Larval growth and perimetamorphosis in the echinoid <Emphasis Type="Italic">Echinocardium cordatum</Emphasis> (Echinodermata): the spatangoid way to become a sea urchin

The prejuvenile development of Echinocardium cordatum (Echinoidea) was investigated by means of scanning electron, confocal and light microscopes, aiming to illustrate the early life history of a spatangoid representative and to compare it with the other major echinoid groups. During the larval development of E. cordatum, two periods follow one another. The first one takes 12 days; it ends with the formation of a complete echinopluteus with twelve elongated larval arms. The second lasts from 3 to 12 days; it is entirely devoted to the building of the echinid rudiment and ends with the acquisition of larval competence. No appendage other than arms develops at the larva’s outer surface. Competent larvae are demersal. They settle onto the substratum and test it for suitability using the five rudiment podia that protrude through the vestibule opening. Metamorphosis is a rapid event that lasts less than an hour. The rudiment does not everse and its spines and podia actively tear up the larval epidermis which is progressively covered by the growing vestibular epidermis. The resulting postlarva is short-lived and morphologically similar to both the late rudiment and the early juvenile, which, however, is exotrophic. Late rudiments in E. cordatum show basic spatangoid features being bilaterally symmetric and having clavulae and sphaeridia. More importantly, they already have the convex shape and the appendage cover of early juveniles. Metamorphosis in E. cordatum appears to be less complex, i.e. no rudiment is everted, and more complete, since, in contrast to “regular” echinoids, no transitory appendages are seen. Metamorphosis/development of E. cordatum, thus, is closer to that of clypeasteroids, since the rudiment of the latter already bears juvenile definitive appendages, when everted during metamorphsis.     

Morphological evolution of newly metamorphosed sea urchins--a phylogenetic and functional analysis

Newly metamorphosed juvenile sea urchins are highly variable across taxa. This contribution documents and illustrates structural, functional, and phylogenetic variation among newly metamorphosed juvenile sea urchins for 31 species from 12 ordinal or familial lineages. The classic juvenile with five primary podia, 20 interambulacral spines, and variable numbers of juvenile spines is found commonly among new metamorphs across lineages, but there are many examples, which depart from this pattern and most likely reflect adaptation to settlement habitats. At metamorphosis juveniles can have 5-25 functional podia. They can have 0-65 spines, 0 or 5 sphaeridia (balance organs). They may have zero or up to eight pedicellariae. While competent larvae that delay metamorphosis may continue to develop juvenile structures, variation across species is much greater than within species and there are strong phylogenetic and functional differences among juveniles. Heterochronic changes in expression of these structures can account for differences among taxa. Based on this sample, juvenile characters such as spines, podia, and larval pedicellariae are expressed in ways that suggest they are developmental modules whose expression can be readily changed relative to one another and to the time of metamorphosis.     

Podial sensory receptors and the induction of metamorphosis in echinoids

Larvae of Strongylocentrotus droebachiensis (Müller), Lytechinus pictus (Verrill), and Lytechinus variegatus (Leske) which are competent to metamorphose display what appears to be substratum-testing behavior prior to metamorphosis. Larvae cease swimming, partially evert the adult rudiment, and walk about examining the substratum with their five primary podia. Larvae eithe r metamorphose or withdraw their podia and resume swimming to settle again elsewhere. Scanning and transmission electron microscopic examinations of the primary podia revealed sensory receptor cells on the rim and on a conical projection at the center of the podial sucker. Each sensory cell has a single short cilium on its apical surface a axonal process at its base which contributes to the basiepithelial nerve plexus. Mature adults of the same species also have comparable sensory structures on their tube feet suckers. It is suggested that the sensory receptors on the primary podia of setting larvae, although they are not specialized larval structures, may be involved in the perception of tactil e stimuli which have been previously demonstrated to be involved in the induction of metamorphosis.     

