Pleurobrachia bachei
From ize2010
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. They are truly beautiful marine invertebrates.
| Sea Gooseberry | |
|---|---|
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| Pleurobrachia bachei with its oral end down | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Ctenophora |
| Class: | Tentaculaa |
| Order: | Cydippida |
| Family: | Pleurobrachiidae |
| Genus: | Pleurobrachia |
| Species: | bachei |
| Binomial name | |
| Pleurobrachia bachei Agassiz, 1860 |
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Contents |
Description
Morphology
An individuals body length can reach up to 20mm (0.8in) across with each of the two tentacles stretching 150 mm (6 in).[1] Their gelatinous globular bodies are composed of 99% water.[2] They have eight rows of well-developed combed plates consisting of thousands of fused macrocilia controlled by an apical organ. Unlike many previous contradictory statements have suggested, Pleurobrachia lacks a conventional photoprotein and is therefore incapable of producing light.[3] 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 stomach has eight branches that are visible through the transparent body and there are numerous anal pores. 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.[4]
Reproduction and Life History
Reproduction
Pleurobrachia lack any sessile (attached) stages and are wholly planktonic in their life cycle. They are hermaphroditic with direct development.[2]
Lifespan
They are relatively short-lived, around 4-6 months.[2]
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.[5]
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[4] and can be construed as an attempt to find an area with more abundant prey.
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.[4]
Ecology and Distribution
Geographic Range
Occurs worldwide but is patchy in its distribution being one of the more common comb jellies found on the West coast.[2]
Habitat
Along with its wide distribution the sea gooseberry is found in pelagic surface waters near shore and in open ocean to considerable depths.[2]
Trophic Strategy
They are insatiable feeders of copepods and other small plankton as well as 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 neccassarily true for all Pleurobrachia. In one experiment the ctenophore favored adult Pseudocalanus minutus more than other forms of zooplankton.[6]
Conservation
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.[7]
Relevance
Economic Importance for humans: Negative
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 Azor 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.[8] Because of their diets Pleurobrachia and other ctenophore species can directiy or indirectly effect trophic cascades and ultimately regulate yield of commercially important fish stocks.
Economic Importance for humans: Positive
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.[4] With the proper management, Pluerobrachia introduction would be very beneficial to regulate copepods and to stop them from overpowering the essential primary producers (phytoplankton) needed for a healthy environment.
References
- ↑ LAMB, A., and B.P. HANBY. 2005. Marine life of the pacific northwest: a photographic Encyclopedia of invertebrates, seaweeds and selected fishes. Harbour Publishing Co.
- ↑ 2.0 2.1 2.2 2.3 2.4 http://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/gooseberry_bg.shtml.
- ↑ HADDOCK, S.H.D., and J.F. CASE. 1995. Not all ctenophores are bioluminescent: Pleurobrachia. Biological Bulletin 189: 356-362.
- ↑ 4.0 4.1 4.2 4.3 GREENE, C.H., M.R. LANDRY, and B.C. MONGER. 1986. Foraging behavior and prey selection by the ambush entangling predator Pleurobrachia bachei. Ecology 67: 1493-1501.
- ↑ PODAR, M., S.H.D. HADDOCK, M.L SOGIN, and G.R. HARBISON. 2001. A molecular phylogenetic framework for the phylum ctenophora using 18S rRNA genes. Molecular Phylogenetics and Evolution 21(2): 218-230.
- ↑ BISHOP, J.W. 1968. A comparative study on the feeding rates of tentaculate ctenophores. Ecology 49: 996-997.
- ↑ http://www.eol.org/pages/393303
- ↑ CHANDY, S.T. and C.H. GREENE. 1995. Estimating the predatory impact of gelatinous zooplankton. Limnology and Oceanography 40: 947-955.
