All-or-none propagated electrical impulses were recorded from the hexactinellid sponge Rhabdocalyptus dawsoni using suction electrodes attached to lumps of aggregated sponge tissue grafted onto the surface of pieces of the same sponge. Impulses were normally evoked by means of externally applied electrical shocks. Recorded externally using an a.c.-coupled amplifier, the electrical event was triphasic and lasted approximately 30 s; integration gave a diphasic waveform. A further integration to give the form of the membrane action potential produced a monophasic signal. Impulses propagated at 0.27+/−0.1 cm s-1 with an absolute refractory period of 29 s and a relative refractory period of approximately 150 s. Concurrent thermistor flow meter recordings confirmed that water flow through the sponge was arrested following the passage of an impulse, presumably as result of the cessation of beating of the flagella in the flagellated chambers. Tactile stimuli also evoked impulses, as did addition of particulate material to the incoming water stream. Impulses continued to propagate through the sponge during arrests, indicating that the conduction and effector systems were independent. Sponges lack nerves, and a variety of evidence indicates that the conducting tissues are the syncytial trabecular reticulum and pinacoderm layers. Na+-deficient solutions had little effect on the action potential, but propagation was blocked by 10 mmol l-1 Co2+, 1 mmol l-1 Mn2+ or 24 micromol l-1 nimodipine. Tetraethylammonium ions at 1–5 mmol l-1 also blocked propagation without prolonging the action potential. Impulse conduction in the sponge is discussed in relation to excitability and conduction in the protozoa and in plants and to non-nervous conduction in more advanced animals.

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Impulse conduction in a sponge.

The swim-control systems of hydrozoan and scyphozoan medusae show distinct differences despite similarity in the mechanics of swimming in the two groups. This dichotomy was first demonstrated by G.J. Romanes at the end of the 19th century, yet his results still accurately highlight differences in the neuronal control systems in the two groups. A review of current information on swim-control systems reveals an elaboration of Romanes' dichotomy, but no significant changes to it. The dichotomy is used to suggest that cubomedusae are more closely aligned with the scyphomedusae, and to highlight areas of future research that could be used to look for common, possibly primitive, features of medusan conduction systems. Resume : Les systemes natatoires des meduses hydrozoaires et scyphozoaires comportent des differences importantes en depit de la similitude des mecanismes de nage chez les deux groupes. Cette dichotomie a ete observee pour la pre- miere fois a la fin du 19 e siecle par G.J. Romanes dont les resultats sont toujours utilises pour mettre en lumiere les differences de controle neurologique chez les deux groupes. Une revue de la litterature recente sur les systemes de con- trole de la nage indique que la dichotomie de Romanes a ete precisee, mais que rien de substantiel n'y a ete ajoute. Cette dichotomie laisse croire que les cubomeduses sont alignees plus etroitement avec les scyphomeduses et met en lumiere de nouvelles avenues de recherche qui pourraient eventuellement reveler l'existence de caracteres communs, possiblement primitifs, dans les systemes de conduction des meduses. (Traduit par la Redaction) Satterlie 1669

Neuronal control of swimming in jellyfish: a comparative story

Predation among pelagic cnidarians and ctenophores is reviewed. The diets of semaeostome scyphomedusae and hydromedusae commonly include other gelatinous zooplanktivores. However, few species of siphonophores and ctenophores are known to consume other gelatinous species. Most of these species can be said to exhibit intraguild predation, since they consume species that potentially compete with them for food. In addition, some hydromedusan and ctenophore species may consume other gelatinous zooplanktivores exclusively. Characteristics of cnidarians and ctenophores as predators and as prey of other gelatinous species are discussed.

A review of cnidarians and ctenophores feeding on competitors in the plankton

The gross and fine morphology of the major effector systems in the anthomedusan, Polyorchis penicillatus, is described and discussed in relation to the known physiological and behavioral properties of these systems. Swimming is controlled by an anastomosing network of giant neurons within the inner nerve ring and radial nerves. Although these neurons may be coupled by gap junctions it is likely that they form a syncytium. The photosensitivity of the "giants" is attributed to reflexive membranes within the cytoplasm. Giant neurons act as both the pre- and postsynaptic cell when forming synapses with other neurons of the inner nerve ring. Neuromuscular synapses between "giants" and the striated swimming muscle are found around the margin and along the radii. Swimming muscle cells are connected laterally by gap junctions and end-to-end by desmosomes which are sometimes elaborated with extra-thick filaments. Unstriated sphincter and radial muscles, the major muscles associated with crumpling, are both greatly folded over mesogloeal ridges and have processes that cross the mesogloea to contact the ring and radial canals, respectively. Synapses or other sites that might be responsible for exciting these muscles during crumpling have not been found. The ability of the endodermal lamella and canals to propagate action potentials can be accounted for by the numerous gap junctions that are seen in these tissues. The precise location where excitation is transferred to the nervous system to initiate crumpling is not known but epithelial bridges crossing the mesogloea are likely routes. Synapses between neurons originating in the outer nerve ring and tentacle longitudinal muscle can account for the control of tentacle length. Neurons of the outer nerve ring also synapse onto velar, radial fibers and the sphincter muscle. The inner and outer nerve rings have nervous connections. The organisation of the outer nerve ring and the arrangement of nerves within the endodermal plexus is described. A diagram showing the major connections and interactions of components of the effector systems is presented.

