Cephalopods are molluscs, but they have many physiological attributes that distinguish them from other molluscs, including predatory lifestyles and sophisticated nervous systems. Historically, cephalopods have been a prolific class, though research is sparse. Today, the class encompasses octopuses, cuttlefish, squid, and nautiluses. Representatives of the class have been found in all the world's oceans.
It is believed that cephalopods evolved from gastropods. So far, more than 10,000 fossils of cephalopod species have been identified in the fossil record. Belemnoids, an extinct species from the Mesozoic era, may be the ancestors of modern squids. Based on fossil record comparisons, it is speculated that there are over eight hundred species of cephalopods that have gone extinct.
Reproduction and Life Cycle
Cephalopods most frequently mate only once, which occurs near the end of their life cycle. Courtship rituals between cephalopods can include a diverse array of color changes and body movements. Male cephalopods have a specialized arm (called a hectocotylus) which is used to transfer spermataphores, or packets of sperm, into the female's genital pore. Sometimes after mating, the hectocotylus is left behind in the female's mantle cavity. Females possess a single oviduct. Females may mate with multiple males and have multiple paternity of offspring, ultimately increasing the genetic diversity of a population.
After being laid by the female, cephalopod eggs are usually covered in a tough coating which helps them attach to hard surfaces. Upon hatching, cephalopods emerge as paralarvae, surpassing a true larval stage. Paralarvae are not morphologically distinct from adults; instead, they are like "miniature adults." Though they are similar in appearance, these paralarvae may occupy different ecological niches from adults. Sexual maturation occurs rapidly, reaching adult size in about one year. Most cephalopods die shortly after mating once, reflective of a monocyclic semelparous life cycle. This uniseasonal breeding can cause extreme fluctuations in cephalopod populations and biomass, as most of a generation will die as a new one arises. These fluctuations are unpredictably variably and inconsistently patterned.
Cephalopods have a buccal cavity, or mouth, that is usually found at the center of their tentacles. Within this mouth is a radula, or a coarse, rasping tongue. Cephalopods are also known for their ink glands, which can expel a dark cloud of melanin through the mantle cavity in the face of a threat. All families of cehpalopods, with the exception of Nautilus, possess cells that contain large amounts of pigments, as well as interacting contractile fibers. Together, these two structures are responsible for the ability of cephalopods to change color. This behavior frequently occurs in response to danger or a change of emotion (e.g. alarm). The majority of cephalopods have reduced or absent calcareous shells, remnants of the prominent shells of their ancestral species. Today, squids and octopuses have completely lost the internal shell and its buoyant properties. Nautiluses have a complete external shell, while cuttlefish have a small, internal "bone." While cephalopods have no true brain, some species exhibit an advanced cluster of ganglionic masses centered around the esophagus, constituting the most complex "brain" of any invertebrate. Cephalopods also have relatively well developed statocysts (gravity and movement senses).
All cephalopods maintain carnivorous diets. At younger ages, cephalopods will most frequently eat macrozooplankton. As they mature, most feed on a wide range of crustacea and fish, consuming up to 15% of their body weight daily. On occasion, cephalopods may exhibit cannibalism on smaller specimens. Prey are usually attacked with a forward strike from the cephalopod and are subsequently pulled toward the buccal cavity by the tentacles. Squid simply bite into their prey with their beaks; octopuses, however, exhibit slightly more complex feeding behaviors. Octopuses often inject a mix of enzymes and toxins into their prey, loosing the tissue for easier extraction (particularly, bivalves and crustaceans).
Cephalopods possess the ability to change colors rapidly. This is most frequently used for crypsis(camouflage), mating displays, and prey/predator response. Jet propulsion and the absence of an external shell has great evolutionary significance because of the enablement of a highly mobile, active lifestyle unique from other molluscs. Cephalopods have a centralized brain with highly developed sense organs. In order to move through water, cephalopods employ jet propulsion, moving water through a ventral funnel to produce maximum velocity in the opposite direction.This is carried out by the muscular mantle of the cephalopod.Coastal octopuses belong to a benthos category and use their arms to move around on the ocean floor. They only occasionally execute jet propulsion. Cephalopods are able to eject ink into their surroundings to confuse predators.
Economically, cephalopods remain an important part of both coastal and inland areas, as many species are eaten by humans. Human consumption of cephalopods dates at least as far back as classical times. Today, as many as 3 million tons are consumed annually by humans, up from about 1 million in the 1970s. Cephalopods are the third most valuable commodity in global fisheries, behind shrimp and tuna.
Cephalopods are an important low level predator in oceanic food chains, as they consume a wide variety of fish, crustaceans, and other invertebrates. In addition to filling a diverse niche as a predator, cephalopods themselves are consumed by fish, marine mammals, and oceanic birds, making them an indispensable food source for tertiary consumers. For example, sperm whales may consume up to 320 million tons of cephalopods annually from pelagic zones.
Human Induced Threats
Cephalopods maintain a relatively high position in trophic feeding levels, placing them at risk for bioaccumulation of toxins. As pollutants are consumed by smaller organisms, the level of toxin exposure is magnified in larger predators. There is also evidence that noise pollution from offshore activities, such as shipping or construction, could damage the sensitive hair fibers and sense organs of cephalopods.Climate change could alter seasonal migration patterns and mating rituals because of ocean warming. Cephalopods could potentially be harmed by overfishing practices, as some cephalopods are considered a human food source or valued for other assets. One of the biggest threats associated with cephalopods is associated with their short life cycles with little overlap of generations to buffer the influence on recruitment of environmental variables. This means that the species lacks resilience to deal with coming environmental changes and human influences. Although cephalopods do not currently have a status as a threatened species, it is speculated that the population of cephalopods is smaller than in the past; however, there is little record from the past to use as a means of comparison.
