Crinoidea, commonly known as sea lilies and feather stars, are marine animals often mistaken for plants due to their graceful, flower-like appearance. These creatures belong to the phylum Echinodermata, a group that also includes sea stars and sea urchins. With a lineage stretching back over 500 million years, crinoids represent one of the oldest living groups of echinoderms. Their delicate structures and varied forms contribute to the diversity of marine life.
Distinctive Body Structure
The central body of a crinoid, known as the calyx, is a cup-shaped structure that houses most of the animal’s internal organs. Extending from the calyx are multiple feathery arms, sometimes numbering ten or more, which can further subdivide into smaller branches called pinnules. These arms and pinnules are lined with specialized tube feet that help in capturing food particles from the water.
Some crinoids, referred to as sea lilies, have a stalk or stem that anchors them to the ocean floor. This stalk, composed of stacked calcite disks, elevates the calyx and arms into the water column. Feather stars lack a stalk in their adult form. Instead, feather stars possess cirri, small, hook-like appendages located on the underside of the calyx, which allow them to grasp onto surfaces. The entire skeletal system of crinoids is made of magnesium-rich calcite, providing both support and protection.
Life in the Ocean
Crinoids inhabit a wide range of marine environments, from shallow coral reefs to the abyssal plains of the deep sea, sometimes found at depths exceeding 9,000 meters. Their feeding strategy is primarily passive suspension feeding, where they filter plankton and small detritus particles from the surrounding water. They achieve this by extending their feathery arms to form a fan-like mesh, positioned perpendicular to the water current. The tube feet on their arms then capture food particles and transport them down grooves towards the mouth, which is located on the upper surface of the calyx.
While some crinoids, like sea lilies, remain sessile, many modern feather stars are mobile. Some species of feather stars can swim for short distances by rhythmically moving their arms up and down. Other mobile crinoids can crawl along the seafloor, using their cirri or arms to navigate and reposition themselves. This mobility allows them to seek out areas with richer food supplies and evade predators such as sea urchins and fish.
From Ancient Seas to Present Day
The fossil record reveals that crinoids have an extensive evolutionary history, with the oldest known fossils dating back to the Tremadocian Stage of the Ordovician Period, 485 to 477 million years ago. They were particularly abundant and diverse during the Paleozoic and Mesozoic eras, with some thick limestone beds almost entirely composed of their skeletal fragments. This long presence has earned them the informal title of “living fossils.”
Many ancient crinoid forms were predominantly stalked, resembling the sea lilies seen today. Over geological time, adaptations led to the emergence of unstalked varieties, known as feather stars. While stalked forms still exist, most of the 700 living crinoid species are unstalked feather stars. This diversification allowed crinoids to exploit a wider range of habitats and ecological niches.
Role in Marine Ecosystems
Crinoids play a role in marine ecosystems as filter feeders, contributing to the clarity and health of the water column. By straining plankton and organic detritus, they help to cycle nutrients through the marine environment.
Their presence also adds to the biodiversity of marine habitats. While not a primary food source for many marine organisms, some fish and sea urchins prey on crinoids. Their forms and functions contribute to the balance of ocean ecosystems.