Cephalopoda
Morphological characteristics of planktonic Cephalopoda

The body of coleoid cephalopods is divided into the head, with extremities, and the body covered by the mantle (Ce 1E). The dorsal mantle length (ML), from anterior to posterior end along the dorsal midline, is the standard measurement of cephalopods. In octopuses lacking a clear boundary between head and mantle, the ML is measured from the midpoint of a line connecting the centers of the eyes.

The head bears two large, well developed eyes (Ce 1E , Ce 1K , Ce 1M) which may be covered by a transparent skin (Sepiida except Spirulidae, Myopsida, Octopoda), or freely communicate with the environment (Spirulidae, Oegopsida, Vampyromorpha). The extremities (8 arms in octopods, 8 arms and 2 tentacles in squids and cuttlefishes) are located on the anteriormost body end as a crown around mouth (Ce 1E). Arms bear suckers along their entire length; in some adult squids some suckers may be transformed into hooks. Tentacles consist of a more or less cylindrical stem and a widened and flattened club (Ce 1C, Ce 1S). Only the club, in adulthood, bears suckers, and sometimes hooks. Larvae are always devoid of hooks. The suckers in squids and cuttlefishes are seated on short thin stalks and equipped with a horny ring, usually with a dentate rim. In vampyromorphs and octopods there are no stalks or horny ring. Arms and tentacles of squids and some cuttlefishes bear, on the outer sides, longitudinal skin keels (swimming keels), and on the inner sides, besides suckers, thin protective membranes supported by muscular trabeculae (Ce 1C, Ce 1O). In some cephalopods the arms are joined by a film (umbrella) along their bases (Histioteuthis bonnellii 4). The arms are numbered from the first to the fourth pair beginning from the dorsalmost pair. Tentacles in squids and cuttlefishes are located between the third (ventrolateral) and the fourth (ventral) pairs (Ce 1E). In some species one or both arms of mature males are modified into a section called the hectocotylus (Ce 1B,Ce 1J, Ce 1N). Some, mostly deepwater, squids lose their tentacles at the end of the larval stage, or during their juvenile or maturing stages, but the larvae always have tentacles.

The mouth, with a parrot-like beak, is located in the centre of the crown of extremities. A funnel connecting the mantle cavity with the environment is located under the head (Ce 1E). The mantle is connected with the head and funnel by three locking cartilages: one nuchal (occipital) and two mantle/funnel ones (Ce 1E , Ce 1G). In Octopoda the mantle is fused with both head and funnel, leaving only one ventral split under the funnel, or two splits on the lateral sides of the head (Amphitretidae). In the squid family Cranchiidae the mantle is also fused but leaves three splits, on the lateral and ventral sides of the head. In some Sepiolidae, and in the squid family Promachoteuthidae, the mantle is fused with the head in the occipital region only, and in postlarval squids Grimalditeuthis (fam. Chiroteuthidae) it is fused with the funnel. The funnel-locking cartilage is a cartilaginous pad with variously shaped furrows, grooves, or pits. It may be simple, with straight or slightly curved furrow, widened posteriorly, or of more complex form: U-like, ear-shaped, triangular etc.; the mantle-locking cartilage is a cartilaginous ridge, knob or swelling that locks into a corresponding furrow in the funnel cartilage and thus looks like its negative. A glandular funnel organ (Ce 1H) is located inside the funnel, on its dorsal and lateral surfaces. It consists of a medial component (in the form of an inverted V) and two lateral components. The medial components may bear ridges and papillae, important features in the taxonomy of some squids (Histioteuthidae, Cranchiidae). Its form (W, VV or IIII-like) is also important in the taxonomy of octopods. The funnel valve, a semilunar muscular flap, is located on the dorsal funnel surface near its opening; it may be absent in some squids.

A pair of fins is located on the posterior end of the body (absent in incirrate octopods). The juveniles of Vampyromorpha have two pairs of fins at certain developmental stages, one larval (which disappears during growth), and one definitive (Vampyroteuthis infernalis 4 , (Ce 1E).

The inner shell (gladius, or pen, Ce 1D), which may be calcified (Spirulidae, Sepiidae) or chitinous, is located under the mantle on the dorsal side. It is extremely reduced or lacking in Octopoda, some Sepiolidae and allied families, and in the Promachoteuthidae.

The tissues encircling the mouth in squids and cuttlefishes are connected with the arm bases by the buccal membrane, with ray-like buccal connectives, the rays numbering 6, 7, or 8 (Ce 1A). Connectives leading to the arms of the 1st and 2nd pairs are attached to the dorsal arm surfaces, those of the 3rd pair to the ventral ones (except in the enoploteuthid genus Enigmoteutis), and those of the 4th pair can connect either dorsally or ventrally. The last character is taxonomically important. Among squids the buccal connectives are attached to the dorsal sides of the 4th pair of arms in 8 families, and to their ventral sides in 16 families.

Many cephalopods, particularly squids and Vampyromorpha, have photophores, sometimes very numerous, complicated and variable in structure. Their distribution on the body is strongly specific and may be a very valuable (sometimes almost the only) taxonomic character. During ontogeny some photophores, or groups of photophores, develop at different times (i.e., at different body sizes), and this may be useful for identification.

The color of cephalopods is determined by chromatophores (Ce 1V, Ce 1WX) which may be black or brown, red, orange, or yellow. Their distribution on the body of larvae is sometimes stable and characteristic, but light-colored chromatophores fade quickly in formalin and become indistinguishable.

Ontogenetic changes in cephalopods proceed continuously and gradually, without sudden metamorphosis. In some genera the larvae resemble small adults (Loligo, Bathyteuthis, Japetella). In others, however, they are very different, and ontogeny includes stages of short and rapid restructuring which may be accompanied by rejection or reduction of some provisional larval structures (Gonatopsis, many Chiroteuthidae). In general, larvae differ from juveniles and subadults by their short and narrow fins, short arms, sometimes by the absence of some arms in newborn larvae, by lack of arm and tentacular hooks and of photophores, and by presence of stalked eyes, arm crown pillar, etc.

In the South Atlantic there are representatives of 3 out of the 5 families of Sepiida, as well species of the families of Teuthida, Vampyromorpha and Octopoda. Species of all families of coleoid cephalopods may be caught in the plankton, at least in their larval stages, except Cirrata, although some of these peculiar animals may be caught as juveniles or adults in deep tows with large trawls (not considered in this review). Representatives of three strictly benthic subfamilies are lacking in the plankton: Rossiinae (Sepiida, Sepiina, Sepiolidae), Bathypolypodinae and Graneledoninae (Octopoda, Incirrata, Octopodidae).

The commonest planktonic cephalopods are oceanic squids (Oegopsida), primarily of:
Family Enoploteuthidae,
Family Onychoteuthidae,
Family Histioteuthidae,
Family Ommastrephidae, and
Family Cranchiidae.

Representatives of the planktonic octopod family Bolitaenidae are rather common in deep tows (meso- and bathypelagic).

Planktonic stages are known for most but not all genera and species. Some families known to occur in the South Atlantic, such as the Psychroteuthidae, Lepidoteuthidae and Walvisteuthidae, are omitted from this review because their larvae and juveniles remain to be discovered.