G. O. Mackie

The biology of glass sponges.

As the most ancient extant metazoans, glass sponges (Hexactinellida) have attracted recent attention in the areas of molecular evolution and the evolution of conduction systems but they are also interesting because of their unique histology: the greater part of their soft tissue consists of a single, multinucleate syncytium that ramifies throughout the sponge. This trabecular syncytium serves both for transport and as a pathway for propagation of action potentials that trigger flagellar arrests in the flagellated chambers. The present chapter is the first comprehensive modern account of this group and covers work going back to the earliest work dealing with taxonomy, gross morphology and histology as well as dealing with more recent studies. The structure of cellular and syncytial tissues and the formation of specialised intercellular junctions are described. Experimental work on reaggregation of dissociated tissues is also covered, a process during which histocompatibility, fusion and syncytialisation have been investigated, and where the role of the cytoskeleton in tissue architecture and transport processes has been studied in depth. The siliceous skeleton is given special attention, with an account of discrete spicules and fused silica networks, their diversity and distribution, their importance as taxonomic features and the process of silication. Studies on particle capture, transport of internalised food objects and disposal of indigestible wastes are reviewed, along with production and control of the feeding current. The electrophysiology of the conduction system coordinating flagellar arrests is described. The review covers salient features of hexactinellid ecology, including an account of habitats, distribution, abundance, growth, seasonal regression, predation, mortality, regeneration, recruitment and symbiotic associations with other organisms. Work on the recently discovered hexactinellid reefs of Canadas western continental shelf, analogues of long-extinct Jurassic sponge reefs, is given special attention. Reproductive biology is another area that has benefited from recent investigations. Seasonality, gametogenesis, embryogenesis, differentiation and larval biology are now understood in broad outline, at least for some species. The process whereby the cellular early larva becomes syncytial is described. A final section deals with the classification of recent and fossil glass sponges, phylogenetic relationships within the Hexactinellida and the phylogenetic position of the group within the Porifera. Palaeontological aspects are covered in so far as they are relevant to these topics.

NERVOUS CONTROL OF CILIARY ACTIVITY IN GASTROPOD LARVAE

1. The locomotory cilia of Mangelia and Pneumoderma larvae undergo arrests spontaneously and in response to tactile stimulation. These events are often associated with muscular contractions in an overall response thought to be protective in nature.2. Isolation of the ciliated bands from the central nervous system abolishes the ability for coordinated ciliary arrests and the cilia show continuous metachronal beating.3. Recordings with suction electrodes attached to the surface show patterns of electrical signals during periods of ciliary arrest. Intracellular recordings with glass microelectrodes from single ciliated cells in Pneumoderma show rapidly rising, slowly decaying, all or none 50 mV spikes when the cilia undergo arrest. There are no fluctuations in membrane potential during metachronal beating.4. The existence of a rich motor innervation supplying the ciliated epithelium in Mangelia has been established using optical and electron microscopy. The nerve endings appear to derive from neurons whose c...

Branchial innervation and ciliary control in the ascidian Corella.

The cilia lining the stigmata of the branchial sac of an ascidian circulate water through the animal. These stigmatal cilia are under nervous control; when either siphon is stimulated, both siphons close by muscular contractions and at the same time the stigmatal cilia stop beating simultaneously in all parts of the branchial sac. Spontaneous ciliary arrests may also occur, with or without associated closure of the siphons. Elements of the branchial nervous system that run in the gill bars are assumed to be concerned in coordination of the ciliary arrests. The majority of the branchial nerve fibres emerge dorsally from the visceral nerves that form the posterior brain roots, although nerves are also believed to enter the branchial sac along its anterior margin. No cell bodies could be found in the branchial nerves or in the visceral nerves, so that the cell bodies of the branchial nerve fibres are assumed to lie in the central nervous system. The branchial nerve fibres form a peripheral conducting net extending throughout the branchial sac. Branches of these nerve fibres terminate in contact with some of the ciliated cells; cell-to-cell conduction (through close junctions?) probably spreads excitation to the other ciliated cells. Nerve-nerve junctions appear to be more sensitive to curare than those between nerves and ciliated cells. Electrical recordings from the branchial sac, obtained with suction electrodes, show that arrest of the cilia is accompanied by electrical activity, and that prolonged arrest is maintained by trains of regular pulses. Intracellular microelectrodes in the ciliated cells indicate that these cells have a negative resting potential of 30-40 mV, and that a ciliary arrest is associated with a positive-going spike of 45-50 mV. The externally recorded ‘ciliary arrest potentials’ probably represent the coordinated depolarization of many ciliated cells. The rhythmical character of the trains of pulses presumably depends on pacemaker activity; this is not localized, since intact organisms or isolated small portions of the branchial sac are capable of generating similar trains of pulses. During the arrest response the stigmatal cilia first perform a reverse beat, then maintain the reverse position for several seconds before slowly relaxing and after several more seconds recommencing to beat with progressively increasing amplitude. The duration of the arrest response varies in media with different concentrations of the common cations, and also varies in response to repetitive stimulation, in a manner which suggests that the depolarization of the ciliated cells is associated with an influx of Ca2+, so that the ciliary control here may have some close parallels with that described for Paramecium.

