Observations of the Deep-Sea Trachymedusa Benthocodon pedunculata

Abstract

Recent observations, from a remotely operated vehicle (ROV) working in the Monterey Submarine Canyon in the northeast Pacific and from the Johnson Sea Link II in the northwest Atlantic, revealed the presence of the trachymedusa Benthocodon pedunculata near the bottom in both oceans. Although feeding was not observed in the field or in the lab, gut contents indicate a preponderance of benthopelagic crustaceans although a wide variety of zooplankton have been found. Two morphologically different tentacle types both possess cnidae with a 3:1 ratio of euryteles to stenoteles. Unencysted metacercariae of a trematode parasite were found in the manubrium of more than half the medusae examined. Additional key words: Cnidaria, Hydrozoa, Monterey Bay, benthic boundary layer, feeding, morphology Knowledge of deep-sea biology has progressed greatly in the last 15 years as advances in technology have increased the amount of time and quality of in situ observations in deep-sea research. Pelagic biomass exponentially declines with depth (Angel & Baker 1982) but diversity and biomass increase near the bottom in a zone termed the benthic boundary layer (Childress et al. 1989; Angel 1990). The benthic boundary layer (BBL) is defined as a layer of resuspended sediments and, with an associated nepheloid layer, may encompass an area from the benthos to 100 meters above the bottom (Childress et al. 1989). There is considerable vertical structure within these layers at bathyal depths (Childress et al. 1989), abyssal depths (Smith et al. 1986), and hydrothermal vents (Wiebe et al. 1988); the causes and effects of this vertical structure and increased diversity and biomass are of great interest to biologists. Large gelatinous zooplankton have been the subject of many in situ studies (Mills & Goy 1988), but large size and fragility also hinder the collection of undisturbed or undamaged specimens (Madin 1988). Medusa diversity increases between 500 m and 800 m, but our knowledge of benthopelagic medusae (i.e., those living up to 100 m from the bottom) is limited as sampling within this zone is difficult (Larson et al. 1992). Observations and collections of benthopelagic medusae have been enhanced with the advent of research submersibles, remotely operated vehicles a Author with whom to correspond. (ROVs), and remote cameras. Yet, in situ observation of small medusae (<3 cm diameter) remains extremely difficult and our knowledge of predator-prey interactions is limited (Gowing & Wishner 1992). Crustaceans dominate the zooplankton in the benthopelagic realm (Angel 1990; Wildish et al. 1992) and benthopelagic medusae are likely to be opportunistic predators of copepods and other crustaceans (Gage & Tyler 1991). The trachymedusa Crossota pedunculata BIGELOW 1913 was collected in Bermuda in 1984 (Larson et al. 1991) and redescribed as Benthocodon pedunculata (LARSON & HARBISON 1990), a benthopelagic medusa with an extremely patchy distribution (Larson et al. 1992). These small medusae have a deeply pigmented (reddish-purple) gut and can reach 3 cm in height and width. They have been observed between 800 and 900 m in the Atlantic Ocean and 175 and 975 m in the Pacific Ocean at densities up to 80 medusae/m2 (Larson et al. 1992) but their impact on the benthopelagic ecosystem is unknown. We observed this benthopelagic medusa in both the Atlantic and Pacific and examined gut contents and prey-capture structures.