A simple visual system without neurons in jellyfish larvae.
Earlier detailed studies of cnidarian planula larvae have revealed a simple nervous system but no eyes or identifiable light sensing structures. Here, we describe the planula of a box jellyfish, Tripedalia cystophora, and report that these larvae have an extremely simple organization with no nervous system at all. Their only advanced feature is the presence of 10-15 pigment-cup ocelli, evenly spaced across the posterior half of the larval ectoderm. The ocelli are single cell structures containing a cup of screening pigment filled with presumably photosensory microvilli. These rhabdomeric photoreceptors have no neural connections to any other cells, but each has a well-developed motor-cilium, appearing to be the only means by which light can control the behaviour of the larva. The ocelli are thus self-contained sensory-motor entities, making a nervous system superfluous.
Nordström
Karin
K
Department of Cell and Organism Biology, Zoology, Lund University, Helgonav. 3, 223 62 Lund, Sweden.
Wallén
Rita
R
Seymour
Jamie
J
Nilsson
Dan
D
D000818:Animals; D007814:Larva / Q000166:cytology / Q000502:physiology / Q000648:ultrastructure; D008854:Microscopy, Electron; D017956:Photoreceptor Cells, Invertebrate; D010860:Pigments, Biological / Q000502:physiology; D037401:Scyphozoa / Q000166:cytology* / Q000502:physiology* / Q000648:ultrastructure*; D014785:Vision, Ocular / Q000502:physiology*
D010860:Pigments, Biological