|
|
Culture
Techniques of Moina :
The Ideal Daphnia for Feeding Freshwater Fish Fry
By:
R. W. Rottmann
Daphnia are small freshwater crustaceans commonly called "water
fleas". This common name is the result not only of their size, but
their short, jerky hopping movement in water. The genera Daphnia and
Moina are closely related cladocerans. They occur throughout the
world and are collectively known as daphnia.
Daphnia have a body consisting of a head and a trunk ( Figure 1 ). The
antennae are the main means of locomotion. Large compound eyes lie under
the skin on the sides of the head. One of the major characteristics of
daphnia is that the main part of the body, the trunk, is enclosed in an
external skeleton (carapace). Periodically, they molt or shed their
external shell. The brood pouch, where the eggs and embryos develop, is on
the dorsal side of the female. In Daphnia , the brood pouch is
completely closed, while Moina have an open pouch.
 |
| Figure 1 . |
There is considerable size variation between the genera. Moina
are approximately half the maximum length of Daphnia . Adult Moina
(700 to 1,000µm) are longer than newly-hatched brine shrimp (500 µm)
and approximately two to three times the length of adult rotifers. Young Moina
(less than 400 µm), however, are smaller than newly-hatched brine
shrimp and approximately the same size or slightly larger than adult
rotifers. As a result, Moina are ideally suited for feeding
freshwater fish fry. The newly-hatched fry of most freshwater species can
ingest young Moina as their initial food. In addition, brine shrimp
die quickly in freshwater. In Singapore, Moina micrura grown in
ponds fertilized with mostly chicken manure or, less frequently, with pig
manure are used as the sole food for fry of many ornamental tropical fish
species, with 95 to 99% survival to 3/4 inch (20 mm) in length quite
common. Unfortunately, there is very little information concerning
practical mass culture methods of Moina , and the available
information is in mimeograph documents, foreign journals, or other scarce
publications.
Physical and Chemical Requirements
Moina appear in high concentrations in pools, ponds, lakes,
ditches, slow-moving streams, and swamps where organic material is
decomposing. They become especially abundant in temporary water bodies
which provide them with suitable conditions for only a brief period. Moina
are generally quite tolerant of poor water quality. They live in water
where the amount of dissolved oxygen varies from almost zero to
supersaturation. Moina are particularly resistant to changes in the
oxygen concentration and often reproduce in large quantities in water
bodies strongly polluted with sewage. Species of Moina have been
reported to play an important role in the stabilization of sewage in
oxidation lagoons. The ability to survive in oxygen-poor environments is
due to their capacity to synthesize hemoglobin. Hemoglobin formation is
dependent on the level of dissolved oxygen in the water. The production of
hemoglobin may also be caused by high temperature and high population
density.
Moina are resistant to extremes in temperature and easily
withstand a daily variation of from 41 to 88°F (5 - 31°C); their optimum
temperature is 75 to 88°F (24 - 31°C). The high temperature tolerance of
Moina is of great advantage for both the commercial fish farmers in
the southern U.S. and hobbyists culturing live food at home.
Food Requirements
Moina feed on various groups of bacteria, yeast, phytoplankton, and
detritus (decaying organic matter). Bacterial and fungal cells rank high
in food value. Populations of Moina grow most rapidly in the
presence of adequate amounts of bacterial and yeast cells as well as
phytoplankton. Moina are one of the few zooplankton which can
utilize the blue-green algae Microcystis aeruginosa . Both plant
and animal detritus may provide for the growth and reproduction of Moina
. The food value of detritus depends on its origin and diminishes with
the age of the detritus.
Life Cycles of Moina
The reproductive cycle of Moina has both a sexual and asexual
phase. Normally, the population consists of all females that are
reproducing asexually. Under optimum conditions, Moina reproduce at
only 4 to 7 days of age, with a brood size of 4 to 22 per female. Broods
are produced every 1.5 to 2.0 days, with most females producing 2 to 6
broods during their lifetime.
Under adverse environmental conditions, males are produced and sexual
reproduction occurs resulting in resting eggs (ephippia), similar to brine
shrimp eggs. The stimuli for the switch from asexual to sexual
reproduction in populations of Moira is an abrupt reduction in the
food supply, resulting in an increase in resting egg production. However,
it is advantageous to keep the population well fed and in the asexual mode
of reproduction, since fewer progeny are produced with resting eggs.
High population densities of Daphnia can result in a dramatic
decrease in reproduction, but this is not apparently the case with Moina
. The egg output of Daphnia magna drops sharply at a density as
low as 95 to 115 mature individuals per gallon (25 - 30/ L). The maximum
sustained density in cultures of Daphnia reported is 1,900
individuals per gallon (500/ L). Moina cultures, however, routinely
reach densities of 19,000 individuals per gallon (5,000 /L) and are,
therefore, better adapted for intensive culture.
