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James G. (James George) Needham.

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

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Plants and animals differ most markedly here. Light
is the prime requisite and source of energy for chloro-
phyl -bearing plants. It is not light but oxygen that
holds many animals at the surface of the water; and
it is indifference to light that allows many other animals
to dwell in the obscurity of the bottom.

Life on the bottom has a number of advantages among
which are the following :

1 . Shelter is available.

2 . Energy is saved when a resting place is found, and
continuous swimming is unnecessary.

3. Gravity brings food down from above.

4. Hiding from enemies is easier in absence of strong
light.

It has also its perils chief among which are :

1 . Failure of oxygen 1 either of which may result

2. Excess of silt in suffocation.



Spatial Relations 327



In the last chapter we have discussed the more
important lines of specialization that have fitted the
members of the bottom population to meet or to
profit by these conditions. Under the subject "pond
societies," further specific illustrations will be cited.

Life at the surface is less tranquil than on the bottom.
There are two kinds of animals that can maintain them-
selves there, (i) Those having bodies (together with
the air they hold about them) lighter than the water;
which rise to the surface like a cork and have to swim
in order to go down below. These are mainly adult
insects whose problem of getting air we have discussed
in the preceding chapter.

(2) Those having bodies heavier than the water,
which maintain themselves at the surface by some sort
of hold on the surface film. If fre^-swimming, they
have to swim up to the surface and break through the
film before they can use it for support. Certain insect
larvae, water-fleas, rotifers, ciliates, etc., are of this
habit. Creeping forms must first climb up some
emergent stem, break through and then glide away sus-
pended underneath the film. Pond-snails and hydras
are of this sort. In an aquarium one may see either,
hanging suspended, and dimpling the surface where the
foot is attached by the downward pull on the film.

The relations of certain water-fleas to the surface film
are particularly interesting. For many of these, such
for example as Bosmina, this is a constant source of
peril. If in swimming a Bosmina accidentally breaks
through this film it falls over on its side and is held there
helpless lying on the surface unable to swim away.
Unless some disturbance dash it again beneath the
water, its only chance for release seems to lie in
moulting its skin and slipping out of it into the
water. Usually when a catch of surface plancton from



328 Aquatic Societies



Cayuga is placed in a beaker, the Bosminas begin
to break through one by one, and soon are gathered in a
little floating company in the center.

Scapholeberis (fig. 192), however, appears to be
especially fitted to take advantage of the surface film.
It is able to maintain a proper position at the surface:
it possesses specialized bristles for breaking the film and
laying hold upon it ; its ventral (uppermost) margin
is straightened and extended posteriorly in a long
spine; as much contact may be had as is needed.
Suspended beneath the surface, where algae from below
and pollen from the air accumulate, Scapholeberis




FlG. 192. Scapholeberis mucronata,
suspended beneath the surface film.
(After Scourfield.)

rows placidly about, foraging ; or it is borne along by the
towing of air currents acting on the surface water a
sort of submarine sailing.

Scapholeberis is unique among water-fleas in this
habit. There is also an Ostracod, Notodromas, of
similar habit; and it is worthy of note that both these
creatures have blackish markings on the ventral edges
of the valves and are pale dorsally. As in the sloths
which climb inverted in trees, the usual coloration of the
body is reversed with reversal of position.

Then there are some little creatures that take advan-
tage of the tenacity of the surface film to cover them-
selves with it as with a veil. Copepods, ostracods,
rotifers and what not, climb up the surface of emergent
stems, pushing a film of water ahead until they are well
above the general surface level, where they rest and



Spatial Relations



329



feed, and find more oxygen. The larva of Dixa is one
of the most interesting of these. It will float in the
surface film, but not for long, if any support be at hand.
Touching a leaf it immediately bends double, and
pushes forward by alternate thrusts at both ends, until
it has lifted a film of water to a satisfactory level.

On the surface are deposited the eggs of many insects
having aquatic larvae, but these eggs are heavier than

water, and unless anchored to
some solid support or buoyed
up with floats (as are such eggs
as those of Culex and Core-
thra) nearly all of them settle
to the bottom. There are,
however, a few midges whose
egg-clusters float freely. A
brief account of the egg-lay-
ing of one of them, Chironomus
meridionalis, will illustrate
several points of dependence
on the surface tension.

