Structure of the digestive tract of the pluteus larva of Dendraster excentricus (Echinodermata: Echinoida)

Summary Histological and ultrastructural observations of the digestive tract of eight-armed plutei of Dendraster excentricus are reported. The esophagus is divided into two regions. The uppermost is a narrow tube comprised of ciliated cells that assist in transporting food to the more bulbous lower esophagus where food particles are formed into a bolus prior to entering the stomach. The esophagus is surrounded by a network of smooth muscle fibers that are predominantly oriented circumferentially in the upper esophagus, and longitudinally in the lower esophagus. The musculature of the upper esophagus produces peristaltic contractions, whereas contractions of the muscle of the lower esophagus open the cardiac sphincter and force food from the lower esophagus into the stomach. Axons are associated with the ciliated cells and the muscles of the upper esophagus. The cardiac sphincter consists of a ring of myoepithelium, with cross-striated myofibrils oriented around the bases of the cells. The gastric epithelium is comprised of two cell types. Type I cells, which predominate, absorb and store nutrients, and may be the source of secreted digestive enzymes. Type II cells apparently phagocytize and intracellularly digest whole algal cells. The intestine is comprised of relatively unspecialized cells and probably functions primarily as a conductive tube for the elimination of undigested materials.     

Functional morphology of regular echinoid tests (Echinodermata,Echinoida): a finite element study

Functional morphology of the calcareous test ofEchinus esculentus was investigated by parametric finite element analysis, an engineering technique developed for numerical analysis of the behaviour of complex structures responding to external forces. Finite element models of the test were generated by methods of Computer Aided Geometric Design (CAGD) to calculate the mechanical responses to different types of loading. The load cases included vertical, concentrated load at the apex, vertical, distributed load on the upper third of the test, internal pressure and tensile forces as introduced into the test by tube feet activity. The objectives were the shape of the test, the distribution of material and the alternating zones of porous and non-porous plates within the test.—Echinoid tests resist external loading without showing any specific points of failure. The thickened margins of the periproct and peristome apertures account for load-bearing capacity as well as the thickned meridional structures which carry a greater portion of stress than the thinner parts of the test. Distribution of material is not a response to concentrated loads on the apex nor to self-weight. Taken strictly, echinoid tests are not thin (or membrane) shells. Under loading, bending moments occur which influence the stress state in the entire test. The pneu hypothesis could not be confirmed. Adaptation of the test shape or of the distribution of material as a response to internal pressure does not exist. Tests of regular echinoids are especially well adapted to the mechanical activity of the ambulacral tube feet, i.e. the shape of the test, its flattening towards the substrate, the outward bulge of the ambulacra and the differential distribution of material within the test.     

The compass depressors ofParacentrotus lividus (Echinodermata,Echinoida): ultrastructural and mechanical aspects of their variable tensility and contractility

Summary The compass depressors are bands of soft tissue which connect the compass ossicles of the echinoid lantern to the inner edge of the test. They are essentially ligaments with on one side a thin layer of muscle cells. The ligamentous component consists mainly of a parallel array of collagen fibrils with interspersed 12 nm microfibrils. The most notable cellular constituents are granule-containing cell bodies and their processes which resemble the juxtaligamental cells that have been found in all echinoderm mutable collagenous tissues and which may control the tensility of these tissues. The muscle cells occupy about 8% of the total cross-sectional area of the compass depressor and are located in a richly innervated pseudostratified myoepithelium. When subjected to constant low loads in creep tests the compass depressor stretches to a fixed length beyond which there is no further extension. The length at this creep limit coincides with the maximum length to which the compass depressor is stretched by natural movements of the intact lantern. Stress-strain tests show that treatment with 1 mM acetylcholine or 100 mM K⁺ ions can increase reversibly the stiffness of the compass depressor to an extent that cannot be due to contraction of the myoepithelium, suggesting that the mechanical properties of the ligament are under physiological control. Tension-length data on the myoepithelium suggest that it generates a maximum active tension when the compass depressor is stretched to the creep limit. The implications of these results for the function of the compass depressors are discussed.     

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea, Echinodermata) during the perimetamorphic period

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.     

