Invertebrate

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Invertebrates
Temporal range: Cryogenian to Present, 665–0 Ma
Diversity of various invertebrates from different phyla (including a invertebrate of the phylum Chordata)
Left to right:
Tardigrada
).
Scientific classificationEdit this classification
Domain: Eukaryota
(unranked): Filozoa
Kingdom: Animalia
Groups included
  • All animal groups not in subphylum
    Vertebrata

Invertebrates is an

cnidarians and sponges
.

The majority of animal species are invertebrates; one estimate puts the figure at 97%.[1] Many invertebrate taxa have a greater number and diversity of species than the entire subphylum of Vertebrata.[2] Invertebrates vary widely in size, from 10 μm (0.0004 in)[3] Myxozoans to the 9–10 m (30–33 ft) colossal squid.[4]

Some so-called invertebrates, such as the

polyphyletic, so the term has little meaning in taxonomy
.

Etymology

The word "invertebrate" comes from the Latin word vertebra, which means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate. The jointed aspect of vertebra is derived from the concept of turning, expressed in the root verto or vorto, to turn.[5] The prefix in- means "not" or "without".[6]

Taxonomic significance

The term invertebrates is not always precise among non-biologists since it does not accurately describe a

Vertebrata or Manidae do. Each of these terms describes a valid taxon, phylum, subphylum or family. "Invertebrata" is a term of convenience, not a taxon; it has very little circumscriptional significance except within the Chordata. The Vertebrata as a subphylum comprises such a small proportion of the Metazoa that to speak of the kingdom Animalia in terms of "Vertebrata" and "Invertebrata" has limited practicality. In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing a cladogram, for example, the presence of a notochord. That would at least circumscribe the Chordata. However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry[7] or perhaps bauplan.[8]

Despite this, the concept of invertebrates as a taxon of animals has persisted for over a century among the

Arthropoda. Arthropoda include insects, crustaceans and arachnids
.

Number of extant species

By far the largest number of described invertebrate species are insects. The following table lists the number of described

IUCN Red List of Threatened Species, 2014.3.[11]

Invertebrate group Phylum Image Estimated number of
described species[11]
Insects
Arthropoda
 1,000,000
Arachnids
Arthropoda
 102,248
Snails Mollusca 85,000
Crustaceans
Arthropoda
 47,000
Clams Mollusca 20,000
Corals Cnidaria 2,175
Octopuses/Squid Mollusca 900
Velvet worms
 Onychophora 165
Nautilus Mollusca 6
Horseshoe crabs
Arthropoda
 4
Others
jellyfish, echinoderms,
sponges, other worms etc. 		— — 68,658
Total: ~1,300,000

The

IUCN estimates that 66,178 extant vertebrate species have been described,[11]
which means that over 95% of the described animal species in the world are invertebrates.

Characteristics

The trait that is common to all invertebrates is the absence of a

Porifera
, invertebrates generally have bodies composed of differentiated tissues. There is also typically a digestive chamber with one or two openings to the exterior.

Morphology and symmetry

The

gastropod species. This is easily seen in snails and sea snails, which have helical shells. Slugs appear externally symmetrical, but their pneumostome (breathing hole) is located on the right side. Other gastropods develop external asymmetry, such as Glaucus atlanticus that develops asymmetrical cerata as they mature. The origin of gastropod asymmetry is a subject of scientific debate.[12]

Other examples of asymmetry are found in

Sessile animals such as sponges are asymmetrical[13] alongside coral colonies (with the exception of the individual polyps that exhibit radial symmetry); alpheidae claws that lack pincers; and some copepods, polyopisthocotyleans, and monogeneans which parasitize by attachment or residency within the gill chamber of their fish hosts
).

Nervous system

Neurons differ in invertebrates from mammalian cells. Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH. The first invertebrate in which a neuron cell was identified was the medicinal leech, Hirudo medicinalis.[14][15]

Learning and memory using nociceptors in the sea hare, Aplysia has been described.[16][17][18] Mollusk neurons are able to detect increasing pressures and tissue trauma.[19]

Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods.[20][21]

Respiratory system

Tracheal system of dissected cockroach. The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm.
The tracheal system branches into progressively smaller tubes, here supplying the crop of the cockroach. Scale bar, 2.0 mm.

