Starfish, also known as sea stars, are marine invertebrates belonging to the phylum Echinodermata (“spiny skin”). These creatures are instantly recognizable by their radial symmetry, typically possessing five or more arms radiating from a central disk. Found in every ocean worldwide, from shallow coastal waters to the deep abyss, starfish are opportunistic predators that occupy a significant ecological role in benthic environments. Their specialized biology includes a unique method of consumption, allowing them to prey on animals much larger than their small, centrally located mouth.
Answering the Question Do Starfish Consume Crabs
The direct answer to whether starfish consume crabs is yes, under certain circumstances and depending on the species. Starfish are generalist predators of benthic invertebrates, and this category includes various crustaceans, which are often slow-moving or trapped. However, the consumption of a large, healthy, and heavily armored crab is a difficult feat for most sea star species. Crabs possess powerful claws and hard exoskeletons, making them a formidable challenge for a predator that relies on slow, sustained pressure. Species like the Sunflower Sea Star (Pycnopodia helianthoides) are more capable of subduing small crabs or vulnerable hermit crabs.
In many marine environments, the relationship is often reversed, with crabs actively consuming smaller or injured starfish. The sea star’s unique feeding strategy allows it to process prey with hard coverings, but actively battling a mobile, well-defended crustacean is not their preferred hunting method. Therefore, crabs are not the primary or routine food source for the majority of sea star populations.
The Primary Prey of Starfish
The typical diet of most sea star species consists overwhelmingly of sessile or slow-moving prey that they can easily envelop or pin down. Their primary food source includes bivalves, such as clams, oysters, and mussels, which often form dense beds on the ocean floor. They also routinely feed on other shelled invertebrates, including gastropods like sea snails and various types of barnacles. Many starfish species are also important scavengers and detritivores, consuming decaying organic matter and dead animals they encounter.
Certain primitive species, such as those in the genera Astropecten and Luidia, do not evert their stomachs like their more advanced relatives. Instead, they swallow smaller prey whole, including worms or tiny crustaceans, and later expel any hard, indigestible shell fragments.
Other species are highly specialized. The Crown-of-Thorns Starfish (Acanthaster planci) exclusively preys on the soft tissues of coral polyps. Species like Henricia and Echinaster are suspension feeders, gathering phytoplankton and organic particles that drift in the water column.
Eversion The Unique Feeding Strategy
The most remarkable feature of a sea star’s diet is the mechanism by which it consumes its prey, a process known as stomach eversion. This strategy allows the sea star to digest organisms much larger than its minuscule mouth opening, which is located on the underside of its central disc. The process begins when the starfish mounts its prey, using the hundreds of tiny, suction-tipped tube feet lining the grooves of its arms to achieve a secure grip. The sea star uses the hydraulic pressure of its water vascular system to apply immense, steady force to the shell of a bivalve. This pressure is sustained until the prey’s powerful adductor muscles eventually fatigue and relax, creating a tiny opening.
Once the shell is slightly compromised, the sea star extends its cardiac stomach out through its mouth and inserts it into the opening, covering the soft tissue of the prey. This action is regulated by neuropeptides, with molecules called SALMFamides triggering the necessary stomach relaxation and eversion. This process represents external digestion, or extracorporeal digestion, because the digestive phase occurs outside the sea star’s body cavity.
Digestive enzymes are secreted directly from the everted stomach onto the prey’s flesh, which rapidly liquefies the soft tissues. This turns the meal into a “soup-like slurry” that the sea star can absorb directly through the walls of its stomach. Once the meal is complete, the stomach is pulled back into the body by the contraction of muscles, a process triggered by another neuropeptide known as NGFFYamide. The nutrient-rich liquid is then transferred to the pyloric stomach and into the pyloric caeca, which are glandular extensions that run down the length of each arm. These structures are responsible for the final stages of digestion and the absorption of nutrients.
Any indigestible hard parts, such as shell fragments from the prey, are either left behind or later expelled once the stomach is fully retracted.