Copyright
James G. (James George) Needham.

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

. (page 11 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 11 of 26)
Font size
QR-code for this ebook


position with the foot projecting still more
deeply into the mud.



Molluscs 181



thus : Unios and their allies are large forms that have
pearly shells and that live mainly in large streams and
lake borders. They produce enormous numbers of
young, and use mostly the outer gill for a brood
chamber. They cast the young forth while still minute
as glochidia, to become attached to and temporarily
parasitic on fishes. The relations of these larval
glochidia with the fishes will be discussed in chapter V.
The lesser mussels (family Sphaeridae) dwell in small
streams and pools and in the deeper waters of lakes.
Their shells are not pearly. They produce but a few
young at a time and carry these until of large size,
using the inner gill for a brood-pouch. The stouter
species, half an inch long when grown, burrow in stream-
beds like the unios. The slenderer species climb up
the stems of plants by means of their excessively mobile
adhesive and flexible foot. On this foot the dainty
white mussel glides like a snail or a flatworm, up or
down, wherever it chooses.

Snails are as a rule more in evidence than are mussels,
for they come out more in the open. They clamber
on plants and over every sort of solid support. They
hang suspended from the surface film, or descend there-
from on strings of secreted mucus. They traverse
the bottom ooze. We overturn a floating board and
find dozens of them clinging to it, and often we find
a filmy green mass of floating algae thickly dotted with
their black shells.

They eat mainly the soft tissues of plants, and micro-
organisms in the ooze covering plant stems. A ribbon-
like rasp (radula) within the mouth drawn back and
forth across the plant tissue scrapes it and comminutes
it for swallowing. Because snails wander constantly
and feed superficially without, as a rule, greatly altering
the form and appearance of the larger plants on which



I 82



Aquatic Organisms



they feed, their work is little noticed; yet they con-
sume vast quantities of green tissue and dead stems.
The commoner pond snails lay their eggs in oblong
gelatinous clumps that are outspread upon the surfaces
of leaves and other solid supports. Other snails are
viviparous.

The two principal groups of fresh-water snails may
roughly be distinguished as (i) operculate snails which
live mainly upon the bottom in larger bodies of water,
and have an operculum closing the aperture of their
shell when they retreat inside, and which breathe by




FIG. 89. Two pond snails (Limnaa palustris) foraging
on a dead stem that is covered with a fine growth of
the alga, Chatophora incrassata.

means of gills: (2) pulmonate snails, which most
abound in vegetation-filled shoals, breathe by means of
a simple lung (and come to the surface betimes, to refill
it with air) and have no operculum.

The snails we oftenest see are members of three
genera of the latter group: Limn&a, shown in the
accompanying figure, having a shell with a right-hand
spiral and a slender point; Physa, having a shorter
spiral, twisted in the opposite way, and Planorbis,
shown in fig. 65 on p. 155, having a shell coiled in a flat
spiral. A ncylus is a related minute limpet-shaped snail,
having a widely open shell that is not coiled in a spiral.
Its flaring edges attach it closely to the smooth surfaces
of plant stems or of stones.



Crustaceans 1 83



ARTHROPODS

We come now to that great assemblage of animals
which bear a chitinous armor on the outside of the
body, and, as the name implies, are possessed of jointed
feet. This group is numerically dominant in the world
today on sea and land. It is roughly divisible into
three main parts; crustaceans, spiders and insects.
The crustaceans are the most primitive and the most
wide-spread in the water- world; so with them we will
begin.

The Crustaceans include a host of minute forms, such
as the water fleas and their allies, collectively known
as Entomostraca, and a number of groups of larger
forms, such as scuds, shrimps, prawns and crabs, col-
lectively known as the higher Crustacea or Malacos-
traca. A few of the latter (crabs, sow-bugs, etc.) live
in part on land, but all the groups are predominately
aquatic, and the Entomostraca are almost wholly so.

The Entomostraca are among the most important
animals in all fresh waters. They are perhaps the chief
means of turning the minute plant life of the waters into
food for the higher animals. They are themselves the
chief food of nearly all young fishes.

There are three groups of Entomostraca, so common
and so important in fresh water, that even in this brief
discussion we must distinguish them. They are:
Branchiopods, Ostracods and Copepods.

The Branchiopods, or gill-footed crustaceans, have
some portion of the thoracic feet expanded and lamelli-
form, and adapted to respiratory use. The feet are
moved with a rapid shuttle-like vibration which draws
the water along and renews the supply of oxygen. The
largest of the entomostraca are members of this group ;
they are very diverse in form.



