Brittle stars can split in half and choose any arm as the front
A brittle star has no head, so when it moves it simply picks one of its five arms to lead and rows with the other four. It can switch leaders whenever it turns. Some species go further, tearing their own bodies in half and regrowing the missing arms, again and again, as their main way to reproduce.
Brittle stars are relatives of starfish, with snake-like arms that give them their scientific name, from the Greek for serpent and tail. More than 2,000 species exist, and over 1,200 of them live deeper than 200 metres, some beyond 6,000. Others crowd tropical reefs, tucked under rocks or even inside other living animals. Unlike a starfish, whose organs extend into its arms, a brittle star keeps its gut and reproductive organs in the central disc. A single opening ringed by five toothed jaws serves as both mouth and exit.
The arms work like a spine. Chains of calcite plates, nicknamed vertebrae, meet at ball-and-socket joints and are moved by muscles, letting the animal wriggle fast across the seafloor in jerky strokes. Most species have no eyes, yet nerve endings in the skin sense touch, chemicals and light, helping them slip into crevices. More than 60 species glow, usually green, probably to put off predators. Lost arms grow back, so shedding one to escape an attacker is a routine trick.
Reproduction is remarkably varied. Most species simply release eggs and sperm into the water, but many brood young inside pouches in the disc, and at least one feeds its embryos through the pouch wall. In a few, a tiny male rides on the female, holding on with his mouth.
The West Indian brittle star is the champion splitter. One side of its disc softens, a groove forms and deepens, and the animal pulls apart, with new arms already sprouting before the division is complete. Populations are full of individuals with three long arms and three short ones. Divisions come roughly every 89 days, so in theory one animal could yield 15 new ones in a year.
Source: Brittle star
