Cephalopoda

Octopuses swim headfirst, with arms trailing behind

While all cephalopods can move by jet propulsion, this is a very energy-consuming way to travel compared to the tail propulsion used by fish.^[18] The relative efficiency of jet propulsion decreases further as animal size increases. Since the Paleozoic, as competition with fish produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with fins and tentacles used to maintain a steady velocity.^[3] The stop-start motion provided by the jets, however, continues to be useful for providing bursts of high speed - not least when capturing prey or avoiding predators.^[3] Indeed, it makes cephalopods the fastest marine invertebrates,^[4]:Preface and they can out-accelerate most fish.^[15] Oxygenated water is taken into the mantle cavity to the gills and 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.^[19]

Some octopus species are also able to walk along the sea bed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of muscle around the mantle.

While most cephalopods float (i.e. are neutrally buoyant; in fact most cephalopods are about 2-3% denser than seawater^[5]), they achieve this in different ways.^[18] Some, such as Nautilus, allow gas to diffuse into the gap between the mantle and the shell; others allow purer water to ooze from their kidneys, forcing out denser salt water from the body cavity;^[18] others, like some fish, accumulate oils in the liver;^[18] and some octopuses have a gelatinous body with lighter chlorine ions replacing sulfate in the body chemistry.^[18]

Shell

Cuttlebone of Sepia officinalis

Nautiluses are the only extant cephalopods with a true external shell. Cuttlefish, squid, spirula, and cirrate octopuses have small internal shells. The majority of octopuses – those in the suborder most commonly known, Incirrina – have almost entirely soft bodies with no vestige of an internal shell.

Females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal.

The largest group of shelled cephalopods, the ammonites, are extinct, but their shells are very common in certain areas as fossils.

A giant squid found in Logy Bay, Newfoundland, in 1873. The two long feeding tentacles are visible on the extreme left and right.

Head Appendages

Cuttlefish and squid have five pairs of muscular appendages surrounding their mouths. The longer two, termed tentacles, are actively involved in capturing prey;^[20]:225 they can lengthen rapidly (in as little as 15 milliseconds^[20]:225). In giant s quid they may reach a length of 8 metres. They may terminate by broadening into a sucker-coated club.^[20]:225 The shorter four pairs are termed arms, and are involved in holding and manipulating the captured organism.^[20]:225 They too have suckers, on the side closest to the mouth; these help to hold onto the prey.^[20]:226

The tentacle consists of a thick central nerve cord (which must be thick to allow each sucker to be controlled independently)^[21] surrounded by circular and radial muscles. Because the volume of the tentacle remains constant, contracting the circular muscles decreases the radius and permits the rapid increase in length. Typically a 70% lengthening is achieved by decreasing the width by 23%.^[20]:227

The size of the tentacle is related to the size of the buccal cavity; larger, stronger tentacles can hold prey as small bites are taken from it; with more numerous, smaller tentacles, prey is swallowed whole, so the mouth cavity must be larger.^[22]

Feeding

All cephalopods have a two-part beak;^[4]:7 most but not all have a radula.^[4]:7 They feed by capturing prey with their tentacles, drawing it in to their mouth and taking bite-size bites.^[8] They have a mixture of toxic digestive juices, some of which are manufactured by symbiotic algae, which they eject from their salivary glands onto their captured prey held in their mouth. These juices separate the flesh of their prey from the bone or shell.^[8] The salivary gland has a small tooth at its end which can be poked into an organism to digest it from within.^[8]

The digestive gland itself is rather short.^[8] It has four elements, with food passing through the crop, stomach and caecum before entering the intestine. Most digestion, as well as the absorption of nutrients, occurs in the digestive gland, sometimes called the liver. Nutrients and waste materials are exchanged between the gut and the digestive gland through a pair of connections linking the gland to the junction of the stomach and caecum.^[8] Cells in the digestive gland directly release pigmented excretory chemicals into the lumen of the gut, which are then bound with mucus passed through the anus as long dark strings, ejected with the aid of exhaled water from the funnel.^[8]

