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Sea Gooseberry - Pleurobrachia bachei
Topic Started: May 22 2014, 03:20 PM (3,383 Views)
Taipan
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Sea Gooseberry - Pleurobrachia bachei

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Scientific classification
Kingdom: Animalia
Phylum: Ctenophora
Class: Tentaculata
Order: Cydippida
Family: Pleurobrachiidae
Genus: Pleurobrachia
Species: Pleurobrachia bachei

Pleurobrachia bachei is a member of the phylum Ctenophora and is commonly referred to as a sea gooseberry. These comb jellies are often mistaken for medusoid Cnidaria, but are not dangerous to handle.

History
Traditionally, Ctenophora has been thought to represent an ancient metazoan phylum. Recent genetic data suggests that all extant Ctenophora taxa may have evolved from a relatively recent common ancestor and that this ancestral ctenophore was tentaculate and cydippid-like. Because of the virtual absence of ctenophores in the fossil record, their evolutionary history holds many more questions than answers.

Morphology
An individual sea gooseberry's body length can reach up to 20 mm (0.79 in) with each of the two tentacles stretching 150 mm (5.9 in). Their gelatinous globular bodies are composed of 99% water. They have eight rows of well-developed comb plates consisting of thousands of fused macrocilia controlled by an apical organ. Unlike most other ctenophores, Pleurobrachia lacks a conventional photoprotein and is therefore incapable of producing light. Their bodies are virtually transparent and the many cilia refract the light, producing rainbow-like colors that can give the false appearance of bioluminescence. The branched tentacles can be white, yellow, pink or orange. They have no nematocysts (stinging cells). Instead, the two long extensile branched tentacles are armed with colloblasts: specialized adhesive cells with which to ensnare their prey.

Lifespan
The sea gooseberry is relatively short-lived, only alive for around 4–6 months.

Reproduction
Pleurobrachia lack any sessile (attached) stages and are wholly planktonic in their life cycle. They are self-fertile hermaphrodites that spawn eggs and sperm freely into the sea, and develop thereafter without any parental protection with direct development.

Feeding - Foraging Behavior
Pleurobrachia bachei is a selective carnivore and its feeding habits are analogous to other ambush "sit and wait" predators, such as the orb-weaving spider. When searching for prey the Pleurobrachia swims with its oral pole forward to set its tentacles. To allow the two main tentacles and numerous lateral tentilla to relax and expand behind it they are often in a curved or helical pathway. Once the tentacles are set, the ctenophore drifts passively. Occasionally, it will retract its tentacles to varying degrees into the sheaths before swimming to another location where it then resets them. This behavior appears to be regulated by its hunger level and can be construed as an attempt to find an area with more prey abundance.

When handling prey both tentacles contract and carry the prey to the mouth. This is achieved by several rapid rotations of the body which swipes the tentacle bearing the food across the oral region. The Pleurobrachia has its oral end opposite of where its tentacles originate.

Trophic strategy
Sea gooseberries are insatiable feeders of copepods and other small plankton, rarely fish eggs and larvae. It has been shown that their prey is more susceptible at an early age (naupliar/larval stages) because of minimal swimming speeds and small size which makes handling more efficient. This generalization is not necessarily true for all Pleurobrachia. In one experiment the ctenophore favored adult Pseudocalanus minutus more than other forms of zooplankton.

Geographic range
P. bachei is found along the West coast of North America from Southeast Alaska to Mexico.

Habitat
The sea gooseberry occurs primarily in surface waters of the coastal NW Pacific within 5 km of shore to about 50 m deep, though is usually in the upper 15 m during the day.

Conservation status
Pleurobrachia bachei has not been evaluated by the International Union for Conservation of Nature (IUCN), but seems to be prevalent and is not considered threatened.

Economic importance for humans
Although Pleurobrachia has not been associated with declines in other populations, a closely related species Mnemiopsis leidyi has. This ctenophore had catastrophic effects on fish catches after its introduction into the Black and Azov Seas. It is believed to have been the main cause of decline in these waters after dissection confirmed its stomach contents had large quantities of the local fish eggs and larvae. Because of their diets Pleurobrachia and other ctenophore species can directly or indirectly affect trophic cascades and ultimately regulate yield of commercially important fish stocks.
As predators, ctenophores have a tremendous capacity to regulate abundance of their prey and therefore help to balance an ecosystem. While they can decimate other populations they can also restrain an overabundance of copepods which, when left to their own devices, could virtually eliminate all phytoplankton from the water column.
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Strange Findings on Comb Jellies Uproot Animal Family Tree
Complete sequence of comb jelly genome reveals a separate course of evolution.


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The new study on ctenophores, such as the American comb jelly above, "really shakes up how we think animal complexity evolved."

Carl Zimmer
for National Geographic
PUBLISHED MAY 21, 2014

A close look at the nervous system of the gorgeously iridescent animal known as the comb jelly has led a team of scientists to propose a new evolutionary history: one for the comb jelly, and one for everybody else.

"It's a paradox," said Leonid Moroz, a neurobiologist at the University of Florida in Gainesville and lead author of a paper in today's Nature about the biology of the comb jelly nervous system. "These are animals with a complex nervous system, but they basically use a completely different chemical language" from every other animal. "You have to explain it one way or another."

The way Moroz explains it is with an evolutionary scenario—one that's at odds with traditional accounts of animal evolution.

Moroz and his colleagues have been studying comb jellies, whose scientific name is ctenophores (pronounced TEN-o-fors), for many years, beginning with the sequencing of the genome of one species, the Pacific sea gooseberry, in 2007. The sea gooseberry has 19,523 genes, about the same number as are found in the human genome.

