Opah (Lampris guttatus). X-ray of a section through an opah, showing its skeleton. This marine fish lives in tropical to temperate waters and has a distinctive round flat silvery body and elongate red fins. (The fins are pressed flat to the body in this specimen.) Commercial fishing boats sometimes accidentally catch these solitary fish along with tuna. The largest specimens weigh more than 90 kg. This specimen measures 21 centimetres in length.