Invertebrate

The common fruit fly, Drosophila melanogaster, has been the subject of much research

An invertebrate is an animal without a backbone. The group includes 97% of all animal species^[1] – all animals except those in the chordate subphylum Vertebrata (fish, amphibians, reptiles, birds, and mammals).

Invertebrates form a paraphyletic group. Given a common multicellular, eukaryotic ancestor, all contained phyla are invertebrates along with two of the three subphyla in Phylum Chordata: Tunicata and Cephalochordata. These two, plus all the other known invertebrates, have only one cluster of Hox genes, while the vertebrates have duplicated their original cluster more than once.

Within palaeozoology and palaeobiology, invertebrates are often studied within the fossil discipline called invertebrate palaeontology.

Etymology

The word invertebrate comes from the word vertebrate, with the prefix in- attached to it.^[2] Thus, the word indicates that which lacks vertebrae.

Characteristics

The trait that is common between all invertebrates is the absence of a backbone: this creates a distinction between invertebrates, and vertebrates. Being animals, invertebrates are heterotrophs, and require sustenance in the form of consuming other organisms. Their cells also lack rigid cell walls. With a few exceptions, such as the Porifera, invertebrates generally have bodies composed of differentiated tissues. There is also typically a digestive chamber with one or two openings in it.

Many invertebrates reproduce through sexual reproduction. They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.^[3] These fuse to form zygotes, which develop into new individuals.^[4] Others are capable of asexual reproduction, or sometimes, both methods of reproduction.

Phyla

The fossil coral Cladocora from the Pliocene of Cyprus

The term invertebrate covers several phyla. One of these are the sponges (Porifera). They were long thought to have diverged from other animals early.^[5] They lack the complex organization found in most other phyla.^[6] Their cells are differentiated, but in most cases not organized into distinct tissues.^[7] Sponges typically feed by drawing in water through pores.^[8] Some speculate that sponges are not so primitive, but may instead be secondarily simplified.^[9] The Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.^[10] Both have distinct tissues, but they are not organized into organs.^[11] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, they are sometimes called diploblastic.^[12]

The Echinodermata are radially symmetric and exclusively marine, including starfish, sea urchins, and sea cucumbers.^[13] Other phyla of invertebrates are the Hemichordata, or acorn worms,^[14] and the Chaetognatha, or arrow worms.

The largest animal phylum is also included within invertebrates: the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. In addition, they possess a hardened exoskeleton that is periodically shed during growth.^[15] Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. The Nematoda or roundworms, are perhaps the second largest animal phylum, and are also invertebrates. Roundworms are typically microscopic, and occur in nearly every environment where there is water.^[16] A number are important parasites.^[17] Smaller phyla related to them are the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom. Other invertebrates include the Nemertea or ribbon worms, and the Sipuncula.

Another phylum is Platyhelminthes, the flatworms.^[18] These were originally considered primitive, but it now appears they developed from more complex ancestors.^[19] Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.^[20] The Rotifera or rotifers, are common in aqueous environments. Invertbrates also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and the Cycliophora.^[21]

Also included are two of the most successful animal phyla, the Mollusca and Annelida.^[22][23] The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented.^[24] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.^[25]

Other phyla include Acoelomorpha, Brachiopoda, Bryozoa, Entoprocta, Phoronida, and Xenoturbellida.

History

Some of the first animal fossils appear to be those of invertebrates. 665-million-year-old fossils in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia have been interpreted as being early sponges.^[26] Some paleontologists suggest that animals appeared much earlier, possibly as early as 1 billion years ago.^[27] Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms.^[28]

Around 453 MYA, animals began diversifying, and many of the important groups of invertebrates diverged from one another. Fossils of invertebrates are found in various types of sediment from the Phanerozoic.^[29] Fossils of invertebrates are commonly used in stratigraphy.^[30]

Classification

Carl Linnaeus divided these animals into only two groups, the Insecta and the now-obsolete Vermes (worms). Jean-Baptiste Lamarck, who was appointed to the position of "Curator of Insecta and Vermes" at the Muséum National d'Histoire Naturelle in 1793, both coined the term "invertebrate" to describe such animals, and divided the original two groups into ten, by splitting Arachnida and Crustacea from the Linnean Insecta, and Mollusca, Annelida, Cirripedia, Radiata, Coelenterata and Infusoria from the Linnean Vermes. They are now classified into over 30 phyla, from simple organisms such as sea sponges and flatworms to complex animals such as arthropods and molluscs.