184 Aquatic Organisms

The fairy shrimp, shown in the accompanying figure,
is one of the largest and showiest of Entomostraca. It
is an inch and a half long and has all of the tints of
the rainbow in its transparent body. It appears in
spring in rainwater pools and is notable for its rapid
growth and sudden disappearance. It runs its rapid
course while the pools are filled with water, and lays
its eggs and dies before the time of their drying up.
The eggs settle to the bottom and remain dormant,
awaiting the return of favorable season. The animal
swims gracefully on its back with two long rows of
broad, thin, fringed, undulating legs uppermost, and
its forked tail streaming out behind, and its rich colors

fairly shimmering in the
light.

Of very different appear-
ance is the related mussel-

FIG " CM "" shrim ? (Estheria) , which has

its body and its long series
of appendages inclosed in a bivalve shell. Swimming
through the water, it looks like a minute clam a centi-
meter long, traveling in some unaccountable fashion;
for its legs are all hidden inside, and nothing but the
translucent brownish shell is visible. This shell is
singularly clam-like in its concentric lines of growth on
the surface and its umbones at the top. This, in
America, is mainly Western and Southern in its distri-
bution, as is also A pus, which has a single dorsal shell
or carapace, widely open below and shaped like a horse-
shoe crab.

These large and aberrant Branchiopods are all very
local in distribution and of sporadic occurrence. As
the seasons fluctuate, so do they. But they are so
unique in form and appearance that when they occur
they will hardly escape the notice of the careful observer
of water life.




Water- Fleas



185



Water-fleas The most common of the Branchiopods
are the water-fleas (order Cladocera) such as are shown
in outline in figure 91. These are smaller, more trans-
parent forms, having the body, but not the head, in-
closed in a bivalve shell. The shell is thin, and finely
reticulate or striated or sculptured, and often armed
with conspicuous spines. The post-abdomen is thin and
flat, armed with stout claws at its tip and fringed with
teeth on its rear margin, and it is moved in and out
between the valves of the shell like a knife blade in its
handle. The pulsating heart, the circulating blood, the
contracting muscles, and the vibrating gill-feet all show
through the shell most
clearly under a microscope.
Hence these forms are very
interesting for laboratory
study, requiring no prepara-
tion other than mounting
on a slide.

Some water-fleas, like
Simocephalus, shown in fig-
ures 91 and 92 swim freely
on their backs, in which
position gravity may aid
them in getting food into their mouths. When the
swimming antennae are developed to great size, as in
Daphne (fig. 91 a), the strokes are slow and progress is
made through the water in a series of jumps. When
the antennae are shorter, as in Chydorus (fig. 916), their
strokes are more rapidly repeated, and progression
steadier.

The Cladocerans are abundant plancton organisms
throughout the summer season. They forage at a little
depth by day, and rise nearer to the surface by night.

The food of water-fleas is mainly the lesser green
algae and diatoms. They are among the most important




FIG. 91. Water-fleas

a, Daphne; b, Chydorus; c, Simocephalus;
d, Bosmina. Note the "proboscis."



1 86



Aquatic Organisms



herbivores of the open water. They are themselves
important food for fishes.

The importance of water fleas in the economy of
water is largely due to their very rapid rate of reproduc-
tion. During the summer season broods of eggs suc-




FIG. 92. A water-flea (Simocephalus vetulus) in its ordinary
swimming position. Note the striated shell, and the ali-
mentary canal, blackish where packed with food-residue in
the abdomen.

cessively appear in the chamber enclosed by the shell
on the back of the animal (see figure 93) at intervals
of only a few days. The young develop rapidly and
are themselves soon producing eggs. In Daphne pulex,
for example, it has been calculated that the possible



Ostracods 187



progeny of a single female might reach the astounding
number of 13,000,000,000 in sixty days.

The Ostracods are minute crustaceans, averaging
perhaps a millimeter in length, having the head, body
and appendages all inclosed in a bivalve shell. The shell
is heavier and less transparent than that of the water
fleas. It is often sculptured, or marked in broad patterns




FIG. 93. One of our largest water-fleas, Eurycerus lamellatus,
twenty times natural size. Note the eggs in the brood chamber
on the back. Note also the short beak and the broad post-
abdomen (shaped somewhat like a butcher's cleaver) by which
this water-flea is readily recognized.

with darker and lighter colors. The inclosed appenda-
ges are few and short, hardly more than their tips show-
ing when in active locomotion. There are never more
than two pairs of thoracic legs. The identification of
ostracods is difficult, since, excepting in the case of
strongly marked forms, a dissection of the animal from
its shell is first required.



