Gastrodon is a sea slug-like Pokémon. Much like its pre-evolved form, it has a slug-like body with a round head and a mantle covering the top half of its body. It has six stubby legs and three black eyes with white pupils. A yellow stripe runs along the edge of its mantle, through its lower two eyes, and around the back of its head. Like Shellos, Gastrodon has two forms based on the sea it lives in.
In its West Sea form, it has a pair of large tentacles on its head that curve upward, as well as a smaller, straighter pair of tentacles on its neck. Its underside is pink, while its mantle is brown with three pink spots on each side. There are two lumps on its back, resembling the cerata on real-life sea slugs.
In its East Sea form, it retains the head tentacles it had as a Shellos, which are now fully green and angled slightly more upward. Its underside is blue, while its mantle is green with two blue spots on each side. It has a pair of yellow, fin-like cerata on its back and a wavy yellow line bordering its side and rear.
Gastrodon and its pre-evolution Shellos are examples of allopatric speciation, although they are still able to breed with each other. It is also a relative of Shellder and Cloyster. When it is threatened, it oozes a purple fluid and flees. The fluid is not poisonous, but it is very bitter and sticky. Its mucus-coated body is soft and squishy, yet surprisingly resilient. It has strong regenerative capabilities that allow it to recover from injuries or dismemberment within a few hours. In ancient times, it had a shell on its back, which gradually became a thin yet hard vestigial plate.
Gastrodon lives in shallow tidal pools and beaches. Its diet consists mainly of plankton, but it is occasionally known to look for other foods such as beach sand on land, leaving behind a trail of sticky slime. Gastrodon cannot stay on land for too long due to its body being highly vulnerable to dehydration; however, it can become more active during cloudy or rainy weather. It has even been found on mountains during times of constant rain. It melts prey with its mucus before eating them.
The two forms of Gastrodon are known to engage in conflict with each other. West Sea Gastrodon's populations were more numerous in the past than it is in recent times. Conversely, East Sea Gastrodon has been increasing in population as of late.
In Diamond and Pearl, no in-game Trainer uses East Sea Gastrodon in battle, even on the east side of Mount Coronet.
Prior to Generation VI, Gastrodon's shape was .
Gastrodon and its pre-evolution were accidentally released in Pokémon GO on April 29, 2019, but were removed shortly after. On May 17, 2019, they made their official debut along with various other Pokémon from Generation IV.
The Pokémon Scarlet and Violet Pokédex entries mentioning a decrease in West Sea Gastrodon and an increase in East Sea Gastrodon may be an inverse on the changes in the populations of real life sea slug species due to climate change — with species that better survive in warmer waters becoming more common.
Gastrodon is based on a sea slug, perhaps one of the order Opisthobranchia. It may also be based on a sea hare, a type of sea slug that is also known to secrete purple fluid in self-defense. The tentacles on Gastrodon's head are shaped like cattle horns, which may be a visual pun on 海牛 umiushi (Japanese for sea slug, derived from 海 umi (sea) and 牛 ushi (cow; cattle)).
Gastrodon may be a combination of gastropod or Gastropteron (genus of sea slug), and don (Spanish for lord) or -don (common suffix in dinosaur names derived from the Ancient Greek word for tooth).
Tritodon may be a combination of Tritoniidae (family of shell-less marine gastropod molluscs) or Triton snail (marine gastropod mollusk named after Triton, Greek god of the sea), and don (Spanish for lord) or -don (common suffix in dinosaur names derived from the Ancient Greek word for tooth). It may also be derived from torito (Spanish for little bull).