Is a Starfish a Crustacean? Key Biological Differences

A starfish is not a crustacean, despite both being marine invertebrates. They belong to entirely distinct branches of the animal kingdom. This article clarifies their fundamental biological differences.

Understanding Starfish: The Echinoderms

Starfish, more accurately called sea stars, are marine invertebrates classified under the phylum Echinodermata. This phylum also includes sea urchins, sand dollars, and sea cucumbers, all characterized by their “spiny skin.” Most sea stars exhibit radial symmetry, with body parts arranged around a central axis, typically with five arms.

A distinctive feature of sea stars is their water vascular system, a hydraulic network of fluid-filled canals used for movement, feeding, and gas exchange. Water enters this system through a sieve-like plate called the madreporite, usually located on the sea star’s upper surface. This water then circulates through a ring canal and radial canals extending into each arm, connecting to numerous tube feet. These tube feet, small, active projections on the underside of the arms, operate by hydraulic pressure, allowing the sea star to move, attach to surfaces, and manipulate food.

Sea stars possess an endoskeleton, an internal skeleton made of calcium carbonate plates called ossicles, located beneath their skin. These ossicles vary in form, from flat plates to granules or spines, contributing to the sea star’s tough or spiny exterior. Sea stars are exclusively marine organisms, found in all oceans from shallow intertidal zones to abyssal depths.

Understanding Crustaceans: The Arthropods

Crustaceans are a diverse group of invertebrates belonging to the subphylum Crustacea, part of the larger phylum Arthropoda. This phylum encompasses animals characterized by segmented bodies, jointed appendages, and an exoskeleton. Familiar examples include crabs, lobsters, shrimp, and barnacles.

A defining characteristic of crustaceans is their hard outer shell, or exoskeleton, which provides protection and structural support. This exoskeleton is primarily composed of chitin and often reinforced with calcium carbonate, making it rigid. Since the exoskeleton does not grow, crustaceans must periodically shed their old shell in a process called molting to increase in size.

Crustaceans typically have segmented bodies that are often divided into a head, thorax, and abdomen, or a fused cephalothorax covered by a single carapace. They possess multiple pairs of jointed appendages, specialized for various functions such as sensing, feeding, locomotion, and reproduction. These appendages include two pairs of antennae for sensory perception, and specialized mouthparts. Most crustaceans are aquatic, inhabiting marine and freshwater environments, though some species are terrestrial.

Why They Are Different: Key Distinctions

The fundamental differences between starfish and crustaceans stem from their classification within separate phyla. Starfish are echinoderms, exhibiting pentamerous radial symmetry with body parts arranged in five sections. In contrast, crustaceans are arthropods, characterized by bilateral symmetry, meaning their bodies can be divided into two mirrored halves.

Their skeletal structures also differ significantly. Starfish have an endoskeleton, an internal framework of calcium carbonate ossicles beneath their skin, allowing flexibility and growth without molting. Conversely, crustaceans possess a rigid exoskeleton, a hard external covering that necessitates periodic molting for growth.

Locomotion and body organization further highlight their distinctions. Starfish move using a water vascular system that operates numerous tube feet through hydraulic pressure. Their bodies are unsegmented and lack a distinct head. Crustaceans move with jointed appendages and have segmented bodies, often with a fused head and thorax, and clearly defined body regions. These distinct biological features place sea stars and crustaceans in separate evolutionary lineages, clarifying why they are not related despite sharing marine habitats.

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