1 88



Aquatic Organisms




FIG. 94. An Ostracod (Cypris
virens), lateral and dorsal views,
(after Sharpe.)



Some Ostracods are free-
swimming (species of Cypris,
etc.) and some (Notodromas)
haunt the surface in sum-
mer; but most are creeping
forms that live among
water plants or that burrow
in the bottom ooze. In pools where such food as algae
and decaying plants abound Ostracods frequently
swarm, and appear as a multitude of moving specks
when we look down into the still water.

Relict pools in a dry summer are likely to be found
full of them. Both sexes are constantly present in
most species of Ostracods, but a few species are repre-
sented by females only, and reproduce by means of
unfertilized eggs.

The Copepods are the perennial entomostraca of open
water. Summer and winter they are present. Three
of the commonest genera are shown in figure 95, toge-
ther with a nauplius the larval form in which the
members of this group hatch from the egg. Nothing is
more familiar in laboratory aquaria than the little

white Cyclops (fig. 96, swim-
ming with a jerky motion,
the female carrying two
large sacs of eggs.

A more or less pear-shaped
body tapering to a bifurcate
tail at the rear, a single
median eye and a pair of
large swimming antennae at
the front, and four pairs of

fh r\r Q r^i r^ cwimmincr
UlOraClC SWimming

"U otn ~ + "U ^"U Q -r o rf P>-ri <7 c*

oeneatn, cnaractenze
members of this group.




FIG. 95. Common copepods

e, Cyclops; /, Diaptomus; g, Canthocamp-
tus; h, a nauplius (larva) of Cyclops.
Figures e and / show females bear ing egg

sacs, while the detached antenna at the
6 form of that appendage



Copepods



189



The species of Diaptomus are remarkable for having
usually very long antennae and often a very lively red
color. Sometimes they tinge the water with red, when
present in large numbers.

Copepods feed upon animals plancton and algae,
especially diatoms. They are themselves important
food for fishes, especially for young fishes.

The higher Crustacea,
(Malacostraca) are rep-
resented in our fresh
waters by four distinct
groups, all of which
agree in having the
body composed of
twenty segments that
are variously fused
together on the dorsal
side, each, except the
last, bearing (at least
during development) a
pair of appendages.
Of these segments five
belong to the head,
eight to the thorax and
the remainder to the
abdomen. Mysis (fig.
97) is the sole represen-




FIG. 96. A female Cyclops, with eggs.



tative of the most primitive of these groups, the order
Mysidacea. Its thoracic appendages are all biramous
and undifferentiated; and still serve their primal
swimming function. Mysis lives in the open waters of
our larger lakes, in their cooler depths. It is a delicate
transparent creature half an inch long.

The Scuds (order Amphipoda) are flattened laterally,
and the body is arched. The thoracic legs are adapted



190 Aquatic Organisms

for climbing, and the abdominal appendages for swim-
ming and for jumping. The body is smooth and pale:
often greenish in color. The scuds are quick and active.
They dart about amid green water- weeds, usually
keeping well to shelter, and they swim freely and
rapidly when disturbed. In figure 98 are shown
three species that are common in the eastern United
States.

The scuds are herbivores, and they abound among
green water plants everywhere. They are of much
importance as food for fishes. They are hardy, and
capable of maintaining themselves under stress of




FIG. 97. My sis stenolepis. (After Paulmier).

competition. They carry their young in a
pectoral broodpouch until well developed ; and
altho they are not so prolific as are many
other aquatic herbivores, yet they have possibilities
of very considerable increase, as is shown by the fol-
lowing figures for Gammarus fasciatus, taken from
Embody 's studies of 1912:

Reproductive season at Ithaca, Apr. i8th to Nov. 3d,
includes 199 days.

Average number of eggs laid at a time 22. Egg lay-
ing repeated on an average of 1 1 days.

Age of the youngest egg-laying female 39 days : num-
ber of her eggs, 6.

Possible progeny of a single pair 24221 annually.

