193
curved around the mouth and movable arms. Their bodies may be spheroid or flat.
There are fins on the sides of the body. On the abdominal side there is a mantle cavity consisting of strong mantle muscles where two gills are located. Cephalopods
swim backwards with the help of water sprinkled from the siphon of the abdomen
located behind the head (Wikipedia 2019j; Atay 1997). Cephalopods occupy most
of the depth of the ocean, from the abyssal plain to the sea surface. Their diversity
is greatest near the equator and decreases towards the poles (Wikipedia 2019k). The
class contains two, only distantly related, extant subclasses: Coleoidea, which
includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus
and Allonautilus. Members of the Cephalopoda class belong to the families Sepiidae,
Sepiolidae, Loliginidae, Ommastrephidae and Octopodidae (Atay 1997).
They have separate sexes and direct development (without a truly larval stage);
they are carnivores or (more rarely) scavengers; and some cephalopods constantly
swim or hover in mid-water, whereas others stay close to the bottom, and some bury
in sandy substrata to remain immobile for large parts of the day (thus hiding from
predators along with minimizing energy expenditures) (Boletzky and Villanueva
2014). Cephalopods are widely regarded as the most intelligent of the invertebrates
and have well developed senses and large brains (larger than those of gastropods).
The nervous system of cephalopods is the most complex of the invertebrates and
their brain-to-body-mass ratio falls between that of endothermic and ectothermic
vertebrates. The brain is protected in a cartilaginous cranium. Some cephalopods
can fly for distances of up to 50 m. While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion; water continues to
be expelled from the funnel while the organism is in the air. One species, Todarodes
pacificus, has been observed spreading tentacles in a flat fan shape with a mucus
film between the individual tentacles while another, Sepioteuthis sepioidea, has
been observed putting the tentacles in a circular arrangement. Cephalopods have
advanced vision, can detect gravity with statocysts, and have a variety of chemical
sense organs. Octopuses use their arms to explore their environment and can use
them for depth perception. Most cephalopods rely on vision to detect predators and
prey, and to communicate with one another. Consequently, cephalopod vision is
acute: training experiments have shown that the common octopus can distinguish
the brightness, size, shape, and horizontal or vertical orientation of objects. The
morphological construction gives cephalopod eyes the same performance as sharks;
however, their construction differs, as cephalopods lack a cornea, and has an everted
retina. The gills of cephalopods are supported by a skeleton of robust fibrous proteins. The lack of mucopolysaccharides distinguishes this matrix from cartilage
(McKenzie and Parker 2019).
Although there is a long fossil record of many different groups, all living cephalopods belong to two ‘subclasses’ the Coleoidea, which includes the major groups
known as squids, cuttlefishes sensulato, octopods and vampires, and the Nautiloidea,
containing two genera, Nautilus and Allonautilus, the only surviving cephalopods
with an external shell. FAO’s fishing catalogue published in 1984 states that the total
number of living species of cephalopods is less than 1000 distributed in 43 families
(Roper et al. 1984). In the FAO’s 2010 fisheries catalogue, the number of commercial
3.2 Cephalopods
curved around the mouth and movable arms. Their bodies may be spheroid or flat.
There are fins on the sides of the body. On the abdominal side there is a mantle cavity consisting of strong mantle muscles where two gills are located. Cephalopods
swim backwards with the help of water sprinkled from the siphon of the abdomen
located behind the head (Wikipedia 2019j; Atay 1997). Cephalopods occupy most
of the depth of the ocean, from the abyssal plain to the sea surface. Their diversity
is greatest near the equator and decreases towards the poles (Wikipedia 2019k). The
class contains two, only distantly related, extant subclasses: Coleoidea, which
includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus
and Allonautilus. Members of the Cephalopoda class belong to the families Sepiidae,
Sepiolidae, Loliginidae, Ommastrephidae and Octopodidae (Atay 1997).
They have separate sexes and direct development (without a truly larval stage);
they are carnivores or (more rarely) scavengers; and some cephalopods constantly
swim or hover in mid-water, whereas others stay close to the bottom, and some bury
in sandy substrata to remain immobile for large parts of the day (thus hiding from
predators along with minimizing energy expenditures) (Boletzky and Villanueva
2014). Cephalopods are widely regarded as the most intelligent of the invertebrates
and have well developed senses and large brains (larger than those of gastropods).
The nervous system of cephalopods is the most complex of the invertebrates and
their brain-to-body-mass ratio falls between that of endothermic and ectothermic
vertebrates. The brain is protected in a cartilaginous cranium. Some cephalopods
can fly for distances of up to 50 m. While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion; water continues to
be expelled from the funnel while the organism is in the air. One species, Todarodes
pacificus, has been observed spreading tentacles in a flat fan shape with a mucus
film between the individual tentacles while another, Sepioteuthis sepioidea, has
been observed putting the tentacles in a circular arrangement. Cephalopods have
advanced vision, can detect gravity with statocysts, and have a variety of chemical
sense organs. Octopuses use their arms to explore their environment and can use
them for depth perception. Most cephalopods rely on vision to detect predators and
prey, and to communicate with one another. Consequently, cephalopod vision is
acute: training experiments have shown that the common octopus can distinguish
the brightness, size, shape, and horizontal or vertical orientation of objects. The
morphological construction gives cephalopod eyes the same performance as sharks;
however, their construction differs, as cephalopods lack a cornea, and has an everted
retina. The gills of cephalopods are supported by a skeleton of robust fibrous proteins. The lack of mucopolysaccharides distinguishes this matrix from cartilage
(McKenzie and Parker 2019).
Although there is a long fossil record of many different groups, all living cephalopods belong to two ‘subclasses’ the Coleoidea, which includes the major groups
known as squids, cuttlefishes sensulato, octopods and vampires, and the Nautiloidea,
containing two genera, Nautilus and Allonautilus, the only surviving cephalopods
with an external shell. FAO’s fishing catalogue published in 1984 states that the total
number of living species of cephalopods is less than 1000 distributed in 43 families
(Roper et al. 1984). In the FAO’s 2010 fisheries catalogue, the number of commercial
3.2 Cephalopods