Cephalopods are renowned for their large brains, well-developed eyes, and complex behavior. Their social organization varies from the solitary life of octopods through small schools of cuttlefish to very large shoals of oceanic oegopsids. Some species of cuttlefishes and squids are known to form seasonal aggregations or gatherings for mating and spawning.

The sexes are separate in cephalopods. Courtship patterns vary from simple male grasping of the female followed by the implanting of spermatophores to complex courtship rituals in which both individuals display elaborate forms of touching prior to mating. Several species produce small "sneaker males" that resemble females; these sneaker males have been shown to be an important alternative to large, dominant mate-guarding males. The mating patterns of most oceanic cephalopods are largely unknown.

In addition to social organization and mating behavior, the use of visual communication among cephalopods has been a subject of debate. Whereas most observers who have watched cephalopods will agree that they perform complex visual signaling, these researchers debate whether such signaling can be considered a language. Cephalopod visual signaling involves a variety of behaviors, some of which are regularly directed toward conspecifics (e.g., mates or potential rivals), and others toward members of other species (e.g., prey or potential predators). The use of ink or other chemicals as an alarm signal to warn conspecifics has been demonstrated but not thoroughly investigated.

As with so many subjects about cephalopods, display patterns have been studied in detail for only a few easily accessible species. In general, these patterns include both acute patterns, lasting for a few seconds to a few minutes; and chronic patterns, lasting several minutes to several hours. The display patterns comprise chromatic (both dark and light), textural, postural, and movement changes. Commonly recorded displays include crypsis (hiding), in addition to what Hanlon and Messenger (1996) refer to as deimatic (threatening, startling, frightening, or bluffing) behaviors and protean (unpredictable or erratic) escape maneuvers.

A squid holds a fish in its tentacles. (Photo by B. Wood. Bruce Coleman, Inc. Reproduced by permission.)

Although some nearshore benthic octopods are known to occupy and defend territories for limited time periods, cephalopods generally are not territorial.

The primary variable in the activity patterns of cephalopods is the 24-hour diel cycle. Some neritic (coastal zone) species forage primarily by daylight, but most are nocturnal. Some species are crepuscular, which means that they are most active at dawn and dusk. Diel activities of oceanic species primarily involve vertical migration. The activity patterns of most deep-sea benthic species are unknown.

Many species of oceanic cephalopods undergo diel vertical migrations, wherein they live at depths of about 1,310-3,280 ft (400-1,000 m) during the day and then ascend into the uppermost 656 ft (200 m) of water during the night. Shifts in distribution over the course of a cephalopod's life history are common. Some oegopsid squids are believed to undertake life-cycle migrations over large geographic distances covering hundreds of miles.

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