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FRESHWATER ANIMAL DIVERSITY ASSESSMENT
The Freshwater Animal Diversity Assessment:
an overview of the results
E. V. Balian ÆH. Segers ÆC. Le
´ve
`que ÆK. Martens
ÓSpringer Science+Business Media B.V. 2007
Abstract We present a summary of the results
included in the different treatments in this volume.
The diversity and distribution of vertebrates, insects,
crustaceans, molluscs and a suite of minor phyla is
compared and commented upon. Whereas the avail-
able data on vertebrates and some emblematic
invertebrate groups such as Odonata (dragonflies
and damselflies) allow for a credible assessment, data
are deficient for many other groups. This is owing to
knowledge gaps, both in geographical coverage of
available data and/or lack of taxonomic information.
These gaps need to be addressed urgently, either by
liberating date from inaccessible repositories or by
fostering taxonomic research. A similar effort is
required to compile environmental and ecological
information in order to enable cross-linking and
analysis of these complementary data sets. Only in
this way will it be possible to analyse information on
freshwater biodiversity for sustainable management
and conservation of the world’s freshwater resources.
Keywords Biodiversity Continental aquatic
ecosystems Endemicity Biogeography
Freshwater Global Assessment
Introduction
The fifty-eight chapters in this compilation aim to
present a comprehensive and up-to-date review of
animal (plus one chapter on macrophyte) diversity and
endemism in the continental waters of the world. The
treatises are diverse, and this is a consequence of the
specific features of the different taxa they deal with.
Nevertheless, owing to the standard approachall experts
agreed to follow, it has, for the first time, become
possible to compare patterns in the biodiversity of
groups as diverse as nematodes, dragonflies and fresh-
water turtles. Clearly, one can imagine numerous
approaches to study these data, and an in-depth analysis
will be presented elsewhere. Here, we restrict ourselves
to presenting a summary overview of the results.
The present overview focuses on species diversity
and endemism. Data on the genus level are available
and presented for all taxa except molluscs.
Guest editors: E. V. Balian, C. Le
´ve
ˆque, H. Segers &
K. Martens
Freshwater Animal Diversity Assessment
E. V. Balian H. Segers
Belgian Biodiversity Platform, Brussels, Belgium
E. V. Balian (&)H. Segers K. Martens
Freshwater Laboratory, Royal Belgian Institute of Natural
Sciences, Vautierstraat 29 B-1000, Brussels, Belgium
e-mail: Estelle.Balian@naturalsciences.be
C. Le
´ve
`que
Antenne IRD, MNHN-DMPA, 43 rue Cuvier,
Case Postale 26, Paris cedex 05 75231, France
K. Martens
Department of Biology, University of Ghent,
K.L. Ledeganckstraat 35, Gent 9000, Belgium
123
Hydrobiologia (2008) 595:627–637
DOI 10.1007/s10750-007-9246-3
An overview of freshwater animal diversity
When we calculate the total number of described
freshwater animal species, we obtain a total of
125,531 species (Tables 1,2; plus one micrognatho-
zoan) or approximately 126,000 species. This figure,
obviously, represents present knowledge and signif-
icantly underestimates real diversity. Most authors,
especially those dealing with less emblematic groups,
point out that significant fractions of species remain
to be discovered, and/or caution that cryptic diversity,
the importance of which we can only speculate about,
remains concealed because of the almost exclusive
morphological approach to taxonomy. The record of
126,000 species represents 9.5% of the total number
of animal species recognised globally (i.e., 1,324,000
species: UNEP, 2002). If it is taken into account that
freshwaters (lakes, rivers, groundwater, etc.) take up
only about 0.01% of the total surface of the globe,
then it becomes evident that a disproportional large
fraction of the world’s total biodiversity resides in
freshwater ecosystems.
