Between the Tides

Chapter 11 (Castro)Between the Tides

The Intertidal ZoneOf all the ocean, the intertidal zone (also called the littoral zone) is the best known.The intertidal zone is the narrow area along the shoreline that lies between the highest high and the lowest low tides. The intertidal is the only area of the marine world that we can experience without leaving our own natural element. Although the intertidal represents only a small fraction of the marine environment, what we have learned about this area has added to our knowledge of marine ecology.

Why is the Intertidal unique?Unlike other areas of the marine environment, the intertidal zone is regularly exposed to the air.

Organisms that live in the intertidal must have a way to cope with this exposure, even if it means giving up characteristics that would be advantageous to the tides.Emersion: being out of the water and exposed to airImmersion: being submerged in the water

The community of organisms, to a great extent, depends on the types of bottom or the substrate.Substrate: the material on or in which an organism lives

Two Main Substrates of Intertidal CommunitiesRocky ShoreSoft Bottom

Mud FlatLess Common Intertidal Substrate

Rocky Shore Intertidal CommunitiesRocky shores generally occur on steep coasts without large amounts of sediment.

Formation of Rocky Shores:Rocky shores have typically been uplifted, or are still rising due to geological events, and have not had much time to erode or accumulate sediments.Waves and currents can carry sediments away, leaving rocks behind. Formation by successive lava flow into the sea, where the coast is being formed by periodic volcanic eruptions (Hawaii).

In North America, rocky shorelines are most common on the western coast and on the eastern coast north of Cape Cod.

Vertical Zonation of the Rocky Shore

Complete Intertidal Zones of the Rocky Shoreline ActivityIntertidal DivisionsRocky intertidal communities vary markedly from place to place, but there is one feature that is remarkably constant. The rocky intertidal community is usually divided into bands, or zones, at characteristic heights within the intertidal.

That said, any given species are usually not found throughout the intertidal, but only within a particular vertical range. This pattern of banding is called vertical zonation.

Vertical Zonation of the rocky shoreline are classified as follows:Splash zoneHigh tide zoneMid tide zoneLow tide zone

Splash ZoneIn the upper intertidal zone, or splash zone, the inhabitants must be well adapted to withstand exposure to air as this zone lies mostly above the high tide mark. That said, organisms are wetted mainly by wave splash and spray.

Main life within the Splash Zone:Algae, Lichens and Cyanobacteria soak up water for long dry periods, protected from drying out by jelly-like coating Periwinkle abundantly occupy the splash zone, grazing on the algae by scraping it up from the rocks. They can breathe air, live out of water for months, and tolerate extreme tempsLimpets

Most common predators come from land such as birds, raccoon, rats, and other land animalsHigh Tide ZoneArea of the intertidal zone that is flooded during the peaks of the tide (once / twice daily) and experiences long periods without water in between these peak times.

Main life forms within the high tide zone:Barnacles Limpets Shore crabs occasionally venture into this zone, mostly scrape algae off rocks with claws but also eat animal matter, dead or alive. Act as predators to periwinkle and limpets.Sea lice / sea roaches breathe air and live above the waters edge, moving into the upper intertidal at low tide.

Mid Tide ZoneArea that is submerged and uncovered by the tides on a regular basis, having a large amount of variation in emersion time.

Main life forms of mid tide zone:Different types of barnacles that can handle the variations of emersionHermit CrabsMussels tend to dominate other life forms in their area, smothering or overcrowding the areaSea stars predators to mussels, inserting their stomachs into the mussel shell and digesting from the inside (do not handle dessication well so must act during times when tide is in

Main predators are the Sea Stars, profoundly affecting the entire community.

Low Tide ZoneImmersed most of the time, making it easy for sea predators to feed. As result, organisms such as mussels and barnacles are rare in this zone.

