Brittle stars, also called serpent stars or ophiuroids, are members of the class Ophiuroidea, the most abundant of all echinoderm groups. They are most closely related to starfish or sea stars of the class Asteroidea, with which they share a number of features. For instance, most brittle stars have five long whip-like arms similar to starfish for crawling. However, brittle stars differ in having a clearly defined central disk.
These animals are named for their ability to voluntarily shed their arms (making them appear brittle) when threatened by predators. Unlike starfish, which crawl using their tube feet, brittle stars move by rapidly wriggling their arms in a snake-like motion.
This group diverged from other echinoderm groups during the Ordovician Period and is currently spread across a wide range of habitats, ranging from tide pools and coral reefs to hydrothermal vents and deep seas. Brittle stars feed on detritus, bacteria, plankton, and even the pollen of certain plants.
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.
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.
The underside of the disk contains the mouth (which also serves the anus) equipped with five toothed jaws composed of skeletal plates. 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.
These animals are internally supported by a skeleton of calcium carbonate (made of calcite) comprising ossicles. These ossicles superficially resemble vertebrae and are hence called vertebral ossicles. They are fused to form armor plates (collectively called the test), which are covered by a smooth, syncytial epidermis. In most species, there are ball-and-socket joints between the ossicles and the plates, which enable bending of the arms in most directions except upwards.
The ossicles are guarded by a thin ring of soft tissue, followed by four series of jointed plates on the upper, lower, and lateral surfaces of each arm. The lateral plates possess long spines that project outwards, aiding the animal in traction against a substrate.
As echinoderms, brittle stars have a characteristic water vascular system comprising a network of fluid-filled vessels.
A central ring canal radiates short lateral canals along the arms, each ending in tube feet or podia. Unlike starfish, the tube feet of these animals are quite reduced and lack the bulbous ampullae that control water pressure.
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, though some lack one entirely. Basket stars, however, have a madreporite on each arm.
They have a simple hemal system comprising a series of sinuses and vessels separate from the water vascular system. This system transports oxygen and nutrients throughout the body.
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, each nestled between two digestive pouches found in the stomach.
In all probability, the bursae are also excretory in function, assisted by phagocytic cells called coelomocytes. These cells collect wastes from the body cavity and send them off to the bursae for expulsion.
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 glandular hepatic cells.
The primary component of the nervous system is the 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 most 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.
Over 2,000 species of brittle stars exist, making class Ophiuroidea the most abundant group of echinoderms. According to the World Register of Marine Species, they are divided into 6 orders and 36 families.
The most abundant of all families are Amphiuridae and Ophiuridae, with 467 and 344 species, respectively.
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 their first pieces of evidence in the Southern Hemisphere, dating back to the Cretaceous Period.
During the Silurian Period, a mass extinction event known as the Mulde Event caused a population bottleneck among these animals, leading to a simplification of their anatomy. Over time, however, they recovered, regaining their size and evolving greater complexity. It is thought that the first large modern brittle star emerged during the Early Carboniferous Period.
They are spread across a variety of marine habitats from the poles to the tropics, including muddy abyssal plains (at over 6,000 m), seamounts, and even hydrothermal vents. Many species, like Asteroschema clavigerum, are an important part of the reef ecosystem and are found attached to deep-sea corals, sponges, urchins, and xenophyophores. Some species, especially those in the family Amphiuridae, are even found to tolerate brackish water environments.
Of about 2,000 species of brittle stars found worldwide, over 1,200 species are found at depths greater than 650 ft (about 200 m). They are generally found beyond the low tide level, with at least six families found at depths of 2 m. Members of the genera Ophiura, Amphiophiura, and Ophiacantha are spotted below 4 m.
Globally, the most widespread species is the long-armed brittle star (Amphipholis squamata), while the green brittle star (Ophioderma brevispina) and the common European brittle star (Ophiothrix fragilis) are the two most common species found in littoral zones.
Brittle stars are primarily scavengers or detritivores, feeding on dead and decaying organic matter. However, they may also prey on small crustaceans or worms. Some groups, such as basket stars, feed on suspended plankton and bacteria.
Certain species within the family Ophiuridae, like Ophiura Linnaeus and Ophiosparte gigas, are carnivorous and hunt epibenthic animals. In contrast, the species Ophionereis reticulata is omnivorous, consuming algae, polychaetes, and detritus.
Ophiopsammus maculata, found in New Zealand, feeds on pollen of southern beeches (plants of the genus Nothofagus).
These animals typically forage at night and hide under rocks or in crevices during the daytime.
Instead of crawling on tube feet like starfish, brittle stars rapidly wriggle their flexible arms, often resembling the movement of snakes. Although they are radially symmetrical, their movement mimics bilateral symmetry. They fix one arm as a leading axis while using the others to propel themselves through the water.
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, others are deposit feeders that scrape organic matter from the sea floor. Basket stars, particularly, have a unique pattern of sweeping their food rhythmically into their mouth using 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.
When threatened, these echinoderms amputate their arms, gut, and gonad fragments to confuse the predators and flee to safety (autotomy). Eventually, they regenerate these lost parts, just like lizards.
Brittle stars typically live up to 5 years, though some basket stars, like Gorgonocephalus, may survive longer.
Most brittle stars 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.
Most brittle stars undergo external fertilization, releasing their gametes (sperm and egg) into the surrounding water through the bursal sacs.
Some species, like Amphipholus squamata, are viviparous and give birth to live young. Their embryo is brooded within the bursae, receiving nourishment through the wall of the bursa.
In contrast, a few species, like the common brittle star (Ophiothrix fragilis), undergo a free-swimming larval stage called the ophiopluteus. These larvae possess four pairs of arms lined with cilia, which help in movement under the water.
Some species, 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, thereby minimizing the time between successive divisions.
This species has an interval of 89 days between simultaneous divisions, allowing a single brittle star to produce up to 15 individuals per year under ideal conditions.
They are preyed upon by fish, harlequin ducks, and aquatic invertebrates, like crabs and other echinoderms, such as sea stars and sea urchins. Moreover, they are also parasitized by a number of invertebrates like protozoans, nematodes, trematodes, crustaceans, and polychaete annelids.
Over 60 shallow-water and deep-sea species, such as Amphiura filiformis, have been found to emit blue and green light (bioluminescence), probably to ward off enemies.
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