It is to be noted, furthermore, that this relation is a
close one between particular species, just as it is be-
tween plants and gall insects. Each attacking species
has its own particular host. Recent careful studies
made by Dr. A. D. Howard and others at the Fairport
Biological Laboratory have shown such relations as
Species of Mussels Host Species
1. Yellow Sand Shell (Lampsilis anodontoides) on the gars
2. Lake Mucket (Lampsilis luteolus) on the basses and perches
3. Butterfly Shell (Plagiola securis) on the sheepshead
4. Warty Back (Quadrula pustulosa) on the channel catfish
5. Nigger-head (Quadrula ebeneus) on the blue herring
6. Missouri Nigger-head (Obovaria ellipsis) on the sturgeons
7. Salamander mussel (Hemilastena ambigua) on Necturus
Some of these mussels infect one species of fish ; some,
the fishes of one family or genus ; a few have a still wider
range of host species, these last being usually the
species having the larger and stronger glochidia with the
best development of clasping hooks on the valve tips.
A very special case is that of Hemilastena, a mussel
that lives under flat stones and projecting rock ledges
in the stream bed. Living in the haunts of the mud-
puppy, Necturus, and out of the way of the fishes, it
infects the gills of this salamander with its glochidia.
The glochidia will grow only on their proper hosts.
They will take hold on almost any fish that touches
them in a manner to call forth their snapping reaction,
but they will subsequently fall off from all but their
proper hosts, without undergoing development.
Whether it be the mussel that reacts only to a certain
kind of fish substance, or the fish that reacts to form a
cyst only for a certain glochidial stimulus is not
known. The relation appears onesided, and beneficial
only to the parasitic mussel; yet moderate infesta-
2 92 Adjustment to Conditions of Aquatic Life
tion appears to do little harm to the fishes. The
cysts are soon grown, emptied and sloughed off, leaving
no scar. And a few fishes, such as the sheepshead
which is host for many mussels, appear to reap an
indirect return, in that their food consists mainly of
these same mussels when well grown.
It may be noted in passing that one little European
fish, the bitterling, has turned tables on the mussels.
It possesses a long ovipositor by means of which it
inserts its own eggs into the gill cavity of a mussel,
where they are incubated.
I RE AT bodies of water
furnish opportunity for
all the different lines
of adaptation discussed
in the preceding chap-
ter. The sun shines
full upon them in all its
life-giving power. The
rivers carry into them
the dissolved food sub-
stances from the land.
Wind and waves and
convection currents dis-
tribute these substances throughout their waters.
Both the energy and the food needed for the main-
tenance of life are everywhere present. Here are
expanses of open water for such organisms as can float
or swim. Here are shores for such as must find shelter
and resting places; shores bare and rocky; shores low
and sandy ; shores sheltered and muddy, with bordering
marshes and with inflowing streams. The character
of the population in any place is determined primarily
by the fitness of the organisms for the conditions they
have to meet in it.
294 Aquatic Societies
For every species the possible range is determined
by climate; the possible habitat, by distribution of
water and land; the actual habitat, by the presence of
available food and shelter, and by competitors and
Our classification of aquatic societies finds its basis
in physiographic conditions. We recognize two princi-
pal ecological categories of aquatic organisms:
I. Limnetic Societies, fitted for life in the open water,
and able to get along in comparative independence of
II. Littoral Societies, of shoreward and inland dis-
FIG. 178. Diagram illustrating the distribution of
aquatic societies, in a section extending from an
upland marsh to deep water. The littoral region
The life of the open water of lakes includes very small
and very large organisms, with a noteworthy scarcity
of forms of intermediate size. It is rather sharply
differentiated into plancton and necton; into small and
large; into free-floating and free-swimming forms.
These have been mentioned in Chapter V, where their
main lines of adaptation were pointed out. It remains
to indicate something of the composition and relations
of these ecological groups.