Main life forms of low tide zone:Seaweed dominant, forming thick surfaces on rocks. Cannot tolerate drying out so they grow profusely. Kelp marks lower limit of the intertidal and continue down into the subtidal zone. Sea urchins common grazers on seaweedAnenomes, worms, snails, and sea slugs

Marine Life within the Rocky ShoreAs it is difficult to burrow through rock, most rocky intertidal organisms live right on the rocks surface.Epifauna: Animals that live on the surface of the substrateMobile Epifauna: Animals that move about over the rocksSessile Epifauna: Animals that stay attached to the rocks

Living on the rocks surface, the organisms in the rocky intertidal are fully exposed to the elements, which subjects them to great physical stress.

Low Tide ExposureLow tide presents many problems for organisms of the rocky intertidal as they are left high and dry (exposed to the air).

Depending on where the organism resides in the intertidal, dictates the degree of difficulty for survival.For those living higher in the intertidal zone, they are only immersed in water for a short period of time at high tide and it may be (if they are at the top of the region) they are only submerged during spring tides. The highest part of the intertidal, in fact, is almost never submerged and is kept wet by wave splash.For those living lower in the intertidal zone, they are submerged most of the time and have to come with emersion for short periods of time. The lowest part of the intertidal, in fact, is almost always submerged and experiences emersion only at the most extreme low tide.

Ultimately, the higher the organism resides within the intertidal zone the more time they have to spend out of the water.Water LossMarine organisms tend to dry out (desiccate) when exposed to air.

In order to survive in the intertidal, an organism must be able to prevent desiccation, tolerate it, or both.

Desiccation is coped with in one of two basic ways:Organisms run and hideOrganisms clam up

Run and Hide MethodWhen the tide goes out, the organism goes somewhere wet and waits for the tide to come back in.Depressions in the rocks that hold seawater after the tide goes out (tidepools) are among a favorite spot for some organisms.An area kept moist by wave sprays or water leaking out from a tide pool.Some smaller organisms hide within other organisms such as mussels

Crabs cope well in the intertidal zone. They are able to run and hide in rock pools or under rocks. This is not an option for most creatures.Central California Coast Tide Poolshttp://www.youtube.com/watch?v=S-_8aP06L6w

Run and Hide Method of Sessile OrganismsSeaweed and sessile organisms cannot run when the tide changes, but many can hide. Instead of moving to moist areas when the tide goes out, they live in a moist area all the time. (This may be that larvae or spores settle in only moist, shady places or because those that settle elsewhere dry out and die.)

Clam-Up MethodOrganisms that use the clam-up method have some sort of protective covering, like a shell, that they can close to hold in water.Some (barnacles and mussels) can completely enclose their shell and seal in moisture, while others (limpets) have an opening that cannot be completely shut. Organisms then clamp themselves tightly to a rock to seal their opening.

BarnaclesLimpetsAdditional Means of CopingAs previously noted, most intertidal organisms either cope with the problem of desiccation through the run and hide or clam-up strategies. Some, however, use a combination of strategies or something completely different:Periwinkle clamp themselves to rocks to seal in moisture AND congregate in moist, shady places.Chitons simply allow themselves to dry out as they can survive loss of 75% of water in their tissues. Some seaweeds, such as rockweed, can withstand water loss of as much as 90%. Both quickly recover when the tide comes in and wets their tissues back up.

PeriwinkleChiton

Extreme TemperaturesEmersion creates problems other than desiccation for marine organisms. Sea temperatures are relatively constant and mild but air temperatures can be much more extreme. At low tide, organisms are at the mercy of the suns heat and the freezing cold of winter. Because tide pools are shallow, they too experience extreme temperatures.Most intertidal organisms can handle a wide temperature range and tolerate temperatures through hiding, having light colored shells, staying in damp areas, placing themselves within the wave spray, and through protective shell ridges that reflect heat.

SalinitySalinity also fluctuates widely in the intertidal area.When it rains, exposed intertidal organisms must endure freshwater which is fatal to most marine organisms. Many keep the freshwater out by closing their shells, although rainstorms during low tide attribute to mass mortality of intertidal organisms.Tide pool residents, as well, face extreme variation in salinity as pools may be diluted by rain during low tide (lowering salinity) and may experience high salinity (due to evaporation) on high, dry days. To cope, tide pool organisms sometimes bury themselves or reduce activity while they await high tide.

