Deep-sea biology

Species that are ontogenetic migrators have early life stages (juveniles) that live
shallower in the water column than the adults and therefore experience a brighter
environment than the adults. This work provides evidence that juveniles and adults of the
ontogenetically migrating crustacean species Gnathophausia ingens, Oplophorus
gracilirostris, and Systellaspis debilis have evolved visual adaptations to their respective
environments. The juveniles use apposition optics that provide greater resolution,
whereas the adults use superposition optics that maximize sensitivity. These animals also
have regional specializations to aid in viewing a light field that is brighter above than
below, such as accessory screening pigments located dorsally and superposition type
optics ventrally. The non-ontogenetic migrators Notostomus elegans and Notostomus
gibbosus possess superposition optics as both juveniles and adults, implying that the
changes seen in ontogenetic migrators are indeed visual adaptations.

Much of the world's oceans lie below a depth of 200 meters, but very little is known about the creatures that inhabit these deep-sea environments. The deep-sea fish family Melamphaidae (Stephanoberyciformes) is one such example of an understudied group of fishes. Samples from the MAR-ECO (www.mar-eco.no) project represent one of the largest melamphaid collections, providing an ideal opportunity to gain information on this important, but understudied, family of fishes. The key to the family presented here is the first updated, comprehensive key since those produced by Ebeling and Weed (1963) and Keene (1987). Samples from the 2004 MAR-ECO cruise and the U.S. National Museum of Natural History provided an opportunity to review two possible new species, the Scopelogadus mizolepis subspecies, and a Poromitra crassiceps species complex. Results show that Scopeloberyx americanus and Melamphaes indicoides are new species, while the two subspecies of Scopelogadus mizolepis are most likely o nly one species and the Poromitra crassiceps complex is actually several different species of Poromitra. Data collected from the MAR-ECO cruise provided an opportunity to study the distribution, reproductive characteristics and trophic ecology of the family Melamphaidae along the Mid-Atlantic Ridge (MAR). Cluster analysis showed that there are five distinct groups of melamphaid fishes along the MAR. This analysis also supported the initial observation that the melamphaid assemblage changes between the northern and southern edges of an anti-cyclonic anomaly that could be indicative of a warm-core ring. Analysis of the reproductive characteristics of the melamphaid assemblage revealed that many of the female fishes have a high gonadosomatic index (GSI) consistent with values found for other species of deep-sea fishes during their spawning seasons.