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Home › Sea Wildlife › Marine Species › Invertebrates › Echinoderms › Sea Stars

Interesting Facts about Sea Stars (Starfish)

[Sea Star Phylum: Echinodermata] [Class: Asteroidea] [Order: Forcipulatida] [Family: Asteriidae]

Sea stars, frequently called starfish due to their star-like body shape, represent about 1,900 of the typical marine invertebrates echinoderm species.

This section contains fun facts and information about starfish (asteroids), including where they live, what they eat, and how they reproduce.

Sea Stars Geographical Range and Habitats

Worldwide, the general distribution of the sea star classification is wide ranging and diversified.

For example:

Starfish are marine invertebrates that are found in all oceans, but most notably in the Atlantic Ocean and the Indian Ocean.

Hence, the ideal sea stars habitats exist even in some polar regions of the Northern and Southern Hemispheres, as well as the tropical waters near the equator.

Even though starfishes are phylum echinodermata animals that occupy an extensive variety of marine ecosystems, they are particularly endemic in the littoral zones (intertidal) and rocky shorelines of shallow coastal areas that experience extended exposure to air and daylight during low tides.

Interesting Fact: Some deep-sea species of sea stars exist in complete darkness and live under intense pressure due to the depth. However, starfish do not exist in any freshwater environments and they would die soon after any exposure to fresh water.

Starfish Physical Characteristics and Traits

Starfish sizes range from as little as ten (10) millimetres wide (the paddle-spined sea star for example) and up to one metre across (e.g. the sunflower sea star).

There are several unusual features and key characteristics of sea stars that help to differentiate them from most of the other saltwater invertebrates in the ocean.

In other words...

Radial symmetry is the anatomical attribute that stands out from the counterparts. The vast majority of all species of sea stars have five (5) rays (arms) with ambulacral grooves that radiate from a centralised body.

Nevertheless, some species can have many more. For instance, the sunflower sea star (Pycnopodia helianthoides) has up to forty (40) arms as an adult - albeit starting life as a juvenile with only five rays.

The water vascular system of starfishes is also unique. The internal system is filled with seawater and it powers their curious movement and feeding habits.

Several hundred flexible tubes, often with 'hydraulic' suction capabilities, create feet that adorn the underside of each ray. Sea stars use their tube feet for walking on the seabed, clinging to slippery rocks, and for gripping their favourite prey - benthic invertebrates.

Sea Stars Anatomy

The mouth of a sea star is situated underneath, and in the centre of its core. The lower part of the anatomy is known as the oral surface and the upper part is called the aboral surface.

The smooth and spiny aboral side of a sea star is often covered with brightly coloured overlapping plates made of calcium. The upper surface displays various shades of red, blue, orange, or brown camouflage.

In a typical sense, starfish do not have a true brain and they circulate nutrients via internal sea water - not blood. Respiration takes place through small structures called papulae (skin gills) and each arm contains a simple nerve ring and radial nerves.

Pro Tip: One of the surprising echinoderms characteristics is being able to regenerate damaged or severed body parts. In fact, they can - and will - sacrifice an arm to try and avoid capture from their main predators (e.g. sea snails and turtles).

What Do Starfish Eat?

Starfish are filter feeders. Plus, they are opportunistic predators that prey on benthic invertebrates, a diversity of bottom-dwelling organisms that live in sediment or sand.

Even though most starfish are carnivorous scavengers, a need for nutrients means their diet could also include:

• Algae
• Barnacles
• Bivalves (e.g. clams)
• Coral polyps (especially the crown of thorns sea star)
• Detritus material
• Mussels
• Oysters
• Plankton
• Sea cucumbers
• Sea snails
• Sea urchins
• Small fish (not common)
• Worms (especially clam worms)

Fun Facts: Despite their slow movement, sea stars are vicious predators and use their tube feet to pry open any tiny gaps in vulnerable or unsuspecting aquatic mollusks. A single sea star is capable of eating more than ten (10) clams in less than one week.

Different Types of Sea Stars

How Do Sea Stars Reproduce?

Most starfish have separate sexes, (male and female) and a few are hermaphroditic. So, sea star reproduction can either be sexual or asexual (less common).

Following a natural trigger for spawning season, such as a rise in water temperature, sexual reproduction results in external fertilisation when millions of eggs and sperm are released into the open ocean.

After fertilisation takes place, the embryos start to develop into free-swimming bipinnaria larvae that will drift with currents for several weeks or months. The larvae eventually settle on the ocean floor before they transform into juvenile sea stars.

In case you were wondering...

The species that reproduce asexually, either through fissiparity or fragmentation (e.g. Coscinasterias), often splits their body in half, and each separate half can regrow the missing parts.

Similarly, if a sea star loses one of its rays and the central disc, it can "clone itself" and grow into a whole new sea star through regeneration, a process that's more common in Linckia starfish.

Threats and Predators

Some species of starfish rank among the most invasive creatures in the world. For example, the tropical starfish Acanthaster planci (known more commonly as the crown-of-thorns) and the sea star of the northern Pacific, are both considered as detrimental invaders with a devastating impact for delicate coral reef ecosystems.

Even though the lifespan of a sea star can be several decades long (up to 35 years in some species), the threats and predators that sea stars face in the ocean are plentiful.

Yet, the most prolific natural predators of sea stars are large fishes, especially pufferfishes (Tetraodontidae) and triggerfish (Balistidae). Moreover, some starfish are cannibalistic animals that will eat each other, particularly the sunflower star.

In addition, many of the sea star species fall victim to marine crustaceans, such as large crabs and lobsters. In the colder regions, sea otters are known to include sea stars as part of their diet.

Here's the thing:

On top of all this, sea star wasting syndrome is a deadly disease caused by the bacterium called Vibrio pectenicida.

As a consequence, sea stars (and other examples of echinoderms) can develop lesions and lose limbs, resulting in complete disintegration.

However, some of the worst human-induced threats to the long term survival of sea stars include:

• Climate change (resulting in varying degrees of coral bleaching).
• Coastal habitat degradation and destruction.
• Underwater garbage pollution.
• Overfishing (particularly in countries where sea stars are considered as a culinary delicacy, such as China and Japan).

They are seen as cultural curios and their symmetry is often copied in art and literature. Some species of starfish are toxic, and yet they are considered a delicacy and eaten in some countries.

Important: In 2020, the IUCN Red List of Threatened Species assessed the conservation status of many starfish families as being of "Least Concern" (LC). But, they list the Sunflower Sea Star (Pycnopodia helianthoides) and the Tasmanian Live-Bearing Seastar (Parvulastra vivipara) as "Critically Endangered" (CR).

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