Significance of the group

Invertebrates are animals without backbones. This has led to the conclusion that invertebrates are a group that deviates from the norm, vertebrates. This has been said to be due to the fact that researchers in the past, such as Lamarck, viewed vertebrates as a "standard": in Lamarck's theory of evolution, he believed that characteristics acquired through the evolutionary process involved not only survival, but also progression toward a "higher form", to which humans and vertebrates were closer than invertebrates were. Although goal-directed evolution has been abandoned, the distinction of invertebrates and vertebrates persists to this day, even though the grouping has been noted to be "hardly natural or even very sharp." Another reason cited for this continued distinction is that Lamarck created a precedent through his classifications which is now difficult to escape from. It's also possible that some humans believe that, they themselves being vertebrates, the group deserves more attention than invertebrates.^[31] In any event, in the 1968 edition of Invertebrate Zoology, it is noted that "division of the Animal Kingdom into vertebrates and invertebrates is artificial and reflects human bias in favor of man's own relatives." The book also points out that the group lumps a vast number of species together, so that no one characteristic describes all invertebrates. In addition, some species included are only remotely related to one another, with some more related to vertebrates than other invertebrates.^[32]

In research

Two of the most commonly studied model organisms are invertebrates: the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. They have long been the most intensively studied model organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but many genes, introns, and linkages have been lost. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to animals.^[33]

Arthropods, especially insects, are often used by forensic scientists. For example, some invertebrates are attracted to dead bodies.^[15]

