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An in vivo comparative study of intersegmental flexibility in the ophiuroid arm.

Introduction

The echinoderm skeleton, like all skeletal systems, constrains and directs the actions of the animal. The skeletal ossicles can be connected rigidly, as in the echinoid test, or flexibly, as in the mobile appendages of crinoids, asteroids, and ophiuroids. Each type of connection imposes certain biomechanical limits on organismal form and function. Only a few of these systems, however, have been studied in detail (crinoids: Meyer, 1973; Macurda and Meyer, 1976; Donovan, 1988; Riddle el al., 1988; Holland el al., 1991; Baumiller et al., 1991; Baumiller and LaBarbera, 1993; echinoids: Telford, 1985; Ellers and Telford, 1991; Carnevali et al., 1991, 1993; asteroids: Eylers, 1976; O'Neill, 1989).

The behavior and ecology of ophiuroids are thought to be influenced by the mobility of their heavily calcified, multisegmented arms [ILLUSTRATION FOR FIGURE 1 OMITTED]. Externally, the ophiuroid arm is covered with a regular arrangement of overlapping plates or scales. Internally, the arm contains a series of disk- or rod-shaped vertebral ossicles that articulate with one another proximally and distally, forming a central "spine" that allows the arm to bend under muscular control (for review, see Byrne, 1994). Coordinated motion at numerous intervertebral joints gives the ophiuroid arm an exceptional range of flexibility, as seen in many locomotory and food-gathering behaviors (Fell, 1966; Magnus, 1967; Pentreath, 1970; Warner, 1982; for review, see Lawrence, 1987). Although the shape of the vertebral ossicle is widely presumed (see below) to affect intervertebral rotation, intersegmental flexibility, and thus the potential behavior and ecology of the animal, the range of vertebral morphologies is little studied and their effects remain largely untested.

Ophiuroids are the most ecologically diverse echinoderm class, encompassing predatory, scavenging, suspension-feeding and deposit-feeding lifestyles (for reviews, see Warner, 1982; Lawrence, 1987). This "high ecological adaptability" (Litvinova, 1989a) has been attributed to the evolutionary innovation of the vertebral ossicle system, which presumably allowed increased arm mobility (Spencer and Wright, 1966) and the subsequent "radiation" of vertebral ossicle morphologies within the established arm skeleton. Thus vertebral variations among both living and fossil ophiuroid taxa are often described as mechanical adaptations for different lifestyles or behaviors (Berry, 1934; Litvinova, 1989a,b).

Such variations have long been noted (Lyman, 1882; Bell, 1892; Matsumoto, 1915, 1917), and recent research has explored the taxonomic distribution and functional implications of different vertebral ossicle forms (Emson and Wilkie, 1982; Robbins, 1986; Bray, 1988; Byrne and Hendler, 1988; Litvinova, 1989a,b; Hendler and Miller, 1991; LeClair, 1994, 1995, 1996). These descriptive studies generally focus on the shape of the intervertebral articulation, which is often assumed to influence rotation between arm segments. Each vertebral ossicle has two articulation surfaces, one proximal and one distal. In basket stars (order Euryalae), these surfaces have an hourglass-shaped, streptospondylous articulation, which allows the arm to coil helically for maintaining posture and obtaining food (Macurda, 1976; Hendler and Miller, 1984; Dearborn et al., 1986; Emson et al., 1991). Other taxa (order Ophiurae) show a more complex zygospondylous articulation, in which each surface bears a bilaterally symmetrical set of projections and depressions [ILLUSTRATION FOR FIGURE 2 OMITTED]. The zygospondylous joint is assumed to be more limited in motion, rotating primarily in a lateral plane (Hyman, 1955; Byrne, 1994). The ophiurans are taxonomically and ecologically more diverse than the euryalans, a fact attributed to greater diversity in the configuration of the zygospondylous joint (e.g., Hendler and Miller, 1991).

