Mermaid trolls are such a fascinating topic! From what I've gathered digging into lore and fan discussions, they seem to occupy this unique space where aquatic mythology and troll traits collide. Unlike land-based trolls who often have shapeshifting abilities tied to forests or mountains, mermaid trolls usually have powers more aligned with water magic—things like controlling tides or communicating with sea creatures. Their forms tend to be more fixed, possibly because their bodies are adapted to underwater life. I remember one obscure comic where a mermaid troll could temporarily grow extra fins or change scale colors, but full shapeshifting? Rare. It makes sense though—imagine the energy needed to reshape gills and flippers into something terrestrial!
That said, there's always room for creative exceptions. In 'Tides of the Abyss,' a niche RPG, certain blessed mermaid trolls could take humanoid forms during lunar eclipses. But even then, it felt more like a celestial loophole than innate ability. Most canon sources stick to the idea that their 'shifts' are subtler—maybe adjusting bioluminescence or skin texture to camouflage. It’s fun to speculate, but I’ve yet to see anything definitive. Maybe future stories will explore hybrids with dual heritage who bridge the gap!