Holothuroidea Have Spongy Bodies Homologous to Spongy Bodies of Echinoidea and Tiedemann’s Bodies of Asteroidea

The spongy body of Davidaster rubiginosa, D. discoidea, and Comactinia meridionalis, is an axial haemal plexus consisting of two structurally similar, but positionally distinct, regions: an oral circumesophageal part and an aboral part which lies lateral to the axial organ. The axial organ is a large axial blood vessel which is infiltrated by hollow cellular tubes lined with monociliated epithelial cells. The spongy body plexus is a tangle of small blood vessels overlain by podocytes and myocytes. The spongy body and the axial organ are situated in the axial coelom, which is confluent with the perivisceral coelom, the water vascular system, and the parietal canals. The parietal canals open to the exterior via ciliated tegmenal ducts and surface pores. The crinoid spongy body is morphologically similar to the axial gland of asteroids, ophiuroids, and echinoids (AOE). Although the axial glands of these three classes of echinoderms are mutually homologous structures, the homology of the crinoid spongy body and the AOE axial gland is questionable because of differences in organization and developmental origin. Alternatively, the crinoid spongy body may be homologous to asteroid gastric haemal tufts, which are podocyte‐covered blood vessels suspended in the perivisceral coelom. The functional organization of the spongy body suggests a filtration nephridium and predicts an excretory function. An alternative hypothesis is that the spongy body is a site of nutrient transfer from the blood vascular system to the perivisceral coelom.

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.

The perivisceral and ambulacral fluids ofAsterias rubenshave been shown to be not only isosmotic but isoionic with sea water, even if that medium is diluted by nearly half. However, there is a slight accumulation and regulation of calcium in the perivisceral fluid and to a much more marked extent, potassium in the water vascular system. The rapidity with which the potassium diffuses away when the ambulacral fluid is dialysed against sea water suggests that its presence in the water vascular system is due to an active accumulatory mechanism. This mechanism is capable of functioning when the animal encounters sea water of significantly reduced salinity, over the temperature range 0—20° C and extends throughout the season apparently uninfluenced by the breeding cycle or sex of the animal. Smaller animals tend to have a higher concentration of potassium in the water vascular system than do larger ones. Direct and indirect measurements of salt loss under various conditions, together with observations on the rate of lithium transfer, suggests that the integument is very permeable to many ions as well as to water. This supports the hypothesis that the ionic regulation observed results from an active process and not merely from an impermeability of the integument, although measurements with potassium itself have not yet been made.

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.

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.

Understanding the water vascular system (WVS) in early fossil echinoderms is critical to elucidating the evolution of this system in extant forms. Here we present the first report of the internal morphology of the water vascular system of a stem ophiuroid. The radial canals are internal to the arm, but protected dorsally by a plate separate to the ambulacrals. The canals zig-zag with no evidence of constrictions, corresponding to sphincters, which control pairs of tube feet in extant ophiuroids. The morphology suggests that the unpaired tube feet must have operated individually, and relied on the elasticity of the radial canals, lateral valves and tube foot musculature alone for extension and retraction. This arrangement differs radically from that in extant ophiuroids, revealing a previously unknown Palaeozoic configuration.

Echinodermata is a bilaterian phylum with a body plan that has diverged significantly from the common bilaterian plan. Echinoderms are pentaradially symmetrical and have a unique type of mesodermal skeleton that lies just under the integument. In addition, they display several more unique structures and systems, such as the water vascular system that functions in gas exchange, excretion, and locomotion, and mutable collagenous tissue that is able to change its physical properties under neuronal control and become alternately rigid or relaxed. The five classes within Echinodermata each use these unusual components differently and have different manifestations of the radial symmetry, sometimes evolving a secondary anterior–posterior axis. The five extant classes are Echinoidea (sea urchins), Asteroidea (sea stars), Ophiuroidea (brittle stars), Crinoidea (sea lilies), and Holothuroidea (sea cucumbers).

