Marine Science Chapters


Deep-sea Benthos

Dark, cold and under high pressures the deep-sea environment is often thought of as having few life forms. But, there are species adapted to this habitat. These species live in water near freezing - usually between -2 and +5 degrees Centigrade. They also live with pressures some 300 to 600 times those at the surface (or up to 1,000 times in the trenches). The bottom is usually composed of mud or clay, the result of eroded sediments sinking to the bottom of the ocean for millions of years.

Without sunlight the bottom of the deep-sea has no living plants. Sometimes drifting pieces of marine plants find their way to the deep-sea bottom but they end up with other dead organisms and decompose on the deep-sea floor.

Brittle Sea Stars and Regular Sea Stars
Brittle sea stars (the ones with the thin legs) cover some areas of the deep-sea as one of the main epifaunal species. These are filter feeders and they trap organic material in the water as their food source. Regular sea stars (the ones with the thick legs) feed on benthic organic matter. (NOAA image)

A large number of individual animals may live in areas under productive surface waters. These species come in both epifaunal (animals living on the surface) and infaunal (animals living under the surface) forms and rely on a constant 'rain' of organic material from the surface waters. This organic material may drift down as dead plankton, fecal pellets, large dead organisms, or marine snow. Marine snow is the gelatinous houses of surface organisms or mucus that is sinking to the bottom and is often covered with bacteria. Marine snow provides a large proportion of the food for many of the deep-sea benthic species.

Sea Cucumber
Sea cucumbers are a common epifaunal animal found on soft bottoms. Many sea cucumbers are deposit feeders - taking in the soft sediments, digesting the organic material and infauna in the sediment and leaving behind cleaned sediment. Notice the cleaned sediment pile left by this sea cucumber on the right. (Image, with permission, from Western Marine Lab)

A large number of infaunal worms are present in deep-sea sediments. These are usually deposit feeders. It is believed that the diversity of infaunal worm species in the deep-sea may be due to the sea cucumber - the infaunal worms rarely get a chance to compete with each other because their habitat (the mud) is constantly being disturbed by a larger epifaunal species (the sea cucumber). These sea cucumbers are called 'croppers' because they are constantly cropping the worm species. Without the 'croppers' it is believed that the number of infaunal worm species would decrease drastically when the species could compete with each other - one would dominate and outcompete others.

Bioluminescence is common in the deep-sea and may be found on benthic species as well as pelagic species. This bioluminescence is usually restricted to a certain area on the organism rather than over the entire surface. It is found as a pattern of luminescent dots, or concentrated in one area, or it can be bioluminescent bacteria harbored within a pocket on the body of the animal (with a movable cover to allow the light to show or not). The bioluminescence itself (a cold, chemical light) is produced by the same chemical reaction of luciferin and luciferase whether it is a bacteria, sea star, or fish. The light itself may be used by the animal to attract food, attract a mate, identify its species, frighten predators, or simply to 'see.'

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(Revised 12October 2004)
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