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Animals of the Benthos

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Animals of the Benthos OS 101—Marine Environment R. Kudela Animals of the Benthos I. Definition - The benthos are organisms (animals and plants) that live on the sea floor. - They are highly diverse - they include 98% of all marine species. - Their substrate determines benthic community composition: - rocky, sandy, or muddy intertidal - muddy deposits or hydrothermal deposits of the deep sea - Benthic organisms are not adapted to wide ranges in pressure - There are very few transparent organisms - Generally stay to a smaller spatial area (they don’t move around as much) - We classify them in relation to the type of shoreline or bottom structure II. Controls on the Benthos A. Biodiversity - diversity is largely controlled by physical factors: - Temperature (more diversity with increasing temperature) - Currents (these affect both the distribution and sediment type) - Wave Energy (high energy vs. low energy environments) - As an example, we would expect more diversity moving from North to South along the east coast (temperature), more diversity in areas of strong longshore currents (more energy, less accumulation of sediments), and more diversity in high-energy (rocky intertidal) shoreline as opposed to low wave-energy (mudflat) environments. - BUT, these are only general patterns! At the same latitude, there is more diversity along the eastern boundary of the North Atlantic than the western boundary, because of the modulating effects of the Gulf Stream, which warms up Europe more than the east coast of the US, even though they are at the same latitude B. Biomass - unlike diversity, biomass is largely controlled by the availability of food…therefore, it is ultimately linked to water column primary production - Benthic biomass is generally higher near the coasts at midlatitudes, and lowest in the open ocean gyres - As with diversity, there are some important exceptions: - vent and seep communities rely on chemosynthetic primary producers, so the patterns of these communities are not well correlated to water column primary production - Similarly, coral reefs have very high biomass and diversity, but are usually found in low growth, low nutrient, tropical waters III. Benthos Classification A. Rocky Intertidal - these areas exhibit some of the greatest extremes in physical conditions: - water cover - temperature - salinity - oxygen - exposure to air - water turbulence - Because of this, there are many ecological niches, and rocky intertidal zones exhibit very strong zonation, and moderate biomass/diversity - The intertidal zone is characterized by: i. The spray zone - Periwinkles and limpets (snails) live here, and they are capable of existence out of water. ii. The high tide zone - It is mostly dry. Barnacles live here, but they must filter-feed and their larval forms are planktonic. Chitons, limpets, and brown algae (see Chapter 14) also live here. iii. The middle tide zone - It is equally exposed and covered and has a greater variety of animals and plants. There is competition for space. Mussels, sea stars, goose barnacles, green, brown, and some red algae live here. Tide pools form in this area, and they contain sea anemones, fish, and hermit crabs. iv. The low tide zone - Here organisms remained submerged except at the lowest low tides. The maximum number of species lives here and the maximum biomass. It is dominated by plants (surf grass/sea grass) and algae, including red algae (from which we obtain thickening agents: agar, carageenan and nori for sushi), hermit crabs, sea anemones, and sea urchins. B. Animals of Sediment Covered Shores ("soft" substrates) - This is a lower energy environment (less longshore currents). - It also has lower diversity (but not necessarily biomass) than other environments. - In this region epifauna live on the sediment. Infauna live in the sediment. - Some organisms are burrowers to avoid drying out. These include clams, worms, and sand crabs. - There are several types of environments in this zone i. Beaches ii. Salt marshes iii. Mud flats - In mud flats there is not much zonation seen since there is little or no slope - In “soft” shores, the dominant biomass is often eel grass and turtle grass, and some species of crab (fiddler, ghost, etc.) - The feeding of animals in this zone is dominated by the easy availability of organic matter (which is why it develops a “soft” sediment): i. Detritus feeding, where plant material is degraded by bacteria and fungi. An example of a detritus feeder is shrimp. ii. ii. Deposit feeding, where organisms engulf sediment and process it in their guts to extract organic matter. Examples include worms (nematods, polychaetes) and sea urchins. iii. Filter feeding, where organisms often bury themselves and protude only their feeding appendages (e.g. clams) C. Shallow Offshore Animals - As with the shoreline, we divide the offshore environment into rocky and soft substrates - Soft substrates are very similar to the soft shoreline…low diversity (but high biomass), low energy, detrital and deposit feeders dominate - A good example of a soft offshore environment is the Atlantic continental margin - In rocky (shallow) offshore environments, there is generally more diversity, and is more “interesting” in terms of biology. - A good example are the kelp forests of the west coast - These are the areas dominated by lobsters, large crabs, etc. and the feeding patterns tend to be more predatorial than in the soft sediments Coral Reefs - This is a special case of a shallow offshore environment - Coral reefs: - have the greatest known diversity in the marine environment. - have 25% of all known marine species. - require warm waters (restricted to tropics, >18 oC) - are clean (low levls of nutrients) - are clear (free of sediment particles) - have shallow water (for sufficient light) - have a firm substrate - Zonation of coral reefs - Due to changes in wave energy, salinity, water depth, temperature, and light, there is a well developed zonation of coral reefs (see Fig. 17-20) which is distinct from all other environments - Coral itself is: i. a colony of animals called polyps, which are related to sea anemones. ii. a symbiosis, e.g., photosynthetic dinoflagellates (zooxanthellae) live within coral, and - Coral provides protection and nutrients. - The zooxanthellae eliminate waste (nutrients and carbon dioxide) and provide conditions favorable for growth. - Hermatypic, or reef-building, corals require light and are limited to the upper 150 meters. - Other organisms also contribute to the reef. These include: - Coralline algae, mollusks, and foraminifera contribute to the "cement" of the reef. - The coral reef community contains the greatest known animal diversity in the ocean. D. Sea Floor (deep sea) Animals - These organisms live on the deep ocean floor (bathyal, abyssal, hadal). - This zone is characterized by: - Low temperature, aphotic, high pressure - High diversity - One type of deep sea community is a chemosynthetic vent community. - It is characterized by a hydrogen sulfide-based chemosynthesis. - They were first discovered in 1977. - Black smokers were first discovered in 1979. - Bacteria are the basis of the food chain here. They also exist as symbionts within tube worms or clams. - Zonation is due to temperature and age of the vent community. - Another type of deep sea community is a cold seep community. - These are (usually) based on methane seeps. - These are quite common in Monterey Bay, and are characterized by many of the same types of organisms (symbiotic clams, tubeworms, etc.) - Unlike hydrothermal vents, the zonation is dependent on age of the vent community and how far away from the seep source you are, rather than temperature - Natural oil and gas (hydrocarbon) seeps are common in the Northern Gulf of Mexico. - The chemical enrichment produced by seeps support lush chemosynthetic communities of tube worms, mussels, and other animals that thrive without light at depths of about 2000 feet - Hypersaline seeps were discovered in 1984, also in the Gulf of Mexico - These seeps are not hot or cold, but have high (46.2 ppt) salinity water - This carries a great deal of hydrogen sulfide, supporting a rich bacterial mat community.
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