The structural origins of brittle star arm kinematics: An integrated tomographic, additive manufacturing, and parametric modeling-based approach

Highlights

• A multi-scale tomography, 3D printing, and parametric modeling workflow is presented.

• 3D-printed skeletal models permit identification of ossicle mechanical interferences.

• Parametric modeling of arm motion reveals how ossicles enable or limit arm bending.

• We demonstrate how ossicle assemblage integration impacts global arm flexibility.

Abstract

Brittle stars are known for the high flexibility of their arms, a characteristic required for locomotion, food grasping, and for holding onto a great diversity of substrates. Their high agility is facilitated by the numerous discrete skeletal elements (ossicles) running through the center of each arm and embedded in the skin. While much has been learned regarding the structural diversity of these ossicles, which are important characters for taxonomic purposes, their impact on the arms’ range of motion, by contrast, is poorly understood. In the present study, we set out to investigate how ossicle morphology and skeletal organization affect the flexibility of brittle star arms.

Here, we present the results of an in-depth analysis of three brittle star species (Ophioplocus esmarki, Ophiopteris papillosa, and Ophiothrix spiculata), chosen for their different ranges of motion, as well as spine size and orientation. Using an integrated approach that combines behavioral studies with parametric modeling, additive manufacturing, micro-computed tomography, scanning electron microscopy, and finite element simulations, we present a high-throughput workflow that provides a fundamental understanding of 3D structure-kinematic relationships in brittle star skeletal systems.

Introduction

Brittle stars (phylum Echinodermata, class Ophiuroidea) are close relatives of sea stars, and are the most speciose group of modern echinoderms, encompassing more than 2000 species (Meglitsch and Schram, 1991, Stöhr et al., 2018, Stöhr et al., 2012). They are globally distributed in marine environments and are important contributors to both shallow and deep benthic communities (Stöhr et al., 2012), but are most abundant in shallow tropical and subtropical seas (Austin and Hadfield, 1980).

Brittle stars are known for their long, slender, and highly flexible arms (Barnes, 1987, Stöhr et al., 2012), as described by J. G. Wood in 1898, who so eloquently wrote, “The whole of the brittle stars are curious and restless beings. They can never remain in the same attitude for the tenth part of a second, but are constantly twisting their long arms, as if they were indeed the serpents with which Medusa’s head was surrounded.”

Due to their lack of cephalization, the terminology used to describe a brittle star’s body axis is defined by the location of the mouth, which is located in the middle of the central disk and faces the substrate, thus defining the “oral” and “aboral” (Hyman, 1940, Meglitsch and Schram, 1991) surfaces (or alternatively “ventral” and “dorsal” (Austin and Hadfield, 1980), respectively).

With a central disk ranging from 3 to 50 mm in diameter, their arm lengths vary from ca. two-times their disk diameter to twenty-times or more in some species (Stöhr et al., 2012). The five arms are symmetrically arranged in a pentaradial fashion around the central disk, and they can bend both in-plane (or “laterally” (LeClair and LaBarbara, 1997), perpendicular to the oral-aboral axis) and out-of-plane (or “vertically” (Barnes, 1987, LeClair and LaBarbara, 1997), parallel to the oral-aboral axis) (Fig. 1).

As a group, brittle stars are considered to be the most mobile echinoderms (Barnes, 1987). Arm movements play a major role in their unique mode of locomotion since, unlike sea stars, most brittle stars do not use their tube feet for this purpose (Lawrence, 1987, Romanes, 1893). The muscular limbs apply forces to the substrate, pushing or pulling the body forward across the sea floor. A decentralized nervous system (Cobb and Stubbs, 1981) coordinates repetitive sinusoidal in-plane movements, often in four of the arms, while one leading arm points in the direction of locomotion (Arshavskii et al., 1975, Glaser, 1907, Romanes, 1893, Smith, 1965). The gait patterns observed in most species show no arm preference or an indication of directional polarity, and as such, brittle stars can quickly change direction (Arshavskii et al., 1976, Astley, 2012, Barnes, 1987, Kano et al., 2017, Watanabe et al., 2012).

