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Abstract.-
Deep-sea submersible observations made in the Bahamas revealed interactions between the stalked crinoid Endoxocrinus parrae and the cidaroid sea urchin Calocidaris micans. The in situ observations include occurrence of cidaroids within "meadows" of sea lilies, close proximity of cidaroids to several upended isocrinids, a cidaroid perched over the distal end of the stalk of an upended isocrinid, and disarticulated crinoid cirri and columnals directly underneath a specimen of C. micans. Guts of two C. micans collected from the crinoid meadow contain up to 70% crinoid material. Two of three large museum specimens of another cidaroid species, Histocidaris nuttingi, contain 14-99% crinoid material.
A comparison of cidaroid gut contents with local sediment revealed significant differences: sediment-derived material consists of single crinoid ossicles often abraded and lacking soft tissue, whereas crinoid columnals, cirrals, brachials, and pinnulars found in the cidaroids are often articulated, linked by soft tissue, and unabraded. Furthermore, articulated, multi-element fragments often show a mode of fracture characteristic of fresh crinoid material. Taken together, these data suggest that cidaroids prey on live isocrinids.
We argue that isocrinid stalk-shedding, whose purpose has remained a puzzle, and the recently documented rapid crawling of isocrinids are used in escaping benthic predators: isocrinids sacrifice and shed the distal stalk portion when attacked by cidaroids and crawl away, reducing the chance of a subsequent encounter. If such predation occurred throughout the Mesozoic and Cenozoic (possibly since the mid-Paleozoic), several evolutionary trends among crinoids might represent strategies to escape predation by slow-moving benthic predators.
Introduction
Biotic interactions, especially predation, have been argued as major driving forces of evolutionary innovation and morphological change among many taxa, and crinoids are no exception. (1) Trends of increasing plate thickness and spinosity among Paleozoic crinoids (Meyer and Ausich 1983; Signor and Brett 1984), (2) offshore displacement of late Mesozoic/Cenozoic stalked crinoids (Bottjer and Jablonski 1988), (3) origin of autotomy (shedding) planes in the stalk and arms of articulates (Oji and Okamoto 1994), (4) crawling and swimming abilities in comatulids (Meyer and Macurda 1977), (5) choice of semi-cryptic habits and nocturnal-diurnal behavior among comatulids (Meyer and Macurda 1977), and (6) planktonic and pseudoplanktonic life styles have all been attributed to predation pressure. Some of these trends, though specific to crinoids, have often been used...