Many people associate crinoids with ancient oceans, often believing these unique marine animals are long extinct. However, this perception overlooks their enduring presence in marine ecosystems around the world. Crinoids continue to thrive, showcasing a remarkable evolutionary journey. These intriguing creatures, often referred to as “sea lilies” or “feather stars,” have successfully adapted to diverse aquatic environments.
Crinoids: A Living Legacy
Crinoids are marine invertebrates belonging to the phylum Echinodermata, which also includes starfish, sea urchins, and sea cucumbers. Their fossil record extends back over 500 million years, making them one of Earth’s oldest animal groups. This history highlights their resilience, including survival through the Permo-Triassic extinction event.
Despite their ancient origins, approximately 600 to 700 species of crinoids are alive today. Their continued existence demonstrates successful adaptation to changing marine conditions. The common misconception of their extinction often arises from their plant-like appearance and fossilized stems in ancient rock formations.
The Two Forms of Modern Crinoids
Modern crinoids exist in two distinct forms: stalked sea lilies and unstalked feather stars. Sea lilies retain a stem that anchors them to the seafloor. This stalk can vary in length, with some deep-sea species having stalks up to a meter long, allowing them to elevate their feeding apparatus. These organisms inhabit deep-water environments, attaching to hard surfaces.
Feather stars lose their stalk during their larval development and become free-living as adults. They possess a cluster of root-like appendages called cirri, which they use to cling to substrates such as rocks, corals, or sponges. Unlike their deep-dwelling stalked relatives, feather stars are found in a broader range of habitats, from shallow coral reefs to deep ocean waters. Their ability to detach and reattach, or even move freely, offers them greater flexibility in seeking optimal feeding grounds.
Life Beneath the Waves: Ecology and Behavior
All crinoids are suspension feeders, meaning they capture food particles suspended in the water column. They achieve this using their feathery arms, which are lined with specialized structures called tube feet. These arms are spread wide to form a net-like structure, often positioned to maximize exposure to passing currents. Microscopic food particles, such as plankton and detritus, become ensnared in sticky mucus on the tube feet and are then transported along grooves in the arms towards the central mouth.
While sea lilies were historically considered sessile, modern observations have revealed that some stalked species can exhibit slow crawling movements across the seafloor. They can also wave their arms in response to irritants or to dislodge debris. Feather stars, however, are considerably more mobile; they can crawl using their cirri and, surprisingly, some species are capable of swimming for short distances by rhythmically flapping their arms. This mobility allows feather stars to relocate to areas with better food availability or to escape potential predators. Crinoids play a role in marine ecosystems by filtering water and contributing to nutrient cycling. They can also provide microhabitats for other small marine organisms.