Crustacea gigantica, a creature added in update 8.7, always had its appearances very rare, but this, at least partially, ended in update 10.70.
In this update mentioned, Seacrest Grounds was added, and in its environment a new mechanic for Crustacea was created: The possibility to "force" its spawn. It took a while for this information to be in the knowledge of the community, but it can still be noted that many people still do not know, because of this, the publication is intended for them.
To force the Spawn, you must look for a creature called Abyssal Calamary:
They spawn in different regions within the Seacrest Grounds, but not all share their respawn with Crustacea Gigantica! When you kill these calamaries in the correct spots, there is a chance, albeit a low one, that at the moment of respawn the Crustacea Gigantica will appear.
Confirmed Data
The success rate in spawning a crustacea is 0.5-1%, so don't be surprised if after 2-4 hours you still don't succeed. In fast respawn events, or when Oramond is in Boosted state (2x), these are the best times to get the mount / bestiary.
Some locations have already been confirmed as "hot", for sharing their respawn [abyssal calamary + crustacea gigantica], and these are shown below:
Location 1, near the stairs to the main area of Seacrest grounds, the closest to the entrance, without the need to enter teleports.
Location 2, west of location 1, just follow the direction to the left, next to that location there will be 2 abyssal calamaries, but only the one on the left (more to the west) shares respawn with a crustacea.
Location 3, also in the main area, but further south, is present in a region with the altar in which Seacrest pearl is used to obtain an orb of life everchanging. Note that this region has 5 abyssal calamaries, but only the one on the far right (east) shares the respawn with a crustacea gigantica.
Location 4, is not in the main area, and is only accessible via the teleport (right at the cave's entrance) on the right. To access it, do the quest "Twenty miles beneath the sea quest". Entering the teleport, just go south and go down the stairs, the place is some sqms to the right, as the image.
Finally ... Can there be more spots? Certainly yes, both to the north and left teleport are likely to also have abyssal calamaries that share respawn with gigantic crustacea, but due to lack of experience in other places, it was not informed in this post, eventually in the future we can add / update the page with these other locations as well.