Water Vascular System Research Articles

Ultrastructure of the tentacles of the apodous holothurian Leptosynapta spp (Holothurioidea: Echinodermata) with special reference to the epidermis and surface coats

The tentacles of the apodous holothurian Genus Leptosynapta have been studied by use of transmission and scanning electron microscopy. The gross anatomy, water vascular system, fibre systems and ectoneural nerve ring are described. A fuzzy coat of attenuated filaments covers the surface of the tentacle, broken only by secretory ducts. A cuticle underlies the fuzzy coat. Bacteria are common in the subcuticular space. Fixation without osmium gives poor preservation of the surface coats. The epidermis consists of a single layer of columnar cells consisting of Type-1, Type-2, support, goblet and uniciliated cells. Type-1 cells secrete electron-dense material and appear to be homologous to adhesive cells of the tentacles of other holothurians. The support cells contain large, granular vesicles not found in other holothurians. Goblet cells contain flocculent mucus and have an apical cilium. Goblet cells are not found in other holothurian tentacles and may function to lubricate and wrap adhering particles to aid their ingestion. The uniciliated cells are rare, poorly developed and the cilium does not extend past the cuticle. The ultrastructure of the tentacles is discussed in relation to those of other holothurians.

The ultrastructure of the tentacles of eleven species of dendrochirote holothurians studied with special reference to the surface coats and papillae

The tentacles of eleven species of dendrochirote holothurians have been studied. The water vascular system, deep fibre system, ectoneural nerve ring and superficial fibre system are described and are similar to those of other holothurian tentacles. A conspicuous fuzzy coat covers the entire tentacular surface except for the tips of cilia. On the basis of its structure it is thought to be an attenuated glycocalyx. Its function is discussed in relation to anti-fouling and surface adhesion. The two surface coats underlying the fuzzy coat are termed the cuticle. Bacteria are found both within the surface coats and in the sub-cuticular space. Primary fixatives lacking osmium give poor preservation of the surface coats. The adhesive papillae of the apices of the tentacles contain elements of support cells and two other cells named Type-1 and Type-2 papillar cells. The secretions of Type-1 papillar cells are dense-cored vesicles and may contain a proteinaceous adhesive. The vesicles fuse with the cuticle and release their products which are apparently disseminated along the fuzzy coat filaments. The secretions of Type-2 papillar cells may have a neurosecretory function. The different models of food capture by dendrochirote tentacles are discussed as are duo-glandular adhesive systems in relation to dendrochirote tentacles.

A new class of Echinodermata from New Zealand

During examination of echinoderms from sunken wood collected from depths between 1,057 and 1,208 m off the New Zealand coast, we discovered nine specimens of a small flattened discoidal invertebrate. These animals, briefly described here, superficially resemble a cnidarian medusa, but their pentamerous morphology is distinctly echinoderm-like. We consider that the features of these newly discovered echinoderms warrant the recognition of a new class. Xyloplax medmiformis n.gen. and n.sp. represents a radical departure in morphology from any other known extant echinoderm. Its obvious tube feet, clearly pentamerous body pattern, and calcite skeleton leave no doubt that the animal is an echinoderm. However, its concentrically arranged skeletal structures and single series of tube feet arranged in a ring are novel. The water vascular system of Xyloplax consists of a double ring of canals which service the tube feet in inter-radial positions; in all other living echinoderms the vascular ring (circum-oral) is single, and the tube feet are serviced from it in radial positions. These specimens represent the first new class of living echinoderms to be described since 1821. A full description of the new taxon is in preparation.

Paramoeba invadens n. sp. (Amoebida, Paramoebidae), a Pathogenic Amoeba from the Sea Urchin, Strongylocentrotus droebachiensis, in Eastern Canada1

ABSTRACTParamoeba invadens n. sp. is described from sea urchin tissues and from culture. Amoebae are 20–40 μm in length with an irregular hyaline region producing short subpseudopodia. One parasome per cell is present. The parasome is bipolar with basophilic, Feulgen‐positive poles and a Feulgen‐negative median segment. The fine structure of the parasome resembles those in other species of Paramoeba and the surface of the amoeba is plain with no hairs or scales. Amoebae dislodged from tissues often adopt a semifloating form; floating forms have been seen in culture. Amoebae circulate within lumina of the water vascular system and can migrate readily into or out of tissues.

