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BIOLOGICAL ,ralorcrtory
Perplexing Problem
The summer sea nettle, Chrysaora quinquecirrha DeSor
(formerly .Daetylatnetra quinquecit-rha) has been a problem since Colonial days. Each year this pest appears in
Chesapeake Bay with regularity and certainty by late June,
.reaching a peak of abundance during July and August.
It may -.disappear in late August, or September in Virginia
waters, but in the upper bay it may vanish anytime from
September to November. When it is abundant, swimming
declines in hay waters. No other animal in Chesapeake Bay
is so detrimental to recreational activities. It also causes con' siderable economic loss to beach operators and to fishermen.
Chrysaora is not unique to Chesapeake Bay. This type
a nettle ranges from New England to the tropics, and
ated species occur in oceanic waters in most temperate
t tropical
seas. •". . However, it is „.. rnore abundant in the
•
ay arid iii the lower parts of its tributary streams and creeks
than in . xxiy other body of:water,
Jellyfish vary in abundance in swimming areas from day
to day, largely because of wave action and water currents.
Strong wave action causes the sea nettle to swim slowly
by feeble rhythmic contractions into deeper water. However,
as soon as the turbulence ceases, it again moves slowly to
the surface and is at the mercy of surface currents that may
transport them to the beaches again.
The sea nettle is well adapted to live and reproduce in
salinities from 7 0/00 (parts per thousand) to 35 0/00, but
it dies when the salinity is lower than 5 0/00.
Maryland and Virginia, assisted by the Federal Government, are studying the biology of the summer sea nettle
and possible methods of controlling its abundance. These
investigations are underway at the Chesapeake Biological
Laboratory, Solomons, Maryland, and at the Virginia Institute of Marine Science, Gloucester Point, Virginia.
Other Jellyfishes Offer No Trouble
Three other prominent jellyfishes are seen in the Bay:
one, the nearly flat, white moon jellyfish, Aurelia aurita
(Linnaeus), which may grow to more than a foot in diameter, is abundant in the lower bay in the summer, but
less so in the upper bay. It is not a problem because its
sting is scarcely felt by swimmers. Another white species,
Rho pilema verrilli (Fewkes) is too rare in the bay to be
of any concern. The third sea nettle, the red winter jellyfish, with short tentacles, Cyanea capillata (Linnaeus),
makes its appearance in December and disappears by the
end of May. Because of its winter occurrence and very weak
sting, it is unimportant in relation to the activities of man.
No Economic Value Here
Attempts have been made to make some economic use of
jellyfishes. For example, in Japan, Hong Kong, and Taiwan,
Rho pilema, when dried, is used for food. Sometimes this
or related kinds of medusae may be bought in local Chinese
markets. However, when Chrysaora medusae become dried
on fish nets the stinging cells can remain for at least 10
years. Then, when the nets are disturbed, the stinging cells
float in the air and fishermen that breathe them sneeze violently, as any old timer will testify. There is no record of
any economically profitable use of the sea nettle.
BIOLOGY
A Simple Animal, Related to the Polyps
The summer sea nettle, which may reach 9 inches in
diameter, is the medusa stage of an organism that belongs
to the Phylum Coelenterata in the Animal Kingdom. It is
a relatively simple animal related to corals, sea anemones,
and hydras, all of which have two layers of differentiated
cells (higher forms have 3 layers); the inner layer surrounds
the digestive cavity and the outer layer contains the stinging
NTER
JELLYFISH
cells. These medusae, bell- or umbrella-shaped, are composed of a translucent gelatinous mass with beautiful radial
symmetry. The sea nettle medusa lacks a brain. It does
have 8 nerve centers in the margin of the bell that serve
to coordinate the pumping contractions of the bell and the
feeding activities. Underneath the edges of the bell are
numerous, long (over 4 feet in large medusae), threadlike,
contractible, fishing tentacles as shown on the cover. They
have thousands of minute stinging cells, the nematocysts, in
their outer tissue layer. There are four long, lacy tissues,
the oral arms, suspended from the center of the bell. In
the bell cavity are tiny finger-like digestive glands.
The summer jellyfishes are milky white in the upper and
middle parts of the bay. However, in the lower bay many
of them are beautifully decorated with 16 short red bars
on the convex side of the umbrella. A chemical analysis
of the medusa reveals that they consist of about 3% protein
and salts and 97% water.
