If you liked the Manhole manga, the Anime-Planet community thinks you'd like:
The world is never quite the same once we grow up. Villains and robots that once ran rampant in our imaginations are reduced to drawings in Manga, and our lives are distilled until there is little left but the daily grind; so it has been for Kenji. After a childhood of dreams he now runs his late father's liquor store and is raising his sister's child. The memories and friends from his early years bring him some happiness, but they become tainted as a string of murders find connections to his past. Kenji and his friends must now fight to save the future from their past and unravel the mystery of "Friend."
If you liked the wide-scale, methodical plotting in Manhole, please read the FAR superior (though less gory) 20th Century Boys. It's hard to explain without spoiling but trust me that you'll find it even better than Manhole.
In 1990 Yashiro Tenshuu murdered his lover and four others, sending him straight to death row. Now, the day of his execution has finally arrived, but at the last second he's given a choice: die, or go to work for a mysterious group of people. Choosing life, Yashiro is taken to a room and left in the company of another death row inmate, where they go from day to day with sake and other niceties, with no idea of why they are there. That is, until from behind a glass window, they see a beautiful girl awaken...
This is kind of a gut feeling recommendation, but Alive! and Manhole have a similar atmosphere (gory, gritty, and dark), a few similarities in plot (what to do with convicts), and will probably appeal to the same audience.
Power corrupts, and absolute power corrupts absolutely. Top-ranked student Yagami Light is disenchanted with the world around him. Through a series of events, he comes into possession of the ultimate power over life and death: a supernatural "death note" which can kill nearly anyone at his whim. As Light sets off on a crusade-turned-killing-spree, investigators from a police task force try to stop the mysterious deaths - including Light's own father, a senior policeman.
Hear me out. Both Death Note and Manhole are about well-planned action and essentially, a person going out of their way to change peoples' lives for better in their eyes. Both will keep you turning page after page to learn more, and death and a sort of chill embodies nearly every single page.
A bloated man with bloodshot eyes wanders to a crowded intersection in Tokyo, blankly staring into the distance before blood explodes from his eyes and his body, killing him on the spot and splashing many onlookers, including a student named Akari. Who was this man, and what killed him? Discovering the mystery behind the body may prove to have deadly consequences not only for Dr. Onotama, but also all of Japan...
Emerging and Manhole start out IDENTICALLY - gross dude wanders out and explodes on someone. The similarities in plot end there, with Manhole being more schemey and Emerging being straight horror/suspense, and admittedly I liked Emerging WAY more. Still, the biological horror and overall feel makes these a good rec for each other.
Okinawa was the first affected; swarms of fish with spindly legs rose from the sea, carrying with them an overpowering stench of death. Kaori and Tadashi find themselves in the midst of the incident and quickly travel to Tokyo to ask for Tadashi's scientist uncle's help - but even Tokyo isn't safe for long, and sea creatures aren't the only beings affected by the horror...