Have a look at these:
Western Conifer BugThey are invasive insect species in the UK. They are also so bright, or large, that they are hard to miss. You'll come across them even if you don't look. Many, many other alien invaders are harder to notice, they arrived, and before anybody other than specialist taxonomists noticed, they have spread across sizeable portions of the country. Many such silent invasions involve invertebrates that are small and or hard to identify.
Clutches of snail eggs in the soil of pot plants, or dormant adults in cracks in stones or other cargo are often sent as inadvertent stowaways across countries thousands of miles away. They arrive in the destination, thrive and begin a quiet invasion. As many snails are tiny, and to the untrained eye they look identical to other snails, they are transported about very often. A sizeable fraction of the snails species of Central Europe is now thought to be of alien origin (about 15%), most of Mediterranean origin and the trend is of a recent increase, aided by increasing temperatures and commerce. In some extreme cases the ability of the snails to disperse is extremely limited, or their requirements for specific substrata so high that they scarcely move after introductions. Such is the case of Papillaria papillaris, a minute snail native of Italy, Sicily and Malta. These snails were likely to have been introduced in the UK in the 18th century with Italian ornaments or stonework, then prized by affluent stately home owners. Just two populations (in Brownsea Island Castle and Cliveden House) have been found so far. Amazingly, they have barely moved a few meters in this time, although they form self-sustaining colonies. Likewise, in Spain, this snail is restricted to walls and ruins from the Roman period, two millennia living practically where they were placed!
Of course, there are many examples of the other extreme of the spectrum. Species that quickly spread on arrival, or after a lag period. An example is the Girdled Snail, Hygromia cinctella, a snail of Mediterranean origin which now is rapidly expanding in the UK. It was first noticed in the South in 1950, and it has now reached Glasgow, that is fast - for a snail. There have been suggestions that the snail might travel as stowaways in cars! Although this might seem outrageous, snails' habit of climbing up vertical surfaces and attaching itself firmly for aestivation or overwintering might facilitate this and it has been documented that this behaviour increases transport by cars. The girdled snail is 1 cm across and triangular when looked at from the side, with a pronounced keel that sports a pale line. At a distance looks like a juvenile garden snail. My daughter, however, noticed this snail on the pavement in my street a few days ago and it was necessary to rescue it from being crushed by passers by. I only noticed it wasn't a garden snail when I picked it up, after it had retreated into its shell. I will keep a close eye and see if it has already arrived in my garden.
Side view showing the keel and pale edge.
Underside showing the lack of umbilicus
Janet Ridout-Sharpe (2010) Papillifera papillaris: a second colony is discovered in England. The Archaeo+Malacology Group Newsletter, 18: 1-6. here.
Alena Peltanova, Adam Petrusek, Petr Kment, Lucie Jurˇicˇkova (2011). A fast snail’s pace: colonization of Central Europe by Mediterranean gastropods Biological Invasions : 10.1007/s10530-011-0121