What is a Crinoid?

A crinoid is a marine animal of the class Crinoidea. There is only one extant subclass of crinoids, the Articulata, consisting of 540 described species, though other subclasses once existed but are now extinct. Crinoids, also called sea-lilies or feather-stars, are feathery or spiny invertebrates consisting of a number of arms around a central, top mouth. They may be fixed to a substrate or free-swimming, and some types of crinoid take both forms at different stages of the life cycle.

Crinoid species are quite diverse, though not nearly as plentiful as they once were. Extinct crinoid species are known from fossils from the Paleozoic era. Today, crinoids may live in very shallow waters or at depths of up to four miles (six kilometers). Crinoids also vary widely in appearance, though many are colorful and reminiscent of flowers.

A crinoid consists of three basic body parts: the calyx, the arms, and the stem. The calyx consists of the digestive and reproductive systems and is surrounded by the arms, or brachials. The brachials typically follow a five-fold symmetry and are covered with thin pinnules, in turn covered with cilia, to increase surface area and help move food toward the center mouth. The stem extends downward from the calyx, opposite from the brachials. Most modern crinoid species, about 85%, lack a stem.

Modern crinoids are the last remaining remnant not only of the class Crinoidea, but of a much more extensive population of filter-feeding echinoderms. Throughout the Paleozoic and Permian eras, Crinoidea had competition from such filter-feeders as blastoids, edrioasteroids, and others. A major extinction event occurring at the boundary between the Permian and Triassic eras, about 251 million years ago, saw the disappearance of about 95% of the world's marine life, including 98% of crinoid species and 100% of other filter-feeding echinoderms. Articulata developed somewhat later than other crinoid species, first appearing in the fossil record during the Triassic period.

Crinoids, often known as sea lilies or feather stars, are marine animals that belong to the phylum Echinodermata, which also includes sea stars and sea urchins. They have a flower-like appearance with a central body and radiating arms, and they are characterized by their unique feeding structure, which resembles a feathery crown used to capture food particles from the water.

Crinoids are among the oldest living marine organisms, with a fossil record dating back over 450 million years. According to paleontological research, they were much more abundant and diverse during the Paleozoic era, particularly in the Mississippian period, often referred to as the "Age of Crinoids".

Crinoids inhabit a wide range of oceanic environments, from shallow reef ecosystems to the deep sea. They are most commonly found attached to the substrate in shallow waters, but some species are free-swimming or can attach to floating debris in deeper parts of the ocean, living at depths of up to 9,000 meters.

Crinoids are suspension feeders, meaning they consume small particles of food suspended in the water, such as plankton, detritus, and other organic matter. They use their feathery arms to create a current that directs food towards their mouth. The sticky tube feet on their arms help to trap and transport food particles to the mouth.

Yes, crinoids are often referred to as living fossils due to their ancient lineage and the fact that they have survived multiple mass extinction events with little morphological change. Their basic body plan has remained relatively unchanged for hundreds of millions of years, providing valuable insights into the evolution of marine ecosystems.

Crinoids reproduce both sexually and asexually. In sexual reproduction, they release sperm and eggs into the water column, where fertilization occurs externally. Some species can also reproduce asexually through fragmentation, where a piece of the arm or stem breaks off and regenerates into a new individual, a process that contributes to their resilience and longevity.