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COMMON FRESHWATER PLANKTONIC ANIMALS
By J.D. Green*
INTRODUCTION
Although the occurrence of zooplankton in Auckland lakes and reservoirs
has been discussed by Bayly (1962), no elementary descriptions of these species
are available and the beginning limnologist often has trouble in identifying the
animals he finds. T o help overcome this difficulty, diagrams o f the most common
genera and species have been included in this paper together with short accounts
of their biology.
A.
CRUSTACEA
1. C O P E P O D A :
Copepods can be distinguished from the other c o m m o n group o f planktonic crustaceans, the Cladocera, by their elongate, clearly segmented bodies,
divided into three regions; cephalothorax, thorax and abdomen (Fig. l c ) . T w o
groups of freeliving copepods are found in the plankton o f lakes, the herbivorous
Calanoida and the carnivorous or herbivorous Cyclopoida.
(a)
Calanoid Copepods: (Figs, l a , b, c & d)
Calanoid copepods have elongated bodies, often of slightly ovoid shape.
The width o f the cephalothorax is similar to that of the thorax, and the
the adult, egg and naupliar stages, are often coloured bright red by stored fat
deposits.
Adult males can be distinguished from females by their geniculate right
antennule (Fig. l b ) which is modified for clasping the female during copulation,
as is the fifth leg (Fig. l b & 3b). Males are also smaller than females (Fig. l a &
b). Both antennules of females are symmetrical, and the animal carries a single
egg sac attached to the genital segment (Fig. l a ) which is often enlarged and
highly coloured. Particularly in Boeckella (Fig. l a ) the postero- lateral edges o f
the last thoracic segment of females are often extended into long projections.
(b)
C y c l o p o i d Copepods: (Fig. le & f).
Unlike calanoids, the body of cyclopoids is pear-shaped, the thorax being
significantly narrower than the cephalothorax which has a broadly rounded
anterior end. ( F i g . If). The antennules are little more than half the body length.
*Department of Zoology, University of Auckland.
FIG. 1.
Copepoda. (a) Boeckella propinqua, female (1.5 —2.0 mm).
(b) B. propinqua, male (1.4 — 1.6 mm), (c) Calamoecia lucasi,
female (0.7 mm), (d) C. lucasi, male (0.65 mm), (e) Mesocyclops
leuckarti, male (0.8 mm). (f)M. leuckarti, female (1.2 mm),
atl = antennule, c = cephalothorax, th = thorax, abd abdomen,
ga = geniculate antennule, gs = gential segment, es = eggsac.
x
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F I G . 2.
Naupliar stages o f Boeckella propinqua (Calanoida) and Mesocyclops
leuckarti (Cyclopoida), showing distinguishing features, (a) Dorsal
view o f body, (b) Anterior view, (c) Lateral view. M
=Mesocyclops,
B = Boeckella, at. = antennule, at = antenna, m d = mandible,
fs = furcal setae.
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Males are very much smaller than females (Fig. l c ) and have two geniculate
antennules. Females carry paired egg sacs. Cyclopoid copepods are generally
more " s p i n y " than calanoids.
Copepod Life Cycles:
The egg hatches into a small larval stage called the nauplius ( F i g . 2) which
has only the first few appendages. By a series of moults five more naupliar
stages are gone through during which more appendages are added. Five
copepodite stages (which are similar in body shape to the adult, but are smaller)
then follow in a similar manner, each moult resulting in an increase in size and
the addition or enlargement o f a pair o f legs until the adult stage is reached.
Naupliar Stages:
Although superficially similar, the nauplii of calanoid and cyclopoid
copepods are fairly easy to tell apart. The following characteristics can be used
to distinguish between the nauplii of Boeckella propinqua and Mesocyclops
leuckarti. They are shown diagrammatically in F i g . 2.
1. Boeckella (Calanoid)
a)
Antennules considerably larger than antenna, with a flattened
terminal segment.
b)
c)
d)
The coxa of the antenna is without large masticatory spines.
The mandible possesses coxal masticatory processes.
The length of the labrum is greater than the width when viewed
ventrally.
e)
When viewed dorsally the body is elongate and flattened anteriorly
(Fig. 2a).
The body is rather laterally compressed (Fig. 2b).
From the lateral aspect the greatest depth occurs midway along the
body and the anterior portion of the head is bluntly flattened
(Fig. 2c).
The furcal setae are asymmetrical.
f)
g)
h)
2. Mesocyclops (Cyclopoid)
a)
b)
c)
d)
e)
The antennule is not larger than the antenna, and is less robust.
The coxa o f the antenna has a large masticatory claw.
The mandible has no coxal masticatory processes.
When viewed ventrally, the labrum is as long as it is wide. It is
larger in relation to body size and much more prominent than in
Boeckella.