The majority of the 126,000 freshwater animal
species are insects (60.4%), 14.5% are vertebrates,
10% are crustaceans. Arachnids and molluscs repre-
sent 5 and 4% of the total, respectively. The
remainder belong to Rotifera (1.6%), Annelida
(1.4%) Nematoda (1.4%), Platyhelminthes (Turbel-
laria: 1%), and a suite of minor groups such as
Collembola (the estimate of this taxon is based on a
restricted subsample of species, see Deharveng et al.,
2008, present volume) and some groups that are
predominantly marine (e.g., Bryozoa, Porifera). On a
regional scale, the Palaearctic appears to be the most
speciose for most taxa, except for insects and
vertebrates. The record for insects is fairly similar
in the Palaearctic, the Oriental and the Neotropical
regions, whereas vertebrates are most diverse in the
Neotropical, followed by the Afrotropical, and
Oriental regions.
Of freshwater macrophytes, there are 2,614 species
distributed over 412 genera. This amounts to ca. 1%
of the total number of vascular plants known to date
(270,000: Chambers et al., 2008, present volume).
This constitutes a considerable fraction, taking into
account that macrophytes are primarily terrestrial. On
the other hand, macrophytes play a key role in
structuring freshwater ecosystems, as they provide
habitat and food to many organisms. Macrophyte
species diversity is highest (ca. 1,000 species) in the
Neotropics, intermediate (ca. 600 species) in the
Oriental, Afrotropical, and Nearctic, and relatively
low (ca. 400–500 species) in the Australasian and the
Palaearctic regions.
The present assessment of freshwater diversity is
incomplete. Our focus is on animal taxa, and only
vascular plants, of all other kingdoms, are also
included. Micro-organisms such as bacteria (s.l.),
viruses, Protozoa, Fungi, and algae are not treated
although these groups clearly are as significant to
freshwater ecology and diversity as the taxa here
considered. Most of these groups, with the exception
Table 1 Total species diversity of the main groups of freshwater animals, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Other phyla 3,675 1,672 1,188 1,337 1,205 950 181 113 6,109
Annelids 870 350 186 338 242 210 10 10 1,761
Molluscs 1,848 936 483 759 756 557 171 0 4,998
Crustaceans 4,499 1,755 1,536 1,925 1,968 1,225 125 33 11,990
Arachnids 1,703 1,069 801 1,330 569 708 5 2 6,149
Collembolans 338 49 6 28 34 6 3 1 414
Insects
a
1,5190 9,410 8,594 14,428 13,912 7,510 577 14 75,874
Vertebrates
b
2,193 1,831 3,995 6,041 3,674 694 8 1 18,235
Total 30,316 17,072 16,789 26,186 22,360 11,860 1,080 174 125,530
a
The distribution of species by zoogeographic regions is incomplete for several families of Dipterans; as a result, the sum of the
regional species numbers is lower than the number of genera known in the world (See chapter on Diptera families excluding
Culicidae, Tipulidae, Chironomidae and Simulidae)
b
Strictly freshwater fish species only are included (there are an additional *2,300 brackish waters species)
628 Hydrobiologia (2008) 595:627–637
123
of algae and cyanobacteria, are dramatically under-
studied in aquatic biodiversity. As the key role of
micro-organisms in ecosystem functioning and health
is becoming more and more obvious, it is to be hoped
that future assessments of micro-organismal diversity
in freshwaters will complete the picture of freshwater
biodiversity. Estimates on some groups are available,
for example, there are 3,047 species on record for
aquatic Fungi, 2,000 of which are probably restricted
to freshwater (Shearer et al., 2007), and 2,392 species
of freshwater protozoans (Finlay & Esteban, 1998).
Problems and knowledge gaps: state of the art
As noted above, the Palaearctic region has the highest
number of species on record, for all taxa except
vertebrates. For most groups, this remarkable result is
very likely not factual, as indicated by many experts.
