Not Thor Heyerdahl
Thor Heyerdahl did see some strange bioluminescent animals whilst on his expedition but that is not the explorer you are thinking of. You are thinking of Eric de Bisschop who did a follow up expedition on a raft Tahiti-Nui. He reported a merbeing type animal coming onboard and attacking a crew member although the account is written slightly ambiguously and is not found in another account of the voyage by Bengt Danielsson which makes me think it was a sort of dream sequence put in the narrative for reasons unknown.
Thor Heyerdahl did see some strange bioluminescent animals whilst on his expedition but that is not the explorer you are thinking of. You are thinking of Eric de Bisschop who did a follow up expedition on a raft Tahiti-Nui. He reported a merbeing type animal coming onboard and attacking a crew member although the account is written slightly ambiguously and is not found in another account of the voyage by Bengt Danielsson which makes me think it was a sort of dream sequence put in the narrative for reasons unknown.