Interspecific variations in ophiuran vertebral ossicle morphology have been linked to unusual forms of locomotion (Hendler and Miller, 1991) and to specific feeding strategies (Robbins, 1986). Emson and Wilkie (1982) interpreted certain vertebral features in the Amphiuridae, Ophiactidae, and Ophiothricidae as modifications for increased flexibility and postural control, and concluded that "arm structure may be correlated with habit, and also with habitat" (p. 17). Bray (1988) described differences between the vertebral ossicles of Ophiocoma echinata and Ophiothrix suensonii, and how these variations might influence arm movement and ecology. Hendler and Miller (1991) suggested that reduced articular surfaces in certain Ophiothrix might allow a greater range of motion between arm segments, facilitating open-water swimming behavior. The most detailed discussions of the functional morphology of the vertebral ossicle system are by Litvinova (1989a,b; 1994), who inferred mechanical consequences for a variety of vertebral ossicle types.

The ophiuroid literature thus contains many working hypotheses about the biomechanics of the intervertebral articulation, its influence on intersegmental flexibility, and how this flexibility may correlate with ecologically interesting behavior. These hypotheses generally assume (1) that the degree of bending between arm segments depends on the morphology of the intervertebral articulation and (2) that the behavior or ecology of the animal depends on the degree of bending between arm segments. Although structural differences are thus used to explain interspecific differences in intersegmental flexibility and ecology, no study has quantified the relative flexibility of different ophiuroid arm joints.

Here we report the first comparative assessment of ophiuroid intersegmental flexibility in vivo. The first goal of our investigation was to measure intersegmental rotations in live, unrestrained animals representing seven ophiurid families and two major vertebral ossicle morphologies. The second goal was to explore how any differences in observed flexibility might correlate with vertebral ossicle shape, taxonomic group, or dominant feeding mode of the animal. If vertebral ossicle morphology is the primary factor dictating intersegmental flexibility, species with very different vertebral morphologies are predicted to show measurable differences in intersegmental rotations. Conversely, without evidence for interspecific variation in arm performance at the level of the segment, it is unnecessary to invoke differences in vertebral morphology as a causal or adaptive explanation.

Materials and Methods

Experimental animals

Epifaunal ophiuroids were collected from backreef areas at Discovery Bay Marine Laboratories, Discovery Bay, Jamaica (March-April 1995; [ILLUSTRATION FOR FIGURE 3A OMITTED]). This area supports at least 15 species of ophiuroids, representing 7 families in the order Ophiurae (Sides, 1981). Animals were captured by hand during snorkel and scuba surveys at four sites previously described by Sides (1981) and at a fifth location immediately north of the laboratory's rock jetty [ILLUSTRATION FOR FIGURE 3B OMITTED]. Large individuals of Ophioderma cinereuro were abundant at the latter site, and Ophiothrix oerstedii was also common. Several individuals of Ophiothrix suensonii were collected by scuba divers at Red Buoy Reef in the eastern half of the bay (depth = 1820 m). Unlike the backreef taxa, which tend to be cryptic during the day, this species was usually conspicuous: individuals could be seen resting, with several arms exposed, in the mouths of large cup sponges.

Live ophiuroids were transferred immediately to running seawater tables for the duration of the field session (2-3 weeks). All animals appeared healthy and active and no arm autotomy was observed. To prevent the animals from becoming acclimated to light, the seawater tables were kept dark with black plastic curtains. No food was added to the tanks, but small particles and sediment were introduced by the constant flow of seawater. All ophiuroids were allowed to adjust for at least 24 h before they were used in any behavioral experiments. Table I lists the species collected, the number of individuals used, and the size of each individual as measured by disk diameter (in millimeters).

