CONTRACTILE CONNECTIVE TISSUE (CCT) IN THE STALK OF THE BOURGUETICRINID CRINOID, DEMOCRINUS: FUNCTIONAL, ECOLOGICAL, AND EVOLUTIONARY IMPLICATIONS

Among the most unusual and functionally important features of the Echinodermata is mutable collagenous tissue (MCT). Although collagen is the main component of animal connective tissues (e.g., ligaments, tendons), only echinoderms actively and rapidly (seconds to minutes) change its mechanical properties. Echinoderm MCT is involved in multiple functions including shedding of body parts (autotomy), often in response to predation, and various aspects of feeding, such as maintaining arm and stalk postures in crinoids. While the mutable nature of echinoderm collagen is well established, a more controversial suggestion is that collagenous tissues can actively contract (contractile connective tissues or CCT of Motokawa et al. 2006). Submersible observations of the stalked bourgueticrinid crinoid, Democrinus at 750 - 800ft off of Roatan, Honduras, support the latter claim. We observed an intentionally dislodged Democrinus raise itself off the substrate on its stalk in the apparent absence of external forces, such as lift, or internal stalk muscles (Grimmer et al. 1984).

Democrinus has 5 short arms, a stalk characterized by synarthrial columnal articulations (with a central fulcrum), and lives on sandy/muddy substrates where low current velocities predominate. The combination of low current and an open filter result in weak lift and drag on the crown, limiting the ability to passively reorient into an optimal feeding posture. The presence of CCT in the stalk may allow Democrinus to actively re-orient in response to changing current directions. Synarthrial (fulcrum-bearing) columnal articulations found in adult Democrinus also characterize juvenile isocrinids and comatulids; because passive reorientation of the crown would be particularly ineffective in these juveniles due to their small size, CCT is likely to be involved. Since ontogenetic, fossil, and molecular data suggest that bourgueticrinids evolved from comatulids through paedomorphosis, CCT may characterize all extant crinoids (Articulata), and perhaps ancient crinoid clades. Many of the latter are thought to have lacked muscular articulations of any type, and in these the functional role of CCT may have been especially important.