The Talmud (Chullin 66b) and Tosefta (Chullin 3:27) mention a fish called achsaftias as being kosher. The word achsaftias is not Hebrew or Aramaic; it is an Aramaicized version of a Greek word. This appears to be the Greek word xiphias, which refers to the swordfish (based on the Greek xiphos, which refers to a sword). From the outset, then, there is reason to believe the swordfish is the achsaftias of the Talmud and is kosher.But then there's the matter of its scales, which is complicated.
The swordfish, Xiphius gladius, is one of a group of large predatory fishes with swordlike projections known as billfish. Other billfish are several species of marlin, sailfish, and spearfish (which have relatively short bills). But whereas the bills of other billfish are round in cross-section – like spears – that of swordfish are flat in cross-section, like swords. Accordingly, any reference to a fish named “sword” presumably refers to the swordfish.
Furthermore swordfish are by far the most common of all billfish species in the Mediterranean. If swordfish weren’t kosher, the Talmud would not describe another billfish as kosher and mislead people into thinking it was talking about the more common swordfish.
Rabbi Dr. Slifkin, a.k.a. "the Zoo Rabbi," is curator of the Biblical Museum of Natural History in Beit Shemesh, Israel. Past PaleoJudaica posts on him and the Museum are here and links.
Visit PaleoJudaica daily for the latest news on ancient Judaism and the biblical world.