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1. GENERAL ASPECTS CONCERNING MARINE
AND TERRESTRIAL BIODIVERSITY
Damià Jaume and Carlos M. Duarte
Mediter
ranean Institute for Advanced Studies (IMEDEA)
Spanish Council for Scientific Research (CSIC) - University of the Balearic Islands
Esporles, Mallor
ca, Spain
1.1. INTRODUCTION
THE OCEANS ARE THE LARGEST BIOME on earth. Totalling 361 million km
2
and
with a mean depth of 3,730 m, they cover 71% of the surface of the planet.
Their volume – 1,348 million km
3
– is immense, and they are also the primeval
scenario for the diversification of life. Thus the oldest known fossils ar
e
marine stromatolites, laminar structures produced by the activity of
cyanobacteria, preserved in Australia and dating back 3,500 million years.
Seemingly, the first animals also appeared in the sea. We know of trace fossils
800 million year old, but the first fossils of “real” animals are dated later; about
640 million years ago at the end of the Proter
ozoic period. These animals
belong to the so-called “Ediacara” fauna of the Vendian system, a name which
recalls the Australian locality where they were discovered, although they are
also pr
esent in other par
ts of the globe. They wer
e soft-bodied organisms that
ar
e hard to attribute to any of our moder
n types.
In comparison, the earliest terrestrial fossil record corresponds to spores, pos-
sibly of bryophytes (mosses, liverworts, etc.) and is datable to the Middle
Ordovician (about 450 million years ago). For animals, the first continental
settlement appears to go back to the Silurian period (a bit over 400 million
years ago), from which we have recovered remains of myriapods (centipedes
and millipedes) and arachnids, although certain trace fossils, probably pro-
duced by terrestrial arthropods, also date to the Ordovician period.
Marine organisms have thus had more time to diversify than their terrestrial
counterparts (about double in the case of animals). And yet the oceans appar-
ently harbour only 2% of the total number of known animal species. Scien
-
tists have resorted to different
ad hoc reasonings to explain this paradox. They
have pointed out the enor
mous potential for dispersal of the pr
opagules of
marine animals (eggs and lar
val stages), which would act against the genetic
1. GENERAL ASPECTS CONCERNING MARINE AND TERRESTRIAL BIODIVERSITY
19
b Photo 1.1: Seahorse (Hippocampus ramulosus) in a seagrass meadow. Seahorses, which are
generally found around our coasts in underwater meadows, are suffering a worldwide decline for unknown
causes.
segregation of their populations in a world apparently without barriers. They
also mention the differing size of primary producers in continents and oceans.
So whereas arboreal terrestrial vegetation can reach a height of more than 100
metres and offers a wide range of niches and microhabitats for other organ-
isms, primary production in the sea relies mainly on bacteria and unicellular
algae, which provide no structural support for diversification. Therefore, the
co-evolutionary processes between insects and angiosperm (= flowering)
plants that have been the driving force of the diversification of terrestrial biota
do not occur in the marine environment (there are only 58 species of marine
angiosperms, versus about 300,000 on the continents).
But is the prevalence of the continental biota a fair reflection of reality? Let us
address this question by first analysing what we know about animal biodiver-
sity on the continents and in the oceans, focusing on certain aspects that limit
the exploration of marine biodiversity and which may go some way to
explaining this paradox.
1.2. A COMPARISON OF BIODIVERSITY ON LAND AND SEA
The number of plant and animal species on the continents is estimated at
around 12 million (see table 1.1). 91% fall within a single phylum, the maxi-
mum categor
y of the taxonomic hierar
chy; namely the arthropods, embracing
creatures such as insects, crustaceans, arachnids, acari and other minor groups.
