UNLIKELY ANIMALS THEY MAY SEEM TO OUR EYES, but feather stars are full-fledged members of the animal kingdom. Here’s a key feather star fact: They may seem to be fixed in place in their perches on coral heads or sponges or sea rods, but these crinoid creatures eat, reproduce and move like other animals.
As a bonus, they’re often beautiful and compelling.
TECHNICALLY, FEATHER STARS HAVE ARMS JUST LIKE THEIR STARFISH AND BRITTLESTAR COUSINS. At first glance, you might think they look like plants with feathery fronds. But those “fronds” are their arms, constructed with zillions of pinnules, the tiny side branches that give the feathery impression.
And, as for arms, there are lots of them, far more than the five typically associated with their fellow echinoderms. In fact, feather star arms come in multiples of five, typically in the 10 to 30 range but up to as many as 200. They earn their livings by waving them in the currents and collecting reams of zooplankton and detritus that pass by.
SEEING STARS: OTHER ECHINODERMS ON POSEIDON’S WEB – EXPLAIN THE REEF!
- Stomach-Everting, Arm-Regenerating Starfish Explained
- Brittlestars: Well-Armed, Laid-Back and Full of Tricks
- Sea Cucumbers – Superheroes of the Seas
- The Sea Urchin’s Tale
Ferny plant resemblance notwithstanding, they’re animals in Phylum Echinodermata. They live by ingesting other organisms, reproduce via eggs and sperm and are capable of independent movement. Some species can swim.
Members of Class Crinoidea, they go by the term crinoids, as well.
FEATHER STAR FACTS 1: SPECIES & DISTRIBUTION
Crinoids are sometimes referred to as “living fossils.” First appearing perhaps some 300 million years ago, they were the first echinoderms in the oceans, and have changed very little since then.
Today, it’s estimated that there are some 550 species found in tropical and warm-temperate waters worldwide. The most species are in the Indo-Pacific basin. Within the echinoderm phylum they’re located in Class Crinoidea, from the Latin word for “lily.” Within Crinoidea, they’re in Order Comatulida, so you may hear them referred to comatulids. But probably not. It’s a biologist’s word.
There is only one other order of crinoids, and they are lilies – “sea lilies.” They’re somewhat similar to feather stars. But while featherstars are capable of free movement, sea lilies live atop stalks attached to a substrate – with the capability of upping the stalk and relocating. Also, as a rule, the 80 species of sea lilies are restricted to deep waters.
FEATHER STAR FACTS 2: BODY ARCHITECTURE
Like other members of Phylum Echinodermata, crinoid bodies are covered with spiny skin overlying bony plates called ossicles. And, they feature vascular-water systems that operate multitudes of tube feet in the arms and pinnules. Unlike starfish, the tube feet aren’t used for locomotion. They and assist in gathering food.
Like brittlestars, a crinoid body starts with a small, central base from which the arms radiate. Crammed inside are a relatively short intestinal tract, a ring-shaped central nerve that manages movements in the arms and the locus of a water vascular system that actually makes the arms and tube feet work.
In contrast to brittlestars, their mouths and anuses are located on the upper side of their discs. On the undersides are claw-like “legs” called cirri that they use to grip whatever substrate they are perched on, coral or sea fan or sponge.
Crinoids are present in male and female sexes and reproduce by broadcast spawning. Larvae – which don’t look anything like the adults they will become – drift with the plankton before settling and morphing into little feather stars. They start life on a stalk attached to the bottom. Eventually, they break loose and become free-swimmers.
FEATHER STAR FACTS 3: ALL ARMS, ALL THE TIME
It’s the arms that make feather stars feather stars. Like all echinoderms, they’re symmetrical along five planes and start with five arms off the central body disc. But the arms each immediately fork, creating more long, jointed arms – supported by ossicles – in multiples of five. Each arm is…well…armed with pinnacles the extend along each side. These give the feathery appearance.
The pinnacles contain tube feet and are covered with sticky mucus that traps any plankton that comes their way. Often, they’re able to adjust their arms’ position to maximize exposure to the water flow, flat fan or parabola shape.
During off hours, when they rest, they’re likely to curl their arms in to look more like a bowl than a fern.
FEATHER STAR FACTS 4: PREY & PREDATORS
Crinoids gather prey by simply finding a good spot to hang out and waving their arms into the passing current. Some situate themselves in a sheltered spot, a crevice or behind a ledge, and just extend their arms out. Others seek out highly public spots with the most promising water flow.
As they wave their arms in the current, anything passing by that they can touch will stick. The effect is often compared to Velcro but I think a more apt description would be flypaper. It’s sticky mucus that does the job. Once caught, the morsel is passed down the arms to the mouth along ambulacral grooves, special channels extending along the arms to the base.
Prominent predators include fishes and sea urchins. But tiny creatures like crinoid shrimps, snails and worms may actually live on these feathery animals, peacefully cohabiting with them, taking advantage of the feather star habitat – and occasionally snacking on them. You have to look carefully. They’re well-camouflaged.
FEATHER STAR FACTS 5: LOCOMOTION
Many (not all) feather stars can swim, generally by coordinated movements of their arms. Their swimming prowess is limited to bursts, often about 30 seconds at a time and covering short distances. Often, swimming is a last resort to escape predators.
In general, they change location or position by using their cirri (their “legs”) to crawl, whether to new, current-friendly high perches or to better face into a changing water flow. The fastest recorded “walking” movement for a feather star is about 2.8 in ft/7 cm per second.
FEATHER STAR FACTS 6: BREAKING UP, GETTING BACK TOGETHER
The first thing to say is that crinoids are very fragile. Their arms fall off very easily. Divers who encounter them should be careful not to touch them (note: you shouldn’t be touching living things on the reef anyway).
Sometimes, a crinoid threatened by a predator will intentionally detach and drop an arm as a distraction while it makes an escape. More than just lying there, the arm continues to twitch as an enhanced diversion during the getaway.
Not to worry, like most other echinoderms, it will most likely will regrow the arm, usually in about a month’s time.
PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Reef Creature Identification, Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Feather Stars, Wild Singapore; Crinoids!, National Oceanic and Atmospheric Administration; What Habitat Do Feather Stars Live In?, Sciencing; In a World of Warming Waters, Feather Stars might be Winners, National Geographic; Marine Species: Feather Star, Scuba Diver Life; Feather Star, Encyclopedia Britannica; Crinoid, Wikipedia; Mesmerizing Video Shows Swimming Feather Star, National Geographic.