The content of Pterioidea: pearl oysters and their relatives is citable because all changes to a page are archived. A combination of web address and date allows for retrieval of any information that was featured on the site, even if a page has changed substantially since it was referenced.
An example of citing the content of the website:
Pterioidea: Pearl Oysters and their Relatives. 2007 Oct 15. CBE home page. <http://pterioidea.myspecies.info>. Accessed 2007 Oct 20.
An example of citing a web discussion forum posting:
Tëmkin I. 2007 Oct 15. . <http://pterioidea.myspecies.info/23354>. Accessed 2007 Oct 16.
The actual cited dates, obviosly, should correspond to the latest on-line publication dates and the date when this information was viewed. The current publication dates are displayed at the Revision tab of every page; web form posts are automatically individually dated. Note, that for citing the web address of a forum entry, the part of the address that follows pterioidea.myspecies.info/ will be different from the given example.