Me and my wife are planning to travel to Japan next september 2014. Unfortunatelly my wife have a big problem with shrimp, she have a huge allergy to it(even the food fried on the same oil that was used to fry shrimp could create a big problem for her). I know that we will find a lot of different foods over that, including MC Donalds and conventional ocidental food, but certainly we will try authentic japanese food(besides everthing, my wife love it), what precautions should we take? There are any japanese food that are mixed with shrimp that we wouldn't(and should) know? It is a common thing on Japan and people are used to deal with it?
Seafood allergies are a big issue in Japan, vegetarianism is often hard to deal with, a strong allergy to shrimp will cause problems unless you know what not to eat and are prepared to tell the waiter your needs in Japanese. If you rely on English skills only and do not avoid special types of food, you will have a problem, guaranteed.
My first recommendation would be to for sure avoid deep fried food such as Tempura, Kushikatsu amd Tonkatsu. Those are all fried in the same oil as other customers food and therefore for sure mixed with shrimp.
Secondly, as long as you go for other food types you might be fine most of the time provided that you prepare a paper for the waiter in Japanese that makes them aware of the issue and thus stay safe of ordering something that contains a sauce or soup made of shrimp. You better also get a proper understanding what causes the allergy so that you know if it's only shrimp or also crabs or other types of shellfish. Shellfish varieties in Japan are numerous and you want to avoid a misunderstanding. Then you will be ok and enjoy a huge variety of Japanese food. Be aware that in many cases waiters do not know too much about the food they are serving and have to check with the cook about the ingredients of what they serve, so you better have something on the paper that says "Please check with the cook that... ".
Last but not least there is the probably most important warning: The Japanese healthcare system is suffering from a slew of litigation cases which causes many hospitals and clinics to refuse emergency cases. A friend of mine had to be brought to 6 (!) different clinics in Tokyo with a bust appendix until he was accepted. So if the consequences of the allergy are life-threatening, you better ask your doctor at home if you can get any emergency treatment injection or pills to stop or delay the effects of the allergy until you can get accepted in a hospital and get proper treatment. Otherwise you might face dire consequences.
My wife is also allergic to seafood and found the last time we went a pain due to dashi (literally stock, but for the most part fish / seafood stock) finding its way into a whole load of unexpected places. My Japanese isn’t awful but I don’t trust myself to be able to explain this properly to a waiter/waitress.
This time we’re taking explanatory flash cards with us. You can buy these commercially (e.g. http://www.selectwisely.com/catalog/Seafood_Fish_and_Shellfish_Allergies ) but there are also free printable versions (e.g. http://www.justhungry.com/japan-dining-out-cards ). It’s easy to print and customise these to exactly what you need.