Sutural loosening and skeletal flexibility during growth: determination of drop-like shapes in sea urchins

Carryover effects of long-term high water temperatures on fitness-related traits of the offspring of the sea urchin Strongylocentrotus intermedius

It is important to study the fitness of marine invertebrates in exposure to high water temperature. We studied whether the long-term high temperatures work on the fitness-related traits (righting behavior, covering behavior, foraging behavior, Aristotle's lantern reflex, body size) of S. intermedius whose parents (males and females) were exposed to ambient or high temperatures (~3 °C higher than the ambient) for a long period of time. The present study found that test diameter, wet body weight and test weight of offspring were not significantly different between temperature treatments, indicating that the parental sea urchins in exposure to high temperatures develop no carryover effects on the body size of the offspring sea urchins. We found no significant difference in foraging behavior, Aristotle's lantern reflex, lantern length and lantern weight of sea urchins after their parents had experienced long-term high temperatures. In addition, no significant change was found in the righting and covering behaviors of sea urchins whose parents were at long-term high temperatures. These results indicate that no significant lasting effects exhibited in the fitness-related behaviors and tissue size after their parents were exposed to high temperatures for a long time. The crushing force of test and test thickness showed no significant difference in the offspring of S. intermedius, no matter whether their parents were exposed to long-term high temperatures or not. The current results enrich our understanding that the parental sea urchin experiencing long-term high temperatures probably develop no carryover effects on the test of their offspring. We found that sea urchins whose parents were exposed to long-term elevated temperatures showed a significantly higher lantern length/test diameter and a significantly lower test height/test diameter in offspring sea urchins due to the thermal experience of their parents, showing the plasticity of lantern and test of offspring sea urchins in response to the thermal experience of their parents. Together with our previous investigation, the present study indicates that small sea urchins are less susceptible to the carryover effects of high temperatures in comparison with the developmental stages of embryos and larvae.

Constructional design of echinoid endoskeleton: main structural components and their potential for biomimetic applications

The endoskeleton of echinoderms (Deuterostomia: Echinodermata) is of mesodermal origin and consists of cells, organic components, as well as an inorganic mineral matrix. The echinoderm skeleton forms a complex lattice-system, which represents a model structure for naturally inspired engineering in terms of construction, mechanical behaviour and functional design. The sea urchin (Echinodermata: Echinoidea) endoskeleton consists of three main structural components: test, dental apparatus and accessory appendages. Although, all parts of the echinoid skeleton consist of the same basic material, their microstructure displays a great potential in meeting several mechanical needs according to a direct and clear structure-function relationship. This versatility has allowed the echinoid skeleton to adapt to different activities such as structural support, defence, feeding, burrowing and cleaning. Although, constrained by energy and resource efficiency, many of the structures found in the echinoid skeleton are optimized in terms of functional performances. Therefore, these structures can be used as role models for bio-inspired solutions in various industrial sectors such as building constructions, robotics, biomedical and material engineering. The present review provides an overview of previous mechanical and biomimetic research on the echinoid endoskeleton, describing the current state of knowledge and providing a reference for future studies.

Fitness benefits and costs of shelters to the sea urchin Glyptocidaris crenularis

Understanding the ecological role of shelters is greatly hampered by the scarcity of long-term laboratory experiments on the trade-off between fitness benefits and costs. This lack probably leads to an underestimation of the negative and/or positive effects on behaviors and growth of marine invertebrates in benthic ecosystems. Although our previous study revealed a significant effect on fitness-related traits of Glyptocidaris crenularis after 31 months, the present study extended it and investigated fitness benefits and/or costs of long-term sheltering on sea urchins to over 7 years. The present long-term study suggests that the previously reported reduction in feeding rate probably resulted from a reduction in reflexive feeding motions (Aristotle’s lantern reflex) rather than changes in foraging behavior. Actively seeking sheltering behavior was negatively impacted in individuals with continuous access to shelters. However, covering and righting behaviors did not differ in sheltered sea urchins, indicating that these behaviors are maintained to escape from adverse environments regardless of shelter. Body size of sea urchins in the group with shelters was significantly lower than those without shelters after 7 years. Weights of gonads and gut were not significantly different after 7 years despite previous observations of differences after ~2.5 years. The present study provides valuable information on the trade-off between fitness benefits and costs to sea urchins residing in shelters. However, the present study is only a laboratory investigation for one urchin species (G. crenularis) which does not consider the complexity of natural environments. Field studies should be carried out with G. crenularis and other sea urchin species, before a more universal conclusion can be drawn.

