James G. (James George) Needham.

The life of inland waters; an elementary text book of fresh-water biology for students online

. (page 14 of 26)
Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 14 of 26)
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All twice natural size.

FIG. 144. The spotted salamander, Ambystoma tigrinum.


Aquatic Organisms

in frogs than in any other vertebrates, involving
profound changes in internal organs and in manner of

The reptiles are mainly terrestrial. Southward there
are alligators in the water, but in our latitude there are

FIG. 145. The common snapping turtle.

only a few turtles and water snakes. These make their
nests on land. They hide their eggs in the sand or in
the midst of marshland rubbish, where the sun's
warmth incubates them.

These also are carnivorous.

Water Birds


The water birds, tho more numerous than the two
preceding groups, are but a handful of this great class
of vertebrates.

The principal kinds of birds that frequent the water
are water-fowl ducks, geese and swans; the shore
birds plover, snipe and rails; the gulls, the herons
and the divers. Some of these that, like the loon, are

FIG. 146. Wild geese foraging in a marsh in Dakota.

superably fitted for swimming and diving, feed mainly
on fishes. Most water birds consume a great variety of
lesser animals. The ducks and rails differ much in diet
according to species. Thus the Sora rail eats mainly
seeds of marsh plants, while the allied Virginia rail in
the same locality eats miscellaneous animal food to the
extent of more than fifty per cent, of its diet.

Only the waterfowl that are prized as game birds are
extensively herbivorous. They eat impartially the
vegetable products of the land and of the water. The


Aquatic Organisms

wild ducks and geese eat great quantities of duckmeat
(Lemna) and succulent submerged aquatics. Canvas-
backs fatten on the wild celery ( Vallisneria) . In
Cayuga Lake in winter they gorge themselves with the
starch-filled winter buds of the pond weed, Potamogeton

FIG. 147. Floating nest of pied-billed grebe (Podilymbus podiceps) in
a cat-tail marsh, surrounded by water.

pusillus. They also dive and pluck up from the bottom
mud the reproductive tubers of the pondweed, Potamoge-
ton pectinatus (see fig. 228 on p. 381).

Water birds, having attained the freedom of the air,
are wide ranging beyond all other animals. They come
and go in annual migrations. They settle here and

Aquatic Mammals 241

there, and commit local and intermittent depredations.
The water birds nest mainly on land, and in their
nesting and brooding habits they differ little from their
terrestrial relatives.

The aquatic mammals of inland waters fall mainly in
two groups, the carnivores and the rodents. Here
again, the carnivores that are more expert swimmers
and divers, such as fisher, martin, otter and mink are
all fish-eating animals. They have become fitted to

FIG. 148. A muskrat, Fiber zibethicus.

utilize the chief animal product of the water. Of these
four the mink alone has withstood the "march of
progress," and retains its former wide distribution.

Of rodents there are two fur-bearers of much import-
ance, the beaver, now driven to the far frontier, and the
muskrat. The muskrat has become under modern
agricultural conditions the most important aquatic mam-
mal remaining. By reason of its rapid rate of repro-
duction, its ability to find a living in any cat-tail marsh,
big or little, and its hardiness, it has been able to main-
tain its place.




O infinitely varied are
the fitnesses of aqua-
tic organisms for the
conditions they have
to meet that we can
only select out of a
worldf ul of examples a
few of the more wide-
spread and significant.
We shall have space
here for discussing

only such adaptations to life in the water as are common
to large groups of organisms, and represent general
modes of adjustment. First we will consider some of
the ways in which the species is fitted to the aquatic
conditions under which it lives, and then we will take
note of some mutual adjustments between different

The first of living things to appear upon the earth
were doubtless simple organisms that were far from


Flotation 243

being so small as the smallest now existing, or so large
as the largest. They grew and multiplied. They
differentiated into plants and animals, into large and
small, into free-swimming and sedentary. Some be-
took themselves to the free life of the open waters and
others to more settled habitations on shores. The
open-water forms were nomads, forever adrift in the
waves: the shoreward forms might find shelter and a
quiet resting place.


In the open water there are certain great advantages
that lie in minuteness and in buoyancy. These quali-
ties determine the ability of organisms to float freely
about in the more productive upper strata of water.
To descend into the depths is to perish for want of
light. So the members of many groups are adapted for
floating and drifting about near the surface. These
constitute the plancton.

