Crinoids (Class Crinoidea) are the oldest extant echinoderm lineage and possess one of the most complete fossil records of any marine animal group, spanning almost half a billion years. Although they dominated many Paleozoic benthic ecosystems, only approximately 731 species survive today, occupying habitats from shallow reefs to the deepest ocean trenches. This chapter provides a comprehensive synthesis of current knowledge on crinoid biology and ecology, integrating evidence from modern and fossil taxa to examine the evolutionary processes that have shaped their remarkable persistence and diversity. We introduce general crinoid systematics, and review global biogeographic and bathymetric patterns, changes in community composition through geological time, and functional morphology, including mutable collagenous tissues, sensory biology, and their exceptional regenerative capacity. We then explore the ecological mechanisms underlying habitat selection, suspension feeding, locomotion, predator-prey interactions, symbioses, and the ecosystem functions of crinoid assemblages. Particular emphasis is placed on behavioural and physiological adaptations that enable crinoids to exploit diverse marine environments, from coral reefs to the deep sea, and on insights gained from comparisons between living species and the fossil record. Finally, we examine the growing range of anthropogenic pressures affecting crinoids, including climate change, pollution, disease, fisheries, and habitat degradation, and highlight major knowledge gaps that continue to limit effective conservation and management. Together, this chapter provides an up-to-date overview of crinoid biology, ecology, and evolution, demonstrating how this ancient lineage continues to inform our understanding of marine biodiversity, ecosystem functioning, and evolutionary resilience.
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