Starfish, often called sea stars, are marine invertebrates that inhabit oceans worldwide. As members of the echinoderm family, they possess a unique body plan. This article explores their distinctive physical appearance, fundamental structure, external features, and the diversity across different species.
Core Physical Characteristics
Most starfish exhibit pentaradial symmetry, meaning their bodies are organized around a central axis with five radiating parts. While five arms are common, some species can have a different number, varying from six or seven arms to more than fifty.
At the center of the starfish is a central disc, from which arms extend outwards. The arms taper towards their ends, forming the characteristic star shape. The starfish body has two main surfaces: the oral surface, the underside containing the mouth, and the aboral surface, the upper side that faces upwards.
External Surface Details
The outer skin of a starfish presents textures ranging from smooth to granular or spiny. This texture is due to an internal skeleton made of calcium carbonate components called ossicles, embedded within the skin, creating bumps or spines. Many species have spines or tubercles that contribute to their rough feel and offer protection against predators. Tiny pincer-like structures called pedicellariae may be present on the surface, helping to keep the starfish clean by removing debris.
Starfish display a wide spectrum of colors, including reds, oranges, blues, purples, greens, and browns, often in mottled or solid patterns. These colors can serve as camouflage or as a warning to potential predators. On the aboral surface, off-center, a small, sieve-like plate called the madreporite is visible, often appearing as a contrasting color like orange or yellow. Along the underside of each arm, rows of tiny, flexible tube feet are visible, used for movement and grasping surfaces.
Diverse Appearances Across Species
Starfish appearance varies widely, beyond the typical five-armed image. While most species have five arms, others like the sun stars can have numerous arms, sometimes exceeding 20 or even 50. Starfish also exhibit a wide size range, from small species just a few centimeters across to much larger ones, with arm spans up to 60 centimeters.
Shapes vary widely; some species are flat, others plump or cushion-like, and some have long, slender arms while others have short, stubby ones. For example, the Crown-of-Thorns starfish is known for its disc-shaped body, multiple arms (up to 21), and sharp, venomous spines. In contrast, the Cushion Star has a rotund, inflated body with almost indistinguishable arms, resembling a pincushion. The Bat Star often has a central disc wider than its short, webbed arms, giving it a distinctive, broad appearance.