Carryover effects of short-term UV-B radiation on fitness related traits of the sea urchin Strongylocentrotus intermedius

Carryover effects of UV-B radiation are largely unknown in marine invertebrates, despite the ecological importance. For the first time, we investigated fitness related traits of the sea urchin Strongylocentrotus intermedius 8 weeks after short-term (1 h) UV-B radiations (0, 10 and 20 µW cm^-2). Short-term UV-B radiations had significant negative effects on survival, food consumption, test diameter, test height, test height:test diameter, gonad weight and crude protein of gonads of S. intermedius, despite of the absence of UV-B radiation for 8 weeks. Survival, food consumption and crude protein of gonads were significantly lowest in S. intermedius exposed to UV-B radiation at 20 µW cm^-2, highlighting that 20 µW cm^-2 is a dangerous UV-B radiation intensity for the fitness of sea urchins (at least S. intermedius). Gonads were significantly more sensitive to UV-B radiation than the gut. The present study increases our understanding of carryover effects of UV-B radiations on sea urchins and provides valuable information into marine environmental safety.

Energy expenditure associated with softening and stiffening of echinoderm connective tissue

Catch connective tissue of echinoderms at rest (in the standard state) either stiffens or softens in response to different kinds of stimulation. The energy consumption associated with the changes was estimated by measurement of the oxygen consumption rate (VO(2)) in three types of connective tissues-echinoid catch apparatus (CA), holothuroid body-wall dermis (HD), and asteroid body-wall dermis (AD). Mechanical stimulation by repetitive compression (10%-15% strain), which increased viscosity measured by creep tests, was employed for inducing the stiff state. Noradrenaline (10(-3) mol l(-1)), which decreased viscosity of CA, and static 80% compressive strain, which decreased viscosity of HD, were used to induce the soft state in the respective tissues. The VO(2) (in μl/g/h) values of the standard state were 2.91 (CA), 1.41 (HD), and 0.56 (AD), which were less than 1/4 of the VO(2) of the resting body-wall muscle of the starfish. The VO(2) of the stiff state was about 1.5 times greater than that of the standard state in all types of connective tissues. The VO(2) of the soft state was 3.4 (CA)-9.1 (HD) times greater than that of the standard state. The economical nature of catch connective tissue in posture maintenance is discussed.

Holotestoid: a computational model for testing hypotheses about echinoid skeleton form and growth

Regular echinoid skeletons, or tests, comprise plate patterns and overall shapes that have proven challenging to analyse solely on the basis of any one approach or process. Herein, we present a computational model, Holotestoid, that emulates four macrostructural ontogenic processes involved in test growth (plate growth, plate addition, plate interaction, and plate gapping). We devise a geometric representation for analysing tests and describe how we use analogies (bubble interactions and close-packing) to emulate the processes. In the computational model, the emulated processes are used to determine the plate size and plate shape and combined to simulate a growth zone. We simulated growth zones for Arbacia punctulata and for Strongylocentrotus franciscanus by changing the value for one parameter, the ambulacral column angle. We quantitatively compared morphological features for simulated forms to those for real specimens to test the computational model. Additionally, we simulated growth zones for A. punctulata, S. franciscanus, Eucidaris thouarsii, and Mellita quinquiesperforata by changing three parameters, ambulacral column angle, peristome radius to apical system radius ratio, and apical system radius to column length ratio. Holotestoid can be used to explain morphological disparity among echinoid tests.

Substratum cavities affect growth-plasticity, allometry, movement and feeding rates in the sea urchinStrongylocentrotus purpuratus

We assessed the influence of rock cavities, or pits, on the growth dynamics and behavior of the purple sea urchin, Strongylocentrotus purpuratus. In a paired-designed, laboratory experiment, sea urchins were assigned to sandstone blocks that were either ‘Flat’ or had a ‘Pit’ drilled into the center. At the start, both groups were approximately the same shape and size. In just 2 months, the shapes of the tests were significantly different between the two treatments, with the Pit urchins having an increased height:diameter profile. This result demonstrates the plastic nature of the sea urchin test and that, despite its apparent rigidity, it is capable of deforming during growth. In addition, the presence of pits modified behavior and food consumption as well as allometric growth of the test and Aristotle's lantern. Sea urchins on Pit sandstone blocks tended to stay in the cavities and not move about the flat areas, whereas individuals on Flat blocks changed position. Sea urchins in the Pit treatment consumed less food and had relatively larger demipyramids (the ‘jaw’ ossicle in Aristotle's lantern). These morphological and allometric changes occurred over a short time-period (8–20 weeks). We conclude that microhabitat is an important factor in controlling the behavior and growth dynamics of the bioeroding sea urchin S. purpuratus.

Waterborne cues from crabs induce thicker skeletons, smaller gonads and size-specific changes in growth rate in sea urchins

Indirect predator-induced effects on growth, morphology and reproduction have been extensively studied in marine invertebrates but usually without consideration of size-specific effects and not at all in post-metamorphic echinoids. Urchins are an unusually good system, in which, to study size effects because individuals of various ages within one species span four orders of magnitude in weight while retaining a nearly isometric morphology. We tracked growth of urchins, Strongylocentrotus droebachiensis (0.013-161.385 g), in the presence or absence of waterborne cues from predatory Jonah crabs, Cancer borealis. We ran experiments at ambient temperatures, once for 4 weeks during summer and again, with a second set of urchins, for 22 weeks over winter. We used a scaled, cube-root transformation of weight for measuring size more precisely and for equalizing variance across sizes. Growth rate of the smallest urchins (summer: <17 mm diameter; winter: <7 mm diameter) decreased by 40-42% in response to crab cues. In contrast, growth rate of larger urchins was unaffected in the summer and increased in response to crab scent by 7% in the winter. At the end of the 22-week experiment, additional gonadal and skeletal variables were measured. Cue-exposed urchins developed heavier, thicker skeletons and smaller gonads, but no differences in spine length or jaw size. The differences depended on urchin size, suggesting that there are size-specific shifts in gonadal and somatic investment in urchins.