The female midge, when
ready to lay her eggs, rests
for a time on some vertical
stem by the water side in the
attitude illustrated in figure
194. She extrudes her eggs
which hang suspended at the
She then flies over the water
carrying them securely in a rounded clump of gelatin.
After a long preparatory flight, consisting of coursing
back and forth in nearly horizontal lines at shoul-
der height above the surface of the water a per-
formance that lasts often twenty minutes she
settles down on the surface and rests there with
outspread feet. The usefulness of her elongate tarsi is




FIG. 193. Larva of a Dixa
midge, inverted, to show: a,
caudal lobe; b, creeping
bristles; c, prolegs. The
arrow indicates the direction
of locomotion, middle fore-
most, both ends trailing.

tip of the abdomen.



330



Aquatic ' Societies



here apparent. They rest like long out-riggers radiately
arranged upon the surface, easily supporting her weight
while she liberates the egg mass and lets it down into
the water. At the top of the egg clump appears a cir-
cular transparent disc from which the egg mass depends.
This disc catches upon the surface film, tho pulled
down into it in a little rounded pit-like depression
by the weight of the eggs. Slowly
the eggs descend pulling out the gelatin
attaching them to the disc into a slender
thread that thus becomes stretched to
a length of several inches. The female
flies away to the shore and leaves them
so. Then they drift about like floating
mines, transported by breezes and cur-
rents. This little disc of gelatin dimp-
ling the surface film is indeed a frail





FIG. 194. The egg-laying of Chironomus meridionalis.

A, The female at rest extruding the egg-mass.

B, The female resting on the surface film, letting the egg mass down into the water.

C and D, The egg mass liberated and hanging suspended from the surface film by a delicate
gelatinous cord attached to a small disc-like float.

bark for their transportation. When driven by waves
and currents, they break their slender moorings and
settle to the bottom, or adhere to floating stems
against which they are tossed.

There is another phenomenon of the water surface so
curious and interesting it merits passing mention here.
There is a black wasp Priocnemis flavicornis, occasion-
ally seen on Fall Creek at the Cornell Biological Field



Spatial Relations 331

Station, that combines flying with water transportation.
Beavers swim with boughs for their dam, and water-
striders run across the surface carrying their booty, but
here is a wasp that flies above the surface towing a load
too heavy to be carried. The freight is the body of a
huge black spider several times as large as the body of
the wasp. It is captured by the wasp in a waterside
hunting expedition, paralyzed by a sting adroitly
placed, and is to be used for provisioning her nest.
It could scarcely be dragged across the ground, clothed
as that is with the dense vegetation of the water-
side; but the placid stream is an open highway. Out
onto the surface the wasp drags the huge limp black
carcass of the spider and, mounting into the air with her
engines going and her wings steadily buzzing, she sails
away across the water, trailing the spider and leaving
awake that is a miniature of that of a passing steamer.
She sails a direct and unerring course to the vicinity of
her burrow in the bank and brings her cargo ashore
at some nearby landing. She hauls it upon the bank
and then runs to her hole to see that all is ready.
Then she drags the spider up the bank and into her
burrow, having saved much time and energy by making
use of the open waterway.

Intermediate between surface and bottom the life of
the water that is not included in either of the two strata
we have just been discussing, but that has continuous
free range of the open water, is still considerable. It
corresponds in part to the plancton of the open waters,
as we have seen. It corresponds in part, also, to the
necton; and, as in the open water, so also in the shoals,
the larger and more important free-swimming animals
are fishes. Its spatial relations are complicated by the
habit some air-breathing forms (especially insects)
have of ranging downward freely thro the depths;



332 Aquatic Societies



also by the way in which forms like Chironomus, that
ordinarily remain in hiding in the bottom, come out
betimes in the open and take a swim. But there yet
remain at least two classes of organisms that belong
neither to the top nor to the bottom, nor yet to the
free-swimming population. These are forms that are
able to sustain themselves above the mud by taking
advantage of plant stems or other solid supports. These
get their oxygen from the water. They are:

1. Climbing forms, that hold on by means of claws,
as do the scuds and some dragonfly, damselfly and may-
fly Iarva3, or by a broad adhesive foot as do certain
minute mussels. Many members of this group find
temporary shelter between the leaves and scales of
plants.

2. Sessile forms that remain more or less per-
manently attached, like sponges, bryozoans, hydras,
etc.

Many members of both these groups construct for
themselves shelters. Chironomus, for example, while
usually living in such tubes as are shown in figure 134
on page 226, is able to creep about freely upon the
stem. Cothurnia (fig. 73) and Stentor, and many
sessile rotifers build themselves shelters.

