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Nematodes in Marine Sediments Nematodes are often the most diverse metazoan group in a habitat. Even habitats typically perceived as having low metazoan diversity, can actually have a high diversity of nematode species. The deep-sea bed, once believed to be devoid of life, is increasingly being seen to be one of the most hyperdiverse ecosystems on the planet, though there is still debate on this issue.
[Lambshead & Boucher 2003] suggest that the "hyperdiversity"
of nematodes on the deep-sea bed may be
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Diversity in Estuarine and Marine Habitats Free-living marine nematodes have been widely studied in north east
European estuaries, where it is common to find 40 - 80 species at a
single Nematodes are typically the most [Bouwman 1983] estimated the number of nematode species in an estuary to be around 200, and that the total diversity for the North Sea was 735 nematode species. |
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Deep-sea Diversity Once regarded as being devoid of life, the deep-sea is now thought
to have high species diversity; indeed [Grassle 1989] suggested that this diversity might rival
or even surpass the biodiversity of
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Estimates of Deep-sea Diversity [Grassle 1991] and [Grassle & Maciolek 1992] sampled a 176 km long 798 macrofaunal species were identified, from 171 families and 14 phyla. After an initial rapid accumulation of new species in the samples, there was, on average, one new species found for every 1 km travelled along the transect. This can be generalised to one new species per km2. Given that the area of seabed below 1000 m is approximately 3 x 108 km2, [Grassle & Maciolek 1992] estimated there to be approximately 108 deep-sea macrofaunal species. However, as the transect was from The final estimate for deep-sea macrofaunal diversity was 1 x 107, or 10,000,000 species. [Lambshead 1993] , while reviewing this macrofaunal species estimate and some terrestrial biodiversity estimates, produced an estimate for nematode deep-sea diversity. [Lambshead 1993] simply used the fact that nematode alpha diversity tends to be an order of magnitude higher than that of the macrofauna, and so produced an estimate of 1 x 108 species of marine deep-sea nematodes.
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Deep Sea Diversity Research over the last 15 - 20 years has revolutionised ideas about the processes which control and maintain diversity in the deep sea. The large area of marine sediments, coupled with spatial and temporal disturbances and habitat heterogeneity, are all strong factors that may promote species diversity in marine sediments. |
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Habitat Size Since 70 % of the Earth's surface is covered by the seas and oceans, it follows that 70 % of the Earth's surface is covered by the seabed - the benthic habitat. The great majority of all benthic habitats are deep-sea, sedimentary muds. It is not surprising that one of the largest habitats on Earth has high species diversity. |
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Structural diversity: muds versus rainforest |
Structural Diversity When compared with tropical rainforests, muds appear to be relatively featureless, and seem not to have the same habitat complexity that is associated with high diversity.
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Sediments often show steep vertical These spatial and |
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Structural Diversity
These structures represent habitat heterogeneity at the small scale
of meiofaunal organisms. |
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Deep-sea Biogenic Structures Biogenic structures on the deep-sea bed at a depth of 1003 m. |
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Worm Biogenic Structures The small spionid polychaete worm, Pygospio elegans, constructs a tube of adhered sand grains within the sediment, which may be packed together. The worm can occur in such high numbers that all the sediment appears bound up in tightly packed tubes |
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The aggregations of tubes can form mounds raised 10 - 15 cm higher than the level of the surrounding sediments. | |||
Diatom cover at top of worm tubes |
The surface of the mound may appear dark brown due to very high densities of diatoms photosynthesising at the sediment surface. | |||
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Estuarine Biogenic Structures Lugworm burrows and faecal mounds, along with amphipod feeding marks and Nereis diversicolor trails, are the biogenic structures seen on this muddy sand. |
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Sampling nematode diversity |
Sampling Nematode Diversity The basic unit for sampling marine nematodes is the core tube. In inter-tidal areas it is a simple matter to take samples by hand, but in the deep-sea more sophisticated equipment is required. Core samples provide good estimates of alpha diversity because they sample the whole habitat. However, they do represent discrete points in space. So, for this reason, they may be less good for assessment of regional diversity as a very large number of samples may be required. |
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Hand sampling |
Hand Sampling Core tubes are usually made from plastics such as perspex. They are bevelled externally to prevent sediment compression and usually sample an area between 5 - 100 m2. |
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Ship To work offshore, and especially in the deep sea, a ship is required which is able to deploy remote sampling gear to great depths. This is the British research vessel the RRS Challenger. |
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Multiple Corer The multiple corer carries a coring head bearing 8 - 12 core tubes. It can be lowered to great depths, limited only by the cable length from the ship. When it reaches the sea bed, the lead-weighted and hydraulically damped coring head can take a series of undisturbed core samples which are then winched back to the surface. When it reaches the sea bed, the lead-weighted and hydraulically damped coring head can take a series of undisturbed core samples which are then winched back to the surface |
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Core Sampling Core samples retrieved from a multicore, clearly showing the collected sediment. These core samples were retrieved from a depth of over 100 m near the Porcupine Seabight in the North East Atlantic. Core Samples |