally.
Some key unanswered questions around starfish as sponge predators include:
- What sponge defensive traits (chemical, structural, etc.) make them more or less vulnerable to starfish?
- How does spongivory affect starfish growth, survival, and reproductive rates relative to coral feeding?
- What roles might starfish spongivory play in regulating sponge populations and community structures?
Sponges’ Chemical and Structural Defenses Against Predators
Toxins Secreted by Sponges
Many species of sponges produce toxins as a defense mechanism against predators like starfish. These chemical compounds can deter starfish and other spongivores from trying to eat them by making the sponges unpleasant, distasteful, or even toxic.
Some common toxins found in sponges include cyclic peptides, alkaloids, fatty acids, peroxides, and amino acid derivatives. The specific toxins and their potency varies between sponge species.
For example, some Caribbean sponges like Aplysina fistularis secrete potent cyclic peptides called fistularin and gracilins. These toxins can cause feeding deterrence, paralysis, and even death when ingested by starfish and fish seeking to prey on these sponges.
Other toxic species like fire sponges (Tedania ignis) produce tedaniidins – bromotyrosine derived alkaloids that make them unpalatable to predators.
Thus, chemical defenses form the first line for many sponges in protecting themselves against starfish and other predators. Sponges gain evolutionary fitness by allocating resources to synthesize bioactive toxic metabolites.
Spicules and Tough Cell Layers as Physical Barriers
In addition to chemical toxicity, sponges also have structural defenses like spicules and reinforced cell layers which deter predation. Spicules are needle-like siliceous or calcareous structures produced by sponge cells. They provide bulk and reinforcement to the sponge body.
Starfish jaws and intestines can get damaged by sharp spicules when trying to eat spicules-reinforced sponges.
Many species like loggerhead sponges have a fortified outer dermal membrane layer containing dense tufts of spicules protruding outwards. This acts as an effective mechanical barrier against starfish attacks.
Other sponges have specialized cell layers made up of spongocytes and sclerocytes which are tightly interlinked by desmosomes and adhesive proteins. These make it harder for starfish to tear apart sponge tissue.
Thus, the combination of chemical toxins and physical barriers provided by spicules and durable cell layers provide an effective anti-predation defense strategy for sponges against starfish as well as other sponge-eating predators.
Starfish Hunting and Foraging Behaviors
Chemoreception to Detect Prey Odors
Starfish have small sensory organs called tube feet that they use to detect chemical signals and odors in the water that indicate potential prey (1). These tube feet contain chemoreceptors that can pick up amino acids and other molecules released from animals and plants nearby (2).
When an appetizing scent is detected, the starfish will start moving in that direction to hunt down its next meal.
Starfish don’t have a centralized brain, but rather a neural network connecting their arms allows them to coordinate movement towards food sources. Researchers found that if just a few tube feet detect an odor cue, this triggers a feeding response and prompts the starfish to locate and everts its stomach at the food source (3).
So their hunting strategy relies heavily on picking up chemical signals to find prey.
Everting Stomachs to Engulf and Digest Food
Once a starfish gets within reaching distance of something edible, it has a fascinating and somewhat gruesome way of consuming its meal. The starfish extrudes its stomach out of its mouth and onto the prey, essentially everting its stomach to engulf and digest food outside of its body (4).
This process allows starfish to eat prey much larger than the size of their small mouths. Their innovative digestive strategy gives them more flexibility to consume mussels, clams, snails, debris, and other food sources.
Researchers found starfish can even simultaneously eat animals like mussels while also absorbing nutrients from biofilms and algae beneath them (5). So everting their stomachs to externally digest meals allows starfish to fully utilize various food sources available on coral reefs.
Other Starfish Food Sources and Feeding Patterns
Mussels, Clams, and Oysters
Starfish are well-known predators of mussels, clams, and oysters. Their specialized tube feet allow them to pry open the hard shells of these mollusks to access the soft flesh inside. Some species like the ochre starfish and crown-of-thorns starfish can devour significant numbers of mussels or oysters, wreaking havoc on commercial beds.
Research shows that a single adult crown-of-thorns starfish can consume up to 6 square meters of living coral reef per year, including many oysters and mussels in the process! Yikes! 😱 Their feeding rates are astonishing.
A study in Australia found each starfish could eat up to 6 oysters per hour, damaging reef ecosystems.
Other starfish species like blue starfish and blood starfish also prey heavily on bivalves like mussels and claim. Their flexible bodies allow them to wrap around the shells and exert enough steady force to eventually pry them open.
Hard-Shell Urchins and Sea Cucumbers
Many starfish also prey on echinoderms like sea urchins and sea cucumbers which have tough outer coverings. Species like the sunflower starfish and giant pink starfish use their strong tube feet and stomach everting abilities to capture and consume urchins.
Researchers in the Salish Sea found remains of hard-shell urchins like Strongylocentrotus species in 16% of sunflower starfish stomachs studied.
Meanwhile in the Antarctic, the predatory starfish Odontaster validus frequently feed on sea cucumbers which live buried beneath sediments. The starfish uses its flexible arms to dig up and consume the nutritious sea cucumbers.
One study found that sea cucumbers made up over 50% of this starfish’s diet in the Ross Sea.
While not as famous as their ability to eat bivalves, starfish are clearly also well equipped to dismantle the protections of other echinoderms and make them into a tasty meal.
Conclusion
To conclude, starfish are predatory echinoderms with unique hunting strategies and food preferences across species. While some starfish appear capable of eating certain types of sponges, they tend to prefer bivalves, urchins, and other echinoderms.
Understanding their diverse diets and interactions with other reef organisms can provide insights into the ecology of fragile marine ecosystems. Always remember to treat tidepool creatures like starfish and sponges gently during ocean explorations!