One type of invertebrate respiratory system is the open

ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their haemolymph.[23]

A tracheal tube may contain ridge-like circumferential rings of

plastron. Despite being internal, the tracheae of arthropods are shed during moulting (ecdysis).[24]

Hearing

This section is an excerpt from Ear § Invertebrates.[edit]

Only vertebrate animals have ears, though many invertebrates detect sound using other kinds of sense organs. In insects, tympanal organs are used to hear distant sounds. They are located either on the head or elsewhere, depending on the insect family.[25] The tympanal organs of some insects are extremely sensitive, offering acute hearing beyond that of most other animals. The female cricket fly Ormia ochracea has tympanal organs on each side of her abdomen. They are connected by a thin bridge of exoskeleton and they function like a tiny pair of eardrums, but, because they are linked, they provide acute directional information. The fly uses her "ears" to detect the call of her host, a male cricket. Depending on where the song of the cricket is coming from, the fly's hearing organs will reverberate at slightly different frequencies. This difference may be as little as 50 billionths of a second, but it is enough to allow the fly to home in directly on a singing male cricket and parasitise it.[26]

Simpler structures allow other arthropods to detect near-field sounds. Spiders and cockroaches, for example, have hairs on their legs, which are used for detecting sound. Caterpillars may also have hairs on their body that perceive vibrations[27] and allow them to respond to sound.

Reproduction

Like vertebrates, most invertebrates reproduce at least partly through

ova.[28] These fuse to form zygotes, which develop into new individuals.[29]
Others are capable of asexual reproduction, or sometimes, both methods of reproduction.

Extensive research with model invertebrate species such as Drosophila melanogaster and Caenorhabditis elegans has contributed much to our understanding of meiosis and reproduction. However, beyond the few model systems, the modes of reproduction found in invertebrates show incredible diversity.[30] In one extreme example it is estimated that 10% of orbatid mite species have persisted without sexual reproduction and have reproduced asexually for more than 400 million years.[30]

Reproductive systems

This section is an excerpt from Reproductive system § Invertebrates.[edit]
Invertebrates have an extremely diverse array of reproductive systems, the only commonality may be that they all lay eggs. Also, aside from
hermaphroditic and exhibit external fertilization
.

Social interaction

Social behavior is widespread in invertebrates, including cockroaches, termites, aphids,

eusocial
species but applies to other invertebrates as well.

Insects recognize information transmitted by other insects.[32][33][34]

Phyla

The fossil coral Cladocora from the Pliocene of Cyprus

The term invertebrates covers several phyla. One of these are the sponges (

diploblastic.[42]

The

sea cucumbers (Holothuroidea) and feather stars (Crinoidea).[43]

The largest animal phylum is also included within invertebrates: the Arthropoda, including insects,

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.[45] A number are important parasites.[46] 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

Cycliophora.[50]

Also included are two of the most successful animal phyla, the Mollusca and Annelida.[51][52] 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.[53] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.[54]

Among lesser phyla of invertebrates are the

Xenoturbellida
.

Classification of invertebrates

Invertebrates can be classified into several main categories, some of which are

taxonomically obsolescent or debatable, but still used as terms of convenience. Each however appears in its own article at the following links.[56]

History

The earliest 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.

triploblastic worms, roughly as large (about 5 mm wide) and complex as earthworms.[59]

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.[60] Fossils of invertebrates are commonly used in stratigraphy.[61]

Classification

sea sponges and flatworms
to complex animals such as arthropods and molluscs.

Significance of the group

Invertebrates are animals without a vertebral column. This has led to the conclusion that invertebrates are a group that deviates from the normal, vertebrates. This has been said to be because 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 is also possible that some humans believe that, they themselves being vertebrates, the group deserves more attention than invertebrates.[62] 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 (see Paraphyly).[63]

In research

For many centuries, invertebrates were neglected by biologists, in favor of big vertebrates and "useful" or

Lamarck in the 18th century.[64] During the 20th century, invertebrate zoology became one of the major fields of natural sciences, with prominent discoveries in the fields of medicine, genetics, palaeontology, and ecology.[64] The study of invertebrates has also benefited law enforcement, as arthropods, and especially insects, were discovered to be a source of information for forensic investigators.[44]

Two of the most commonly studied model organisms nowadays 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.[65] Invertebrates are also used by scientists in the field of aquatic biomonitoring to evaluate the effects of water pollution and climate change.[66]

See also

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Further reading

External links

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