An evolutionary scenario for the origin of pentaradial echinoderms-implications from the hydraulic principles of form determination

The early evolutionary history of echinoderms was reconstructed on the basis of structural-functional considerations and application of the quasi-engineering approach of 'Konstruktions-Morphologie'. According to the presented evolutionary scenario, a bilaterally symmetrical ancestor, such as an enteropneust-like organism, became gradually modified into a pentaradial echinoderm by passing through an intermediate pterobranch-like stage. The arms of a pentaradial echinoderm are identified as hydraulic outgrowths from the central coelomic cavity of the bilateral ancestor which developed due to a shortening of the body in length but widening in the diameter. The resulting pentaradial symmetry is a consequence of mechanical laws that dictate minimal contact surface areas among hydraulic pneumatic entities. These developed in the coelomic cavity (metacoel) in the bilaterally symmetrical ancestor, when from the already U-shaped mesentery with the intestinal tract two additional U-shaped bows developed directly or subsequently. During the subsequent development tensile chords of the mesentery 'sewed' the gut with the body wall first in three and secondly in five 'seams'. During the direct development five 'seams' between tensile chords and body wall developed straightly. These internal tensile chords subdivide the body coelom into five hydraulic subsystems ('pneus'), which eventually arrange in a pentaradial pattern. The body could then enlarge only between the tensile chords, which means that five hydraulic bulges developed. These bulges initially supported the tentacles and finally each of them enclosed the tentacle until only the feather-like appendages of the tentacles projected over the surface. The tentacles with their feathers were transformed into the ambulacral system, and the bulges become the arms. These morphological transformations were accompanied and partly determined by specific histological modifications, such as the development of mutable connective tissues and skeletal elements that fused to ossicles and provided shape stabilization in form of a calcareous skeleton in the body wall. The organism resulted was an ancestral echinoderm ('Ur-Echinoderm') with an enlarged metacoel, stabilized by hydraulic pressure working against a capsule of mutable connective tissue, skeletal elements and longitudinal muscles. In regard to these reconstructions, the body structure of echinoderms can be understood as a hydraulic skeletal capsule.

Mutable collagenous tissue: overview and biotechnological perspective

The mutable collagenous tissue (MCT) of echinoderms can undergo extreme changes in passive mechanical properties within a timescale of less than 1 s to a few minutes, involving a mechanism that is under direct neural control and coordinated with the activities of muscles. MCT occurs at a variety of anatomical locations in all echinoderm classes, is involved in every investigated echinoderm autotomy mechanism, and provides a mechanism for the energy-sparing maintenance of posture. It is therefore crucially important for the biology of extant echinoderms. This chapter summarises current knowledge of the physiology and organisation of MCT, with particular attention being given to its molecular organisation and the molecular mechanism of mutability. The biotechnological potential of MCT is discussed. It is argued that MCT could be a source of, or inspiration for, (1) new pharmacological agents and strategies designed to manipulate therapeutically connective tissue mechanical properties and (2) new composite materials with biomedical applications.

Distinct maturations of N-propeptide domains in fibrillar procollagen molecules involved in the formation of heterotypic fibrils in adult sea urchin collagenous tissues

We have characterized the primary structure of a new sea urchin fibrillar collagen, the 5alpha chain, including nine repeats of the sea urchin fibrillar module in its N-propeptide. By Western blot and immunofluorescence analyses, we have shown that 5alpha is co-localized in adult collagenous ligaments with the 2alpha fibrillar collagen chain and fibrosurfin, two other extracellular matrix proteins possessing sea urchin fibrillar modules. At the ultrastructural level, the 5alpha N-propeptide is detected at the surface of fibrils, suggesting the retention of this domain in mature collagen molecules. Biochemical characterization of pepsinized collagen molecules extracted from the test tissue (the endoskeleton) together with a matrix-assisted laser desorption ionization time-of-flight analysis allowed us to determine that 5alpha is a quantitatively minor fibrillar collagen chain in comparison with the 1alpha and 2alpha chains. Moreover, 5alpha forms heterotrimeric molecules with two 1alpha chains. Hence, as in vertebrates, sea urchin collagen fibrils are made up of quantitatively major and minor fibrillar molecules undergoing distinct maturation of their N-propeptide regions and participating in the formation of heterotypic fibrils.