As it says on the tin, when you Team Up successfully, you also get a sunlight.
Thanks to Octo from the BrainGoodGames discord for the suggestion.
(Currently on Windows and the beta branch only)
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Rolling in the Reef update for 21 April 2021
1.02c (Team Up gives one sunlight)
Wendy
Patchnotes via 
Steam Community
View more data in app history for build 6577720
Rolling in the Reef Windows Depot
Depot 884961
Changed depots in beta branch