Blue Zoo: Crinoids
Featuring one amazing marine animal per week.
The crinoids - which include sea lilies and feather stars - are distinctive and ancient ocean animals that come in all colors imaginable. They have long, feathery arms that they extend into the water column and they use their tiny, sticky tube feet to pick up particles for food.
Feather stars (left) are mobile and tend to live in reef habitats; when they settle on a solid surface, they can hold on using appendages called cirri. Sea lilies (right) tend to live in deep sea habitats and spend most of their lives fixed to the seafloor with a stalk.
Photos by Andrew Vasenin via Wikimedia Commons and Neptune Canada via Flickr (Creative Commons License).
Feather stars are most commonly found in tropical coral reef habitats. They have no stalk and can move around freely, and can even swim by undulating their many arms (watch here).
Sea lilies, on the other hand, often live in deeper water, and have a long stalk that attaches them to the seafloor – although if they need to escape from a predator such as a sea urchin, they can break off the stalk and crawl away.
Evidence from fossils such as these shows that crinoid species have existed for hundreds of millions of years.
Photo by Ghedoghedo via Wikimedia Commons, Creative Commons License.
The class Crinoidea belongs to the phylum Echinodermata, which means they’re related to sea stars, cucumbers, and urchins. The crinoids are the oldest class of echinoderms and fossils of extinct species show that they existed 570 million years ago.
Like other echinoderms, crinoids have pentaradial symmetry – that is symmetry that extends in five parts from a central point, like a pie cut into five identical slices. They may have just five arms or they may have many more arms than that, but they often grow in multiples of five. Also like other echinoderms, if a crinoid loses an arm, it can grow it back.
To feed, crinoids collect particles on tiny sticky tube feet. The particles are passed from the tube feet to the small branches or pennules, and then to the main arm or tegmen. They then move along a groove in the tegmen all the way to the mouth in the center.
Photos by Sunphol Sorakul and Elias Levy via Flickr, Creative Commons License.
The "feather" on a crinoid consists of the tegmen (the main branch of the arm) and the pinnules (the smaller fringing branches). On each pinnule are tiny tube feet. These sticky tube feet catch small particles like plankton from the water. The particles are then moved along the pinnule to the tegmen, and eventually they make it all the way to the mouth, which is on the top of the center of the animal.
Feather stars sometimes stop feeding and roll up their arms, for example to hide from predators. Photo by Nhobgood via Wikimedia Commons, Creative Commons License.
Some species keep their arms curled up during the day. Then at night, when predators are less likely to see them, they unfurl their arms to feed.
Feather stars commonly serve as a safe home for many other animals. Often, tiny critters such as shrimp, can be found living among the arms of a feather star – perfectly camouflaged.
Certain shrimp species specialize in living on crinoids and are able to camouflage perfectly with their hosts. Clockwise from top left, photos by Lakshmi Sawitri, Lakshmi Sawitri, and prilfish (via Flickr, Creative Commons License) and courtesy of MacGillivray Freeman Films.
Primary source: Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7th ed. Brooks Cole/Thomson, Belmont, CA.
Photo by Alexander Vasenin via Wikimedia Commons, Creative Commons License.
Photo by Dr. Dwayne Meadows, NOAA/NMFS/OPR via Wikimedia Commons, Public Domain.