I saw this a few days ago – https://phys.org/news/2019-06-tube-anemone-largest-animal-mitochondrial.html.
The tube anemone Isarachnanthus nocturnus is only 15 cm long but has the largest mitochondrial genome of any animal sequenced to date, with 80,923 base pairs. The human mitochondrial genome (mitogenome), for example, comprises 16,569 base pairs.
The shape of the mitogenomes in these two species of tube anemone and the gene sequences they contain were more surprising than their size.
Because they are closely related species, their gene sequences should be similar, but I. nocturnus has five chromosomes while P. magnus has eight, and each has a different composition in terms of genes. This kind of variation had previously been found only in medusozoans, sponges, and some crustaceans.
However, more data will be needed before a definitive conclusion can be reached. The necessary data could come from the sequencing of these species’ nuclear genomes, which Stampar and his group intend to complete by the end of 2019.