The scientists enlarged their library to the genes of ten other species of comb jelly (out of the 150 or so species known to exist) and compared them to the analogous genes in other animals. And when they looked at the genes involved in the nervous system, they found that many considered essential for the development and function of neurons were simply missing in the comb jelly.

Some of those missing genes are involved in building neurons in embryos. The cells in any animal start out in the embryo as stem cells, looking pretty much identical to one another and capable of turning into any particular type of cell. Only later in embryonic development do some stem cells switch on specific genes that transform them into neurons. This process is much the same in humans as it is in flies, slugs, and just about every other animal with a nervous system.

But comb jellies, Moroz and his colleagues found, lack those neuron-building genes altogether. Which means that comb jelly embryos must build their neurons from a different set of instructions—instructions no one yet understands.

Nor do comb jellies use the standard complement of neurotransmitters found in other animals, the scientists found. The genes for most of the neurotransmitters in other animals are either missing or silent in the comb jelly—except for one, the gene for the neurotransmitter glutamate. No wonder Moroz likes to call these creatures "aliens of the sea."

Instead of the typical neurotransmitter genes, the scientists found, comb jellies produce a huge diversity of receptors on the surface of their neurons. Moroz can't say yet what the receptors are doing there, but he says they're probably grabbing neurotransmitters, maybe as many as 50 to 100 neurotransmitters in all (comparable to the number of neurotransmitters in the human brain).

Rewriting Evolutionary History

The unique nature of the comb jelly nervous system led the Florida scientists to hypothesize a new evolutionary history for these marine animals, which they laid out in the Nature paper. The earliest animals, according to this new theory, had no nervous system at all. The cells of these early animals could sense their environment directly, and could send signals directly to neighboring cells.

Millions of years later, those signals and receptors became the raw material for the nervous system. But its evolution, according to Moroz, took place in two separate lineages. One led to today's ctenophores. The other led to all other animals with nervous systems—from jellyfish to us.

If there was indeed a parallel evolution with two separate lineages, the split would have happened long ago. Fossils that look a lot like modern-day ctenophores date back some 550 million years, making them among the oldest traces of complex animal life.

But precisely how and when the comb jelly split off from other animal lineages remains controversial. To draw the animal evolutionary tree, Moroz and his colleagues analyzed the similarity of DNA in different species. According to the authors, ctenophores belong to a lineage all their own that split off from the others at the tree's base.

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Comb jellies, like this one at Monterey Bay Aquarium, California, are missing many genes considered essential for the development and function of neurons.

In finding that relationship, the new paper confirms the findings of a team led by Andy Baxevanis, head of the Computational Genomics Unit at the National Human Genome Research Institute, who arrived at a similar conclusion in December after sequencing the genome of another ctenophore species, the American comb jelly (Mnemiopsis leidyi). "You couldn't ask for a better outcome," he said about Moroz's research. "It really shakes up how we think animal complexity evolved."

Gert Woerheide, an evolutionary geobiologist at Ludwig-Maximilians-Universität in Munich, who was not involved in the research, agreed that Moroz and his colleagues have made a thorough case for their revised view of brain evolution. "I think, in this respect, this is a great paper," he said.

But in terms of the actual shape of the animal family tree, Woerheide is less convinced. He isn't sure that comb jellies branched off at the base of the tree, he said; sponges, for example, might have branched off first. In Woerheide's view, the exact reconstruction of the tree reaching so far back in evolutionary history remains an open question.

No matter how the nervous systems of comb jellies evolved, though, everyone agrees that they are weird—and thus worth getting to know better. As Casey Dunn, an evolutionary biologist at Brown University in Providence, Rhode Island, who was not involved in the research, pointed out, comb jellies are turning out to be "even more different from other animals than had previously been appreciated."

http://news.nationalgeographic.com/news/2014/05/140521-comb-jelly-ctenophores-oldest-animal-family-tree-science/




The ctenophore genome and the evolutionary origins of neural systems

Leonid L. Moroz, Kevin M. Kocot, Mathew R. Citarella, Sohn Dosung, Tigran P. Norekian, Inna S. Povolotskaya, Anastasia P. Grigorenko, Christopher Dailey, Eugene Berezikov, Katherine M. Buckley, Andrey Ptitsyn, Denis Reshetov, Krishanu Mukherjee, Tatiana P. Moroz, Yelena Bobkova, Fahong Yu, Vladimir V. Kapitonov, Jerzy Jurka, Yuri V. Bobkov, Joshua J. Swore, David O. Girardo, Alexander Fodor, Fedor Gusev, Rachel Sanford, Rebecca Bruders et al.

Nature (2014) doi:10.1038/nature13400
Received 15 September 2013 Accepted 23 April 2014 Published online 21 May 2014

Abstract
The origins of neural systems remain unresolved. In contrast to other basal metazoans, ctenophores (comb jellies) have both complex nervous and mesoderm-derived muscular systems. These holoplanktonic predators also have sophisticated ciliated locomotion, behaviour and distinct development. Here we present the draft genome of Pleurobrachia bachei, Pacific sea gooseberry, together with ten other ctenophore transcriptomes, and show that they are remarkably distinct from other animal genomes in their content of neurogenic, immune and developmental genes. Our integrative analyses place Ctenophora as the earliest lineage within Metazoa. This hypothesis is supported by comparative analysis of multiple gene families, including the apparent absence of HOX genes, canonical microRNA machinery, and reduced immune complement in ctenophores. Although two distinct nervous systems are well recognized in ctenophores, many bilaterian neuron-specific genes and genes of ‘classical’ neurotransmitter pathways either are absent or, if present, are not expressed in neurons. Our metabolomic and physiological data are consistent with the hypothesis that ctenophore neural systems, and possibly muscle specification, evolved independently from those in other animals.

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13400.html
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