The purported overwhelming biodiversity of the
Palaearctic probably results from the fact that most
taxonomic expertise and research efforts are centred in
this region. Similarly, several authors highlight the
lack of data from the Afrotropical and Oriental realms
(e.g. Central Africa, parts of South America and
Southeast Asia) The geographical gaps in knowledge
are often linked to the extent (or limitation) of
taxonomic expertise, which is greatly unequal from
one group to another. On the other hand, there are
several groups for which the current, Holarctic-centred
distribution of species richness is suspected to be
accurate: amphipods are typical of cool temperate
climates and are notably rare in the tropics. Epheme-
roptera or Plecoptera are predominantly Palaearctic
and also this is congruent with the environmental
preferences of these groups.
Similarly, a lack of knowledge on autecology of
many species makes it difficult to decide whether a
taxon is a true freshwater species or not, and hence
whether they are to be included in the count. Such is
the case for springtails, many water beetles and
rotifers, amongst others. The current estimate for
Collembola is based on the subset of species for
which ecological information exists. It is likely that
this number is an underestimate of the global number
of freshwater-dependent springtails. In rotifers the
problem is especially acute for bdelloids, often semi-
terrestrial, many of which are known from single
records only.
Diversity and distribution of vertebrates are clearly
better documented than for other groups and even
though it can be seen that new species of freshwater
fish or even amphibians are still being described
regularly, experts of all vertebrate groups are able to
supply a fairly reliable estimate of the true number of
extant species. Molluscs and crustaceans are gener-
ally also quite well documented, despite some
geographical gaps in tropical areas. For insects, the
situation is very different from one group to the next.
The emblematic dragonflies are exemplary of an
Table 2 Total genus diversity of the main groups of freshwater animals, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Other phyla 573 372 286 300 284 205 76 42 778
Annelids 190 121 78 109 90 77 4 11 354
Molluscs
a
137 351 117 226 150 43 2 0 1,026
Crustaceans 634 294 288 424 381 325 76 25 1,533
Arachnids 152 148 171 120 102 139 5 2 456
Collembolans 71 22 5 15 10 3 2 1 78
Insects
b
1,366 1,160 871 1,269 1,159 909 132 10 4,395
Vertebrates
c
497 426 590 974 626 183 6 1 2,768
Total 3,620 2,894 2,406 3,437 2,802 1,884 303 92 11,388
a
Gastropoda genera are not included
b
The distribution of genera by zoogeographic regions is incomplete for several families of Dipterans; as a result, the sum of the
regional genus numbers is lower than the number of genera known in the world (See chapter on Diptera families excluding Culicidae,
Tipulidae, Chironomidae and Simulidae)
c
Strictly freshwater fish genus number is estimated at around 2,000 (there are an additional *500 brackish waters genera)
Hydrobiologia (2008) 595:627–637 629
123
extensively studied group, and the current estimate of
ca. 7,000 species can be considered reliable. Het-
eroptera and Culicidae (Diptera) also seem well
documented. On the other hand, the knowledge and
taxonomic expertise available for most of the
numerous dipteran families vary a lot depending on
the group, and it is clear that our current estimate of
their diversity should be interpreted with care.
Amongst the least known groups are some phyla of
primitive invertebrates such as Platyhelminthes/Tur-
bellaria, Gastrotricha or Nematoda, to name a few,
for which taxonomic knowledge and available data
are critically limited. Problems relate to data mass,
reliability and repeatability: unique, unvouchered or
plainly dubious records are common in these little-
studied groups. In addition, some of these taxa are
often primarily marine or terrestrial and most of the
available knowledge therefore concerns these habi-
tats. Nematodes, for example, are likely to be the
least known of all metazoan phyla. Experts currently
estimate that the total diversity of extant nematodes
stands at about one million species, 97% of which are
undescribed (Hugot et al., 2001). As freshwater
nematodes are relatively poorly studied when com-
pared to marine or terrestrial ones, and as they
represent only 7% (1,800 species) of the total number
of described nematode species (27,000 species), the
true diversity of freshwater nematodes is likely to be
one or two orders of magnitude higher.
