Echinoderm Skeletons Research Articles

Proteins and Saccharides of the Sea Urchin Organic Matrix of Mineralization: Characterization and Localization in the Spine Skeleton

Properties of the echinoderm skeleton are under biological control, which is exerted in part by the organic matrix embedded in the mineralized part of the skeleton. This organic matrix consists of proteins and glycoproteins whose carbohydrate component is specifically involved in the control mechanisms. The saccharide moiety of the organic matrix of the spines of the echinoid Paracentrotus lividus was characterized using enzyme-linked lectin assays (ELLAs). O-glycoproteins, different types of complex N-glycoproteins, and terminal sialic acids were detected. Sialic acids are known to interact with Ca ions and could play an important role in the mineralization process. Some of the carbohydrate components detected by ELLAs as well as two organic matrix proteins (SM30 and SM50) were localized within different subregions of the spine skeleton using field-emission scanning electron microscopy. The mappings show that some of these components are not homogeneously distributed in the different skeletal subregions. For example, some N-glycoproteins were preferentially located in the putative amorphous subregion of the skeleton, whereas some O-glycoproteins were localized in the subregion where skeletal growth is inhibited. These results suggest that the biological control exerted on the skeletal properties can be partly modulated by local differences in the organic matrix composition.

The design of mineralised hard tissues for their mechanical functions.

Most hard tissues have as their primary purpose to be stiff. Outside the arthropods, mineralisation of a soft organic matrix is the almost universal method of producing high stiffness. However, stiffening brings with it the undesirable mechanical result of brittleness (lack of toughness). The mineralisation of some tissues, such as bone and dentine, can be modified rather easily, in evolutionary terms, to produce the optimum mix of stiffness with bending strength (which, except at the highest mineralisations, go together) on one hand and toughness on the other hand. However, in most other tissues, such as mollusc shell, echinoderm skeleton, brachiopod shell, barnacle shell and enamel, mineralisation is almost all-or-none, and no subtle gradations seem possible. In such cases, other features, such as architecture, must be modified to produce a useful skeleton. Not only the mechanical properties of the skeletal tissue, but its cost, mass and time taken for production will, biologists tend to assume, be balanced by natural selection to produce a satisfactory result. However, such complexity makes it difficult to be sure that we understand the extent to which mineralised skeletal materials are the best possible solution to the problems facing the animals and that we are not just telling 'Just-So' stories. Furthermore, there are some skeletal materials that do not seem to make much sense at the moment, although no doubt all will become clear eventually.

Echinoderm taphonomy, taphofacies, and Lagerstätten

Taphonomic study of echinoderms provides useful information on sedimentary conditions before, during, and after burial. Taphonomic studies of Recent echinoderms indicate that much skeletal disarticulation occurs within a few days after death. However, experiments also indicate that within a short period after death echinoderm carcasses remain rather resistant to disarticulation, and thus may be transported a considerable distance by currents; following periods of a few hours of decay, more delicate portions of echinoderm skeletons are readily disarticulated. Some skeletal modules (e.g., crinoid pluricolumnals) may resist disarticulation for periods of months in quiet- and or cool-water environments. Anoxia promotes intact preservation by excluding scavenging metazoans. Echinoderm ossicles may undergo minor abrasion and/or corrosion if left exposed, and less dense stereom corrodes much more rapidly than dense plates, such as echinoid spines. However, heavily abraded ossicles may indicate prefossilization and reworking.Various groups of echinoderms (e.g., pelmatozoans, asterozoans, echinoids) have differing propensities for degradation and, therefore, produce different arrays of preserved fossil material primarily depending upon the relative rates of burial, bottom-water oxygenation, and turbulence. Echinoderms may be divided into three groups based upon the relative ease of skeletal disarticulation. Type 1 echinoderms include weakly articulated forms (e.g., asteroids and ophiuroids) that rapidly disintegrate into individual ossicles. Type 2 includes those echinoderms whose bodies contain portions in which are more tightly sutured, as well as portions in which the ossicles are somewhat more delicately bound (e.g., crinoids, regular echinoids). Such echinoderms display more varied taphonomic grades from fully intact to mixtures of isolated ossicles and articulated modules. Type 3 comprises those echinoderms (e.g., irregular echinoids) in which major portions of the skeleton are so resistant to disarticulation that they may be broken across sutures rather than coming apart at plate boundaries.Comparative taphonomy of particular types of echinoderm skeletal remains leads to recognition of distinctive taphofacies that characterize particular depositional environments. Taphofacies include two types of characteristic modes of fossil preservation: event taphonomic signatures and background taphonomic signatures. Depending upon normal conditions of environmental energy and rates of sedimentation, the background condition of various types of echinoderms for a given facies may range from articulated, unabraded skeletal modules (in Types 2 and 3) to highly corroded and/or abraded ossicles. Conversely, the occurrence of fully intact fossil echinoderms provides unambiguous evidence of rapid and deep burial of benthic communities. Such well-preserved fossil assemblages can provide a wealth of information regarding the paleobiology of echinoderms, as well as the nature of the depositional events and burial histories.This paper presents a preliminary classification and characterization of background and event aspects of echinoderm taphofacies for carbonate- (9 taphofacies, including reefs and hardgrounds) and siliciclastic-dominated (5 taphofacies) environments. In each case, we recognize a spectrum of echinoderm taphofacies that coincides with a gradient of environments, ranging from nearshore, high energy shoreface through proximal and distal storm-influenced shelf, to deeper ramp and dysoxic basinal settings. Most taphofacies also feature particular styles of obrution (smothered bottom) Lagerstätten. These range from scattered lenses of articulated fossils in some high energy sandstone and grainstone facies to bedding planes of articulated, pyrite coated specimens in dark shales. We classify and discuss the genesis of these types of Lagerstätten and list typical examples. Finally, we present a simple model that integrates the occurrence of various echinoderm taphofacies with concepts of cyclic and sequence stratigraphy.