Excretory System

Most cephalopods possess a single pair of large nephridia. Filtered nitrogenous waste is produced in the pericardial cavity of the branchial hearts, each of which is connected to a nephridium by a narrow canal. The canal delivers the excreta to a bladder-like renal sac, and also resorbs excess water from the filtrate. Several outgrowths of the lateral vena cava project into the renal sac, continuously inflating and deflating as the branchial hearts beat. This action helps to pump the secreted waste into the sacs, to be released into the mantle cavity through a pore.^[23]

Nautilus, unusually, possesses four nephridia, none of which are connected to the pericardial cavities.

Reproduction and Life Cycle

Female Argonauta argo with eggcase and eggs

With a few exceptions, Coleoidea live short lives with rapid growth. Most of the energy extracted from their food is used for growing. The penis in most male Coleoidea 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 tran sfers the spermatophores directly to the female. They tend towards a semelparous reproduction strategy; they lay many small eggs in one batch and die afterwards. The Nautiloidea, on the other hand, stick to iteroparity; they produce a few large eggs in each batch and live for a long time.

External sexual characteristics are lacking in cephalopods. So cephalopods use color communication. A courting male will approach a likely looking opposite number flashing his brightest colors, often in rippling displays. If the other cephalopod is female and receptive, her skin will change color to become pale, and mating will occur. If the other cephalopod remains brightly colored, it is taken as a warning^[24].

The male has a sperm-carrying arm, known as the hectocotylous arm, with which to impregnate the female. In many cephalopods, mating occurs head to head and the male may simply transfer sperm to the female. Others may detach the sperm-carrying arm and leave it attached to the female. In the paper nautilus, this arm remains active and wriggling for some time, prompting the zoologists who discovered it to conclude it was some sort of worm-like parasite. It was duly given a genus name Hectocotylus, which held for some time until the mistake was discovered^[25].

The eggs may be brooded, the female either making a shelter for them as in the paper nautilus, or else laying them under rocks and aerating them until they hatch. Often though, the eggs are left to their own devices, as for example in squids, which lay sausage-like bunches of eggs in crevices or occasionally on the sea floor. Cuttlefish lay their eggs separately in cases and attach them to coral or algal fronds.^[26] Fossilised egg clutches show that ammonites also laid clutches of eggs.^[27]

Cephalopods are occasionally long-lived, especially in the deep water or polar forms, but most of the group live fast and die young, maturing rapidly to their adult size. Some may gain as much as 12% of their body mass each day.^[8] Most live for one to two years,^[8] reproducing and then dying shortly thereafter.^[28]

In order to free up resources for reproduction, many squid are known to resorb the muscle tissue of their mantle and tentacles, breaking down the tissue and using the energy contained therein to produce more gametes.^[29]

Embryology

Unlike most other molluscs, cephalopods do not have a distinct larval stage. The fertilised ovum initially divides to produce a disc of germinal cells at one pole, with the yolk remaining at the opposite pole. The germinal disc grows to envelop and eventually absorb the yolk, forming the embryo. The tentacles and arms first appear at the hind part of the body, where the foot would be in other molluscs, and only later migrate towards the head. ^[23][30]

The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the oppo site surface.^[31]:86 The early embryological stages are reminiscent of ancestral gastropods and extant monoplacophora.^[30]

Evolution

The class developed during the Late Cambrian, and underwent pulses of diversification during the Ordovician period^[32] to become diverse and dominant in the Paleozoic and Mesozoic seas. Small shelly fossils such as Tommotia were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals,^[33] and the earliest accepted cephalopods date to the Late Cambrian Period.^{[note 1][34]} During the Cambrian, cephalopods are most common in shallow near-shore environments, but they have been found in deeper waters too.^[35] Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid monoplacophoran clade.^[36] However genetic studies suggest that they are more basal, forming a sister group to the scaphopoda but otherwise basal to all other major mollusc classes.^[37] The internal phylogeny of mollusca, however, is wide open to interpretation - see Mollusca#Phylogeny.