Common Octopus Octopus vulgaris
The common octopus has a lifespan around twelve to twenty-four months in length. It's geographic range is varied across the world's oceans, including the Eastern Atlantic Ocean. The common octopus can typically be found living in tropical or subtropical and temperate waters. The common octopus is rarely found outside of coastal waters or the upper continental shelf, occupying shallow waters almost exclusively. Specifically, it frequents depths of 100-150 meters. The common octopus reaches about one to three feet in length. It has smooth skin with the absence of an external shell. Mating occurs between male and female members of the species. Females will lay eggs in shallow water, often near reefs. After egg-laying, the female will engage in isolated brooding during which she does not leave her eggs. Shortly after the final embryos hatch, the female will die. The common octopus is solitary and territorial in its behavior. It may occupy the same crevice for days at a time before migrating to a different location. Mostly sedentary creatures, they are mainly nocturnal except for short daytime ventures for food. The common octopus feeds primarily on gastropods and bivalves. During the paralarvae stage, the common octopus consumes plankton, but switches to an adult diet upon reaching a size of approximately 0.2g. At this point in time, there is no specific risks outlined in the conservation status of the common octopus; however, there is a potential for the overfishing of this creature.
Caribbean Reef Octopus Octopus briareus
The Caribbean reef octopus is found in warm, shallow waters, typically near coral reefs. Members of this species are quite reclusive and often reside in lairs near rocks and seagrass. Like other octopuses, the Caribbean reef octopus has eight arms of varying length and diameter. It has a chunky body, and relatively thin arms by comparison. The Caribbean reef octopus has a wide range of color, from brown to green, blue, or reds.It exhibits minimum sexual dimorphism. Like other octopuses, the males of this species have a hectocotylusMales initiate mating, which usually lasts between 30 and 80 minutes. Females can store sperm in oviducts for more than 3 months.Females brood multiple egg clusters of about 25, totaling anywhere between 100-500 eggs.The entire process takes anywhere from fifty to eighty days, but is accelerated in warmer watersThe Caribbean reef octopus then hatches as a miniature adult, capable of swimming, eating, inking, and changing color. Males will reach sexual maturity around 140 days; females will in about 150 days. The Caribbean reef octopus is not a social animal, often fighting over territory with other members of the species in a close vicinity. Standard distance between living spaces is typically about 60ft.The Caribbean reef octopus frequently changes dens, only remaining in one place during the brooding process. As a predatory defense, these octopuses swell their mantle with water before jet-propelling themselves away behind a cloud of ink. Members of this species eat mostly crab, which is the most preferred food source, and they also consume shrimp.In some cases, the Caribbean reef octopus may eat fish and lobsters as well. It hunts in early morning and evening hours. It's current conservation status does not outline any threats or dangers.
Caribbean Two-Spot Octopus The Caribbean two-spot octopus is often found in the Bahamas, Caribbean, and Florida. It typically inhabits shallow coral reefs. It's physical appearance is characterized by a morphology, skin texture, and color similar to that of the Common Octopus. The Caribbean two-spot octopus, however, has distinct blue rings called ocelli that make false eye-markings. Like other octopuses, the Caribbean two-spot octopus has the ability to change colors; however, it's color is typically mottled. Little other information is known regarding this species. 
Caribbean Reef Squid Sepioteuthis sepioidea
The Caribbean reef squid can be found throughout the Caribbean. It lives in clear, shallow waters. Newly hatched organisms will live very close to the shore. Specifically, they can be found anywhere between .2-10m deep and frequently near vegetation. Older specimens are denizens of turtle grass in the shallows near shore. The Caribbean reef squid will often stay as many as two meters deep to avoid predation from birds. Similarly, it avoids the ocean floor to stay away from other predators. At night, it swims to deeper water to engage in hunting. Breeding adults spend their time in coral reefs, reaching depths of no greater than 8m. Non-breeding adults prefer open water, enjoying ample space in depths of up to 100m at night. The adult Caribbean reef squid resembles a cuttlefish. It is less elongated and arrow-shaped, making it more streamlined than other species of squid. Females tend to be slightly larger than males. The color of the Caribbean reef squid is mottled with browns and whites and tans. A white line runs longitudinally on its dorsal side. Brow ridges are found above its eyes. The Caribbean reef squid will die after reproducing once. Females die immediately after laying eggs. Males can mate many times in a small window of time before death. Females do not engage in care for their young. The Caribbean reef squid is a highly social animal. It possesses a complex set of signal communications, involving color changes and posture alterations. It can change the color of their brow ridges and central arms to express alarm. It also can eject ink in open water as a means of evading predation. Younger squid often display bars that make them more closely resemble vegetation, presumably a defense mechanism. The diet of the Caribbean reef squid consists primarily of sardines and other small fish, with the occasional shrimp. Hunting is dependent on its vision. The Caribbean reef squid currently has no specific conservation status, although it is important as a food source for humans. 
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