When viewed dorsally, the body is more or less pear-shaped with a
broadly rounded anterior end (Fig. 2a).
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f)
g)
h)
The body is not laterally compressed, rather it is somewhat flattened
dorsoventrally (Fig. 2b).
The greatest depth occurs towards the posterior end o f the body
where there is a distinct hump just anterior to the furcae (Fig. 2c).
The furcal setae are symmetrical.
Local Copepods: (Fig. 1)
In the Auckland area, both the New Zealand genera o f calanoid copepods,
Boeckella and Calamoecia, are found. Calamoecia is represented by C. lucasi
(Fig. l c & d) in Lake Pupuke in particular, while o f the species of Boeckella, B.
propinqua (Fig. l a & b) which is found in the Waitakere reservoirs and various
small ponds throughout the area, is perhaps the most common. These two
species differ greatly in size (Fig. l a & l c ) and the females o f Boeckella
propinqua often carry much larger clutches o f eggs than do those o f Calamoecia
lucasi. However correct identification can only be made by dissecting off the
modified male fifth leg which has a distinctive structure in each calanoid species
(Fig. 3).
The most common planktonic cyclopoid is Mesocyclops leuckarti (Fig. l e
& f) although various other littoral cyclopoids are often found in plankton
samples. Cyclopoid copepods are difficult to identify, and those interested
should consult Ward and Whipple or Pennak. The fifth legs o f some c o m m o n
pond species are shown in Barclay (1964).
a
F I G . 3.
b
Fifth legs o f Calanoid Copepods. (a) Female Boeckella
(b) Male B. propinqua. (c) Male Calamoecia lucasi.
e
propinqua.
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2. C L A D O C E R A :
The Cladocera lack any obvious segmentation, are laterally compressed
and covered with a folded carapace (Fig. 4b). The second antennae are large
and used for swimming, while the antennules are reduced. The large postabdomen (Fig. 4b) bears two terminal claws, lateral teeth and two large setae.
It is an important aid in identification. Dorsal to the body and beneath the
carapace is a brood pouch (Fig. 4a) where eggs are deposited and incubated
after laying.
Cladoceran Life Cycles:
For most of the year only females are present in many cladoceran populations, and the eggs develop without fertilization (parthenogenesis). In many
species males are unknown, but when they do occur they are most abundant at
times of low food levels and population crowding. Males are smaller than
females, have larger antennules and a conspicuous hook projecting from the
first legs (Fig. 4c). Sexual reproduction results in the production of resting eggs
able to survive adverse conditions.
Local Planktonic Cladocera:
In the Auckland area only three species o f cladocera are regularly found in
the plankton. These are Bosmina meridionalis (Fig. 4a), Ceriodaphnia dubia
(Fig. 4e) and Daphnia carinata (Fig. 4b). Occasionally cladocerans which
normally lead a littoral existence are found in the plankton, two of the most
common of these being Simocephalus spp. (Fig. 4f) and the ubiquitous
Chydorus sphaericus (Fig. 4d). Descriptions of other littoral cladocerans can be
found in Barclay (1964).
B. R O T A T O R I A ( R O T I F E R S )
Rotifers, being small, are often overlooked, yet they are extremely
common members of the planktonic community. Their most characteristic
feature is the whorl of cilia on the head (Fig. 5b). Vortices in the water set up
by this so called "wheel organ" capture food particles and deposit them in the
region o f the mouth. The jaws (mastax) are noticeable by their constant movement as food is ground up before entering the stomach. Many littoral rotifers
possess a foot which contains adhesive organs for attachment to the substrate
(Fig. 7c), but this is reduced or lost in many planktonic forms (Fig. 5a, 6a).
Rotifers exhibit an enormous range of body form. Planktonic species are
often saccomorph (Figs. 5 & 6) although many (as do littoral types) possess a
hardened shell, or lorica, which often has characteristic markings (Fig. 7a).
F I G . 4.
Cladocera. (a) Bosmina meridionalis, (0.2 — 0.6 mm).
(b)Daphnia
carinata, (1.9 - 2.2 mm), (c) Male Ceriodaphnia dubia, note modified antennules and thoracic spine, (d) Chydorus sphaericus,
(0.35 mm), (e) Ceriodaphnia dubia, female (0.3 - 1.15 mm),
(f) Simocephalus exspinosus, (1.6 mm), bp = brood pouch,
cp = carapace, pa = postabdomen, atl = antennule, sp = spine from
1st thoracic leg, ant = antenna.
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F I G . 5.