Reproduction
—
bryos, or planulae, become covered with cilia and then begin
to swim. They escape from the female into the water, where
they are dependent on currents for movement from place
to place. During this pelagic larval stage, the planulae are
white in color and smaller than a grain of salt. After a few
days they settle and attach to hard objects on the bottom.
The attached planula looks like a tiny white speck, the
center of which soon bulges outward and then develops
into a flower-like polyp shaped like an ice cream cone.
This is the sessile, or attached, stage in the life cycle of
jellyfishes. They are sometimes abundant on the underside
of oyster shells, on bottles, cans and anything else available
in shallow water in the numerous, small, protected creeks
and in the bay. They should be looked for especially
on oyster shells.
The growing polyp (1/25 inch in diameter and 1/8 inch
tall) first forms 8 small tentacles around its outer free margin. There is a small cup-shaped mouth at the center, thus
the name "scyphistoma" (cup and mouth) for this stage
of sea nettle. As the polyp grows, 16 tentacles are formed.
They are greatly extensible and retractile, and bear numerous stinging cells. These nematocysts are discharged when
touched by an organism. For example, tiny protozoans are
paralyzed and held by the tentacles, then quickly moved
toward and deposited in the mouth of the polyp. Polyps will
eat any organism they can sting and hold with the tentacles.
Tentacles
Nerve
Centers
Sexual and Asexual
Medusae of Chrysaora have separate sexes. The ovaries
and testes are shaped like a 4-leaf clover within the bell.
The olive- to dusky-colored ovaries of the mature female
are distinguishable from the males, which have whitish or
pinkish testes. The entire bell of the male appears more
whitish in the water than that of females when viewed
from above.
The gonads are sexually mature from July onward to
the end of the season. Sperm apparently are set free in
the water from the testes. They fertilize the eggs within the
ovaries, where development begins. Soon the developing em-
MOON JELLYFIS
Asexual reproduction in Chrysaora is highly developed.
One common method is by the formation of stolons, These
are small tubular projections from the side of a polyp that
extend outward then downward to touch the hard substrate,
at which point a new polyp, or cyst, grows.
The cyst is shaped like a cupcake and is always attached
to the hard substrate. It is reddish brown and measures
1/2 mm thick and about 2 mm (1/16 inch) in diameter.
Another method of reproduction, strobilation, occurs
mostly in the spring and early summer. The distal or outer
2/3 of the polyp forms into disk-like buds, which measure
1 to 3 mm in diameter. Soon the outer one begins to pulsate
and then breaks away to swim freely in the water. It is
called an ephyra. The latter has 8 flap-like extensions,
called lappets, around the margin. When this ephyra
reaches 1/4 inch in diameter (6 or 7 mm), tentacles begin
to form and soon it looks like the adult medusa or jellyfish,
which is the sexual stage.
The summer sea nettle has remarkable ability to reproduce itself. Each polyp may produce about 5 ephyral discs
(some form over 10) at a time, and this may occur
as many as 4 times in a season. Thus, if a polyp produced
a total of 50 cysts and polyps in one season, and then in
the spring and summer if each of the 50 polyps formed
20 discs, it is possible that 1,000 medusae could result
from a polyp within a year. When we consider that the
bay has literally millions of shells, many of them bearing
numerous polyps, it is easy to understand why we have
a problem with jellyfishes.
The abundance of polyps and cysts varies greatly under
natural conditions. One shell had 1200 polyps on it; however, usually there are fewer than 25 polyps and cysts. Many
shells have none. A polyp that sets in July may reproduce
itself more than 50 times by the start of the winter. Cysts
are made at the base, or foot, of the polyp, and the polyp
then moves a little distance away and forms a new cyst. If
living conditions for the polyp are unfavorable, the polyp
can withdraw completely into a cyst and remain protected
until conditions improve. Chrysaora polyps thus may pass
the winter in an encysted condition. Polyps can survive for
several years, and perhaps even longer. When the water
temperature rises to about 17°C (63°F) in the spring, a
new polyp grows out of the center of the cupcake-shaped
cyst and within a few days strobilation starts.
The male and female medusae reproduce sexually, forming sperm and eggs and then larvae, whereas the polyp
reproduces asexually by forming cysts, stolons, and ephyrae.
These two alternating methods of reproduction, asexual and
sexual, illustrate an interesting well-known phenomenon
in nature, the "alternation of generations".
