Starfish, or sea stars, are marine invertebrates characterized by their radial symmetry and tough, spiny skin. Sponges are simpler, filter-feeding invertebrates that are permanently attached to the seafloor. While many starfish are generalist predators, some species are specifically adapted to consume these sessile creatures, making them dedicated spongivores. This predation is a significant ecological interaction, as these starfish utilize a highly specialized feeding mechanism to access the nutritional value locked within the sponge body.
Specific Starfish Species That Prey on Sponges
Many starfish species include sponges in their diet, but certain ones exhibit a strong preference for this food source. The Cushion Star (Culcita novaeguineae), found across the tropical Indo-Pacific, consumes sponges among other reef organisms. In contrast, the Antarctic sea star (Perkinaster fuscus antarcticus) is a highly specialized spongivore, feeding almost exclusively on a single species of deep-sea sponge.
Other species, such as the Crown-of-Thorns Starfish (Acanthaster planci), primarily feed on coral but will turn to sponges when their preferred prey is scarce. Even the common Blue Star (Linckia laevigata) includes sponges in its omnivorous diet of bacterial films and detritus. The existence of these dedicated and opportunistic spongivores underscores the importance of sponges as a calorie source in marine food webs.
Unique Starfish Feeding Method
To consume a sponge, which is too large to be swallowed whole, spongivorous starfish employ a physiological process known as stomach eversion. The sea star first positions itself over its prey using its tube feet to secure a hold on the sponge’s surface. It then pushes its cardiac stomach—one of its two stomachs—out through a central opening on its underside.
The soft stomach tissue wraps around the exterior of the sponge, a process that is mechanically facilitated by a decrease in muscle tension triggered by neuropeptides called SALMFamides. Once the stomach is pressed firmly against the sponge, the starfish releases powerful digestive enzymes directly onto the prey tissue. This external digestion breaks down the sponge’s soft organic material into a nutrient-rich “soup.” Finally, another neuropeptide, NGFFYamide, triggers the contraction of the stomach muscles, allowing the sea star to retract its stomach and absorb the partially digested meal internally.
Overcoming Sponge Defenses
Sponges rely on two primary mechanisms to deter predation: structural components and chemical compounds. Structurally, sponges are supported by a skeleton of microscopic, needle-like elements called spicules, which can make the tissue unpalatable or physically irritating to swallow. Chemically, many sponges produce potent secondary metabolites, which are toxic or foul-tasting compounds that make the organism inedible to most generalist predators.
Specialized spongivorous starfish have evolved specific biological countermeasures to neutralize these defenses. The external digestion process leaves the indigestible skeletal matrix behind, allowing the sea star to bypass the structural defense. These dedicated sponge-eaters possess a high tolerance for the chemical toxins, suggesting the evolution of detoxification enzymes or specialized receptors within their digestive systems. Consequently, a sponge species that is highly toxic to a reef fish can be safely consumed by a starfish that has adapted to metabolize its defensive chemistry.