A major reorganization of marine life occurred during the Mesozoic Marine Revolution (MMR), with diversifying predators hypothesized to have influenced the evolution of some prey species and forced others to respond by retreating to low predation refuges. Stalked crinoids, which in the Mesozoic inhabited a wide depth range but only occur today deeper than 100m, have been considered as an example of the latter. The absence of stalked crinoids from shallow water today implies that predation on crinoids varies inversely with depth. Numerous studies have relied on using the proportion of individuals with arm injuries in a population, often referred to as regeneration frequency (RF), as a measure of predation, with high RF interpreted as equivalent to intense predation. However, due to the ephemeral nature of crinoid injuries, RF is influenced by two processes: directly by the rate at which injuries are incurred (injuries/time) and inversely by the rate at which injuries regenerate, leading to the injuries’ “disappearance” (cm/time). In the context of the MMR hypothesis and the bathymetric predation gradient, reliance on RF without considering both processes can result in misleading interpretations regarding predation. We illustrate this by examining injuries and regeneration in Democrinus
an extant species of deep-water Bourgueticrinidae, a family of stalked crinoids known to have inhabited shallow-water habitats during the Mesozoic.
Thousands of D. cf. brevis anchor via terminal rootlets on gentle sediment slopes at depths of 200-400m off Isla Roatán, Honduras, and are accessible by submersible. RF was obtained from video transects, and rate of arm regeneration by identifying injured individuals and revisiting them over a 6-month period to measure arm regrowth. Our results indicate that, while the observed RF in D. cf. brevis is high (~10-20%), comparable to RFs in some shallow-water crinoids, this is due to low regeneration rate rather than high rate of incurred injuries: we estimate that on average, an individual D. cf. brevis suffers an arm injury only about once every ten years. Intensity of predation on this deep-water crinoid is much lower than on investigated shallow-water crinoids, consistent with the hypothesis that the deep sea represents a refuge from predation for stalked crinoids.