Tentacle Musculature in the Cubozoan Jellyfish Carybdea marsupialis

The diploblastic cnidarian body plan comprising the epidermis and gastrodermis has remained largely unchanged since it evolved roughly 600 Ma. The origin of muscle from the mesoderm in triploblastic lineages is a central evolutionary question in higher animals. Triploblasts have three embryonic germ layers: the endoderm, mesoderm, and ectoderm, which develop into organs, muscle, and skin, respectively. Diploblasts lack the mesoderm, the layer thought to give rise to the skeletomuscular system. However, phyla such as Cnidaria and Ctenophora, which are typically classified as diploblasts, possess striated musculature. Within phylum Cnidaria, class Cubozoa includes carnivorous box jellyfish, which are capable of extending and contracting their tentacles for predation and defense mechanisms, thus suggesting a well-organized system of muscles. Here, the tentacle musculature of the cubomedusae Carybdea marsupialis is investigated using transmission electron microscopy in conjunction with light microscopy to further understand the arrangement of musculature in these primitive animals. Cross sections of tentacles confirmed that the gastrodermis is separated from the epidermis by a collagenous mesogleal layer containing numerous longitudinal muscle cells arranged in fascicles. Longitudinal muscles permit the tentacle to retract toward the bell during fast tentacle shortening and crumpling behavioral responses. Circular muscle cells were found in the gastrodermis and epidermis, encircling the layer of longitudinal muscle. These circular muscles likely enable the elongation process that allows the tentacles to return to a resting state after contraction. The presence of a definitive muscle cell layer within the mesoglea suggests that C. marsupialis has an advanced muscle morphology that is similar to triploblastic animals.