The clubs on stalks along its sides are the reason to know this genus. They contain light-producing tissue, and a disturbed animal flashes from them: one of a very small number of sea slugs anywhere that make their own light, and a genuinely startling thing to trigger with a dive torch on a night dive.
What the light is for has never been settled. A flash may startle a predator, or it may work the way a lizard's shed tail does, drawing a strike toward a detachable part and away from the body, since these clubs come off readily. Both explanations are plausible and neither is proven.
This is the large species, six centimeters (2.4 in), against relatives in the same genus that stay around two. It grazes bryozoans, the fine encrusting lace on rubble and dead coral, and it does it after dark, spending the day out of sight.
It also swims. Off the bottom the body flexes from side to side in slow deep curves and the animal travels through open water, which is uncommon among sea slugs and a strange sight from something that spent the previous ten minutes creeping over rubble.
The name honors Wilhelm Gottlieb Tilesius, a German naturalist and artist on a Russian circumnavigation at the start of the nineteenth century. It has been photographed on the sandy ground off Abu Dabab, which is as good a place as any to start looking, with a torch, after dark, on the rubble at the reef edge.
- three pairs of stalked clubs standing out along the sides of the body
- gills exposed at the rear, flanked by the rearmost pair of clubs
- large for the family at six centimeters, and out only at night
- swims by flexing the body when it leaves the bottom