Restriction of FeedingBecause little sediment accumulates in the rocky intertidal, deposit feeders are uncommon and most sessile animals are filter feeders.Deposit feeders: animals that eat organic matter that settles to the bottomFilter feeders: animals that actively filter food particles from the water.

Filter feeders are unable to feed when the tide is out because they must be under water to filter the food. Also, many clam-up during low tide to avoid water loss and cant filter with their shells closed.

Even animals that are not filter feeders struggle with feeding during low tide. Many are grazers and feed on pieces of seaweed and dead organic matter that drift up from deeper waters. Others are predators and move over rocks in search of prey. At low tide, mobile animals seek shelter to avoid water loss, which prevents them from moving around to find food.

Then Dungeness Crab resides in the lower intertidal zone and does not have much difficulty feeding as it is immersed most of the day. As result, the Dungeness Crab grows at faster rates than other animals living higher up within the intertidal zoneThe Power of the SeaAs weve discussed, life when the tide is out is extremely rough on intertidal organisms. When the tide is in however, life isnt necessarily easy.

Ocean waves expend tremendous energy as they crash onto the shore and rocky intertidal organisms are exposed to the full power of the sea.

Distribution of Wave Energy along the ShoreThe impact of the waves varies along the shoreline. Some areas are sheltered from the surf, others are fully exposed. Ultimately, there is tremendous variation in the intensity of wave impactWaves slow down as they feel the bottom of the floor (same wave travels faster in deeper water)Waves almost never approach the shore straight on, but come at an angleThe end of the wave reaches shallow water prior to the other partsRefraction the bending of the wave causes waves to become nearly parallel to the shore, although they are never completely parallelRefraction produces complex wave patterns where the coast is not a straight lineOffshore bottom features can influence the effect of waves on the coastCoping with Wave ShockWave Shock: variation in the intensity of wave impact

Some intertidal organisms simply cannot withstand wave shock and are only found in sheltered locations.

Organisms that are exposed to the force of the waves need some way to deal with wave shock. Sessile organisms anchor themselves firmly to the rocks to keep from being washed awayMobile organisms cling strongly to rocks through suctioned feetIntertidal fishes sink and stay to the bottom as they lack swim bladders (gas-filled sac that provides bony fishes with buoyancy)Some intertidal organisms have thicker shells than their counterparts from less exposed areas, which is an adaptation to wave shockThe Battle for SpaceDespite the extreme conditions of the intertidal area, organisms generally have plenty of food.High light levels and nutrient-rich coastal waters supply the basic requirements for photosynthesis, so there is vigorous algal growth.High tide brings in abundant plankton and drifting seaweed fragments, which is the most abundant food source in some communities. That said, rocky intertidal populations are often limited by space, not food or nutrients.

Unoccupied space is often in short supply. Even in sheltered areas, intertidal organisms will drift away or be smashed against the rocks if they cant attach themselves to the substrate. Often, there is not enough room to go around and the lack of space limits rocky intertidal populations.Competition for space is extreme in the rocky shore intertidal area.

Competition for SpaceCompetition for space is a dominant biological factor in the rocky intertidal and there are many ways to compete for space:Be first to get to open spots Having effective means of dispersal (getting themselves or offspring to new locations quickly) and then either hold onto the space or reproduce rapidlyTake over space already occupiedBarnacles undercut neighbors, loosening them from rocksOwl limpets bulldoze out their intrudersMany grow over their competitors, smothering them or blocking precious sunlight needed for growthGrow species into colonies, increasing space occupied by speciesRocky Intertidal FishMost rocky intertidal fish live in the lower intertidal or tide pools.

Among the most common are gobies, clingfishes, sculpins, pricklebacks, and gunnels which are all small fishes that are adapted to the environmental extremes of the intertidal.

Most intertidal fish are carnivorous.

Intertidal FishSoft Bottom Intertidal Communities Any bottom composed of sediment, as opposed to rock, is considered a soft bottom.

