23. The Animal Kingdom


23.5. Marine Lifestyles


All groups of animals originated in the ocean, and most still call the oceans home. Marine animals exhibit several kinds of lifestyles.



Zooplankton is a mixture of different kinds of small animals that drift with currents and feed on phytoplankton and other zooplankton. Crustaceans make up approximately 70% of the zooplankton. These include copepods, krill (small, shrimplike organisms), and other kinds of crustaceans. In addition, jellyfish and comb jellies are common. Other kinds of zooplankton include protozoa, several kinds of worms, a group of mollusks, and the larvae (immature, free-living offspring) of many animals, including fish and primitive chordate relatives.



Nekton includes many kinds of aquatic animals that are large enough and strong enough to be able to swim against currents and tides and go where they want to. They are carnivores that feed either on plankton or other nekton. Like the zooplankton, this is a diverse group of organisms, including some actively swimming jellyfish, squid and cuttlefish, shrimp, sharks, bony fish, turtles, sea snakes, birds (penguins, cormorants, and other diving birds), and several kinds of mammals (whales, porpoises, seals, sea otters, walruses, and so on).


Benthic Animals

A major ecological niche in the oceans includes large numbers of bottom-dwelling (benthic) organisms. Among the benthic organisms are such animals as sponges, corals, and sea anemones; segmented worms; clams and snails; lobsters, crabs, and shrimp; starfish and sea urchins; and many kinds of fish. Benthic marine organisms fall into two general categories: those that move about in search of food and those that are filter feeders. Common animals that move about the bottom are crabs, many worms, fish, starfish and some mollusks.

Filter feeders are sessile—permanently attached to the substrate. They feed by creating water currents with cilia or appendages that bring food to them. Sponges, corals, tube worms, clams, barnacles, and crinoids are examples of filter feeders.

Sexual reproduction presents special problems for sessile animals because they cannot move to find mates. However, because they are in an aquatic environment, the sperm can swim to the egg and fertilize it. The fertilized egg develops into a larval stage—the juvenile stage (figure 23.9). The larvae are usually ciliated or have appendages that enable them to move, even though the adults are sessile. The free-swimming larval stages allow the animal to disperse through its environment.



FIGURE 23.9. The Life Cycle of an Oyster

Each oyster can be either male or female at different times in its life. Sperm are released and swim to the egg. Fertilization results in the formation of a free-swimming larva. This larva undergoes several changes during the first 12 to 14 days and eventually develops into an immature oyster, which becomes attached and develops into the adult oyster.


The larva differs from the sessile adult not only because it is free-swimming but also because it usually uses a different source of food and often becomes part of the plankton community. The larval stages of most organisms are subjected to predation, and the mortality rate is high. The larvae move to new locations, settle down, and develop into adults.

Even many marine animals that do not have sessile stages produce free-swimming larvae. For example, crabs, starfish, and eels move about freely and produce free-swimming larvae.



12. What is a sessile filter feeder?

13. How do plankton and nekton organisms differ?

14. What is a benthic organism?