If one draw a net of fine silk bolting-cloth through
the clear water of the open lake, where no life is visible,
he will soon find that the net is straining something out
FIG. 179. "Water bloom" from the surface of Cayuga
Lake. The curving filaments are algae of the genus
Anabsena. The stalked animalcules attached to the
filaments are Vorticellas. The irregular bodies of
small flagellate cells, massed together in soft gelatine,
of the water. If he shake down the contents and lift
the net from the water he will see covering its bottom a
film of stuff of a pale yellowish green or grayish or brown-
ish color, having a more or less fishy smell, and a
gelatinous consistency. If he drop a spoonful of this
freshly gathered stuff into a glass of clear water and
296 Aquatic Societies
hold it toward the light, he will see it diffuse through
the water, imparting a dilution of its own color; and in
the midst of the flocculence, he will see numbers of
minute animals swimming actively about. Little can
be seen in this way, however. But if he will examine a
drop of the stuff from the net bottom under the micro-
scope, almost a new world of life will then stand
It is a world of little things ; most of them too small
to be seen unless magnified; most of them so trans-
parent that they escape the unaided eye. Here are both
plants and animals; producers and consumers; plants
with chlorophyl, and plants that lack it; also, parasites
and scavengers. And it is all adrift in the open waters
of the lake.
Tho plancton-organisms are so transparent and
individually so small, they sometimes accumulate in
masses upon the surface of the water and thus become
conspicuous as "water bloom." A number of the
filamentous blue-green algae, such as Anabaena, fig. 179,
and a few flagellates, accumulate on the surface during
periods of calm, hot weather. Anabaena rises in August
in Cayuga Lake, and Euglena rises in June in the back-
waters adjacent to the Lake (see fig. i, on page 15).
The plants of the plancton are mainly algae. Bacteria
and parasitic fungi are ever present, though of little
quantitative importance. They are, of course, import-
ant to the sanitarian. Of the higher plants there are
none fitted for life in the open water; but such of their
products as spores and pollen grains occur adventi-
tiously in the plancton. It is the simply organized
algae that are best able to meet the conditions of open-
water life. These constitute the producing class.
These build up living substance from the raw materials
offered by the inorganic world, and on these the life of
all the animals of both the plancton and the necton,
These are diatoms, blue-green and true-green algas,
and chlorophyl-bearing flagellates. Concerning the
limnetic habits of the last named group, we have spoken
briefly in Chapter IV (pp. 102-108). Being equipped
with flagella, they are nearly all free-swimming.
Most important among them are Ceratium, Dinobryon
Most numerous in individuals of all the plancton
algae, and most constant in their occurrence throughout
the year, are the diatoms (see fig. 35 on p. 1 1 1). Wher-
ever and whenever we haul a plancton net in the open
waters of river, lake or pond, we are pretty sure to get
diatoms in the following forms of aggregation:
I. Flat ribbons composed of the thin cells of Dia-
toma, Fragillaria, and Tabelaria.
2. Cylindric filaments composed of
the drum-shaped bodies of Melosira and
3. Radiating colonies of Asterionella.
4. Slender single cells of Synedra.
And we may get less common forms
showing such diverse structures for flota-
tion as those of Stephanodiscus (fig. 351)
and Rhizosolenia (fig. 180); or we may
get such predominantly shoreward forms
as Navicula and Meridion.
The blue-green algae of the plancton
are very numerous and diverse, but the
more common limnetic forms are these :
i . Filamentous forms having :
(a) Stiff, smoothly-contoured fila-
ments; Oscillatoria (see fig. 34
on p. 109) and Lyngbya, etc.
(b) Sinuous nodose filaments, Ana-
baena (fig. 179), Aphanizomenon,
FIG. 1 80.
FIG. 181. Rotifers.
(c) Tapering filaments that are immersed in more
or less spherical masses of gelatine, their points
radiating outward; Gloiotrichia, Rivularia (see
fig. 51, onp. 133, and 52), etc.
2. Non-filamentous forms having:
(a) Cells immersed in a mass of gelatine, Micro-
cystis (including Polycystis and Clathrocystis,
see fig. 51 on p. 133), Coelosphaerium, Chrooco-
b) * Cells arranged in a thin flat plate. Tetra-
pedia (fig. 51), Merismopasdia (see fig. 53 onp.
Representatives of all these groups, except the one
last named, become at times excessively abundant in
lakes and ponds, and many of them appear on the
surface as "water bloom."
Of the green algae there are a few not very common
but very striking forms of rather large size found in the
plancton. Such are Pediastrum (see fig. 44 on p. 123)
and the desmid, Staurastum. There are many minute
green algae of the utmost diversity in form and arrange-
ment of cells. Most of those that are shown in figure
50 on page 129 occur in the plancton; Botyrococcus is
the most conspicuous of these. A few filamentous
green forms such as Conferva (see fig. 45 on p. 124) and
the Conjugates (fig. 41 on p. 119), occur there adventi-
tiously, their centers of development being on shores.
