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| Musing over the medusae of transparent Moon jellies (Aurelia sp.), at Monterey Bay Aquarium |
ETHEREAL JELLIES
While I’ve never seen an organism I would call ugly, some
are certainly more pleasing to the eyes than others and among the most
beautiful of all are jellyfish. I
recently visited the Monterey Peninsula in California and had the opportunity to
spend a day in the Monterey Bay Aquarium, as spectacular an array of living
exhibits of sea life as one might ever hope to see.
I was particularly intrigued by the extensive exhibits of
living jellyfish. These diaphanous,
almost otherworldly, creatures, are mesmerizing to watch as they gently drift
and pulsate in their large tanks. Some
are transparent and nearly invisible without the aquarium lighting while others
are opaque or even somewhat brightly colored.
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| Unidentified jelly, Monterey Bay Aquarium. |
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White spotted jelly (Phyllorhiza punctata), Monterey Bay Aquarium.
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| Moon jelly (Aurelia sp.) Monterey Bay Aquarium |
It was exciting to see many
hundreds of jellies while kayaking in the opaque waters of the kelp forest at Monterey. However,
watching them in large exhibit tanks where currents, water filtration, and
lighting are controlled allow you to a more intimate, close up, and more
dramatic view.
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| Blue blubber jellyfish (Catostylus mosaicus), Monterey Bay Aquarium |
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| Pacific sea nettle (Chrysaora fuscescens) Monterey Bay Aquarium |
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| Crown jellyfish, Monterey Bay Aquarium. |
These are among the most
popular attractions, which is a good thing because maintaining a collection of
jellies is a complex and expensive proposition.
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| Pacific sea nettles spark the imagination and wonder of a future marine biologist at Monterey Bay Aquarium. |
Although primitive, jellies were likely
the first group of multicelluar organisms to achieve movement through muscular
activity. First appearing in the
Cambrian, some 500+ million years ago,
jellies are a very successful group and an important component of modern marine
ecosystems. They are universally
carnivorous, feeding on a wide range of organisms, from small invertebrates to
boney fish, with some species specializing in feeding on other jellies. Jellies have exploited an amazing range of
environments, from cold arctic oceans to warm tropical seas and from sunny,
surface waters to the abyss, 25,000+ feet deep with frigid cold, immense
pressure, and eternal darkness. There are even freshwater species.
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| A swarm of golden jellyfish, Mastigias cf. papua etpisoni in the freswater lake of Palau. |
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The crown or helmet jellyfish (Periphylla periphylla) inhabits ocean depths down to 7,000 meters (23,000 feet)!
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The smallest jelly, the Irukandji, is a mere 5 millimetres (0.20 in) wide, with
tentacle close to 1 meter in length. The
lion’s mane (Cyanea capillata),
on the other hand, reaches a body diameter of 2.29 m (7 feet
6 inches) with tentacles 37 m (120 feet) long.
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| The Irukandji (Malo kingi) is both the smallest and deadliest jellyfish. |
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| Cyanea capillata, the lion's mane is the largest known jellyfish and inhabits cold waters. |
Remarkably, jellyfish have
diversified and survived with a very primitive body plan. The umbrella shaped body, or bell, pulsates for locomotion while the long
trailing tentacles bear stinging sells for capturing and killing prey. Jellies
lack a centralized brain, instead having a diffuse network of nerves running
across their epidermis. However, their behavioral responses to tides, currents,
other jellies (friendly and predatory), daily vertical and horizontal
migration, navigation, etc. suggest that this is due to more than just simple
reflexes. There is certainly some
coordination of stimulus and response within the nervous system.
Jellies don’t need gills or a
respiratory system because they absorb oxygen directly through the body wall.
They also lack a circulatory system. The mass of the bell is composed of a
non-living jelly-like substance, the mesoglea, sandwiched between two layers of
skin, each only one or a few cells thick. To top it off, they are 99% water! Most jellies have light sensitive organs
around the margin of their bells that can discern patterns of light and dark
but little detail. Yet in spite of this simplicity and apparent fragility,
jellyfish have weathered the five great mass extinctions in the history of life
on Earth and continue to flourish and survive.
STONE
JELLIES
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| Rhizostomites admirandus, a large, late Jurassic jellyfish fossil from the Solnhofen Limestone. |
Often, while I am looking at
living organisms several questions come to mind. What are these things related
to? How far back in time does this group
go? What is their fossil record like? In
the case of jellies, which lack any hard parts and seem to be composed of
almost nothing, the fossil record question seems particularly intriguing.