Evaluation of the attachment strength of individuals of Asterina gibbosa (Asteroidea,Echinodermata) during the perimetamorphic period

Abstract

A turbulent channel flow apparatus was used to determine the adhesion strength of the three perimetamorphic stages of the asteroid Asterina gibbosa, i.e. the brachiolaria larvae, the metamorphic individuals and the juveniles. The mean critical wall shear stresses (wall shear stress required to dislodge 50% of the attached individuals) necessary to detach larvae attached by the brachiolar arms (1.2 Pa) and juveniles attached by the tube feet (7.1 Pa) were one order of magnitude lower than the stress required to dislodge metamorphic individuals attached by the adhesive disc (41 Pa). This variability in adhesion strength reflects differences in the functioning of the adhesive organs for these different life stages of sea stars. Brachiolar arms and tube feet function as temporary adhesion organs, allowing repetitive cycles of attachment to and detachment from the substratum, whereas the adhesive disc is used only once, at the onset of metamorphosis, and is responsible for the strong attachment of the metamorphic individual, which can be described as permanent adhesion. The results confirm that the turbulent water channel apparatus is a powerful tool to investigate the adhesion mechanisms of minute organisms.     

First Insights into the Biochemistry of Tube Foot Adhesive from the Sea Urchin Paracentrotus lividus (Echinoidea, Echinodermata)

Sea urchins are common inhabitants of wave-swept shores. To withstand the action of waves, they rely on highly specialized independent adhesive organs, the adoral tube feet. The latter are extremely well-designed for temporary adhesion being composed by two functional subunits: (1) an apical disc that produces an adhesive secretion to fasten the sea urchin to the substratum, as well as a deadhesive secretion to allow the animal to move and (2) a stem that bears the tensions placed on the animal by hydrodynamism. Despite their technological potential for the development of new biomimetic underwater adhesives, very little is known about the biochemical composition of sea urchin adhesives. A characterization of sea urchin adhesives is presented using footprints. The latter contain inorganic residues (45.5%), proteins (6.4%), neutral sugars (1.2%), and lipids (2.5%). Moreover, the amino acid composition of the soluble protein fraction revealed a bias toward six amino acids: glycine, alanine, valine, serine, threonine, and asparagine/aspartic acid, which comprise 56.8% of the total residues. In addition, it also presents higher levels of proline (6.8%) and half-cystine (2.6%) than average eukaryotic proteins. Footprint insolubility was partially overcome using strong denaturing and reducing buffers, enabling the visualization of 13 proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis. The conjugation of mass spectrometry with homology–database search allowed the identification of six proteins: alpha and beta tubulin, actin, and histones H2B, H3, H2A, and H4, whose location and function in the adhesive are discussed but require further investigation. For the remaining unidentified proteins, five de novo-generated peptide sequences were found that were not present in the available protein databases, suggesting that they might be novel or modified proteins.     

Gametogenesis correlated with steroid levels during the gonadal cycle of the sea urchin Paracentrotus lividus (Echinodermata: Echinoidea)

The specific mechanism regulating reproduction in invertebrates is a field of topical interest which needs to be explored in detail considering also the intriguing possible comparison with vertebrates. In this paper levels of Testosterone (T) and Estradiol (E2) and their reciprocal ratios were determined in ovaries and testis of the echinoid model species Paracentrotus lividus during the year 2004 by taking into account a putative relationship between steroid levels and reproductive cycle. T levels appeared to significantly vary during male reproductive cycle, thus suggesting a possible role of this hormone in regulation of spermatogenesis as demonstrated for other echinoderms. E2 levels were lower in males with respect to females; consequently E2 involvement in oogenesis is hypothesized. In parallel with steroid levels evaluation, variations in P450-aromatase activity and its possible role on regulation of gametogenesis were also considered. Clear correlations between steroid levels and gonad index (GI), as well as between GI and reproductive cycle were not detected, suggesting that GI alone is not a reliable parameter in describing the reproductive status of the gonads. Altogether the results obtained so far confirm the presence of a relationship between steroid levels and reproductive cycle as suggested by previous results on different echinoderm species.     

Gonadal growth and reproduction in the commercial sea urchin Paracentrotus lividus (Lamarck, 1816) (Echinodermata: Echinoidea) from southern Spain

The reproductive state of six populations of the commercial sea urchin P. lividus was investigated. Specimens were sampled monthly from six sites of southern Spain. The development of the gonad during the period of study was measured using histological methods and categorised by four stages for females and two for male specimens. A Gonadosomatic Index was also established. The six populations exhibited similar reproductive patterns, having mature gonads during almost all months while reconstitution and spent were present mainly in autumn. The population from Sancti Petri had the highest Gonadosomatic Index value. Higher food availability may explain this difference.     

