I shall summarize my lay understanding of my question before going into details. To my lay reading, the sex of a vast diversity of animal kingdom species can be thought of as arising from the extent that the process of Aromatization - the catalyic conversion of testosterone produced by the gonads to oestrogen - happens. This extent of aromatization can be egg temperature determine, or genetically determined.
The German "Aromatase" Wikipedia page makes the following curious, unsubstantiated claim (translation mine, so apologies in advance):
"Aromatase (aka CYP19A1) is the Enzyme that in backboned animals catalyses the conversion from testosterone to oestradiol ....[emphasis mine]"
Which raises the question in the title. My understanding was that the aromatization process as the sex determining process was wider spread in the animal kingdom than backboned animals, at least as wide as chordates. Since the claim is unsubstantiated, i put the question in my title to this community.
Another take on the question: Can anyone give me examples of animals which are not backboned animals and for whom sex is determined by the extent of the aromatization reaction?