ChapterChapter 11 11

Between the TidesBetween the Tides

What is the intertidal zone?What is the intertidal zone?

• The The intertidal zoneintertidal zone is is the area between the the area between the mean low tide and mean low tide and mean high tide. mean high tide.

• By contrast, the By contrast, the subtidal zone is the subtidal zone is the area that is always area that is always submerged. submerged.

• The intertidal zone, The intertidal zone, therefore, borders the therefore, borders the subtidal zone.subtidal zone.

• The intertidal zone will The intertidal zone will be be exposedexposed during low during low tide.tide.

SubstrateSubstrate

• Substrate refers to the type of Substrate refers to the type of “bottom” that is present in a “bottom” that is present in a community. community.

• In the intertidal zone, substrate types In the intertidal zone, substrate types can be rocky or soft bottom.can be rocky or soft bottom.

• Soft bottoms can vary from sand to Soft bottoms can vary from sand to silt or a mixture (mud). silt or a mixture (mud).

• Rocky substrate can vary as to the Rocky substrate can vary as to the type of rock present and the degree type of rock present and the degree of height/slope.of height/slope.

What type of organisms live in the What type of organisms live in the intertidal zone?intertidal zone?

• EpifaunaEpifauna – organisms that live on the – organisms that live on the surface of the substrate (ex: mud snails surface of the substrate (ex: mud snails on soft bottoms, barnacles on hard on soft bottoms, barnacles on hard substrates)substrates)

• InfaunaInfauna – organisms that live in the – organisms that live in the substrate (ex: clams burrowed in soft substrate (ex: clams burrowed in soft bottoms)bottoms)

• MeiofaunaMeiofauna – organisms that are so small – organisms that are so small that they live between the grains of soft that they live between the grains of soft substratesubstrate

• These organisms can be sessile or These organisms can be sessile or motile.motile.

Problems Associated with the Problems Associated with the Intertidal ZoneIntertidal Zone

• Due to the exposure seen in the intertidal Due to the exposure seen in the intertidal zone, organisms face a variety of zone, organisms face a variety of challenges, including:challenges, including:– DesiccationDesiccation– Temperature changes (can be extreme)Temperature changes (can be extreme)– Salinity changes (can be extreme)Salinity changes (can be extreme)– Interrupted feedingInterrupted feeding– Wave action and tidesWave action and tides– Oxygen availability and build-up of COOxygen availability and build-up of CO22

– Limited spaceLimited space

DessicationDessication

• When exposed, organisms must deal When exposed, organisms must deal with potential water loss. with potential water loss.

• Water loss is more pronounced on Water loss is more pronounced on hot, dry days or windy days.hot, dry days or windy days.

• Organisms can deal with the Organisms can deal with the potential water loss by hiding or potential water loss by hiding or “clamming up.”“clamming up.”

DesiccationDesiccation

• Hiding may involve Hiding may involve moving to a tide pool moving to a tide pool or an area with more or an area with more moisture (motile moisture (motile organisms)organisms)

• Hiding may also mean Hiding may also mean that some organisms that some organisms only live in areas only live in areas where moisture will where moisture will remain when the tide remain when the tide is out (such as is out (such as crevices in rocks or crevices in rocks or low spots in soft low spots in soft bottoms)bottoms)

Devil’s Churn Tidal Devil’s Churn Tidal Pool in Oregon-Sea Pool in Oregon-Sea anemones, bivalves anemones, bivalves and barnacles in and barnacles in the deepest parts of the deepest parts of pools and bivalves pools and bivalves and barnacles and barnacles “clammed up”“clammed up”

DesiccationDesiccation

• ““Clamming up” consists of closing Clamming up” consists of closing shells or otherwise walling yourself shells or otherwise walling yourself off from the environment in an off from the environment in an attempt to conserve moisture (such attempt to conserve moisture (such as an oyster closing its shell or a as an oyster closing its shell or a snail walling itself off using its snail walling itself off using its operculum).operculum).

• While moisture can be conserved in While moisture can be conserved in this way, there is a down side – no this way, there is a down side – no exchange of gases or feeding occurs.exchange of gases or feeding occurs.

Changing TemperaturesChanging Temperatures• The wide variety of temperatures that must The wide variety of temperatures that must

be tolerated by organisms in the intertidal be tolerated by organisms in the intertidal can be severe. can be severe.

