A recent run to Zaonia resulted in a tet crab female coming onboard and laying a clutch of eggs. She died at the pincers and palps of her voracious children since she’d already eaten their father. At that point the young went into hiding and began maturing or being eaten by their siblings.
Luch the steward’s cat, Rockit was in charge of vermin control. Being a young cat he assumed the Profit Rockit was his ship. It was named after him after all. Since the humans had their uses he let them remain. At the moment the situation was developing he entered the lower hold, which was nearly empty. That was why he spotted the tet crab so easily. The other reason was she was an arrogant young crab that considered the ship her property and ate whoever disputed it.
Rockit attempted a pouncing maneuver, but came up short when four pincers began snapping in his face. The two began circling, Rockit hissing, the crab whistling. The commotion brought Luna, the ship’s dog.
Luna was an anomaly. The ship already had a cat for pest control and anti-hijacking apps for security. She was really not needed. But, when was a dog ever really needed unless you were herding or hunting? All that mattered was that Skipper the deckhand wanted her and after a great deal of fuss she was allowed to keep Luna.
Luna still had a lot of puppy in her but was an exceptional dog, even in Rockit’s opinion, considering she’d learned to use the ladders on the ship by herself. Rockit still didn’t think much of her of course since he was a cat.
Luna saw Rockit and the pinchy thing with many legs circling and went for it. Dogs have an innate loyalty but even their biggest fans admit they have no sportsmanship. Luna came at the thing from the side hoping for a quick kill but the crab had several more eyes and very good peripheral vision and Luna got a pincer clamped onto her muzzle. Her fur saved her from laceration but it still hurt like blazes. She tried shaking the tet crab off and the crab’s pincer caught Rockit by the tail as the cat turned to get out of the way.
Canine, crustacean, and feline did a sad and painful dance on the deck, like a small tornado with pincers and fur. The commotion brought Skipper and Luch. Luch immediately ran to his cat’s aid and stomped the alien intruder.
The crab latched onto Luch’s slipper-clad foot with a free pincer. Skipper fled as the steward joined the dance. At this point Captain and Sandoval arrived. Sandoval was the first to climb the ladder to the deck and found herself at floor level with a whirling ball of feet andpincers. She did what any good spacer would do, screamed like a little girl and got the hell out of their way. Captain was next up the ladder and he dodged the falling Second Tier Navigator.
Captain was a Zaonian and Zaonians don’t knuckle under. This one almost did. Then he heaved himself up onto the deck and began seeking a weapon. That was when Skipper came down the ladder from the upper with Captain’s revolver sidearm. She took careful aim fired and missed completely, the bullet burying itself in a deckplate. Captain grabbed the revolver from her before she could ruin another gravity generator, gripped it by the barrel and attempted to pistol whip the tet crab.
Tet crabs also don’t knuckle.
Vermin on ocean going ships is a given. The same will most likely be true of space going ships. Both afford plenty of small dark places to hide and edibles. Unlike terrestrial ocean going rats and roaches any SF pests may have to adapt to the environment and diet of the ship's crew. There's not going to be any fluorine or levo-protein based life on a human ship for example. But then most SF settings have a lot of planets with compatible environments and biologies. And the player characters thought this was for their convenience. Heh heh.
On the other hand vermin breed rapidly, otherwise they aren't vermin. A bear rummaging in your pantry isn't vermin, it's an animal encounter. Rapid breeders may adapt quickly as subsequent generations grow in unusual conditions. A good example of this is the flea. Fleas could cover the earth in a month unchecked and breed so rapidly using the same toxins against them for more than a couple of months can result in them becoming immune. Your crew's referred methods of dealing with pests may become useless at the worst time.
A bear is probably less destructive to a ship than most vermin. Roaches, rats and such can not only make your galley fail a health inspections, they can destroy wiring, including warning sensors. As for fouling a galley think of telling a high passenger that you all have to eat prepackaged rations on your next trip out because you failed a health inspection.
There are many and numerous methods of pest control. The TL 0 solution is a cat. Cats are pound for pound very efficient little killers (just ask one). Dogs generally speaking come in a far second, unless your crew is savvy enough to get breeds specialized for ratting, like terriers. Then again some alien pests might make a ship's mascot earn hazard pay. Genemodded cats and dogs are also possible. I wouldn't get any pets cybernetic enhancements. I wouldn't trust a cat with laser eyes and a dog wth laser eyes would take its begging to a whole new level. Just step away from the pot roast.
There are many and numerous poisons and traps. Doing a web search for pest control can give all manner of devices. Checking out an exterminator's web page could give plenty of ideas and they generally give you cogent reasons why you should leave the pest control to professionals.
Some starports, of course, will seal and bug bomb your ships for a reasonable rate. Reasonable to the folks who sell you a ton of the most common element in the universe for 500 cr. that is.
Of course space is not an ocean. One resource spacecraft all have easy access to is vacuum (sometimes the access is too easy but by then the pests are very far down your list of concerns!) Lifting a ship and opening the airlocks is pretty cheap. Of course it requires the crew and any passengers have spacesuits or survival bubbles. Remember you can shove two middle passengers in a survival bubble but high passengers get their own. This also will not likely win you repeat business but in the example above, tet crabs might make a few minutes in a bubble time well spent.
Vacuum will also get into places poison will not and it pretty much kills everything outright, unless you have some really hardcore pests. Just make sure the cats and dogs are safe as well as any fresh foods or other commodities that will not react well to vacuum, like bottled wine. Also make sure there are no pests hiding out in the pressurized cages and cargo pods.
A far future sort of pest might be destructive nanites. Heinlein help you. Immune to vacuum, breeds like mad and might have a go at eating everything. You might have to shut everything down and drop an EMP bomb or buy some hunter killer nanites.
Uncharitable types may note many of these ideas apply to stowaways.