The Zaldans are a minor Federation species, visually indistinquishable from humans save for a webbed membrane between their fingers. They are amphibious in nature, but have adapted to life in an oxygen-nitrogen enviroment as well, even though prefering to live underwater.

The peculiarity about the Zaldan culture, which often sees them separated from serving on mostly-human starships within the fleet, is that they are infuriated by courtesy, viewing it as a form of phony social behavior designed to camouflage true feelings. Thusly, the best way to communicate with a Zaldan is to be as direct and aggressive as is possible without descending into fist-fight.

Or at least be direct.

Source: Coming of Age (TNG). Text in italic is speculation.