Males and females gonad fatty acids of the sea urchins Paracentrotus lividus and Arbacia lixula (Echinodermata)

The aim of this study was to analyze male and female gonad fatty acids of two sea urchin species, Paracentrotus lividus and Arbacia lixula, from the south coast of Spain. Additionally, we investigated possible differences between two locations. The ovaries of both species showed higher percentages of 14:0, 16:0, 16:1n-7, 18:2n-6, 18:3n-3 and 18:4n-3 than testes and lower levels of 18:0, 22:1n-9, 20:4n-6 and 22:5n-3. In P. lividus but not in A. lixula, the level of 20:5n-3 was higher in testes than in ovaries. These differences between sexes probably indicate different requirements of males and females during gametogenesis although the presence of a large number of gametes in the mature gonad may also have influences on fatty acid composition. Significant differences in gonad fatty acid profiles where also found when individuals of P. lividus collected at a location of the Mediterranean region were compared with specimens collected at the Atlantic coast. The most remarkable changes were the lower levels of 14:0, 18:1n-7, 20:1n-9, 20:4n-6 and 22:4n-6 and the higher values of 20:1n-11, 20:5n-3 and 22:6n-3 found in males and females of the Mediterranean specimens compared to those of the Atlantic coast. These differences probably reflect the differences in potential food sources at each location.     

Ultrastructural investigation of matrix-mediated biomineralization in echinoids (Echinodermata,Echinoida)

Summary The formation of echinoderm endoskeletons is studied using echinoid teeth as an example. Echinoid teeth grow rapidly. They consist of many calcareous elements each produced by syncytial odontoblasts. The calcification process in echinoderms needs (1) syncytial sclerocytes or odontoblasts, (2) a spacious vacuolar cavity within this syncytium, (3) an organic matrix coat in the cavity. As long as calcite is deposited, the matrix does not touch the interior face of the syncytium. The cooperation between syncytium, interior cavity and matrix coat during the mineralization process is discussed. The proposed hypothesis applies to the formation of larval skeletons, echinoderm ossicles and echinoid teeth.When calcite deposition ceases the syncytium largely splits up into filiform processes, and the skeleton is partly exposed to the extracellular space. However, the syncytium is able to reform a continuous sheath and an equivalent of the cavity and may renew calcite deposition.The tooth odontoblasts come from an apical cluster of proliferative cells, each possessing a cilium. The cilium is lost when the cell becomes a true odontoblast. This suggests that cilia are primitive features of echinoderm cells. The second step in calcification involves the odontoblasts giving rise to calcareous discs which unite the hitherto single tooth elements. During this process the odontoblasts immure themselves. The structures necessary for calcification are maintained until the end of the process.The mineralizing matrix is EDTA-soluble. The applied method preserves the matrix coating the calcite but more is probably incorporated into the mineral phase and dissolved with the calcite.Abbreviations A adhesive point (LNC) - B adaxial bag - bb basal body (ci) - CA calcareous deposits - cb cytoplasmic bladder (cp) - ce centriole - ci cilium - cp cable-like cell process - cv condensing vacuole - dp distal processes (sh) - E epithelium of the tooth - ex extracellular space - f extracellular fibrils - ga gasket (sh) - ic interior cavity - L lamellae (LNC) - LNC lamellae needle complex - m mitochondrium - mc matrix coat - MF main fold (P) - MI mitosis - mt microtubules - N nucleus - O odontoblast - P primary plate - Ph phagocyte - PR proliferative cell - pr prism - rb reserve body - RER rough endopl. reticulum - rl rootlet (ci) - RY relatively youngest plate - s satellite (bb, ce) - sh synplasmic sheath (O) - SP secondary plate - sv smooth-walled vesicle - TF transversal fold (P) - U umbo (P) - v Golgi vesicle - Y youngest tooth element     

Ultrastructure and cytochemistry of the early calcification site and of its mineralization organic matrix in Paracentrotus lividus (Echinodermata: Echinoidea)

 The ultrastructure and cytochemistry of skeleton formation sites prior to mineralization are described for the first time in echinoderms. These early sites are intracellular vacuoles located in syncytial pseudopodia of skeleteton-forming cells. They contain a mineralization organic matrix, which shows a calcium-binding ability and is framed in a tridimensional structure made of concentric layers bridged by radial threads. This organic matrix presents repetitive structures which could be implicated in mineralization control. Both the tridimensional organization of the organic matrix and its framing, before mineralization starts, question the current theories which suggest that the echinoderm organic matrix is soluble at the onset of mineralization and adsorbs on the forming crystal. Accepted: 19 March 1998     

