Exploring the Paramount Seamount
This expedition’s inaugural dive was on the Paramount Seamount, an underwater mountain with a summit less than 300m below the sea surface. We were able to characterize the biological communities on this seamount, and make some interesting comparisons with last year’s expedition to the Sangihe Talaud region in Indonesia.
Seamounts are isolated topographic features found on the seafloor – literally, mountains in the sea – and they provide important hard substrate for colonisation by sessile organisms, such as deep-sea corals, sea-lilies, and sponges, amongst others. Unlike chemosynthetic organisms found on hydrothermal vents, these communities are dependent on the rain-down of suspended organic particles from surface waters for food. Seamounts typically experience accelerated water flows, and as a result, they are an ideal setting for suspension feeders to passively intercept suspended food particles.
The dive started at ~800m. The ROV Little Herc moved upslope and explored the rim of the feature, which probably used to be an island shore a long time ago, before resuming movement towards the summit. Moving upslope, there was an increase in abundance (number of species per unit area) of organisms, and there was a general trend of high abundance of corals, but with low diversity – that is, we were encountering lots of the same species. There was a clear difference in the types of species colonising the summit margin, compared to lower depths, which were separated by a flat area. The summit margin was characterized by a high abundance of white brachiopods (these shelled organisms look superficially like clams, but are actually not at all related) and pink brittle stars. The rocks were a puzzling red color, prompting several hypotheses about their origin. One hypothesis from shore-side scientists was that the rocks are actually fossil shallow-water corals that were formed during a period at which the seamount was above or near the sea surface.
Only limited comparisons may be made between a single seamount from the Galápagos Rift and the many seamounts that were explored during the INDEX-SATAL expedition last year. Nevertheless, for scientists involved in both expeditions, it is hard not to consider how the regions compare. Indonesia, being within the coral triangle, had exceptionally high abundance and diversity of corals, sea-lilies and sponges. These sessile organisms also had a high abundance and diversity of associate fauna living upon them. By contrast, Paramount Seamounts typically had lower diversity and abundance compared to many of the seamounts in the Sangihe Talaud region. One interesting phenomenon was that we encountered a Brachyuran crab carrying some sort of egg case – almost identical to something that we had puzzled over previously in the INDEX-SATAL expedition. We still have much to learn about reproductive strategies in the deep sea.
This one dive on the Paramount Seamounts provided a tantalizing glimpse of the communities and species found in non-hydrothermal settings along the Galápagos Rift.
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