In North America, soft bottoms dominate on the east coast, south of Cape Cod, and virtually all of the Gulf coast. We have already discussed the rocky west coast, but failed to mention the rocky coast is often interrupted by sandy beaches and mudflats, especially near river mouths.

The type of sediment depends on both the amount of water motion and the source of the sediment. The type of sediment also strongly influences the type of community.Shifting SedimentsSoft bottoms are unstable and constantly shift in response to waves, tides, and currents.

Very few seaweed have adapted to this environment, which is why the east coast beaches are not overpowered with seaweed as the west coach beaches are. Sea grasses are the most common large primary producers, and will be discussed in our next unit.

Main characteristics of soft-bottom communities are as follows:Lack solid attachment sitesMost burrow in sediment for protection and to keep from being washed away (infauna)

Kind of SedimentThe kind of sediment on the bottom, especially the size of the grain, is one of the most important physical factors affecting soft-bottom communities.Examples include sand, silt, clay, and mud which ultimately refer to sediments of particular sizes, with sand being the most coarse followed by silt and then clay. The term mud refers to silt and clay together.Most sediments are mixtures of different particle sizes and are described according to grain size, with composition directly related to the degree of water motion. Fine sediments remain suspended with a small amount of water motion, whereas coarse sediments settle out unless there is considerable flow. Living in the SedimentLiving in sediment has its advantages in the intertidal. Soft bottoms stay wet after the tide is out, so the problem of desiccation is not as critical as in the rocky intertidal.

The grain size however, effects this as coarse sands drain and dry out, quite rapidly. As result, coarse sand beaches have relatively little animal life.Oxygen AvailabilityThe amount of organic matter in bottom sediments is important to deposit feeders, who extract the organic matter from the sediments. The amount of matter, again, depends on grain size.

The grain size also affects the amount of oxygen that is available in the sediments. Oxygen in sediments is used by the respiration of animals and decay bacteria. Below the sediment surface, there is no light and no photosynthesis, so circulation of the water must replenish the oxygen supply.

Getting AroundSoft bottom animals use a variety of methods to burrow through sediments. Clams and cockles are able to change the shape of their muscular foot, using it as an anchor while they pull the rest of their body along. Some worms force their pharynx into cracks in the sediment to expand the crack before pulling their bodies along behind.Heart urchins burrow or plough through the sediment with their spines and tube feetCrustaceans, such as amphipods or sand crabs and ghost shrimp use their jointed appendages to dig.Deposit feeders solve two problems at once by eating their way through the sediment.

The clam or cockle pushes its foot down through the sediment, then expands the end of the foot. The expanded foot anchors the clam as it pulls its body down.FeedingAs already noted, particles of dead organic matter are the main source of food in the soft bottom intertidal. Single-celled algae sometimes form highly productive mats on the sediment surface, but most often they dont account for much primary production. Most animals dont distinguish between the organic matter and the algae, and plankton brought in by the tides make a contribution to the food supply.

Ways of feeding:Some animals take in sediments as they burrow, digest it, and leave the rest of the sediment behindSome animals use their tube feet to pick up organic particlesSome animals siphon food particles from the surfaceSome animals catch particles as they settleSome animals catch particles in suspensionSome filter food

Soft Bottom PredatorsSoft bottoms also have their share of predators.Moon snails burrow through the upper sediments looking for clams. Once they find the clam, they drill a hole in the shell and eat it.Polychaetes and other worms are predatorsBirds can be major predators during low tideFish can come in at high tide, although they do not often eat the entire animal and instead nip off siphons or other bits that stick outSoft Bottom Zonation

ZonationBecause organisms live in the sediment, where they cant be seen, zonation is not as obvious as that of the rocky intertidal. Zonation does exist, especially on sandy beaches. As water drains rapidly from the sand, and the beach slopes, the upper part is drier than the lower part. The upper beach is inhabited by beach hoppers (sand fleas) and by isopods. In warmer areas, ghost and fiddler crabs occupy this area. The lower beach is inhabited by clams and other animals.

Zonation is less obvious in muddy areas where the bottom is flat and fine sediments retain water. The habitat in these places does not change much between high and low tide.