Walking versus breathing: mechanical differentiation of sea urchin podia corresponds to functional specialization

The podia of sea urchins function in locomotion, adhesion, feeding, and respiration; but different podia on a single urchin are often specialized to one or more of these tasks. We examined the morphology and material properties of podia of the green sea urchin, Strongylocentrotus droebachiensis, to determine whether, despite apparent similarities, they achieve functional specialization along the oral-aboral axis through the differentiation of distinct mechanical properties. We found that oral podia, which are used primarily for locomotion and adhesion, are stronger and thicker than aboral podia, which are used primarily for capturing drift material and as a respiratory surface. The functional role of ambital podia is more ambiguous; however, they are longer and are extended at a lower strain rate than other podial types. They are also stronger and stiffer than aboral podia. In addition, all podia become stronger and stiffer when extended at faster strain rates, in some cases by nearly an order of magnitude for an order of magnitude change in strain rate. This strain-rate dependence implies that resistance to rapid loading such as that imposed by waves is high compared to resistance to slower, self-imposed loads. Thus, the serially arranged podia of S. droebachiensis are functionally specialized along an oral-aboral axis by differences in their morphology and mechanical properties.     

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.     

A comparative study of the life style of two Jurassic irregular echinoids

The irregular echinoids Plesiechinus ornatus (Buckman) (Pygasteroida) and Galeropygus agariciformis (Forbes) (Cassiduloida) occur together in beds of the murchisonoe Zone, Bajocian, outcropping in the Cheltenham region of Gloucestershire. These species were largely restricted to different lithofacies within the carbonate shelf environment. Both adopted a hidden mode of life but achieved this by different techniques. Plesiechinus had fairly short spines and strongly muscular podia over the whole corona and was able to cover itself with coarse substrate particles. The oral tubercles are bilaterally symmetrical and are radially arranged. The oral spines are thought to have pulled sediment out from beneath the test, excavating a small depression for it. Galeropygus bore a dense covering of very small spines and its tube feet were differentiated into aboral respiratory podia and oral suckered podia. It had a preferred anterior direction of locomotion and is thought to have buried itself completely by excavating and ploughing into the substrate as it moved forward. Plesiechinus fed using only its lantern and postulated peristomial tube feet, whereas Galeropygus was a continugus sediment swallower and used its phyllode tube feet and peristomal lip spines in transferring particles towards the mouth.     

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.     

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)     

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.     

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.     

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.     

Host-parasite associations of Eimeria spp. (Apicomplexa:Eimeriidae) in kangaroos and wallabies of the genus Macropus (Marsupialia:Macropodidae)

Faecal samples from 514 kangaroos and wallabies representing 12 species of the genus Macropus were examined for oocysts of Eimeria spp. Six species of Eimeria were redescribed from their type hosts, and on the basis of finding homologous oocysts in the faeces of other Macropus spp., host ranges for these coccidia were extended. Eimeria hestermani Mykytowycz, 1964 is redescribed from M. giganteus (eastern grey kangaroo) and is described from M. fuliginosus (western grey kangaroo), M. rufogriseus (red-necked wallaby), M. dorsalis (black-striped wallaby), and M. eugenii (tammar wallaby). E. toganmainensis Mykytowycz, 1964 is redescribed from M. rufus (red kangaroo) and the host range is extended to M. giganteus, M. fuliginosus, M. rufogriseus and M. eugenii. E. wilcanniensis Mykytowycz, 1964 is redescribed from M. rufus, and the host range is extended to M. giganteus, M. fuliginosus and M. robustus (euro or wallaroo). E. macropodis Wenyon & Scott, 1925 is redescribed from M. rufogriseus, and is described from M. giganteus, M. fuliginosus, M. rufus, M. irma (western brush wallaby), M. parryi (whip-tailed wallaby), M. dorsalis, M. eugenii, and M. parma (parma wallaby). E. fausti Yakimoff & Matschoulsky, 1936, E. cunnamullensis Mykytowycz, 1964 and E. purchasei Mykytowycz, 1964 are synonymized with E. macropodis. E. marsupialium Yakimoff & Matschoulsky, 1936 is redescribed from M. giganteus, and from M. fuliginosus. E. gungahlinensis Mykytowycz, 1964 is redescribed from M. fuliginosus, and from M. giganteus. Seven new species of Eimeria are described. E. flindersi, new species, is described from M. eugenii, M. rufogriseus, and M. antilopinus (antilopine wallaroo). E. prionotemni, new species, is described from M. eugenii, M. parryi, M. rufogriseus, M. agilis (agile wallaby) and M. dorsalis. E. mykytowyczi, new species, is described from M. agilis, M. antilopinus, and M. parryi. E. parryi, new species, is described from M. parryi. E. yathongensis, new species, is described from M. fuliginosus and M. giganteus. E. parma, new species, is described from M. parma, and E. desmaresti, new species, is described from M. rufogriseus. E. kogoni Mykytowycz, 1964, and E. rufusi Prasad, 1960 are considered species inquirendae. The host-parasite associations of these coccidia, and of similar species of Eimeria in other genera of Macropodoid marsupials, are discussed in relation to the postulated phylogeny of the hosts.     

