Anatomy & Physiology
As a cnidarian, a characteristic anatomical feature of Myrionema hargittis is the presence of numerous cnidocytes located on each tentacle. While some aquarium users have experienced painful stings while handling colonies of this genus—likely Myrionema ambionense (Ronchi 2012)—there were no such outcomes of touching hargittis . This does not however rule out the use of toxic nematocytes, as they may simply be too weak to affect larger organisms.
Figure 7. The presence of cnidocytes on the tentacles of Myrionema hargitti.
Arguably the most important physiological feature of this hydrozoan is its symbiosis with the Symbiodinium zooxanthellae. Research has suggested that up to 90% of the nutritional requirements of a colony are met by the photosynthetic activity of these algal cells (Ruppert et al 2004).
As Figure 8A illustrates, the green-gold colouration of the cells within the tentacles suggest photosynthetic capabilities, based on the presence of chloroplasts. This is confirmed through a fluorescence microscope, highlighting extensive photosynthetic activity (Figure 8B).
 
Figure 8A. Myrionema tentacle with Symbiodinium. Figure 8B. Fluorescence image of Symbiodinium.
These chloroplasts have been identified in high concentrations throughout the whole specimen, with the greatest density in the tentacles (Figure 9A and 9B).
 
Figure 9A and 9B. Fluorescence image of Symbiodinium activity in tentacles and body.
The activity of the chloroplasts within the Symbiodinium cells can be clearly observed in Figure 10, as seen in blue.
Figure 10. The activity of chloroplasts within Symbiodinium cells in the tentacles.
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