Absence of postzygotic isolating mechanisms:evidence from experimental hybridization between two species of tropical sea urchins

Two reef margin species of tropical sea urchins,Echinometra sp.C(Ec) and Echinometra oblonga(Eo),occur sympatrically on Okinawa intertidal reefs in southern Japan.Hybridization between these species was examined through a series of cross-fertilization experiments.At limited sperm concentrations,where conspecific crosses reached near 100% fertilization,both heterospecific crosses showed high fertilization rates(81%-85%).The compatibility of the gametes demonstrated that if gamete recognition molecules are involved in fertilization of these species,they are not strongly species-specific.We found that conspecific crosses reached peak fertilization levels much faster than did heterospecific crosses,indicating the presence of a prezygotic barrier to hybridization in the gametes.Larval survival,metamorphosis,and juvenile and adult survival of hybrid groups were nearly identical to those of their parent species.Hybrids from crosses in both directions developed normally through larval stages to sexually mature adults,indicating that neither gametic incompatibility nor hybrid inviability appeared to maintain reproductive isolation between these species.In adults,Ec×Ec crosses gave the highest live weight,followed by Eo(ova)×Ec(sperm),Ec(ova)×Eo(sperm),and Eo×Eo.Other growth performance measures(viz.,test size,Aristotle's lantern length,and gonad index) of hybrid groups and their parental siblings showed the same trends.The phenotypic color patterns of the hybrids were closer to the maternal coloration,whereas spine length,tube-foot and gonad spicule characteristics,pedicellaria valve length,and gamete sizes showed intermediate features.Adult F 1 hybrids were completely fertile and displayed high fertilization success in F 1 backcrosses,eliminating the likelihood that hybrid sterility is a postzygotic mechanism of reproductive isolation.Conversely,intensive surveys failed to find hybrid individuals in the field,suggesting the lack or rarity of natural hybridization.This strongly suggests that reproductive isolation is achieved by prezygotic isolating mechanism(s).Of these mechanisms,habitat segregation,gamete competition,differences in spawning times,gametic incompatibility or other genetic and non-genetic factors appear to be important in maintaining the integrity of these species.

Sea Urchin Spines as a Model-System for Permeable,Light-Weight Ceramics with Graceful Failure Behavior. Part I. Mechanical Behavior of Sea Urchin Spines under Compression

The spines of pencil and lance urchins Heterocentrotus mammillatus and Phyllacanthus imperialis were studied as a model of light-weight material with high impact resistance.The complex and variable skeleton construction ('stereom') of body and spines of sea urchins consists of highly porous Mg-bearing calcium carbonate.This basically brittle material with pronounced single-crystal cleavage does not fracture by spontaneous catastrophic device failure but by graceful failure over the range of tens of millimeter of bulk compression instead.This was observed in bulk compression tests and blunt indentation experiments on regular,infiltrated and latex coated sea urchin spine segments.Microstructural characterization was carried out using X-ray computer tomography,optical and scanning electron microscopy.The behavior is interpreted to result from the hierarchic structure of sea urchin spines from the rnacroscale down to the nanoscale.Guidelines derived from this study see ceramics with layered porosity as a possible biomimetic construction for appropriate applications.

Sea Urchin Spines as a Model-System for Permeable,Light-Weight Ceramics with Graceful Failure Behavior.Part Ⅱ.Mechanical Behavior of Sea Urchin Spine Inspired Porous Aluminum Oxide Ceramics under Compression

Sea urchin spines were chosen as a model system for biomimetic ceramics obtained using starch-blended slip casting. Porous alumina ceramics with cap-shaped layers with different alternating porosities were found to have superior fracture behavior under bulk compression compared to ceramics with uniform porosity.They fail in a cascading manner,absorbing high amounts of energy during extended compression paths.The porosity variation in an otherwise single phase material mimicks the architectural microstructure design of sea urchin spines of Heterocentrotus mammillatus,which are promising model materials for impact protection.

