The intertidal zone is the dynamic interface between land and sea, which is constantly shifting and changing with the tide.

This extreme ecosystem is divided into four vertical zones, based on the amount of time each section is submerged. There are the low tide zone, middle tide zone, high tide zone and splash/spray zone. 

Anything that lives in the intertidal zone must withstand dramatic changes in moisture, temperature, salinity, and wave action.  

Marine life and ecosystems respond to these challenges either by short-term, reversible adjustments (phenotypic plasticity), or by long-term adaptations that involve heritable genetic changes (evolution). 

Let’s explore the challenges of living in this dynamic environment, and some of the ingenious ways that intertidal animals have adapted to survive here.

Intertidal animals are exposed to air for a large portion of their lives

The drying out effect of air exposure poses a challenge for these animals, and they must find ways to reduce water loss to survive here.

The most common adaptation is to avoid water loss altogether. Intertidal molluscs (like mussels and oysters) retain water within their shells and tightly seal them shut. Others seal off their shell opening with a door-like structure called an operculum (snails do this).  

Limpets seal themselves against the hard substrate using suction and produce a mucous layer at this interface to create a watertight seal.  

Air exposure leads to greater temperature fluctuations and extremes. 

Mobile animals such as crabs avoid the largest temperature changes by shuttling between cooler and warmer environments. This technique is called behavioural thermoregulation.

For less mobile animals, thermal regulation becomes more challenging. 

More sensitive animals such as sea stars/ starfish are limited to the low tide zone where they’re more frequently submerged.

Others, such as some species of intertidal mollusc, have evolved internal mechanisms such as heat shock proteins and a high freeze tolerance. These help the animals to cope with extreme temperature variations in the high tide zone. 

The intertidal zone is characterised by severe changes in oxygen availability. 

Particularly in overcrowded tide pools, when many animals are aggregated together during low tide, increased respiration can reduce water oxygen content to critical levels. 

In response to this, some intertidal animals can switch to aerial respiration and take in oxygen from air instead of water.  

For example, the high intertidal porcelain crab (Petrolisthes) has an aerial gas exchange organ on each of their walking legs for air-breathing during periods of emersion.

On sandy shores, some species of mudskipper will repeatedly emerge at the surface of their burrows and take in mouthfuls of air. They deposit this air into a specialised chamber within their burrows to protect themselves and their eggs against hypoxic (low oxygen) conditions. 

On sandy shores, activities of some intertidal creatures can influence local oxygen availability.  

Lugworms burrow deep into the sediment, feeding on organic material in the sand and creating U-shaped burrows deep below the surface. These burrows help to rework and ventilate the sand. This process is known as bioturbation, which increases localised oxygen availability.

They spend their lives filtering out organic material from the sand, pooping out wiggly mounds of undigestible sand to the surface, known as ‘casts’. 

All intertidal habitats are impacted by fluctuations in salinity.  

Heavy rain or river inputs can make coastal water fresher, and evaporation and droughts can cause hypersaline (saltier) conditions.

On sandy and muddy shores, intertidal worms move vertically up and down their burrows along a salinity gradient until they reach more favourable conditions.

Other animals maintain their internal osmotic (salt and water) balance in the same way that they protect themselves from water loss. Some clamping to the substrate (limpets); others closing their operculum (snails); or moving to more favourable environments (crabs).

The shallows are exposed to ultraviolet (UV) radiation from the sun. 

This can cause DNA mutations and damage to molecules needed for key biological pathways.  

To combat this, the aggregating anemone (Anthopleura elegantissima) contracts during peak levels of UV radiation. It attaches debris to its column for extra protection against harmful sun rays: Just like putting on a sunhat.

Other intertidal animals have even evolved ‘sunscreens’. These are absorptive, reflective, or light-scattering pigments in their skin and mucus which help to protect them from the damaging effects of sun exposure. 

For example, Irish moss (Chondrus crispus) is the reddish leafy seaweed that can be found on rocky shores and tide pools across the UK and Ireland. If you look closely, you’ll see that it looks slightly iridescent in the light.

This is because the tips of the growing fronds are covered in multiple, transparent layers. When sunlight hits these layers, it’s reflected away from these delicate regions, protecting this alga from harmful UV rays.

To avoid being swept away by wave action, intertidal animals must hold on tight. 

Barnacles secrete cement, while mussels produce a sticky thread called Byssus threads to attach themselves to the rock. Once attached, these sessile animals remain in position for most of their lives.  

Other intertidal creatures rely on suction. Sea stars (starfish) have rows of tube feet on their underside. Each of their feet has a sticky, suction cup at its end that help it hold on to the rocky substrate. These also come in useful to prise apart the shells of bivalves to eat.

Many adaptations are shared among diverse animal groups.  

This is an example of convergent evolution: when similar features independently evolve among different species under the same pressures.

