Introduction Cusk eels of the family Ophidiidae are a diverse group of fishes found throughout the world oceans at depths ranging from the shallow subti- dal to the oceanic trenches. In general, little is known about the biology and ecology of most of the deep-sea species (Nielsen et al. 1999), although reproductive mode was examined in some (Nielsen 1986). Our knowledge of the pat- terns of variation in population dynamics and the mechanisms behind such variation is absent for most species, especially in bathyal and abyssal regions. While behavior mediates rates of sur- vivorship for predator avoidance and of growth rates for prey capture, an understanding of these components is lacking in Ophidiidae in part because of the extremely limited time available for direct observation underwater. Here we report, we believe for the first time, on the relationship between juvenile cusk eels and pancake urchins (Family Echinothuriidae) and interpret these observations in the context of shelter resources and foraging opportunities. Materials and Methods We conducted imaging surveys (i.e., with sub- mersible, remotely operated vehicle (ROV), autonomous underwater vehicle, and camera sled deployments) during several cruises from 2003 through 2005 at 13 seamounts across the New England and Corner Rise chains, with total bot- tom time exceeding 450 hours (see Appendix 1 in Moore et al. 2008 for detailed dive information). This report focuses on direct underwater observa- tions made during two cruises (RB04-04 and RB05-03) by the National Oceanic and Atmos- pheric Administration (NOAA) Ship Ronald H. Brown. Both cruises used the ROV Hercules and the depressor camera sled Argus in a dual body configuration (Evers and Manley 2007). Two Commensalism Between Juvenile Cusk Eels and Pancake Urchins on Western North Atlantic Seamounts Jon A. Moore 1 and Peter J. Auster 2 1 Wilkes Honors College, Florida Atlantic University, 5353 Parkside Drive, Jupiter FL 33458 USA —email: jmoore@fau.edu 2 National Undersea Research Center and Department of Marine Sciences, University of Connecticut at Avery Point, Groton CT 06340 USA Abstract We describe a commensal relationship between small (approximately 6 to 9 cm) deep-sea cusk eels, one tentatively identified as Barathrites sp. (Osteichthyes: Ophidiidae), and pancake urchins, both Hygrosoma petersi and Phorosoma placenta (Echinodermata: Echinothuriidae). These inter- actions were observed in situ using a remotely operated vehicle (ROV), deployed at depths of 1410 to 1775 m on three seamounts in the western North Atlantic (Bear and Rehoboth seamounts in the New England Seamounts and Yakutat Seamount in the Corner Rise Seamounts). Full motion video and still frames documented individual cusk eels associated with urchins in open fine-grained sediment and coral rubble habitats. Cusk eels foraged around the periphery of urchins and took refuge at the base and among the long spines on the aboral sur- face. We suggest that such associations provide juvenile cusk eels with shelter from predators or flow (or both) and access to prey far from other forms of refugia. Keywords Ophidiidae, Echinothuriidae, deep-sea, ROV, fish, behavior. Bulletin of the Peabody Museum of Natural History 50(2):381–386, October 2009. © 2009 Peabody Museum of Natural History, Yale University. All rights reserved. • http://www.peabody.yale.edu