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

Fun and Interesting Facts about Brittle Stars

[Brittle Star Phylum: Echinodermata] [Subphylum: Asterozoa] [Classification: Ophiuroidea]

The class Ophiuroidea signifies that brittlestars are echinoderms and closely related to sea stars due to their radial symmetry and five (5) long flexible arms.

This segment contains information about brittle stars (ophiuroids), including where they are found, what they eat, and how their reproductive strategies differ from their starfish relatives.

Brittle Stars Global Distribution and Habitat

Ophiuroidea flourish in all oceans, including the polar seas and tropical waters.

Even though most brittle star species thrive in shallow intertidal zones, others can tolerate depths over 6,000 metres below sea level.

As a consequence of that, brittlestars are one of the most abundant of all deep-sea animals.

During the day time, they hide inside the crevices and rocky outcrops that are commonly found around most coral reef formations.

So, because the brittle star is a nocturnal creature, they are difficult to spot when they are partly buried in the sand and mud bottoms of the seafloor. Thus, they emerge during the dark hours to capture food from the water with their long slender arms.

Interesting Fact: Like many symbiotic relationships that occur in marine ecosystems, some species of brittle star live interdependently with corals, sea sponges, and even some of the other examples of echinoderms (crinoids).

Brittle Star Characteristics and Traits

Scientists recognise about two thousand (2,000) living species of brittle stars within the classification Ophiuroidea (within the subphylum Asterozoa).

Yet, this number increases to more than 3,000 when you include brittle star fossils found in sedimentary rocks that date back over 450 million years ago.

In terms of species diversity, they're the largest class of all surviving echinoderms, more than sea cucumbers, sea stars (starfish), and sea urchins.

Put another way...

Even though brittle stars look the same as many of the sea star species, in fact they are not "true" sea stars. It's fair to say the main distinctions are anatomical.

In general, brittle stars have a smaller central disk and longer arms, which can reach up to sixty (60) centimetres (24 inches) in some species.

So, while sea stars have fleshy arms that tend to be continuous with the central disc, brittle stars have thin slender arms (usually five) covered in delicate spines and they are more distinct from the scale-like central core.

Furthermore, their arms are jointed (similar to vertebrae) which allows them to move much faster than starfish, in undulating snake-like and agile movements.

Fun Fact: One of the surprising echinoderms characteristics is being able to regenerate damaged or severed body parts. They can - and will - sacrifice an arm to try and avoid capture from their natural predators in the wild (e.g. snails and sea turtles).

What Do Brittle Stars Eat?

Brittle stars use their lithesome slender arms to wedge themselves into tiny cracks as far away from predators as possible. But, these opportunistic scavengers need to come out into the open to eat.

In areas with strong water currents, they're mostly suspension feeders that consume planktonic matter drifting in the lower water layer.

In contrast, the deposit feeders eat aquatic animals that live within the soft sediments (infaunal), such as carrion, detritus material, and other organic particles.

Even though most brittle stars are carnivorous scavengers, the need for nutrient-rich food means their diet could also include:

• Algae
• Barnacles
• Coral polyps (especially the crown of thorn starfish)
• Mussels
• Oysters
• Sea cucumbers
• Sea snails
• Sea urchins
• Worms (phylum Annelida)

Fun Facts: Despite their ungainly movements on the ocean floor, brittle stars are vicious predators that use tube feet to pry open gaps in vulnerable or unsuspecting aquatic mollusks.

Types of Brittle Star Species

How Do Brittle Stars Reproduce?

Most starfishes have separate sexes, (male and female) and a few are hermaphroditic. So, brittle star reproduction can also be sexual or asexual (albeit 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 brittle stars.

In case you were wondering...

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

So, if a brittle star loses one of its rays and the central disc, it can "clone itself" and grow the split half into a whole new creature through regeneration.

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 average lifespan of brittle stars is about five (5) years, the threats and predators that they face in the ocean are plentiful.

Yet, the most prolific natural predators of brittle stars are large fishes, especially pufferfishes (Tetraodontidae) and triggerfish (Balistidae). In addition, many of the brittle star species are prey for marine crustaceans, such as large crabs and lobsters.

Nonetheless, 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 and brittle star families as being of "Least Concern" (LC).

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