Neurobiology of Polyorchis. II. Structure of effector systems

1.

Swimming in the cubomedusaCarybdea rastonii is controlled by a subumbrellar nerve net. Neurons that make up this net, including “giant” neurons, make random synaptic contacts with each other and with the circular subumbrellar swimming muscles (Figs. 1–3).




2.

Extracellularly recorded swimming impulses originate in the rhopalia and spread throughout the subumbrellar nerve net, initiating contractions of the subumbrellar musculature (Fig. 4).




3.

Intracellular recordings from the subumbrellar giant neurons indicate that all-or-none overshooting action potentials precede each swimming contraction (Fig. 6). Synaptic depolarizations were occasionally recorded alone, and triggering an action potential (Fig. 6C, E).




4.

Extracellularly recorded muscle potentials exhibit frequency-dependent facilitation, with normal swimming at about 80% of the maximal contraction of the muscle sheet (Figs. 4 and 5).




5.

Intracellular recordings from subumbrellar muscle cells reveal graded depolarizations with each contraction of the muscle sheet. The jagged potentials are initially small and increase in amplitude with the first few contractions in a series (Fig. 7). The increases in muscle cell depolarizations may be related to the facilitation in the size of extracellularly recorded muscle potentials.




6.

Pacemakers of the four rhopalia interact by a dominance hierarchy; the rhopalium with the highest firing frequency controls swimming. Dominance shifts have been observed in two-rhopalia preparations (Fig. 8 A), and facilitation of the musculature often occurs independently of rhopalial sequence (Fig. 8B).




7.

The swimming system ofCarybdea is comparable to the “giant fiber nerve net (GFNN)” of other scyphomedusae.

Central control of swimming in the cubomedusan jellyfishCarybdea rastonii

1. Feeding in the medusa of Proboscidactyla flavicirrata is accompanied by local, small amplitude impulses recorded from the bases of perradial tentacles.2. Medusae, both attached and free, swim spontaneously with electrodes in place thus giving data on the temporal patterns of contractions of the swimming muscle.3. Swimming pulses (SP's) can be recorded from the circular muscle of the subumbrella. Each SP is preceded by a pre-swim pulse (PSP) which is a neuronal pulse conducted in the marginal nerve(s).4. Marginal pulses (MP's) are neuronal pulses conducted in the marginal nerve(s) which can trigger synchronous tentacle contraction.5. MP's originate from pacemaker sites located in tentacle bulbs. When new tentacles first appear the firing of MP's from these new sites is not synchronized with established pacemakers: eventually they become linked to the original pacemaker system.6. Synchrony between MP pacemakers is lost under Mg++ anaesthesia.7. Tentacle contraction pulses (TCP's) are the electrical accom...

Behavior and electrical activity in the hydrozoan proboscidactyla flavicirrata (brandt). ii. the medusa.

1. P. flavicirrata feeds on material collected from the brachiolar food-grooves of the host sabellid worm.2. The feeding cycle involves wiping the tentacles through the mouth. An abortive feeding cycle can also be seen when the tentacles are not wiped through the mouth.3. The frequency of the feeding cycle is in part controlled by activities of the host worm.4. Tentacle contraction pulses and column contraction pulses are the electrical accompaniment to feeding and abortive feeding. Their properties are described.5. Colonial pulses, conducted in part by epithelial cells at velocities between 1.9 and 7.3 cm/sec (13° C), cause protective retraction of all the individuals in the colony. The varying response of medusa buds as they develop is described. The colonial pulse conducting system fatigues rapidly after repetitive stimulation.6. Colonial pulses continue to propagate after exposure to excess Mg++ for 3 hr.

Andrew N. Spencer

Papers

Behavior and electrical activity in the hydrozoan proboscidactyla flavicirrata (brandt). ii. the medusa.

Behavior and electrical activity in the hydrozoan Proboscidactyla flavicirrata (Brandt). I. The hydroid colony.