On the other hand, large size has its advantages when
coupled with good ability for swimming and food
gathering. In the rough world's strife the battle is
usually to the strong. It is the larger, wide-ranging,
free-swimming organisms that dominate the life of the
open water. These constitute the necton.

Plancton and necton will be discussed in the next
chapter as ecological groups, but in this place we may
take note of the two very different sorts of fitness, that
they have severally developed for life in the open water,
the plancton organisms being fitted for flotation, and
the necton for swimming.

Flotation All living substance is somewhat heavier
than water (i. e. has a specific gravity greater than i)
and therefore tends to sink to the bottom. The veloc-

244 Adjustment to Conditions of Aquatic Life

ity in sinking is determined by several factors, one of
which is external and the others are internal:

The external factor is the varying viscosity of the

The internal factors are specific gravity, form and

We have mentioned (p. 30) that the viscosity of the
water is twice as great at the freezing point as at
ordinary summer temperatures ; which means, of course,
that the water itself would offer much greater resistance
to the sinking of a body immersed in it. We are here
concerned with the internal factors.

Lessening of specific gravity The bodies of organisms
are not composed of living substance alone, but con-
tain besides, inclusions and metabolic products of
various sorts, which oftentimes alter their specific
gravity. The shells and bone and other hard parts of
animals are usually heavier than protoplasm; the fats
and gelatinous products and gases are lighter. We
know that the fats of vertebrates, if isolated and thrown
upon the water, will float; and that a fat man, in order
to maintain himself above the water, needs put forth
less effort than a lean one. There are probably many
products of the living body that are retained within
or about it and that lessen its specific gravity, but the
commonest and most important of these seem to fall
into three groups:

1. Fats and oils, which are stored assimilation
products. These are very easily seen in such plancton
organisms as Cyclops (see fig. 96 on p. 189) where they
show through the transparent shell as shining yellowish
oil droplets. Most plancton algae store their reserve
food products as oils rather than as starches.

2. Gases, which are by-products of assimilation, and
are distributed in bubbles scattered through the tissue

Flotation 245

where produced, or accumulate in special containers.
These greatly reduce the specific gravity of the body,
enabling even heavy shelled forms (see p. 159) to float.

3. Gelatinous and mucilaginous products of the body
which usually form external envelopes (see fig. 10 on
p. 52) but which may appear as watery swellings of the
tissues. Their occurrence as envelopes is very common
with plants and with the eggs of aquatic animals ; they
may serve also for protection and defense, and for
regulating osmotic pressure, but by reason of their low
specific gravity they also serve for flotation.

Improvment of form We have already called atten-
tion (p. 42) to the fact that size has much to do with the
rate of sinking in still water. This is because the
resistance of the water comes from surface friction and
the smaller the body the greater the ratio of its
surface to its mass. Given a body small enough, its
mere minuteness will insure that it will float. But in
bodies of larger size relative increase in surface is
brought about in various ways:

1. By extension of the cell in slender prolongations
(see fig. 50, j, k, 1, on p. 129).

2 . By the aggregation of cells into expanded colonies :

a. Discoid colonies, as in Pediastrum (fig. 44 on

P- 123).

b. Filaments, as in Oscillatoria (fig. 34 on p. 109).

c. Flat ribbons of innumerable slender cells placed
side by side, as in many lake diatoms (Fragil-
laria, Tabelaria, Diatoma).

d. Radiate colonies as in Asterionella (fig. 35 n on
p. in).

e. Spherical colonies as in Volvox (fig. 31, p. 105:
see also a b c of. fig. 50 on p. 129), wherein the
cells are peripheral and widely separated the

246 Adjustment to Conditions of Aquatic Life

interstices and the interior being filled with
gelatinous substances of low specific gravity.
f. Dendritic colonies, as in Dinobryon (fig. 32 on
p. 106).

