The glow squid was one of three Minecraft Earth mobs in the Minecraft Live mob vote last year. In two rounds of voting, the glow squid came out victorious, and has now been added to the game.
The glow squid does not yet spawn naturally in your world, so you will have to go into Creative mode and use a glow squid spawn egg to get one. They behave like regular squids, except, as the name suggests, they are brightly lit up in contrast to the darkness of the deep water.
The axolotl, added in the last snapshot in December, will attack any glow squid it sees, so be sure to keep them separate if you want to keep a glow squid.
When killed, glow squids drop special glowing ink sac items. These can be used to make item frames or signs glow in the dark.
Following the glowing theme, there is a new type of cave flora to be found in your world, the glow lichen. These spawn rarely in caves and light up the area around them.
Glowing things weren't the only features added in this snapshot, as it includes a few other changes, such as a new death message for when you fall to your death onto a stalagmite, "player was impaled on a stalagmite", to broadcast your demise to all players online.
As always, there have been a couple dozen bugs fixed in this snapshot, many relating to the dripstone caves biome and the shulker mobs.
For a visual overview of the changes in this snapshot, watch my short video below: