Parasitism and Predation on Fossil and Extant Crinoids
January 21st (Tuesday), 6 p.m., Teton Co. Library Auditorium – Open to Public. Presentation: “Parasitism and Predation on Fossil and Extant Crinoids”, Presented by Forest Gahn, BYU Idaho
Crinoids are marine animals related to sea stars and urchins. The earliest known fossil crinoids are from the Early Ordovician of Idaho, and over 600 species are extant today. By the Late Ordovician, fossil evidence suggests crinoids were occasionally infested by parasites and attacked by predators. In today’s oceans, nearly every individual crinoid shows some evidence of parasitism or predation.
In the Paleozoic, crinoids were regularly infested by a group of extinct snails (platyceratids) that typically attached over their anal vents. These gastropods were likely stealing partially to undigested food directly from the guts of their hosts as evidenced by reduced growth rates among infested crinoids. Crinoids may have responded to such exploitation through the evolution of long anal “chimneys”, with the anus at the distal end. In turn, their infesting parasites responded by drilling through the body wall of their hosts.
Paleozoic crinoids also show evidence of attacks by predators in the form of regenerating arms and severe physical trauma to their calyxes (central body). Like their starfish cousins, crinoids have remarkable regenerative abilities. Generally, the frequency of individuals with regenerating arms positively correlates with predation intensity. Data on crinoid arm regeneration suggest predation on crinoids, presumably by fish, increased three-fold in the Devonian. Crinoids may have adapted to this increase in predation by developing defensive spines and thicker calyx plates. Many of the attacks on fossil and extant crinoids by fish are likely the result of collateral damage; fish are targeting the epibionts hiding amongst the crinoids’ arms rather than the crinoids themselves.
By the Mesozoic, sea urchins increasingly became a threat to stalked crinoids. Evidence from fossils, aquarium experiments, and modern marine environments demonstrates that sea urchins will aggressively consume whole crinoids. Crinoids likely adapted to this increased threat by developing autotomy planes in their arms and stalks, in addition to engaging in cryptic and swimming behaviors. Today, most reef-dwelling crinoids (feather stars) lose their stalks as juveniles, maximizing their mobility. In addition, all stalked crinoids (sea lilies) live below the photic zone where they are nearly invisible to predators that hunt by sight. These changes are best explained as strategies for predator avoidance.