When and why did Captain Kirk turn into a giant crab?

Question

It was in a Star Trek book but not a recent one. Definitely not based on any of the TV episodes or movies.

• They are exploring a strange world. I know this doesn't narrow it down.
• Kirk and some other crew are transported away and transformed into giant crabs. The rest don't know what's going on. They have a mixture of alien memories and human memories.
• Kirk is able to hang on to his human self, but another one, I think Chekhov (???) was more crab-like and aggressive. They end up fighting. I think there was also a female crew member who was crabified and they were fighting over her.
• The carapace changes color when they feel different emotions. Kirk Manages to communicate with Spock by getting angry and happy so that his shell pulses in morse code.

I can't remember how they fixed it but they did turn back into people.

As well as identifying the story I would like to remember why they became crabs. It was something like the species was dying out and they needed more crabs?

Its not on https://memory-alpha.fandom.com/wiki/Crab but probably I just don't know the species or planet name to look up.

Answer 1

Sounds like Windows on a Lost World (1993) by V.E. Mitchell.

From Memory Alpha:

While Captain Kirk and a landing party from the Starship Enterprise explore the ruins of an ancient civilization on the uninhabited planet Careta IV, they discover strange devices that appear to be windows. But the mysterious windows prove to be more than they seem when Kirk, Chekov, and two security guards enter them and disappear.

Suddenly, Kirk and his team find themselves trapped in a strange alien environment and must fight with all their strength to survive and keep their sanity. Now Spock must locate his missing comrades and solve the window's ancient mysteries before his captain and crewmates are lost forever…

From a review.

Kirk and the others are not only transported by the apparatus, but transformed by it as well, into members of the Kh!lict, the alien crustaceans that invented it. Kirk essentially rides shotgun in the Kh!lict’s bicameral brain, his consciousness occupying the higher brain while the lower brain runs the show by default. Kirk quickly finds he can fairly easily take the wheel at will, but decides to hang back until he gets a better feel for how the alien arthropod’s body works.

As Kirk carefully observes the lower brain’s reactions to its surroundings and external stimuli, he and we learn more about the Kh!lict. That they believed themselves the only intelligent life in the universe. That the transit frame turned them into Kh!lict because the Kh!lict believed only Kh!lict, being again the only intelligent life in the galaxy by their own estimation, could or would ever use it. That the Kh!lict were one of the most deeply xenophobic and savage races to ever exist, slaughtering any civilization they came across. Continued habitation of the higher brain yields one unsavory revelation after another, and if Spock can’t figure out how to revert the transformed crewmen to their human forms, the racial memory of the Kh!lict will consume them entirely.