Starfish, also known as sea stars, are captivating marine invertebrates with distinctive radial symmetry and vibrant colors. These invertebrates lack an obvious head, presenting a body plan highly adapted to their underwater existence. A common inquiry is whether starfish have mouths.
Locating the Starfish Mouth
Starfish do possess a mouth, though its location differs significantly from many familiar animals. Positioned centrally on their underside, the mouth is a key component of their unique feeding strategy, allowing the starfish to position itself directly over its prey. It is surrounded by a tough membrane that can expand and contract.
Connecting to the mouth is a short esophagus, which leads to the starfish’s two-part stomach. The outer portion is the cardiac stomach, which can extend outside the body. This eversion of the cardiac stomach through the mouth is a distinctive initial step in how many starfish feed.
Starfish use their numerous tube feet, located on their underside, to manipulate and hold prey. These appendages, part of their water vascular system, allow them to grasp objects and pry open bivalve shells. Some starfish can exert considerable pressure to open shells just enough for their stomach to enter. This positioning and the action of the tube feet are crucial for directing food into the everted stomach.
How Starfish Consume Food
The diet of starfish is quite varied, often including bivalves like clams, mussels, and oysters, as well as snails, crustaceans, and worms. Some species are scavengers, consuming detritus or carrion, while others, like the crown-of-thorns starfish, specialize in feeding on coral polyps. Some primitive starfish swallow their prey whole, rather than using stomach eversion.
For many species, after the cardiac stomach is everted and positioned over the prey, digestive enzymes are released directly onto the food. These enzymes work externally, breaking down the prey’s tissues into a liquid form. This external digestion allows the starfish to consume prey larger than its mouth opening. The partially digested material is then absorbed through the lining of the cardiac stomach as it is retracted back into the body.
Once inside, the liquefied food passes into the pyloric stomach, which is the second part of their digestive system. From the pyloric stomach, two extensions, known as pyloric caeca or digestive glands, extend into each arm. These glands are responsible for further digestion and the absorption of nutrients. While many starfish species possess a small anus on their upper surface for waste expulsion, it is often not highly functional. Undigested hard materials, such as shell fragments, are commonly expelled back out through the mouth.