Zooid arrangement and colony growth in Porpita porpita.

BACKGROUND

The blue button, Porpita porpita (Porpitidae), is a highly integrated colonial animal-i.e., a superorganism. It has multiple genetically identical bodies (zooids) that arise from the same embryo and are functionally specialized for distinct tasks and arranged in precise patterns. Their colonies include a float, coenosarc, gastrozooid (feeding polyp), gonozooids (reproductive polyps), and dactylozooids (tentacle polyp). Colonies are fragile and difficult to culture, leaving much about their development and lifecycle unknown. We provide new insight into colony development of P. porpita with morphological observation and histological analysis using histological sections and micro-CT technology.

RESULTS

From 2019 to 2024, we collected over 267 P. porpita specimens of varying sizes to study colony development. Morphological investigation revealed that the number and length of gastrozooids, gonozooids and dactylozooids increased with float size. Further observation by histological section and micro-CT technique revealed the internal structures of colonies, including gastrozooid, floats, and aboral chambers that connect various zooids. Immature gonozooids and dactylozooids were observed near mature ones, providing insight into their colony level development. In addition, some colonies showed irregular shapes, but still contained at least one gastrozooid, illustrating the structural variation within the species.

CONCLUSIONS

Our study revealed that gonozooids and dactylozooids increased in both number and size as the colony develops. Moreover, the growth zones for dactylozooids are located at the boundary of the mantle and coenosarc, and gonozooids emerge along the entire epithelium between the gastrozooid and dactylozooids. Colony growth generally follows a pattern proportional to colony circumference and area, and some colonies show irregular shapes, suggesting they have high regenerative capabilities. Taken together, these findings enhance our understanding of the ecology and life history of P. porpita.