The Darwin Online website

is a truly fantastic resource: Darwin's papers, notebooks, letters and manuscripts online. Free and searchable. Treat yourself to a browse here. The field notebooks are Charles Darwin's on-the-spot records of what he saw, recorded and collected. From these we know that the first specimen collected by Darwin when Beagle made landfall was:

Eel dark reddish purplish brown with pale or whitish brown spots. Eyes Bluish. Darwin had caught a Jewel Moray Eel.

Then:
Saturday left our anchorage & stood out to outside of Island, did not anchor
(Sunday) Continued to beat to windward...

Brief, but vivid and straight from the mind of the great man. From the Galapagos, Otahaite, Lima notebook (1835) Beagle field notebook EH1.17, page 17b.