On the sites of secondary podia formation in a juvenile echinoid: growth of the body types in echinoderms

The growth of the adult echinoderm body is addressed here in the echinoid Holopneustes purpurescens in a study of the early development of the secondary podia along the five radial canals of the adult rudiment. At a stage when the first four secondary podia have formed along each radius oral to the primary podium, two podia are on one side of the radius and two are on the other side, all at a different distance from the primary podium. The pattern of the connexions of these secondary podia to the radial canals changes in successive radii in a manner similar to Lovén’s law for skeletal plates and matches the reported sequence in the times at which the first ambulacral skeletal plates form in the adult echinoid rudiment. A similar pattern is described for the reported origins of the secondary podia in apodid holothurians. A common plan for the growth of the body types is described for echinoids, asteroids, holothurians and concentricycloids. The five metameric series of secondary podia formed in echinoderms have a coelomic developmental origin like the single metameric series of somites formed in the axial structures of chordates.     

The attachment complex of brachiolaria larvae of the sea star <Emphasis Type="Italic">Asterias rubens</Emphasis> (Echinodermata): an ultrastructural and immunocytochemical study

The attachment complex of brachiolaria larvae of the asteroid Asterias rubens comprises three brachiolar arms and an adhesive disc located on the preoral lobe. The former are used in temporary attachment and sensory testing of the substratum, whereas the latter is used for permanent fixation to the substratum at the onset of metamorphosis. Brachiolar arms are hollow structures consisting of an extensible stem tipped by a crown of dome-like ciliated papillae. The papilla epidermis is composed of secretory cells (type A, B and C cells), non-secretory ciliated cells, neurosecretory-like cells and support cells. Type A and B secretory cells fill a large part of the papilla epidermis and are always closely associated. They presumably form a duo-gland adhesive system in which type A and B cells are respectively adhesive and de-adhesive in function. The adhesive disc is an epidermal structure mainly composed of secretory cells and support cells. Secretory cells produce the cement, which anchor the metamorphic larva to the substratum until the podia are developed. The relatedness between the composition of the adhesive material in the brachiolaria attachment complex and in the podia of adults was investigated by immunocytochemistry using antibodies raised against podial adhesive secretions of A. rubens. Type A secretory cells were the only immunolabelled cells indicating that their temporary adhesive shares common epitopes with the one of podia. The attachment pattern displayed by the individuals of A. rubens during the perimetamorphic period—temporary, permanent, temporary—is unique among marine non-vertebrate Metazoa.     

Embryonic,Larval, and Juvenile Development of the Sea Biscuit Clypeaster subdepressus (Echinodermata: Clypeasteroida)

Sea biscuits and sand dollars diverged from other irregular echinoids approximately 55 million years ago and rapidly dispersed to oceans worldwide. A series of morphological changes were associated with the occupation of sand beds such as flattening of the body, shortening of primary spines, multiplication of podia, and retention of the lantern of Aristotle into adulthood. To investigate the developmental basis of such morphological changes we documented the ontogeny of Clypeaster subdepressus. We obtained gametes from adult specimens by KCl injection and raised the embryos at 26C. Ciliated blastulae hatched 7.5 h after sperm entry. During gastrulation the archenteron elongated continuously while ectodermal red-pigmented cells migrated synchronously to the apical plate. Pluteus larvae began to feed in 3 d and were 20 d old at metamorphosis; starved larvae died 17 d after fertilization. Postlarval juveniles had neither mouth nor anus nor plates on the aboral side, except for the remnants of larval spicules, but their bilateral symmetry became evident after the resorption of larval tissues. Ossicles of the lantern were present and organized in 5 groups. Each group had 1 tooth, 2 demipyramids, and 2 epiphyses with a rotula in between. Early appendages consisted of 15 spines, 15 podia (2 types), and 5 sphaeridia. Podial types were distributed in accordance to Lovén''s rule and the first podium of each ambulacrum was not encircled by the skeleton. Seven days after metamorphosis juveniles began to feed by rasping sand grains with the lantern. Juveniles survived in laboratory cultures for 9 months and died with wide, a single open sphaeridium per ambulacrum, aboral anus, and no differentiated food grooves or petaloids. Tracking the morphogenesis of early juveniles is a necessary step to elucidate the developmental mechanisms of echinoid growth and important groundwork to clarify homologies between irregular urchins.     

