Juvenile cephalopod from plankton
Cephalopods (Greek plural Κεφαλόποδα (kephalópoda); "head-foot") are marine mollusks (phylum Mollusca) of the class Cephalopoda, a group that includes the familiar octopus and squid. Cephalopods are characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot into the form of arms or tentacles surrounding the mouth, which has beak-like jaws. Cephalopods have the most complex nervous system among invertebrates, and large and image-focusing, vertebrate-like eyes. The highly developed neural organs allows them to have an excellent capacity to learn and remember; nonetheless, most are short-lived, in the order of months or a few years.
The class Cephalopoda contains two extant (living) subclasses. In the Coleoidea, the mollusk shell has been internalized or is absent; this subclass includes the octopuses, squids, and cuttlefish. In the Nautiloidea, the shell remains; this subclass includes the nautilus. There are around 800 known extant species of cephalopods, although an estimate 10,000 or more are known from the fossil record. Almost all belong to Coleoidea; Young (2006) recognizes only six definitive living species of the once abundant and diverse Nautiloidea. Two important extinct taxa are Ammonoidea, the ammonites, and Belemnoidea, the belemnites.
Cephalopods are important ecologically, commercially, and scientifically. Ecologically, they have key roles in food chains, preying on fish, crabs, shrimps, and other mollusks, and being consumed by whales, birds, fish, and seals. Commercially, cephalopods are increasingly being targeted by the fishing industry. Such has been the case for thousands of years; Plato and Aristotle both describe fishing methods (Lane 1960; Wood 2001). Today, well over one million metric tons are caught each year (Vecchinone 2006). Cephalopods protein is considered to be low calorie and includes unsaturated fatty acids, which are considered to have health benefits, including lowering risk of heart disease (Okuzumi & Fujii 2000). Because of their highly developed neural organs, cephalopods are valuable experimental animals in biomedical and neurophysiology research.
Beyond these values, cephalopods provide aesthetic value, serving as popular focal points in movies, animated cartoons, books, artistic drawings, and marine aquariums, and touching upon people's imagination and ability to experience beauty.
Cephalopods are found in all the oceans of the world and at all depths, from 0 to over 5,000 meters (Vecchione 2006). Cephalopods cannot tolerate freshwater, but a few species do tolerate more or less brackish water; none are found at salinities less than 17.5 PSU (practical salinity units) (Vecchione 2006). (Ocean water is about 35 PSU, or 35 percent salinity, or 3.5 percent salt.)
The name cephalopod ("head-foot") refers to the large, well-developed head and the prominent foot of tentacles. Octopuses have eight arms, squids and the squid-like cuttlefish have eight arms and two tentacles, and nautiluses have up to 90 tentacles. These typically have suckers or hooks, with the exception of Nautilus. Teuthology is the study of cephalopods.
Adult cephalopods range from about 2 centimeters (less than one inch) to possibly over 14 meters (over 40 feet) in total length, and there have been reports of the largest being over 60 feet and weighing over one ton (Vecchione 2006). The colossal squid (Mesonychoteuthis hamiltoni) and giant squids (Architeuthisare sp.) are the world's largest invertebrates. O'Shea (2006) discounts reports of Architeuthisare sp. being 60 feet in length as a myth, noting exaggeration of size by lengthening of specimens, by stretching like rubber bands the tentacular arms. He likewise discounts reports of specimens weighing up to one ton. The size of Mesonychoteuthis hamiltoni, considered to be the largest squid, is based on estimates.
Cephalopods are soft-bodied and generally without a hard covering. Only Nautilus has an external shell, although female argonauts have a shell-like egg case (Vecchione 2006).
Cephalopods have one pair of gills, with the exception of Nautilus, which has two pairs (Vecchione 2006). Cephalopods have a close circulatory system. They have two gills hearts (also known as branchial hearts) that move blood through the capillaries of the gills. A single systemic heart then pumps the oxygenated blood through the rest of the body (Wells 1980). Like most mollusks, cephalopods use hemocyanin, a copper-containing protein, rather than hemoglobin to transport oxygen. As a result, their blood is colorless when deoxygenated and turns blue when exposed to air (Ghiretti-Magaldi and Ghiretti 1992).
With the exception of Nautilus, cephalopods have special skin cells called chromatophores that change color and are used for communication and camouflage. Cuttlefish are known as "chameleons of the sea" for their ability to change skin color and patterns. (See camouflage.)
The nervous system of cephalopods is generally highly developed. Cephalopods are widely regarded as the most intelligent of the invertebrates and have well developed senses and large brains; larger than the brains of gastropods or bivalves. The giant nerve fibers of the cephalopod mantle have been a favorite experimental material of neurophysiologists for many years.
