Rhopalium

Rhopalia (sing. rhopalium; from Ancient Greek ῥόπαλον (rhópalon) 'club, mace') are small sensory organs of certain Scyphozoan (true jellyfish) and Cubozoan (box jellyfish) species.

Description The structures typically occur in multiples of four, are bell shaped and face outward from invaginations around the bell of the jelly's mantle. They are each connected ectodermally to the periphery of other rhopalia by a stalk-like projections which join extremities in a skirt-like shape. These connections form the junctions of the cnidarian 'central nervous system', which synapse within the rhopalial centers. Rhopalia vary in form, size and number, but ubiquitously consist of specialized structures to sense light (ocelli), which line the structure, and regions to perceive gravity (statoliths) at their terminal tip. Rhopalia are unique to the medusoid forms of Cnidarians and are best studied in Scyphozoa, within the genus Aurelia, which exhibits the most typical arrangement and structure of rhopalia in marginal indentations around the skirt of the bell which are flanked by rhopalial lappets. However, studies about the organ in Cubozoa, which exhibit the most complex rhopalial structure and visual sensory mechanisms, have been few, but are coming to reveal more detail about the mechanisms and origins of these structures. Sensory input from rhopalia are not only crucial for Cnidaria to sense light and spatial orientation, but help to gauge and control the pace of swimming and muscle contraction. In continuing to increase the breadth and depth of scientific knowledge about the origins of rhopalia and how they manifest during the process of metamorphosis is crucial to understanding sensory evolution in all metazoa, and could contribute to the progression of knowledge about the beginnings of nervous system evolution.

Morphology and function

Studies observing the genus Aurelia indicate that development of their rhopalial nervous system occurs in an organized, staged manner beginning in the strobila phase and results in bilaterally symmetrical organization of the organs and the stalk-like projections that connect them inwardly. Rhopalia uniformly manifest in multiples of four, regardless of their variations in morphology among the species that possess them. Each rhopalium is capped by a lithocyst, or statocyst, at the terminal end, which is known to sense gravitational change and orientation. On the oral side of the rhopalium near the lithocyst is a mass of sensory cells called the "pigment-cup ocellus", while on the aboral side, there is a “pigment-spot ocellus” just proximal to the lithocyst, formed by a patch of pigment cells. In tandem with a “touch plate” which is situated near the spot-ocellus, tilt and gravity during the jellies' movements pulls downward on the dense lithocyst, which bends the entire rhopalial body such that the cells of the touch plate are either distant from or making contact with the lithocyst hood; this physical mechanism is how the rhopalia deduct sensory information into gravity-sensate behavioral responses As cnidarians lack centralized ganglia and cephalization, the centralization of sensory mechanisms divided up among connections within the rhopalial centers is the nearest concept to a brain that we can place within the phylum. While rhopalia are minute and separated by number and distance across the body, they are extraordinary in the variability of sensory cells they possess in such small areas of tissue. The RNS (rhopalial nervous system) consists of roughly 1000 neurons, excluding photoreceptive cells, and possesses a neuropil, a tightly bundled cluster of cells at its center, serving as the junctions of synapse between adjacent rhopalia. In its totality, the rhopalial nervous system is involved in deduction of the sensation of light (the complexity of which approaches visual processing in some species) and in spatial-behavioral control; because the sensory cells of both sight and physical touch are located within such proximity, studies have explored how related and potentially integral these two mechanisms are to one another in terms of cnidarian behavior. It has thus been observed that the rate of muscle contraction and swimming speed increases when adult Aurelia medusae are exposed to higher levels of light. Additionally that this positive phototactic behavior is absent when the same conditions are presented to jellies whose pigment-cup ocelli have removed (but not the spot-ocellus), insinuating that the pigment-cup ocellus on the oral side is primarily photosensory yet intrinsically related to normal functional behavior In recent years, it has been found that rhopalia retain a sizable reservoir of undifferentiated, stem-like cells, which are thought to be poised for potential use in both cell turnover and regeneration in the rhopalia.

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