Kin Demes was a quiet backwater world. Mostly ocean, settlement concentrated on a chain of fertile islands. The latest settlers had no hstory of warfare. Barring some police actions against raids by the Ingoko indigenes (in other words people settled there long enough to forget they settled) it was a peaceful planet. In recent years even the Ingoko united into a loose commonwealth with the settlers providing cheap labor and trade in pearl-gems and seafood. It recently upgraded its bare dirt landing field to a Class C starport after several decades of labor took a deep breath and entered the interstellar community.
The planet was invaded almost immediately.
It was not ready. It had defenses around the starport and capital. It had a defense boat in orbit. It had a small respectable militia designed to deter invasion. They were of no use. The invaders were tenacious, merciless, and hungry! Kin-Demes defeat was made more galling by the fact the invaders had never mastered fire.
A bare 1.2 parsecs from Kin-Deme is Yu, a harsh unforgiving world with high gravity, windy deserts and brackish seas. Most humans there belong to nomadic tribes that can only be described as savage. They raid each other nearly as much as the starport and trade various rare minerals for guns and more guns. They'd roll over Kin-Demes in a minute if they were organized and united. But they weren't. They couldn't and they didn't. Forget them.
In the brackish waters lives the bloat fish. Bloat fish lived there a long time evolving and thwarted by high gravity and a low oxygen count. Eventually their fins evolved into tentacle like feelers and they were able to scramble on land in search of food until they found it or got their fishy brains blown out by an irate native who woke up to find their pet's leash trailing from a daddy bloat fish''s mouth.
Then a captain landed his free trader on Yu, was too cheap to buy fuel at the starport, took on water from the brackish sea, and lifted ship.
Starships have a great many fuel tanks. Some are to hold water for trim (even reactionless drive ships worry about their center of mass as they boost). Into these tanks went the bloat eggs and fry and they survived to go where n bloat fish had gone before: Kin-Demes.
Cheapskate repeated his offshore fueling on Kin-Demes. He later misjumped and wound up 4 parsecs from the nearest star. His crew spaced him and climbed int their low berths and left the distress beacon on. But before that, while refueling bloat fish eggs and fry were released when a trim tank was blown to right the ship in some waves.
The bloat fish had a ball. They thrived in the less salty and benign ocean. In the lighter gravity they made leaps and bounds (sometimes literally) in practicing walking on shore and they ate anything that swam or walked near the beach. The Ingoko told their new allies something was up with the fishing. Then it collapsed. Then a boat hauled up a school of bloat fish wh were the size of tuna and not amused. They tore through the fishing nets and the crew.
By the time the celebrations about the new starport ended and hangover therapy began working the bloat fish were established and on the move. Rivers, streams, even lakes a short walk apart were getting infested. Crops were being devoured. There was always the fishing industry to fall back on ... no wait.
Kin-Demes had a few weeks between ships calling. When the next merchant ship came calling famine and riot had taken over. Soldiers were barely holding the capital and port people were screaming for passage offworld. They'd pay anything. Unfortunately the leaders of the military and government would do anything to get offworld and tried to take the ship. The captain lifted ship and got ready to leave, but he hadn't time to refuel.
He took on water in the ocean.
There's a reason starports charge you so much for fuel. Yes, a good deal of that is price gouging (100 cr. per ton hydrogen? It's the most common element in the Universe!). Part of it is the cost of honest labor to clean your ship's tanks out and ensure no clever little beasties make their way into an open eco system. Your ship is essentially a closed system until it does something like refueling, or letting off passengers and cargo. Customs deals with that but some people forget that so many little cryptids and refugees from a monstrous tome of beasts will find their way into landing gear, airlocks, and other nooks and crannies where they could survive or at least lay durable eggs or spores.
This does not begin to consider the smugglers, poachers, pirates and ne'er do wells who perform ocean refueling and never think about it again. Invasive species are a problem here on Earth -a single planet. Commerce between worlds really opens up a can of worms (note these worm eat metal and spit hydrochloric acid at you).
Most space opera assumes that earth-like worlds have similar biologies: in other words you or I can get something to eat that won't kill you. They don't address invasive species. The thing is invasive species thrive when the environmental factors limiting them are removed. Go read Chamax Plague/Horde if you doubt me. A planet hop can leave an animal's predators behind but it might still perish due to other environmental factors (of course baby Kal-El did okay.)
An invasion of this sort could spur a new business for characters: pest control! This falls squarely under business plans for non merchant ships. Take a lab ship (or hunting ship) fit it with bio labs and head into the danger zone. You can have field missions to get specimens, hunting trips to get bigger specimens, military missions for the reeeeeeeeally big specimens, all the while dealing with displaced, desperate or chewed on survivors. Meanwhile the scientist types can whip up a toxin or virus or pest catching robot as the situation demands.
The crisis can take a darker turn when lab tests of an invader reveal it is genetically modified to survive on the afflicted world. Enter the mad scientist: a truly underused enemy of SF RPGS. Ecological sabotage can be a cheap alternative to war in virtually any setting and very hard to get caught at. In fact local scientists making that discovery before the characters might start pointing fingers. Maybe they introduced the bloat fish to drum up business?
And wait 'til the flying monkeys attack!