Energy Absorption in Functionally Graded Concrete Bioinspired by Sea Urchin Spines

Functionally Graded Concrete （FGC） is fabricated at the Institute for Lightweight Structures and Conceptual Design （ILEK） by using a layer-by-layer technique with two different technological procedures： casting and dry spraying. Functional gradations are developed from two reference mixtures with diametrically opposed characteristics in terms of density, porosity, compression strength and elasticity modulus. In this study the first mixture consists of Normal Density Concrete （NDC）, with density about 2160 kg·m^-3 while the second mixture helps to obtain a very lightweight concrete, with density about 830 kg·m^-3. The FGC specimens have layers with different alternating porosities and provide superior deformability capacity under bulk compression compared to NDC specimens. In addition, the FGC specimens experienced a graceful failure behaviour, absorbing high amounts of energy during extended compression paths. The porosity variation inside the layout of tested specimens is inspired by the internal structure of sea urchin spines of heterocentrotus mammillatus, a promising role model for energy absorption in biomimetic engineering.

Higher dietary protein increases growth performance,anti-oxidative enzymes activity and transcription of heat shock protein 70 in the juvenile sea urchin(Strongylocentrotus intermedius)under a heat stress

This study was conducted to investigate the effect of dietary protein concentration(12%,18%,24%,30% and 36%)on the growth performance,activity of anti-oxidative enzymes and heat shock protein 70(HSP70)transcription in the sea urchin(Strongylocentrotus intermedius)under a heat stress.After 112 days of feeding trial the sea urchins were heat stressed(26
C)and the coelomic fluid and intestine sampled at time 0 and 15 min,2 h and 6 h.The results showed that an increase in dietary protein(12%-24%),significantly increased(p<0.05)the sea urchin weight gain rate(WGR).As dietary protein increased(from 18% to 36%),the gonadosomatic index(GI)of juvenile sea urchins also significantly increased(p<0.05)from 18.0%to 22.6%.Superoxide dismutase(SOD)activity increased with dietary protein increase(12%-30%)and the enzyme activity was significantly higher(p<0.05)in the coelomic fluid of sea urchins that were fed with 30% diets when compared to 12% and 36% protein diets at all time points after the heat stress.Catalase(CAT)activity showed a similar tendency with the increase in dietary protein concentration at time 0 and 15 min after the heat stress(p<0.05).Transcription of HSP70 in the intestine also showed a similar trend to SOD and was highest in the animals that were fed with 30% protein diets(p<0.05).Our results suggest that 24% protein diets could meet the requirements for growth performance but a 30% protein diet resulted in improved gonad development and anti-heat stress capacity in this sea urchin species.

Growth Ring-dependent Fracture Toughness of Sea Urchin Spines Estimated by Boundary Effect Model

Although the fracture behavior of sea urchin spines has been extensively investigated,there is as yet a lack of quantitative estimation on the effect of growth rings on the fracture properties of sea urchin spines.In sea urchin spines,much denser pores present in growth rings rather than porous layers.The tensile strength and fracture toughness of sea urchin spine samples with different numbers of growth rings are measured by the Boundary Effect Model(BEM).The experimental results of single-edge notched three-point bending tests indicate that the BEM is an appropriate method to estimate the fracture toughness of the present porous sea urchin spines,and the number of growth rings plays an important role in the fracture properties of spines.Specifically,the tensile strength and fracture toughness of sea urchin spines can be significantly improved with the increase in the number of growth rings,and their fracture toughness can even reach a relatively high value compared with some other porous materials with an identical porosity.The present research findings are expected to provide a fundamental insight into the design of high-performance bionic materials with a highly porous structure.