See also

• Marine invertebrates
• Pain in invertebrates

References

1. ^ Encarta Reference Library Home Premium 2005 DVD. Article – Invertebrate.
2. ^ Skeat, Walter William (1882). An etymological dictionary of the English language. Clarendon Press. p. 301. 
3. ^ Schwartz, Jill (2010). Master the GED 2011 (w/CD). Peterson's. p. 371. ISBN 9780768928853. 
4. ^ Hamilton, Matthew B. (2009). Population genetics. Wiley-Blackwell. p. 55. ISBN 9781405132770. 
5. ^ Bhamrah, H. S.; Kavita Juneja (2003). An Introduction to Porifera. Anmol Publications PVT. LTD.. p. 58. ISBN 9788126106752. 
6. ^ Sumich, James L. (2008). Laboratory and Field Investigations in Marine Life. Jones & Bartlett Learning. p. 67. ISBN 9780763757304. 
7. ^ Jessop, Nancy Meyer (1970). Biosphere; a study of life. Prentice-Hall. p. 428. 
8. ^ Sharma, N. S. (2005). Continuity And Evolution Of Animals. Mittal Publications. p. 106. ISBN 9788182930186. 
9. ^ Dunn et al. 2008. "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature 06614.
10. ^ Langstroth, Lovell; Libby Langstroth, Todd Newberry, Monterey Bay Aquarium (2000). A living bay: the underwater world of Monterey Bay. University of California Press. p. 244. ISBN 9780520221499. 
11. ^ Safra, Jacob E. (2003). The New Encyclopaedia Britannica, Volume 16. Encyclopaedia Britannica. p. 523. ISBN 9780852299616. 
12. ^ Kotpal, R. L.. Modern Text Book of Zoology: Invertebrates. Rastogi Publications. p. 184. ISBN 9788171339037. 
13. ^ Alcamo, Edward (1998). Biology Coloring Workbook. The Princeton Review. p. 220. ISBN 9780679778844. 
14. ^ Tobin, Allan J.; Jennie Dusheck (2005). Asking about life. Cengage Learning. p. 497. ISBN 9780534406530. 
15. ^ ^{a b} Gunn, Alan (2009). Essential forensic biology. John Wiley and Sons. p. 214. ISBN 9780470758045. 
16. ^ Prewitt, Nancy L.; Larry S. Underwood, William Surver (2003). BioInquiry: making connections in biology. John Wiley. p. 289. ISBN 9780471202288. 
17. ^ Schmid-Hempel, Paul (1998). Parasites in social insects. Princeton University Press. p. 75. ISBN 9780691059242. 
18. ^ Gilson, Étienne (2004). El espíritu de la filosofía medieval. Ediciones Rialp. p. 384. ISBN 9788432134920. 
19. ^ Ruiz-Trillo, I., I; Ruiz-Trillo, Iñaki; Riutort, Marta; Littlewood, D. Timothy J.; Herniou, Elisabeth A.; Baguñà, Jaume (March 1999). "Acoel Flatworms: Earliest Extant Bilaterian Metazoans, Not Members of Platyhelminthes". Science 283 (5409): 1919–1923. doi:10.1126/science.283.5409.1919. ISSN 0036-8075. PMID 10082465. 
20. ^ Todaro, Antonio. "Gastrotricha: Overview". Gastrotricha: World Portal. University of Modena & Reggio Emilia. http://www.gastrotricha.unimore.it/overview.htm. Retrieved 2008-01-26. 
21. ^ Kristensen, Reinhardt Møbjerg (July 2002). "An Introduction to Loricifera, Cycliophora, and Micrognathozoa". Integrative and Comparative Biology (Oxford Journals) 42 (3): 641–651. doi:10.1093/icb/42.3.641. http://icb.oxfordjournals.org/cgi/content/full/42/3/641. Retrieved 2008-01-26. 
22. ^ "Biodiversity: Mollusca". The Scottish Association for Marine Science. Archived from the original on 2006-07-08. http://web.archive.org/web/20060708083128/http://www.lophelia.org/lophelia/biodiv_6.htm. Retrieved 2007-11-19. 
23. ^ Russell, Bruce J. (Writer), Denning, David (Writer) (2000). Branches on the Tree of Life: Annelids (VHS). BioMEDIA ASSOCIATES. 
24. ^ Eernisse, Douglas J., D. J.; Eernisse, Douglas J.; Albert, James S.; Anderson , Frank E. (1 September 1992). "Annelida and Arthropoda are not sister taxa: A phylogenetic analysis of spiralean metazoan morphology". Systematic Biology 41 (3): 305–330. doi:10.2307/2992569. ISSN 10635157. JSTOR 2992569. 
25. ^ Eernisse, Douglas J.; Kim, Chang Bae; Moon, Seung Yeo; Gelder, Stuart R.; Kim, Won (September 1996). "Phylogenetic Relationships of Annelids, Molluscs, and Arthropods Evidenced from Molecules and Morphology" (–^{Scholar search}). Journal of Molecular Evolution (New York: Springer) 43 (3): 207–215. doi:10.1007/PL00006079. PMID 8703086. http://www.springerlink.com/content/xptr6ga3ettxnmb9/. Retrieved 2007-11-19. ^{[dead link]}
26. ^ Maloof, Adam C.; Rose, Catherine V.; Beach, Robert; Samuels, Bradley M.; Calmet, Claire C.; Erwin, Douglas H.; Poirier, Gerald R.; Yao, Nan et al. (17 August 2010). "Possible animal-body fossils in pre-Marinoan limestones from South Australia". Nature Geoscience 3 (9): 653. doi:10.1038/ngeo934. http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo934.html. 
27. ^ Campbell. Neil A.; Jane B. Reece (2005). Biology (7 ed.). Pearson, Benjamin Cummings. p. 526. ISBN 9780805371710. 
28. ^ Seilacher, A., Bose, P.K. and Pflüger, F., A (Oct 1998). "Animals More Than 1 Billion Years Ago: Trace Fossil Evidence from India". Science 282 (5386): 80–83. doi:10.1126/science.282.5386.80. ISSN 0036-8075. PMID 9756480. http://www.sciencemag.org/cgi/content/abstract/282/5386/80. Retrieved 2007-08-20. 
29. ^ Clarkson, Euan Neilson Kerr (1998). Invertebrate palaeontology and evolution. Wiley-Blackwell. ISBN 9780632052387. 
30. ^ Kummel, Bernhard (1954). Status of invertebrate paleontology, 1953. Ayer Publishing. p. 93. ISBN 9780405127151. 
31. ^ Barnes, Richard Stephen Kent (2001). The Invertebrates: A Synthesis. Wiley-Blackwell. p. 3. ISBN 9780632047611. 
32. ^ Barnes, Robert D. (1968). Invertebrate Zoology (2nd ed.). W.B. Saunders. OCLC 173898. 
33. ^ N.H. Putnam, et al., NH (July 2007). "Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization". Science 317 (5834): 86–94. doi:10.1126/science.1139158. ISSN 0036-8075. PMID 17615350. 

Further reading

• Hyman, L. H. 1940. The Invertebrates (6 volumes) New York : McGraw-Hill. A classic work.
• Anderson, D. T. (Ed.). (2001). Invertebrate zoology (2nd ed.). Oxford: Oxford University Press.
• Brusca, R. C., & Brusca, G. J. (2003). Invertebrates (2nd ed.). Sunderland, Mass. : Sinauer Associates.
• Miller, S.A., & Harley, J.P. (1996). Zoology (4th ed.). Boston: WCB/McGraw-Hill.
• Pechenik, Jan A. (2005). Biology of the invertebrates. Boston: McGraw-Hill, Higher Education. pp. 590 pp. ISBN 0072348992. 
• Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). Invertebrate zoology: a functional evolutionary approach. Belmont, CA: Thomas-Brooks/Cole.

External links

• A. R. Maggenti & S. Gardner (2005). Online Dictionary of Invertebrate Zoology. http://digitalcommons.unl.edu/onlinedictinvertzoology/. 
• Support for endangered invertebrates
• African Invertebrates