The ultrastructure and formation of the calcareous ossicles in the body wall of the sea cucumber Leptosynapta clarki (Echinodermata, Holothuroida)

The calcareous ossicles of the burrowing sea cucumber Leptosynapta clarki have been examined by scanning and transmission electron microscopy. The ossicles occur in the dermis of the body wall and comprise three main types: 1) curved rods; 2) miliary granules; and 3) anchorshaped structures that are paired with oval plates. Rods average about 80 μm in length, and miliary granules are typically 20–30 μm long. Both of these ossicles appear to form a protective skeleton in regions where the water vascular system and accompanying nerves are located. Anchors and plates are scattered throughout the interambulacra of the body at densities ranging from 2–8/mm2. Each anchor measures about 145 μm long and is attached to the plate underlying it by a flexible ligament that is composed of collagen fibrils. Tetracycline labeling studies indicate that anchors and plates take several months to reach full size. All developing ossicles appear to be surrounded by a syncytial network of sclerocytes that characteristically possess numerous mitochondria and a conspicuous external lamina. Fully formed anchors lie directly beneath the epidermis and do not protrude through the outermost layer of the body wall. During burrowing, the curved flukes of the anchorshaped ossicles may provide added traction as the buccal tentacles dig through the sediment.

Histopathology of the disease causing mass mortality of sea urchins ( Strongylocentrotus droebachiensis) in Nova Scotia

The disease causing mass mortalities of Strongylocentrotus droebachiensis off Nova Scotia, Canada, from 1980 to 1983 is described. Diseased urchins were characterized by loss of preipheral muscle function in tube feet, spines, and mouth. Signs occurred primarily in the body wall and associated tissues (water vascular system, nerves, spine bases) and coelomic fluid. These symptoms were diffuse and included a general infiltration of tissues with amoebocytes. The coelomic fluid often contained reduced numbers of red and white spherule cells, and clotting was incomplete. Progressive breakdown and fragmentation of muscle cells in tube feet and spine bases resulted in destruction of coherent muscle layers and their replacement by numerous spindle-shaped fibrillar muscle remnants. Coelomic lining cells in the tube feet sloughed off into the lumen, but remained in clumps and phagocytosed muscle remnants.

THE EARLY RADIATION AND PHYLOGENY OF ECHINODERMS

THE EARLY RADIATION AND PHYLOGENY OF ECHINODERMS

A Re-Appraisal of the Fluid Loss Resulting from the Operation of the Water Vascular System of the Starfish,Asterias Rubens

In 1955, Kerkut recorded the spontaneous activities of a tube foot ofAsterias rubensL. Such activity consisted of cyclical protraction and retraction phases, one complete cycle lasting on average some 6·5 min (S.D. 2·5). The duration of the protraction phase was on average 4·3 min. It is reasonable to assume that under the conditions of this experiment a similar level of hydrostatic pressure was attained as was found by Binyon (1964), namely 30 cm water. There then followed a fairly rapid retraction of the podium and a quiescent period of some 2·2 min duration. An individual podium would thus execute some 9·2 cycles/h.

TRANSLOCATIVE FUNCTIONS OF THE ENIGMATIC ORGANS OF STARFISH—THE AXIAL ORGAN, HEMAL VESSELS, TIEDEMANN'S BODIES, AND RECTAL CAECA: AN AUTORADIOGRAPHIC STUDY

Starfish (Echinaster graminicolus) were fed C14-labeled clams or liquid glucoseamino acid medium, and subsequently examined as autoradiographic sections. Ingested tracer was first incorporated into the cardiac stomach and lower-middle digestive gland. By 8 to 12 hours, increasing amounts were located throughout the axial organ, aboral hemal ring, radial hemal strand, and mesenteric hemal vessel. Subsequently (22 to 48 hours), label progressed into the genital hemal connectives and the gonads, as well as the connective tissue-hemal plexus of the tube feet. C1early, hemal tissues, together with associated perihemal spaces, play major roles in the translocation of nutritive materials to the gonads and parts of the tube feet. These nutritive materials, however, may not come directly from the digestive system, which only has poorly developed hemal connections, but from coelomic sources.Also noted was a gradual build-up of labeled material in the rectal caeca, suggesting that these organs remove nutrients from circulating digestive fluid before its evacuation through the anus or mouth. In animals fed liquid medium, tracer rapidly appeared in all parts of the water vascular system. showing that inflow of sea water does occur through the madreporite. Intense, rapid uptake by the Tiedemann's bodies revealed that they must filter a portion of this inflow, possibly producing coelomic fluid for body turgor and stomach inflation.