Fossil orthoconic nautiloid from the Ordovician of Kentucky; an internal mold showing siphuncle and half-filled camerae, both encrusted.

The cephalopods are thought to have evolved from a monoplacophoran-like ancestor^[38] with a curved, tapering shell,^[39] and to be closely related to the gastropods (snails).^[40] The development of a siphuncle allowed their shells to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separates the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle.^[40] Negative buoyancy (i.e. the ability to float) came later, followed by swimming in the Plectroneocerida and eventually jet propulsion in more derived cephalopods.^[41] However, because chambered shells are found in a range of molluscs - monoplacophora and gastropods as well as cephalopods - a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda.^[39]:57 The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity.^[39]:57

The earliest cephalopod order to emerge was the Ellesmerocerida, which were quite small organisms; their shells were slightly curved, and the internal chambers were closely spaced. The siphuncle penetrated the septa with meniscus-like holes.^[32] Early cephalopods had fine shells which could not cope with the pressures of deep water.^[32] In the mid Tremadoc, these were supplemented by larger shells around 20 cm in length; these larger forms included straight and coiled shells, and fall into the orders Endocerida (with wide siphuncles) and Tarphycerida (with narrow siphuncles).^[32] By the mid Ordovician these orders are joined by the Orthocerids, whose first chambers are small and spherical, and Lituitids, whose siphuncles are thin. The Oncocerids also appear during this time; they are restricted to shallow water and have short conchs which surround the stomach.^[32] The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m.^[32] The wide-siphuncled Actinocerida and the Discocerida both emerged during the Darriwilian.^[32] The direction of coiling would prove to be crucial to the future success of the lineages; endogastric coiling would only permit large size to be attained with a straight shell, whereas exogastric coiling - initially rather rare - permitted the spirals familiar from the fossil record to develop, with their corresponding large size and diversity.^[42] (Endogastric means that the tip of the coiling shell points in the same direction as the funnel; exogastric shells coil the other way, allowing the funnel to be pointed backwards beneath the shell.)^[42]

Early cephalopods were likely predators near the top of the food chain.^[8]

An ammonitic ammonoid with the body chamber missing, showing the septal surface (especially at right) with its undulating lobes and saddles.

The ancient (cohort Belemnoidea) and modern (cohort Neocoleoidea) coleoids, as well as the ammonoids, all diverged from the external shelled nautiloid during the middle Paleozoic Era, between 450 and 300 million years ago,^{[verification needed]} although the coleoids may be polyphyletic.^[20]:289 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. It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters.^[40]:36 However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid and cuttlefish.^{[verification needed]} The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit.^[20]:289 This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure.^[20]:289 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.

The tentacles of the ancestral cephalopod developed from the mollusc's foot;^[30] the ancestral state is thought to have had five pairs of tentacles which surround the mouth.^[30] Smell-detecting organs evolved very early in the cephalopod lineage.^[30]

The earliest cephalopods^{[note 2]}, like Nautilus and some coeloids, appeared to be able to propel themselves forwards by directing their jet backwards.^[20]:289 Because they had an external shell, they would not have been able to generate their jets by contracting their mantle, so must have used alternate methods: such as by contracting their funnels or moving the head in and out of the chamber.^[20]:289

The preservation of cephalopod soft parts is not entirely unusual; soft-bodied fossils, especially of coeloids (squid), are relatively widespread in the Jurassic,^[29] but phosphatised remains are unknown before this period.^[43]

Classification

Chambered Nautilus (Nautilus pompilius)

Common Cuttlefish (Sepia officinalis)

Atlantic Bobtail (Sepiola atlantica)

European Squid (Loligo vulgaris)

Common Octopus (Octopus vulgaris)

The classification as listed here (and on other cephalopod articles) follows largely from Current Classification of Recent Cephalopoda (May 2001), plus fossil groups from several sources. The three subclasses are traditional, corresponding to the three orders of cephalopods recognized by Bather.^[44] Parentheses indicate extinct groups.

