The ultrastructure of nematocysts from the fishing tentacle of the Hawaiian bluebottle

Fishing tentacle nematocysts of the hydrozoan, Physalia utriculus, from Hawaiian coastal waters, were examined by light microscopy, as well as by scanning and transmission electron microscopy (SEM and TEM). Hawaiian P. utriculus has been assumed to be conspecific with the Australian morph based on general macroscopic descriptions. Ultrastructural examination using SEM and TEM revealed that the cnidome was composed of two sizes of heterotrichous anisorhizas as well as rare rhopoloids (less than one observed per 1,000 nematocysts). Beyond classification of the cnidome of Hawaiian P. utriculus, an unprecedented morphological structure was observed. Specifically, SEM revealed fibers (< 1 μm in diameter) along the spiral crest of spines of some discharged tubules of P. utriculus anisorhizas. A hypothetical model of spine morphogenesis was formulated based on comparative analysis of hundreds of discharged tubules.