Distribution of Sodium, Potassium and Chloride in the Ophiuroid,Ophiocomina Nigra(Abildgaard)

Echinoderms are not generally considered to experience significant dilution of the surrounding medium in their usual marine habitats and so it is to be expected that, due to the absence of any obvious excretory organ, the ionic composition of the body fluid would be similar to that of the ambient sea water. In general this has been confirmed by previous workers for many species of echinoderm. Binyon (1966) lists the results of workers who have determined osmotic pressure and/or ionic concentrations in the perivisceral and/or ambulacral fluids of many echinoderms. With reference to the perivisceral fluid in asteroids there is usually a small excess of potassium (9–16%) which is maintained in the more euryhaline species and in those acclimatized to reduced salinity. Generally, the chloride content is similar to, or a little higher than, that in the surrounding sea water. In echinoids there is little difference in the potassium content and chloride concentrations with respect to sea water though there may be a small increase in the former ion. In the holothurians, there is little regulation of potassium ions. Such concentrations which have been determined for the chloride ion in this group are found to be a little higher than in sea water. The sodium concentration in the perivisceral fluids of echinoderms tends to be similar to, or slightly lower than, that in the surrounding sea water. However in the ambulacral fluids of the water vascular system it has been shown that the potassium concentration is 20–90% higher in some species of asteroids and echinoids (Robertson, 1949; Binyon, 1962,1966). The fluids of the water vascular system of echinoderms, analysed to date, have concentrations of sodium ions lower than in the ambient sea water. From the available evidence, there appears to be little difference between environmental and ambulacral chloride concentrations.

The basiepithelial nerve plexus of the viscera and coelom of eleutherozoan Echinodermata.

The organisation of the basiepithelial nerve plexus in the alimentary canal of a starfish and the water vascular system of a sea-urchin is described. The plexus contains varicose aminergic neurones which terminate adjacent to the ciliated epithelial cells. It is proposed that the basiepithelial plexus innervates these cells and controls ciliary beating. The distribution of the basiepithelial plexus in various tissues described by other workers is discussed particularly in relation to whether it is the coelomic epithelium or the luminal epithelium which is innervated. It is concluded that where there is both an endothelium and a coelomic epithelium only one is innervated. The muscles, where present, of the viscera are innervated by a separate nervous system. The muscles are always on the opposite side of the non-cellular connective tissue sheath to the basiepithelial plexus.

Deposit-feeding mechanisms and resource partitioning in tropical holothurians

Aspidochirote holothurians found on tropical reef flats feed on particulate deposits which form a variety of substrata. The synaptid holothurian Opheodesoma grisea (Semper) feeds in a similar manner by scraping deposits from the surfaces of sea grasses. Distributional and gut content analyses showed that species partitioning is on the basis of substratum and particle size preference. Scanning electron microscopy revealed that the tentacles of aspidochirotes have a nodular surface while those of O. grisea have a tessellated surface structure. The twelve different species examined were shown to have different tentacular surface textures which bore an apparent relationship with the mean particle sizes selected by the different species. Light microscope studies of tentacle sections confirmed earlier observations on the extent of the water vascular system in aspidochirote and pinnate tentacles. From these observations a functional interpretation is proposed for tentacular operation and the means of particle selection in such holothurians.

Fine structure of the axial complex of Sphaerechinus granularis (Lam.) (Echinodermata: Echinoidea).

Three regions of the axial complex in Sphaerechinus granularis can be distinguished: 1) The axial organ which protrudes from one side of the axial sinus; the sinus septum which separates the sinus from the body cavity and encloses the stone canal; the pulsating vessel which runs along the inside of the axial organ. 2) The blindly-ending terminal sinus in which the pulsating vessel broadens out to the contractile terminal process. 3) The ampulla of the stone canal which connects the axocoel and water vascular system and which opens out through the madreporite. A single-layered, monociliated coelomic epithelium surrounds all regions of the axial complex. This epithelium contains smooth muscle cells at the contractile areas. Canaliculi, surrounded by basal lamina, are formed through infolding of epithelia; they end blindly in the fluid- and connective tissue-matrix of the inner structures. The lacunae of the dorso-ventral mesentery connect the periesophageal and the perianal haemal ring with the axial organ. The axial organ contains many coelomocytes rich in pigment and granules. These coelomocytes are separated into compartments by elastic fibres. Phagocytosis of whole cells and transformational stages of coelomocytes suggest storage and degradation functions. An excretory function via the water vascular system is also suggested.

