Overview on Hyperdiversity
Speciose Taxonomic Groups
Comparative Hyperdiversity
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Nematodes in marine sediments

 

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.

  • What factors have led to a high diversity amongst meiofaunal species, particularly nematodes, in marine sediments?

[Lambshead & Boucher 2003] suggest that the "hyperdiversity" of nematodes on the deep-sea bed may beoverstated compared to coastal waters. [Gray et al. 1997] also found comparable levels of meiofaunal diversity on the continental shelf compared to deep-sea samples. These studies show the importance of considering scale when sampling and estimating diversity, and the problems associated with global estimates extrapolated from local samples.

  • How can nematode diversity be sampled in deep-sea and other habitats?
 
 

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 singlelocation.

Nematodes are typically the mostspeciose animals in these habitats.

[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.

   

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 ofrainforest.

  • What are the estimates of diversity in the deep-sea?
   
 

Estimates of Deep-sea Diversity

[Grassle 1991] and [Grassle & Maciolek 1992] sampled a 176 km longtransect of the continental slope off New Jersey, USA.

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 frombathyal depths, where macrofaunal diversity is known to be higher than atabyssal andhadal depths, [Grassle & Maciolek 1992] reduced the estimate, by a factor of ten.

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.

  • What is thesignificance of this estimate?
   
 

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.

       
   

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.

 
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.

  • What happens if a more appropriate scale is chosen to look at sedimentary muds?
       
 


Muddy sand

 

Sediments often show steep vertical gradients of declining oxygen, and other chemicals associated with a transition from oxic to anoxic conditions.

These spatial and temporal gradients result in small scale habitat heterogeneity.

       
 


Biogenic structures

 

Structural Diversity

Larger animals may create small-scale habitat structure and complexity, as a result of the biogenic structures they produce.

These structures represent habitat heterogeneity at the small scale of meiofaunal organisms.

       
 


Deep-sea Biogenic Structures

 

Deep-sea Biogenic Structures

Biogenic structures on the deep-sea bed at a depth of 1003 m.

       
 


Worm tubes packed together

 

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

       
 


Mounds caused by worm tubes

  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.

       
 


Lugworm burrows and faecal mounds

 

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.

       
 
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.

       
 
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.

       
 


RRS Challenger

 

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.

       
 


Multiple corer

 

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

       
 


Core Sampling

 

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
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.