First results of the Freshwater Animal Diversity
Assessment
In the following sections we summarise the informa-
tion on species diversity and endemicity for five
major groups above the level of the different
chapters: vertebrates, insects, crustaceans, molluscs
and a collection of several primitive phyla. Further,
in-depth analyses on the FADA data will be presented
elsewhere. All information and data have been
extracted from the different contributions included
in this special issue.
Vertebrates
The total number of freshwater vertebrate species,
including water birds but excluding brackish fish
species, is 18,235 species (Tables 3,4). This repre-
sents 35% of all described vertebrates (52,000
species). Of these, a majority (69%) are fishes,
followed by amphibians (24%). Considering that the
total global number of fish species is presently
estimated at ca. 29,000 species (Le
´ve
ˆque et al.,
2008, present volume), this means that nearly 50%
of all fish species inhabit fresh and brackish waters
(15,062 species, 12,470 of which are strictly fresh-
water). Freshwater habitats support 73% of all
amphibian species; other groups are less represented
in freshwaters. Freshwater vertebrates are most
diverse in the Neotropical region, followed by the
Oriental and the Afrotropical regions, and this holds
for both generic as well as species diversity (Fig. 1).
The Palaearctic is more speciose than the Nearctic,
but this holds for fishes and birds only; amphibians,
reptiles and mammals are more diverse in the
Nearctic. Australasia stands out by its relatively low
vertebrate diversity, especially of fishes (Tables 3,4).
The highest number of vertebrate endemics is
found in the Neotropics, and, again, regards mostly
fishes. Here, the Amazonian province is an endemic-
ity hotspot for fishes: 2,072 of the 2,416 species
recorded from the region are endemic. The Afrotrop-
ical ichthyofauna is notorious for the presence of
several endemic species-flocks in a number of ancient
lakes, complemented by high rates of endemicity in
certain invertebrate groups. For birds, amphibians
and reptiles, endemicity is highest in the Afrotropical
region. The Oriental region is richest in endemic
turtles, which also have an endemicity hotspot in the
eastern Nearctic. Most species of mammals, amphib-
ians and reptiles are endemic to a single continent or
zoogeographical region; hence their diversity hot-
spots coincide with endemicity hotspots, which, for
mammals, are the Neotropical and Afrotropical
regions.
On a subregional scale, the island fauna’s are
notable as centres of endemicity for birds and
amphibians. The Malagasy example is significant by
its endemicity rates of 90–100% for fishes, amphib-
ians and birds.
Insecta
Diptera, Coleoptera and Trichoptera are the major
representatives of freshwater insects with 43, 18 and
630 Hydrobiologia (2008) 595:627–637
123
Table 3 Species diversity of the main groups of freshwater vertebrates, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Amphibia 160 203 828 1,698 1,062 301 0 0 4,294
Crocodilians 3 2 3 9 8 4 0 0 24
Lizards 0 0 9 22 28 14 2 0 73
Snakes 6 22 19 39 64 7 153
Turtle 8 55 25 65 73 34 260
Fish (FW only) 1,844 1,411 2,938 4,035 2,345 261 12,740
Mammals 18 22 35 28 18 11 0 0 124
Aves 154 116 138 145 76 62 6 1 567
Total 2,193 1,831 3,995 6,041 3,674 694 8 1 18,235
Table 4 Genus diversity of the main groups of freshwater vertebrates, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Amphibia 26 27 89 127 71 20 0 0 348
Crocodilians 2 2 2 4 4 1 0 0 8
Lizards 0 0 4 7 7 4 2 0 19
Snakes 5 6 8 13 12 7 44
Turtle 6 16 6 16 34 8 86
Fish (FW only) 380 298 390 705 440 94 2,000
Mammals 10 15 18 15 10 7 0 0 65
Aves 68 62 73 87 48 42 4 1 198
Total 497 426 590 974 626 183 6 1 2,768
Fig. 1 Distribution of
freshwater vertebrate
species and genera, by
zoogeographic regions
(number of species/number
of genera). Numbers
include strictly freshwater
fish (not brackish),
amphibians, mammals,
reptiles and water birds as
defined in each specific
contribution
Hydrobiologia (2008) 595:627–637 631
123
15%, respectively, of the total of almost 76,000
freshwater insect species (Tables 5,6). These num-
bers include some families of Diptera, such as
Tabinidae, which are not addressed in specific
chapters and whose diversity is estimated at around
5,000 species. Other important taxa are Heteroptera
(6%), Plecoptera (5%), Odonata (7%) and Epheme-
roptera (4%). In insects, there is a remarkable
discrepancy between species- and genus-level diver-
sity: Diptera account for 43% of total insect species-
level diversity, against only 22% for genera. On the
other hand, in Ephemeroptera, Odonata and Heterop-
tera, the genus-level diversity contributes about twice
that of species-level diversity to total insect diversity.