The continents are thus scantly diverse as regards animal body plans, and the
lar
ge number of species present is attained thr
ough vir
tually infinite combina-
tions of a single pair of body plans, namely: (1) ar
thropods with a single pair
of antennae and only three pairs of thoracic limbs (what we call insects); and
(2) ar
thropods that advance by means of their perioral appendages (the che-
licerates: arachnids, acari, etc.). The quantity of continental species still await-
ing discovery and description is simply overwhelming. Entomological expedi-
THE EXPLORATION OF MARINE BIODIVERSITY: SCIENTIFIC AND TECHNOLOGICAL CHALLENGES
20
Taxon Number of species after Briggs (1995)
Insecta 10,000,000
Acari
750,000
Ar
achnida
170,000
Nematoda 1,000,000
Mollusca 20,000
Other groups
100,000
TOTAL
12,040,000
Table 1.1: Estimated number of species per taxonomic group on the continents
tions to tropical rain forests continue coming up with thousands of new insect
species, at so fast a rate that many cannot be described by conventional meth-
ods, due to lack of time and/or resources, and are identified only by a number
or registration code. Thus, in a study confined to ten trees in a rain forest in
Borneo, the British entomologist Nigel Stork collected a mean of 580 species
of insect per tree. By comparison, a European oak harbours between 100 and
200 species. In tropical rain forests, tree diversity is much higher (up to 250
species per hectare) than in temperate forests, and the specificity of insects to
their host trees falls to between 3% and 20% (Ødegaard et al. 2000). These
data equate to extr
emely high numbers of species per hectar
e, without consid
-
ering insects of a terrestrial as opposed to arboreal habit, which are also far
mor
e diverse in tropical rain forests than in forests elsewhere.
But it is also tr
ue that the discover
y of higher
-rank taxa is only rar
ely report-
ed nowadays. Recently (2002), we hear
d of the discover
y in Namibia and Tan-
zania of a new order of insects, the Mantophasmatodea, resembling preying
mantis, although specimens of this group, wrongly identified, had formed part
of the collections of South African museums for more than a century. The last
description of a new order of insects dates back to 1914 (Notoptera).
1. GENERAL ASPECTS CONCERNING MARINE AND TERRESTRIAL BIODIVERSITY
21
Photo 1.2: Stromatolites in Hamelin Pool, Shark Bay, Australia. Stromatolites are the earliest liv-
ing structures known to man, formed from the growth of microorganism communities. Their oldest fossils,
dating to around 3,500 million years, were discovered in Australia.
On the continents, the discovery of new large-size species is likewise an infre-
quent event. Findings always take place in extremely remote areas, or territo-
ries where human conflict or isolationist political regimes have hampered zoo-
logical exploration. Among the most spectacular discoveries in recent years we
can cite
Dendrolagus mbasio, an arboreal kangaroo from New Guinea,
described in 1995, and the bovid
Pseudoryx nghetinhensis (1993) and cervid
Megamuntiacus vuquangensis (1996), inhabiting the forests of Vietnam and
Laos respectively.
The case of the sea is different. The number of marine species currently
described stands around 212,000 only, but there are eight, rather than one, ani-
mal phyla accounting for 90% of the total species (table 1.2). The diversity of
body plans is therefore much higher than on the continents: of the 30 phyla
reported, 15 (including groups like echinoderms, urochordates or ctenophores)
are exclusive to this biome. In comparison, of the mere 15 phyla reported from
land, only one, the Onychophora, a type of worm with legs, mandibles and a
velvet texture (hence the name “velvet worm”) is exclusive to this medium. For
some time, marine fossils from the Cambrian, such as
Aysheaia, were consider
ed
to be onychophorans, although now they are classified in a separate group
vaguely known as the “lobopods”. Apparently, the invasion of continental
waters by various phyla which r
emain typically marine has been halted by phy-
siological or str
uctural constraints. Thus the Ur
ochor
dates (sea squirts) have a
need for vanadium, a component of their blood pigments which is widely avail-
able in sea water but present in much lower and irregular concentrations in con-
tinental waters. Seemingly, the direct connection of the ambulacral system of
echinoderms to the exterior hinders osmorregulation in non-marine waters.