Given that jellies are, well
gelatinous, they would seem to have little chance of being fossilized. A quick glance of the scientific literature might
give the impression that many fossil jellies have, in fact, been discovered,
going back into the latter part of the 19th century. However, more recent analyses with a more
critical eye have shown that many of these circular and/or domed fossils have
other origins. A wide range of things
have been identified as fossil jellies, such as trace fossils of marine invertebrates
(like infilled feeding and dwelling burrows of worms and crustaceans), fossil algae, and sedimentary structures of
inorganic origin(such as sediment disruption from ancient releases of gases), concretions,
sand volcanoes, evaporite pseudomorphs, etc.
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| The supposed jellyfish Laotira cambria ultimately turned out to be fossilized, infilled burrows of ancient marine invertebrates. |
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Sometimes the jelly is really the partial remains of other kinds
of animals, such as Peytoia, the Cambrian
jellyfish (below left) that turned out to be the peculiar circular mouthparts of the giant
marine arthropod Anomalocaris (below right), the 2 meter predatory
terror of the Cambrian seas.
Nevertheless, there are still
some fossils that are bonafide jellies.
The record is sparse, no more than nine jelly bearing rock units can be
identified with certainty (
Young and Hagadorn 2010).
Some of these rock units may be the same unit,
but are physically
separated and not
continuous, which means that distinct jelly bearing rock units may well be less
than nine. In many cases the units have yielded less than 30 specimens in
total.
“This is a shockingly small
fossil record for groups with such importance in modern seas (Young and
Hagadorn 2010).
The beautiful, graceful jelly
bell-shape in the ocean often transforms into an amorphous gelatinous
blob once it washes onshore. Tentacles
are delicate and are the first parts of a jelly to disappear as decomposition
proceeds. The mesoglea rapidly
dehydrates when out of water so the bell may become distorted or shriveled on
the beach. So recognizing a fossil jelly
(and being able to differentiate it from other kinds of fossils and sedimentary
structures) can be a difficult undertaking, often requiring the careful study
of multiple specimens and considerably
more detective work than, say, recognizing a bone is from a dinosaur.
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| A modern swarm of jellies stranded on the beach by a retreating tide. |
Jellies occur in enormous numbers
under the right conditions in right environment. These swarms sometimes come in with the tides
and are stranded on the beach when the sea retreats. As they continue to pump
their bells in an attempt to escape, they often fill their stomach and other
internal cavities with sand. Similar
ancient strandings can produce spectacular fossil concentrations. In Wisconsin and New
York on ripple marked intertidal beds of Cambrian age
(more than 510,000,000 years old), have produced tens of thousands of jellyfish fossils.
A fossilized swarm of stranded jellyfish from the Cambrian of Wisconsin. A: rippled beach sand with numerous jellyfish impressions. D, E, F closeup of individual jelly fossils.
Interestingly, there are no mass
strandings of jellies known in the fossil record after the Cambrian
period. This absence is not because
jellies no longer occurred in mass groups, we know they continued to do so right
up to the present. Rather this peculiar
record is the result of other biological factors. During the Cambrian ALL life was in the
oceans. There was no plant or animal life on land. I don’t mean no reptiles or amphibians on
land, I mean NO life on land. Hence
there were no shoreline scavengers to feed on the dead jellies. Furthermore, during the Cambrian organisms
occupied only the surface of the sea floor --- no groups had yet evolved to
burrow and feed within sea floor and beach sediments. So there were no invertebrate sediment
dwellers to consume the jellies after they were buried.
Jelly fossils are also known from
other kinds of ocean environments. Cambrian subtidal deposits of the Marjum
Formation have yielded exceptional jellies that, unlike the Wisconsn and New York localities,
have soft anatomy, such as tentacles, muscles, and reproductive organs, clearly
preserved.
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| Unnamed fossil jellies (left) and living representatives of the groups each fossil belongs to, Marjum Formation, UT. |
The ironstone concretions of the Mazon Creek of Illinois, approximately 300 million years old, have
yielded more than
35,000 specimens of jellyfish!!!.
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| The fossil cubozoan jellyfish Anthracomedusa turnbulli, from Mazon Creek with a life restoration. |
The Late Jurassic Solnhofen
limestone, most famous for producing skeletons of Archaeopteryx, the oldest bird, contains abundant large jellies, up
to 50 cm in diameter. Like the Marjum, these fossils also show remarkable soft
tissue preservation.
