Eat your jellyfish!
2 September 2011 by Liz Fisher
A large bloom of jellyfish in the Arabian Sea turns out to have provided food for micro-organisms on the deep-sea floor. Liz Fisher explains how this could help us understand how seabed communities adapt to changes in the supply of food and oxygen.
In December 2002, scientists aboard the RRS Charles Darwin were surveying the geology and biology of the Arabian Sea off the coast of Oman. They were using a deep-towed video and camera imaging system called SHRIMP (Seafloor High Resolution IMaging Platform) to examine the sea floor. What they saw was a surprise - a large number of dead and decaying jellyfish (Crambionella orsini), some in deep-sea canyons, others rolling down the sloping seabed into the abyss. Jellyfish usually live at the surface, but carcasses were seen on the seabed as deep as 3·2km.
Crambionella orsini is a native species of the Indian Ocean, living in surface waters but only very occasionally in large numbers. It is edible, and is exported from the Arabian Sea to China and Japan. Similar blooms have occurred elsewhere. For example, a mass deposition of jellyfish-like creatures called pyrosomes (Pyrosoma atlanticum) was discovered near the Ivory Coast in 2006. The patches of jellyfish found at the Oman Margin were several metres across, at least 7cm thick and covered about a fifth of the seabed. Other areas of sea floor were covered in a continuous layer of jelly 'slime'.
Jellyfish carcasses on the sea floor at 1,400m depth. The weight (seen to the right of the image) is dangled from the SHRIMP camera system which is positioned approx 2-3m above the sea surface.
The scientists reported "curious aligned striated patterns", which may have been formed by jellyfish dragging through the soft sediment surface as they tumbled down to the sea floor. They took images of the weight dangling below SHRIMP, dipping into the jelly slime, which let them gauge how deep the jelly detritus was. They estimated that this jelly detritus would provide ten times more food on the seabed than is normally produced each year by phytoplankton - the tiny algae that form the base of the marine food chain.
We're not sure why jellyfish blooms occur, but some people think they are becoming more common and that human activity may be partly to blame. Fish and jellyfish compete for the same prey. Fish also eat jellyfish, so overfishing would mean jellyfish have more food and fewer predators, so their numbers would rise. Global warming and pollution of coastal waters by fertilizers stimulate blooms of phytoplankton. Jellyfish prefer feeding on the smallest types of plankton, which tend to dominate these blooms, so they flourish.
Living jellyfish were seen only in the well-oxygenated surface waters off Oman. It is possible that they were present in large numbers on the seabed because of the very low oxygen waters which occur in the Arabian Sea, forming a region known as the oxygen minimum zone (OMZ).
Oxygen minimum zone
This zone is caused by strong monsoon winds which blow from the north-east between December and March and from the south-west between June and September. This allows nutrient-rich water to well up into shallower regions from below, stimulating phytoplankton growth. Once the plankton die and sink, they decay, using up oxygen. This causes the OMZ to form, with very little dissolved oxygen (20 times less than in surface waters). The OMZ at the Oman continental margin (the region of the ocean between the deep sea and the shore) extends from 100m to around a kilometre deep.
The dead carcasses presumably fall into the OMZ for a variety of reasons, including natural death. The OMZ would have slowed down their rate of decay as they sank through it, so more of the jelly detritus was deposited on the sea floor. However, jellyfish remains are not a long-term feature of the seabed because jellyfish blooms are rare and last only a few months at a time.
Scientists using a multi-corer to take sediment samples. (Left to right) Xana da Silva, Kate Larkin and Rachel Jeffreys.
What effect does all this jellyfish detritus have on the creatures that live on the seafloor? This is where our work came in. We hypothesised that the clumps of rotting jellyfish might have caused areas of the seabed to become starved of oxygen and that this led to the rise of specialist bacteria. These microbes live on chemical energy from hydrogen sulphide (produced from the decay of organic matter, like jellyfish) rather than on the energy in sunlight, making their food by a process called chemosynthesis. Their chemical expertise lets them obtain energy with no need for oxygen. Other organisms can then feed on these specialised bacteria.
To test our hypothesis we decided to examine two similar environments - the Oman Margin and the Pakistan Margin. Both border the Arabian Sea and both have OMZs, but only the former had been affected by the jelly fall. In normal circumstances - that is, without a jelly fall - we would expect to find chemosynthesis occurring on both margins within the OMZs, but not above or below them. Would the jelly change this?
We examined foraminiferans from the Oman and Pakistan sediments. These are single-celled organisms which live in shells called tests. Depending on the species, the test may be made of organic material, sand grains and other particles cemented together, or secreted calcium carbonate. Radiating from the test's small opening are fine hair-like extensions of the cell known as reticulopodia, which the foraminiferans use to find and capture food. Some deep-sea species feed on the remains of phytoplankton which make their food using photosynthesis, but where there is little or no oxygen, they can also eat bacteria.
Photosynthesis or chemosynthesis?
We used a technique called stable isotope analysis to find out what the foraminiferans were eating. Carbon has two stable forms: Carbon-13 (heavy) and Carbon-12 (light). Stable isotope analysis lets us measure the ratios of these 'isotopes' by comparing them to the ratio of isotopes in a standard substance. If there is a higher proportion of the heavier isotope, the values are positive; if there is a lower proportion it is negative. The differences are so small that the values (called isotopic signatures) are measured in parts per thousand (o/oo). Chemosynthesis produces a different ratio from photosynthesis. For example, foraminiferans consuming food that was made by photosynthesis would give a value of -15 to -20 o/oo, whereas those feeding on chemosynthetic food sources would have isotopic values outside this range.
We expected to find that foraminiferans living in the OMZ feed on both phytodetritus and chemosynthetic bacteria, whereas those outside it feed only on phytodetritus. This was the case off Pakistan where there had been no jelly fall. Yet off Oman, the results implied that foraminiferans were feeding on chemosynthetic bacteria even below the OMZ. This was exciting - it suggested that some species may be feeding on bacteria that live on the jelly detritus.
To confirm this, we also measured nitrogen stable isotopes. Nitrogen's stable isotopes are Nitrogen-15 (heavy) and Nitrogen-14 (light). If foraminiferans were eating phytodetritus, we would expect their signature to be 10-12 o/oo. If they were eating chemosynthetic bacteria, the signature would be lower. Once again, in both places this was the case in the OMZ, confirming the bacteria were a food source. Yet at Oman, there were low nitrogen values below the OMZ, confirming our carbon results.
It certainly seems likely that the jelly has indirectly nourished certain foraminiferans at the Oman margin. If jellyfish do become more common in world oceans, then more of the seabed may be affected by their jelly remains. We don't know what this will mean for many deep-sea fauna, but it is likely that microbial communities will adapt readily to the new conditions. This could also have wider implications for how carbon moves through the ocean - it will greatly increase the amount of carbon delivered, via jelly detritus, to the seabed.
Dr Liz Fisher is a post-doctoral research assistant working in the Oceans & Ecosystems Research Cluster in the School of Environmental Science at Liverpool University.
'Mass deposition of jellyfish in the deep Arabian Sea' - DSM Billett, BJ Bett, et al (2006), Limnology & oceanography 51(5), 2077-2083.
'The jellyfish joyride: causes, consequences and management responses to a more gelatinous future' - AJ Richardson, A Bakun, et al (2009). Trends in Ecology & Evolution 24(6): 312-322.