High Biodiversity in the Deep-Sea
Today we had one of the most interesting dives so far during this expedition. The location was the volcanic feature termed “Target K” in the southwestern part of our area of operations. The summit of this seamount reaches a depth of approximately 460m below sea level. During ROV dives at similar features in the same depth range as Target K, we have seen that the abundance (amount of animals per area unit) of benthic animals (that live on the sea floor) at the summit are relatively high when compared to the abundance found in deeper areas. This is not surprising considering that these non-chemosynthetic deep-sea communities are composed mainly of suspension feeders (animals that feed on microscopic suspended particles) such as corals, sponges and sea lilies.
Suspended food particles are mostly composed of body parts and secretions of planktonic organisms that sink from the surface waters. As these particles sink into the ocean depths they become consumed and dissolved (transported). As you go deeper, the concentration of food particles in the water column becomes smaller. Food availability is one of the main factors limiting the number of animals that can co-exist in a given area. So, in general, animals at the top of a seamount, such as Target K, obtain more food than the animals on the flanks or base of it; therefore the animals at the top can grow larger and reproduce more.
On most seamount sites the communities are dominated by a small number of groups of organisms and are often unique in composition. For example, the summit of the seamount Naung, which has a depth of approximately 500 meters and is separated from Target K by approximately 90km, has a distinctive community that is dominated by paramuricid sea-fan corals and a few species of sea-lilies. A great contrast to this trend was found at Target K. At this seamount, both the abundance AND diversity of species was exceptionally high! High numbers of individuals and species of at least 10 large groups of corals were observed, including: black corals, hydrocorals, bamboo corals, golden corals, bubblegum corals, soft corals, sea pens, stony corals and other bottle-brush and fan-like corals. A yet-to-be-determined number of sea lilies and glass sponges were extremely abundant as well. But it is not only the diversity of these sessile animals (fixed in one place) that is high, it is also the diversity of animals living on them. Many species of brittle stars, anemones, hermit crabs, barnacles, worms, squat lobsters, fish and shrimp use the corals, sponges and sea lilies as their home. In many cases these relationships between host (animal providing house) and associate (animal using the house) seemed to be specific (certain associate species restricted to certain host species). In other parts of the world it has been found that certain deep-sea coral communities have greater diversities than their shallow water equivalents. Could this also be the case in the Coral Triangle?
The exploration of the Sangihe Talaud region using the underwater and communication technologies available on board the Okeanos Explorer is allowing us to make very detailed observations of the deep-sea animal communities of this region. These observations constitute fundamental material for planning, proposing and executing future scientific research that will help us answer already stated, and other interesting questions such as:
Which of the host/associate relationships are species-specific? How many species are endemic (unique, not found anywhere else) to this region? What are the evolutionary histories of these species? Why is there a higher diversity in Target K compared to other nearby similar seamounts such as Naung? How many species are shared among these seamounts? Are the populations of these shared-species actively exchanging individuals? Are there any hydrographic barriers limiting the dispersal of certain species more than others?
These are just some of the many questions generated by our exploration so far.
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