The animals of the plancton are mainly protozoans,
rotifers and crustaceans. The protozoans of the open
I, Philodina. 2, 3, Rotifer. 4, Adineta. 5, Floscularia. 6, Stephanoceros. 7,Apsilus.
8, Melicerta. 9, Conochilus. 10, Ramate jaws. 11, Malleo-ramate jaws. 12, Micro-
codon. 13, Aspl.inchna. 14, 15, Synchaeta. i6,Triarthra. 17, Hydatina. 18, P9ly-
arthra. 19, Diglena. 20, D urella, 21, Rattulus. 22, Dinocharis. 23. 24, Salpina.
25, Euchlanis. 26, Monostyla. 27, Colurus. 28, 29, Pterodina. 30. iBrachionus.
31, Malleate jaws. 32, Noteus. 33, 34, Notholca. 35, 36, Anuraea. 37, Ploesoma.
38, Gastropus. 39. Forcipate jaws. 40, Anapus. 42, Pedalion.
From Genera of Plancton Organisms of the Cayuga Lake Basin, by
O. A. Johannsen and the junior author.
water are few. If we leave aside the chlorophyl-
bearing flagellates already mentioned (often considered
to be protozoa) the commoner forms among them are
such other flagellates as Mallomonas (see fig. 185 on
page 309), such sessile forms as Vorticella (fig. 179)
FIG. 182. Plancton Cladocerans from Cayuga Lake,
larger, Acroperus harpa; the smaller, Chydorus sp.
and such shell-bearing forms as Arcella and Difflugia
(see fig. 69 on p. 159).
The rotifers of the plancton are many. The most
strictly limnetic of these are little loricate forms such
as Anuraea and Notholca, two or three species of each
genus. When one looks at his catch through a micro-
scope nothing is commoner than to see these little thin-
shelled animals tumbling indecorously about. Some-
times almost every female will be carrying a single large
egg. Several larger limnetic rotifers, such as Triarthra,
Polyarthra and Pedalion, bear conspicuous appendages
by which they may be easily recognized. The softer-
bodied Synchaeta will be recognized by the pair of ear-
like prominences at the front. Other common limnetic
forms are shown at 2 (Rotifer neptunius), 21 and 25 of
The Crustacea of fresh-water plancton are its largest
organisms. They are its greatest consumers of vege-
table products. They are themselves its greatest con-
tribution to the food of fishes. Most of them are
herbivorous, a few eat a mixed diet of algae and of the
smaller animals. The large and powerful Leptodora is
strictly carnivorous. The following are the more
truly limnetic forms :
I. Cladocerans ; species of
Daphne (fig. 234) Diaphanosoma
Chydorus Ceriodaphnia (fig. 165)
Bosmina (fig. 91) Polyphemus
Acroperus (fig. 182) Leptodora. (fig. 186)
II. Copepods; species of
Canthocamptus (see figures 95 and 96)
Of plancton animals other than those of the groups
above discussed, there are no limnetic forms of any
great importance. There is one crustacean of the
Malacostracan group, My sis relicta, that occurs in the
deeper waters of the great lakes. There is one trans-
parent water-mite, A tax crassipes, with unusually long
302 Aquatic Societies
and well fringed swimming legs, that may fairly be
counted limnetic. There is only one limnetic insect.
It is the larva of Corethra a very transparent, free
swirnming larva, having within its body two pairs of
air sacs that are doubtless regulators of its specific
FIG. 183. The larva of the midge, Corethra. (After Weismann.)
Seasonal Range. There is no period of absence of
organisms from the open water, yet the amount of life
produced there varies, as it does on land, with season
and temperature. In winter there are more organisms
in a resting condition, and among those that continue
active, there is little reproduction and much retardation
of development. Life runs more slowly in the winter.
Diatoms are the most abundant of the algae at that
There is least plancton in the waters toward the end
of winter February and early March in our latitude.