Understanding the challenges overcome by animals living at the interface between land and sea may allow us to better understand our own historical transition from Ocean to land, millions of years ago.  

So next time you’re at the coast, see if you can spot some of these creatures and their adaptations. (Remember, they’re under enough stress already, so be respectful and don’t touch, poke or prod at them).

Take a moment to appreciate the incredible feat of survival achieved by the Ocean creatures that not only survive but thrive in this extreme environment.

Post by

Jemma Sargeant

Thousands of metres down in the Galápagos Rift valley, a deep-sea camera is towed along the seafloor, capturing our first glimpse of an extraordinary and alien world. 

Towering chimneys pumping out plumes of black smoke cover the seabed; these are hydrothermal vents.

Despite low oxygen levels, high toxicity and fluid temperatures of up to 350°C, hydrothermal vents host a remarkably diverse array of Ocean life.

These Ocean creatures are specially adapted to these extreme conditions: Giant tubeworms, beds of mussels and clams, fluffy crabs, pink vent fish and more.

The discovery of hydrothermal vents in 1976/ 1977 prompted a new branch of deep-sea biology. Since then more and more species have been discovered. 

Where are hydrothermal vents found?

Hydrothermal vents were one of the first environments to have existed on Earth and have been bubbling away for over 4 billion years. 

Hydrothermal vents can form anywhere a heat source meets a fluid system. They often occur on the seafloor at tectonic plate boundaries. The hot, upwelling magma heats up seawater which is ejected as mineral-rich plumes. 

They are mostly found in the abyssal zone of the Ocean (3,000 – 6,000m). While the majority (65%) of the hydrothermal vents are located close to the tectonic plate boundaries, they are also common (12%) along chains of underwater volcanoes, called volcanic arcs.  

In 2000, a new type of vent was discovered, located several kilometres from a divergent plate boundary (tectonic plates that are moving apart) called Lost City vents. They resemble the spires of an underwater metropolis like Atlantis. 

Take a look at some of the weird and wonderful Ocean life found in the deep-sea: 

Annelid tubeworms (Riftia pachyptila) 

Also called ‘giant tubeworms’, these are extremophiles, meaning they’re able to live in extreme environments, and can reach over 1.8 metres (six feet) tall.

They have a unique body plan with no mouth or anus and their lifestyle is unique, too as they rely entirely on symbiotic bacteria as a food source.  

The Yeti Crab (Kiwa hirsuta) 

This new family of crab was discovered in 2005 and has claws covered in dense setae (stiff bristles). They get almost all their food from the chemoautotrophic bacteria (bacteria that can turn inorganic chemicals into energy) that live in these bristly structures.

These furry crabs have been seen to wave their claws to help provide a flow of oxygen and minerals to their symbiotic bacteria.  

Pompeii worm (Alvinella pompejana)  

Named after the explosive eruption of Mount Vesuvius in Pompeii, the Pompeii worm is the most heat tolerant animal we know of. They can survive temperatures at high as 80°C. One physiological adaptation Pompeii worms have evolved to survive these extreme temperatures are heat shock proteins. These’re specific proteins which provide cells with thermal stability.  

So how are these marine animals living in such extreme conditions? 

Photosynthesis often gets all the credit for providing the energy that flows through food webs by converting light energy into food.

However, there is another lesser-known reaction. Chemosynthesis does the same thing but draws from chemical energy instead. This reaction is what supports the diverse communities we see at hydrothermal vents.

Could hydrothermal vents have sparked the origin of life on Earth?  

Chemosynthetic bacteria found in these communities use the toxic hydrogen sulphide released by hydrothermal vents to convert carbon dioxide into organic carbon molecules.

These form the building blocks to all life on Earth.  

It’s this nifty reaction that’s enabled deep-sea organisms to adapt and survive at hydrothermal vents.  

Animals living here have formed symbiotic relationships with these bacteria which can be incorporated into tissues (endosymbiosis) or on the animal surfaces (ectosymbiosis).

These chemosynthetic bacteria provide energy from the environment for their host. This can be so efficient that some creatures (such as the annelid tubeworm) don’t need to feed at all. 

The discovery of these self-sufficient ecosystems cast new light on the origins of life on Earth. It was here that the unique conditions were suitable for a spontaneous metabolism (the spontaneous formation of molecules that are essential for all life) to occur. 

This discovery gave rise to the question: Does the Ocean hold secret to the origin of life on Earth? 

Theories on the origin of life range from lighting speeding up reactions to comets delivering organic molecules from outer space.

The ancient process of chemosynthesis precedes photosynthesis, and likely sustained the earliest life on Earth. 

Bacteria were some of the first life forms to emerge. The most striking piece of evidence are the parallels between the chemistry spontaneously occurring at these vents and the core metabolic reactions found in these single-celled organisms. 