Non-junctional modulation of neurogenic twitches of the guinea-pig ileum by some peptides and other compounds in the triple bath

The effects of some neuropeptide transmitter candidates and of some other neurotoxins or drugs on conduction of neural excitation were studied in myenteric plexus-longitudinal muscle strips from the guinea-pig ileum. A preparation in a special triple bath was drawn through two rubber membranes dividing the strip into three segments. Neurogenic stimulation of the oral segment set up nerve action potentials propagating aborally across the middle segment so that the aboral segment might also be invaded. Drugs were added to the middle segment to affect neuronal propagation (non-junctional effects) which was monitored by twitch amplitude of the aboral segment. The application of bradykinin and cromakalim did not affect aboral twitches although strong contractile and relaxatory effects were observed when the drugs were applied directly to the aboral segment; no neurogenic effects thus manifested. Capsaicin and neurotensin, when applied both to the middle and aboral segments, elevated the tone of the preparations accompanied with a decrease in twitch amplitude; these effects may have been due to neurogenic stimulation and release of other motor neurotransmitters. The application of VIP, apamin and dendrotoxin to the middle as well as to the aboral segments augmented aboral twitches, which might be at least partly due to facilitation of nerve action potential propagation in nerve terminals of cholinergic motor fibres.     

The feeding activity of Echinostrephus molaris (de Blainville) in the central red sea

Echinostrephus molaris (de Blainville) is a small Indo‐pacific echinoid which burrows in coral reef limestone. Normally individuals do not leave the burrows so they cannot graze on algae growing around the burrow mouth. E. molaris catches floating algal particles with its long aboral spines. When a particle touches one of these organs or a tube‐foot in the area, the surrounding spines converge and grip it. Captured fragments are lowered to the test by further tube‐foot and spine action and are then passed across the ambitus towards the mouth. They are held by the oral tube‐feet and the shorter curved oral spines which aid ingestion. The behavioural and structural modifications shown for this habit are discussed. Burrowing and particle collecting have allowed E. molaris to occupy a particular niche on the reef. A similar method of food gathering is reported for Echinometra mathaei (de Blainville).     

Early development of coelomic structures in an echinoderm larva and a similarity with coelomic structures in a chordate embryo

Early coelomic development in the abbreviated development of the sea urchin Holopneustes purpurescens is described and then used in a comparison with coelomic development in chordate embryos to support homology between a single arm of the five-armed radial body plan of an echinoderm and the single bilateral axis of a chordate. The homology depends on a positional similarity between the origin of the hydrocoele in echinoderm development and the origin of the notochord in chordate development, and a positional similarity between the respective origins of the coelomic mesoderm and chordate mesoderm in echinoderm and chordate development. The hydrocoele is homologous with the notochord and the secondary podia are homologous with the somites. The homology between a single echinoderm arm and the chordate axis becomes clear when the aboral to oral growth from the archenteron in the echinoderm larva is turned anteriorly, more in line with the anterior–posterior axis of the early zygote. A dorsoventral axis inversion in chordates is not required in the proposed homology.     

水螅的反口孔结构和功能

在２３０个水螅（Ｐｅｌｍａｔｏｈｙｄｒａｐｓｅｕｄｏｌｉｇａｃｔｉｓ）中观察到３８例反口孔排物过程。反口孔静止时,基盘附着层外表中心见不到孔迹。反口孔开启时,有废物或气体从孔内排出。连续切片检查,发现反口孔排物前,基盘腔内堆积有食泡残骸等许多废物,孔周死亡细胞和断裂废弃的中胶层在反口孔开启时脱落。非排物期,基盘附着层中心处缺少中胶层。根据观察结果,对反口孔的结构特点及其生理功能作了讨论。     

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.     

Expression of Hox4 during development of the pentamerous juvenile sea star, Parvulastra exigua

Expression of Hox4 during development of the bilateral larva and pentameral juvenile sea star was investigated in Parvulastra exigua. The role of Hox4, possibly the anterior-most gene in the echinoderm Hox cluster, in the formation of the echinoderm adult body plan has not been examined previously. In the larva of P. exigua, PeHox4 is expressed in the developing coeloms—the anterior and the right and left coeloms that generate the aboral and oral coeloms of the juvenile. At the rudiment stage, PeHox4 was expressed in the five primary lobes of the hydrocoel that give rise to primary podia, the foundation of the adult body plan. This suggests a role for this gene in the development of the echinoderm body plan. In contrast to other bilaterians, Hox4 was not expressed in the developing asteroid central nervous system.     