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| A partial fossil of the large Solnhofen jelly Rhizostomites admirandus |
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Close up of Rhizostomites admirandus specimen above, showing impressions of circular muscles around the margin of the bell.
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The soft tissue preservation in
the Marjum and Solnhofen are the result of peculiar bottom conditions in each
ocean. The Marjum has anoxic bottom
waters, meaning that the oxygen levels in the water were extremely low. In the
Solnhofen the bottom waters were hypersaline.
In each case, the result was that there were no scavenegers or bottom
dwelling fauna to feed on or decompose the jelly carcasses after they sunk to
the bottom. The Francis Creek
jellies were preserved in the brackish waters of an estuary, where iron mineral
formation around dead organisms preserved many soft body animals.
Surprisingly, no unambiguous
jelly fossils have been found in post-Jurassic rocks. Although some have been
reported closer study has revealed them to be feeding and dwelling trace
fossils. So for the last 150,000,000 years of Earth history there is no fossil
record for these beautiful carnivores! Hopefully
future discoveries will begin to lift the veil on this dark and unknown chapter
of jelly evolution.
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| Pacific sea nettle tank, Monterey Bay Aquarium. |
A
NOTE TO NITPICKERS
I have used jelly and jellyfish
interchangeably here and both in the popular sense. I am aware that true jellies are members of
the class Scyphozoa, that hydromedusan jellies (including the Portugese Man of War
) are Class Hydrozoa, and cubozoan jellies are, well, Class Cubozoa. I’ve
written about “jellies” in the sense that they are all medusae, i.e. umbrella
shaped free swimming cnidarians, because most general followers of Land of the
Dead would think of them as jellyfish.
I haven’t included comb jellies
in the discussion because they belong to an entirely different phylum of
animals (Ctenophora) with only a distant relationship to the Cnidaria. Yes,
most people would think of a comb jelly as a jellyfish, but that was just too much
of a phylogenetic stretch for me.
And yes, I know that jelly fish
are not fish.
SOURCES
Albert, D.J. 2011. What’s on
the mind of a jellyfish? A review of behavioural observations on Aurelia sp. jellyfish. Neuroscience and Biobehavioral Review 35(3):
474-482.
Graham Young: Ancient Shore Blog: Recognizing Ancient Jellyfish
Graham Young: Ancient Shore Blog: Anthracomedusa
Angier, N., June
6, 2011. So Much More Than Plasma and Poison
Cartwright, P., Halgedahl, S.L., Hendricks, J.R.,
Jarrad, R.D., Marques, A.C., Collins, A.G., and Lieberman, B.S. 2007. Exceptionally preserved jellyfishes from the
Middle Cambrian. PloS One 2(10): e1121
Garm, A., Oskarsson, M., and Nilsson, D.E. 2011. Box jellyfish use terrestrial visual cues for
navigation. Current Biology 21(9):
798-803.
Hagadorn, J.W. and Belt, E.S. 2008. Stranded in upstate New York: Cambrian scyphomedusae from the
Potsdam Sandstone. Palaios 23: 424-441.
Hagadon, J.W., Dott, R.H., and Damrow, D. 2002. Stranded on a Late Cambrian shoreline:
medusae from central Wisconsin.
Geology 3092): 147-150.
Selden, P. and Nudds, J. 2004. Evolution of Fossil
Ecosystems. Chicago University
Press: 160 pages. (see chapter on Mazon Creek Beds, pages 59-70)
Young, G.A. and Hagadorn, J.W. 2010.
The fossil record of cnidarian medusae.
Palaeoworld 19: 212-221.
PHOTOS
All photos in Monterey Bay Aquarium by the blogger.
Golden jellyfish: Credit: Michael Dawson, University of
California at Merced
Periphylla periphhylla
Irukandji
lion’s mane jelly
Rhizostomites
admirandus
Laotira cambria
http://ancientshore.com/2010/12/05/recognizing-fossil-jellyfish/
Peytoia
Anomalocaris
http://paleoking.blogspot.com/2009/09/anomalocaris-first-super-predator.html
Modern jellies stranded on beach:
Vultures and Dead Jellyfish: Loose Leaf Noes Blog
Cambrian jelly stranding in Wisconsin:
Hagadorn, J.W. and Belt, E.S. 2008 (see citation in Sources
below).
Marjum Formation fossil jellies from Utah
Anthracomedusa
