Kelim, Chapter Seventeen, Mishnah Thirteen
Today’s mishnah deals with one who makes vessels out of the bones or skin of fish or other creatures that come from the sea.
1) All that live in the sea are clean, except the sea-dog because it seeks refuge on dry land, the words of Rabbi Akiba.
2) If one made vessels from what grows in the sea and joined to them anything that grows on land, even if only a thread or a cord, if it is susceptible to uncleanness, they are unclean.
Section one: Vessels made of skin and bones of all creatures of the sea are not susceptible to impurityonly of those of land creatures. Rabbi Akiva notes that the one exception is the sea-dog (probably a seal) because it lives both in the sea and on land. Since it lives partly on the land, vessels made from its skin and bones are susceptible to impurity.
Section two: If one made a vessel that is mostly of material from the sea, if even a small part of the vessel was made from something that comes from land, it is susceptible to impurity.