People are fascinated by the largest of something. Hydra is no exception. The largest Hydra species is Hydra oxycnida, a member of the Oligactis clade. H. oxycnida was first described by Schulze (Schulze, 1914). Adult polyps of this species can be as long as 3 cm when extended. Compared to some other Hydra species, the distribution of H. oxycnida is relatively restricted. It has been collected only in Germany, Scotland, Belgium, the Netherlands, Hungary, Finland, Austria, and Russia. The closest relative to H. oxycnida in the Oligactis clade is Hydra canadensis (Martínez et al., 2010). H. canadensis is also quite large. H. canadensis is found only in North America. The divergence of H. canadensis and H. oxycnida is the most recent split in the Oligactis clade. Interestingly, the polyp of Branchiocerianthus imperator, a close marine relative of Hydra, can be 1 meter in length. Wouldn’t you love to see a 1-meter long Hydra!

REFERENCES

Martínez, D.E., Iñiguez, A.R., Percell, K.M., Willner, J.B., Signorovitch, J., Campbell, R.D., 2010. Phylogeny and biogeography of Hydra (Cnidaria: Hydridae) using mitochondrial and nuclear DNA sequences. Mol. Phylogenet. Evol. 57, 403-410.

Schulze, P., 1914. Bestimmungstabelle der deutschen Hydraarten. Sber. Ges. naturf. Freunde Berl. 9, 395-398.