Starfish

Not actually fish

The name “starfish” is biologically inaccurate — these animals are not fish. They’re echinoderms, sharing more ancestry with sea urchins and sand dollars than with fish.

Most marine biologists prefer the term “sea star” to avoid the confusion. The “starfish” name persists in popular usage but increasingly is being replaced by “sea star” in educational and scientific contexts.

Five-arm symmetry (mostly)

Most sea stars have five-arm radial symmetry — but not always:

• Most species — 5 arms
• Some species — 6, 7, 9, or more arms
• Sun stars (Solaster) — up to 24 arms
• Crown-of-thorns (Acanthaster) — typically 11-21 arms
• Maximum recorded: 50+ arms in some Pacific species

The arms aren’t separate appendages but integrated parts of the body, each containing complete organ systems including digestive tubes and reproductive organs.

Regeneration champions

Sea stars have extraordinary regenerative abilities:

• Lost arm regrows within weeks to months
• Some species can regrow entire body from a single severed arm with central disc
• One arm + disc fragment can become a new individual
• Regeneration happens through tissue dedifferentiation and regrowth

This regeneration ability is studied intensively for medical applications. Understanding sea star regeneration could provide insights for human tissue regeneration research.

A practical implication: cutting up sea stars to control them doesn’t work. Each piece (with central disc material) can become a new sea star.

Tube feet hydraulics

Sea stars move via hydraulically-powered tube feet beneath their arms:

• Hundreds to thousands of tiny tube feet per individual
• Each operates by water pressure in the species’ water vascular system
• Suction cups at tips for gripping surfaces
• Coordinated movement allows slow but precise locomotion
• Speeds typically a few cm per minute

The tube feet system is unique to echinoderms — found nowhere else in the animal kingdom. It enables sea stars to climb vertical surfaces, hold position in current, and pry open clams.

Stomach extrusion feeding

Sea stars have one of the strangest digestive strategies in the animal kingdom:

1. Sea star locates clam or mussel
2. Pries shell open slightly using tube feet (only needs millimeters of opening)
3. Pushes its stomach out through its mouth opening
4. Stomach enters the bivalve’s shell
5. Digests prey externally within the shell
6. Pulls digested food back by retracting stomach

This external digestion allows sea stars to feed on prey larger than their entire body opening. It’s a remarkable evolutionary solution to consuming hard-shelled prey.

Crown-of-thorns crisis

The crown-of-thorns starfish (Acanthaster planci) has caused major coral reef destruction in recent decades:

• Massive population outbreaks (“blooms”) periodic in Pacific reefs
• Each starfish consumes coral polyps
• Bloom can destroy entire reef sections
• Great Barrier Reef especially affected
• Multiple control efforts with varying success

Causes of population blooms remain debated — overfishing of natural predators, agricultural runoff increasing food sources, climate change effects. Multiple factors likely contribute.

Sea star wasting disease

Since 2013, sea star wasting disease has caused massive Pacific Coast die-offs:

• First detected in British Columbia and Washington
• Spread along entire Pacific Coast — from Alaska to Mexico
• Mortality rates 90%+ for some species
• Sunflower star (Pycnopodia helianthoides) particularly devastated
• Cause uncertain — likely viral, possibly climate-stress amplified

The disease has fundamentally altered Pacific Coast ecosystems. The sunflower star’s decline has caused kelp forest collapses in some regions, as the species was a major sea urchin predator and urchins now overgraze kelp.

Color and pattern diversity

Sea stars come in astonishing variety:

• Bright orange (Pacific blood star)
• Vibrant red (Caribbean cushion stars)
• Brown and tan (most temperate species)
• Blue and purple (some tropical species)
• Multi-colored (decorator sea stars)
• Camouflaged patterns (rocky-bottom species)

The color diversity reflects camouflage adaptations for specific habitats and substrate types. Many colorful tropical species are also poisonous, with bright colors warning predators.

Aboral and oral surfaces

Sea stars have distinct top and bottom surfaces despite their radial symmetry:

• Aboral (top) surface — facing upward; usually contains anus (small central opening)
• Oral (bottom) surface — facing downward; contains mouth at center; tube feet beneath arms

This terminology is used because sea stars don’t have traditional “front” or “back” — their radial symmetry means any direction can be the leading direction during movement.

Ecosystem roles

Sea stars are important ecosystem components:

• Predator population control — many serve as keystone predators
• Prey for various species — fish, otters, gulls
• Bioturbation — disturb sediment as they feed
• Reef structure — both maintenance and destruction roles

The “keystone species” concept was actually first developed using Pisaster ochraceus (the Pacific purple sea star) — researcher Robert Paine’s 1966 experiments showed how removing this single species cascaded through the entire intertidal community.