Fast Growth, Wide Variety of Food
Growth from the ephyra stage to a mature jellyfish is
very rapid. Diameter increases as much as 1 or 2 inches
per week in the early part of the season. Under favorable
conditions, from the jellyfish point of view, a size of 6 to 8
inches across is reached in 5 weeks. By late summer most
of the medusae are less than 6 inches across, indicating
slower growth. Possibly this results from a reduced food
supply through overpopulation of jellyfish, or the medusa
may become mature at smaller sizes later in the summer.
Sea nettles do not seek or voluntarily swim to their food
but drift with water movements. They extend their long
thread-like tentacles in many directions. When touched,
they discharge a highly toxic venom which almost instantly
paralyzes small fishes and other aquatic organisms that adhere to the tentacles. The latter contract and draw the food
organism into the oral arms. Gradually, the food item is
moved by ciliary action into the mouth, thence into the
gastric cavity of the bell, where powerful juices, secreted
by the digestive glands, digest the food in 8 to 12 hours
at room temperature. A small fish will be completely absorbed, except for the undigestible, veil-like remains that
are later ejected by the medusa. The commonst food of this
sea nettle consists of small fishes, worms, oyster and other
shellfish larvae, sea walnuts, and many kinds of small marine
invertebrates and protozoans.
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The Stinging Cells Are Poisonous
When vacationers touch a sea nettle, the stinging cells
or nematocysts are stimulated to discharge microscopic
"darts," which can penetrate the thinner skin of legs, arms,
or exposed parts of the body. Each stinging nematocyst is
a lemon-shaped cell with a trigger-like hair. This large cell,
situated in the surface layer of the tentacles, oral arms and
bell, contains a coiled thread. The thread, coated with the
cenom, is quickly everted when the "trigger" is touched.
The venom discharged is a complex protein substance
and not "formic acid," as sometimes reported. It irritates
the affected skin, producing a stinging sensation almost instantly. A few people have an allergic reaction that may
hospitalize them. Most people stung show only inflamed
skin that may last for a few hours. Various substances have
been used by bathers in an attempt to reduce the burning
sensation, for example, baking soda, salt, ammonia water,
and sand rubbed on the stung area. Each of these home
remedies helps but none is fully satisfactory. Ointments,
such as vaseline, if spread thickly on the skin, are effective
shields, but such substances are so messy that few people
use them. Recently it was reported that the proteolytic en-
zyme, papain, one of the active ingredients of most commercial meat tenderizers, gave almost instant relief for most
people when applied immediately. It is more effective if
applied quickly to the skin after an attack. The stung area
should be rnoisetened, then richly sprinkled with the tenderizer, and briskly rubbed. A second application is often
needed to stop the burning sensation, but some itching may
still remain in more severe attacks.
Since the stinging thread can penetrate only the thinner
layers of the skin, one can usually grasp the bell side of a
jellyfish with the hands without danger. The skin of the
palm side of the hand is usually too thick to be penetrated
by the "dart" of the nematocyst.
The Sea Nettle Population Keeps Changing
Records of relative abundance of the summer sea nettle
have been kept by the staff of the Chesapeake Biological
Laboratory since 1960. These data show considerable fluctuation. This sea nettle was scarce in 1960 and 1961, but
has been abundant since then. Medusae usually reach a
peak of abundance in July and August and then decrease
gradually and disappear by November. However, in 1963,
1966 and 1969, they disappeared suddenly by early September. Marine biologists are trying to discover the causes of
such changes. They have speculated that heavy rains, considerable sedimentation, some unknown jellyfish disease, or
perhaps a predator might cause these changes. Not enough
knowledge is as yet available to determine the correct
answers. Perhaps each affects this species.
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1962
1963
1.1 64
1966
1965
1000
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1967
1968
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1970
1500
500
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ANNUAL SEASONS MONTHS MAY TO DECEMBER
Enemies of the Jellyfish
An important predator that might influence relative
abundance is the shell-less nudibranch, or sea slug, a marine
mollusk about 1/4 to 3/4 of an inch long. It prefers to eat
jellyfish polyps but also feeds on hydroids. Nudibranchs
actively seek the polyps and are able to detect them at a
distance of several inches. One large individual of Cratena
pilata (Gould) kept in the laboratory ate 500 polyps in a
3-day period. Biologists believe that if the nudibranchs
became abundant, jellyfish might be reduced in numbers.