In addition to locomotion, brittle stars use their arms to hold onto substrates, coiling their flexible arms tightly around structurally complex objects such as kelp, sponges, or corals (Austin and Hadfield, 1980, Lawrence, 1987, Mosher and Watling, 2009), with spines located along the arms aiding in the anchoring process (Austin and Hadfield, 1980). In strong currents, brittle stars have also been observed to interlock arms between adjacent individuals to form stable aggregations (Warner and Woodley, 1975). Arm movements also play a key part in the process of burying themselves in mud or the crevices of rocks (Barnes, 1987, Stöhr et al., 2012). When hiding in small openings, for example, the animals coil their arms, resulting in the formation of robust anchoring postures (LeClair and LaBarbara, 1997).

The arms of brittle stars also operate as feeding structures (Lawrence, 1987, Warner and Woodley, 1975), with both the tube feet and spines playing important roles in the detection of chemical stimuli during feeding (Pentreath, 1970, Sloan and Campbell, 1982). While holding on to the substrate with one or two arms, suspension-feeding species such as the long-armed Ophiothrix spp., for example, extend the remaining arms above the disk and expose them to the prevailing currents, capturing small passing particles with their spines and tube feet, which are then transferred to the mouth (Austin and Hadfield, 1980, Barnes, 1987, Lawrence, 1987, Warner, 1982, Warner and Woodley, 1975). In species such as Ophiopteris papillosa, which exhibit more targeted feeding behaviors, the arm spines are used to break off small pieces from larger food items (Austin and Hadfield, 1980), while carnivorous species can actively grab large prey items directly through rapid arm coiling, and carry them to the mouth (Barnes, 1987, Hyman, 1940, Lawrence, 1987, Warner, 1982, Warner and Woodley, 1975).

Brittle stars and other echinoderms possess a dermal endoskeleton consisting of numerous small porous calcareous skeletal elements (ossicles), which exhibit a wide range of species-specific geometries (Stöhr et al., 2012). Their positions are maintained by connective tissues (Bray, 1985), and the interaction between the arms’ internal skeleton, the muscles, and soft connective tissues allows for a large number of degrees of freedom during arm bending (Byrne and Hendler, 1988, Lawrence, 1987, LeClair and LaBarbara, 1997). The extent of skeletal development is often associated with a brittle star’s overall level of activity, with delicate skeletal elements facilitating more agile motions, while heavily calcified forms exhibit more sluggish behaviors (Lawrence, 1987).

Historically, the structural diversity of brittle star ossicles have primarily been used for taxonomic purposes (LeClair, 1996, Lyman, 1882, O’Hara et al., 2014, Smith et al., 1995, Stöhr et al., 2012, Thuy and Stöhr, 2011), and the species-specific spiny ornamentations on the arms are thought to be closely tied to the structural complexity of the habitats in which each species is found (Austin and Hadfield, 1980, Hendler and Miller, 1984). Hard substrate dominated rocky reefs are commonly home to the smooth-armed and heavily armored brittle stars, whereas spiny brittle stars frequently live in more structurally complex or soft substrate-dominated communities (Fig. 2).

Brittle star studies from the past few decades have primarily focused on arm regeneration and the formation of the calcitic skeleton (Carnevali, 2006, Czarkwiani et al., 2016, Czarkwiani et al., 2013, Skold and Rosenberg, 1996, Zeleny, 1903), nervous system structural complexity (Cobb and Stubbs, 1981), and the relationships between neurophysiology, arm movement, and locomotion (Astley, 2012, Glaser, 1907, Kano et al., 2017, Moore and Cobb, 1986, Watanabe et al., 2012). From a historical perspective, while much has been learned regarding ossicle structural diversity (Lyman, 1882), their impact on the arms’ range of motion is still poorly understood (Thuy and Stöhr, 2011).

Inspired by their impressive flexibility, we set out to develop an integrated approach to identify how ossicle morphology and skeletal organization dictate the kinematics of brittle star arms, a study that builds on recent advances in this field (Clark et al., 2018). Here, we present the results of an in-depth analysis of three brittle star species, purposely chosen for their distinctive level of agility and extent of skeletal development. By combining behavioral studies with parametric modeling, additive manufacturing, micro-computed tomography, scanning electron microscopy, and finite element simulations, we present a high-throughput workflow that provides critical kinematic insights into structure-function relationships in brittle star skeletal systems.