Over 60 species of brittle stars glow in the dark, producing light in the green wavelengths to deter predators in the deepest, darkest corners of the ocean. These creatures, known scientifically as ophiuroids, are echinoderms closely related to sea stars, yet they possess a unique set of adaptations that set them apart from their more famous cousins. Unlike sea stars, which often rely on their tube feet for movement, brittle stars use their long, slender, whip-like arms to crawl across the sea floor with remarkable speed and agility. Their arms can reach lengths of up to 60 centimeters on the largest specimens, allowing them to navigate complex underwater terrains with ease. The Ophiuroidea class contains two large clades, Ophiurida and Euryalida, with over 2,000 species currently known to exist. More than 1,200 of these species thrive in deep waters, greater than 200 meters below the surface, where sunlight never reaches. Despite their abundance, brittle stars remain one of the least understood groups of marine life, hidden beneath rocks, within living organisms, and in the abyssal depths of the ocean.
Anatomy of a Serpent
The body of a brittle star is sharply divided into a central disk and five distinct arms, a design that maximizes both flexibility and protection. The central disk contains all the viscera, meaning the internal organs of digestion and reproduction never enter the arms, as they do in sea stars. This unique anatomical arrangement allows the arms to remain free for locomotion and feeding without the burden of internal organs. The mouth, located on the underside of the disk, is rimmed with five toothed jaws formed from skeletal plates, and the madreporite, a key component of the water vascular system, is usually found within one of these jaw plates rather than on the upper side of the animal. The ophiuroid coelom is strongly reduced, particularly when compared to other echinoderms, and the water vascular system generally has only one madreporite, though some species, such as certain Euryalina, have one per arm on the aboral surface. The nervous system consists of a main nerve ring that runs around the central disk, with radial nerves extending to the end of each limb, allowing for precise control of movement and sensory perception. Most ophiuroids lack eyes or other specialized sense organs, but they possess sensitive nerve endings in their epidermis that can detect chemicals, touch, and even the presence or absence of light. These sensory capabilities are especially pronounced at the ends of their arms, where they can detect light and quickly retreat into crevices to avoid danger.
Feeding and Survival
Brittle stars are primarily scavengers or detritivores, consuming small organic particles that are moved into the mouth by their tube feet. However, their dietary habits vary widely among species, with some being carnivorous predators and others acting as suspension feeders. Basket stars, for instance, use their mucus-coated arms to trap plankton and bacteria, extending one arm to sweep food rhythmically to the mouth while using the other four as anchors. In large, crowded areas, brittle stars feed on suspended matter from prevailing seafloor currents, while species like Ophiura Linnaeus hunt epibenthic animals, and the Antarctic Ophiosparte gigas is an active predator. Some species, such as Ophiopsammus maculata, consume Nothofagus pollen in the New Zealand fjords, taking advantage of the trees that hang over the water. The digestive system of a brittle star includes a short esophagus and a stomach cavity that occupies much of the dorsal half of the disk, with digestion occurring within 10 pouches or infolds of the stomach. These pouches, known as ceca, almost never extend into the arms, unlike in sea stars. Gas exchange and excretion occur through cilia-lined sacs called bursae, which open between the arm bases on the underside of the disk. Typically, ten bursae are found, each fitting between two stomach digestive pouches, and water flows through them by means of cilia or muscular contraction. The bursae are also the main organs of excretion, with phagocytic coelomocytes collecting waste products in the body cavity and then migrating to the bursae for expulsion from the body.
Movement and Defense
The reproductive strategies of brittle stars are as diverse as their habitats, with most species having separate sexes, though a few are hermaphroditic or protandric. The gonads are located in the disk and open into pouches between the arms, called genital bursae, and fertilization is external in most species, with the gametes being shed into the surrounding water through the bursal sacs. An exception is the Ophiocanopidae, in which the gonads do not open into bursae and are instead paired in a chain along the basal arm joints. Many species brood developing larvae in the bursae, effectively giving birth to live young, and a few, such as Amphipholus squamata, are truly viviparous, with the embryo receiving nourishment from the mother through the wall of the bursa. However, some species do not brood their young and instead have a free-swimming larval stage, referred to as an ophiopluteus, which has four pairs of rigid arms lined with cilia. These larvae develop directly into an adult, without the attachment stage found in most starfish larvae, and the number of species exhibiting ophiopluteus larvae is fewer than those that directly develop. In a few species, the female carries a dwarf male, clinging to it with the mouth. Brittle stars generally sexually mature in two to three years, become full grown in three to four years, and live up to five years, though members of Euryalina, such as Gorgonocephalus, may live much longer. Reproduction and Longevity
Some species, such as the six-armed members of the family Ophiactidae, exhibit fissiparity, or division through fission, with the disk splitting in half and regrowth of both the lost part of the disk and the arms occurring to yield an animal with three large arms and three small arms during the period of growth.
The first known brittle stars date from the Early Ordovician, but the study of past distribution and evolution of brittle stars has been hampered by the tendency of dead brittle stars to disarticulate and scatter, providing poor brittle star fossils. Until discoveries in the Agrio Formation of Neuquén Basin in the 2010s, no fossil brittle star was known in the Southern Hemisphere, nor was any brittle star of Cretaceous age known. Silurian fossils from a minor mass extinction called the Mulde event show that the ancestors of modern brittle stars went through a bottleneck, where a miniaturization caused by paedomorphosis led to structural simplification of their skeletal anatomy. These traits affected their further evolution, and as they began to increase in size again, so did their complexity. The first large-sized modern brittle star originated in the Early Carboniferous. The ophiuroids diverged in the Early Ordovician, and they can be found today in all of the major marine provinces, from the poles to the tropics. Basket stars are usually confined to the deeper parts of this range, while Ophiuroids are known even from abyssal depths greater than 6,000 meters. However, brittle stars are also common members of reef communities, where they hide under rocks and even within other living organisms. A few ophiuroid species can even tolerate brackish water, an ability otherwise almost unknown among echinoderms. The skeleton of a brittle star is made up of embedded ossicles, which are calcium carbonate plates fused to form armor plates known collectively as the test. Ecology and Diversity
The plates are covered by the epidermis, which consists of a smooth syncytium, and in most species, the joints between the ossicles and superficial plates allow the arm to bend to the side, but cannot bend upwards. However, in the basket stars, the arms are flexible in all directions.