Paleozoic Mg calcite preserved: Implications for the Carboniferous ocean

Modern Mg calcite echinoderm skeletons are thought to lose Mg so rapidly that no pre-Pleistocene ossicles retain their original Mg concentrations; however, Carboniferous echinoderm skeletons from the Imo Formation of Arkansas and the Holder Formation of New Mexico are reported herein that are still composed of Mg calcite (7.8 to 14.5 mol% MgCO 3 ). Their Sr concentration is also high (620 to 3710 ppm). The pore system in these echinoderm skeletons is filled with ferroan calcite or siderite that has not grown in optical continuity with the skeleton. The discovery of Carboniferous Mg calcite confirms an interpretation already made from their diagenetic replacements. Marine Mg calcite has been associated with proposed icehouse conditions, and although the Carboniferous echinoderm skeletons are compatible with this as they formed during Gondwana glaciation, they are too few to be of significance. Also they are organic secretions and so can override secular mineral trends based on inorganic precipitates. The Carboniferous echinoderm skeletons, produced in tropical waters, have a concentration of Mg similar to values for modern echinoderms. The simplest explanation for this similarity is that both precipitated by similar processes from oceans of similar composition. This does not support predictions, based on mass-balance modeling, that the Mg/Ca ratio of the Carboniferous ocean was half that of today9s. The Sr content of Carboniferous echinoderms is less than that for modern echinoderms, leading to a prediction of 6 ppm Sr concentration for the Carboniferous ocean. Further examples of Mg calcite are required to substantiate these interpretations.

Abundance and arm loss of the infaunal brittlestar Ophiophragmus filograneus (Echinodermata: Ophiuroidea), with an experimental determination of regeneration rates in natural and planted seagrass beds

The regeneration rate of arms of the infaunal brittlestar Ophiophragmus filograneus was determined in natural and planted beds of the seagrass (Halodule wrighttii) in Tampa Bay, Florida, USA in 1990. Annually, brittlestar densities did not differ on average between the two beds, but they did show marked differences on individual dates. Rates of arm loss could not be determined for the two sites separately, but were estimated for the entire area. Between 52 and 94% of the brittlestars collected showed evidence of arm regeneration. Approximately one-third of those collected were regenerating one arm, with smaller proportions regenerating two or more arms. Regeneration rate was greater overall for individuals that had lost more tissue; however, the rate of soft-tissue regeneration per arm regenerated was significantly greater for those that had lost one arm compared to those that had lost three arms. The rate of soft-tissue regeneration did not differe between individuals from the two seagrass beds, and regeneration rate was lower than reported for other infaunal brittlestars. Skeletal regeneration per arm was similar in both arm-removal treatments. This result supports the hypothesis that echinoderm skeleton is energetically inexpensive and may help to lengthen arms during early regeneration. The rate of skeletal production was three-fold greater in individuals from the planted seagrass than in those from the natural. bed. As increased skeletal regeneration requires some additional energy, this indicates a greater energy expenditure and differential allocation by brittlestars regenerating in the planted seagrass areas.

