Brittle star
The first known brittle stars date from the Early Ordovician period. These ancient creatures diverged from other echinoderms during that time. Study of past distribution and evolution has been hampered by a tendency for dead brittle stars to disarticulate and scatter. This behavior provides poor fossil evidence compared to other marine life. Until discoveries in the Agrio Formation of Neuquén Basin in the 2010s, no fossil brittle star was known in the Southern Hemisphere. No brittle star of Cretaceous age was known until recently either.
Silurian fossils from a minor mass extinction called the Mulde event show ancestors went through a bottleneck. A miniaturization caused by paedomorphosis led to structural simplification of their skeletal anatomy. These traits affected further evolution significantly. As they began to increase in size again, so did their complexity. The first large-sized modern brittle star originated in the Early Carboniferous.
Ophiuroids possess a skeleton of calcium carbonate in the form of calcite ossicles fused into armor plates. These plates are collectively known as the test and covered by epidermis consisting of smooth syncytium. In most species, joints between ossicles allow arms to bend sideways but not upwards. Basket stars differ with flexibility in all directions.
Vertebral ossicles support five long slender whip-like arms up to 60 cm in length. These vertebrae articulate via ball-and-socket joints controlled by muscles. They occur along the median of the arm in modern forms. Ophiuroida move horizontally while Euryalina species move vertically. Adults do not use tube feet for locomotion though very young stages use them as stilts or adhesive structures.
One arm presses ahead while other four act as two pairs of opposite levers. This thrusts the body in rapid jerks when disturbed. Lateral plates often have elongated spines projecting outward providing traction against substrate. Spines compose rigid borders to arm edges whereas euryalids transform them into downward-facing clubs or hooklets.
Gas exchange occurs through cilia-lined sacs called bursae opening between arm bases on disk underside. Typically ten bursae fit between stomach digestive pouches. Water flows through bursae by means of cilia or muscular contraction. Oxygen transports through hemal system series of sinuses and vessels distinct from water vascular system.
Most ophiuroids lack eyes or specialized sense organs yet possess sensitive nerve endings in epidermis. They can sense chemicals touch and presence absence of light. Tube feet may also sense light and odors especially at arm ends detecting light and retreating into crevices. These sensory capabilities allow survival without complex visual systems.
The madreporite usually locates within jaw plate rather than upper side animal as found in starfish. Some Euryalina species possess one madreporite per arm on aboral surface while others have none entirely. Suckers and ampullae absent from tube feet distinguish this group further.
Some brittle stars such as six-armed members family Ophiactidae exhibit fissiparity division through fission with disk splitting in half. Regrowth of lost disk part and arms yields animal with three large arms and three small arms during growth period. West Indian brittle star Ophiocomella ophiactoides frequently undergoes asexual reproduction by fission of disk with subsequent regeneration of arms.
In both summer and winter large numbers individuals with three long arms and three short arms found. Other individuals have half disk and only three arms. Study of age range population indicates little recruitment and fission primary means reproduction in this species. Fission starts with softening one side of disk initiating furrow deepening widening until extending across disk splitting animal in two.
New arms begin growing before fission complete minimizing time between possible successive divisions. Time period between successive divisions 89 days theoretically allowing each brittle star produce 15 new individuals during course year. Ophiuroids can readily regenerate lost arms or segments unless all arms lost. Amphiuridae can regenerate gut gonad fragments lost along with arms discarded arms not shown to regenerate ability.
Ophiuroids generally scavengers or detritivores moving small organic particles into mouth via tube feet. They may also prey on small crustaceans worms. Basket stars capable suspension feeding using mucus coating arms trap plankton bacteria. They extend one arm out use other four as anchors retrieving food rhythmically to mouth.
Brittle stars eat small suspended organisms if available. Large crowded areas see brittle stars eating suspended matter from prevailing seafloor currents. Many species family Ophiuridae carnivorous hunting epibenthic animals Antarctic Ophiosparte gigas active predator. Ophiura albida Forbes and Ophiura sarsii Lütken eat both infaunal prey carrion seafloor organic matter. Ophionereis reticulata omnivorous feeding algae polychaetes detritus.
Eurylina clings coral branches browse polyps. Ophiopsammus maculata consumes Nothofagus pollen New Zealand fjords trees hanging over water. Two best-known shallow species green brittle star Ophioderma brevispina found Massachusetts Brazil common European brittle star Ophiothrix fragilis.
Between 2000 and 3000 species of brittle stars currently known total number modern species may exceed 4000. This makes brittle stars most abundant group current echinoderms before sea stars. Around 270 genera distributed 16 families making relatively poorly diversified group structurally compared other echinoderms.
467 species belong sole family Amphiuridae frail brittle stars living buried sediment leaving arms stream capture plankton. 344 species exist family Ophiuridae. Six families live at least 2 m deep genera Ophiura Amphiophiura Ophiacantha range below 4 m. Shallow species live among sponges stones coral under sand mud arms protruding only.
Most widespread species long-armed brittle star Amphipholis squamata grayish bluish strongly luminescent species. Deep-water species tend live sea floor adhere coral urchins xenophyophores. Over 60 species known bioluminescent producing light green wavelengths few blue-emitting discovered shallow water deep-sea species produce light presumably deter predators.
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Common questions
When did the first known brittle stars appear in history?
The first known brittle stars date from the Early Ordovician period. These ancient creatures diverged from other echinoderms during that time.
Where were fossil brittle stars discovered before the 2010s discoveries in the Southern Hemisphere?
Until discoveries in the Agrio Formation of Neuquén Basin in the 2010s, no fossil brittle star was known in the Southern Hemisphere. No brittle star of Cretaceous age was known until recently either.
How do brittle stars move without using tube feet for locomotion?
Vertebral ossicles support five long slender whip-like arms up to 60 cm in length and articulate via ball-and-socket joints controlled by muscles. One arm presses ahead while other four act as two pairs of opposite levers to thrust the body in rapid jerks when disturbed.
What is the primary method of reproduction for West Indian brittle star Ophiocomella ophiactoides?
West Indian brittle star Ophiocomella ophiactoides frequently undergoes asexual reproduction by fission of disk with subsequent regeneration of arms. Fission starts with softening one side of disk initiating furrow deepening widening until extending across disk splitting animal in two.
Which family contains the most species among brittle stars according to current counts?
467 species belong sole family Amphiuridae frail brittle stars living buried sediment leaving arms stream capture plankton. 344 species exist family Ophiuridae.