Figure 1

The hydrozoan cnidarian Hydractinia symbiolongicarpus. (A) Cladogram depicting the phylogenetic position of Cnidaria as sister group of Bilateria, as well as the phylogenic relationships between Hydrozoa and other cnidarian groups, based on Kayal et al.⁵⁰. (B) Photo of a female adult colony of H. symbiolongicarpus. Orange arrowhead points to a feeding polyp and green arrowhead points to a reproductive polyp. (C) H. symbiolongicarpus life cycle. Images show key stages of embryonic development as well as stages of metamorphosis from a larva to a primary polyp and ultimately the adult colony. Mature female and male colonies spawn hundreds to thousands of gametes daily following a light cue. Spawned eggs are fertilized in the water column, and proceed through embryonic development⁶. About 3 days post-fertilization (3 dpf), fully-developed larvae are competent to receive a natural or artificial stimulus that will induce settlement and metamorphosis^51,52. Metamorphosis is completed within 24 h, and this process transforms a mouthless larva into a feeding primary polyp. The animal then expands clonally by stolonal elongation and asexual budding of new polyps. The polyps are interconnected by a stolonal mat which holds gastrovascular canals, enabling food distribution and transfer of stem cells throughout the growing colony⁵³. Two to three months after metamorphosis, reproductive polyps start to bud, allowing the colony to reproduce sexually.