Such support may be found on the bottom itself
where that is hard ; but the bottom is soft where most
seed-plants grow. Furthermore, to ascend and remain
above the level of the hordes of voracious bottom
dwellers must be a means of safety. It is clear, there-
fore, that plants rising from the bottom and branching
extensively must add enormously to the biological rich-
ness of the shoals, by the support and shelter they
afford to such animals as these.

Size As on land a weed patch is a miniature jungle,
having a population of little insects roughly correspond-



Life in Some Typical Lenitic Situations 333

ing in social functions to the larger beasts of the forest,
so in the water there are large and small, assembled in
parallel associations. The larger, as a rule, inhabit the
more open places. Paddle-fish and sturgeons and gars
belong to the rivers; the quantative demands of their
appetites exclude them from living in the brooks There
is not a living there for them. Little fishes belong to
the brooks and to the shoals. In our diagram on page
233 we have already shown how in a small lake shore-
ward distribution of the fishes corresponds roughly with
their size, the largest ranging farthest out, and the
smallest sticking most closely to shelter. The senior
author has shown (07) a parallel to this in the distribu-
tion of diving beetles in an angle of the shore of a weedy
pond. Here the most venturesome beetle was Dytiscus
(see fig. 129 on p. 221). It was taken at the front of
the cat-tails in about three feet of water. The associa-
ted species were disposed closely, tho not strictly in
accordance with their size, between that outer fringe
and the shore, Acilius, Coptotomus, Laccophillus,
Hydroporus, (see fig. 130) Ccelambus and Bidessus
following in succession, the last named (a mere molecule
of a beetle, having but ?ifW the weight of Dytiscus)
being found only among the trash at the very shore line.

LIFE IN SOME TYPICAL LENITIC SITUATIONS

The association of organisms in natural societies is
controlled by conditions; but conditions intergrade.
Lakes, ponds, rivers, marshes all merge insensibly,
each into any of the others; and their inhabitants
commingle on their boundaries. Yet these names stand
for certain general average conditions that we meet
and recognize, and with which certain organisms are
regularly associated. It will be worth while for us to
note the main characteristics of the life of several of the
more typical of such situations.



334



Aquatic Societies



I

Pond societies The kind of associations we now come
to discuss are typically represented in ponds, but they
occur also in any bodies of standing fresh water, that
are not too deep for growth of bottom herbage, nor too
exposed to wind and wave for the growth of emergent




FIG. 195. Where marsh and pond meet. The head of "the cove" at the Cor-
nell Biological Field Station. Beds of spatterdock backed by acres of cat-
tail flag. Neguena valley in the distance.

aquatics along shore. The same forms will be found
in ponds, lagoons, bayous, sheltered bays and basin-
like expansions of streams. The bordering aquatics
will tend to be arranged in zones, as discussed in the
preceding pages, according to the closeness of their
crowding.

I. The shoreward zone of emergent aquatics will
include, in our latitude, species of cat-tail (Typha), of



Pond Societies



335



bur-reed (Sparganium) , of bulrush (Scirpus), of spike-
rush (Eleocharis) , of water plantain (Alisma) , of arrow-
head (Sagittaria) , and arrow-arum (Peltandra) , of pick-
erel-weed (Pontederia), of manna grass (Glyceria), etc.

2. The intermediate zone of surface aquatics will
include such as:

(a). These rooted aquatics with floating leaves:
white water-lily (Castalia), spatterdock (Nymphaea),
water shield (Brasenia), pondweed (Potomogeton), etc.




FIG. 196. A spray of the sago pondweed, Potampgeton, coated with
slime-coat diatoms, its leaf tips bearing dwelling tubes of midge
larvae (Chironomus).



336 Aquatic Societies



(b). These free-floating aquatics; species of duck-
weed (Lemna, Spirodela), water fernworts (Azolla,
Salvinia), liverworts (Riccia), etc.

3. An outer zone of submerged plants will include
such forms as pondweeds (Potamogeton) , hornwort
(Ceratophyllum) , crow-foot (Ranunculus), naiad
(Najas), eel-grass (Zostera), stonewort (Chara), etc.