Cyanobacteria are an ancient group of photosynthetic microbes which represent one of the earliest forms of life on Earth. With fossils dating back to 2000 – 3500 million years ago, these single-celled organisms evolved photosynthesis, allowing life to rise up from the darkness below.  

The rest is history. 

Since their discovery, hydrothermal vents have become the most popular theory among scientists for explaining the origins of life on Earth. Yet much remains to be discovered. Secrets still held within these mysterious ecosystems have the potential to revise our life-on-Earth theories once again. 

Cover image via Research Feature.

Post by

Olivia Jones

Bioluminescence: Lighting up a lightless world. 

While bioluminescence is everywhere throughout our Ocean, it’s the only source of light in the deep-sea. 

A staggering 76% of all Oceanic marine animals are capable of bioluminescence, which means that they can produce their own light through chemical reactions inside their body.  

How does bioluminescence work in the deep Ocean?  

Bioluminescence is a chemical reaction that occurs when the light-emitting molecule called luciferin reacts with a luciferase enzyme, releasing energy in the form of light. 

It’s an active process, meaning it can be turned on/off, as opposed to the passive traits of fluorescence and phosphorescence. 

Some bioluminescent organisms generate their own light. Others take up bioluminescent bacteria from the water column and house it in their light organs in a symbiotic relationship. 

Marine bioluminescence is commonly expressed as blue/green light. This is most likely because these wavelengths travel further distances through the water. They’re more also easily visible in the deep Ocean.   

In rarer cases, red and yellow bioluminescence have also been observed in marine creatures.  

Why do marine animals emit light at all?   

In the lightless world of the deep Ocean, marine creatures have adapted to use bioluminescence to their advantage:  

1. Deep-sea anglerfish have a specialised lure to attract prey. 

Perhaps the most famous bioluminescent predator is the deep-sea anglerfish.

This ferocious hunter has a large head, incredibly sharp teeth and a long, fishing-rod-like structure that extends out from the top of its head. At the end of this rod is a ball (called the esca) which contains glowing bacteria called Photobacterium. Ringing any bells? You may recognise her from Finding Nemo.

This lure is used to attract curious prey and is also useful for finding a mate in the vast, dark expanse of the deep Ocean. 

2. Vampire squid expel bioluminescent mucus to deter predators. 

When threatened, the vampire squid inverts its body, raising its arms over its head to expose rows of spikes to deter attackers.

And if that’s not deterrent enough, they also eject a sticky, bioluminescent mucus which can startle, disorient, and confuse predators.

This defensive tactic can buy the squid enough time to escape, while also covering its predator in brightly lit fluid, leaving them vulnerable to attack.  

3. Stoplight loosejaw dragonfish have red flashlights to see in the dark. 

Stoplight loosejaw dragonfish have special red-emitting light organs beneath their eyes that can be activated to look for prey.

The stoplight loosejaw is the only known animal to use chlorophyll pigments (usually found in plants) inside its eyes, which allows it to see red wavelengths of light. 

They use these red beams as a flashlight to search for prey. Since most deep-sea fish can only see blue light, these predators have a huge advantage. They can see their prey, but their prey can’t see them.

4. Lanternfish use light to blend in. 

Lanternfish have adapted an ingenious ability to camouflage themselves using light. 

These masters of disguise have rows of photophores (light-emitting organs) on their underside. They emit a faint glow which allows them to blend in with any remaining light that filters down from the surface.

This process is known as counter-illumination and renders them almost invisible to attackers hunting from below.  

Some marine animals use counter measures against bioluminescence in the deep Ocean.   

Light from bioluminescence has the potential to reveal the whereabouts of creatures that hide in the darkness of the deep Ocean. 

To counter this, many take measures to disguise themselves or break up their outline. 

Many deep-sea creatures are dark red in colour. Red wavelengths of light are the first to be absorbed in the Ocean, and very few deep-sea creatures can see red light (the stoplight loosejaw being a notable exception). Red-coloured creatures therefore appear black and blend in against the near-lightless backdrop.  

Others have ultra-black skin that can absorb light from bioluminescence. For example, pelican eels are found in the midnight zone (where there’s no sunlight, and life exists in complete, constant darkness). Their skin can absorb up to 99.7% of light, rendering them virtually undetectable, even when exposed to bioluminescence. 

Transparency is another technique used for camouflage in the deep Ocean. The glass squid has been observed as deep as 2,000m, and is almost completely transparent. The only organ visible through the tissue of this small-tentacled, swollen-bodied squid is the red-coloured digestive gland. This makes it difficult to be spotted by even the most astute predator. 

Human ingenuity often takes inspiration from nature, and bioluminescence is no exception. 

Due to its unique ability to produce light without the need for an external light source, bioluminescence has been utilised in the field of medical research.