The taxa studied also represent a diversity of feeding strategies, including scavenging (Ophioderma appressum, O. cinereum), deposit-feeding (Ophiocoma pumila), obligate suspension-feeding (Ophiothrix oerstedii, O. suensonii), and facultative deposit-/suspension-feeding (Ophiocoma echinata, O. pumila). These behavioral categorizations are drawn from previous studies of the same species at the same locality (Sides, 1981, 1984; Sides and Woodley, 1985). Table II lists the vertebral ossicle morphologies and predominant feeding strategies of the species sampled. Two vertebral types, keeled and non-keeled, are represented. A detailed description of these types and an analysis of variation within types is given elsewhere (LeClair, 1994, 1995, 1996; see also Robbins, 1986; Smith et al., 1995). In contrast to nonkeeled vertebral ossicles [ILLUSTRATION FOR FIGURE 4A OMITTED], keeled vertebral ossicles have reduced articulating projections, a large notch on the proximal aboral surface, and an aboral, distal-pointing projection, or "keel" [ILLUSTRATION FOR FIGURE 4B OMITTED]. Each distal keel fits into the proximal notch of the adjacent vertebral ossicle, creating an overlapping, or "imbricated," skeletal series that non-keeled morphs lack [ILLUSTRATION FOR FIGURE 4C OMITTED]. Keeled taxa also feature "accessory aboral muscles" between the proximal depression and distal keel that are absent in non-keeled taxa ([ILLUSTRATION FOR FIGURE 4B OMITTED]; Robbins, 1986). This dichotomy in skeletal structure led us to predict that species with different vertebral ossicle morphologies would show measurable differences in intersegmental flexibility - specifically, that the reduced articular surfaces and additional muscles present in keeled species would allow greater lateral bending, as proposed by Hendler and Miller (1991).

Photographic apparatus

Intersegmental flexibility was recorded by photographing unrestrained ophiuroids moving in a shallow glass aquarium (30 X 23 X 8 cm). A front-surface mirror was mounted underneath the aquarium, angled 45 [degrees] to the tank bottom. A Nikon 6006 camera equipped with a macro lens was aimed at this mirror to provide a view perpendicular to the tank floor and the oral surface of the animal. Two Vivitar strobe lights were angled upwards beneath the tank to briefly (1/10,000 s) illuminate the animal from below during each exposure (1/125 s). All exposures were recorded on Kodak Tech Pan film (ASA 25). Images obtained by this method show excellent detail of the oral arm surface and good contrast at the boundaries between external plates [ILLUSTRATION FOR FIGURE 5 OMITTED].

Arm-flexing behaviors

All experiments were conducted in a darkened laboratory after dusk (1830-0200 h) when these species are most active (Sides, 1981). Ophiuroids generally show negative phototaxis, rapidly crawling away from any light source. This tendency was exploited to elicit two types of arm-flexing behaviors in the laboratory:

Coiling: One ophiuroid was placed in the center of the tank next to a small, slightly concave rock or coral chip. Most animals, with some prodding, quickly occupied this artificial crevice. Once inside, the ophiuroid was illuminated from above by a small 40-watt incandescent lamp. In response to the light, the animal would generally shelter its disk under the crevice and quickly retract its arms, forming multiple arm loops. When the animal had arranged itself into a stable configuration of coiled arms (after 1-5 min), the camera shutter was fired, triggering the strobes. The incandescent light was then extinguished, the covering rock removed, and the animal allowed to roam freely in darkness for 2-3 min before another trial. Ten trials were recorded for each individual.

Locomotion: One ophiuroid would be gently dropped upside down in the center of the tank, exposed to a faint illumination from one side. The animal would right itself and, using rhythmic arm strokes, walk away from the [TABULAR DATA FOR TABLE I OMITTED] light source. By observing the repeated excursions of the arms, the experimenter could fire the shutter at the approximate moment of the power stroke, during maximum curvature of the arms. After a pause of 2-3 min in darkness, the trial was repeated, for a total of 10 trials per individual. This technique is more prone to error than photography when the animal is stationary, because capturing the moment of maximum arm curvature is not guaranteed. Ophiuroid locomotion is relatively slow, however, so shutter-release errors should account for only small variations in the recorded position of the arm. Because all animals were photographed by the same experimenter (E.E.L.) under the same conditions, the tendency to err with this technique is not expected to bias comparisons among species.
Table II