The sphaeridia of sea urchins: ultrastructure and supposed function (Echinodermata,Echinoida)

Summary Sphaeridia are minute skeletal appendages of the echinoid test which are considered to be sense organs, organs of equilibrium, according to their shape. The sphaeridium forms a functional unit with the tubercle to which it adheres. The tubercle is encircled by a basiepithelial nerve ring of the epidermis. A circle of regularly arranged myocytes stretches from the tubercle to the sphaeridium. The muscles are distant from each other. The myofibrillar processes enter the pore space of the sphaeridial skeleton to which they are anchored by tendons; tendons are absent in the tubercle region. The cell bodies of the myocytes lie opposite to the nerve ring outside the skeleton. In this region the muscle cells and the nerve ring are in contact with each other, their basal laminae fuse. Tensions of the various myocytes are dependent on the position of the top-heavy sphaeridium. The nerve ring contains neurones which are provided with a cilium which lies close to the contact region with the myocytes. This arrangement leads to the assumption that the nerve cells in question have a proprioceptor function. Unique filter cells are present in the pore spaces of the sphaeridium and the tubercle. They possibly detoxicate the extracellular fluid that surrounds the myocytes. Phagocytes loaded with spacious phagosomes are crowded in the adjacent pore spaces. They are possibly extruded via the epidermis. Filter cells and phagocytes have obviously to do with the metabolism within the sphaeridium-tubercle-system.     

Macrostructure and evolution of the digestive system in Echinoida (Echinodermata)

The structure of the digestive system in Echinoida has long been puzzling since comparative studies have suggested that a derived structure, the siphon, has apparently evolved twice independently. New observations on the digestive system in five species of Cidaroida, four species of the Diadematoida and three species of Echinothurioida are presented. The results show that the four diadematoid species have a siphon and the three species of Echinothurioida have a siphonal groove, contrary to previous assertions. These observations make the macrostructure of the echinoid digestive system fully consistent with more recent phylogenetic hypotheses based on molecular and general morphological data, and support the idea that a siphon has evolved only once, in the stem lineage of the Acroechinoidea.     

Mechanical Properties of the Compass Depressors of the Sea-Urchin Paracentrotus lividus (Echinodermata,Echinoidea) and the Effects of Enzymes,Neurotransmitters and Synthetic Tensilin-Like Protein

The compass depressors (CDs) of the sea-urchin lantern are ligaments consisting mainly of discontinuous collagen fibrils associated with a small population of myocytes. They are mutable collagenous structures, which can change their mechanical properties rapidly and reversibly under nervous control. The aims of this investigation were to characterise the baseline (i.e. unmanipulated) static mechanical properties of the CDs of Paracentrotus lividus by means of creep tests and incremental force-extension tests, and to determine the effects on their mechanical behaviour of a range of agents. Under constant load the CDs exhibited a three-phase creep curve, the mean coefficient of viscosity being 561±365 MPa.s. The stress-strain curve showed toe, linear and yield regions; the mean strain at the toe-linear inflection was 0.86±0.61; the mean Young’s modulus was 18.62±10.30 MPa; and the mean tensile strength was 8.14±5.73 MPa. Hyaluronidase from Streptomyces hyalurolyticus had no effect on creep behaviour, whilst chondroitinase ABC prolonged primary creep but had no effect on secondary creep or on any force-extension parameters; it thus appears that neither hyaluronic acid nor sulphated glycosaminoglycans have an interfibrillar load transfer function in the CD. Acetylcholine, the muscarinic agonists arecoline and methacholine, and the nicotinic agonists nicotine and 1-[1-(3,4-dimethyl-phenyl)-ethyl]-piperazine produced an abrupt increase in CD viscosity; the CDs were not differentially sensitive to muscarinic or nicotinic agonists. CDs showed either no, or no consistent, response to adrenaline, L-glutamic acid, 5-hydroxytryptamine and γ-aminobutyric acid. Synthetic echinoid tensilin-like protein had a weak and inconsistent stiffening effect, indicating that, in contrast to holothurian tensilins, the echinoid molecule may not be involved in the regulation of collagenous tissue tensility. We compare in detail the mechanical behaviour of the CD with that of mammalian tendon and highlight its potential as a model system for investigating poorly understood aspects of the ontogeny and phylogeny of vertebrate collagenous tissues.     

