I found there are two characters for "xia1" (ghra as in Middle Chinese), 蝦 and 鰕.
Could someone explain what the difference is?
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This can help you to distinguish 蝦 and 鰕 http://dict.variants.moe.edu.tw/yitia/fra/fra03660.htm
蝦 the toad. The toad can be found in Compendium of Medical Herbs. It has black spots at the back. Small one. Can jump to catch many insects.
鰕 the shrimp. In three characters. (this is 說文解字 typical wording) Other book says this is a kind of spotted fish. Now is corrected to shrimp. 鰕, is the character 蝦 we use now.
so the character 蝦 came from 蝦蟆 (toad), but somehow it replaced 鰕 and become the right character. So if you want to have some shrimp, you should use 蝦.
Though I'm a native speaker, I'm still amazed that 鰕 is original character since I use 蝦 all the time. Thanks for your question.
According to modern science, shrimp is not a kind of fish, and it is not even Vertebrate，so you shall use 蝦 rather than 鰕. Because 虫 usually used to build glyphs related to worm-like animals, and 魚 used for fishes.