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life history and effects on the great lakes
of the spiny tailed bythotrephes
SPINY TAILED BYTHOTREPHES: ITS LIFE
HISTORY AND EFFECTS ON THE GREAT LAKES
During the 1980s the exotic zooplankton Bythotrephes cederstroemi
entered the waters of the Great Lakes. Scientific study suggests
that Bythotrephes has become a permanent member of the Great
Lakes ecosystem, and has carved out a niche for itself at some cost
to lake fisheries. The quick success of the animal in colonizing
all of the Great Lakes raises the possibility that it may soon invade
smaller, inland lakes in the Great Lakes basin.
EXOTIC INTRODUCTIONS
The Great Lakes have experienced invasions by dozens of exotic
species during the last century. Some of the introduced species
were the result of human planning and intensive management, as was
the case with the coho and chinook salmon. Other additions were
unintentional. The alewife and the sea lamprey, which entered the
upper Great Lakes through the Welland Canal, are two examples of
accidental additions.
Although most planned introductions have been fish species, accidental
introductions have involved vertebrates and invertebrates alike.
The ruffe and zebra mussel are two examples of exotic species that
have recently captured public attention. Both invaded the Great
Lakes from their native habitats in Europe. The mussel has become
a nuisance by clogging the intakes of water pipes and outboard boat
engines. The ruffe possesses protective, bony fins which discourage
bigger fish from eating it. Thus it may survive better than native
species of small fish, to the eventual detriment of the game fish
that rely on the small fish for food.
Invaders like the zebra mussel and the ruffe receive much attention
because they can be common in shallow water near shore and also
because they are large enough to be easily seen. Less well-known,
but smaller, invaders are no less important within the complex ecological
communities of the Great Lakes. For example, during the 1980s the
zooplankton Bythotrephes cederstroemi, (sometimes called
the "spiny waterflea," although it is not an insect) entered the
waters of the Great Lakes from a European source. Although its average
length is scarcely more than one centimeter (0.4 inch), Bythotrephes
can have as profound an effect on an ecosystem as a larger invader.
UNIQUE BODY STRUCTURE
Bythotrephes (bith-o-TREH-feez) is easily recognized by
its unique body shape. The tail spine is its distinguishing feature
and separates it from all other free-swimming lake invertebrate
animals, or zooplankton. The spine is proportionately long, often
comprising over 70 percent of the animal's total length. The spine
contains from one to four pairs of thorn-like barbs. Juveniles are
born with just one pair, so these barbs can be used to determine
the age of the animal.
The head consists primarily of a single, large eye filled with
black pigment. Also present are a pair of mandibles, or jaws. Bythotrephes
uses these sickle-shaped mandibles to pierce and shred its prey.
The animals possess four pairs of legs, the first pair being much
longer than the others. These first legs are used for catching prey,
whereas the other pairs of limbs are designed for grasping prey
while they are being consumed. Just behind the head is a pair of
swimming antennae, which propel the animals through the water. Bythotrephes
are good swimmers, moving several times their body length in
a second. The ability to swim, as opposed to merely drifting with
the current, helps Bythotrephes to encounter prey and to
move between shallow and deeper lake waters.
Bythotrephes belongs to the class Crustacea, a group of
animals such as crabs and shrimp that possess a hard exoskeleton
(outer shell). Like all other Crustacea, its exoskeleton is molted
in order to grow. Bythotrephes is unique for it sheds only
the exoskeleton that covers its body, retaining the exoskeleton
that covers the tail spine. The animal is never without its long,
stout spine, a fact which suggests to biologists that the tail serves
a vital protective function.
female Bythotrephes
Figure 1
Figure 1. Alternating asexual and sexual life cycles of Bythotrephes.
The top loop illustrates the progression of embryos as they develop
asexually. Females hatched from resting eggs can develop as many
as four pairs of barbs. The presence of the male in the bottom loop
indicates the sexual production of resting eggs.
UNUSUAL REPRODUCTIVE CYCLE VARIES WITH ENVIRONMENT
Bythotrephes is able to have a remarkable influence on
the biological communities of the Great Lakes, largely because of
its rapid reproduction rate (Figure 1). Reproductive females
carry their offspring on their backs in a balloon-like brood pouch,
which can be filled with either developing embryos or resting eggs.
Most of the time, female Bythotrephes exhibit a rapid and
unusual method of reproduction known as parthenogenesis, or asexual
reproduction. By this method, females produce from one to ten eggs
that are able to develop into new females without mating or fertilization.
The new females are genetic replicas, or clones, of the mother.
The generation time of this parthenogenic life cycle (embryo to
adult female) varies with water temperature because as with all
Crustacea, rates of metabolism rise and fall with temperature. During
the summer, when the surface water of the lake is warm, Bythotrephes
can produce a new generation without fertilization (parthenogenesis)
in less than two weeks. Since males are not needed for parthenogenesis,
they are rarely found when food is plentiful, or when environmental
conditions favor rapid population growth.