These grow lustily and produce great quantities of
aquatic stuff which serves in part while living, but prob-
ably in a larger part when dead, for food of the animal
population, and the ultimate residue of which slowly
fills up the pond. These plants contribute largely to
the richness and variety of the life in the pond, by
offering solid support to hosts of sessile organisms, both
plants and animals. Their stems are generally quite
encased with sessile and slime-coat algae, rotifers,
bryozoans, sponges, egg masses of snails and insects
and dwelling tubes of midges (fig. 196). Especially do
floating leaves seem to attract a great many insects to
lay their eggs on the under surface. This is doubtless
a shaded and cleanly place, so elevated as to be favor-
able for the distribution of the young on hatching.

The alga of ponds are various beyond all enumerating.
It is they, rather than the more conspicuous seed-plants,
that furnish the basic supply of fresh food for the animal
population. Small as they are individually, their rapid
rate of increase permits mass accumulation which
often become evident enough. Such are:

(i). The masses of filamentous algae, (Spirogyra and
its allies; Ulothrix, Conferva, etc.) collectively called
"blanket algae" that lie half-floating in the water, or are
buoyed to the surface by accumulated oxygen bubbles.

(2). The beautiful fringes of branching sessile algae
(Chaetophora, fig. 198, Cladophora, etc.) that envelop
every submerged stem as with a drapery of green.



Pond Animals



337




(3). The lumps of brownish gelatin inclosing com-
pound colonies (Rivularia, see fig. 52 on p. 134, etc.),
that are likely to cover the same stems later in the
season, and that sometimes seem to smother the green
vegetation.

(4). The spherical lumps of greenish gelatin that lie
sprinkled about over the bottom rather hard lumps
inclosing compact masses of fila-
ments of Nostoc, etc.

(5). The accumulated free-
swimming forms that are not
seen as discrete masses, but that
tint the water. Volvox tints it
a bright green; Dinobryon, yel-
lowish; Trachelomonas, brown-
ish; Ceratium, grayish, etc.

Such differences as these in
superficial aspect, coming, as
many of them do, with the regu-
larity of the seasons, suggest to
one who has studied them the
principal component of the
masses ; but one must see them
with the microscope for certain
determination.

The animals of the pond that breathe free air are a few
amphibians (frogs and salamanders), a few snails
(pulmonates) and many insects. The insects fall into
four categories according to their more habitual posi-
tions while taking air:

(i). Those that run or jump upon the surface.
Here belong the water-striders and their allies long
legged insects equipped with fringed and water-repel-
lent feet that take hold on the surface film, but do not
break through it. Here belong many little Diptera that
rest down upon the surface between periods of flying.



FIG. 197. Diagram of a lily-
pad, inverted, showing
characteristic location and
arrangement of some
attached egg clusters.

a, Physa; 6, Planorbis; c, Triae-
nodes; d, Donacia; e, Hydro-
campa; /, Enallafema (inserted
into punctures); g, Notonecta
(laid singly) ; h, Gyrinus.



338 Aquatic Societies



Here belong the hosts of minute spring-tails that gather
in the edges in sheltered places, often in such numbers
as to blacken the surface as with deposits of soot.
Minute as these are they are readily recognized by
their habits of making relatively enormous leaps from
place to place.

(2). Those that lie prone upon the surface. Best
known of these because everywhere conspicuous on stiL 1




FIG. 198. Two fallen stems enveloped with a rich growth
of the alga, Chcetophora incrassata.

waters, are the whirl-i-gig beetles. Less common and
much less conspicuous are the pupae of the soldier-flies
(Stratiomyia, etc.) and the larvae of the Dixa midges.

(3). Those that hang as if suspended at the surface,
with only that part of the body that has to do with
intake of air breaking through the surface film. Here
belong by far the larger number of aquatic insects.
Here are the bugs and the adult beetles, alertly poised,



Pond Animals



339



with oar-like hind legs swung forward, ready, so that a
stroke will carry them down below in case of approach
of danger. Here hang the wrigglers larvae and pupae
of mosquitoes. Here belong the more passive larvae of
many beetles and flies and the pupae of swale-flies and
certain crane-flies.

(4). Those that rest down below, equipped with a
long respiratory tube for reaching up to the surface for




FIG. 199. Diagram of distribution of pond life. The right side
illustrates the zonal distribution of the higher plants. /, shore
zone; 2, standing emergent aquatics; 3, aquatics w i 'th floating
leaves; 4, submerged aquatics; 5, floating aquatics; 6, free swim-
ming algae of the open water.

