The ocean is home to some pretty amazing creatures. For example, have you ever seen a feather star? You are probably more likely to see one if you potter around in your favourite rock pool at night. That’s when they come out of hiding for their favourite snacks drifting by in the gentle current. They walk on tiny tube feet and can even swim with undulating movements of the many ‘feathered’ arms.
Photo: OK Divers
The lineage of “feather stars” (members of the crinoid class) goes back 485 million years, give or take a million. Despite their resemblance to seed weed, they are not plants or algae but animals. They are echinoderms, relatives of sea stars, brittle stars, sea urchins and sea cucumbers. Juvenile feather stars sit atop a stalk but, once mature, they detach from it and crawl down to perch directly on the sea floor!
Feather stars tend to hide in plain sight among bright corals and anemones, firmly anchored to the sea floor by numerous small legs. Their slender, branching limbs billow like colourful fern fronds. Those “fronds” are their arms, constructed with zillions of pinnules – the tiny side branches that give the impression of feathers.
Feather stars come in different colours, depending on what they eat.
According to Prof. Charles Griffiths the animal is “turned upside down, so instead of its mouth and tube feet facing downwards, the mouth and tube feet face upwards into the water. It has actually developed another set of limbs on its back and those are used to hold onto rocks or other hard surfaces.”
Like other echinoderms, the body of the feather star displays five-fold radial symmetry. That means that its body has five identical sides – a bit like the Castle of Good Hope in Cape Town. The feathery arms radiate out from this small central body. Feather stars have lots of arms, far more than the five typically associated with their fellow echinoderms, as feather star arms come in multiples of five, typically in the 10 to 30 range but up to as many as 200. They wave in the currents, collecting reams of zooplankton and detritus that pass by.
Image showing anchoring legs – Photo: SCUBA News
So the animal faces upwards and holds its feathery arms with their tube feet out into the water to catch any bits of food floating by. The tube feet are rather slimy so that plankton and other bits of organic matter stick to them. The food is passed from one tube foot to another until it reaches the mouth at the centre of the body. Now here is another interesting fact. The mouth and the anus of a feather star are next to each other – which means that the digestive tract is U-shaped. I have not discovered why, but suspect that, in evolutionary terms, when the mouth migrated to the bottom of the animal (now on top of the upside down animal), there was no reason to move the anus as well – just bend the gut to follow the mouth.
The central body of a feather star is quite small with no space in it for the gonads. Instead, the gonads are displaced into additional arm-like structures that grow from the central body. Reproduction is through so-called “broadcast spawning”. That means that the eggs and sperm are released into the surrounding water where the sperm has to find the eggs by happy accident. To increase the chances of fertilization, all the feather stars cast their seed into the water at the same time in a synchronised spawning. The feather stars communicate the exact date and time with each other through hormones released into the water. Who decides when to release the signalling hormone is not clear to me, but it’s thought to be triggered by a sudden upwelling of water, with a corresponding change in water temperature. I suppose it’s the more mature stars that initiate the signal.
When the fertilised eggs hatch, larvae emerge that float in the currents and eventually float down to the sea floor to grow into fully-formed stars.
Many feather stars provide a home for small animals like shrimp and crabs. It is not a symbiotic relationship – the little squatters are actually freeloaders that help themselves to the food caught by the feather star! Note the small arm-like structures arising from the central body that carry the gonads.
Feather stars are famous for being able to regenerate their arms if they should be lost (most likely to a fish). Although regeneration is slow in cold water (warmer water can speed it up), the star can regenerate limbs repeatedly over its lifetime. No-one is quite sure how long feather stars live, but the central body is certainly not immortal.
What a remarkable species this is, with relatives dating back 485 million years. It has built up a few defences over that time which include being able to regenerate arms and deterring predators by having a body that has little nutritional content, is hard, and may taste bad. That same body however is strong enough to withstand the current that delivers snacks, but light enough to allow swimming either as an escape response or in search of more food. A truly fabulous animal.