The podial pit — a new structure in the echinoidDiadema antillarum Philippi

Summary A glandular pit with an orally directed opening has been revealed at the base of the aboral podia, in the precise region known to be the most photosensitive area of the skin. The pit is associated with a thickening of the superficial nerve felt to form the podial organ. The fine structure is unremarkable aside from the large number of fine nerves housed in spaces between the cells of the lining epithelium. The underlying nerve felt is composed of densely packed varicose axons with little evidence of synapses and no indication of sub-cellular photosensory specialization. It is tentatively suggested that the remarkably superficial location of the fine nerve fibres may be correlated with the relatively high photosensitivity of this region.One of us (N. Millott) is greatly indebted to the Science Research Council for grants providing most of the equipment used. We are also grateful to the Zoological Society of London, especially to Dr.H. G. Vevers. Our thanks are also due to Messrs.R. L. Jones andZ. Podhorodecki for assistance.     

Distal cis-acting elements restrict expression of the CyIIIb actin gene in the aboral ectoderm of the sea urchin embryo

The distal region of the S. purpuratus actin CyIIIb gene, between −400 and −1400 nucleotides, contains at least three distinct cis-acting elements (C1R, C1L and E1) which are necessary for correct expression of fusion reporter genes in transgenic sea urchin embryos. The contribution of these elements in the temporal and spatial regulation of the gene was analyzed by single and double site-directed mutagenesis in fusion constructs which carry the bacterial chloramphenicol acetyl transferase (CAT) gene as a reporter. Following microinjection of the transgenes in sea urchin embryos, the activity of the mutants was compared to the wild type in time and space by measuring CAT activity at the blastula and pluteus embryonic stages and by in situ hybridization to the CAT mRNA at pluteus stage. Our results indicate that E1 involved in the temporal regulation of CyIIIb and that all three elements are necessary and sufficient to confer aboral (dorsal) ectoderm specificity to the proximal promoter. This is achieved by suppressing the promoter's activity in all other tissues by the cooperative interaction of the cis-acting elements. The C1R element, binding site of the nuclear receptors SpCOUP-TF and SpSHR2, is by itself sufficient to restrict expression in the ectoderm, whereas the aboral ectoderm restricted expression requires in addition the presence of both C1L adn E1. It is therefore evident, that the actin CyIIIb gene is exclusively expressed in the aboral ectoderm by a combinatorial repression in all other cell lineages of the developing embryo.     

The effects of three-dimensional cell culture on single myoblasts

Identification of cellular and morphological changes in myoblasts during three-dimensional (3D) culture may provide novel insight into skeletal muscle morphogenesis. One particular morphological change that occurs during the transition from monolayer culture to the 3D environment is the appearance of cytoplasmic projections (podia). The purpose of these studies was to determine if: (1) 3D culture increased podia formation in single cells, and (2) podia were F-actin dependent. C2C12 cells were grown in 3D conditions using a rotary cell culture system (RCCS) for 3, 6, and 9 h, fixed, and stained. Analysis of confocal images revealed that podia were significantly more numerous on RCCS cultured cells than those on suspension controls. Further, the podia of RCCS cultured cells decreased in number and increased in length during the time intervals examined. RCCS cultured cells showed no significant changes in viability, Annexin V staining, and activated Caspase 3 expression over time. In contrast, significant decreases in viability of suspension controls occurred. The application of 2 μM Latrunculin A (Lat A), an actin depolymerizing agent, significantly reduced the number of cells with podia. The number of cells with podia recovered with Lat A removal. Changes in viability and apoptosis markers were not significant during Lat A application or washout experiments. These observations reveal that: (1) culture conditions in the RCCS increase the quantity of podia formation; (2) these podia increase in length with time; and (3) F-actin plays a role in podia formation.     

From competent larva to exotrophic juvenile: a morphofunctional study of the perimetamorphic period of Paracentrotus lividus (Echinodermata, Echinoida)

 The perimetamorphic period in Paracentrotus lividus lasts for 8–12 days. It starts from the acquisition of larval competence, includes the change in form (metamorphosis) and the endotrophic postlarval life, and stops with the appearance of the exotrophic juvenile. All major postlarval appendages already occur in competent larvae being either grouped into the echinoid rudiment (terminal plates, early spines and primary podia) or scattered within the larval integument (genital plates and sessile pedicellariae). Competent larvae show particular behaviour which brings them close to the substratum. The latter is tested by primary podia protruding through the vestibular aperture of the larva. Primary podia are sensory–secretory appendages that are deprived ampullae. They are able to adhere to the substratum in order to allow evagination of the echinoid rudiment (i.e. metamorphosis) and substatum adhesion of the postlarva. Particular spines are borne by the postlarva; these are multifid non-mobile appendages forming a kind of protective armour. Like those of the larva, all characteristic structures of the postlarva (primary podia, multified spines and sessile pedicellariae) are transitory and regress either at the end of postlarval life (primary podia) or during early juvenile life (multifid spines and sessile pedicellariae). Other appendages that develop during postlarval life (i.e. podia with ampulla, point-tipped spines and sphaeridiae) are similar to those borne by the adults and become functional when the individual enters its juvenile life. Thus, the perimetamorphic period appears to be a fully fledged period in the life-cycle of P. lividus, and presumably in the life-cycle of any other sea-urchin species. Accepted: 7 October 1997