Rotifers, (a) Asplanchna brightwellii, (0.8 mm), (b) A. priodonta,
lateral view, (0.6 mm), (c) mastax o f A. priodonta,
(d) mastax o f
A. brightwellii. (e) Conochiloides sp. (0.25 mm), (f) Conochilus
sp. (0.25 mm). (Modified after Hudson & Gosse). c = whorl o f cilia
on head (= corona), mx = mastax, st = stomach, o = ovary,
ant = antenna, e = egg.
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eye
Rotifers, (a) Polyarthra sp., (0.1 mm), (b) Synchaeta pectinata,
(0.25 mm), (c)Hexarthra mira, (0.15 mm), (d) Filinia terminalis,
(0.4 mm), (a + b after Grasse. modified; c modified after Hudson
& Gosse.) bl = lateral blades, ft = foot, Is = lateral spine,
ts = terminal spine.
F I G . 7.
Loricate Rotifers, (a) Keratella valga, (0.12 mm), (b) K. cochlearis,
( 0 . 1 m m ) . (c) Platyais quadricornis, (0.3 mm), (d) Brachionus
calyciflorus, (0.4 mm). (c)B. bidentatus, (0.4 m m ) .
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Long spines are also common features.
Rotifer Life Cycles:
The life cycles o f most rotifers are similar to those o f the cladocera, with
normal parthenogenetic development o f eggs, and the production o f small,
reduced males during unfavourable conditions. Eggs are often carried till they
hatch (Fig. 7d & e) but some species are viviparous (e.g. Asplanchna, Fig. 5) and
the young can be seen inside the adult body cavity.
Identification:
If possible, rotifers should always be studied alive as on preservation most
of the soft bodied species retract the wheel organ and foot, and in this state they
are difficult to recognise. The mastax is an important aid in identification (e.g.
compare Figs. 5, c & d) and can be removed by eroding the tissues o f the animal
away with sodium hypochlorite. Further information on techniques and specific
identification o f rotifers can be found in Ward and Whipple.
Common Local Rotifers:
Two species o f the genus Asplanchna, a large, saccomorph, carnivorous
rotifer, are found around Auckland. These differ both in the shape o f the ovary
(horseshoe shaped in A. brightwellii (Fig. 5a) and spherical in A. priodonta
(Fig. 5b) and in the structure of the mastax (Fig. 5c & d). The colonial
Conochilus sp. (Fig. 5f) is very abundant in the Waitakere reservoirs and can be
distinguished from the similar, but solitary Conochiloides sp. (Fig. 5e) by the
position o f the dorsal antenna. This is located within the corona in Conochilus,
and on the dorsal body surface, outside the cornona, in Conochiloides.
Polyarthra (Fig. 6a), a small rotifer, has a number o f lateral feather-like blades,
while Hexarthra (Fig. 6c) as well as being somewhat larger, has six prominent,
fleshy, lateral arms.
Synchaeta (Fig. 6b) is another clear, soft bodied, carnivorous rotifer. It
has a characteristic triangular shape and is found in more acid environments
than Asplanchna. Filinia (Fig. 6d) is often very abundant, and in preserved
samples it is usually in the retracted state, with the two lateral spines pointed
anteriorly. It is often found sticking to the carapaces of any cladocera which
are present. Perhaps the most common planktonic rotifer is Keratella cochlearis
(Fig. 7b) which has one median posterior spine on the lorica. Keratella valga
(Fig. 7a) which has two spines which are often unequal. A', quadrata also has
two spines, but differs from K. valga in that the width o f the body at the base
of the posterior spines is as great as the width at the base of the anterior spines.
In K. valga the body is narrower at the base o f the posteroir spines.
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Some littoral species of rotifers sometimes found in the plankton are also
shown in Fig. 7. These are Brachionus calyciflorus (Fig. 7d), B. quadridentatus
(Fig. 7e) and the related Platyais quadricornis (Fig. 7c).
ACKNOWLEDGEMENTS
1 wish to thank Dr M . A . Chapman for critically reading the manuscript.
REFERENCES
B A Y L Y , I.A.E.
1962
Ecological studies on New Zealand Lacustrine
Zooplankton, with special reference to Boeckella
propinqua Sars (Copepoda, calanoida). Aust. J.
Mar. Freshwat. Res. 13(2): 143-197.
BARCLAY, M.H.
1964
Some common pond Arthropoda o f the Auckland
district. Tane 10: 40 - 48.
GRASSE,P.P.
1965
"Traite de Zoologie, Tome I V . " Paris.
H U D S O N , C.T.
GOSSE, P.H.
1886
"The Rotifera; or wheel animalicules, both British
and Foreign." 2 vols. Longmans, Green. L o n d o n .
P E N N A K , R.W.
1953
"Freshwater Invertebrates o f the United States."
New Y o r k .
WARD, H.B.
WHIPPLE, G.C.
1959
2nd cd: Edmonson W.T. ed. "Freshwater Biology"
Wiley.