INTRODUCTION

One of the better known traits of modern stalkless crinoids, the comatulids, is their ability to locomote. Not only can they use their arms to crawl along the bottom, but some are capable of swimming for short distances (Clark 1915, 1921; Breimer and Lane 1978; Meyer and Macurda 1977, 1980; Macurda and Meyer 1983; Meyer et al. 1984; Shaw and Fontaine 1990). It has been argued that comatulid mobility is "an essential ingredient of the success of the group" (Meyer and Macurda 1977, p. 76). Not only are comatulids by far the most diverse crinoids today, with more than 500 species recognized compared to fewer than 100 stalked species (Roux et al. 2002), but they occupy a broader range of environments than stalked crinoids, which today are restricted to depths greater than 100m (Oji 1986). It has been claimed that locomotion in comatulids is associated with seeking a more advantageous habitat with, for example, more favorable flow and suspended food-particle regimes or less competitive overlap with suspension feeding neighbors. However, perhaps most importantly, comatulid locomotion has been associated with escaping from predators (Meyer and Macurda 1977; Shaw and Fontaine 1990). Not surprisingly, textbook treatments often compare diversity, abundance, and habitat between extant stalked and stalkless crinoids in the context of mobility and predation. For example, Prothero (2004, p. 230) contrasted "the rare stalked crinoids...found in low diversity (only 25 living genera) in habitats that have few predators" with the "more common (130 living genera) stalkless crinoids...which are highly mobile."

The characterization of stalkless forms as highly mobile need not imply that all stalked crinoids are necessarily sessile, as there has been a long history of conjecture on the topic of stalked crinoid motility. For example, Buckland (1837, vol. 1, p. 436) considered fossil Pentacrinus "a locomotive animal" because of its mode of attachment and association with driftwood. Likewise, a free-living existence for extant isocrinids was proposed by Thomson (1873, p. 444), who thought their locomotory abilities were intermediate between comatulids, such as Antedon, and "permanently fixed crinoids." Perhaps the most extreme view with regard to stalked crinoid "free mode of life" was espoused by Kirk (1911, p. 30) who argued that "[T]he number of stalked genera ... which were and are truly eleutherozoic ... is quite large."

In these examples, the underlying argument used for inferring stalked crinoid locomotion was the mode of attachment—a crinoid lacking a stalk structure such as cementing holdfast that permanently fixed it to the substrate could be considered "free living." Of course, although the free-living state is a necessary condition for locomotion, it is not sufficient. Some mechanism by which the animal moves is necessary and many ingenious hypotheses have been postulated with crinoids moving either actively or passively as part of the plankton, pseudoplankton, nekton, or vagile benthos (Kirk 1911; Breimer and Lane 1978). These scenarios were supported by arguments from functional morphology and taphonomy. Nevertheless, because direct proof of active locomotion was lacking, the general opinion was that stalked crinoids were sessile, as evidenced by Lawrence's (1987) statement that the isocrinids, the most likely candidates for locomotion among extant stalked crinoids, are "free-living without the capacity to locomote" (p. 187, italics added).

In the late 1980s, however, direct evidence of isocrinid crawling became available through in situ observations (Messing et al. 1988) and laboratory flow-tank studies (Baumiller et al. 1991). These studies revealed that isocrinids could relocate by crawling with their arms, dragging the stalk behind them. Subsequently, Birenheide and Motokawa (1994) provided a detailed account of crawling in laboratory specimens of the isocrinid Metacrinus rotundus. These observations indicated that crawling was exceedingly slow, roughly 0.1 mm sec^-1. Also, the biological role of crawling was not obvious, even though laboratory observations identified "current velocity and current regime" (Baumiller et al. 1991, p. 739) as potentially important factors.

These direct observations proved once and for all that at least some stalked crinoids, the isocrinids, could crawl. Nevertheless, the mechanism and speed of crawling in situ and its biological role remained unanswered. In this contribution, we address these questions relying on data collected by submersibles. These data reveal that isocrinids can crawl much faster than previously recognized, and that one biological role of crawling might be similar to that of comatulids, that is, an escape strategy from predators. Furthermore, crawling may be a plesiomorphic feature of the isocrinid-comatulid clade and might extend to the base of the advanced cladids/articulates in the late Paleozoic.