Particularly in imaging and probe techniques for cancer detection and cell culture research, bioluminescence has helped us to detect and respond to disease more effectively.  

With so much of the deep Ocean left to discover, each unique finding may lead to new and exciting medical applications.  

Bioluminescence, therefore, not only lights up the lightless world of the deep Ocean but can also shine a light on our human mysteries too.  

Post by

Jemma Sargeant

A dead whale descends into the darkness of the deep-sea abyss.

In life, these majestic creatures travel vast distances playing an important role in surface ecology. But even in death, their decaying remains become a haven of life on the black Ocean floor.

Here in the deep Ocean the environment is sparse, offering fewer resources to sustain life. What falls from above, marine snow, is the steady trickle of dead organic material and supports an array of life on the seabed. 

A dead whale is a 30-tonne avalanche of fat and organic carbon, equivalent to more than 1000 years’ worth of marine snow across 100 square meters. 

Eventually, a whale fall (a whale’s death) becomes an island of biodiversity in the deep-Ocean.

1. It all starts with a feeding frenzy. 

Soon after the whale falls, a variety of species descend upon it and the dinner party begins.

The first to arrive are the large Ocean wanderers such as hagfish (eel-shaped jawless fish) or gigantic sleeper sharks. These mobile scavengers remove soft tissue by rasping or tearing at the flesh exposing the energy-rich skeleton, giving the name of this phase the mobile-scavenger stage.

2. As the pieces get smaller so do the scavengers. 

It can take up to two years for the mobile-scavengers to finish feeding on the whale, where the next wave of guests arrives in a second phase known as the enrichment-opportunist stage.

Animals like polychaetes (a class of marine worms) and crustaceans including amphipods (shrimp-like crustacea) will move in to feed on remaining blubber and burrow into the nutrient enriched sediments surrounding the whale.  

3. Finally, only the bones of the whale remain.

These would seemingly have no further use. However, ecological diversity is about to flourish in the sulfophilic stage of the whale fall. The whale’s bones provide a large reservoir of energy-rich lipids, a shining prize to deep-sea organisms. 

Bacteria break down fatty lipids in the bones, releasing sulphides. The sulphides can be used to generate energy, in a process called chemosynthesis (producing food using chemicals as an energy source instead of sunlight).

These chemosynthetic bacteria have become resistant to sulphides’ toxicity and can establish bacterial mats which act as a foundational food source, supporting a huge array of marine biodiversity: sponges, mussels, limpets, sea spiders and snails.

The breakdown of bone-lipids can take 50-100 years and these mini-ecosystems are highly significant for seabed ecology. Even then, after the complete extraction of nutrients, it isn’t over.  

Decades after a whale dies, the whale-fall is still essential to marine ecosystems:  

Some scientists believe there’s a further stage of succession: the reef stage. Even after the feeding frenzy, the whales’ bones can remain for more than 100 years, acting as hard surface for suspension feeders to settle.

These ‘habitat islands’ act as evolutionary stepping stones between other seafloor ecosystems like hydrothermal vents. This may have allowed sulphide-specialised organisms to spread across the seafloor and diverge into new species. 

Despite whale-fall ecosystems being poorly sampled, 407 species have been found living off the carcasses globally, which is high for the bottom of the sea. Of these, 21 species can only be found on whale-fall, known as whale-fall specialists.

Whale-fall specialists are species that require a whale carcass to complete their lifecycle and maintain their populations. These marine organisms will jump from habitat island to island to survive.

For example, Osedax, Latin for “bone-eater”, are a genus of polychaetes (marine worms) found worldwide.  They are important ecosystem engineers by eroding whale bones and allowing rarer species to colonise the whale skeleton.  

How whale populations impact the global Ocean? 

Whale-falls also contribute to the conversion of inorganic carbon (CO2) into organic carbon (marine life), a set of processes known as the Biological Carbon Pump (BCP). This carbon is sequestered (stored) in the deep Ocean.  

Whales deliver huge amounts of carbon in their biomass to the seafloor, which is then locked-away for centuries within deep-sea sediments.

Any threat to whale populations will threaten entire ecosystems and disrupt the process of carbon sequestration.

Commercial whaling, for example, has been depleting whale populations for around 1000 years, beginning in 1000CE. Experts agree that tens of millions of whales were likely killed during this period, pushing many whale species to extinction and causing the extinction of whale-fall specialist species, who rely on whale falls for survival.

A single whale-fall can provide everything a whale-fall specialist needs for 50-100 years, meaning there is a lag-time of at least 30-40 years before the decline in whale populations is felt. Which is to say, if whale populations can recover, we may be able to mitigate the impacts on deep-sea ecosystems

As we follow the timeline of a whale’s life, we can see the incredible contribution whales make to the Ocean.

From enhancing surface ecology in life, to supporting entire ecosystems in death. 

Post by

Olivia Jones