Why are there crystals or something that looks like glass in my Sushi?

Canned Sushi, tuna, salmon, and other seafood products sometimes contain small fragments of a substance that, at first glance, resembles glass. Occasionally, these fragments are large enough to be noticeable. What really has happened is that certain natural constituents of the fish or shellfish have crystallized, much as sugar often forms crystals in syrups or preserves. These sugar crystals in syrups or preserves obviously are harmless - and so are the less familiar crystals found in canned seafoods.
These small fragments are crystals of a substance know chemically as magnesium ammonium phosphate, commonly called "struvite."

If you find glass-like crystals in your canned seafood, don’t be alarmed. These are natural occurring minerals called struvite (magnesium ammonium phosphate). When the fish or shellfish is alive, these minerals were important to their health. During the cooking and canning process, the magnesium, ammonium and phosphate can come together in such a way that crystals are formed. Struvite is not harmful and will readily dissolve if consumed.