New phyla ar
e still being discovered in the sea, which indicates that the cata-
logue of marine biodiversity is far from being complete. The latest additions
THE EXPLORATION OF MARINE BIODIVERSITY: SCIENTIFIC AND TECHNOLOGICAL CHALLENGES
22
Taxon
Number of species after Bouchet (see chapter 2)
Porifera 5,500
Cnidaria 10,000
Nematoda 12,000
Annelida 17,000
Arthropoda 45,000
Mollusca 52,500
Bryozoa 15,000
Chordata 21,000
Other groups 20,500
TOTAL 212,000
Table 1.2: Number of species per taxonomic group present in the oceans
are the Cicliophora, recorded in 2000; a group of aschelminth worms living as
commensals in the perioral region of the Norwegian
(Nephrops norvegicus),
common (Homarus gammarus) and American (H. americanus) lobsters (Obst,
Funch and Kristensen 2006). Befor
e them (1983) came the Loricifera, animals
similar to rotifers which live among the non-consolidated grains of marine
sediments at all depths (Kristensen 1983).
As regards large-sized animals, remember that none of the ca. 10 species of
giant squid (over 20 m in length) (photo 1.3) recorded to date has ever been
observed alive, despite their apparent abundance (sperm whales frequently
show the imprints of their suckers on their skin). Or recall the description in
1983 of the “Megamouth” shark,
Megachasma pelagios (4.5 m long) (photo
1.4), discover
ed in Indo-Pacific waters, or that of
Balaenoptera omurai (2003),
a small rorqual (reaching up to 9 m in length) from the same ocean.
The inventory and description of smaller animals is far from being complete,
even in shallow waters easily accessible fr
om the coast. Hence our knowledge
of groups like the meiofauna – the animal community dwelling in between
grains of unconsolidated sediments – r
emains fragmentary even on the Euro-
1. GENERAL ASPECTS CONCERNING MARINE AND TERRESTRIAL BIODIVERSITY
23
Photo 1.3: Giant squid (Architheutis) found off the coast of Asturias (Spain). These mythical
cephalopods, although relatively abundant, remain largely a mystery, as none has yet been seen live in
its natural habitat.
pean coasts of longest naturalistic tradition. In fact, some estimates put the
per
centage of new species of copepods (tiny crustaceans that are the main
component of zooplankton, but also very abundant in marine sediments) on
Belgian sandy beaches at somewhere between 35% and 45% (Rony Huys,
pers. comm.). Other less accessible coastal habitats ar
e also yielding unexpect
-
ed results, including new taxa of higher rank. Recent explorations of anchia
-
line caves – located inland, but flooded by marine or brackish water – have
shown the existence of a new class of crustaceans (of a total of five), the Remi-
pedia (1980) resembling swimming centipedes; and two new orders of per-
acarids (relatives of amphipods, isopods and mysids), the Mictacea and the
Bochusacea (1985), as well as many new families and genera of other crus-
taceans. In all, eight of the 28 new families of copepods described between
1980 and 1999 came from anchialine caves, compared to only three from
marine plankton, which lives in a comparatively immense space (Geof
f
Boxshall, pers. comm.).
The majority of benthic organisms from surface waters appear to exhibit high-
ly discontinuous distributions, so sampling pr
ogrammes have to be well
THE EXPLORATION OF MARINE BIODIVERSITY: SCIENTIFIC AND TECHNOLOGICAL CHALLENGES
24
Photo 1.4: Megamouth shark (Megachasma pelagios) in North American Pacific waters. Discov-
eries in marine biodiversity are not all small-size species. They also include mighty creatures like this shark
of over 4 m length, first spotted 23 years ago.
designed and intensive in order to assess their true diversity. Thus Cunha et al.
(2005) used molecular techniques to r
eveal the extraordinary diversity of gas-
tropods of the genus
Conus present in the Cabo Verde Archipelago (52
species, 49 endemic); some of them restricted to a single bay and with vicari-
ants pr
esent in adjacent bays. In tr
opical seas, a r
ecent study of molluscs in a
292 km
2
area in New Caledonia (SW Pacific), a zone outside the Indo-Pacific
biodiversity hotspot for hermatypic corals, unveiled 2,738 species from 42
sampling stations dotted across all types of habitats, and the accumulation
curves suggest the occurrence of 3,900 species (Bouchet et al. 2002). This is
more than has ever been recorded in an area of comparable size, and more
exciting still: only 36% of species shared another area of New Caledonia a
mer
e 200 km away!