Class Cephalopoda

• Subclass Nautiloidea: all cephalopods except ammonoids and coleoids
  • (Order Plectronocerida): the ancestral cephalopods from the Cambrian Period
  • (Order Ellesmerocerida): include the ancestors of all later cephalopods
  • (Order Endocerida)
  • (Order Actinocerida)
  • (Order Discosorida)
  • (Order Pseudorthocerida)
  • (Order Tarphycerida)
  • (Order Oncocerida)
  • Order Nautilida: nautilus and its fossil relatives
  • (Order Orthocerida)
  • (Order Ascocerida)
  • (Order Bactritida): include the ancestors of ammonoids and coleoids
• (Subclass Ammonoidea): extinct ammonites and kin
  • (Order Goniatitida)
  • (Order Ceratitida)
  • (Order Ammonitida): the true ammonites
• Subclass Coleoidea
  • (Cohort Belemnoidea): extinct belemnites and kin
    • (Genus Jeletzkya)
    • (Order Aulacocerida)
    • (Order Phragmoteuthida)
    • (Order Hematitida)
    • (Order Belemnitida)
    • (Genus Belemnoteuthis)
  • Cohort Neocoleoidea
    • Superorder Decapodiformes (also known as Decabrachia or Decembranchiata)
      • (?Order Boletzkyida)
      • Order Spirulida: Ram's Horn Squid
      • Order Sepiida: cuttlefish
      • Order Sepiolida: pygmy, bobtail and bottletail squid
      • Order Teuthida: squid
    • Superorder Octopodiformes (also known as Vampyropoda)
      • Order Vampyromorphida: Vampire Squid
      • Order Octopoda: octopus

Other classifications differ, primarily in how the various decapod orders are related, and whether they should be orders or families.

Shevyrev Classification

Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms.^{[45] [46]}

Class Cephalopoda Cuvier 1795

• Subclass Ellesmeroceratoidea Flower 1950
  • Order Plectronocerida
  • Order Protactinocerida
  • Order Yanhecerida
  • Order Ellesmerocerida
• Subclass Endoceratoidea Teichert, 1933
  • Order Endocerida
  • Order Intejocerida
• Subclass Actinoceratoidea Teichert, 1933
  • Order Actinoceratoidea
• Subclass Nautiloidea Agassiz, 1847
  • Order Basslerocerida
  • Order Tarphycerida
  • Order Lituitida
  • Order Discosorida
  • Order Oncocerida
  • Order Nautilida
• Subclass Orthoceratoidea Kuhn, 1940
  • Order Orthocerida
  • Order Ascocerida
  • Order Dissidocerida
  • Order Bajkalocerida
• Subclass Bactritoidea Shimansky, 1951
• Subclass Ammonoidea Zittel, 1884
• Subclass Coleoidea Bather, 1888^[47]

Cladistic Classification

Pyritized fossil of Vampyronassa rhodanica, a vampyromorphid from the Lower Callovian (164.7 million years ago)

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 ^{[48] [49]}

Monophyly of Coeloids

The coeloids may represent a polyphyletic group.^[20]:289

Post-Mortem Decay

After death, if undisturbed, cephalopods^{[note 3]} decay relatively quickly.^[29] Their muscle softens within a couple of days, and may swell; egg sacs can swell so much that they rip through the mantle.[29] Subsequently, the organs shrink again; at this point the organism may start to break up into fragments. The eyes retain their size while the head shrinks around them. The gills may remain swollen at this point.^[29] After around a week, the carcass collapses in on itself and begins to disintegrate. The ink sac solidifies around this point.^[29] After a fortnight little is left but a blob with eyes, arms and ink sac visible.^[29] After a couple of months, these are only recognisable as flattened dark stains - although in some cases the eye lenses can remain intact for up to a year.^[29]