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Brittle Star

Ophiuroidea

Brittle stars, also known as serpent stars or ophiuroids, belong to Ophiuroidea, the most species-rich class of echinoderms, with around 2,000 known species. They are most closely related to starfish (class Asteroidea) and, like them, have five long, whip-like arms used for crawling. However, brittle stars differ in having a clearly defined central disk.

These animals can voluntarily shed their arms as a defensive strategy when threatened by predators, a trait that gives them their name. Unlike starfish, which crawl using their tube feet, brittle stars move by rapidly wriggling their arms in a snake-like motion.

Fossil records suggest that this group diverged from other echinoderm groups during the Ordovician Period. Brittle stars are spread across a wide range of habitats, ranging from tide pools and coral reefs to hydrothermal vents and deep seas.

Description

Size

These animals are measured based on both their disk diameter and arm length. Their arms typically measure 5 to 8 times the diameter of the disk, with a total length ranging from 8 to 24 in (20 to 60 cm). In basket stars (order Euryalida), an outstretched arm can extend up to 3 ft (1 m) in length.

Body Plan

They are characterized by a distinct, flat central disk and, in most species, five slender, whip-like arms (pentaradial symmetry). However, members of the family Ophiactidae have six arms and are an exception to the pentaradial symmetry.

The underside of the disk contains the mouth equipped with five toothed jaws composed of skeletal plates. The mouth is the only external opening in the body and helps expel waste. The gonads are located within the central disc, nestled in pouches (genital bursae) found between the bases of the arms. 

When compared to other echinoderms, the body cavity or coelom of brittle stars is much reduced. It is confined to the central disk and does not extend into the arms.

Endoskeleton

These animals are supported by an internal skeleton of calcium carbonate in the form of calcite ossicles. In the arms, these ossicles are arranged in a series resembling vertebrae in structure and function (hence the name vertebral ossicles), though they are not homologous to vertebrate bones. These ossicles articulate via ball-and-socket joints, allowing the arms to bend in most directions except upward. Externally, the ossicles form prominent dorsal, ventral, and lateral plates, which are covered by a thin, syncytial epidermis. The lateral plates often bear long spines that project outward, helping these echinoderms gain traction on substrates.

Organ System

Water Vascular System

As echinoderms, brittle stars have a characteristic water vascular system comprising a network of fluid-filled vessels.

The network comprises a central ring canal that radiates five radial canals extending into each of the five arms. The radial canals end in reduced tube feet that lack the bulbous ampullae present in other echinoderms, like sea stars. Thus, the tube feet are not locomotory in function and instead may assist in feeding or sensory functions.

The system opens to the exterior through a calcareous opening called the madreporite, which is usually located within one of the jaw plates on the lower side of the animal (unlike starfish, in which the madreporite is found on the upper surface). Most species possess a single madreporite on the underside of the disk, though in some deep-sea brittle stars, this opening is absent or not externally visible[1]. Basket stars, however, have a single madreporite on each of their arms.

Circulatory

They do not have a true circulatory system with blood and a heart like vertebrates. Instead, they rely on the coelomic and hemal systems for the transport of nutrients.

Respiratory and Excretory

These animals respire through specialized ciliated sacs called bursae, which are found between the bases of the arms on the underside of the disk. There are usually ten bursae found adjacent to digestive pouches in the stomach.

The bursae are also considered to be excretory in function, assisted by phagocytic cells (coelomocytes) that collect waste from the body cavity and direct it to the bursae for expulsion.

Digestive

The mouth, functioning as both an inlet and an outlet, is equipped with five jaws and leads to a short esophagus. The stomach, located in the dorsal part of the disk, contains 10 pouches or infolds and is lined with specialized glandular hepatic cells that are similar in function to liver cells in vertebrates. These cells secrete digestive enzymes and absorb the nutrients from partially digested food.

Nervous

Brittle stars lack a centralized nervous system like vertebrates. The primary component of their nervous system is a nerve ring that circles the central disk. From the ring, radial branches spread out along the arms of the animal, reaching the tip of each limb. 

While brittle stars lack specialized sense organs, their epidermis is rich in nerve endings, particularly concentrated at the tips of their arms. These nerve endings detect light, tactile stimuli, and chemicals in the water.

Taxonomy

Over 2,000 species of brittle stars exist, making the class Ophiuroidea the most abundant group of echinoderms. According to the World Register of Marine Species (following O’Hara 2017), they are divided into 6 orders and 36 families. 

Brittle Star (Ophiuroidea)

The most abundant of all families are Amphiuridae and Ophiuridae, with around 470 and 340 species, respectively.

Evolution and Fossil Records

Fossils of brittle stars are rare because their bodies tend to disarticulate quickly after death. However, the limited fossil evidence suggests that these echinoderms first appeared during the Early Ordovician Period, approximately 500 million years ago.

In the 2010s, specimens were unearthed from the Agrio Formation of Neuquén Basin, Argentina. These were the first pieces of evidence of brittle stars in the Southern Hemisphere, dating back to the Cretaceous Period. During the Silurian Period, a minor mass extinction event known as the Mulde Event resulted in a population bottleneck in brittle star ancestors. This event caused a significant reduction in body size and structural simplification in these ancestors, driven by paedomorphosis (retention of juvenile traits), particularly in the fossil species Muldaster haakei. Such miniaturization marked the onset of the modern brittle star body plan. Over time, however, they gradually regained their size and became more structurally complex. It is believed that the first large-sized modern ophiuroid emerged during the Early Carboniferous Period[2].

