AMER. ZOOL., 29:1075-1084 (1989)
Diversity of Organisms: How Much Do We Know?1
ROBERT D. BARNES
Department of Biology, Gettysburg College,
Gettysburg, Pennsylvania 17325
SYNOPSIS. The history of Invertebrate Zoology over the past 40 years can be used to
illustrate interest in organisms and some of the ways in which the symposium's question
may be interpreted. The study of animal organisms from a holistic perspective has progressed enormously as reflected in changes in described and estimated numbers of species,
in the discovery of new higher taxa and in the growth of literature. Generalizations on
the biology of animal organisms, however, rest on relatively small samples, and many of
the same organisms that have received the most attention in the past continue to receive
the most today. Symbiosis and colonial organization have been two important means
whereby new organizational levels for organisms have evolved. Ultrastructural research
over the past 20 years has provided new evidence in support of the hypothesis promulgated
long ago that multicellular animals (metazoans) may have evolved from colonial protistans.
Some polymorphic, colonial metazoans have approached or crossed the threshold to a
still more complex level of organism.
but, surprisingly, the numbers were not
always
smaller.
There has been a great accummulation
Compare
Mayr et al. (1953) with the most
of information about the diversity of aniof Barnes (1987) whose
recent
edition
mal organisms over the past 40 years. The
numbers
(Table
1) are derived from vargrowing understanding of this diversity
ious
sources,
but
especially from the
reflects not only interest in the study of
organisms but in some of the ways of inter- McGraw-Hill Synopsis and Classification of
preting the symposium's question—Is the Living Organisms (Parker, 1982). In 1953
Mayr, Linsley and Usinger estimated
organism necessary?
There are almost one and a half million 1,090,235 species of animals having been
described species of eukaryotic organisms described as compared to 1,035,185 today,
living on our planet. Most are animals, about 55,000 less. In this comparison,
of which there are over a million. Two mollusks, myriapods and insects show
hundred thirty-five thousand are plants; decreased numbers. Arnett's (1985) cur80,000 are fungi; and 87,082 are protis- rent figure of 751,000 species of insects is
tans (algae and protozoans) (Fig. 1). The a very careful estimate. Some groups like
diversity is enormous, especially among cnidarians and rotifers show little change,
animals. Animal motility and heterotro- and sponges, nematodes, annelids, bryophic nutrition have led to many different zoans and echinoderms show moderate
increases. The really large increases are in
life styles and body plans.
flatworms, crustaceans and arachnids.
Yet even for groups that show little
NUMBERS OF DESCRIBED SPECIES
in the two lists, a large number of
change
How many species of animals have been
described to date? A reasonably good esti- new species have been described over this
mate is that there are about 1,035,250. The period. For example, the number of new
five largest groups in order of magnitude species of polychaetes described between
would be insects, arachnids, mollusks, ver- 1972 and 1983 averaged 92 a year. But the
tebrates and crustaceans (Fig. 2). The esti- total increase in the estimates of described
mates have changed over the past 35 years, species of annelids is only 1,700 and that
includes oligochaetes and leeches, as well.
Several factors may contribute to the
foregoing, apparent paradox. Earlier esti1
From the Symposium on Is the Organism Necessary?
mates vary in their accuracy and a considpresented at the annual meeting of the American
Society of Zoologists, 27-30 December 1987, at New erable number of new species as well as old
Orleans, Louisiana.
ones fall into synonymy. Herbert Levi at
INTRODUCTION
1075
1076
ROBERT D. BARNES
off the coast of New Zealand (Baker et al.,
1986). Prior to that representatives of a
new phylum, the phylum Loricifera, were
6%
discovered by Kristensen (1983) in the
interstitial spaces of marine gravel off the
coast of France. Loriciferans belong to the
aschelminth assemblage and look somewhat like a cross between a rotifer and a
kinorhynch.