The highest diversity of freshwater insects is
recorded from the Palaearctic (20%), closely fol-
lowed by the Neotropical (18.5%) and the Oriental
realms (18.3%) (Fig. 2). The Afrotropical and Aus-
tralasian regions represent 12 and 10%, respectively,
of extant insect species diversity. As several experts
did not treat the Pacific Oceanic Islands and Antarctic
region separately, we here refrain from further
commenting on the insect diversity of these regions.
The data on insect diversity should be interpreted
with caution, as many experts report a strong
sampling and study bias. Especially, the Holarctic
insect fauna is notoriously better studied than that of
the Neotropical, Afrotropical and Oriental regions,
and this for most groups. This bias is less pronounced
in two emblematic insect groups, namely butterflies
and moths (Lepidoptera) and dragonflies (Odonata),
and is reflected in the fact that for these groups, the
Holarctic is not the most diverse region: Lepidoptera
species diversity is highest in the Neotropical (30%),
Australasian (23%) and Oriental (23%) realms,
whereas for Odonata the Neotropical and Oriental
regions have the most diverse fauna. In contrast, the
fact that Hymenoptera are most diverse in the
Holarctic region (Table 5) is most likely owing to a
study bias. For insects, there are few species that
occur in more than one region; hence hotspots of
endemicity and diversity largely coincide.
Table 5 Species diversity of insect orders, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Coleoptera 3,346 1,419 2,507 2,693 2,189 1,334 13,514
Diptera other families
a
2,458 2,045 2,623 933 909 945 143 2 13,454
Diptera—Chironomidae 1,231 1,092 618 406 359 471 155 9 4,147
Diptera—Culicidae 492 178 1,069 795 1,061 764 3,492
Diptera—Simulidae 699 256 355 214 321 195 55 2 2,000
Diptera—Tabanidae
b
5,000
Diptera—Tipulidae 1,280 573 805 339 925 385 4,188
Ephemeroptera 787 650 607 390 390 219 3,043
Heteroptera 496 424 1,289 799 1,103 654 37 4,801
Hymenoptera 57 53 17 1 28 8 9 147
Lepidoptera 81 49 219 64 169 170 9 737
Mecoptera 3 5 8
Megaloptera-Neuroptera 78 99 52 18 144 50 1 0 446
Odonata 560 451 1,636 889 1,665 870 168 1 5,680
Orthoptera 9 10 54 14 98 5 188
Plecoptera 1,156 650 474 95 828 295 3,497
Trichoptera 2,370 1,461 2,100 944 3,723 1,140 11,532
Total 15,190 9,410 14,428 8,594 13,912 7,510 577 14 75,874
a
The distribution of species by zoogeographic regions is incomplete for several families of Dipterans; as a result, the sum of the
regional species numbers is lower than the number of species known in the world (See chapter on Diptera families excluding
Culicidae, Tipulidae, Chironomidae and Simulidae)
b
Estimated
632 Hydrobiologia (2008) 595:627–637
123
Crustacea
The different chapters dealing with freshwater crus-
taceans report on a total of 11,990 described species,
distributed over 1,533 genera (Tables 7,8). This
constitutes 30% of the total known diversity of
crustaceans, which is estimated at about 40,000
species (Groombridge & Jenkins, 2002). Amongst
Table 6 Genus diversity of insect orders, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Coleoptera 209 152 175 204 167 138 710
