The trick with dealing with alien lifeforms in the hold is to do a really thorough clean-out just before taking on anything valuable. Disintegrator rays tend to take care of alien intruders very nicely. The trouble is that they also take care of the average cargo (especially the really valuable ones) even nicelier. You so badly do not want to be returning to New Sevastopol with a cargo full of energy-blasted gaps. Customers take that kind of thing quite humourlessly. So, while you have an (intentionally) empty cargo bay, send your security folks in, to zap anything that has no right to be there. Stunrays for sentients, max-lev for the creepy-crawlies. If you leave it until the last minute and do a thorough job, the space-slugs won't have a chance of damaging your merchandise and your customers will be more than pleased. Your security people will even have worked off their excess aggression for another voyage.