Shardnias are an aquatic species that goes through two different life stages before reaching maturity, and are renowned for their ability to filter magic out of the water during their "sea pen" stage of development.
Most wild shardnias develop a Water affinity, unless they develop in waters that contain higher amounts of another element. Those kept in captivity will not develop an affinity unless the water they live in magically-charged in some form.
Their main use lies in this "sea pen" stage of development, as sea pens with an affinity can be harvested and subsequently used in magical writing (for spellbooks or scrolls or runes or what have you). They can also suck the magic out of an artifact one wishes to return to an everyday object; simply place the unwanted magical item within the water tank or enclosure that houses the sea pens, and watch as they slowly drain it of any use (or lack thereof)! Perfect for repurposing magic that otherwise might have been forever trapped in some kind of nonsense trinket.
Sea pens will develop into shardpoles if not harvested early enough, which has frustrated many an absentminded witch or wizard hoping to gain a new pen. Fortunately, elemental shardpoles still have a use, and serve as potent ingredients in magical tonics and tinctures. While more magically potent, adult shardnias require more maintenance to keep alive, as they require larger amounts of elemental magic to feed from than sea pens or shardpoles.
Shardnias are a Kadath staple, and shardnia starter kits are an incredibly popular DIY way to get into artificing and elemental-repurposing among novice magic workers.—glitchedpuppet