a C. xamachana releasing mucus (yellow arrows) following collection in the field by authors (Bonaire, The Netherlands). Cassiosome nests (pink arrows) appear as light bulging spots at the termini of vesicular appendages (cyan arrow). b Mucus (yellow arrow) released into the water by C. xamachana in the lab—small white flecks correspond to live cassiosomes (green arrows). c Live cassiosomes (green arrows) suspended in mucus (yellow arrow) harvested after release from C. xamachana. d Multiple motile cassiosomes isolated from C. xamachana mucus. e Live cassiosome close-up (green arrows), showing irregular shape and centralized Symbiodinium dinoflagellates as amber spheres (red arrows). f Confocal image of highly motile cassiosome immobilized on MatTek glass bottom dishes coated with Cell Tak adhesive (Corning); image collected with ×60 objective (oil) reveals organization of the peripheral cell layer: NucBlue-Hoescht 33342 (1,100) (ThermoFisher) stains nuclei (blue) of nematocytes with peripheral nematocytes bearing O-isorhiza nematocysts (blue arrows, DIC) and non-nematocyte ectodermal cells (purple). DIC shows centrally Symbiodinium (dark spheres, red arrows) occupy presumptive C. xamachana amoebocytes in an otherwise acellular core. Scale bars: a = 3 cm; b = 3 mm, c = 1 mm;, d = 5 mm; e = 300 μm, f = 50 μm.