Diptera other families
a
227 158 114 198 107 115 29 2 457
Diptera—Chironomidae 181 211 104 154 105 116 29 6 339
Diptera—Culicidae 19 13 15 24 25 22 42
Diptera—Simulidae 12 13 2 10 1 2 1 1 26
Diptera—Tipulidae 45 38 23 36 45 30 115
Ephemeroptera 77 94 93 84 78 405
Heteroptera 60 67 96 105 123 87 16 553
Hymenoptera 29 33 1 10 13 6 5 51
Lepidoptera 12 17 11 21 14 21 4 53
Mecoptera 1 2 2
Megaloptera-Neuroptera 14 10 5 11 16 10 1 0 45
Odonata 137 89 132 186 235 169 47 1 642
Orthoptera 7 6 5 20 20 2 50
Plecoptera 108 102 8 57 41 46 286
Trichoptera 229 157 87 148 169 143 619
Total 1,366 1,160 871 1,269 1,159 909 132 10 4,395
a
The distribution of genera by zoogeographic regions was not complete for several families of Dipterans, (See chapter on Diptera
families excluding Culicidae, Tipulidae, Chironomidae et Simulidae)
Fig. 2 Distribution of total
insect species and genus
diversity by zoogeographic
regions (number of species/
number of genera).
Numbers do not include
some dipteran families (i.e.
Tabanidae) that are not
addressed in the specific
contributions
Hydrobiologia (2008) 595:627–637 633
123
freshwater crustaceans, the most speciose taxa are the
decapods (24%) and copepods (23%), closely fol-
lowed by the ostracods and amphipods (both 16%).
Branchiopods, Isopods and syncarids represent 9, 8
and 2%, respectively, of the total number of species.
The remaining 2% is composed of representatives of
Table 7 Species diversity of crustaceans, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Amphipoda 1,315 236 56 127 17 107 10 1,866
Branchiopoda 175 93 81 61 47 75 2 1 508
Branchiura 8 18 40 33 16 3 1 0 113
Cladocera 245 189 134 186 107 158 33 12 620
Copepoda 1,204 347 405 561 381 205 29 17 2,814
Cumacea & Tanaidacea 20 2 2 3 1 25
Isopoda 475 130 22 109 31 134 5 942
Mysidacea 39 11 1 20 7 1 0 0 72
Ostracoda 702 298 455 275 199 176 5 3 1,936
Spelaeogriphacea – – 1 1 – 2 – 4
Syncarida 128 12 27 29 12 33 0 0 240
Thermosbaenacea 6 1 1 8 1 1 – 18
Aeglidae 63 63
Astacidea 38 382 9 64 151 638
Brachyura 97 19 149 340 818 89 24 1,476
Caridea 47 17 92 109 349 87 25 – 655
Decapoda 182 418 313 513 1,167 327 49 0 2,832
Total 4,499 1,755 1,536 1,925 1,985 1,225 135 33 11,990
Table 8 Genus diversity of crustaceans, by zoogeographic region
PA NA AT NT OL AU PAC ANT World
Amphipoda 185 23 17 35 10 34 9 293
Branchiopoda 28 20 14 18 14 12 2 1 43
Branchiura 1 1 3 3 1 2 1 0 4
Cladocera 60 52 46 50 44 52 21 7 95
Copepoda 134 87 60 104 79 50 15 14 257
Cumacea & Tanaidacea 10 2 2 2 1 14
Isopoda 45 18 8 42 11 50 4 194
Mysidacea 15 7 1 6 6 1 0 0 26
Ostracoda 87 57 73 55 46 57 4 3 189
Spelaeogriphacea – – 1 1 – 1 – 3
Syncarida 30 6 18 18 9 15 0 0 78
Thermosbaenacea 5 1 1 2 1 1 – 6
Aeglidae 1 1
Astacidea 6 11 1 6 9 33
Brachyura 14 4 27 65 139 24 13 238
Caridea 14 5 17 17 21 15 6 – 59
Decapoda 34 20 46 88 160 48 19 0 331
Total 634 294 288 424 381 325 76 25 1,533
634 Hydrobiologia (2008) 595:627–637
123
smaller groups: mainly Branchiura and Mysidacea,
with a few species of Cumacea, Tanaidacea, Spelae-
ogriphacea and Thermosbaenacea.
