烏帽子鯛

烏帽子(えぼし) (eboshi, “black-lacquered cap”) + 鯛(たい) (tai, “sea bream”). The tai changes to dai as an instance of rendaku (連濁).

烏(え)帽(ぼ)子(し)鯛(だい) or 烏帽子鯛(エボシダイ) • (eboshidai) 

1. a man-of-war fish

As with many terms that name organisms, this term is often spelled in katakana, especially in biological contexts (where katakana is customary).