The family Hepadnaviridae contains Hepatitis B viruses that are enveloped viruses with reverse-transcribed DNA genomes.
However, recently scientists reported the discovery of a new family of fish viruses, designated Metahepadnaviruses, which lack one of the four Open Reading Frames, the ORF X, compared to Hepadnaviriruses.
Otherwise these Metahepadnaviruses exhibit key characteristics of Hepatitis B viruses, including genome replication via protein-primed reverse-transcription and utilization of structurally related capsids.
The very first of these Metahepadnaviruses is the Tetra Metahepadnavirus (TMDV) of the Mexican tetra (Astyanax mexicanus). Collectors covet this species because it consists of distinct two subspecies: a 'normal' and a blind version.
Thousands of years ago, a population of Mexican tetras in northeastern Mexico swam (or was swept) from its hospitable river home into underwater caves and became trapped. Facing a dramatically different environment of near total darkness and hardly any food, the fish had to adapt very fast. Among other changes, these 'cavefish' swiftly (in evolutionary terms, within just a few thousand years) lost their pigmentation and their eyes. Regressive evolution is the correct term.
 Liang: Hepatitis B: The Virus and Disease in Hepatology – 2010
 Lauber et al: Deciphering the Origin and Evolution of Hepatitis B Viruses by Means of a Family of Non-enveloped Fish Viruses in Cell Host & Microbe – 2017
 Protas et al: Regressive Evolution in the Mexican Cave Tetra, Astyanax mexicanus in Current Biology - 2008