A comparison of the production of Daphnia magna and Moina
macrocopa cultures fertilized with yeast and ammonium nitrate, showed
that the average daily yield of Moina (1.42 to 1.47 ounces / 100
gallons) (106 - 110 g/m 3 ) is three to four times the daily
production of Daphnia (0.33 to 0.53 ounces / 100 gallons) (25 - 40
g/m 3 ). Moina cultures fed phytoplankton cultured on
organic fertilizer have been reported to exceed 5 ounces / 100 gallons
(375 g/m 3 ) daily yield.
Nutritional Value of Moina
The nutritional content of Moina varies considerably depending on
their age and the type of food they are receiving. Although variable, the
protein content of Moina usually averages 50% of the dry weight.
Adults normally have a higher fat content than juveniles. The total amount
of fat per dry weight is 20% to 27% for adult females and 4% to 6% for
juveniles.
Procedure for Moina Culture
The batch culture method of producing Moina uses a continuous
series of cultures. Briefly, a new culture is started daily in a separate
container using the procedures outlined below. When all the fungal,
bacterial, and algal cells are consumed, usually about 5 to 10 days after
inoculation, the Moina are completely harvested, and the culture is
restarted. This method is particularly applicable when a specific quantity
of Moina is needed each day, because daily production is much more
controlled. Batch culture is also useful for maintaining pure cultures
because there is less chance of the cultures becoming contaminated with
competitors (e.g., protozoans, rotifers, copepods) or predators of fish
larvae or fry (e.g., Hydra , back-swimmers, diving beetles,
dragonfly larvae).
Semi-continuous cultures can be maintained for two months or more by
daily partial harvests of Moina , water changes, and regular
feeding, keeping the population in a state of rapid growth. Eventually,
the Moina cultures will fail to respond to additional
fertilization. When it is evident that they are not reproducing well, the Moina
should be completely harvested and a new culture started. Moina can
be produced either in combination with their food or as separate cultures.
Combined culture is the simplest, but production from separate cultures
has been reported to be approximately 1/3 higher. For separate culture,
the phytoplankton tank is positioned so that it can be drained into the Moina
culture tank ( Figure 2 ). Production from separate cultures has the
disadvantage of requiring additional space for the cultivation of
phytoplankton. Consequently, production per unit volume, especially in
large scale cultures, is equivalent to when the animals are grown together
with their food. However, there are advantages of separate culture of Moina
and phytoplankton such as: 1) less chance of contamination; 2) greater
degree of control; and 3) more consistent yield.
 |
| Figure 2 . |
- Regardless of the culture method, always maintain several Moina
cultures to ensure a supply in case of a die off.
Containers
Cultures have been maintained in 10-gallon (38-L) aquaria. However, this
volume is usually too small to yield enough Moina to satisfy
demand. Tanks or vats (concrete, stainless steel, plastic, or fiberglass),
and earthen ponds can be used. Wading pools, plastic sinks, old bathtubs,
discarded refrigerator liners, and cattle watering troughs also work well.
Do not use unpainted metal containers unless they are stainless steel.
Water depth should be no greater than 3 feet (0.9 m). A maximum depth
of 16 to 20 inches (0.4 - 0.5 m) is probably optimum. The shallow water
depth allows good light penetration for photosynthesis by phytoplankton
and provides a large surface to volume ratio for oxygen diffusion.
Diffuse light or shade over 1/3 of the water surface of the Moina culture
container is recommended. A greenhouse covered with shade cloth (50-80%
light reduction) is ideal. Outdoor cultures should be protected from rain
to help stabilize production and screened to prevent entry of predacious
aquatic insects.
Containers to be used, whether aquaria, tanks, vats, or ponds, need not
be particularly clean. However, filamentous algae and predators of fish
larvae or fry (e.g., Hydra , back-swimmers, diving beetles,
dragonfly larvae) can be especially troublesome in Moina cultures.
Tanks can be disinfected with a 30% solution of muriatic acid or by drying
in sunlight. Earthen ponds should be drained and sun dried.
Water
Moina are extremely sensitive to pesticides, metals (e.g., copper
and zinc, which may be prevalent in municipal or well water), detergents,
bleaches, and other toxic materials in the water supply. Insure that
toxins are not inadvertently introduced into the culture container. Well
water should be aerated for at least two hours. Municipal water should be
aerated for at least two days to neutralize the chlorine, or sodium
thiosulfate or a commercially available chlorine neutralizer can be added
to shorten this process. Natural spring water is ideal. Rain water is also
excellent for Moina cultures, if it is collected from an area that
does not have excessive air pollution. Filtered lake or stream water may
also be used.
The optimum water temperature for Moina is 75 to 88°F (24 - 31°C).
Moina continue to thrive at temperatures in excess of 90°F (32°C)
for short periods. However, low temperatures reduce production.
