Radioactive Hermit Crab Shell

[Zaxh]

All jokes aside, can someone identify what is growing on my red-leg hermit shell? It's growing a nice orange color under blue light.

 

[WheatToast]

It is hard to tell for certain without clearer photos, though it may be dying red encrusting macroalgae (ex. Hildenbrandia, Peyssonnelia, and Coralline algae):

yes. phycoerythrin is an accessory pigment and normally does a handoff of energy from phycoerythrin through other pigments to chl a in an intact phycobilisome

now imagine in a dead cell or otherwise damaged phycobilisome, the coupling between phycoerythrin and the other pigments is broken. So instead of handing off that energy down the chain, it fluoresces at ~570nm and longer which looks orange.

you can see this by putting a little vinegar on some coralline algae...




maybe, but probably not. it might just generate more sunscreen pigments (carotenoids or similar). Also photosynthetic organisms have ways of turning down their photosynthetic machinery when the light is too high. This might change the look.