The left side represents the principal features of the distribution of
animals, r, s, t, u, forms that breathe air; v, w, x, y, and z, forms
that get their oxygen from the water.

(From the Senior Author's General Biology)

air. Such are Ranatra, and the rat-tailed maggots of
syrphus-flies.

The animals of the pond that are more strictly aquatic
in respiratory habits (being able to take their oxygen
supply from the water itself) are so numerous that we
shall be able to mention only a few of the larger and
more characteristic forms. First there are the inhabi-
tants of the bottom. These fall into two principal cate-
gories, the free-living and the shelter-building forms.
The free-living forms may be grouped as follows:



340



Aquatic Societies



(i). Bottom sprawlers that lie exposed, or only
covered over with adherent silt. These are character-
ized by a marked resemblance to their environment.
Such crustaceans as the crawfish and Asellus, such
insects as Ephemerella, Casnis and other mayfly nymphs

Libellula, Didymops,
Celithemis (fig. 200)
and other dragonfly
nymphs, and certain
snails and flatworms
belong here.

(2). Bottom dwel-
lers that descend more
or less deeply into the
mud or sand, by the
various means already
discussed in the pre-
ceding chapter.
Among the shallow
burrowers are many
shell-bearing molluscs,
both mussels and
snails; a few may-
fly and dragonfly
nymphs. Descending
more deeply in muddy
beds are some true worms and horsefly larvae.

The shelter-building forms of the bottom may be
grouped as:

(i). Forms making portable shelters. These are
mainly caddis-worms that construct cases of pieces of
wood or grains of sand.

(2). Forms making fixed shelters. These are
such caddis- worms as Polycentropus, such worms as
Tubifex (see fig. 83 on p. 174) and such midges as
Chironomus (see fig. 134 on p. 220).




FIG. 200. A bottom sprawler: nymph of
of the dragonfly, Celithemis eponlna.



Marsh Societies 341

It is some of these animals of the pond bottom that
give to the littoral region its great extension out tinder
the open waters of the lakes. It is only a few members
of the population that are able to endure conditions in
the depths far out from shores. These are such as:

Small mussels of genus Pisidium.

Mayfly nymphs of the genus Hexagenia.

Midge larvae of the genus Chironomus.

Caddis-worms in the cylindric cases of sand, not yet
certainly identified, etc.

The larger animals of the pond that belong neither
to surface nor bottom and that correspond to neither
plancton nor necton of the open water may be grouped
as:

(i). Climbing forms (most of which can swim on
occasion), such as the scuds (Amphipods), the nymphs
of dragonflies such as Anax, of damselflies such as Lestes
and Ischnura, of mayflies such as Callibaetis, larvae of
caddisflies such as Phryganea and of moths such as
Paraponyx, mussels such as Calyculina, and many
leeches, entomostracans and rotifers.

(2). Sessile forms such as hydras, sponges, bryzoans
and rotifers.

II

Marsh Societies. We come now to consider the
associations of organisms in waters that are not too deep
for the growth of standing aquatics. Shoalness of
water and instability of temperature and other physical
conditions at once exclude from residence in the marsh
the plants and animals of more strictly limnetic habits ;
but it is doubtless the presence of dense emergent plant
growth that most affects the entire population. This
gives shelter to a considerable number of the higher
vertebrates, and these rather than the fishes are the
large consumers of marsh products. The muskrat



342



Aquatic Societies



breeds here and builds his nest of rushes. He prefers,
to be sure, the edge of a marsh opening, where in deep
water he may find crawfishes and molluscs, with which
to vary his ordinary diet of succulent shoots and tubers.




FIG. 201. The eggs of the spotted salamander, Ambystoma punctatum.

(Photo by A. A. Allen.)

Deep in the marsh dwell water birds, such as grebes,
rails, coots, terns, bitterns, and in the north, ducks and
geese as well. Such non-aquatic birds as the long-billed
marsh-wren and the red-winged blackbird use the top
of the marsh cover as a place to build their nests and



Marsh Piants



343



use also the leaves of marsh plants for building materials.
Several turtles and water snakes are permanent resi-
dents as are also a few of the frogs. Most of the frogs
visit the marsh
pools at spawn-
ing time, making
the air resound
with their nup-
tial melodies.
The spotted sal-
amander is the
earliest amphi-
bian to spawn
there. Though
the adult is but
a transient, its
larvae remain in
the marsh pools
through the sea-
son.

The plants are
the same kinds


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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 19 of 26)