Molecular evidence for a single origin of ultrafiltration-based excretory organs

How sea urchins face microplastics: Uptake, tissue distribution and immune system response

Biological characterization of the reproductive cycle of the sea urchin (Paracentrotus lividus) in the western central region of Portugal (Peniche)

Sea urchins are emblematic models in developmental biology and display several characteristics that set them apart from other deuterostomes. To uncover the genomic cues that may underlie these specificities, we generated a chromosome-scale genome assembly for the sea urchin Paracentrotus lividus and an extensive gene expression and epigenetic profiles of its embryonic development. We found that, unlike vertebrates, sea urchins retained ancestral chromosomal linkages but underwent very fast intrachromosomal gene order mixing. We identified a burst of gene duplication in the echinoid lineage and showed that some of these expanded genes have been recruited in novel structures (water vascular system, Aristotle's lantern, and skeletogenic micromere lineage). Finally, we identified gene-regulatory modules conserved between sea urchins and chordates. Our results suggest that gene-regulatory networks controlling development can be conserved despite extensive gene order rearrangement.

The madreporite is one of the most enigmatic organs of the echinoderms. It connects the internal cavity of the water-vascular system to the external seawater through its many pores which are lined with ciliated epithelium. Its physiological function, even the nature of transport, if any, through its pores has been controversial since the 19th century. We report here that the pores of the echinoid madreporite are capable of changing their diameter in response to stimulation and that their cilia support bidirectional transport, drawing water into the water-vascular system while expelling larger particles. A reversible constriction of the pore was induced by acetylcholine (ACh). At > 10 -7 M, ACh reduced the pore diameter to about 70% in 2 mins and to 60% in 6 mins. Atropine (10 -4 M), but not d -tubocurarine (10 -4 M), blocked the response to ACh (10 -7 M). Adrenaline (10 -5 M) had no effect on the pore diameter. These results suggest that the size of the pore is under cholinergic control. Observations of isolated pore-canal tissue indicated that the changes in pore size are not accompanied by changes in volume of the cells surrounding the pores. Electron microscopy showed no muscle cells in or near the pore-canal tissue. In the apical region of the ciliated columnar epithelial cell lining the pore canal, there are fine filaments attached to the adherens junctions. This region of the cell was positively stained with rhodamine-phalloidin, suggesting that the filaments consist of F-actin. It is possible that the filaments are involved in the pore closure response. We studied the water flow through isolated single pore canals, using Indian ink, and found that the water flows inwards while particles larger than ca. 1 μM move in the opposite direction. When the pores became narrower by the action of ACh, complete blocking of the water flow was observed. It is possible that the madreporite controls the volume or the pressure of the fluid in the water-vascular system by both the ciliary-driven water flow and the pore closure response.

The 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.

While vertebrate hemoglobins typically are tetrameric and show highly regulated and cooperative ligand binding, little is known of the evolution of these properties. We are studying the structural and functional properties of the hemoglobins from Caudina arenicola, an echinoderm. The echinoderms are in the lineage most closely related to the vertebrates to express hemoglobin. C. arenicola has three sets of red cells, in the water vascular system, the coelomic cavity, and in an intestinal vein. Each of these expresses a distinct array of globins. The hemoglobins are cooperative and exhibit unusual ligand-linked associative properties, being dimeric when oxygenated and forming tetramers and higher aggregates on deoxygenation. The major coelomic hemoglobins have been subjected to a detailed examination by a combination of ligand binding analyses and protein and DNA sequencing, as well as X-ray crystallography. Two typical globin introns were identified, along with a unique intron that bisects an N-terminal extension of the globin from the remainder of the gene. X-ray crystallographic analysis shows that the subunit interfaces of C. arenicola hemoglobins differ radically from those of vertebrate hemoglobins and indeed from some other invertebrate hemoglobins, but closely resemble the packing arrangements found in a clam hemoglobin (Scapharca). However, the residues implicated in cooperativity in these two types of hemoglobins differ substantially.

Much literature in marine biology describes the extraordinary behaviour of sea urchins, e.g., Paracentrotus lividus, who cover their body with shells, stones and debris. The function of this strange behaviour, described as 'masking', is still a puzzle. Our experiment shows that sea urchins are loaded with more mussel shells when the delicate apical openings of their water vascular system which powers all their movements, are in danger of being occluded by floating sand. 'Masking' shells appear to function as an umbrella against floating particles.