Over the past 45 years representatives
of four new classes of crustaceans have been
EMBRYOPHYTES
discovered. All are minute animals; most
16%
are less than a millimeter in length. A number of species of the class Remipedia have
METAZOAN ANIMALS
been collected from marine caves. First
1.035.250
72%
described by Yager in 1981, these highly
metameric arthropods, which look somewhat like polychaetes, are perhaps the most
primitive known crustaceans. The Tantulocarida is a class of marine ectoparasites
FIG. 1. Estimated numbers of species comprising the
eukaryotic kingdoms. Based on figures from Barnes related to copepods, first described by
Boxshall and Lincoln in 1983. Members of
(1987), Bold et al. (1987), and Corliss (1984).
the class Cephalocarida are primitive crustaceans first described in 1955 by Sanders
the Museum of Comparative Zoology has from sediments in Long Island Sound and
informed me that in his revisions of neo- since taken from many other locations.
tropical orbweaving spiders, he has found Finally, the class Mystacocarida, reported
that about 70% have been unnamed, but by Pennak and Zinn in 1943, contains elonof the 30% that have names, they have been gate, interstitial species related to copenamed three or four times! This is not true pods.
of all groups. For example, relatively few
In 1980 Rieger described a strange
polychaetes fall to synonomy (K. Fauchald, interstitial worm, Lobatocerebrum, that is cilpersonal communication). Almost all of the iated and acoelomate like flatworms but
small groups show increases in estimated has metameric ventral ganglia and protonumbers. But this is not surprising; the nephridia like annelids. It has arbitrarily
smaller the number of described species, been placed with the annelidan oligothe more accurately can new species chaetes but may eventually have a higher
descriptions be evaluated and counted.
taxon of its own.
Among mollusks the first living monoNEW HIGHER TAXA
placophorans were discovered in 1952 in
The increase in numbers of described deep water off the coast of Chile. They
species over the past 45 years has been have since been taken from a number of
impressive. Just as impressive, however, is sites in the world's oceans and eleven species
the increase in descriptions of new species belonging to three genera (Neopilina,
that have been assigned to new higher taxa. Monoplacophorus and Vema) are now known
Since 1950 eight new classes and phyla of (Wingstrand, 1985).
animals have been discovered. The most
To these new groups we should add sevrecent is a class of tiny echinoderms (2.6- eral others, which although discovered
9.0 mm in diameter) which look a little like earlier, have really only become known to
medusae, and have been assigned to the any degree during the past 35 years.
new taxon, the class Concentricycloidea. Included here are the marine Trichoplax,
The class was first described in 1986 from representing the monotypic phylum Plaspecimens collected on wood in deep water cazoa (Grell, 1982) and the pterobranchs,
80,000
DIVERSITY OF ORGANISMS
1077
FIG. 2. Estimated numbers of species of the major groups of metazoan animals. From Barnes (1987).
whose biology has only received attention
during the past 15 years and most recently
by Lester (1985). Up until about 1950 the
pogonophorans, a deep water gutless group
of worms related to annelids, were represented by a relatively small number of specimens found in miscellaneous dredging
samples. With improved collecting techniques and wider oceanographic sampling
the collection of pogonophorans increased
dramatically. Then, beginning in 1970s our
knowledge of the biology of pogonophorans began to move forward.
NUMBERS OF UNDESCRIBED SPECIES
How many species are yet to be
described? A report from the Office of
Technological Assessment (1987) suggests
5-10 million. If so, only 15 to 30% of
organisms have been described. The great
numbers that have been postulated are
largely arthropods. Certainly the most
mind-boggling are the projections of Erwin
(1983) for insects of the rainforest canopy,
one of the few remaining frontiers of
unknown organisms.