1.3. BIODIVERSITY A
T THE DEEP-SEA FLOOR
The coastal mar
gins with their wide variety of habitats (coral r
eefs, man
-
groves, seagrass meadows, estuaries, soft and rocky bottoms, etc.) harbour an
1. GENERAL ASPECTS CONCERNING MARINE AND TERRESTRIAL BIODIVERSITY
25
Photo 1.5: Coral reefs in the Red Sea. Coral reefs are diverse and highly productive ecosystems found
along shallow waters in tropical seas. Vast extensions of white coral (unpigmented) have recently been
discovered living at depths of up to 1,000 m, even in polar waters.
immense biodiversity. We might assume that, in comparison, the oceanic
floor below 1,000 m, supposedly uniform and covered mainly by soft sedi-
ments, could have nothing like the same number of species present. This is
the most extensive habitat on Earth, covering around 300 million km
2
, yet its
biodiversity remains practically unprospected due to technical and economic
constraints. Precision machinery, nets or vehicles are hard to operate at such
large depths, and their deployment is time consuming. Getting a dredge
down to 4,000 m takes about two hours, with another two for its recovery.
And using oceanographic vessels suited to these depths is a very costly enter-
prise (about
€50,000/day for the German R/V Polarstern; one of the best
equipped ships currently in existence for the study of deep oceanic floors).
Reckoning on the 0.5 m
2
of oceanic floor sampled by the larger dredges (of
the “van Veen” type), and five deployments per day (with scientific staff
working non-stop for 24 hours), sampling 2.5 m
2
of oceanic floor would take
up an entire working day and cost a minimum of
€50,000! There is little
chance, therefore, of deploying dredges or nets in modern oceanographic
cruises, which are generally devoted to objectives other rather than pure fau-
nistic prospection.
Mor
eover, the deep ocean is a dark world. At around 900 m depth, darkness is
total for the human eye, and what can be dir
ectly observable thr
ough cameras,
ROVs or submarines is limited to the ar
ea cover
ed by their artificial light
beams.
The study of this medium started late. During the first half of the 19th cen-
tury, the ocean was considered devoid of life below 300 fathoms (ca. 550 m)
depth. This was believed by the British naturalist Edward Forbes (1815-54),
an eminent marine biologist and author of
Natural Histor
y of the European
Seas
(1859; published posthumously), the most complete marine biology
handbook of its day. In 1834, Forbes published a report on the molluscs,
cnidarians and echinoderms of the Aegean Sea, which testified to finding no
trace of animal life in soundings up to 230 fathoms (about 420 m) depth. He
then generalized this situation to the entire ocean, and his authority was such
that no one seriously tried to refute his theory despite indications to the con-
trary. Indeed in these years several British scientists and explorers had report-
ed the presence of animal life at great depths; among them John Ross, who
recovered a starfish at 1,800 m depth in the Bay of Baffin, or James Clark
Ross, who r
ecorded the existence of animals on the sea floor at 730 m depth
during soundings off New Zealand in 1843. Some time later (1860), George
C. Wallich caught 13 brittlestars at 2,293 m depth between the Labrador
THE EXPLORATION OF MARINE BIODIVERSITY: SCIENTIFIC AND TECHNOLOGICAL CHALLENGES
26
Peninsula and Iceland. The recovery, also in 1860, of sessile fauna attached to
a damaged telegraphic cable set at 2,184 m depth between Cape Bon (Tunisia)
and Sardinia should have been still more conclusive in discrediting the azoic
theory. But these facts were not considered until 1868-69, when fellow
Britons Charles W
yville Thompson and William Carpenter embarked on
their famous pr
ospections of the Atlantic deep floor on board the
Lightning
and the Porcupine, discovering animal life at 4,289 m depth. Forbes may be
part way exonerated by the conclusions drawn from a later (1870)
Porcupine
dredging cruise to the Mediterranean, which noted that animal life at 2,744 m
was ver
y scant compared to that of the Atlantic, and in fact some areas were
practically azoic.