Asellus is the commonest representative of the order
Isopoda; broad, dorsally-flattened crustaceans of some-



Decapoda



191



what larger size, that live sprawling in the mud of the
bottom in trashy pools. Their long legs and hairy
bodies are thickly covered with silt. Two pairs of
thoracic legs are adapted for grasping and five pairs for
walking, and the appendages of the middle abdominal
segments are modified to serve for respiration. Asellus
feeds on water- cress and on other soft plants, living and
dead, are found in the bottom ooze. It reproduces
rapidly, and, in spite of cannibal habits when young,




FIG. 98. Three common Amphipods.

A, Gammarus limn&us; B, Gammarus fasciatus; C, Eucrangonyx gracilis.
(Photo by G. E. Embody).

often becomes exceedingly abundant. An adult female
of Asellus communis produces about sixty eggs at a
time and carries them in a broodpouch underneath her
broad thorax during their incubation. There is a new
brood about every five or six weeks during the early
summer season.

Both this order and the preceding have blind
representatives that live in unlighted cave waters, and
pale half -colored species that live in wells.

The crawfishes are the commonest inland representa-
tives of the order Decapoda. These have the thoracic



192 Aquatic Organisms

segments consolidated on the dorsal side to form a hard
carapace, and have but five pairs of walking legs (as
the group name indicates), the foremost of these bear-
ing large nipper-feet. This group contains the largest
Crustacea, including all the edible forms, such as crabs,
lobsters, shrimps, and prawns, most of which are marine.
Southward in the United States there are fresh-water
prawns (Palczmonetes) of some importance as fish food.

The eggs of crawfishes are carried during incubation,
attached to the swimmerets of the abdomen, and the
young are of the form of the adult when hatched. They
cling for a time after hatching to the hairs of the swim-
merets by means of their little nipper-feet, and are
carried about by the mother crawfish.

Crawfishes are mainly carnivorous,
their food being smaller animals,
dead or alive, and decomposing flesh.
In captivity they are readily fed on
scraps of meat. Southward, an omni-
vorous species is a great depredator
in newly planted fields of corn and
cotton. Hankinson ('08) reports
that the crawfishes "form a very Fl 9-99 .

, .,. .-, i . ~ ~ + r icus, (x2, after Sars).

important if not the chief food of

black bass, rock bass, and perch" in Walnut Lake,

Michigan.

Spiders and Mites are nearly all terrestrial. Of the
true spiders there are but a few that frequent the water.
Such an one is shown in the initial cut on page 158.
This spider is conspicuous enough, running on the
surface of the water, or descending beneath, enveloped
in a film of air that shines like silver ; but neither this
nor any other true spider is of so great importance in
the economy of the water as are many other animals
that are far less conspicuous. In habits these do not
differ materially from their terrestrial relatives.




Spiders and Mites 193

Of mites there is one rather small family (Hydrach-
nidae) of aquatic habits. These water-mites are minute,
mostly rotund (sometimes bizarre) forms with unseg-
mented bodies, and four pairs of long, slender, radiating
legs. One large species (about the size of a small pea)
is so abundant in pools and is so brilliant red in color
that it is encountered by every collector. Others, tho




FIG. 100. An overturned female crawfish (Cambarus bartont), showing
the eggs attached to the swimmerets (four thoracic legs broken off).

smaller, are likewise brilliant with hues of orange,
green, yellow, brown and blue, often in striking patterns.
Water-mites, even when too small to be distinguished
easily by their form from ostracods or other minute
Crustacea are easily distinguished by their manner of
locomotion. They swim steadily, in one position;
not in the jerky manner of the entomostraca. The
strokes of their eight hair-fringed swimming feet come



194 Aquatic Organisms



in such rapid succession that the body is moved
smoothly forward. A few water-mites that dwell in the
open water of lakes are transparent, like other
members of open- water planet on.

Water-mites are nearly all parasitic: they puncture
the skin and suck the blood of larger aquatic animals.
Certain of them are common on the gills of mussels:
others on the intersegmental membranes of insects.




FIG. 10 1. Water mites of the genus Limnochares

Nothing is more common than to find clusters of red
mites hanging conspicuously at the sutures of back-
swimmers and other water insects.

Many mites lay their minute eggs on the surface of
the leaves of water plants. Their young on hatching
have but three pairs of legs.



Aquatic Insects 195



INSECTS

This is the group of animals that is numerically
dominant on the earth today. There are more known
species of insects than of all other animal groups put
together. The species that gather at the water-side
give evidence, too, of most extraordinary abundance of
individuals. Who can estimate the number of midges
in the swarms that hover like clouds over a marsh, or
the number of mayflies represented by a windrow of
cast skins fringing the shore line of a great lake? The
world is full of them. Like other land animals they are
especially abundant about the shore line, where condi-
tions of water, warmth, air and light, favor organic
productiveness .

