Covering two-thirds of the earth’s surface, the deep sea is the largest sedimentary habitat, consisting of fine muddy sediments and soft clays (Gray, 2002). The majority of the deep-sea environment is abyssal plain, but also incorporates expansive continental ridges, deep trenches and canyons to form a complex habitat (McClain and Hardy, 2010).
At depths of over 3000m the abyssal plain experiences extremes of temperature (average 1-2°C), pressure (300 atmospheres) and darkness with the exception of bioluminescence (Figure 1; Van Dover, 2000). Despite the extreme conditions, deep-sea benthic exploration has resulted in high species discovery rates indicating a high biodiversity supported by the abyssal plain (Fiege et al. 2010).
The abyssal plain itself attributes to 54% of the earth’s surface and at depths of 3000-6000m receives no exposure to light required for photosynthetic primary production (Scheckenbach et al. 2010). With the exclusion of hydrothermal vent areas, the majority of energy supplied to the abyssal plain benthos is derived for primary production at the sea surface (Rex and Etter, 2010).
However, on average only 1% of sea surface water production will reach the sea floor due to processing of material by organisms in the upper water column (Lampitt and Antia, 1997). The flux of this surface derived material will also vary with depth, seasonality and distance from productive coastal and surface water, creating a spatial and temporal patchiness in food input (Billet et al. 1983). Although the deep-sea benthos energy input is supplemented by large marine mammal cadavers and sinking plant material from marine and terrestrial sources, it poses a challenging environment to inhabit due to limited food resources (Klages et al. 2002).
Despite the extreme conditions, the abyssal plain supports a wide diversity of fauna such as Sponges, Anemones and Corals but is visually dominated by Echinoderms (Ruhl and Smith, 2004). Echinoderms occupy both the deep sea benthic (crawling Brittlestars) and pelagic (swimming Sea cucumbers, genus Paelopatides) environment (Gage and Tyler, 1991). Deep-sea Echinoderms show unique adaptations in feeding methods, morphology, movement and reproduction will be discussed in subsequent sections.