Life in the deep sea was not dir
ectly obser
ved until 1934, when W
illiam Beebe,
a zoologist, and Otis Bar
ton, an engineer
, descended to 923 m depth off
Ber
muda in the
Bathyspher
e
, a claustr
ophobic steel chamber with port holes
communicating with its support vessel via a telephone cable. It took another
26 years for man to reach the bottom of the deepest oceanic trenches, when
the bathyscaph
Trieste, crewed by Swiss national Jacques Pickard and Ameri-
can Don Walsh, landed on the floor of the Marianas Trench; at 10,915 m, the
deepest oceanic floor on Ear
th.
The total of oceanic floor deeper than 3,000 m that has been adequately sur-
veyed for fauna is less than 30 m
2
, and shows a wide heterogeneity in species
1. GENERAL ASPECTS CONCERNING MARINE AND TERRESTRIAL BIODIVERSITY
27
Photo 1.6: Marine copepod. The copepods, planktonic crustaceans, are the most individually numerous
group of marine organisms; the marine equivalent of insects.
THE EXPLORATION OF MARINE BIODIVERSITY: SCIENTIFIC AND TECHNOLOGICAL CHALLENGES
28
Photo 1.7: Fan mussel (Pinna nobilis) amid a Posidonia oceanica meadow in the Spanish
Mediterranean.
The fan mussel is the fastest growing biv
alv
e (up to 1 mm of shell per da
y) and can grow
to 1 m in height. A dweller of seagrass meadows, its abundance has declined dramatically and harvesting
is now strictly prohibited.
composition. The number of new species gathered in dredge or net deploy-
ments is extremely high at these kinds of depths, and invariably accounts for
over 50% of total captures. Recently, the sampling of 1 m
2
of oceanic floor at
5,000 m depth in the Angola Basin (South Atlantic) rendered 600 new species
of harpacticoid copepods (Pedro Martínez-Arbizu, pers. comm.). If we con-
sider that the number of described copepod species is currently around 12,500
(including the numerous parasitic forms in fishes and other invertebrates), the
estimates of up to 10 million species on the deep ocean floor (or even 100 mil-
lion, if we factor in the meiofauna; Lambshead 1993) begin to look distinctly
credible. Having said that, the methods underlying these estimates are some-
what naïve, and the resulting conjectures must be handled with care. Hence in
a study that is now a classic, Grassle and Maciolek (1992) established a model
of spatial correlation between the number of species collected and the geo-
graphic distance covered along a deep-sea transect between 1,200 and 2,100 m
depth at the continental rise off the east coast of North America. Based on the
obser
vation that about one new species was added per square kilometre of
oceanic floor, the 798 species of macrofaunal invertebrates found in 21 m
2
could be extrapolated to 100 million species in the world oceanic floor below
1,000 m. There is no need to add that evaluating the strength of this kind of
assessment would call for many more studies dealing with faunistic prospec-
tion and the spatial heterogeneity of marine species composition at all geo-
graphical scales.
We can see that it is still hard to venture any figure for the species richness of
the oceans, although it may well be comparable to that of the continents. Esti-
mations to date rely on poor statistics and incomplete taxonomic and geo-
graphic baseline data. Furthermore, there are huge operational constraints to
obtaining such data, especially in the deep sea. The oceans are nevertheless an
extraordinary reservoir of biodiversity. Life there has had roughly four times
the time for diversification as life on the continents. Oceans harbour 30 phyla
of metazoans, of which 15 ar
e exclusive to the medium (compar
ed to only one
on the continents), and are home to the largest animal on the face of the Earth:
the blue whale (up to 36 m in length).
ACKNOWLEDGEMENTS
This chapter is a contribution to the MarBEF Network of Excellence on
Marine Biodiversity and Ecosystem Functioning, funded by the Eur
opean
Commission.
1. GENERAL ASPECTS CONCERNING MARINE AND TERRESTRIAL BIODIVERSITY
29
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