that may be what's called 'liver mortis,' somethiing whereby the blood in a cadaver settles in the bottom-most area, because of gravity. the poor thing, it really tugs at my heart. it reminds me of my girl jeeves (the first gecko i ever had, the only one i have lost), a big, healthy-looking gecko, lost in a heartbeat.
the only way to know is to refridgerate the corpse (good for max 3 days), and take it into a vet for necropsy (as others have recommended). it really is worth it, i found it helped calm my mind & stay informed for my other geckoes. i know that what turned out to have been the COD for my jeeves was an infection from an inflamed, undeposited egg yolk.
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marla
currently: 0.0.9 catfish, 0.1 ferrets, 2.8.3 leopard geckoes, 0.0.13 oriental fire-bellied toads, and 0.2.0 sugar gliders