Protein-coding introns in mitochondrial genomes
Mitochondrial genomes differ greatly in size, coding potential and even whether they are circular or linear. Mammalian mitochondrial DNA is small (11–28 kbp) and intronless. However the mitochondria of certain other organisms range up to 1000 kbp in size.

Certain sponges (demosponges) with large mitochondrial genomes contain type I introns and type II introns.  Although introns were initially thought to have no other function than to separate exons, the introns of certain nuclear genes have been found themselves to contain genes. This turns out also to be true of some of the mitochondrial introns. To quote from then introduction to a paper I found through an internet search:


  Most Group I introns encode homing endonuclease genes (HEG) and/or
  maturase of the LAGIDADG † family, while most Group II introns encode a
  reverse transcriptase (RT).


These introns are found in genes such as that encoding cytochrome oxidase subunit 1 (COI).



† This appears to be a misprint for the LAGLIDAD mutases, which are also endonucleases, and are involved in splicing the introns in which they reside.