Extreme Adaptation in Caves. 
Cave adaptation leads to unique anatomical specializations in many taxonomic groups.  As the role of vision is reduced or disappears in a subterranean environment, other specializations arise to allow the organism to successfully detect and interact with their environment.  A suite of unique, convergent phenotypes associated with subterranean adaptation has emerged (termed troglomorphy), with reduction or loss of pigmentation and eyes being the most conspicuous.  Two vertebrate groups that have successfully colonized and adapted to subterranean environments are cavefishes and cave salamanders.  There are many shared troglomorphic anatomical characters shared between these two groups, and we describe herein the morphological traits that are unique to fishes and salamanders that are adapted to caves and other subterranean habitats.  Troglobionts, animals strictly bound and adapted to underground habitats, are outcomes of not just regressive evolution, but also constructive adaptation.  There are skeletal changes, such as broadening and flattening of the head, as well as hypertrophy of non-visual modalities.  Cavefishes and salamanders have lost eyes and pigmentation, but also enhanced mechanosenzation, chemosenzation and, in some cases, electroreception.  Both cavefishes and cave salamanders have become important models in the study of the ecology, behavior, and evolution of subterranean colonization and adaptation.  However, our knowledge is primarily limited to a few taxa and many questions remain to be studied.  Anat Rec, 2018.  © 2018 American Association for Anatomy.