雌雄差や成長に伴う変化のほか、ホソツツムシ属（Cerapus）やホソヨコエビ属（Ericthonius）、カマキリヨコエビ属（Jassa）などの雄では二型が知られ（adult formとhyper-adult form）、同じ種でも多様な形態を示す。
＜参考＞
・Benoit Gouilleux, Hiroyuki Ariyama, Ana Cristina Costa, Guillemine Daffe, Agnese Marchini, Joana Micael and Aylin Ulman (2020) New records of Ericthonius didymus Krapp-Schickel, 2013 (Crustacea: Amphipoda: Ischyroceridae) in European waters with a focus in Arcachon Bay, France and key to Ericthonius species, Journal of the Marine Biological Association of the United Kingdom, 100 (3) , pp. 401-412.