Starfish are marine invertebrates recognized by their distinctive shape. They possess a unique body plan: “pentaradial symmetry.” This article explores what pentaradial symmetry means for starfish, how they acquire it, its advantages, and how it manifests in their anatomy.
Understanding Pentaradial Symmetry
Symmetry in biology describes how an organism’s body parts are arranged. While many animals, including humans, exhibit bilateral symmetry (divisible into two mirror-image halves), starfish display pentaradial symmetry. This means their body parts are arranged in five equal sections around a central axis. If cut through its center, a starfish yields five identical “slices” radiating outwards.
This five-part arrangement is unique to echinoderms, a group including starfish, sea urchins, and sea cucumbers. Other radially symmetrical animals, like jellyfish, might have body parts arranged in multiples of four or six, but the pentaradial pattern defines starfish. The term “pentaradial” combines “penta” (five) and “radial” (arrangement around a central point, like spokes on a wheel).
From Larva to Adult: A Shift in Symmetry
Adult starfish do not begin life with pentaradial symmetry. Starfish start as tiny, free-swimming larvae, typically bipinnaria or brachiolaria. These larval stages are bilaterally symmetrical, having a distinct left and right side like many other animal larvae. This bilateral form suits their planktonic lifestyle, allowing efficient, directed movement through water to find food.
During metamorphosis, the bilaterally symmetrical larva reorganizes its body plan into the adult pentaradial form. The larva settles onto the seabed, undergoing a significant rearrangement of tissues and organs. Larval structures are reabsorbed as new adult structures develop, leading to the characteristic five-part radial symmetry. This shift highlights pentaradial symmetry as an acquired adult characteristic, shaped by developmental processes.
How Pentaradial Symmetry Benefits Starfish
The pentaradial body plan offers several advantages for starfish on the ocean floor. Their five arms allow movement in any direction without a distinct “front” or “back,” benefiting slow-moving, bottom-dwelling organisms. This radial arrangement also aids feeding, enabling prey capture from multiple angles.
Starfish also benefit from this symmetry for sensory input. Light-sensing eyespots at the tip of each arm detect changes in light from various directions. Their decentralized nervous system, with a nerve ring and radial nerves extending into each arm, supports this all-directional awareness. The five-part symmetry also contributes to their regenerative capabilities, allowing them to regrow lost arms or even an entire body from a single arm and a portion of the central disc. This radial arrangement also provides stability, helping them maintain a firm grip on underwater surfaces.
Anatomical Manifestations of Pentaradial Symmetry
Pentaradial symmetry is evident throughout a starfish’s internal and external anatomy. Externally, the central disc typically radiates five arms, though some species have more. Along the underside of each arm (the oral surface) are ambulacral grooves containing rows of tube feet. These tube feet are arranged to reflect the five-part symmetry, allowing coordinated movement and attachment.
Internally, the water vascular system, a hydraulic network unique to echinoderms, demonstrates pentaradial organization. Water enters through a sieve-like madreporite on the aboral (upper) surface, traveling through a stone canal to a ring canal encircling the mouth. From this ring canal, five radial canals extend into each arm, supplying water to the tube feet. Similarly, the nervous system consists of a central nerve ring and five radial nerves running down each arm, coordinating tube feet functions and sensory input in a radially distributed manner.