Like vertebrates, cephalopods can focus their eyes. Cephalopod vision is acute, and training experiments have shown that the common octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects. Cephalopods' eyes are also sensitive to the plane of polarization of light. Surprisingly in light of their ability to change color, most are probably color blind (Hanlon and Messenger 1996). When camouflaging themselves, they use their chromatophores to change brightness and pattern according to the background they see, but their ability to match the specific color of a background probably comes from cells such as iridophores and leucophores that reflect light from the environment (Hanlon and Messenger 1996). Evidence of color vision has been found in only one species, the Sparkling Enope Squid (Hanlon and Messenger 1996).
Cephalopods' primary method of movement is by jet propulsion, a very energy-consuming way to travel compared to the tail propulsion used by fish. The relative efficiency of jet propulsion degrades with larger animals. This is probably the reason why many cephalopod species prefer to use their fins or arms for locomotion if possible.
In cephalopod locomotion, oxygenated water is taken into the mantle cavity. Through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions.
Some cephalopod species undergo daily migrations, such as from depths of 400 to 1,000 meters during the day and 200 meters or so during the night (Vecchinone 2006).
Cephalopod sexes are separate, being male and female, with many exhibiting external sexual differentiation, whether size differences or structural differences.
With a few exceptions, Coleoidea (octopus, squid, cuttlefish) live short lives with rapid growth and high metabolism. Most of the energy extracted from their food is used for growing. They tend towards a semelparous reproduction strategy; they lay many small eggs in one batch and die afterwards. For most species, life expectancy appears to be about one or two years, but the poorly known giant squid (Architeuthis spp.) and the giant octopus (Enteroctopus spp.) seemingly have at least somewhat longer lives (Vecchione 2006). Some species such as pyroteuthids complete their life cycles in less than six months, and some species die after spawning (Vecchione 2006).
The Nautiloidea, on the other hand, stick to iteroparity. They produce a few large eggs in each batch and live for a long time.
In Coleoidea, the penis in the males is a long and muscular end of the gonoduct used to transfer spermatophores to a modified arm called a hectocotylus. That in turn is used to transfer the spermatophores to the female. In species, where the hectocotylus is missing, the penis is long and able to extend beyond the mantle cavity and transfers the spermatophores directly to the female.
Cephalopoda is an ancient and successful group, including some of the dominant large marine predators during various periods in geological history (Young et al. 1996). Cephalopods appeared 500 million years ago during the late Cambrian and were dominant and diverse during the Paleozoic and Mesozoic eras. Tommotia, a basal cephalopod, had squid-like tentacles but also a snail-like foot it used to move across the seabed. Early cephalopods were at the top of the food chain.
The ancient (cohort Belemnoidea) and modern (cohort Neocoleoidea) coleoids, as well as the ammonoids, all seemed to have diverged from the external shelled nautiloid during the middle Paleozoic era, between 450 and 300 million years ago. While Nautiloid was dominant in the Paleozoic, ammonites were dominant during the Mesozoic.
Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous.
|Phanerozoic eon (542 mya - present)|
|Paleozoic era||Mesozoic era||Cenozoic era|
Wood (2001) reports that 786 distinct living species of cephalopods have been identified. However, there are often new species of cephalopods discovered or delineated, and taxonomic changes made. Brune (2004) estimates there may be as many as 1,000 to 1,200 living species.
There are many more fossil species identified. It is estimated there are around 11,000 extinct taxa of cephalopods (Ivanov et al. 2001).
The classification as listed here is one possible arrangement. The three subclasses (one extinct) are traditional, corresponding to the three orders of cephalopods recognized by Bather (1888b). The first mention of Coleoidea appears in Bather (1888a). Parentheses indicate extinct groups.
Other classifications differ, primarily in how the various decapod orders are related, and whether they should be orders or families.
Young et al. (1996) report that the classification of modern cephalopods is "unstable," with various authorities suggesting "highly variable" arrangements. The following is the classification derived from their taxonomic scheme. Note that Order Oegopsida and Order Myopsida are sometimes recognized as suborders Oegopsina and Myopsina within the Order Teuthida (squid) in some schemes.
Class Cephalopoda (Cuvier, 1795)
Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms.
Class Cephalopoda (Cuvier, 1795)
Another recent system divides all cephalopods into two clades. One includes nautilus and most fossil nautiloids. The other clade (Neocephalopoda or Angusteradulata) is closer to modern coleoids, and includes belemnoids, ammonoids, and many orthocerid families. There are also stem group cephalopods of the traditional Ellesmerocerida that belong to neither clade (Berthold & Engeser, 1987; Engeser 1997).
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