Erwin conjectures that there may be as
many as 30 million species of insects, a staggering figure given the 750,000 described
species estimated by Arnett (1985). Erwin's
projections are based on canopy collections
made in Brazil and other parts of the
American tropics. An insecticide fogging
device was elevated to various levels in the
rainforest. The insects thus disturbed and
killed rain downward and are collected on
plastic trays containing a collecting bottle
in the center. The beetle fauna, with which
Erwin was largely concerned, consists of
small species belonging to about six families. His projections are based on the enormous number of endemic species charac-
1078
TABLE 1.
ma Is.*
ROBERT D. BARNES
Numbers of described species of living ani-
Porifera
Cnidaria
Ctenophora
Platyhelminthes
Nemertea
Mesozoa
Acanthocephala
Rotifera
Gastrotricha
Kinorhyncha
Nematoda
Nematomorpha
Annelida
Pogonophora
Echiura
Sipuncula
Mollusca
Tardigrada
Onychophora
Chelicerata
Crustacea
Insecta
Myriapoda
Bryozoa
Entoprocta
Phoronida
Brachiopoda
Echinodermata
Chaetognatha
Hemichordata
Urochordata
Vertebra ta
Total
1953, Mayr
rtal.
1987, Barnes
4,500
9,000
5,000
9,000
+ 500
—
90
50
-40
6,000
750
12,700
900
+6,700
+ 150
50
50
Change
300
150
1,500
1,500
175
100
460
100
+ 285
10,000
12,000
+ 2,000
100
230
7,000
8,700
1
60
250
80
140
320
80,000
50,000
180
65
400
+ 130
+ 1,700
+ 79
+ 80
+ 70
-30,000
+ 220
+5
+ 33,000
+ 17,000
-99,998
-2,500
+700
+ 90
+6
+ 75
-150
—
in the years ahead. Although even with
regard to these marine groups, the projections of various workers vary. For example,
Kristian Fauchald (personal communication) speculates that only about 60% of the
polychaetes are known. Echinoderms are
about 90% known, according to Cynthia
Ahern (personal communication). Interestingly, coral reefs are providing many of
the new species of polychaetes, but coral
reef echinoderms, such as brittle stars and
feather stars are fairly well known, and it
is the deep sea that is the last frontier of
new echinoderms.
LITERATURE GROWTH
35,000
25,000
850,000
13,000
3,300
70
68,000
42,000
751,012
10,500
4,000
60
4
250
150
10
325
4,000
6,000
70
+ 2,000
85
+5
-350
+ 12,143
-55,050
30
80
1,600
37,790
1,090,235
1,250
49,933
1,035,185
+40
* Changes in estimates between 1953 and 1987.
teristic of each of the four forest types he
surveyed. Fifty-eight to 78% of the species
of beetles within each type were endemic.
Mites number about 30,000 species, and
this is believed to represent only about 20%
of the actual fauna. Moreover, since the
bulk of these species is thought to live in
tropical forests, many are predicted to
become extinct before they are ever discovered. But insects and arachnids are
probably the only two groups for which
there are still very large numbers of undescribed species. I expect the larger marine
groups will exhibit only modest increases
How much do we know about the biology of this enormous diversity of animals?
If the volume of literature is any indication, we know a great deal. In 1985, for
example, out of about 220,000 publications in Biology, approximately 62,000,
one-fourth, were devoted to animals. Of
these 62,000 articles concerned with animal organisms, over a quarter were on
insects. Articles on birds, mammals, fish,
mollusks and crustaceans, in that order,
together had half.
The growth of literature on organisms
in this century can be illustrated by considering publications on a few groups of
animal species. The graphs in Figures 3
and 4 show the number of papers published each year for bryozoans, arachnids
and myriapods over the past 85 years. The
tabulations are derived from Zoological
Record, and these groups were selected
because of the ease of obtaining the figures. Groupings of phyla have continually
changed in Zoological Record. The annual
output of papers on bryozoans (Fig. 3) has
fluctuated but within a relatively modest
range of about 25 to 75 papers during the
first part of this century. Following World
War II, the number increased several fold,
reaching a peak of 282 papers in 1970.
More recently it has dropped to less than
200. Myriapods had a similar history up to
World War II, but subsequently rose markedly, exceeding 300 papers a year since
1979. The growth of papers on arachnids
is much more spectacular. In 1900 there
1079
DIVERSITY OF ORGANISMS
280
240
200
160
120
80
40
1900
1908
1916
1924
1932
1940
1948
1956
1964
1972
1980
FIG. 3. Annual publications on bryozoans from 1900 to 1985. Based on figures from Zoological Record.
were 131 papers published; after 1978
there has been an output of about 2,000
papers a year.
