This hitchhiker just showed up about 2 weeks ago. At first I thought he only came out at night but he is out in full light today.
In the first photo he is working along a Koralia 1. He is about 7mm long and 4mm wide with a hard shell but way to small for his entire body. He can move a pretty fast clip for a "snail" type creature.
Although it is a bit dificult to make out in the photo his shell is quite flat similar to a limpet, not voluminous like a snail.
This is what he normally looks like working his way along the glass. He has two antennae that are at least as long as his body.
