(Pssst! T-T-Trigger was making a joke...)
The biggest that I have heard tell of is about a foot and a half, and that's a Spearer, I do believe, which are far less common in aquaria.
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Quote:
The largest of all mantis shrimp, Lysioquillina maculata, can reach up to 40 centimeters in length. It is monogamous and the male (seen here) has enlarged spearing appendages for impaling passing fish.
All images are courtesy Roy L. Caldwell, University of California Berkeley.
http://www.genomenewsnetwork.org/articles/2004/04/30/shrimp.php
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If you can get a picture of the Mantis, specifically the meral spots from it's foelimbs or a clear picture of the telson (the tail), Dr. Roy will ID it for you.
Welcome aboard, btw. I have a little G. Viridis that is about 2.5 inches or so (Baron von Munchausen).
He lives with a green mithrax crab (only whacked twice, when the crab foolishly tried to swoop down upon the Baron's shrimp platter), about 5 hermits (hardly ever touched for food, as he vastly prefers snails) and a constantly rotating cast of snails...rotating on a spit,with a bit of B-B-Q sauce and a side of cole slaw, that is.
Ahem.
-Ron