We discovered for the first time a mitochondrial intron in a non-tetillid demosponge, which sheds new light on the interpretation of mitochondrial intron evolution among non-bilaterian animals and has consequences for phylogenetic and DNA barcoding studies. The newly discovered class 1 intron of Aplysinella rhax (Verongida) CO1 has an ORF for a putative LAGLIDADG-type and resembles other sponge and cnidarian mitochondrial introns. Our analysis of the Aplysinella rhax intron underlines that the patchy distribution of introns in sponges is caused by a combination of horizontal and vertical transmission. Further implications for CO1 phylogenetic and barcoding projects are discussed.