Central control of swimming in the cubomedusan jellyfishCarybdea rastonii

Summary

• 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.