Marine Biology

Invertebrates

Molluscs :  Nektonic molluscs include octopus and squid. Unlike clams and oysters, squids  have no external shells.They have a shell inside their bodies called a pen. There are about 375 species of squids. They possess 10 arms, of which two are longer than the others and are called tentacles. Squid range in size from less than  an inch to more than 60 feet in length. They have long, tubular bodies and short heads. They swim  very fast using a kind of jet propulsion to move. Squids have some unique adaptations. Some can change colour, some use bioluminescence to create light, and some shoot ink to cloud the water and escape from predators. They usually travel in groups and can be found in the euphotic zone and the twilight zone. 

           There are two large groups of octopus. The cirrata or finned octopuses live in the deep sea at depths  between 1000 and 24,000 feet. About 85% of octopuses are in the incirrata group. They have no fins and live in shallow water in caves or crevices. Octopuses have no shell at all, not even an inner one. They have eight tentacles. The tentacles have suction cups on them and are used to hold onto prey. The tentacles also have taste sensors that let the octopus to know  whether  grabbed  ones are  worth eating.  It has a sharp beak on its mouth which is used for cracking  shells. Some species may also inject prey with a toxic substance. Because it has no shell, an octopus can squeeze into very small spaces. Octopuses live alone and, like the squid, some species can shoot ink and change colours. 

ii. Arthropods:   Among arthropods, shrimps are the major nektonic organisms.