Investigation on the Effect of the Multilayered Porous Structure of Sea Urchin Skeleton on Its Mechanical Behavior

In this paper,the effect of stereom structure on the mechanical behavior of the Sea Urchin Inorganic Skeleton(SUIS)has been studied.The stereom microstructure of both Anthocidaris crassispina and Tripnenstes gratilla was characterized by Scanning Electron Microscopy(SEM).Results indicate that a three-layer porous structure consisting of a growth,a support,and a resorption(GSR)layer is a common denominator for both species.The effect of GSR layer order on the mechanical behavior of the SUIS was studied by a finite element method.The results show that the GSR model could effectively reduce the maximum tensile stress on its meridional sutures under unidirectional pressure,hydrostatic pressure,and self-weight situation.For a fabricated three-layered ceramic test strips with different layer orders,the mechanical properties have a completely opposite performance compared with the compressive properties of the calculated SUIS-Iike models.This indicates that the GSR structure can effectively improve the mechanical properties of the SUIS,but it cannot be applied to bionics without considering its synergistic effect with the macro-structure of the SUIS.This is a typical example of bionic invalidation by single structure,where multi-level structure bionics may be an effective solution.

Conversion of natural marine skeletons as scaffolds for bone tissue engineering

Marine CaCO3 skeletons have tailored architectures created by nature, which give them structural support and other functions. For example, seashells have dense lamellar structures, while coral, cuttlebone and sea urchin spines have interconnected porous structures. In our experiments, seashells, coral and cuttlebone were hydrothermaily converted to hydroxyapatite （HAP）, and sea urchin spines were converted to Mg-substituted tricalcium phosphate, while maintaining their original structures. Partially converted shell samples have mechanical strength, which is close to that of compact human bone. After implantation of converted shell and spine samples in rat femoral defects for 6 weeks, there was newly formed bone growth up to and around the implants. Some new bone was found to migrate through the pores of converted spine samples and grow inward. These results show good bioactivity and osteoconductivity of the implants, indicating the converted shell and spine samples can be used as bone defect fillers. The interconnected porous HAP scaffolds from converted coral or cuttlebone that have pore size larger than 100μm likely support infiltration of bone cells and vessels, and finally encourage new bone ingrowth.

Morphogenesis: a focus on marine invertebrates

Morphogenesis is a process describing how the shapes of living tissues and bodies are created during development. Livingand fossil organisms exhibit enormously diverse tissue architecture and body forms, although the functions of organs areevolutionally conserved. Current knowledge reveals that relatively conserved mechanisms are applied to control developmentamong different species. However, the regulations of morphogenesis are quite diverse in detail. Animals in the oceandisplay a wide range of diversity of morphology suitable for their seawater environment. Nevertheless, compared with theintensive studies on terrestrial animals, research on marine animal morphogenesis is still insufficient. The increasing genomicdata and the recently available gene editing methods, together with the fast development of imaging techniques, quantitativeanalyses and biophysical models, provide us the opportunities to have a deeper understanding of the principles that drivethe diverse morphogenetic processes in marine animals. In this review, we summarize the recent studies of morphogenesisand evolution at molecular, cellular and tissue levels, with a focus on three model marine animals, namely ascidians, seaurchins and sea anemones.

Active bleb formation is abated in Lytechinus variegatus red spherule coelomocytes after disruption of acto-myosin contractility

Red spherule coelomocytes are immune cells in the sea urchin Lytechinus variegatus that have been characterizedas motile O2 transport cells. Video microscopy of living red spherule coelomocytes reveals a constitutive, dynamicarray of cellular morphologies and movements. Cells continuously send out and retract membrane blebs all over thecell surface as part of their normal cellular physiology. Disruption of microtubules by perfusion with either nocodazoleor taxol had no effect on bleb formation or motility. Perfusion with cytochalasin B abated bleb formation andrevealed cells that exhibited multiple small spheres attached by short membrane extensions. Attenuation of blebbingand intracellular organelle motility were restored by washing out with cytochalasin B. Treatment with phalloidinalso abated bleb formation and revealed a smooth, spherical cellular morphology. The effects of phalloidin werecompletely reversible after washout. Red spherule coelomocytes treated with blebbistatin rounded up with anirreversible retraction of blebs into surface blebs that were greatly reduced in size, number and motility. Normal cellsurface bleb formation and intracellular organelle motility were not restored after washout of the drug. These resultsindicate that the acto-myosin contractile mechanism contributes to the dynamics of constitutive cell surface membraneblebbing in invertebrate immune cells.