Starfish are marine invertebrates recognized for their distinct radial symmetry. Their body plan, typically star-shaped, features arms radiating from a central disc. These appendages are intricate, playing a comprehensive role in the starfish’s survival and interaction with its marine habitat.
Anatomy of Starfish Arms
Most starfish species possess five arms, though some have more, all extending from a central body disc. Along the underside of each arm runs an ambulacral groove, housing rows of tube feet. These arms are not only for locomotion; they also contain various internal organs. The madreporite, a sieve-like plate on the central disc’s upper surface, acts as the entry point for water into the animal’s water vascular system, which extends throughout the arms.
The starfish’s internal skeleton, an endoskeleton, is composed of small calcareous plates called ossicles, giving them a firm, spiny texture. Within each arm, beneath the ambulacral groove, lies an ambulacral ridge, forming a roof over the groove. Each arm also contains paired digestive glands (pyloric caeca) that produce digestive enzymes and absorb nutrients. Gonads are also present within each arm, highlighting their reproductive role.
How Tube Feet Work
The primary functions of a starfish’s appendages, including locomotion, feeding, and attachment, are powered by its tube feet. Water enters the water vascular system through the madreporite on the starfish’s upper surface. It then flows through a stone canal to a ring canal encircling the mouth. From the ring canal, five radial canals extend into each arm, running along the ambulacral grooves.
Numerous lateral canals branch off the radial canals, each connecting to a tube foot. Each tube foot consists of a bulb-shaped ampulla inside the arm and a flexible podium with a sucker at its tip, protruding from the ambulacral groove. When the ampulla contracts, it forces water into the podium, causing it to extend and adhere to a surface via its sucker. The coordinated contraction and relaxation of these ampullae and tube feet allow the starfish to move across various substrates, climb, and pry open bivalve shells for feeding by creating strong suction and leverage.
Regeneration of Arms
Starfish can regenerate lost or damaged arms, a survival mechanism against predation or injury. When an arm is lost, the starfish initiates wound healing by contracting the stump and forming a thin wound epidermis through epidermal cell migration. This initial repair phase is followed by an early regenerative phase where tissue reorganization begins.
Cellular proliferation occurs in the sub-epidermal tissues, including cells from the parietal peritoneum, radial water canal lining, and dermis. A small new arm tip develops, complete with a terminal ossicle, tentacle, and optic cushion, as the radial water canal and nerve extend into the regenerating tissue. In some species, a single arm, if it retains a portion of the central disc, can regenerate into an entire new starfish. This process typically takes weeks to months, or up to a year for a full arm to regrow, depending on the species and environmental conditions.