3. In the Metazoa, by the expansion of the external
armor and appendages into bristles, spines and fringes.
Thus in the rotifer Notholcalongispina (fig. 149),
a habitant of the open water of lakes, there is a
great prolongation of the angles of the lorica,
before and behind; and in the Copepods (fig. 95,
p. 1 8 8) there is an extensive development of
bristles upon antennae and caudal appendages.
Expansions of the body, if mere expansions,
serve only to keep the body passively afloat; but
many of them have acquired mobility, becom-
ing locomotor organs. Cilia and flagella are the
simplest of these, and are common to plants and
animals. Almost all the appendages of the
higher animals, antennas, legs, tails, etc., are
here and there adapted for swimming. A body
whose specific gravity is but little greater than
that of the water may be sustained by a mini-
mum use of swimming apparatus. The lesser
iong- flagellate and ciliate forms, both plant and
spin e d animal , maintain their place by continuous lash -
rotifer. j n g Q ^ Q water. If we watch a few waterfleas
in a breaker of clear water we shall see that their swim-
ming also, is unceasing. Each one swims a few strokes
of the long antennae upward, and then settles with
bristles all outspread, descending slowly, as resistance
yields, to its former level. This it repeats again and
again. It may turn to right or to left, rise a little
higher or sink a little lower betimes, but it keeps in
the main to its proper level. Its swimming powers
are to an important degree supplemental to its inade-



quate powers of flotation. The strokes of its swim-
ming antennae are, like the beating of our own hearts,
intermittent but unceasing, and when these fail it falls
to its grave on the lake bottom.

Flotation devices usually impede free swimming,
especially do such expansions of the body as greatly
increase surface contact with the water. It is in the
resting stages of animals, therefore, that we find the
best development of floats: such, for example, as the
overwintering statoblasts of the Bryo-
zoan, Pectinatella, shown in the accom-
panying figure. Here an encysted mass
of living but inactive cells is sur-
rounded by a buoyant, air-filled an-
nular cushion, as with a life preserver,
and floats freely upon the surface of
the water, and is driven about by the

Too great buoyancy is, however, as
much a peril to the active micro-organ-
isms of the water as too little. Contact
with the air at the surface brings to soft
protoplasmic bodies, the peril of evap-
oration. Entanglement in the surface
film is virtual imprisonment to certain
of the water-fleas, as we shall see in
the next chapter. It is desirable that
they should live not on but near the
surface. A specific gravity about that
of water would seem to be the optimum
for organisms that drift passively about: a little greater
than that of water for those that sustain themselves in
part by swimming.

Terrestrial creatures like ourselves, who live on the
bottom in a sea of air with solid ground beneath our
feet, have at first some difficulty in realizing the nicety

FIG. 150. The over-
wintering stage
of the bryozoan,
Pectinatella; a
statoblast or
gemmule. The
central portion
contains the liv-
ing cells. The
dark ring of min-
ute air-filled cells
is the float. The
peripheral an-
chor-like pro-
cesses are attach-
ment hooks for
securing distribu-
tion by animals.

248 Adjustment to Conditions of Aquatic Life

of the adjustment that keeps a whole population in the
water afloat near to, but not at the surface. This comes
out most clearly, perhaps, in those minor changes of
form that accompany seasonal changes in temperature
of the water. In summer when the viscosity of the
water grows less (and when in consequence its resist-

a b c

FIG. 151. Summer and winter forms of plancton animals : sum-
mer above, winter below, a, the flagellate Ceratium; b, the
rotifer Asplanchna; c, d, e t water- fleas; c and d, Daphne;
e, Bosmina. (After Wesenberg-Lund).

ance to sinking is diminished) the surface of many
plancton organisms is increased to correspond. The
slender diatoms grow longer and slenderer, the spines
on certain loricate rotifers grow longer. Bristles and
hairs extend and plumes and fringes grow denser. Even
the form of the body is altered to increase surface-
contact with the water. A few examples are shown in



the accompanying figures. These changes when fol-
lowed thro the year show a rather distinct correspond-
ence to the seasonal changes in viscosity of the water.

FIG. 152. Seasonal form changes of the water-flea, Bosmina coregoni. The
fractional figures above indicate date: those below indicate corresponding
temperatures in C. (After Wesenberg-Lund.)

