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3.2.1 Squids
3.2.1.1 General Information About Squids
Biology of Squids
Squid are cephalopods in the superorder Decapodiformes with elongated bodies,
large eyes, eight arms and two tentacles. Like all other cephalopods, squid have a
distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like
octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen,
made of chitin. Squids are soft-bodied molluscs whose forms evolved to adopt an
active predatory lifestyle. The head and foot of the squid are at one end of a long
body, and this end is functionally anterior, leading the animal as it moves through
the water. A set of eight arms and two distinctive tentacles surround the mouth; each
appendage takes the form of a muscular hydrostat and is flexible and prehensile,
usually bearing disc-like suckers (Wikipedia 2019k).
The body of the squid is covered with a thick mantle of strong muscles. The shell
is embedded in the upper part of the mantle and is pen shaped. It has a pair of eyes
on the sides of the head. There are eight arms of the same size around the mouth and
two longer arms, also called catchers. There are two rows of suction cups in the
short arms. The forward or backward movement of the squid occurs when the swimming siphon changes direction. Movement is provided by the strong discharge of
water taken from the edge of the head through the siphon. It can change its colour
in less than a second by the contraction of black, red, and yellow colour cells (pigment) under the skin. It catches its prey with tentacles and arms. Digestion begins
in the mouth. The nutrient, which is broken down by a pair of jaws and radula, is fed
through the cylindrical esophagus into the stomach. The intestine where the stomach is opened is thrown out through the anus behind the siphon. A pair of gills
located behind the mantle cavity is connected to the heart and kidney (nephridium),
which consists of a pair of atria and a ventricle (Anonymous 2011).
Squids are major predators in the world’s oceans. Generally thought of as predators of micronekton, they can also act as top predators (Young et al. 2013).
Distribution of Squids
Squids occur in almost all marine habitats of the world. Salinity is a limiting factor
in squid’s distribution. They are generally restricted to salinity concentrations
between 27 and 37‰. However, Lolliguncula brevis, which lives and reproduces in
waters of 17‰, demonstrates a capacity for a higher degree of salinity tolerance.
Some species in habit the Red Sea and the southern coasts of the Iberian Peninsula,
where the salinity is higher than 37‰ and other species have been found in waters
where salinity ranges between 25 and 18‰. The habitat depth range extends from
the intertidal to over 5000 m. Many species of oceanic squids undergo diel vertical
3.2 Cephalopods
3.2.1 Squids
3.2.1.1 General Information About Squids
Biology of Squids
Squid are cephalopods in the superorder Decapodiformes with elongated bodies,
large eyes, eight arms and two tentacles. Like all other cephalopods, squid have a
distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like
octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen,
made of chitin. Squids are soft-bodied molluscs whose forms evolved to adopt an
active predatory lifestyle. The head and foot of the squid are at one end of a long
body, and this end is functionally anterior, leading the animal as it moves through
the water. A set of eight arms and two distinctive tentacles surround the mouth; each
appendage takes the form of a muscular hydrostat and is flexible and prehensile,
usually bearing disc-like suckers (Wikipedia 2019k).
The body of the squid is covered with a thick mantle of strong muscles. The shell
is embedded in the upper part of the mantle and is pen shaped. It has a pair of eyes
on the sides of the head. There are eight arms of the same size around the mouth and
two longer arms, also called catchers. There are two rows of suction cups in the
short arms. The forward or backward movement of the squid occurs when the swimming siphon changes direction. Movement is provided by the strong discharge of
water taken from the edge of the head through the siphon. It can change its colour
in less than a second by the contraction of black, red, and yellow colour cells (pigment) under the skin. It catches its prey with tentacles and arms. Digestion begins
in the mouth. The nutrient, which is broken down by a pair of jaws and radula, is fed
through the cylindrical esophagus into the stomach. The intestine where the stomach is opened is thrown out through the anus behind the siphon. A pair of gills
located behind the mantle cavity is connected to the heart and kidney (nephridium),
which consists of a pair of atria and a ventricle (Anonymous 2011).
Squids are major predators in the world’s oceans. Generally thought of as predators of micronekton, they can also act as top predators (Young et al. 2013).
Distribution of Squids
Squids occur in almost all marine habitats of the world. Salinity is a limiting factor
in squid’s distribution. They are generally restricted to salinity concentrations
between 27 and 37‰. However, Lolliguncula brevis, which lives and reproduces in
waters of 17‰, demonstrates a capacity for a higher degree of salinity tolerance.
Some species in habit the Red Sea and the southern coasts of the Iberian Peninsula,
where the salinity is higher than 37‰ and other species have been found in waters
where salinity ranges between 25 and 18‰. The habitat depth range extends from
the intertidal to over 5000 m. Many species of oceanic squids undergo diel vertical
3.2 Cephalopods