ECHINODERM CALCITE: A MECHANICAL ANALYSIS FROM LARVAL SPICULES

The flexural stiffness (EI) was measured for simple and fenestrated spicules in echinoid larvae. A Young's modulus (E) of 36.3 x 109 N/m2 was estimated for these calcitic spicules by EI/I where I was calculated independently from measurements made by SEM. The flexural stiffness of fenestrated spicules is approximately three times greater than that of simple spicules. This increased flexural stiffness is due to structural and not material differences between the spicules. At the material level, this calcitic tissue behaves like a composite which will reduce stiffness but increase strength compared to inorganic calcite. At the structural level its porous nature increases its stiffness and buckling strength over that of a solid structure of similar weight. These characteristics should also increase the tensile strength of this skeletal component and increase its usefulness as a strong, stiff element in most echinoderm skeletons.

Biocompatibility of echinoderm skeleton with mammalian cells in vitro: preliminary evidence.

The physical and chemical properties of echinoderm skeleton are reviewed. A method is described for preparing cell-free, sterile echinoderm skeletal plates (ossicles) which were used as porous substrates for cell cultures. Ossicles of the starfish Pisaster ochraceus were evaluated as substrates for the culture of three mammalian cell lines. Each line grew vigorously on ossicles, and fibroblasts quickly infiltrated their porous microstructure. Echinoderm skeletal plates provide a simple, convenient alternative to coverslips and porous membranes for SEM or correlated SEM/TEM studies of cell behavior. More importantly, the preliminary evidence for biocompatibility presented suggests that native echinoderm skeleton has potential use as a biomaterial and, because of its microstructure and relative solubility; deserves evaluation as a kind of biodegradable ceramic.

Determination of the rate constants of effective collisions of erythrocyte aggregates in the blood

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A comparison of the strength of echinoderm spines and mollusc shells

The echinoderm skeleton is interesting because its material is so different from that of other invertebrates. It is full of interconnecting cavities, and each piece of skeleton seems to be made of a single crystal of rather impure calcite (Nichols & Currey, 1968). An obvious question for a biologist is whether echinoderm skeleton is in some way better mechanically than other, more conventional, skeletal materials.

Unusual strength properties of echinoderm calcite related to structure

Echinoderm skeletons are composed of high-magnesium calcite having a fenestrate structure. Each skeletal ossicle appears to be a single crystal of calcium carbonate rather than a polycrystalline aggregate as, for example, are mollusc shells. Measurements of crushing strength for spines of three species of sea urchin demonstrate that the strength to weight ratio of these spines is equivalent to or greater than that of mollusc skeletons and most calcareous rocks. Echinoderms utilize calcium carbonate with unusually high efficiency, fulfilling the strength and volumetric requirements of a suitable skeleton with a minimum amount of material. Comparative dimensional analysis of structural and mechanical properties of these skeletal materials may provide further insight into the evolution of echinoderms.

Anelasticity in Bone and Echinoderm Skeletons

ABSTRACT Bones and echinoderm skeletons have been tested for anelasticity. Bone shows considerable, largely anelastic strains, continuing for at least 54 days, by which time the extra deflexion may be 130% of the original deflexion. The rate at which the anelastic deflexion appears is proportional to the applied stress. Between 2 and 50° C. there is a 300% increase in the rate at which anelastic deflexions in bones appear. Drying seems not to affect irreversibly the anelastic properties of bone. Echinoderm spines show no anelastic deflexion on prolonged loading. The mechanical behaviour of the skeletal materials of the two phyla investigated are such as would be expected from their constitution. The biological significance of these findings is discussed. It is concluded that anelasticity will, except perhaps in a few special cases, be of only marginal importance in vertebrates in life. The adaptive significance of the difference between bone and the skeletal tissue of echinoderms is as yet unknown. It does not lie in their anelastic properties.

Crystallography of Echinoid Calcite

Each element of an echinoderm skeleton is a single crystal of calcite. The universal stage method is used to determine orientations of crystals in coronal plates of fossil and Recent regular echinoids. Stereographic diagrams are presented which relate crystallographic orientation to the symmetry of the echinoid skeleton. Most species exhibit one of two preferred orientations and show little variation within a single individual. In one of the preferred orientations, c-axes are perpendicular to the test surface; in the other, they are tangent to the test surface and are aligned longitudinally. In the present sample, crystallographic orientation shows no variation within single genera, and most echinoid families studied have but one orientation. The two preferred orientations appear to have evolved independently several times during echinoid history. Preferred orientation, as an adaptation to the environment, may be related to the organisms' sensitivity to light.