Again, the region with the highest number of
species is the Palaearctic (37%). Second and third are
the Oriental and Neotropical regions (both ca. 16%).
This holds for most crustacean taxa, except for
Brachyura and Caridea decapods, which are most
diverse in the Oriental region, and Astacidea, which
exhibit a diversity and endemicity hotspot in the
Nearctic, and which are absent from the Oriental
region. Aeglidae (Anomura) crabs form an endemic
family in the Afrotropical region. All other crusta-
cean taxa (Copepoda, Ostracoda, Branchiopoda,
Isopoda, Amphipoda, Syncaridea) are most diverse
in the Palaearctic. As for insects, sampling and study
gaps most likely account for this.
Remarkable endemic crustacean faunas occur in
the ponto-caspian basin and in Lake Baı
¨kal. These are
identified as hot spots of richness and endemicity for
several crustacean taxa, including amphipods, ostrac-
ods, copepods and branchiopods. In amphipods, there
is a large group of endemic taxa inhabiting subter-
ranean habitats in the west Palaearctic, whereas
crayfish exhibit a different pattern of endemicity,
with a centre in the southeast of the Nearctic region,
notably in the south of the Appalachean range.
Mollusca
The ca. 5,000 species of freshwater molluscs repre-
sent 4% of the total number of freshwater animal
species, and account for only about 7% of the global
total of described mollusc species, estimated at about
80,000 species (Groombridge & Jenkins, 2002).
Eighty percent of the freshwater molluscs are
gastropods, whereas 20% are bivalves. Gastropods
and bivalves attain their highest diversity in the
Palaearctic and Nearctic regions, respectively. How-
ever, the bivalve Unionidae family, of great
economic importance, is most diverse in the Oriental
region.
Freshwater gastropod faunas of underground
systems, springs and small rivers are particularly
rich, both in terms of species diversity and
endemicity. Further noteworthy habitats are ancient
oligotrophic lakes (e.g. Baikal, Ohrid, Tanganyika),
which are key hotspots of gastropod diversity. The
lower reaches of some river basins (e.g. Congo,
Mekong, Mobile Bay) are also identified as areas
of high species richness.
Minor invertebrate phyla
The most speciose amongst the ‘‘minor’’ invertebrate
phyla are Rotifera (1,948 species), Nematoda (1,808
species), Annelidae (1,761 species) and Turbellaria
(Platyhelminthes: 1,297 species). Gastrotricha, Nem-
atomorpha and Porifera are less species rich in
freshwater habitats (200–300 sp.), although they are
very successful in marine environments. The same
holds for Bryozoa and Tardigrada (60–80 species).
The least diverse groups in freshwater are Nemertea
(22 species) and Cnidaria (18 species). Rotifera,
Nematomorpha and Annelida-Hirudinea are mainly
freshwater, but there are also generally species-rich
groups like Cnidaria (7,000+ species), or Annelida-
Polychaeta (9,000+ species) that are, however, poorly
represented in freshwater (Fig. 3).