Central Neural Circuitry in the Jellyfish Aglantha

Like other hydrozoan medusae, Aglantha lacks a brain, but the two marginal nerve rings function together as a central nervous system. Twelve neuronal and two excitable epithelial conduction systems are described and their interactions summarized. Aglantha differs from most medusae in having giant axons. It can swim and contract its tentacles in two distinct ways (escape and slow). Escape responses are mediated primarily by giant axons but conventional interneurons are also involved in transmission of information within the nerve rings during one form of escape behavior. Surprisingly, giant axons provide the motor pathway to the swim muscles in both escape and slow swimming. This is possible because these axons can conduct calcium spikes as well as sodium spikes and do so on an either/or basis without overlap. The synaptic and ionic bases for these responses are reviewed. During feeding, the manubrium performs highly accurate flexions to points at the margin. At the same time, the oral lips flare open. The directional flexions are conducted by FMRFamide immunoreactive nerves, the lip flaring by an excitable epithelium lining the radial canals. Inhibition of swimming during feeding is due to impulses propagated centrifugally in the same epithelium. Aglantha probably evolved from an ancestor possessing a relatively simple wiring plan, as seen in other hydromedusae. Acquisition of giant axons resulted in considerable modification of this basic plan, and required novel solutions to the problems of integrating escape with non-escape circuitry.

APPARENT ABSENCE OF GAP JUNCTIONS IN TWO CLASSES OF CNIDARIA

Study of the literature and new observations by electron microscopy suggest that gap junctions are absent in the anthozoa and scyphozoa, but present in the hydrozoa. While this may help to explain the marked electrophysiological differences known to exist between the hydrozoa and the other two groups, it raises questions about how intercellular metabolic communication is achieved in the groups lacking gap junctions.

Nitric oxide regulates swimming in the jellyfish Aglantha digitale.

The cnidarian nervous system is considered by many to represent neuronal organization in its earliest and simplest form. Here we demonstrate, for the first time in the Cnidaria, the neuronal localization of nitric oxide synthase (NOS) in the hydromedusa Aglantha digitale (Trachylina). Expression of specific, fixative‐resistant NADPH‐diaphorase (NADPH‐d) activity, characteristic of NOS, was observed in neurites running in the outer nerve ring at the base of the animal and in putative sensory cells in the ectoderm covering its tentacles. At both sites, diphenyleneiodonium (10‐4 M) abolished staining. Capillary electrophoresis confirmed that the NO breakdown products NO2‐ and NO3‐ were present at high levels in the tentacles, but were not detectable in NADPH‐d–negative areas. The NADPH‐d–reactive neurons in the tentacles send processes to regions adjacent to the inner nerve ring where swimming pacemaker cells are located. Free‐moving animals and semi‐intact preparations were used to test whether NO is involved in regulating the swimming program. NO (30–50 nM) and its precursor L‐arginine (1 mM) stimulated swimming, and the effect was mimicked by 8‐Br‐cGMP (50–100 μM). The NO scavenger PTIO (10–100 μM) and a competitive inhibitor of NOS, L‐nitroarginine methyl ester (L‐NAME, 200 μM), significantly decreased the swimming frequency in free‐moving animals, while its less‐active stereoisomer D‐nitroarginine methyl ester (D‐NAME, 200 μM) had no such effect. 1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxaline‐1‐one (ODQ, 5–20 μM), a selective inhibitor of soluble guanylyl cyclase, suppressed spontaneous swimming and prevented NO‐induced activation of the swimming program. We suggest that an NO/cGMP signaling pathway modulates the rhythmic swimming associated with feeding in Aglantha, possibly by means of putative nitrergic sensory neurons in its tentacles. J. Comp. Neurol. 471:26–36, 2004.