Observations on the clypeasteroid Echinocyamus pusillus (O. F. Müller)

The tiny echinoid Echinocyamus pusillus (O. F. Müller) is equipped with specialized external structures that suit it for a wide variety of environments. Special features include the ability to burrow in sediments of fine sand to shell gravel and to climb vertically.Specimens dredged off the west coast of Scotland were observed in aquaria and with the SEM. E. pusillus is characterized by three kinds of spines, and by two of pedicellariae. In contrast to sand dollars, the spines play a passive rôle in the feeding and burrowing operation, probably retaining a defensive nature as in the regular urchins. It is the podia that are chiefly involved in climbing, burrowing, righting, and probably feeding. Surface ciliary currents transport particles, but not to the mouth; they may have a respiratory or cleansing function. Experimental animals did not burrow in either very fine or very coarse sand, probably because a certain relationship exists between particle weight and podia size.E. pusillus shares behavioural and structural characteristics with regular and irregular urchins. It is not a true sand dollar, but may illustrate an evolutionary stage towards such a form.     

Loss and gain of the juvenile rudiment and metamorphic competence during starvation and feeding of bryozoan larvae

SUMMARY In many animals, larval structures and juvenile rudiments develop independently. One advantage of this independence is that juvenile rudiments can be expended as a nutrient reserve or for energy conservation. When bryozoan cyphonautes larvae were starved, structures required for settlement and metamorphosis shrank. When the larvae were again fed, these structures grew back. Starvation reduced the size of both the internal sac, a rudiment of postlarval juvenile structures, and the pyriform organ, which functions in sensing and crawling on the substratum at settlement. In contrast, starvation affected neither the size of the larval shell nor the lengths of the ciliary bands used in swimming and feeding. Starved larvae that had reduced the pyriform organ and internal sac did not metamorphose in response to stimuli from a laminarian alga. The laminarian alga did stimulate metamorphosis of the same larvae after renewed feeding, when the larvae had regrown these structures. Thus starved larvae expended body parts needed for settlement and metamorphosis when food was scarce while retaining structures for feeding, swimming, and defense. Starved larvae thereby retained the capacity to regrow structures needed for settlement and metamorphosis when they again encountered food. Advantages from expendable juvenile rudiments may enhance selection for their being developmentally distinct from structures for larval swimming and feeding.     

Development of nervous systems to metamorphosis in feeding and non-feeding echinoid larvae,the transition from bilateral to radial symmetry

The development of nervous system (NS) in the non-feeding vestibula larva of the sea urchin, Holopneustes purpurescens, and the feeding echinopluteus larva of Hemicentrotus pulcherrimus was examined by focusing on fate during metamorphosis. In H. purpurescens, the serotonergic NS (SerNS) appeared simultaneously and independently in larval tissue and adult rudiment, respectively, from 3-day post-fertilization. In 4-day vestibulae, an expansive aboral ganglion (450 × 100 μm) was present in the larval mid region that extended axons toward the oral ectoderm. These axons diverged near the base of the primary podia. An axonal bundle connected with the primary podia and the rim of vestopore on the oral side. Thus, the SerNS of the larva innervated the rudiment at early stage of development of the primary podia. This innervation was short-lived, and immediately before metamorphosis, it disappeared from the larval and adult tissue domains, whereas non-SerNS marked by synaptotagmin remained. The NS of 1-month post-fertilization plutei of H. pulcherrimus comprised an apical ganglion (50 × 17 μm) and axons that extended to the ciliary bands and the adult rudiment (AR). A major basal nerve of serotonergic and non-serotonergic axons and a minor non-serotonergic nerve comprised the ciliary band nerve. In 3-month plutei, axonal connection among the primary podia in the neural folds completed. The SerNS never developed in the AR. Thus, there was distinctive difference between feeding- and non-feeding larvae of the above sea urchins with respect to SerNS and the AR.     

