Marine Biology

Introduction

The organisms occurring in the ocean are generally divided into pelagic and benthic organisms based on their selection of habitats.

Pelagic organisms

     Pelagic organisms are those that live or occupy the pelagic realm of the ocean (living in the water column). They represent only two percent of marine species. The pelagic zone can be divided into photic and aphotic zones, a distinction that is especially important for photosynthetic organisms. The photic zone is the shallower part of the ocean that receives enough sunlight to support photosynthesis. This zone is about two hundred meters deep in the clearest waters, and as shallow as three meters in turbid coastal waters. The aphotic zone is where there is no light to support photosynthesis, and extends from the bottom of the photic zone to the ocean floor.

       Pelagic organisms include plankton, which float along with currents, and nekton, which are active swimmers. 

The marine habitat is said to be the ‘mother of life’ as the earliest known life forms are marine. The average depth of oceans is about 3,800 meters, which means that they represent 99 percent of the living space on the planet. Despite having 99 percent of the planet’s living space, only 250,000 of approximately 1.8 million described living species (14 percent) are marine. While the oceans lack diversity at the level of species, they are home to members of thirty-one of the thirty-four animal phyla, about twice the number of phyla that are found on land or in freshwater. Because of its vastness and humans’ inability to easily visit deep waters, the oceans remain the least studied habitats on earth.