Aeration
Gentle aeration of the Moina pools oxygenates the water, keeps food
particles in suspension, and increases phytoplankton production; this
results in an increase in the number of eggs per female, the proportion of
egg-bearing females in the population, and the population density. A small
trickle of fresh water into the culture container may also improve
production of Moina . Only one or two aquarium air lines are
required in culture containers up to 400 gallons (1.5 m 3 ).
Extremely small bubbles should be avoided; they can get trapped under the
carapace. This causes Moina to float at the surface, eventually
killing them.
Feeding or Fertilizing
Listed below are some common fertilizer materials and application rates.
Try several of these culture medias to determine which works best in your
situation. The initial fertilization rates provided are only a starting
point and will probably need to be adjusted depending on individual
culture conditions.
The following quantity of fertilizer materials should be added
initially for each 100 gallons (379 L) of water. Additional feed or
fertilizer, approximately 50 to 100% of the initial amount, should be
added about 5 days later.
- Yeast: 0.3 - 0.5 ounce (8.5 - 14.2 g) of baker's yeast.
- Yeast and mineral fertilizer: 0.3 - 0.5 ounce (8.5 - 14.2 g) of
yeast, and 0.5 ounce (14.2 g) of ammonium nitrate.
- Alfalfa, bran, and yeast: 1.5 ounces (42.5 g) of alfalfa pellets or
meal, 1.5 ounce (42.5 g) of wheat or rice bran, and 0.3 ounces (8.5 g)
of yeast.
- Cow manure or sewage sludge, bran, and yeast: 5 ounces (142 g) of
dried manure or sewage sludge, 1.5 ounces (42.5 g) of wheat or rice
bran, and 0.3 ounces (8.5 g) of yeast.
- Cow manure or sewage sludge, cotton seed meal, and yeast: Use 5
ounces (142 g) of dried manure or sewage sludge, 1.5 ounce (42.5 g) of
cotton seed meal and 0.3 ounces (8.5 g) of yeast.
- Horse or cow manure or sewage sludge: Combine 20 ounces (567 g) of
dried manure or sewage sludge.
- Chicken or hog manure: Combine 6 ounces (170 g) of dried manure.
Organic fertilizers are usually preferred to mineral fertilizers because
they provide bacterial and fungal cells and detritus as well as
phytoplankton as food for the Moina . This variety of food items
more completely meets their nutritional needs, resulting in maximum
production. Mineral fertilizers may be used alone; however, they work
better in earthen ponds than in tanks or vats.
Fresh manures are preferred because they are richer in organic matter
and bacteria. However, some farm animals are given feed additives that
control fly larvae in their manure; these may inhibit the production of Moina
. Although not absolutely necessary, the manure is frequently dried
before use. Commercially available organic fertilizers such as dehydrated
cow manure and sewage sludge may be used for Moina cultures.
Although manure is widely used to culture Moina , yeast,
alfalfa, and bran are less objectionable to use and work well. Activated
yeast (baker's yeast) is readily available from wholesale food
distributors in 2-pound (0.9-kg) bags. Bran and alfalfa meal or pellets
can be purchased in 50-pound (22.7-kg) bags from livestock feed stores.
Coarse organic materials such as manure, sewage sludge, hay, bran, and
oil seed meals are usually suspended in the water column in mesh bags.
Cheese cloth, burlap, muslin, nylon, or other relatively loose weave
fabrics may be used. Nylon and other synthetic fabrics, however, do not
deteriorate in water as do cotton or burlap. For smaller culture
containers, nylon stockings work well for this purpose: they are
inexpensive, and readily available. The use of a bag prevents large
particles from being a problem when the Moina are harvested and
allows greater control of fertilization.
Overfeeding can quickly cause problems in water quality. Regardless of
the type of media used, start with small amounts of feed or fertilizer
added at frequent intervals; slowly increase the amount used as you gain
experience. If fungus occurs in the culture container due to
over-fertilization, the bag containing the organic material should be
removed from the culture. If fungus persists in large quantities, the
culture should be discarded and restarted.
Excessively high pH (greater than 9.5) due to a heavy algae bloom and
the resulting increase in the proportion of the toxic form of ammonia
(un-ionized), may inhibit the production of Moina . The pH of the
culture can be adjusted to 7 to 8 with vinegar (acetic acid).
Inoculating
Use pure live cultures to inoculate. Avoid using animals for inoculation
from poor or declining cultures, cultures producing resting eggs, or
cultures containing predators of fish larvae or fry. Inoculate with
approximately 100 Moina / gallon (25/ L). Although a culture can
theoretically be started with a single female, always use an adequate
number to develop a harvestable population quickly. If fewer are used, the
population in the culture will increase more slowly, therefore, the
initial quantity of fertilizer or food should be reduced to prevent
overfeeding. A greater number used for inoculation reduces the time to
harvesting and lessens the chance of contamination by competitors.
