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We have sequenced and characterized the complete mitochondrial genome of the sea slug, Aplysia californica, an important model organism in experimental biology and a representative of Anaspidea (Opisthobranchia, Gastropoda). The mitochondrial genome of Aplysia is in the small end of the observed sizes of animal mitochondrial genomes (14,117 bp, NCBI Accession No. NC_005827). The Aplysia genome, like most other mitochondrial genomes, encodes genes for 2 ribosomal subunit RNAs (small and large rRNAs), 22 tRNAs, and 13 protein subunits (cytochrome c oxidase subunits 1-3, cytochrome b apoenzyme, ATP synthase subunits 6 and 8, and NADH dehydrogenase subunits 1-6 and 4L). The gene order is virtually identical between opisthobranchs and pulmonates, with the majority of differences arising from tRNA translocations. In contrast, the gene order from representatives of basal gastropods and other molluscan classes is significantly different from opisthobranchs and pulmonates. The Aplysia genome was compared to all other published molluscan mitochondrial genomes and phylogenetic analyses were carried out using a concatenated protein alignment. Phylogenetic analyses using maximum likelihood based analyses of the well aligned regions of the protein sequences support both monophyly of Euthyneura (a group including both the pulmonates and opisthobranchs) and Opisthobranchia (as a more derived group). The Aplysia mitochondrial genome sequenced here will serve as an important platform in both comparative and neurobiological studies using this model organism.

Ctenophores are one of the most basally branching lineages of metazoans with the largest mitochondrial organelles in the animal kingdom. We sequenced the mitochondrial (mtDNA) genome from the Pacific cidipid ctenophore, Pleurobrachia bachei. The circular mitochondrial genome is 11,016 nts, with only 12 genes, and one of the smallest metazoan mtDNA genomes recorded. The protein coding genes are intronless cox1–3, cob, nad1, 3, 4, 4L and 5. The nad2 and 6 genes are represented as short fragments whereas the atp6 gene was found in the nuclear genome. Only the large ribosomal RNA subunit and two tRNAs were present with possibly the small subunit unidentifiable due to extensive fragmentation. The observed unique features of this mitochondrial genome suggest that nuclear and mitochondrial genomes have evolved at very different rates. This reduced mtDNA genome sharply contrasts with the very large sizes of mtDNA found in other basal metazoans including Porifera (sponges), and Placozoa (Trichoplax).