Solute Secretion by the Tube Foot Epithelium in the Starfish Asterias Forbesi

ABSTRACT The K+ and Cl− levels and osmotic concentration of the tube feet fluid in intact starfish (Asterias forbesi) are higher than that in the external sea water or perivisceral (coelomic) fluid. The increase in K+ in the fluid in the tube feet lumen is brought about by the active secretion of K+ by the tube feet epithelium. No exchange of material from the external environment with the more distal portions of the water vascular system across the madreporite was observed in this study. It is suggested that the fluid volume in the tube feet is maintained by active K+ secretion, by the tube feet epithelium, which creates an osmotic driving force for water into the lumen of the tube feet.

Maintenance of fluid volume in the starfish water vascular system.

LOCOMOTION in the starfish Asterias forbesi involves many tube feet, each functioning independently as a hydrostatic skeleton; the circular muscles of the ampulla acting antagonistically to the longitudinal muscles of the tube foot itself through the constant volume of fluid contained in the ampulla–foot unit1. The fluid for each tube foot comes from the water vascular system to which each foot is connected through its own lateral canal. The water vascular system in Asterias consists of three interconnecting series of canals: (1) radial canals running the length of each arm; (2) a circular canal running around the gut at the base of the arms, and (3) the stone canal which runs from the radial canal up to the aboral (“dorsal”) surface, terminating in the madreporite. The madreporite, an orange disk, is porous and associated with several sets of cilia. For some time it has been presumed, and is still presented or indicated in some textbooks, that the fluid contained within the starfish water vascular system is pumped by ciliary activity through the madreporite into the canal system, although it has been pointed out that no experimental evidence supports this assumption2. lonically, the fluid within the water vascular system is nearly identical to the external seawater with the exception of internal K+, which is present in a concentration up to 60% higher than that of the seawater3,4. On the basis of this difference, it was suggested that K+ accumulation by the water vascular system is responsible for water uptake by this system, either by creating a slight osmotic gradient within the tube feet or by direct movement of water with hydrated K+ ions, as opposed to direct uptake of seawater through the madreporite5. We have investigated the generation of the fluid in the water vascular system more thoroughly by determining the osmotic and ionic characteristics of the fluid within the tube feet and the ionic transport characteristics of the isolated tube foot epithelium.

Limitations on Diversity of Forms: Branching of Ambulacral Systems of Echinoderms

In most well-skeletalized echinoderms of radial form, extension and branching of ambulacra occurs at or near the ends of ambulacra farthest from the mouth. In most echinoderms of compact body form, aboral coverage of ambulacra cannot be combined with the more even coverage of ambulacra which is provided by branching at the widest part of the animal. There are three ways in which echinoderms of compact body form can circumvent this limitation by a change in growth pattern: (1) freeing the water vascular system from a one-to-one correspondence with the ambulacral skeletal system, (2) adding side branches along older parts of the ambulacram, or (3) possessing separate oral and aboral ambulacral systems. The first has occurred in clypeasteroids (sand dollars) and some cystoids. The second may have occurred in some cystoids, in which ambulacral skeletal plates can slide relative to the underlying plates which support and encase the body, but has not evolved through more elaborate control of differential resorption and growth of interambulacral skeletal plates. The third appears to provide the greatest versatility with the simplest growth pattern and functional unity of skeletal and water-vascular systems, but occurs in no echinoderm, perhaps because the alteration in early development which would produce such a form has a lethal side effect (two oral sides). Thus there appears to have been no evolutionary route to this adaptive peak. Stellate forms and compact forms without aboral coverage both have the potential advantage of terminal branching of ambulacra, but stellate forms also (1) can change ambulacral distribution simply by moving the arms; (2) can alter ambulacral distribution by intercalation of arms, subterminal branching, or casting off of arms; and (3) can minimize the disadvantages associated with restriction of the ambulacral system to the oral side by reduced arm width or by bending the arms. Differences in long-term success among classes which probably had podia and lacked brachioles are consistent with the assumptions that aboral coverage by ambulacra and versatility in distributing ambulacra have both been of general adaptive importance. Blastoids were less versatile in distributing ambulacra than the cystoids; yet blastoids appeared later and became extinct later. This provides an apparent exception to Vermeij's hypothesis on increase in potential morphological versatility.