Cultures are usually inoculated 24 hours or more after fertilization.
However, when yeast is used, Moina can be added to the culture
after a few hours of aeration, assuming good water quality and proper
temperature. This is because the yeast cells are immediately available to
the Moina as food. The small amount of phytoplankton present in the
water and digestive tract of the Moina used to inoculate the
culture is usually sufficient to initiate a phytoplankton bloom. Sometimes
the mortality of the initial inoculation is high and an additional
inoculation is required.
Monitoring
The culture should be inspected daily to determine its health. The
following observations should be made.
- The health of the culture is determined by stirring the culture,
removing 1 tablespoon (15 ml) of the culture, and examining the sample
with a 8X to 10X hand lens or dissecting scope. Green or brown-red Moina
with full intestinal tracts and active movement indicate a healthy
culture. Pale Moina with empty digestive tracts or Moina producing
resting eggs are indications of suboptimum environmental conditions or
insufficient food.
- The population density of Moina is determined by killing the Moina
in the sample with a 70% alcohol solution and counting all Moina
in a petri dish with a hand lens or dissecting scope. Cultures
ready for harvest should contain 45 to 75 Moina in the
1-tablespoon sample (3 to 5/ml). With experience, population density
can be estimated visually without the need for counts.
- The food concentration in the culture water, when examined in a
clear glass, should appear slightly cloudy and tea colored or green.
Clear culture water is an indication of insufficient food. The
cultures should be fed or fertilized with approximately 50-100% of the
initial quantity whenever the transparency is greater than about 12 to
16 inches (0.3 to 0.4 m). This can be determined with a white plastic
or metal lid approximately 4 inches (100 mm) in diameter, attached to
the end of a yard stick. The depth of transparency is where the disk
is just barely visible when lowered into the tank.
- If predators of fish larvae or fry (e.g., Hydra ,
back-swimmers, diving beetles, dragonfly larvae) are observed, discard
the culture and clean and disinfect the tank or pool to avoid
contaminating other cultures.
Harvesting
Moina can be harvested by simply dipping out the required number
with a brine shrimp net or re-usable coffee filter as they concentrate in
"clouds" at the surface. Cultures may also be harvested by
draining or siphoning the culture water into a plankton collector equipped
with 50 to 150-µm mesh netting net suspended in a container of water.
Turn off the aeration and allow the food particles to settle before
harvesting. For semi-continuous culture, do not harvest more than 1/5 to
1/4 of the population each day, unless you are restarting the culture.
Harvesting by draining the culture tank allows for a partial water
exchange, improving water quality. Harvest only small quantities at a time
and transfer the Moina to containers with fresh water to keep them
alive.
The bottom sediments should be stirred up manually every day following
harvest, thoroughly mixing the culture, to prevent anaerobic conditions
and re-suspend food particles.
Additional points
Differences in size, brood production, and optimum environmental
conditions exist between different species and varieties of Moina .
Adjustments will need to be made in the culture technique depending on the
particular species or variety you wish to produce.
Additional surfaces in the culture tank may have a positive effect on
the production of Moina . For Daphnia , a four-fold increase
of surface area, in the form of plastic sheets, has been shown to result
in a four-fold increase in the density, biomass, and harvest. It is
unknown whether this is the result of improved water quality due to
nitrifying bacteria on the substrate, a change in the spatial distribution
of the Daphnia , or improved nutrition.
It may not always be possible to match Moina production to the
food demand of the fish fry. Harvested Moina can be kept alive for
several days in clean water in a refrigerator. They will resume normal
activity when they are again warmed. The nutritional quality of the stored
Moina probably will not be optimal because of the period of
starvation, so the Moina should be enriched with algae and yeast
before feeding them to fish. Moina can be stored for long periods
by freezing in low salinity water (7 ppt, 1.0046 density) or by
freeze-drying. Both methods kill the Moina , so adequate
circulation is required to keep them in suspension after thawing so they
will be available to the fish fry. Frozen and freeze-dried Moina are
not as nutritious as live animals, and they are not as readily accepted by
fish fry. Although freezing or freeze-drying does not significantly alter
the nutritional content of Moina , nutrients do leach out rapidly
into the water. Nearly all of the enzyme activity is lost within ten
minutes after introduction in fresh water. After one hour, all of the free
amino acids and many of the bound amino acids are lost.
R. W. Rottmann, Senior Biological
Scientist, Department of Fisheries and Aquatic Sciences, Cooperative
Extension Service, Institute of Food and Agricultural Sciences, University
of Florida, Gainesville, 32611.
Copyright:
This document is copyrighted by the University of Florida, Institute
of Food and Agricultural Sciences (UF/IFAS). Published: May, 1992. |
