Since the family Lysianassidae is one of the largest and most diverse in the order Amphipoda it is suitable for the study of adaptations to different zoogeographic regions. The fauna is best known in regions of cold water (polar oceans and deep sea). Here most species are relatively small (6-13 mm) but a small number of large species are also found. In tropical waters such as the Indo-Pacific lysianassids are less well known and less conspicuous. The species are smaller and there is an apparent lack of large species. The problem is therefore not the occurrence of giants in cold water habitats but rather the small size of tropical species. While increased predation or competition could be responsible it is suggested here that the differences are probably due to shorter life spans at the higher temperatures in tropical waters. The species mature at a small size and produce small clutches of small eggs. In cold water survival is greater and the species mature at a larger size and infrequently produce large clutches of large eggs.