Stalked crinoids are marine invertebrates, often referred to as “sea lilies” due to their resemblance to a flower. These animals are part of the phylum Echinodermata, which also includes starfish, sea urchins, brittle stars, and sea cucumbers. Stalked crinoids are characterized by their attachment to the seafloor via a stem, a trait that sets them apart from their more mobile relatives. They represent an ancient lineage, with a fossil record extending back hundreds of millions of years.
Anatomy and Appearance
A stalked crinoid’s body plan exhibits distinct sections, contributing to its lily-like form. The base of the organism features a holdfast, a root-like structure that anchors the crinoid to the substrate. Extending upwards from the holdfast is the stalk, or column, which is composed of numerous disc-shaped, calcareous plates called ossicles, held together by ligaments. This stalk serves to elevate the feeding portion of the animal into the water column.
At the top of the stalk sits the calyx, also known as the cup or theca, a cup-shaped central body containing the crinoid’s internal organs. From the calyx emerge five arms, which often branch multiple times, giving them a feathery appearance. These arms are lined with smaller, jointed appendages called pinnules. The entire organism displays pentaradial, or five-fold, symmetry, a common characteristic among echinoderms.
Habitat and Lifestyle
Stalked crinoids inhabit deep-sea environments, including abyssal plains and continental slopes, found at depths greater than 100 meters. Their lifestyle is sessile; they remain attached to the seafloor, utilizing their stalk and holdfast for stability. Some species can also anchor themselves to hard elevated substrates like deep-sea corals.
To obtain food, stalked crinoids are passive suspension feeders, relying on water currents to bring microscopic organisms and detritus within reach. Their feathery arms, lined with finger-like tube feet and cilia, are spread into a fan shape, held perpendicular to the current. As food particles drift past, the tube feet capture them with sticky mucus and then flick them into ambulacral grooves, where cilia transport the food towards the central mouth located on the upper surface of the calyx. This efficient filter-feeding mechanism allows them to thrive in deep-sea environments where food sources can be sparse.
Ancient Lineage and Modern Relatives
Stalked crinoids’ earliest forms appeared in the fossil record during the Ordovician Period, approximately 485.4 to 477.7 million years ago. They were particularly abundant and diverse during the Paleozoic Era, so much so that some ancient limestone beds are almost entirely composed of their skeletal fragments. Despite suffering near-complete extinction during the Permo-Triassic extinction event, modern stalked crinoids persist in today’s oceans.
The class Crinoidea encompasses both stalked crinoids and their more mobile counterparts, the feather stars. While both share a common ancestry and exhibit similar feathery arms and five-fold symmetry, adult feather stars lack the permanent stalk found in sea lilies, instead using hook-like cirri to cling to surfaces or move across the seafloor. Juvenile feather stars develop a stalk, but they shed it as they mature, allowing for a free-swimming or crawling lifestyle.