• Imagine, one morning the temperature may Imagine, one morning the temperature may hover around 60 degrees with an afternoon hover around 60 degrees with an afternoon spent in temperatures exceeding 100 spent in temperatures exceeding 100 degrees. degrees.

• Some organisms have mechanisms to help Some organisms have mechanisms to help stay cool such as body ridges to allow heat stay cool such as body ridges to allow heat pass out of shell and light colors to reflect pass out of shell and light colors to reflect heat. heat.

Changing SalinityChanging Salinity

• Salinity can change dramatically due to Salinity can change dramatically due to temperatures or weather.temperatures or weather.

• Normal ocean salinity is around 33-35 ppt.Normal ocean salinity is around 33-35 ppt.

• Estuarine salinity normally varies between Estuarine salinity normally varies between 5 ppt and 30 ppt.5 ppt and 30 ppt.

• Organisms in the intertidal are normally Organisms in the intertidal are normally euryhaline (can tolerate a wide variety of euryhaline (can tolerate a wide variety of salinities); subtidal organisms by contrast salinities); subtidal organisms by contrast are normally stenohaline (tolerate a very are normally stenohaline (tolerate a very low range of salinities) low range of salinities)

Changing SalinityChanging Salinity• Salinity may begin at 20-25 ppt and Salinity may begin at 20-25 ppt and

climb dramatically during the day due to climb dramatically during the day due to water loss when the intertidal is exposed.water loss when the intertidal is exposed.

• Salinity may also drop during the day Salinity may also drop during the day due to a sudden influx of freshwater due to a sudden influx of freshwater provided by a passing thunderstorm. provided by a passing thunderstorm.

• Mollusks “clam up” to Mollusks “clam up” to avoid fresh water and avoid fresh water and those without shells those without shells “ “run and hide” in run and hide” in pockets of seawaterpockets of seawater

Interrupted FeedingInterrupted Feeding

• If an organisms “clams up,” feeding will If an organisms “clams up,” feeding will cease.cease.

• Feeding will also cease for filter feeders Feeding will also cease for filter feeders when exposed to air.when exposed to air.

• A few organisms such as snails can still A few organisms such as snails can still feed during low tide by scraping algae feed during low tide by scraping algae off surfaces.off surfaces.

• organisms may spend nearly half of their organisms may spend nearly half of their day un-submerged and not feeding.day un-submerged and not feeding.

Wave ActionWave Action• Waves can be a problem for organisms Waves can be a problem for organisms

that live in intertidal communities directly that live in intertidal communities directly exposed to them. This action may dislodge exposed to them. This action may dislodge organisms from their habitat.organisms from their habitat.

• Rocky shorelines are often exposed to Rocky shorelines are often exposed to significant wave action. significant wave action.

• In soft bottomed communities, the open In soft bottomed communities, the open beach intertidal zone is exposed to beach intertidal zone is exposed to significant wave action.significant wave action.

• Marsh communities are normally not Marsh communities are normally not exposed to as much wave action due to exposed to as much wave action due to their position behind the open beach.their position behind the open beach.

Giant green sea _____ on Pacific Coast are taller in sheltered areas but have adapted to areas of heavy wave action by being shorter

Other organisms like this kelp have adapted to wave action by being flexible.

Oxygen AvailabilityOxygen Availability• Oxygen can be exhausted if an organism “clams up” Oxygen can be exhausted if an organism “clams up”

during low tide.during low tide.

• It can also be exhausted in tidal pools if many It can also be exhausted in tidal pools if many organisms have sought refuge there. organisms have sought refuge there.

• Carbon dioxide can also build up to toxic levels when Carbon dioxide can also build up to toxic levels when organisms are not able to exchange these gases with organisms are not able to exchange these gases with their environments. their environments.

• Some organisms have Some organisms have

evolved the capability evolved the capability

to exchange gases in to exchange gases in

air and water (such as air and water (such as

many crab species).many crab species).

Limited SpaceLimited Space

• In some intertidal communities, proper In some intertidal communities, proper space may be limited.space may be limited.

• This is particularly true in rocky intertidal This is particularly true in rocky intertidal environments where the amount of environments where the amount of surface area is limited. surface area is limited.

• However, space can be limited in soft However, space can be limited in soft bottomed communities as well. Organisms bottomed communities as well. Organisms in these areas prefer hard substrate such in these areas prefer hard substrate such as living on an “oyster reef” because they as living on an “oyster reef” because they do not wash away due to water action. do not wash away due to water action.

