Coleoidea[From Greek ; sheath][
] or Dibranchiata is one of the two subclasses of cephalopod containing all the various taxa Popular science of as "soft-bodied" or "shell-less" (i.e. octopus, squid and cuttlefish). Unlike its , the mollusc shell Ammonoidea and Nautiloidea, the coleoids have at most an internal shell called cuttlebone or gladius that is used for buoyancy or as muscle attachment. Some species, notably the Incirrata (including commonly known varieties living in the shallows), have lost their internal shell altogether, while in some it has been replaced by a support structure.This is an example.
Evolution
The earliest certain coleoids are known from the Mississippian sub-period of the
Carboniferous Period, about 330 million years ago. Some older fossils have been described from the
Devonian,
[
]
but paleontologists disagree about whether they are coleoids.
[
] Other cephalopods with internal shells, which could represent coleoids but may also denote the independent internalization of the shell, are known from the
Silurian.
[
] It has been hypothesized that the Early–Middle
Cambrian fossil
Nectocaris represents a coleoid (or other cephalopod) that lost its shell, possibly
secondary loss,
although it is more likely that
Nectocaris represents an independent lineage within the
Lophotrochozoa and not a cephalopod.
By the Carboniferous, coleoids already had a diversity of forms, but the major radiation happened during the Tertiary period.[
]
Classification
The major divisions within Coleoidea are based upon the number of
cephalopod arm or
and their structure. The extinct and most primitive form, the
Belemnoidea, presumably had ten equally-sized arms in five pairs numbered as (counting from dorsal to
ventral pairs) I, II, III, IV and V. More modern species either modified or lost a pair of arms; the
superorder Decapodiformes (literally "ten arms") has arm pair IV modified into long tentacles with suckers generally only on the club-shaped
distal end. Superorder
Octopodiformes has modifications to arm pair II; it is significantly reduced and used only as a sensory filament in the
Vampyromorphida, while species of
(literally "eight arms") have totally lost that arm pair. The inner surface of the suckers (acetabulum) are reinforced with rigid sucker rings which are smooth in
Sepiolida, have blunt teeth in
Sepiida and sharply pointed teeth in
Loliginidae and
Oegopsida. The arms and/or tentacles of some oegopsid families have also evolved claw-like hooks, such as the
hooked squid and the colossal squid.
Subordinate to Coleoidea is the division/cohort; Neocoleoidea, containing two extant groups: Decapodiformes (squid, cuttlefish, and relatives) and Octopodiformes ( and the vampire squid). Species within this group exist in all major habitats in the ocean, in both the Southern Ocean and Arctic Ocean, and from to the deep sea.
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Class
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Subclass Nautiloidea: nautilus
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Subclass †Ammonoidea: ammonites
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Subclass Coleoidea
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Division †Belemnoidea: extinct belemnoids
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Division Neocoleoidea
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(uncertain order)
Reproduction
The majority of coleoid species are
semelparous; dying after reproducing once, with males dying after
insemination, and females dying after
Oviparity/
brooding their clutch.
A few species do not conform to this trend however;
vampire squid, large Pacific striped octopus, and the West Atlantic scaled squid are thought to be iteroparous.
This is somewhat comparable to extant
nautilus, which are also iteroparous, being long-lived animals.
Some authors prefer to separate coleoid reproduction using other terms:
1. One-time reproduction (formerly semelparity); being the occurrence of "simultaneous terminal spawning" (terminal: occurring at the end of life). This group is characterized by synchronous ovulation (the ova all ripen prior to spawning), single-cycle spawning, and the absence of growth between egg batches.
2. Multiple reproductive events (formerly iteroparity). This category is divided further into:
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(i) polycyclic spawning; where single egg batches/clutches develop and are laid multiple times during the spawning season, with growth occurring between production of egg batches and and the gonads regenerate/ripen between clutches, e.g. Nautilus ;
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(ii) multiple spawning; where multiple clutches of eggs develop simultaneously, which can be differentiated in the ovaries through their development stage (also known as group-synchronous ovulation). This method is defined by monocyclic spawning and the mother's growth between egg batches, e.g. Octopus chierchiae, Sthenoteuthis oualaniensis, Ommastrephes bartramii, and Dosidicus gigas ;
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(iv) continuous spawning; where oocyte developed without any apparent "batches", with all stages of development potentially being present (or asynchronous ovulation), monocyclic spawning and growth between egg batches, e.g. Cirrothauma murrayi, Opisthoteuthis agassizii, Opisthoteuthis grimaldii and Grimpoteuthis, likely also includes Argonauta bottgeri, Argonauta hians, and Idiosepius pygmaeus.
Paralarva
Paralarvae (:
paralarva) are young cephalopods immediately after hatching, prior to the development of adult diagnostic features and before exhibiting a similar ecology to older members of the same species. The term was introduced by Richard E. Young and Robert F. Harman in 1988; the term "
larva" had been used previously, but it fell out of use as the term implied a
metamorphosis occurred where larval body parts are lost completely and adult body parts developed from some "rudiments" left in an
state; in contrast, young cephalopods do not undergo metamorphosis, they mostly grow
Morphometrics, though as this process still transforms the animal significantly, being comparable to the development of
Juvenile fish (referred to as
Fish larva), the term "paralarva" (
: near, almost; near-larva/almost larva) was thus coined.
Paralarvae have been observed only in members of the Octopoda and Teuthida (which constitutes the modern definition of Coleoidea).
File:Psychroteuthis glacialis paralarva2.jpg| Psychroteuthis glacialis
File:Taningia_persica.jpg| Taningia
File:Fish3566 - Flickr - NOAA Photo Library.jpg|Octopus paralarva
File:Grimalditeuthis bonplandi paralarva (ROTATED).jpg| Grimalditeuthis bonplandi
External links