All of these groups are generally ill-studied, and
this was clearly emphasised by all experts. Never-
theless, Lake Baikal appears to have been studied
more intensively for most of these groups and is
identified as a hotspot of endemicity. Further gener-
alisations are hard to make considering the lack of
data, although the analysis of rotifer diversity and
endemism reveals some intriguing patterns (Segers &
De Smet, 2007; Segers, 2008, present volume).
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d
i
ordy
H-air
a
d
in
C
alle
br
uT r
ai (ni cara
p
l)
Bryzo oa
ate
ah
c
og
il
O-
n
nA
Roi
tef ra
re
meN e
ta
dr
a
Tia
da
r
g
Gat
srhc
i
r
t
oa
H-
n
nA iaen
i
d
u
r
Nematrom
opa
h
Fig. 3 species diversity in freshwater compared to total
number of described species
Hydrobiologia (2008) 595:627–637 635
123
Comparison with marine and terrestrial species
diversity
As early evolution of all major animal phyla took
place in the sea, it is not surprising that marine
systems show higher diversity at the phylum and
class level than terrestrial or freshwater systems. Of
the total 33 metazoan phyla, 31 are found in the sea,
with 11 being exclusively marine; whereas 17 phyla
are present in freshwater and 12 on land (only 2
phyla, freshwater Micrognathozoa and terrestrial
Onychophora have no marine species). At the species
level, the diversity of terrestrial ecosystems, with
more than 1.5 million species, largely exceeds the
280,000 species of marine organisms currently
known. At habitat levels, the most diverse marine
habitats—coral reefs—are far less diverse in terms of
species number than the moist tropical forests that are
often taken as their terrestrial counterparts.
Conclusion
A clear result of our survey is that increased sampling
efforts are needed to address the obvious gaps, both
geographical and taxonomical, the current assessment
of freshwater biodiversity reveals. Especially in terms
of richness and endemicity, hot spots are often
located in less-studied areas of the Oriental, the
Neotropical and the Afrotropical regions. The situa-
tion is especially critical for the least-known groups
such as Nematoda. One possible cost-effective way to
improve this situation is to make better use of the
existing knowledge, shelved in museum collections,
local laboratories or in scientists’ drawers. This on-
going task is being carried out by several interna-
tional initiatives including GBIF and the IUCN
Freshwater Biodiversity Assessment Programme.
However, additional surveys are also needed and
will require a new generation of taxonomic experts
and increased financial means.
This global assessment of freshwater species
diversity and distribution is thus but a first step in
the process of compiling and upgrading our knowl-
edge on freshwater biodiversity. The regional or
global-scale approach used here allows for the
identification of knowledge gaps and is critical to
come to a better understanding of evolutionary
patterns in freshwater diversity and endemicity, in
particular, for less-known invertebrate taxa.
In order to complement the present database on
diversity and endemicity, a similar effort focussing
on environmental information, from geographical to
sociological, will be needed. It is clear that the results
presented in this volume, apart of their inherent
scientific value, should be interpreted in a broader
ecological and evolutionary context, if they are to
play a role in the development or improvement of
sustainable management and conservation of fresh-
water resources. Indeed, the challenges society is
confronted with in the face of global change and
increased human utilisation of natural resources, are
daunting and can only be dealt with successfully on
the condition that sufficient and credible scientific
knowledge is made available as a basis for action, in
addition to the political will to implement the
necessary measures (Dudgeon et al., 2006).
To facilitate usage and analysis of the data
collected during the present Freshwater Animal
Diversity Assessment (FADA) project, an on-line
database is presently being developed. This resource,
which can be consulted on http://www.FADA.
biodiversity.be, will offer additional services includ-
ing extraction of name lists, visualisation of
geographical (GIS) records in an interactive envi-
ronment and link to other datasets containing
information of freshwater systems. All data will be
made freely and universally accessible through the
Internet. For this, FADA is developing links with
global initiatives in the field, like the Global
Biodiversity Information Facility (GBIF), Catalogue
of Life (CoL), SpeciesBase and Encyclopedia of
Life.
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