Non-respiratory podia of clypeasteroids (Echinodermata,Echinoides): II. Diversity

Summary There are five major types of non-respiratory podia in the Order Clypeasteroida: accessory, barrel-tipped, food groove, large food groove, and buccal. The anatomy of each type is intimately related to its function in the feeding mechanism of clypeasteroids. Accessory podia are found aborally and orally in some species, only aborally and ambitally in others. Accessory podia are largely sensory and manipulatory, but can be locomotory in the small fibulariids and juvenile sand dollars. Barrel-tipped podia have expanded disk muscles and connective tissue, and are usually found in two sizes, large and small. In species that have them they are usually restricted to the oral surface. These podia collect food and pass it towards the food grooves in the manner of a bucket brigade. Food groove podia are found only in species with food grooves. These podia are small, with reduced tip musculature and expanded secretory tissue for coating food with mucus. They transport food down the food grooves to the mouth. Large food groove podia are simply large versions of ordinary food groove podia. They help move the clumped food into the mouth area towards the buccals, and are found only in the Clypeasteridae and some scutellines. Buccal podia lack tip musculature, but possess tip support fibres and a single type of small secretory cell. They are sensory, and capable of manipulating particles into the mouth. Buccals are present in all families except the Clypeasteridae. Juvenile Echinarachnius less than 3 mm in diameter have only respiratory, accessory and buccal podia. Food groove and barrel-tipped podia start to differentiate from the accessories as the juvenile approaches a diameter of 4 to 5 mm. Clypeasteroid podial diversity increases the efficiency of the food collecting mechanism. The anatomy and distribution of podia on the oral surface of scutellines supports the fact that this surface is the prime food collecting area in all true sand dollars. The podia (not miliary spines) are the major source of mucus used during the feeding process and are the primary feeding appendages.     

淡水青虾幼体发育的初步研究

作者观察了淡水青虾在实验室条件下幼体发育的情况。在幼体发育的全过程中,有的个体蜕皮9次,有的蜕皮11、14次。本文是描述蜕皮9次的个体的发育情况。从第一至第九期幼体以及后期幼体都附有详细的附肢图和部分整体图。幼体发育所需的时间随水温而变,在26.5±1.3℃条件下,幼体完成变态达到后期幼体需21天,在30.9±1-2℃仅需16天。       

Involvement of two Hox genes and Otx in echinoderm body-plan morphogenesis in the sea urchin Holopneustes purpurescens

The expression of Hox11/13 and Hox5 orthologues in the adult echinoid rudiment in the vestibula larva of Holopneustes purpurescens is described from whole mounts and sections of whole mounts after mRNA in situ hybridization. The Hox5 orthologue is HpHox5, which was isolated here. The expression of HpHox11/13 in the epithelium of the vestibule is aboral to the expression of HpHox5. HpHox5 is expressed in the epithelium of the vestibule floor where the secondary podia develop. The expression of HpHox11/13 and HpHox5 contrasts with the expression of an Otx orthologue, HprOtx, in the circum-oral nerve ring, the radial nerves and the neuroepithelium around the bases of the primary podia. From the expression patterns, we conclude that the two Hox genes are involved in the growth of a metameric series of secondary podia from a growth zone aboral to each primary podium, with the older podia nearer the circum-oral nerve ring. With respect to echinoderm body-plan polarities, we conclude that the growth zone is posterior relative to the anterior circum-oral nerve ring. The metamerism generated in this echinoderm from a posterior growth zone thus might not be generated differently from the way it is generated in bilateral animals.     

Expression of an Otx gene in the adult rudiment and the developing central nervous system in the vestibula larva of the sea urchin Holopneustes purpurescens