Mechanical properties of sea-urchin lantern muscles: a comparative investigation of intact muscle groups in Paracentrotus lividus (Lam.) and Stylocidaris affinis (Phil.) (Echinodermata, Echinoidea)

The jaw apparatus, or lantern, of sea-urchins contains five pairs of retractor and protractor muscles which are responsible for lantern displacement. Using intact retractor or protractor groups, the force-length relations of these muscles were compared in two taxonomically distant species, Paracentrotus lividus and Stylocidaris affinis. The total contractile forces generated by the muscles can be resolved into vertical and horizontal components. It was found that the vertical component of the retractors is maximal at a lantern position which is significantly lower (i.e. more protruded) in Paracentrotus than in Stylocidaris. Total forces generated by the retractors were in both species maximal at or above the lantern `resting positions'. In Paracentrotus alone, the total force-displacement curves tended to be bimodal. It is hypothesized that the retractors of Paracentrotus contain two populations of muscle fibres, one adapted for jaw opening and one for lantern retraction. No significant differences in the properties of the protractors of the two species could be identified. The lantern of Paracentrotus is more mobile than that of Stylocidaris and is able to exploit a wider range of food sources. This investigation has shown that the force-length relations of the lantern muscles match their differing working conditions. Accepted: 3 November 1997     

Fine structure and presumed functions of the pedicellariae of Echinocardium cordatum (Echinodermata,Echinoida)

Summary Tridactylous, trifoliate, and globiferous pedicellariae occur on the body surface of Echinocardium cordatum. Tridactyles have three forms: the typical, the rostrate, and the large forms. Both typical and rostrate tridactyles and trifoliates occur all around the echinoid body (trifoliates are, however, 4 times more numerous than tridactyles). Large tridactylous and globiferous pedicellariae are restricted to the peribuccal area.As a general rule tridactyles and trifoliates are similar in morphology. The distal part of the valves forms an open blade and bears lateral teeth and/or denticles (single or in combs). The stalk consists of a rigid proximal part supported by an axial rod and a flexible distal part which includes an axial fluid-filled cavity. The cavity is surrounded by muscle fibers and acts as an hydroskeleton, allowing the undulating-coiling movements of the flexible part of the stalk. Trifoliates are always active while tridactyles react only to direct or indirect mechanical stimulation.The valves of the globiferous pedicellariae have a tubular distal part whose upper opening is surrounded by teeth. There is no differentiated venom gland but a cluster of epithelial glandular cells located at the level of the valve upper opening. A small ciliary pad occurs just below the glandular cluster. Globiferous stalks are not flexible, being supported for their full length by an axial rod. Globiferous pedicellariae appear to be sensitive only to chemical stimulation.The presumed functions of E. cordatum pedicellariae are (1) cleaning of the body surface and ciliary structures (trifoliates), (2) protection against sedimenting particles (tridactyles), and (3) defense of the peribuccal area against potential small predators (globiferous pedicellariae).     

Functional morphology of coronal and peristomeal podia in Sphaerechinus granularis (Echinodermata,Echinoida)

Summary Coronal podia of Sphaerechinus granularis are anchoring (adhering) appendages involved in either locomotion or capture of drift materials. Adhesion is not due to the presumed sucker action of the disc but relies entirely on secretions of the disc epidermis. Peristomeal podia function in wrapping together food particles or food fragments in an adhesive material thus facilitating their capture by the Aristotle's lantern. In both types of podia, the disc epidermis is made up of four cell types: non-ciliated secretory cells (NCS cells) that contain graules whose content is at least partly mucopolysaccharidic in nature, ciliated secretory cells (CS cells) containing granules of unknown nature, ciliated non-secretory cells (CNS cells) and support cells. The cilia of CS cells are subeuticular whereas those of CNS cells, although also short and rigid, traverse the cuticle and protrude in the outer medium. All these cells are presumably involved in an adhesive/de-adhesive process functioning as a duogland adhesive system. Adhesive secretion would be produced by NCS cells and de-adhesive secretion by CS cells. These secretions would be controlled through stimulations by the two types of ciliated cells (receptor cells) which presumably interact with the secretory cells by way of the nerve plexus. This model of adhesion/de-adhesion fits well with the activities of both coronal and peristomeal podia. The secretion of NCS cells would make up a bridge of adhesive material between a podium and the substratum (coronal podia) or would coat and gather food particles (peristomeal podia), respectively. The de-adhesive material enclosed in the granules of CS cells would allow the podia (either coronal or peristomeal) to easily become detached from the substratum and to always remain clear of any particles.Research Assistant, National Fund for Scientific Research (Belgium)