Swimming For rapid locomotion through the water
there are numberless devices for propulsion, but there
is only one thoroly successful form of body; and that
is the so-called "stream-line form" (fig.
153). It is the form of body of a fish:
an elongate tapering form, narrowed
toward either end, but sloping more
gently to the rear. It is also the form
of body of a bird encased in its feathers.
It is probably the form of body best
adapted for traversing any fluid medium
with a minimum expenditure of energy.
The accompanying diagram explains its
efficiency. The white arrow indicates
direction of movement. The gray lines
indicate the displacement and replace-
ment of the water. The black arrows
indicate the direction in which the
forces act. At the front the force of
the body is exerted against the water;
at the rear the force of the water is exerted against the
body. The water, being perfectly mobile, returns

FIG. 153. Stream-
line form. For
explanation see

250 Adjustment to Conditions of Aquatic Life

after displacement; and much of the force expended
in pushing it aside at the front is regained by the
return-push of the water against the sloping rearward
portion of the body.

The advantage of stream-line form is equally great
whether a body be moving through still water, or
whether it be standing against moving water. A
mackerel swimming in the sea is benefited no more than
is a darter holding its stationary position on the stream
bed. To this we shall have occasion to return when
discussing the rapid-water societies.

Apparatus for propulsion is endlessly varied in the
different animal groups. Plants have developed hardly
any sort of swimming apparatus beyond cilia and
flagella. These also serve the needs of many of the
lower animals the protozoa, the flat worms, the roti-
fers, trochophores and other larvae, sperm cells generally,
etc. But more widely ranging animals of larger size
have developed better swimming apparatus, either with
or without appendages. Snakes swim by means of
horizontal undulating or sculling movements of the
body, and so also do many of the common minute
Oligochaete worms. Horseleeches swim in much the
same manner, save that the undulations of the body are
in the vertical plane. Midge larvae (" blood worms")
swim with figure-of-8-shaped loopings of the body that
are quite characteristic. Mosquito larvae are
" wrigglers, " and so also are many fly and beetle larvae,
tho each kind wriggles after its own fashion. Dragon-
fly nymphs swim by sudden ejection of water from the
rectal respiratory chamber.

All of these swim without the aid of movable appen-
dages ; but the larger animals swim by means of special
swimming organs, fringed and flattened in form and
having an oar-like function. These may be fins, or

Life on the Bottom 251

legs, or antennae, or gill plates, in infinite variety of
length, form, position and design.

Great is the diversity in aspect and in action of the
animals that swim. Yet it is perfectly clear, even on a
casual inspection, that the best swimmers of them all are
those that combine proper form of body stream-line
form with caudal propulsion by means of a strong


Shoreward, the earth beneath the waters gives
aquatic organisms an opportunity to find a resting place,
a temporary shelter, or a permanent home. Flotation
devices and ability at swimming may yet be of advan-
tage to the more free-ranging forms ; but the existence
of possible shelter and of solid support makes for a line
of adaptations of an entirely different sort. Here dwell
the aquatic organisms that have acquired heavy armor
for defense; heavy shells, as in the mussels; heavy
carapaces as in the crustaceans ; heavy chitinous armor
as in the insects ; or heavy incrustations of lime as in the
stone worts.

The condition of the bottom varies from soft ooze in
still water to bare rocks on wave washed shores. The
differences are very great, and they entail significant
differences in the structure of corresponding plant and
animal associations. These have been little studied
hitherto, but a few of the more obvious adaptations to
bottom conditions may be pointed out in passing.

First we will note some adaptations for avoidance of
smothering in silt on soft bottoms ; then some adapta-
tions for finding shelter by burrowing in sandy bottoms
and by building artificial defenses: then some adapta-
tions for withstanding the wash of the current on hard

252 Adjustment to Conditions of Aquatic Life

Avoidance of silt Gills are essentially thin- walled
expansions of the body, that provide increased surface
for contact with the water, and thus promote that
exchange of gases which we call respiration. Gills
usually develop on the outside of the body; for it is

only in contact with the water
that they can serve their func-
tion. In most animals that live
in clear waters they are freely
exposed upon the outside; but
in animals that live on soft
muddy bottoms they are with-
drawn into protected chambers
(or, rather, sheltered by the
outgrowth of surrounding parts)
and fresh water is passed to
them thro strainers. Thus the
gills of a crawfish occupy capa-
cious gill chambers at the sides
of the thorax, and water is
admitted to them thro a set
of marginal strainers. The gills
of fresh -water mussels are located
at the rear of the foot within the
inclosure of valves and mantle,
and water is passed to and from
them thro the siphons . The gills
of dragonfly nymphs are located
on the inner walls of a rectal

respiratory chamber, and water to cover them is slowly
drawn in thro a complicated strainer that guards the
anal aperture, and then suddenly expelled thro the
same opening, the valves swinging freely outward.