Nine orders of insects (as orders are now generally
recognized) are found commonly in the water. These
are the Plecoptera or stoneflies; the Ephemerida or
mayflies; the Odonata or dragonflies and damselflies;
the Hemiptera or water bugs; the Neuroptera or net-
winged insects; the Trichoptera or caddis-flies; the
Lepidoptera or moths ; the Coleoptera or beetles ; and
the Diptera or true flies. These, together with the
Thysanura or springtails, which hop about upon the
surface of the water in pools, and the Hymenoptera,
of which a few members are minute egg-parasites and
which, when adult, swim with their wings, represent
the entire range of hexapod structure and metamor-
phosis. Yet the six-footed insects as a class are pre-
dominantly terrestrial. It is only a few of the smaller
orders, such as the stoneflies and the mayflies, that
are wholly aquatic. Of the very large orders of moths,
beetles and true flies only a few are aquatic.

Aquatic insects are mainly so in their immature
stages; the adults are terrestrial or aerial. Only a few
adult bugs and beetles are commonly found in the



Aquatic Organisms



water. Other insects are there as nymphs or larvae;
and, owing to the great change of form that is undergone



ll




FIG. 1 02. The green darner dragonfly, Anaxjunius; adult and nymph
skin from which it has just recently emerged. Save for the displaced
wing cases the skin preserves well the form of the immature stage.
Photo by H. H. Knight

at their final transformation, they are very unlike the
adults in appearance. How very unlike the brilliant



Aquatic Insects



197



adult dragonfly, that dashes about in the air on shim-
mering wings, is the sluggish silt-covered nymph, that
sprawls in the mud on the pond bottom! How unlike
the fluttering fragile caddis-fly is the
caddis-worm in its lumbering case!

As with terrestrial insects, so with
those that are aquatic, there are
many degrees of difference between
young and adult, and there are two
main types of metamorphosis, long
familiarly known as complete and
incomplete. With complete meta-
morphosis a quiescent pupal stage is
entered upon at the close of the
active larval life, and the form of
the body is greatly altered during
transformation. Adults and young
are very unlike. Caddis-worms, for
example, the larvae of caddis-flies, are
so unlike caddis-flies in every exter-
nal feature, that no one who has not
studied them would think of their
identity.

The caddis-fly shown in the accom-
panying figure is one that is very
common about marshes, where its
larva dwells in temporary ponds and
pools. Often in early summer, the
bottom will be found thickly strewn
with larvae in their lumbering cases.
Then they suddenly disappear.
They drag their cases into the shelter
of sedge clumps bordering the pools,
and transform to pupae inside them. A fortnight later
they transform to adult caddis-flies, and appear as
shown in figure 103, pretty soft brown insects marked
with straw-yellow in a neat pattern. The larva is
of the form shown in figure 104, a stocky worm-like




FIG. 103. Caddis-fly.
(Limnophilus sp,)



198



Aquatic Organisms



1




L. ,



FIG. 104. Caddis- worms : larvag of Halesus guttifer.

creature, half soft and pale
where constantly protected by
the walls of the case in which
it lives, and half dark colored
and strongly chitinized where
exposed at the ends. There
are stout claws at the rear
for clutching the wall of the
case; there are soft pale fila-
mentous gills arranged along
the side of the abdomen, and
there are three spacing tuber-
cles upon the first segment
of the abdomen for insuring
that a fresh supply of water
shall be admitted to the case
to flow over the gills. The
legs are directed forward, for




FIG. 105. The larval case of
Limnophilus, attached end-
wise to a submerged flag leaf,
in position of transformation.



A Caddis-fly



199




FIG. 106. End view of pupal case of Litnno
philus showing silken barrier; enlarged.



readier egress from
the case ; they reach
forth from the front
end, clutching any
solid support.

The larva of Lim-
nophilus lives in the
case shown in figure
105. Thisisadwel-
ling composed of flat
plant fragments
placed edgewise and
attached to the out-
side of a thin silken
tube.

The larva, living
in this tube, clam-
bers about over the vegetation, jerkily dragging its
cumbrous case along, foraging here and there where
softened plant tissues offer, and when disturbed, quickly
retreating inside. It frequently makes
additions to the front of its case, and
casts off fragments from the rear; so


1 2 3 4 5 6 7 8 9 11 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 11 of 26)