Is the study of all aspects of ecology and life histories of marine organisms. The primary focus is on the organisms and their local interactions with their physical, chemical, and biological environments. Many marine biologists study the natural history of organisms that live in coastal and inshore environments, and so are more readily accessible and can be used in experimental studies. So the scope of marine biology as a scientific discipline overlaps that of biological oceanography. Biological oceanographers tend to be more concerned about large-scale distribution patterns, how substances like carbon flow through the ocean system and how the living communities are responding to the motion of the ocean and different spatial scales. They often use ‘proxies’, for example using the volume of sound back-scattered echo-sounding to track and quantify fish and plankton, and estimating the quantities of the marine plant called phytoplankton in a water sample by measuring the amount of chlorophyll it contains. Marine biologists are much more concerned with the organisms themselves and their responses to other species, and their chemical and physical environment.
Thus, on a rocky beach the seaweeds and their grazers tend to be vertically zoned as a result of their differing abilities to withstand exposure to the air at low tide. The species also vary in their abilities to withstand being battered by waves, so different types of animal and plants are found on beaches that are either sheltered from, or exposed to, heavy surf. Predation and grazing pressures alter the structure of habitats. For example kelp, the straplike seaweeds that grow profusely in places at, and below, the low tide mark on rocky shores, form complex three-dimensional forests. These forests are full of local microhabitats each of which tends to be inhabited by different types of animals ranging from sea anemones to worms and Crustacea. However, echinoderms like sea urchins (Echinus spp.) can graze down these forests and reduce their diversity. Off California the populations of sea urchins are kept in check by one of the most charismatic of marine mammals, the sea otter. When the sea otter populations crashed, because they were over-exploited for their fur, the sea urchins became so abundant they ate down the kelp forests along many shores.
Starfish (asteroids), another group of echinoderms, are active predators that consume many of the snails that graze down the seaweeds on the shore. They often feed selectively on the most abundant species and as a result the less common species are more successful, and the numbers of species able to live on the shore increases. In experimental areas on a rocky shore from which marine biologists removed all starfish, the populations of grazing snails became far less diverse.
Behaviour can be important to those studying marine biology. For example, the crustacean sandhoppers (Talitrus spp.) that are often found feeding in large numbers on the dead seaweed that piles up along the strandline on sandy beaches lay their eggs in the sand. The young hatchlings have to find their way down to the sea, but how do they know which way to hop? The answer seems to be that they navigate according to the patterns of polarized light in the sky, and the correct way down the beach is passed to the egg as chemical information by the female as she is laying. If a polarizing filter is placed over the eggs to that the sky pattern is reversed, the newly hatched youngsters hop inland.