The number of books and reviews has
also increased dramatically. In 1960 when
I was working on the first edition of my
Invertebrate Zoology, the books in English
covering the general biology of free-living
invertebrates other than insects were
largely limited to five volumes of Libbie
Hyman's series (1940-1959; she had not
yet completed the volume on mollusks), two
volumes on the physiology of crustaceans
and a few British volumes on the arachnids,
some of which were already quite old at
that point. In the intervening 37 years the
output of books on invertebrates has been
enormous. Almost every phylum of any size
has been covered, even relatively small ones
like nemerteans and brachiopods. There
are volumes on pycnogonids, leeches and
tunicates. Still missing are a general biology of the cnidarians and one on rotifers.
However, they will soon be covered in the
multivolume work on the Microscopic
Anatomy of Invertebrates now in process
under the editorship of Frederick Harrison.
So, how much do we know? I believe that
we know a great deal. Of course, there has
been a great accumulation of information
about individual species. But as I look back
over the past thirty years, I am impressed
with major areas of knowledge about animal organisms for which we had little or
no information thirty years ago. Let me
point out just four. A whole new world has
been revealed in the interstitial fauna, the
animals that live between sand grains. Our
knowledge of animal symbiosis, especially
mutualistic symbiosis with unicellular
organisms, has greatly increased. There has
been the recognition of the fact that shells
and skeletons secreted by animals provide
a record of their age and environmental
conditions. For example, there are intertidal bivalves which record by fine lines on
their shells, not only the daily occurrence
of low tides, but of spring and neap tides
(Evans, 1972). The series of lines can therefore be read like a tide chart with the
sequence of spring and neap tides quite
visible.
A most important contribution to the
advancement of our general knowledge of
animals, especially to our better understanding of their evolutionary relationships, has come from ultrastructural
1080
ROBERT D. BARNES
400
Combined with
300
arachnids
100
1900
1908
1916
1972
1980
1972
1980
2400
2000
1600.
Combined with
myriapods
800
1900
1908
1916
1924
FIG. 4. Annual publications on myriapods (upper
graph) and arachnids (lower graph) from 1900 to 1985.
Based on figures from Zoological Record. The figures
for myriapods also include a small number of publications on some very small groups, such as tardigrades
and pycnogonids, which are sometimes contained in
this section.
research. Through electron microscopy
great advances have been made in our
understanding of animal ciliation, protonephridia, podocytes, vascular and coelomic linings or their lack, to name but a few.
KNOWLEDGE SAMPLE SIZE
In all of these advances in our knowledge
of the biology of metazoans, one might ask
how broadly based is the advance. I have
often wondered how far our knowledge of
the biology of species extends beyond the
familiar names that appear over and over
again in the literature. This symposium
prompted me to investigate. Using polychaete annelids again, the lists in Table 2
show the number of papers published per
species in 1983 and 1984 and the names
of the species with the record number of
papers. The reader may note that many
are familiar. And if you compare them with
the polychaetes that won the publication
race in the first quarter of the twentieth
century, you will find that many are the
same. This is not surprising. The animals
that we know best are those that are at
hand, those easiest to obtain and maintain
in a laboratory. Among marine animals
they are the common, easily collected
species around marine laboratories or academic institutions located near the coast.
Of course there has been some increase in
the variety of species studied over the past
thirty years, but much attention continues
to be focused on a group of favorites.
How broadly based are the generalizations we make about animals? The information to answer that question is not easily
obtained, but some data are available for
a few areas. For example, textbooks make
the generalization that the ophiuroids, or
brittle stars, possess pluteus larvae, a larval
type similar to that of sea urchins, and that
some brood their eggs. There are about
2,000 species of brittlestars. On what is
that generalization based? In 1975 Hendler surveyed the literature and found that
of the 2,000 species of brittlestars 71 are
known to have larvae and 55 to brood.
That means we know something of the
developmental pattern of about 6% of the
ophiuroids. The figure is probably a little
higher today.
