The most distictive feature of decapods is the modification of the fourth pair of arms into tentacles (see title photographs). In some species these are long slender structures that are several times the length of the body and probably function much like a fishing line. In some others, they are short, muscular structures that can be quickly extended (literally shot forward) to grasp prey then completely retracted into pockets in the head.
The Decapodiformes includes species with a wide range of body forms and habits. Muscular sepioids bury in the sand during the day; gelatinous squids swim via slow jet propulsion in the deep-sea while in the same habitat some squids with weak mantle muscles use large muscular fins for swimming; surface-dwelling squid with powerful jet propulsion may undertake migrations of a thousand miles and, when disturbed, dart from the water and glide over the ocean surface like a flying fish. In size they range from Idiosepius with a mantle length of 8 mm at maturity to the giant squid Architeuthis which reaches nearly 18 m in total length. Cephalopods are primarily visual animals even though they possess lateral line analogs, very low frequency hearing and olfaction. The dominance of the visual system is particularly true of decapods. For example, the eye diameter of the small Iridoteuthis is over 40% of the mantle length and large squids have eyes that surpass the eye-size of all other animals big or small. Among decapods, the oegopsid squids characteristically produce small pelagic eggs and the paralarvae develop in the near-surface plankton whereas myopsids and sepioids have benthic eggs that often produce large demersal hatchlings.
The suckers of all decapods have horny rings. These rings often carry sharp claw-like teeth and in some species these have been modified into hooks. The advantages of hooks are uncertain. They could be designed for sinking into soft-bodied prey, such as other squid, or they could act as grappling hooks for hard-body spiny prey where suckers would be ineffective (Young, et al., 1999).
Figure. Oral-lateral view of a portion of the tentacular club of a preserved Ancistrocheirus lesueurii showing hooks with sheaths. Photograph by R. Young.
A large number of fossil species possessed a gladius but lacked a phragmocone. They have been included by various authors in the Decapodiformes and/or the Vampyromorpha. Resolution of their relationships is badly needed (Young, et al., 1999).
Arms IV modified as tentacles (see title photographs).
Suckers bilaterally symmetrical.
Suckers with horny rings.
Suckers stalks with constricted necks.
Figure. Arm suckers. Left - Oral view of arm suckers of Sthenoteuthis oualaniensis, preserved. The reddish, toothed portion is the inner horny ring; the yellow surrounding slats form the outer horny ring. Right - Lateral view of an arm sucker and stalk of Histioteuthis hoylei, preserved. The skin has been removed from the sucker stalk to show its shape: a broad base tapering to a constricted neck where the stalk attaches to the sucker. Photographs by R. Young.
Arms IV without connecting web.
Buccal crown present.
Figure. Oral view of the brachial (=arm) crown surrounding the buccal crown of Histioteuthissp. Arrow points to one of seven buccal supports of the buccal crown.
Statocyst with single capsule.
Superior and posterior buccal lobes widely separated.
Inferior frontal lobe system of brain absent.
Photosensitive vesicles lie within cephalic cartilage.
Fins present but without cartilagenous core.
Fin cartilage present at base of fins often as a plate.
Shell a phragmocone, gladius or absent.
Nidamental glands present.
Oviducal glands bilaterally symmetrical.
Digestive gland duct appendages surrounded by nephridial coelom.
Nephridial coeloms joined to form a single coelom.
Visceropericardial coelom extensive.
Comparisons of the major taxa within the Decapodiformes
At present we recognize four major lineages within the Decapodiformes but the relationships between them are unresolved. Two additional taxa, Bathyteuthoidea and Idiosepiidae, which have subordinal rankings are included here as their ordinal relationships are uncertain. The Bathyteuthoidea shows relationships with both the Oegopsida and the Myopsida while the Idiosepiidae shows relationships with the Myopsida and the Sepioidea. Among the orders, the Myopsida shows relationships with both the Oegopsida and the Sepioidea and the Sepioidea and the Spirulida show relationships.
Among the latter two orders, the closeness of the relationship is uncertain. The Sepiidae of the Sepioidea and the Spirulidae of the Spirulida have substantial fossil records and these records argue for a close relationship between these two orders. In both cases, the presence of an internal calcareous shell with a phragmocone, however, represents the retention of an ancient and plesiomorphic character state, and does not indicate that the animal is an early derivative of the decapod clade or close relationships. The details of the shell structure do support a close relationship.
Spirula spirula, the only living member of the Spirulida, is a peculiar cephalopod that has stimulated much discussion regarding its phylogenic position. While the relationships are still unresolved, Young, et al. (1998) support its close relationship to the Sepiidae and other sepioid families based on arguments mostly put forward by Naef (1922). A cladistic study based on morphology has not been made.
Figure. Drawings from Young et al. (1998) modified from Naef (1922) show the similarities between Spirulirostra,Spirulirostrina(fossil relatives of Spirula) and Belosaepia(a fossil relative of Sepia) and Sepia. Note the gray-colored "guard-like sheath" on all species illustrated (it forms the rostrum, shield,etc. on the Sepiacuttlebone). The cuttlebone of Belosaepialacks the cross-struts of the Sepiacuttlebone and has distinct septa on the ventral side of the siphuncle. The siphuncle is also more ventrally curved than in Sepia. The presumed attachment of the mantle muscle (red) to the ventral process of the sheath is shown in Spirulirostraand Spirulirostrina. The mantle attaches similarly to the recurved ventral process of the cuttlebones.
Both the spirulid and sepiid shells have a distinctive ventral curvature of the apical end and the asymmetrical elaboration of the sheath that includes such similarities as a rostrum and a ventral process (Young, et al., 1998).