Skin impulses and locomotion in Oikopleura (tunicata: larvacea).

The skin covering the tail and hinder trunk region of Oikopleura propagates impulses at 15-21 cm/sec. Spread is non-decremental and unpolarized. The impulses are of short duration (8-12 msec) and in general resemble skin impulses in hydromedusae more than those of amphibian and tunicate tadpole larvae.The skin was examined by optical and electron microscopy. The cells are connected by gap junctions. Impulses are assumed to spread by direct current flow from cell to cell.Electromyograms of tail activity during three behavior patterns (pumping, swimming, and house rudiment expansion) are analyzed in relation to neuromuscular histology. There appear to be at least two classes of pacemakers, both located in the caudal ganglion. There is no evidence that proprioceptive feedback is required for maintenance of rhythmic activity; and isolation of the tail from the trunk, which contains the cerebral ganglion, does not affect rhythmicity. The system differs fundamentally from the locomotory systems of Amphioxus and...

Coronal organ of ascidians and the evolutionary significance of secondary sensory cells in chordates

A new mechanoreceptor organ, the coronal organ, in the oral siphon of some ascidians belonging to the order Pleurogona has recently been described. In contrast to the known mechanoreceptor organs of ascidian atrium that consist of sensory neurons sending their own axons to the cerebral ganglion, coronal sensory cells are secondary mechanoreceptors, i.e., axonless cells forming afferent and efferent synapses with neurites of neurons located in the ganglion. Moreover, coronal cells exhibit an apical apparatus composed of a cilium accompanied or flanked by rod‐like microvilli (stereovilli). Because of the resemblance of these cells to vertebrate hair cells, their ectodermal origin and location in a linear array bordering the bases of the oral tentacles and velum, the coronal organ has been proposed as a homologue to the vertebrate acousticolateralis system. Here we describe the morphology of the coronal organs of six ascidians belonging to the suborders Phlebobranchia and Aplousobranchia (order Enterogona). The sensory cells are ciliated, lack typical stereovilli, and at their bases form synapses with neurites. In two species, the sensory cells are accompanied by large cells involved in synthesis and secretion of protein. We hypothesize that the coronal organ with its secondary sensory cells represents a plesiomorphic feature of ascidians. We compare the coronal organ with other chordate sensory organs formed of secondary sensory cells, i.e., the ventral lip receptors of appendicularians, the oral secondary sensory cells of cephalochordates, and the acousticolateralis system of vertebrates, and we discuss their homologies at different levels of organization. J. Comp. Neurol. 495:363–373, 2006.

Sequence of two gonadotropin releasing hormones from tunicate suggest an important role of conformation in receptor activation

The primary structure of two forms of gonadotropin releasing hormone (GnRH) from tunicate (Chelyosoma productum) have been determined based on mass spectrometric and chemical sequence analyses. The peptides, tunicate GnRH‐I and ‐II, contain features unprecedented in vertebrate GnRH. Tunicate GnRH‐I contains a putative salt bridge between Asp5 and Lys8. A GnRH analog containing a lactam bridge between Asp5 and Lys8 was found to increase release of estradiol compared with that of the native tunicate GnRH‐I and ‐II. Tunicate GnRH‐II contains a cysteine residue and was isolated as a dimeric peptide. These motifs suggest that the conformation plays an important role in receptor activation.

BIOLUMINESCENCE AND OTHER RESPONSES SPREAD BY EPITHELIAL CONDUCTION IN THE SIPHONOPHORE HIPPOPODIUS

1. Four responses are spread by through-conducting excitable epithelia in the nectophores: luminescence, blanching, muscular involution, and secretion. Swimming, which is independently controlled by the nervous system, is inhibited by epithelial impulses.2. Luminescent flashes are correlated one for one with epithelial impulses. At least three impulses must be propagated before the first flash is recorded. Flashes sum and facilitate. Pacemaker-like after-discharges may continue after stimulation has ceased. Not all regions of the epithelium luminesce equally, and the active area can shift during a single luminescent episode, although the excitatory impulses pass across all regions equally. No steady luminescent glow has been observed. Comparisons are drawn with other luminescent systems.3. Luminescence is generated intracellularly within the exumbrellar epithelium, but blanching (opacity) is associated with formation of granules in the adjacent mesogloea. The response builds up to saturation level within ...