Crinoids are ancient marine animals that often resemble plants, leading to their common name, sea lilies. Their fossilized remains are abundant in rock formations around the world, suggesting a past dominance in prehistoric oceans. This extensive fossil record frequently leads to the misconception that these unique creatures are extinct.
Crinoids Are Still Alive
Crinoids are very much alive today, thriving as a vibrant component of marine ecosystems. They belong to the phylum Echinodermata, making them relatives of familiar creatures such as starfish, sea urchins, and sea cucumbers. Modern crinoids exist in two primary forms: the stalked “sea lilies” and the unstalked, free-moving “feather stars.” Approximately 600 to 700 recognized living species of crinoids are found across the globe’s oceans.
Where Modern Crinoids Live
Modern crinoids live in diverse marine environments, from shallow tropical reefs to the extreme depths of oceanic trenches. Sea lilies, which retain their stalk throughout adulthood, primarily inhabit deeper waters, typically found at depths of 100 meters (300 feet) or more, with some species observed exceeding 9,000 meters (30,000 feet).
Feather stars are more common in shallower waters, often in coral reef ecosystems. They are prevalent in regions like the Caribbean Sea and South Pacific, though found globally, including polar waters. These unstalked forms use root-like appendages, called cirri, to cling to substrates like rocks, sponges, and coral heads.
Diversity and Lifestyle of Modern Crinoids
Modern crinoids have diverse physical characteristics and behaviors. Their bodies typically feature a cup-like central structure called the calyx, from which numerous arms extend. Though they exhibit the basic echinoderm five-fold symmetry, their arms often branch extensively, sometimes numbering over 200 in certain feather star species.
All crinoids are passive suspension feeders, capturing food particles that drift in water currents. They spread their feathery arms, lined with microscopic tube feet. These tube feet, covered in sticky mucus, trap plankton and detritus, moving them by cilia along arm grooves towards the mouth.
Sea lilies remain attached to the seafloor, using a holdfast or root-like structures for anchorage. Feather stars are mobile and crawl across the ocean bottom using their arms. Some feather star species are even capable of swimming through coordinated arm movements to reposition themselves for better feeding opportunities or to evade predators. Many modern crinoids have striking coloration, appearing in shades of green, black, yellow, orange, and gold.
The Fossil Record Connection
The question about crinoid existence stems from their extensive fossil record. Crinoids were abundant in ancient seas, particularly during the Paleozoic Era, 250 to 540 million years ago. Their skeletal remains, especially the disc-like stem segments known as columnals, are so prevalent in some regions that they form thick limestone deposits, showing their past dominance.
Over 6,000 fossil species of crinoids have been identified, significantly outnumbering living species today. This abundance contributes to the perception of their extinction. While ancient forms were widespread in shallow, continental shelf environments, many modern sea lilies are relegated to deep-sea habitats, less accessible for direct observation. Feather stars, though more numerous, typically inhabit coral reefs and other marine environments where their remains are less likely to wash ashore, contributing to their inconspicuous nature compared to their fossilized ancestors.