Regression of juvenile tentacles coincides with loss of cell proliferation in Haliclystus sanjuanensis, a cnidarian with limited metamorphosis

Medusozoan cnidarians (e.g., jellyfish) metamorphose from a benthic juvenile polyp into a pelagic adult medusa, providing a well-known example of a clade that uses tissue remodeling to create distinct juvenile and adult body plans. Staurozoans (i.e., stalked jellyfish) are an atypical lineage of medusozoans that have lost their medusa stage; thus, their juvenile and adult body plans look remarkably alike. Their limited metamorphosis is characterized by the regression of primary (juvenile) tentacles and the development of secondary (adult) tentacles. In some staurozoan lineages, metamorphosis also involves development of novel adhesive structures (anchors), which are built on top of the regressing primary tentacles. Understanding how cells are partitioned from making juvenile tissues to making adult tissues is important for understanding how animals can make adult structures in the absence of complete metamorphosis. We compared the abundance and distribution of proliferative cells in tissues undergoing regression (primary tentacles) and development (secondary tentacles and anchors) during the juvenile to adult transition in the San Juan Island stalked jellyfish, Haliclystus sanjuanensis . We show that proliferative cells are lost in regressing primary tentacles but are gained in anchors, consistent with a shift in investment from juvenile to adult tissue. Prior to regression, primary and secondary tentacles show similar patterns in their proliferative cell distribution and in the identity of their cnidocytes (stinging cells), indicating that adult tentacles are made by re-deploying a juvenile tentacle program. Finally, we demonstrate that unlike secondary tentacles, primary tentacles cannot regenerate, illustrating that the temporary investment in this tissue coincides with their loss of proliferative cells. We propose that continued investment in a population of proliferating cells is an important mechanism for segregating temporary tissues (primary tentacles) from long-term tissues (secondary tentacles). These observations of cell dynamics in H. sanjuanensis suggest that temporary investment into juvenile structures may be used to pattern novel adult tissues, providing an important mechanism for diversifying adult body plans.