In Bythotrephes, sex of offspring is not determined genetically,
but rather by environmental factors. So, when food becomes limited
or when the lake cools in the fall, males begin to appear. Declining
environmental quality can be sensed by adult females, who respond
by producing male rather than female offspring. These males are
able to mate with surviving females, producing resting eggs. The
resting eggs are first carried as orange-brown spheres in the female
brood pouch. They are later released and fall to the lake bottom
where they can survive the cold winter. In spring or early summer,
these eggs hatch into juvenile females that begin parthenogenic
reproduction again.
Resting eggs can remain dormant for long periods of time, and
they offer an explanation for the arrival of Bythotrephes
in North America. The animal is native to northern Europe, including
the British Isles, Scandinavia, and the Soviet Union. The first
living specimen in the New World was found in Lake Huron in December
1984. The most likely mode of transport was in fresh water or mud
brought to the Great Lakes from Europe in the ballast water of merchant
ships. By 1985, Bythotrephes had spread to Lakes Erie and
Ontario. It had invaded Lake Michigan by 1986 and Lake Superior
by 1987.
DISRUPTING THE BALANCE
Biologists have documented the arrival of Bythotrephes into
Lake Michigan, and they have studied the subsequent effects of this
new predator on other species in the lake. As a consequence of these
scientific investigations, much has been learned about the complex
biological community of Lake Michigan, and about the interactions
and special dependencies of various predators and prey, particularly
fish and plankton.
Bythotrephes eat smaller herbivorous Crustacea, including
the common zooplankton, Daphnia. Daphnia, however, are also
an important food item for small, juvenile fish such as the bloaterchub.
Bythotrephes thus competes directly with young fish for food.
Because of their unique reproductive pattern, Bythotrephes can
reproduce many times faster than the fish. Their rapid population
growth enables Bythotrephes to monopolize the food supply
at times, to the eventual detriment of the fish.
Although Bythotrephes can fall prey to fish, its spine
seems to frustrate most small fish, which tend to experience great
difficulty swallowing the animal. In Lake Michigan, Bythotrephes
can rarely be found in stomachs of fish less than 5 centimeters
(2 inches) in length, although fish of that size avidly consume
Daphnia when that food item is available. There are indications
that the growth rates and survival of these young fish may be adversely
affected by the presence of Bythotrephes in the ecosystem,
owing to competition for food. In general, the more abundant Bythotrephes
becomes, the less food will remain available for juvenile fish.
BEHAVIOR PATTERNS REDUCE PREDATION
In European lakes, populations of Bythotrephes are often
suppressed as the result of predation by larger fish. The large
black eye and full brood pouch makes adult females quite visible
to fish, and fish prefer them over smaller species of zooplankton.
Faced with possible predation by fish, Bythotrephes has been
found to adopt a behavior called diel (daily) vertical migration.
Adult females move deeper in the water during daylight hours, where
less light penetrates and visibility to fish is reduced. At night,
they rise closer to the surface, where there is abundant food and
the warmer water helps to quicken metabolism and growth.
male Bythotrephes
Figure 2 represents graphically how in the offshore waters of
Lake Michigan, Bythotrephes is found mainly from 10 to 20
meters (33 to 66 feet) deep during the day, while at night the majority
of the population occupies the water from 0 to 10 meters (0 to 33
feet) deep.
Figure 2. Percentage of the Bythotrephes population occupying
each depth interval in Lake Michigan. The majority of the population
is found below 10 meters during the day, but at night over 50 percent
of the population is in the surface water. The chart above shows
the rapid change in water temperature between 10 meters and 20 meters.
ARE THEY HERE TO STAY?
Scientific study suggests that Bythotrephes has become
a permanent member of the Great Lakes ecosystem, and has carved
out a niche for itself at some cost to lake fisheries. The quick
success of the animal in colonizing all of the Great Lakes raises
the possibility that it may soon invade smaller, inland lakes in
the Great Lakes basin. Studies are continuing to identify effects
of this and other introduced species on the food webs and the fish
communities of the lakes. The public can help these investigations
by reporting any occurrences of Bythotrephes, or other alien
species in inland lakes, to the Michigan Department of Natural Resources,
the authors, or to Michigan Sea Grant.
Authors
Carla E. Caceres
John T. Lehman
Department of Biology
Natural Science Building
The University of Michigan
Ann Arbor, Ml 48109-1048
Illustrations by Todd Buck
Printed by the Michigan Sea Grant College Program, a cooperative
program of The University of Michigan and Michigan State University,
with grant number NA89AA-D-SGO83 from the National Sea Grant College
Program National Oceanic and Atmospheric Administration(NOAA), U.S.
Department of Commerce, and funds from the State of Michigan.
Copies are available from any Michigan Sea Grant staff member.
Michigan Sea Grant Communications should be contacted for bulk quantities.
Michigan Sea Grant Communications
The University of Michigan
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Ann Arbor, MI 48109-2099
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