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The low-mass low-surface brightness (LSB) disc galaxy Arakelian 18 (Ark 18)resides in the Eridanus void and because of its isolation represents an idealcase to study the formation and evolution mechanisms of such a galaxy type. Itscomplex structure consists of an extended blue LSB disc and a bright centralelliptically-shaped part hosting a massive off-centered star-forming clump. Wepresent the in-depth study of Ark 18 based on observations with the SCORPIO-2long-slit spectrograph and a scanning Fabry-Perot interferometer at the Russian6-m telescope complemented by archival multi-wavelength images and SDSSspectra. Ark 18 appears to be a dark matter dominated gas-rich galaxy without aradial metallicity gradient. The observed velocity field of the ionised gas iswell described by two circularly rotating components moderately inclined withrespect to each other and a possible warp in the outer disc. We estimated theage of young stellar population in the galaxy centre to be ~140 Myr, while thebrightest star-forming clump appears to be much younger. We conclude that theLSB disc is likely the result of a dwarf-dwarf merger with a stellar mass ratioof the components at least ~5:1 that occurred earlier than 300 Myr ago. Thebrightest star forming clump was likely formed later by accretion of a gascloud.

Search for gas accretion imprints in voids: II. The galaxy Ark 18 as a result of a dwarf-dwarf merger