The base composition of the Hydra genome is very A+T-rich. The base composition was measured some years ago by Dr. Joseph G. Gall (then at Yale University) using buoyant density centrifugation and determined to be 71.4% A+T (please note that this result has never been published, so it should be cited as a personal communication).
Subsequent cloning of Hydra genes revealed, as expected from the A+T-rich base composition of the genome, that codon usage is very biased in favor of codons with an A or T at the third position. This has been seen for every gene that has been cloned from Hydra. A codon usage table for Hydra genes was first published in January 1993. The reference for this table is:
Galliot, B. and Schummer, M. (1993). 'Guessmer' screening strategy applied to species with AT-rich coding sequences. Trends in Genetics 9, 3-4.
There is now a web site of codon usage tables for a large number of species that includes the following cnidarians:
ANTHOZOA
CUBOZOA
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| Tripedalia cystophora |
SCYPHOZOA
HYDROZOA
Examination of these tables reveals that some other hydrozoans (Hydractinia, Podocoryne, Eleutheria, and Polyorchis), like Hydra, have very A+T biased codon distributions. Such a clear codon bias is not obvious in the other cnidarian classes. However, in the cases of Scyphozoa and Cubozoa, only a few genes have been analyzed. Since a particular gene may exhibit a bias distinct from the bulk of the genes in an organism, conclusions about codon usage in these two classes must await sequencing of more genes.