The study of protonephridia provides
another example. Protonephridia are blindending excretory tubules found in some
ten phyla. The terminal cells bear one or
more cilia. The general structure of these
organs was delineated by the end of the
last century, but the function of protonephridia has been little understood since
protonephrida commonly occur in animals
that are too small to require organs for the
removal of metabolic wastes. In 1958 the
first EM work on protonephridia was
undertaken by Kummel on the sheep liver
fluke. Subsequent investigators examined
the protonephridia and found fenestrations around the barrel of the cell. Showing
some resemblance to podocytes, the fenestrations appear to be the sites of passage
1081
DIVERSITY OF ORGANISMS
TABLE 2. Numbers of papers published per species of living polychaetes in 1983 and 1984.
1983
One paper each for
Two papers each for
Three papers each for
Four papers each for
Five papers each
Six papers each
220 species
36 species
13 species
4 species
Glycera dibranchiata
Neanthes succinea
Neanthes arenaceodenta
Pomatoceras lamarckii
Chaetopterus variopedatus
Capitella capitata
Perinereis cultrifera
Eight papers
Ten papers
Eleven papers
Twelve papers
Fourteen papers
Nineteen papers
Twenty papers
1984
466 species
27 species
8 species
3 species
Sabellaria alveolata
Capitella capitata
Neanthes diversicolor
Neanthes virens
Arenicola marina
Neanthes diversicolor
Arenicola marina
Neanthes virens
of fluid into the interior of the tubule, the mation accumulate in animal biology, gencilia providing the filtration pressure. How eralizations have usually become refined,
broadly are these generalizations based? Is not discarded. Structures and processes that
our sample size adequate? The ten phyla depart markedly from what has been preof animals with protonephridia include viously known are usually described and
more than 17,000 species excluding poly- those that follow the pattern are usually
chaetes. Since Kummel's first paper in 1958 not. Therefore our knowledge is probably
there have been at least 44 papers pub- greater than the numbers of published
lished on protonephridia from about 40 descriptions might lead us to believe. We
species that belong to 9 of the 10 phyla in are never going to have a complete record;
which protonephridia occur (Fig. 5). generalizations are always going to be based
Although the sample size is still small, I on just a sample. Nevertheless, we must be
think this is a remarkable record.
careful not to invest our generalizations
A final example relates to the feeding with more status than they merit.
mechanism of crinoids. The crinoids
ORGANISM BOUNDARIES
include about 80 species of sea lilies and
450 species of feather stars. In 1960 David
Finally, I would like to look at the way
Nichols published one of the first detailed our growing knowledge of animal diversity
papers on the feeding mechanism of a cri- has contributed to concepts of organism
noid, Antedon bifida, the European feather boundaries. Of the many contributions EM
star. Since that time there have been obser- studies have made to animal biology, one
vations on the feeding posture of a rela- of the most interesting, from an evolutiontively large number of species, probably ary point of view, has been studies of cilaround thirty, largely from the work of D. iation. The work of Rieger (1976) has
L. Meyer and D. B. Macurda. However, provided convincing evidence that monopublished work on the actual feeding ciliated epidermal cells, i.e., one cilium per
mechanism has been limited to some seven cell, is the primitive condition for animals.
studies that have been made on only two Monociliated cells are characteristic of plaspecies, the European Antedon bifida and cozoans, sponges, cnidarians, gnathostothe North Pacific Florometra serratissima. mulids, and some gastrotrichs. They are
This investigational base needs broaden- also found among deuterostomes, such as
ing.
echinoderms and pterobranchs. They do
I do not feel uneasy about the empirical not occur in the flatworm, mollusk, annelid
data base of our generalizations about ani- and arthropod assemblage (protostomes).