The life history of the nudibranch is not yet well understood. They appear to be abundant in June and again in
October and November. Nudibranchs are hermaphroditic
(each individual is both male and female), but selffertilization is not possible. They mate and then both lay
eggs in a jelly-like coiled string that is draped loosely on
plants, hydroids, or shells, from late May through November. They hatch in less than a week. The larvae, or veligers,
each bearing a tiny snail-like shell, are set free. For about
two weeks they swim freely in the water and then lose their
shell. Soon they begin to crawl about on the bottom. They
grow rapidly and become mature adults within two months.
Probably they pass the winter hidden in the bottom and
emerge when the water reaches about 11°C. (52°F). The
life span of Cratena may be less than one year.
There are other predators that feed on jellyfishes, but
none of them is effective in reducing sea nettle abundance.
One, the juvenile harvestfish, Peprilus ale pidotus, swims in
and out of the jellyfish tentacles with impunity. In some
years about 1 in 100 medusae will be accompanied by one
or more harvestfish. These fish feed on the tentacles and
bell of jellyfishes. Larger harvestfish do not stay under the
bell of a sea nettle, but travel in schools. They can destroy
several medusae in a few minutes. The juvenile orange
filefish, Alutera schoepfi, also feeds on these medusae.
At least 4 small fishes that clear off part of a shell for
their nests compete with Chrysaora for living space and
destroy some polyps in the process. These are the clingfish,
Gobiesox strumosus Cope; the striped blenny, Chasmodes
bosquiannus Lacepede; the feather blenny, Hypsoblennius
hentzi Le Sueur; and the naked goby, Gobiosoma bosci
(Lacepede). None of them are abundant enough to
measurably reduce the population of polyps.
CONTROL EFFORT
Man-Made Barriers
Chemical and Biological Controls Under Study
Numerous protective devices have been built to keep
summer sea nettles out of swimming areas along the shores
of the bay. However, most of them have been unsuccessful
because they were not properly designed nor given adequate
daily attention after installation. Bubble screens and water
jets have been used experimentally to divert sea nettle away
from bathing areas.
So far, there is no known method of controlling the
abundance of the summer sea nettle. The area of the bay
is so large, and the volume of water so great, that it would
be very difficult and expensive to change the abundance of
sea nettles with chemicals or by scooping them up in large
nets. Chemicals so far tested that kill the sessile polyp stage
also kill oysters, crabs, and other marine organisms.
It has been experimentally determined that sediment
covering polyps to a depth of 1/8 inch stops asexual reproduction within a period of 3 or 4 weeks and that longer
exposure causes a very high mortality. However, the application of silt to control the abundance of sea nettles may
not be practical because the layer of sediment also kills
small oysters and other sessile organisms.
There is, however, a possibility that biological control
might give some relief from the nuisance. This approach
would attempt to attack some vulnerable stage in the life
cycle of Chrysaora. The polyp, although perhaps not the
weakest. stage, is one which is sessile or fixed in position on
the bottom and may be attacked. The only predator so far
discovered in the bay that voraciously eats the polyp stage,
is Cratena, the local sea slug. It is being studied to determine its value for possible control of the abundance of sea
nettles. Attempts are underway to culture it in the laboratory for this purpose. Continued research on the biology,
on chemical and biological controls, on hormones that
might retard or stop reproduction, on sea nettle barriers, and on substances that might neutralize the effect
of the venom may help reduce the discomfort and considerable economic loss resulting from Chesapeake Bay's
most troublesome pest.
Successful sea nettle barriers along the open bay shore
have been constructed of galvanized wire hardware cloth,
fastened to wooden frames. The swimming area protected
is encircled on 3 off-shore sides by posts set at 10-foot intervals, to which wooden frames are attached. The frames
are removed each autumn, repaired, and then again placed
on the posts in the spring.
Fish netting hung on ropes or cables has been used with
partial success in coves and other areas that are protected
from strong waves and strong currents. The bottom line
of the net is held down by lead weights or a heavy chain.
These devices, however, become clogged with jellyfishes
and other floating debris, which often pulls the net off the
bottom or submerges the "corkline." Drifting timbers or
trees tear holes in these nets, letting jellyfish into the swimming area. Strong wave action breaks up the medusa and
its tentacles into small parts, which pass through the netting.
The detached tentacles can still sting swimmers, as much
as when attached to the bell. .A survey of these installations
made it clear that whatever type of barrier is built, it
requires daily servicing to be effective. The best type of barrier, still in the experimental stage, may be a combination
of water jets and netting.