Zonation in the Rocky Zonation in the Rocky IntertidalIntertidal

• Zonation in Zonation in the rocky the rocky intertidal intertidal consists of consists of upper upper intertidal intertidal (most (most exposed) to exposed) to middle and middle and lower lower intertidal intertidal (least (least exposed).exposed).

Zonation in the Rocky Zonation in the Rocky IntertidalIntertidal• Competition will be greater in the lower Competition will be greater in the lower

intertidal because it is the least “severe” intertidal because it is the least “severe” of the zones since it is exposed to a lesser of the zones since it is exposed to a lesser degree than the upper intertidal.degree than the upper intertidal.

• The lower intertidal is always more species The lower intertidal is always more species rich for the same reason (although species rich for the same reason (although species will vary greatly by location).will vary greatly by location).

• Species include seaweeds and seagrasses Species include seaweeds and seagrasses with many types of small invertebrateswith many types of small invertebrates

CompetitionCompetition• Some organisms are Some organisms are

better competitors than better competitors than others or will exclude others or will exclude other organisms if the other organisms if the community is left community is left undisturbed. undisturbed.

• If a habitat is disturbed, If a habitat is disturbed, organisms move in and organisms move in and are later excluded in a are later excluded in a predictable pattern predictable pattern known as succession. known as succession.

• The climax community The climax community (end result) will form (end result) will form when the community is when the community is undisturbed for a long undisturbed for a long period of time. (Seen at period of time. (Seen at right.) Seastars are a right.) Seastars are a keystone predator that keystone predator that maintains the middle maintains the middle intertidal community.intertidal community.

Middle Rocky Intertidal Community

Upper intertidal of rocky shores is the harshest environmentAnd includes lichens, encrusting algae, and various gastropods

Soft bottomed Intertidal Soft bottomed Intertidal Areas like Padre Island Areas like Padre Island National SeashoreNational Seashore

Substrate in the Soft Bottomed Substrate in the Soft Bottomed IntertidalIntertidal

• In the soft bottomed In the soft bottomed intertidal intertidal community, grain community, grain size can vary greatly size can vary greatly between sand, silt, between sand, silt, clay or a mixture clay or a mixture (mud). (mud).

• The smaller the The smaller the particle size, the less particle size, the less oxygen that can fit oxygen that can fit in those spaces.in those spaces.

Substrate in the Soft Bottomed Substrate in the Soft Bottomed IntertidalIntertidal

• The grain size is so small in most soft bottomed The grain size is so small in most soft bottomed intertidal communities that all except the top intertidal communities that all except the top few inches will be anoxic and be dominated by few inches will be anoxic and be dominated by anaerobic bacteria that decompose bits detritus.anaerobic bacteria that decompose bits detritus.

• Therefore, _____ (animals that live on surface) Therefore, _____ (animals that live on surface) like moon snails and _____ (microscopic animals like moon snails and _____ (microscopic animals between the grains of substrate) are usually between the grains of substrate) are usually restricted to the oxygenated zone.restricted to the oxygenated zone.

• _____ (animals that burrow in the substrate) like _____ (animals that burrow in the substrate) like polychaetes and bivalves must maintain a polychaetes and bivalves must maintain a connection with the oxygenated zones via tubes, connection with the oxygenated zones via tubes, siphons or extended burrows if they are located siphons or extended burrows if they are located in the anoxic zone.in the anoxic zone.

Soft Bottom Intertidal Soft Bottom Intertidal OrganismsOrganisms• Diversity Diversity

of infauna of infauna and and epifauna epifauna that may that may be found be found in this in this communitcommunity.y.

Sandy Atlantic Coast Community

Photo of Donax (coquina clam) by Dr. Don Keith

Soft Bottom Food WebsSoft Bottom Food Webs• Detritus Detritus

(broken down (broken down organic organic particles) and particles) and producers such producers such as as phytoplankton phytoplankton and benthic and benthic diatoms form diatoms form the basis for the basis for the food web in the food web in this community.this community.

Rocky Intertidal Food WebsRocky Intertidal Food Webs• The food web in The food web in

the rocky intertidal the rocky intertidal is based on is based on producers such as producers such as seagrasses, algae seagrasses, algae and diatoms.and diatoms.

• Top carnivores in Top carnivores in any community any community that have the that have the ability to change ability to change community community composition composition significantly are significantly are known as keystone known as keystone predators.predators.