mal organisms. As I have watched inforRecent work on choanoflagellates has
1082
ROBERT D. BARNES
CEPHALOCHOROATES
45
POLYCHAETES
II
PRIAPULIDS
9
ACANTHOCEPHALANS
1,150
KINORYNCHS
100
I III I II
ROTIFERS
1,500
GASTROTRICHS
460
GNATHOSTOMUL1DS
80
NEMERTEANS
900
J
CESTODES
4,000
I
J I
FLUKES I
8,000
TURBELLARIANS (
3,000 1958
J-M
1962
,—L-, 1 I , ' , r1966
1970
1974
1978
1982
1986
FIG. 5. Publications on the ultrastructure of protonephridia. Publications are indicated by bars. Figure under
name of group indicates approximate number of described species. The number of species of polychaetes
possessing protonephridia as opposed to metanephridia was not known to author.
demonstrated that their mitochondria and
ciliation are strikingly similar to those of
metazoans (Nielsen, 1985). Indeed, the
similarities indicate that the choanoflagellates may be the protozoan flagellate ancestorsofthe Animal Kingdom. Nielsen (1985)
even assigns the choanoflagellates to the
Animal Kingdom, but I think this is going
too far. Such a decision requires a fundamental change in the way the Animal Kingdom is currently defined.
|
I have called the reader's attentiori/tcr
current research findings indicating a phylogenetic relationship of choanoflagellates
to metazoans because it has provided a reaffirmation of a position long held by many
biologists—that the multicellular condition of metazoans is derived from a protistan colony. Following the origin of the
first cells, evolution to a new level of organism may have occurred in at least four ways:
(1) by symbiosis, (2) by intracellular differentiation, (3) by unicellular, colonial organization with intercellular differentiation,
and (4) by multicellular, colonial organization with polymorphism.
Thus the first metazoan animals—motile,
multicellular heterotrophs—evolved by
increasing differentiation and interdependence of cells within a flagellate colony,
probably a choanoflagellate colony. At
some point the high degree of cellular
interdependence resulted in the colony
becoming a multicellular organism.
Within the Metazoa, various groups have
established mutualistic symbiotic relationships with bacteria, cyanobacteria, green
^lgae, diatoms and dinoflagellates. The host
groups are fairly restricted: sponges, cnidarians, flatworms, pogonophorans and
mollusks. In some sponges and corals the
unicellular symbionts contribute a large
part of the total biomass. Have any of these
symbiotic relationships approached the
threshold of a new level of organism?
Polymorphism, whereby the members of
a colony become structurally specialized for
different functions, has evolved in a number of colonial groups: hydroid cnidarians,
siphonophoran cnidarians, hydrocorals,
pennatulacean cnidarians, bryozoans, and
social insects. In all but the social insects,
DIVERSITY OF ORGANISMS
the individuals of these polymorphic colonies are attached together. When a feeding individual is predominant and has
retained the primitive ground plan of the
phylum, such as the gastrozooid of hydroids
and the autozooids of bryozoans, colonial
organization remains distinct. Where specialization has blurred that original ground
plan, then the colony could cross the
threshold to a new level of organization
and organism. I believe there is one group
of animals which approaches that threshold. These are the siphonophores. These
marine colonies have developed extreme
polymorphism. Their composite individuals have commonly lost the distinctive,
radial, tentaculate form and the colony with
its swimming bells,floats,fishingpolyps and
reproductive individuals functions as an
integrated whole, one organism. I would
wager that if all cnidarians were extinct but
siphonophores, we would be hard pressed
to recognize the individuals even if we
guessed it was a colony.
There is still another, less familiar,
example of how our increasing knowledge
of diverse kinds of animals can disturb our
conventional ideas about the boundaries of
the organism. The old debate as to whether
a leuconoid sponge with many oscula is one
individual or a colony, is rarely heard today.
The notion that multiple oscula, like
mouths, must indicate more than one individual, has disappeared with the recognition that multiple oscula are simply a feature of leuconoid architecture. However,
it is now known that where larval settlement is dense in some species, adjacent
developing individuals will fuse together
(Fell and Jacob, 1979). The resulting
sponge is indistinguishable from one which
arises from a single egg, but it is a genetic
mosaic. Is it one organism or two?
ACKNOWLEDGMENTS
1 am grateful to Dr. Ralph A. Sorensen
for reviewing the manuscript.
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