The returning sun quickens the over- wintering forms,
according to their habits, into renewed activity, and
up to the optimum degree of warmth, hastens reproduc-
tion and development. With the overturn of the
waters in early spring com.es a great rise in the produc-
tion of diatoms, these reaching their maximum often-
times in April. This is followed by a brisk develop-
ment of diatom-eating rotifers and Crustacea. Usually
the entomostraca attain their maximum for the year in
May. This rise is accompanied by a marked decline
in numbers of diatoms and other algae, due, doubtless,
to consumption overtaking production. The warmth
Seasonal Range 303
of summer brings on the remaining algae, first the greens
and then the blue-greens, in regular seasonal succession.
It brings with them a wave of the flagellate Ceratium,
which, being much less eaten by animals than they,
often gains a great ascendency, just as the browsing of
grass in a pasture favors the growth of the weeds that
are left untouched. Green algae reach their maximum
development in early summer, and blue-greens, in mid
or late summer, when the weather is hottest.
With the cooling of the waters in autumn, reproduc-
tion of summer forms ceases and their numbers decline.
The fall overturning and mixing of the waters usually
brings on another wave of diatom production, followed
by the long and gradual winter decline. This is often
accompanied, as in the spring, by abundance of Dino-
bryon. The flagellate Synura (see fig. 30 on p. 103) is
rather unusual in that its maximum development occurs
often in winter under the ice.
The coming and going of the plancton organisms
has been compared to the succession of flowers on a
woodland slope; but the comparison is not a good one;
for these wild flowers hold their places by continuously
occupying them to the exclusion of newcomers. The
planctonts come and go. They are rather to be
likened to the succession of crops of annual weeds in a
tilled field; crops that have to re-establish themselves
every season. They may seed down the soil ere they
quit it, but they may not re-occupy it without a strug-
gle. And as the weeds constitute an unstable and
shifting population, subject to many fluctuations, so
also do the plancton organisms. They come and go;
and while on their going we know that when they come
again, another season, they will probably present col-
lectively a like aspect, yet the species will be in different
,304 Aquatic Societies
There are probably many factors determining this
annual distribution ; but chief among them would seem
to be these three:
1. Chance seeding or stocking of the waters.
Each species must be in the waters, else it cannot
develop there; and for every species, there are many
vicissitudes (such as famine, suffocation, and parasitic
diseases) determining the seeding for the next crop.
2. Temperature. Many plants and animals, as we
have seen, habitually leave the open waters when they
grow cooler in the autumn, and reappear in them when
they are sufficiently warmed in the spring. They pro-
vide in various ways (encystment, etc.) for tiding over
the intervening period. Some of them appear to be
attuned to definite range of temperature. Thus the
Cladoceran, Diaphanosoma, as reported by Birge for
Lake Mendota, has its active period when the tempera-
ture is about 20 C. (68 F.). For this and for many
other entomostraca reproduction is checked in autumn
by falling temperature while food is yet abundant.
3. Available Food. Given proper physical condi-
tions, the next requisite for livelihood is proper food.
For the welfare of animal planctonts it is not enough
that algae be present in the water; they must be edible
algae. The water has its weed species, as well as its
good herbs. Gloiotrichia would appear to be a weed,
for Birge reports that no crustacean regularly eats it,
and it is probably too large for any of the smaller ani-
mals. Birge says also ('96 p. 353) , ' 'I have seen Daphnias
persistently rejecting Clathrocystis, while greedily
collecting and devouring Aphanizomenon." Yet
Strodtmann ('98) reports Chydorus sphcericus as feeding
extensively on Clathrocystis, even to such extent that
Plancton Pulses 305
its abundance in the plancton is directly related to the
abundance of that alga. Each animal may have its
food preference. The filaments of Lyngbya are too
large for the small and immature crustaceans to handle.
Ceratium has too hard a shell; it appears to be eaten
only by the rather omnivorous adult Cyclops. For
animal planctonts in general Anabaena and its allies
and the diatoms and small flagellates appear to be the
Obviously, the amount of food available to any
speciea is in part determined by the numbers of other
species present and eating the same things.
Plancton pulses The organisms of the plancton
come in waves of development. Now one and now
another appears to be the dominant species. In most
groups there are a number of forms that are competitors
for place and food. The diatoms Asterionella,
Fragillaria and Tabelaria may fill the upper waters of
a lake together or in succession. A species of Diap-
tomus may dominate the waters this May, and
species of Cyclops may appear in its stead next May.