There is probably no better illustration of parallel
adaptation for silt avoidance than that furnished by the

FIG 154. The abdomen of
Asellus, inverted, showing
gill packets.

Avoidance of Silt


crustacean, Asellus, and the nymph of the mayfly,
Caenis. Both live in muddy bottoms where there is
much fine silt. Both possess paired plate-like gills.
In Asellus they are developed underneath the abdomen ;
in Caenis upon the back. In Asellus they are double ;
in Caenis, simple. In
Asellus they are blood
gills; in Casnis, tracheal
gills. In both they are
developed externally in
series, a pair correspond-
ing to a body segment.
In both they are soft and
white and very delicate.
But in both an anterior
pair has been developed
to form a pair of enlarged
opercula or gill covers.
These are concave pos-
teriorly and overlie and
protect the true gills.
The gills have been ap-
proximated more closely,
so that they are the more
readily covered over ; and
they have developed in-
terlacing fringes of radi- FIG. 155. The nymph of the mayfly
ating marginal hairs, Caenis, showing dorsal gill packets.

which act as strainers,

when the covers are raised to open the respiratory


Such are the mechanical means whereby suffocation
in the mud is avoided. It must not be overlooked that
there is a physiological adaptation to the same end. A
number of soft bodied thin-skinned animals have an
unusual amount of haemoglobin in the blood plasma

254 Adjustment to Conditions of Aquatic Life

enough, indeed, to give them a bright red color. This
substance has a great capacity for gathering up oxygen
where the supply is scanty, and of yielding it over
to the tissues as needed. True worms that burrow in
deep mud, and Tubifex (see fig. 83 on p. 174) that bur-
rows less deeply and the larger bright red tube making
larvae of midges known as "blood worms'* (see fig. 236
on p. 393) are examples. Since these forms live in the
softest bottoms, where the supply of oxygen is poorest,
where few other forms are able to endure the conditions,
their way of getting on must be of considerable efficiency.


Burrowing The ground beneath the water offers
protection to any creature that can enter it ; protection
from observation to a bottom sprawler, that lies littered
over with fallen silt ; protection from attack about in
proportion to its hardness, to anything that can bur-

Animals differ much in their burrowing habits and in
the depth to which they penetrate the bottom. Many
mussels and snails burrow very shallowly, push-
ing their way along beneath the surface, the soft foot
covered, the hard shell-armored back exposed. The
nymphs of Gomphine dragonflies (fig. 116 on p. 209)
burrow along beneath the bottom with only the tip
of the abdomen exposed at the surface of the mud.
Other insect larvae descend more deeply into burrows
which remain open to the water above : while horsefly
larvae and certain worms descend deeply into soft mud.

The two principal methods by which animals open
passageways thro the bottom are (i) by digging, and
(2) by squeezing thro. Digging is the method most
familiar to us, it being commonly used by terrestrial
animals. Squeezing thro is the commonest method of
aquatic burro wers.



FIG. 156. A nymph of a burrowing mayfly, Ephemera. (From Annals
Entom. Soc. of America: drawing by Anna H. Morgan).

The digging of burrows requires special tools for mov-
ing the earth aside. These, as with land animals, are
usually flattened and shovel-like fore legs. The other
legs are closely appressed to the body to accommodate
them to the narrow burrow. The hind legs are directed
backward. The head is usually flattened and more or
less wedge-shaped, and often specially adapted for
lifting up the soil preparatory to advancing thro it
(see fig. 116 on p. 209).

One of the best exponents of the burrowing habit is
the nymph of the may-
fly, Hexagenia, whose
innumerable tunnels
penetrate the beds of
all our larger lakes and
rivers. It is an un-
gainly creature when
exposed in open water;

but when given a bed
of sand to dig in, it
shows its fitness. Be-
sides having feet that

FIG. 157. The front of a burrowing may-
fly nymph, Hexagenia, much enlarged,
showing the pointed head, the great
mandibular tusks and the flattened
fore legs.

are admirably fitted for

scooping the earth aside, it has a pair of enormous

256 Adjustment to Conditions of Aquatic Life

mandibular tusks projecting forward from beneath
the head. It thrusts forward its approximated blade-

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Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 14 of 26)