Species list and ecological information

                          Vertebral
Species                   ossicle type           Feeding mode

OPHIOCOMIDAE

Ophiocoma echinata       non-keeled      deposit-/suspension-feeder
Ophiocoma pumila         non-keeled      deposit-/suspension-feeder
Ophiocoma wendtii        non-keeled      deposit-/suspension-feeder

OPHIODERMATIDAE

Ophioderma appressum     non-keeled      arm-loop scavenger
Ophioderma cinereum     non-keeled      arm-loop scavenger

OPHIOCHITONIDAE

Ophionereis reticulata   keeled          deposit-feeder

OPHIOTHRICIDAE

Ophiothrix oerstedii     keeled          suspension-feeder
Ophiotrhix suensonii     keeled          suspension-feeder




Digitizing technique

To quantify the degree of intersegmental rotation, more than 1300 photographic negatives were digitized using BioScan OPTIMAS software (ver. 3.1). The portions of the arm with the sharpest lateral curvature were visually selected from each frame. In the locomotion experiments, the region of sharpest curvature was selected from each arm participating in the locomotory "stride." These regions were generally in the middle third of the arm, where the segments were sharply bent to push against the substrate. In the coiling experiments, the region of sharpest curvature was selected from the proximal or middle portion of each arm that appeared tightly coiled. In this treatment, there were often several tight bends per arm, and most (but not necessarily all) of the arms were involved in this form of bending.

From each selected region, line segments were digitized along the proximal borders of 10 adjacent oral plates, and the angles between adjacent plates were calculated [ILLUSTRATION FOR FIGURE 5 OMITTED]. Oral plate borders were used as landmarks for comparing segment rotations because these boundaries appeared sharpest on the photographic negatives. Because the oral arm plates are not rigidly connected to the vertebral ossicle series, we use the term "intersegmental angles" to refer to these data. Although we did not visualize the vertebral series directly, in adjacent segments the oral intersegmental angle and the underlying intervertebral angle are probably quite similar, as both series of ossicles must follow essentially the same overall arm curvature. Even if oral arm plates are free to move independently of the vertebral series, the angle between adjacent oral plates cannot consistently be larger (or smaller) than the underlying intervertebral angle, else the oral and intervertebral series would quickly diverge. Any offset between these two series of ossicles is thus likely to be small, and unlikely to introduce any persistent bias in comparisons between species. Thus intersegmental angles, measured externally from adjacent oral plate boundaries, are probably good estimators of intervertebral rotations in vivo.

For each series of 10 oral plate boundaries, the digitizing program automatically calculated all intersegmental angles ([Theta]) and the maximum intersegmental angle achieved in that region ([[Theta].sub.max]). When the segments are aligned along the proximal-distal axis, [Theta] has a value of zero; it increases to a maximum as the segments rotate.

The metric of interest is [[Theta].sub.max], because we wish to know whether different ophiuroid species are differently limited in their ability to flex between segments. Measurement error was determined by repetitive digitizing and was estimated to be [+ or -]2 [degrees] (n = 900, SD = 2.1 [degrees]).

Results

Results from this photographic survey allow interspecific comparisons of lateral flexibility between ophiuroid arm segments. These data do not address other plausible planes of rotation, such as oral-aboral flexibility or torsion. Figure 6 compares maximum intersegmental angles pooled separately from all of the locomotion and coiling experiments. The largest intersegmental rotations were observed during coiling trials [ILLUSTRATION FOR FIGURE 6B OMITTED], where both the minimum and the maximum observations exceed those seen in unrestrained locomotion. This suggests that the direct illumination and limited refuge size used to elicit coiling prompts an increase in joint flexion. Because the observed rotations depend on the experimental conditions, data from the locomotion and coiling trials were analyzed separately.