Expression of the Otx gene, HprOtx, from the sea urchin Holopneustes purpurescens, is described during the development of the adult echinoid rudiment in the vestibula larva of this species. The adult rudiment forms directly after gastrulation in the vestibula larva since, unlike the pluteus larva of most other sea urchin species, it is not a feeding larva. The expression is described during the period from hatching to a late vestibula larva. At hatching, HprOtx is expressed throughout the ectoderm of the gastrula. A short time later, expression is absent from the ectoderm on the oral side of the gastrula where the vestibule will form. In an early vestibula larva, HprOtx is not expressed in the ectodermal floor of the vestibule but is expressed in an asymmetric pattern in the aboral ectoderm. As the vestibule invaginates, HprOtx is newly expressed in the ectodermal floor of the vestibule as it develops into the neuroectoderm that is the anlage of the circum-oral central nervous system. The expression is at first in the central part of the floor, then it extends outwards to the ectoderm around the five primary podia and to the epineural folds between the podia. The epineural folds later close to form the radial nerves and the circum-oral nerve ring. In a late vestibula larva, HprOtx is expressed in the radial nerves and the nerve ring. The expression of an Otx gene in the developing echinoid central nervous system is interpreted as an instance of conserved gene expression in echinoderm development.     

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)     

How do changes in parental investment influence development in echinoid echinoderms?

SUMMARY Understanding the relationship between egg size, development time, and juvenile size is critical to explaining patterns of life-history evolution in marine invertebrates. Currently there is conflicting information about the effects of changes in egg size on the life histories of echinoid echinoderms. We sought to resolve this conflict by manipulating egg size and food level during the development of two planktotrophic echinoid echinoderms: the green sea urchin, Strongylocentrotus droebachiensis and the sand dollar, Echinarachnius parma . Based on comparative datasets, we predicted that decreasing food availability and egg size would increase development time and reduce juvenile size. To test our prediction, blastomere separations were performed in both species at the two-cell stage to reduce egg volume by 50%, producing whole- and half-size larvae that were reared to metamorphosis under high or low food levels. Upon settlement, age at metamorphosis, juvenile size, spine number, and spine length were measured. As predicted, reducing egg size and food availability significantly increased age at metamorphosis and reduced juvenile quality. Along with previous egg size manipulations in other echinoids, this study suggests that the relationship between egg size, development time, and juvenile size is strongly dependent upon the initial size of the egg.     

Rostrate pedicellariae: A morphologically distinct form of echinoid test appendage

The term rostrate was introduced by Mortensen ('07) to describe a type of pedicellaria he found in spatangoids. These pedicellariae resemble tridentate ones but have arching valves. Unlike the main categories of echinoid pedicellariae, no clear diagnosis of the rostrate form exists. This work examines the detailed morphology of the valves of rostrate pedicellariae observed by light and scanning electron microscopy and compares the shapes and dimensions of their component parts with tridentate pedicellariae. The data reveal considerable differences between the two, which warrant the recognition of rostrate pedicellariae as a distinct form. A diagnosis is given. © 1993 Wiley-Liss, Inc.     

Early activation of adult organ differentiation during delay of metamorphosis in solitary ascidians, and consequences for juvenile growth

Abstract. Many animals have the ability to delay metamorphosis when conditions are unfavorable. This strategy carries obvious benefits, but may also result in severe consequences for lecithotrophic larvae that run low on time and energy. Precocious activation of postlarval developmental programs—so-called anticipatory development—may be adaptive and increase the survival of older, energy-depleted larvae by allowing more rapid metamorphosis. Three of six solitary ascidian species displayed extensive anticipatory development of postlarval structures, similar to heterochronies normally observed for colonial species. The capacity for anticipatory development may be linked to the length of competent period, taxonomic group, or both: members of suborder Phlebobranchia exhibited extensive anticipatory development and long competent periods, but members of suborder Stolidobranchia exhibited little or no anticipatory development and had shorter competent periods. Delay of metamorphosis of up to 3 d did not negatively impact postlarval and juvenile growth rates for any of three species tested, regardless of taxonomic group or length of competent period, although a longer, 7-d, delay resulted in slower postlarval growth in Ciona intestinalis . Anticipatory development of postlarval structures may ameliorate the negative consequences of delay of metamorphosis in C. intestinalis and Ascidiella aspersa , but Molgula socialis showed neither anticipatory development nor a negative impact of metamorphic delay on postlarval fitness. This is the first demonstration that anticipatory development of postlarval structures, normally associated only with colonial ascidians, can occur as a normal part of the development of solitary ascidians.