Distribution and Habitat

These echinoderms inhabit all oceans worldwide, ranging from tropical to polar waters. They are typically found in shallow coastal waters below the low-tide level, with at least six families recorded at depths as shallow as 2 m. Members of the genera Ophiura, Amphiophiura, and Ophiacantha are commonly found at depths greater than 4 m. However, some species have been recorded at significantly greater depths, between 2,500 and 4,000 m[3].

Many species, like Asteroschema clavigerum, are an important part of the reef ecosystem and are found attached to deep-sea corals, sponges, and urchins. Some species, especially those in the family Amphiuridae, are capable of thriving in brackish water environments.

Of around 2,000 species of brittle stars found worldwide, over 1,200 species have been recorded at depths greater than 650 ft (about 200 m). Some species, such as Spinophiura jolliveti, have been recorded at hydrothermal vent sites (2,500 to 2,700 m) along the East Pacific Rise[4]. Globally, the most widespread species is the long-armed brittle star (Amphipholis squamata), and the green brittle star (Ophioderma brevispina) and the common European brittle star (Ophiothrix fragilis) are the two most common species found in littoral zones.

Diet

While some species of brittle stars, such as Amphiura filiformis, are detritivores feeding on benthic organic matter, others, like Ophiura albida, are scavengers that consume dead, decaying animal matter and macroalgal detritus.

Certain species within the family Ophiuridae, like Ophiosparte gigas, are carnivorous and hunt epibenthic animals, such as bivalves, polychaete worms, and crustaceans[5]. The species Ophionereis reticulata is omnivorous, consuming algae, polychaetes, and detritus.

Ophiopsammus maculata, found in New Zealand, feeds on the pollen of southern beeches (plants of the genus Nothofagus) that is washed into coastal waters from nearby forests.

Behavior

Most shallow-water species typically forage at night, hiding under rocks or in crevices during the daytime. In contrast, deep-sea brittle stars do not follow a photoperiodic pattern, and they forage whenever food becomes available.

Locomotion

Instead of crawling on tube feet like starfish, brittle stars rapidly wriggle their flexible arms, often resembling the movement of snakes. They fix one arm as a leading axis while using the others to move across the ocean floor. The leading arm points in the direction of travel, whereas the two adjacent arms on either side push against the substrate. The remaining arms trail behind and help steer the animal forward.

Feeding and Digestion

The feeding strategies vary across different brittle stars. While some, like basket stars, are suspension feeders and trap suspended matter using their mucus-laden arms, many other brittle stars are deposit feeders that collect organic matter from the sea floor. Basket stars, in particular, have a unique pattern of sweeping their food into their mouth using rhythmic, coordinated movement of their arms. Active predators like Ophiosparte gigas rely on sensors at the tips of their arms to detect prey. Upon detecting prey, they extend one arm to capture it while using the other arm to anchor themselves.

Autotomy and Regeneration

When threatened, these echinoderms defend themselves by shedding one or more arms (autotomy), which continue to wriggle after detachment, distracting predators and giving the animal a chance to escape. If a brittle star loses an arm to injury, it can gradually regenerate it over a period ranging from a few weeks to more than 18 months, depending on the species and environmental conditions. Some members of the family Amphiuridae, like Amphiura filiformis, can also regenerate gut and gonad fragments, which may be lost along with the arms[6].

Lifespan

The lifespan of brittle stars varies considerably among the different species. While some, like Amphipholis squamata, have an average lifespan of 3 to 5 years, others, like Amphiura filiformis, are estimated to have a maximum lifespan of 25 years[7].

Reproduction and Life Cycle

Most brittle star species have separate sexes (gonochoric), though a few, like Ophiothrix synoecina, are protandric hermaphrodites that start as males but switch to females at some point in their lifecycle. 

Sexual

Most brittle stars undergo external fertilization, releasing their gametes (sperm and egg) into the surrounding water through the bursal sacs.

Some species, like Amphipholis squamata, are viviparous, and their embryos are brooded within the bursae, where they receive nourishment by absorbing organic molecules from the bursal fluid. In contrast, a few species, like the common brittle star (Ophiothrix fragilis), undergo a free-swimming, planktotrophic larval stage called the ophiopluteus. These larvae possess four pairs of arms lined with cilia, which help in movement under the water.

Asexual

Only a few species of brittle stars, especially the six-armed members of the family Ophiactidae, undergo asexual reproduction through fission. In this process, the disk of the animal splits into two halves. Each half regrows the disk, as well as the three lost arms. This leads to the formation of two individuals, each with a complete disk and six arms (three large and three small).

One species that frequently undergoes fission is the West Indian brittle star (Ophiocomella ophiactoides). The process begins with the softening of one side of the disk, followed by a deepening furrow that eventually splits the animal into two parts. New arms start growing even before the fission is complete, resulting in shorter intervals between successive divisions. This species has an interval of 89 days between two consecutive divisions, allowing a single brittle star to produce up to 15 individuals per year under ideal conditions[8].

Predators and Parasites

They are typically preyed upon by fish, including some species of wrasses, pufferfish, and triggerfish. Some crustaceans, such as velvet crabs, hermit crabs, and mantis shrimp, also feed on these echinoderms. Occasionally, some species of starfish may also feed on brittle stars. These echinoderms are parasitized by several invertebrates, including protozoans such as Coccomyxa ophiurae, and crustaceans like the copepod Collocheres gracilicauda.

Interesting Facts

References Article last updated on 4th June 2025
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