Locomotion trials

Six species provided sufficient data for analysis of flexion during locomotion. Individuals within species showed similar means and variances for intersegmental angles, so all measurements from conspecifics were [TABULAR DATA FOR TABLE III OMITTED] pooled. The resulting histograms of maximum intersegmental angles are shown in Figure 7. Note that each species exhibits a wide range of maximum angles in lateral bending, and that among species there is considerable overlap in the range of maximum angles measured. There are small interspecific differences in the mean maximum angle, or central tendency of each range. Because each distribution closely approximates a normal curve (Komolgorov-Smirnov tests, each P [less than] 0.05), parametric statistics were used to describe and compare these distributions. For multiple post hoc tests of the data, we used the Bonferroni correction (Rice, 1988) to adjust the within-test P value required for statistical significance.

Table III lists basic statistics for the distribution of maximal angles in each species. In lateral flexion during locomotion, most species exhibit the same range of maximal intersegmental angles (10 [degrees]-15 [degrees]) between arm segments. Five of the six species show no significant differences in variance (multiple F tests, each P [greater than] 0.003); the sixth species, Ophiothrix suensonii, is significantly less variable (P [less than] 0.0001) than the others. Ophioderma appressum and O. cinereum have the lowest mean rotations (10 [degrees] - 11 [degrees]), Ophionereis reticulata and Ophiothrix suensonii are intermediate ([approximately]12 [degrees]), and Ophiocoma echinata and Ophiothrix oerstedii show the highest mean rotations ([approximately] 14 [degrees]). Figure 9A illustrates these differences by showing the species ranked according to mean maximum angle and grouped according to significant differences among means; each box encloses taxa whose means are not statistically different (multiple t tests, P [greater than] 0.003). Although these results show some significant interspecific differences in mean maximal rotation, the absolute differences in this measure are small, and the functional significance of such differences is uncertain.

Coiling trials

Seven ophiuroid species are represented in the analysis of flexion during coiling. As before, maximal angles within species were normally distributed, and measurements from conspecific individuals were pooled. Species histograms of maximum intersegmental angles during coiling are shown in Figure 8. Each species exhibits a wide range of maximal angles, and there is considerable interspecific overlap in the maximal angles measured.

Descriptive statistics for the coiling distributions are shown in Table III. The coiling experiments tend to elicit mean lateral rotations of about 14 [degrees] to 23 [degrees] between arm segments. Ophiothrix suensonii tends to have the least rotation between segments (13.8 [degrees]), whereas Ophionereis reticulata tends to have the greatest ([approximately]24 [degrees]). The congenerics Ophioderma appressum and O. cinereum have similar mean intersegmental angles, as do Ophiocoma echinata and O. pumila. The two species of Ophiothrix, however, exhibit different responses; O. oerstedii has an intermediate mean rotation, but the mean rotation of O. suensonii is rather low. Again, some interspecific differences in mean maximum angle are statistically significant, as shown by a ranking of species [ILLUSTRATION FOR FIGURE 9B OMITTED]. Note that this order of species, from the lowest mean maximum to the highest, differs considerably from the order observed in the locomotion experiments [ILLUSTRATION FOR FIGURE 9A OMITTED]. Thus the observed flexion depends not only on the species, but on the circumstances used to provoke arm flexion.

Interspecific differences in mean lateral flexibility do not clearly correspond to differences in vertebral ossicle morphology or feeding ecology. In the locomotion experiments, Ophiocoma echinata (a non-keeled, suspension/deposit feeder) and Ophiothrix oerstedii (a keeled, obligate suspension-feeder) both share the highest mean intersegmental rotations observed (about 14 [degrees]). Ophionereis reticulata (a keeled deposit-feeder) and Ophiocoma pumila (a non-keeled deposit/suspension-feeder) are the most flexible species in the coiling experiments; their mean rotations (23.1 [degrees] and 23.5 [degrees], respectively) are not significantly different. Thus species of different sizes, ecologies, and arm construction can achieve comparable intersegmental rotations in both of the activities measured here.

