Amphicutis stygobita Pomory, Carpenter & Winter, 2011 was the world’s first known cave brittle star. It has been found only in two anchialine caves: Bernier Cave (type locality and current study area) and Lighthouse Cave on San Salvador Island, The Bahamas. Bernier Cave’s low salinity (14–28 ppt) reduces ionic precipitation in A. stygobita’s endoskeleton to produce fewer and lighter ossicles. Scanning electron microscopy (SEM) revealed details of internal skeletal structures including elongated arm segment ossicles with greatly reduced density and increased fenestration. The large ceiling entrance of Bernier Cave is directly above the water allowing abundant growth of algae and accumulation of detritus. Small (disk diameter = 3-4 mm) microphagous deposit-feeding brittle stars survived and grew in captivity by consuming energy-rich detritus containing algae, bacteria, invertebrates, and a sticky biofilm containing extracellular polymeric substances (EPS). Reproductive structures are described for this hermaphroditic brooding species, as are morphology and growth rates for adults and three babies born in captivity. Comparisons are made to three recently described cave species that appear to be cave endemics and to several epigean brittle stars including the brackish-water species Ophiophragmus filograneus and two deep-sea species: Amphilepis patens and Amphilepis platytata herein removed from synonymy. Several of these species show paedomorphy, including reduced mouth structures and arm ossicles. Paedomorphy conserves energy by not producing, maintaining, and transporting adult structures not needed for survival. Paedomorphic traits that are adaptive and occur in cave organisms are considered troglomorphic traits, as in A. stygobita. Correlations are made between specific paedomorphisms and environmental features.