Yet while species fluctuate, the representation of the
groups to which they belong remains fairly con-
These sudden waves of plancton production are
made possible, as every one knows, by the brief life
cycle of the planctonts, and by their rapid rate of
increase. If a flagellate cell, for example, divide no
oftener than every three days, one cell may have more
than a thousand descendants, within a month. The
rotifer, Hydatina is said to have a length of life of some
thirteen days, but during most of this time it is rapidly
producing eggs, and the female is mature and ready to
begin egg laying in 69 hours from hatching. Some of
the larger animals live much longer and grow more
306 Aquatic Societies
slowly, but even such large forms as Daphne have an
extraordinary rate of increase, as we have already
indicated on pages 186 and 187. The rises in produc-
tion grow out of :
1. Proper conditions of temperature, light, etc.
2. Abundant food
3. Rapid increase
Declines follow upon failure of any of these, and
upon the attack of enemies. So swift are the changes
during the growing season that those who systematically
engage in the study of a lake's population take plancton
samples at intervals of not more than fourteen days,
and preferably, at intervals of seven days.
Local Abundance Plancton organisms tend to be
uniformly distributed in a horizontal direction. Al-
though many of them can swim, their swimming, as
we have noted in the preceding chapter, is directed far
more toward maintenance of level, than toward change
of location. There are, however, for many plancton
organisms, well authenticated cases of irregular hori-
zontal distribution, one of which, for Carteria, we
quoted on pages 103 and 104. Alongside that record
for a Mttle flagellate, let us place Birge's ('96) record for
the water-flea, Daphne pulicaria, in Mendota Lake.
"The Daphnias occurred in patches of irregular extent
and shape, perhaps 10 by 50 meters, and these patches
extended in a long belt parallel to the shore. The
surface waters were crowded by the Daphnias, and
great numbers of perch were feeding on them. The
swarm was watched for more than an hour. The water
could be seen disturbed by the perch along the shore
as far as the eye could reach. * * * * On this
occasion the number was shown to be 1,170,000 per
cubic meter of water in the densest part of the swarm."
Distribution in Depth 307
Shoreward Range Few plancton organisms are
strictly limited to life in open water. Most of them
occur also among the shore vegetation in ponds and
bays and shoals. They are very small and swim but
feebly, and there is room enough for their activities in
any pool. They mostly belong in the warm upper
strata of the lake, and similar conditions of environ-
ment prevail in any pond. It is the deep waters of the
lake that maintain uniform conditions of low and
stable temperature, and scanty light; and it is the
organisms of the deeper strata that do not appear in the
Hence, though the aquatic seed-plants pushing out
on a lake shore are stopped suddenly at given depth,
as with an iron barrier, the more simple and primitive
algae of the plancton range freely into all sorts of suit-
able shoreward haunts. Wo shall meet with them
there, commingled with numberless other forms that
have not mastered the conditions of the open water.
In each kind of situation (pond, river or marsh has each
its plancton) we shall find a different assemblage of
species. In all of them we shall find the planctonts are
less transparent; in none of them will there be quite
such uniformity, from place to place, as is found in the
population of the open waters of the lake.
Distribution in Depth. Since plancton organisms
tend to be uniformly distributed in a horizontal plane
one may ply his nets at any point on a lake with the
expectation of obtaining a fair sample ; but not so with
depth, except at times when the waters are in complete
circulation. A net drawn at the surface would make
a very different catch from one drawn at a depth of
fifty feet. Certain species found in abundance in the
one would not be represented in the other. The
organisms of the lakes tend to be horizontally stratified.
Each species has its own level; its own preferred habi-
tat, where it finds optimum conditions of pressure, air,
temperature and light. Fig. 184 is a diagram of the
midsummer distribution in depth of seven important
synthetic planctonts of Cayuga Lake.
FIG. 184. Diagram illustrating midsummer distribution of
seven important synthetic organisms in the first one hundred
feet of depth of Cayuga Lake. A, Ceratium; B, Dinobrypn;
C, Mallomonas; D, Anabaena; , Microcystis (Clathrocystis) ;
F, Asterionella; G, Fragillaria.
(Based in part on Juday 15)
Light is the principal factor determining distribution
in depth. This we have touched upon in Chapter II,
under the subject of "Transparency." It is only in the
upper strata of lakes, within the reach of effective light,
that green plants can grow. Animals must likewise
remain where they can find their food; whence it