Slugs

Symptoms
Soft succulent plant growth, including seedlings, young tender shoots, leaves and flowers are damaged or eaten, particularly in wet conditions. Underground parts of some plants, especially potato tubers and tulip bulbs can be tunnelled by black keeled slugs. Slime trails may be visible near the damaged tissues. Can cause additional damage, such as bacterial soft rot on vegetables and ornamentals.

Spot them
Slugs vary in size and colour, from yellowish grey to brown/black.