From Physorg, we learn that the spotted salamander is “the first known vertebrate to have an endosymbiont”, in the form of an alga conferring a benefit on the eggs by living inside them:
Naturalists first noticed an association between spotted salamander eggs and green algae more than 100 years ago. This relationship was formalized by name in 1927 by Lambert Printz, who named the algal species Oophilia amblystoma. The genus name means “egg loving.” The nature of that symbiosis was not known until the 1980s, when experimentation revealed the salamander embryos do not develop as quickly or as fully in the absence of the green algae. Likewise, algae grown separately from the embryos but in the presence of water exposed to the embryos also grew more robustly.
Endosymbiosis has been associated in the past with single-celled creatures.
Lynn Margulis must be happy. with this.
Some wonder why it wasn’t investigated for 100 years.