Discussion

Although there are significant variations in mean intersegmental angles among Discovery Bay ophiuroids, the absolute angular differences are quite small, and the degree of lateral flexibility is not obviously correlated with vertebral form or trophic mode. These data contrast with previous studies that have claimed mechanical significance for small interspecific variations in vertebral ossicle morphology. We tested this assumption by comparing species with non-keeled vertebral ossicles (Ophiocoma, Ophioderma), and species with keeled vertebral forms (Ophionereis, Ophiothrix). If these two vertebral types differ greatly in their capacity for lateral bending, we might expect to see species from one group consistently out-bend those of the other.

The observed differences among ophiuroid species are more complex, as shown in a summary of the angular measurements [ILLUSTRATION FOR FIGURE 10 OMITTED]. First, the overall ranges of maximal rotations exhibited by different species greatly overlap. Second, a high mean maximal rotation is not confined to species of a particular vertebral type or feeding strategy. Third, although the difference among means is significant for some species, the groups formed by this quantitative criterion do not clearly separate species according to either morphology or general ecology.

Species vary not only in their mean rotations, but also in the apparent extremes of rotation [ILLUSTRATION FOR FIGURE 10 OMITTED]. The largest single deflection between arm segments (32.4 [degrees]) was observed in Ophionereis reticulata during coiling (Table III). This amount of lateral rotation is only slightly greater than that observed in Ophiocoma pumila (29.8 [degrees]), Ophiothrix oerstedii (29.6 [degrees]), and Ophiocoma echinata (29.4 [degrees]) under the same conditions. Other species in this trial show much lower extremes, i.e., Ophiothrix suensonii (20.8 [degrees]).

Though one might interpret these extreme values as species-specific "limits" on intersegmental flexibility, several caveats apply. (For a recent treatment of extremes in ecology, see Gaines and Denny, 1993.) As in any behavioral study, we can measure only the "actual" or observed performance of adjacent segments, not the "potential" performance of which they are theoretically capable. Actual intersegmental rotations clearly depend on experimental circumstances; nearly all species show greater rotations in the coiling trials, and some increase flexion more than others. Ophiocoma pumila and Ophionereis reticulata seemed particularly sensitive to this treatment, vigorously retracting their arms into multiple loops and coils. The single species that is exposed during daylight (Ophiothrix suensonii) shows the least change between experimental treatments. These behavioral variations may confound comparisons of musculoskeletal performance, even when ophiuroids perform the "same" activities. For two species under the same conditions, actual bending may be close to the potential limit in one, but not in the other. Despite these limitations, our quantitative data suggest that there is nothing inherently limiting in either type of ophiuroid vertebral ossicle: within each treatment, species with keeled and non-keeled morphologies can achieve similar extremes of lateral rotation.

The lack of correspondence between vertebral form and maximal intersegmental angles in these species argues against the notion that vertebral morphology is a dominant factor in limiting intersegmental rotation. As others have suggested, there is likely to be a complex interaction between the intervertebral articular surfaces, the external plates, and associated soft tissues in the ophiuroid arm (for review, see Byrne, 1994). Biomechanical influences on intersegmental rotation may include the extensibility of the integument (Byrne and Hendler, 1988), the neurally mediated stiffness of the connective tissue (Wilkie, 1978, 1979, 1992), the arrangement of the intervertebral musculature (Emson and Wilkie, 1982), and the conformation of the external arm plates (Litvinova, 1994). For example, Litvinova (1994) recognizes 15 types of ophiuroid arm structure on the basis of the size and shape of the external plates and their overall pattern of contact. These plate configurations are divided into three groups: those that "do not restrict," those that "slightly restrict," and those that "strongly restrict" arm movements. This classification, like most functional discussions of the ophiuroid vertebral system, seems to presuppose that there are inherent, species-specific differences in the capacity for intersegmental rotation.