Tube-Foot Ultrastructure of an Ophiuroid Echinoderm

The tube-feet of asteroids and echinoids, highly extensible tubular structures expanded distally into suckers, are important in gas exchange, locomotion, feeding and sense-reception. Their ultrastructure has been thoroughly examined. The less extensible, suckerless tube feet of ophiuroids have been shown to serve the same functions, but their ultrastructure has not been examined. I report here on ultrastructural features of the tube-feet of the ophiuroid Hemipholis elongata.The wall of the tube-foot is continuous with the water vascular system. The lumen of the tube-foot contains the same spherical hemoglobin-containing cells found elsewhere in the water vascular system. The innermost layer of the wall is composed of flagellated, choanocyte-like cells which form cytoplasmic extensions into the cavity of the tube-foot (Figure 3). Attachment of these cytoplasmic extensions to the hemoglobin-containing cells has been observed. Septate desmosome cell junctions are frequently seen in this inner epithelial layer and less frequently in the surface epithelial layer.

Regeneration in the sea cucumber Leptosynapta. II. The regenerative capacity.

AbstractThe dependence of survival and regeneration in the holothuroid Leptosynapta crassipatina (Apoda) on the oral complex—the mouth, the nerve ring, the calcareous ring, and the water vascular system—was investigated. The effect of surgical deletion of parts on survival and regeneration was determined. In isolated sections of the oral complex, the disk including the nerve ring and calcareous ring was able to survive and regenerate. Even small fractions of this disk began regeneration. Regeneration capacity and survival capacity were tested independently by deletion of parts from half animals. Regardless of the deletion, normal morphallactic posteriad regeneration occurred in animals surviving beyond five days. This was true even when the entire oral complex was deleted. Thus, no part of the oral complex is necessary for regeneration of posterior parts.Survival of a high percentage of the population was restricted to populations lacking a coelomic cavity (i. e., isolated pieces of the oral complex), or to those with a complete water vascular system. Simple section of the connection between the tentacular ampullae and the water ring leads to death in all animals with a coelomic cavity. A role for these parts in oxygenation of the coelomic fluid is suggested.Finally, the ability of the organism to regenerate deleted parts was tested. Missing parts posterior to the oral complex can be regenerated. In addition, lateral parts of the oral complex and the tentacles can be regenerated. Regeneration away from the disk containing the calcareous ring and the nerve ring always occurs. Regeneration towards this disk never occurs.

The lantern of Echinus esculentus (L.) III . The fine structure of the lantern retractor muscle and its innervation

The lantern retractor muscles are attached to the calcareous pyramid structures by a diffuse connective tissue into which the proximal ends of the muscle fibres are inserted. The lantern musculature is invested by a compartment of the water vascular system, and the limiting layer is a thin sheet of ciliated cubical epithelium. The muscle fibres are about 8 to 12 μm in diameter and several millimetres in length. They contain many short myofilaments arranged longitudinally in the cell; these are 1 to 2 μm in length and 45 nm in diameter. The muscle cells show much folding of the surfaces, and there is interdigitation of the surfaces of two contiguous cells. The finger-like projections are often bulbous at the end, containing mitochondria and glycogen droplets. The attachment area of the muscles also contains many nerve fibres. These arise at the hyponeural ganglion, and pass in tracts up to the muscles. The fibres fall into two populations, one (85%) of presumed motor fibres being about 0.8 μm in diameter, the minority (15%) reaching only 0.25 μm. Each muscle cell is innervated by a single neuron at the proximal end. The neuromuscular junction is a short dilation of the nerve wrapped in wing-like process of the surface of the muscle cell. In this area the nerve terminal contains small synaptic-type vesicles, and the muscle cell is possessed of many mitochondria.

On the Mode of Functioning of the Water Vascular System ofAsterias RubensL.

SUMMARYExperiments reported in a previous paper have indicated a considerable water loss by isolated preparations of the tube feet ofAsterias rubenswhen ligatured in the protracted state. Because of its specialized structure, the podial diameter remains constant throughout the stepping cycle and this simplifies the procedure for measuring the surface area. Measurements of the internal hydrostatic pressure within the tube foot ampulla systems shows a maximum of about 30 cm water to be reached during the protraction phase of the stepping cycle, falling to nearly zero during the resting period. From these data it is possible to calculate a value for the permeability to water of the tube foot membrane, and such a value is compared with other membranes.By measurement of the total area of protracted tube foot membrane exposed by the animal when supporting itself in sea water, it is possible to arrive at a figure for the total water loss by the water vascular system. For a 50 g animal this amounts to some 0·5 ml./h. Experiments involving blocking or extirpating the madreporite, and with isolated arms, make it extremely improbable that this loss could be made good from this route.As alternative mechanisms, a slight hypertonicity due to the presence of excess potassium, or water movement as a direct consequence of the operation of an ion pump are discussed. In view of the hydrostatic pressure within the water vascular system the possibility of its functioning as a physiological kidney is also discussed. A possible alternative function for the madreporite is put forward.