Mysterious marks in algae

[PeteTheBloke]

I noticed this regular pattern of marks on an old desk out in the garden (it was supposed to be in the bin, but I keep forgetting). The surface has a plastic "wood-effect" finish and the algae film has built up over a longish (!) period. Recently I noticed these pear-shaped marks in the algae. Each pear is 4mm at its longest point, they're very regular and they appear to follow wandering tracks. I hope the pics show you what you need.

Location: N. Ireland.

Slugs? Snails? Mouses? Not birds, I think??

[Wild-Woman]

Algae crop circles?

[triops]

Snails browsing algae. To see how the marks are made, rather than a complicated explanation, put a snail in a glass or plastic tank and watch its radula at work from the underside. Fascinating!

[PeteTheBloke]

Crop circles! Brilliant!

Thanks Triops, I thought snails but was slightly thrown by the arrangement of "mouthfuls" into a series of arcs.

[Dogghound]

I agree this is the work of snails or slugs.