Although we found no outstanding differences in intersegmental flexibility among the ophiuroids in our sample, this study is limited to the measurement of lateral flexion. The vertebral variants tested here may somehow contribute to interspecific differences in the range of dorsoventral bending or torsion. Until such differences can be measured, however, we must be more skeptical of the notion that vertebral ossicle morphology dictates flexibility between segments, and less speculative about how some types of segments are more (or less) flexible than others. If interspecific variation in intersegmental flexibility is more apparent than real, the present functional diversity of the Ophiuroidea cannot be explained solely by the evolutionary accumulation of vertebral ossicle variations that modify joint biomechanics. The ecological breadth of ophiuroids may be due to many combined factors of arm construction, including segment size; segment number; the control of muscles and mutable collagenous tissues; and the behavioral coordination of accessory spines, scales, and tube feet. The tendency to link vertebral variation with higher-level ecology in these animals is also at odds with several vivid accounts of how species can be ecologically labile, facultatively assuming three or four feeding modes (Fontaine, 1965; Fell, 1966; Reese, 1966; Magnus, 1967; Pentreath, 1970; Woodley, 1975; Feder, 1981). An alternative hypothesis is that vertebral ossicle morphology is variable because it has little or no influence on intersegmental rotation, and does not affect how (or how well) the ophiuroid makes its living.

Diversity in the external aspect of the skeleton has been the foundation of ophiuroid taxonomy. Diversity in the internal skeleton remains poorly understood both in its morphological variance and in its functional effects. Although keeled and non-keeled ossicles are the major vertebral variants in these epifaunal species, other ophiuroid families show even more diverse arm morphologies and lifestyles. These include infaunal amphiurids (which have extremely long, slender arms), epifaunal hemieuryalids (which have highly flexible, coiled grasping arms with a streptospondylous articulation), and the enigmatic ophiomyxids (which have a reduced axial skeleton and large amounts of connective tissue; see Byrne and Hendler, 1988). These groups are open for the further testing of form-function relationships.

Although the ophiuroid axial skeleton is conspicuously variable in morphology and obviously functional in its anatomical role, at the mechanical level we know little about how morphological variations influence intersegmental flexibility. At the ecological level, we lack convincing arguments to link flexibility per se to the style or success of an ophiuroid's locomotory or food-gathering movements. Further work must establish what are the differences in intersegmental flexibility, if any, among ophiuroid species, and in what way these differences are meaningfully correlated with ecologically relevant behaviors. More detailed behavioral visualization, mechanical testing of individual arm segments, and physiological recording of muscle activity may further elucidate the range and response of the intersegmental junction. Future discussions of the vertebral ossicle system should also make clear whether physical capabilities or biological roles are implied when skeletal morphology is described in functional terms (Gans and Gasc, 1992). Although ophiuroids are "flexible in feeding habits" (Fontaine, 1965) as well as flexible in motion, these two flexibilities may be only distantly related.

Acknowledgments

E. LeClair thanks Elizabeth Sides (Trinity College, Ireland) for her directions on how to stalk the more elusive Jamaican ophiuroids. Michael Hale and the entire staff of Discovery Bay Marine Laboratories were essential hosts during fieldwork. Dive buddies Matt Mills and Jean-Luc Solandt got wet for the underwater echinoderm-hunting. Linda Collins and Walter Ambrosius (Univ. of Chicago Statistics Dept.) consulted on the analysis. Digitizing equipment used in this work was funded by NSF grant #EAR-84-177011 (to D. Jablonski). This research was conducted in partial fulfillment of the requirements of the doctoral program in Evolutionary Biology at the University of Chicago. Financial support for E. LeClair was provided by an American Fellowship from the American Association of University Women.

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Author:LeClair, Elizabeth E.; LaBarbera, Michael C.
Publication:The Biological Bulletin
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