Marine Biology (2+1)

Benthic organisms (Benthos)

Benthic organisms

Organisms that live in, on, or near the ocean floor are appropriately called benthic organisms. There represent 98 percent of all marine creatures. They occur in such familiar marine habitats as intertidal rocky shores, mud flats, sandy beaches, coral reefs, and kelp forests. The main primary producers in benthic habitats are macroscopic seaweeds that grow attached to the bottom or microscopic algae that grow within the tissues of animals such as corals, sponges, and bryozoans. Benthic animals include mobile creatures such as fish, crabs, shrimp, snails, urchins, sea stars, and slugs. Additionally, there are numerous animals that never move around as adults. These sessile animals include barnacles, sponges, oysters, mussels, corals, gorgonians, chrinoids, hydroids, and bryozoans. The commonness of sessile animals in the  bottom suggests that it is a successful way of life. Their way of life combines facets of plant and animal lifestyles. Sessile invertebrates are plant like in that they obtain some of their energy from sunlight (the animals themselves do not photosynthesize, but they house photosynthetic symbionts), they are anchored in place, and they grow in a modular fashion just as the branches of a tree do. They are animal-like in that they capture and digest prey and they undergo embryonic development, often involving metamorphosis. In fact, nearly all benthic animals start life in the pelagic realm, drifting around as planktonic larva, dispersing to new habitats as they develop and feed. After a few hours to weeks of pelagic living, they sink to the ocean floor to complete life as adults. Being stuck in one place presents special challenges for sessile animals, including food acquisition, predator avoidance, and mating.  Sessile animals feed by having symbiotic algae and by filtering organic particles from passing water currents. Like plants, sessile animals use structural and chemical defenses against predators, and have tremendous regenerative abilities to recover from partial predation events. Most benthic animals reproduce via external fertilization. Sperm and eggs are spawned into the water column and fertilization occurs outside the body of the female. Surprisingly, sessile barnacles must copulate to achieve internal fertilization. These animals increase their reproductive success by being hermaphroditic, thus assuring that any neighbour is a potential mate; being gregarious to assure a high density of mates; and by having a penis long enough to deliver sperm to an individual seven shell lengths away. 

   Certain deep-sea habitats can be highly diverse. In the deep-sea vents, for example, chemosynthetic bacteria (rather than photosynthetic species) form the basis of the food chain. These bacteria obtain energy from chemical sources such as hydrogen sulfide instead of from sunlight.