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I’ve always been fascinated with what goes on in the ocean. There’s a world under the waves that is filled with amazing creatures that display some incredible behaviors. One great example of this is the ability of some creatures to seemingly become architects of their marine environments and are able to construct nests and shelters to keep themselves safe.
Types of Structures Built & Used by Marine Creatures
Every animal has its own unique needs and this means that not all of the underwater structures we see are the same. Depending on what the animal is trying to achieve, it may create a burrow, a nest, or even make use of something constructed by another creature.
Most people are familiar with the term nest but just what exactly is a nest and how do animals use them?
The primary function of a nest is somewhere for a creature to lay its eggs and raise its young. These structures are designed to offer protection from extreme conditions, the ocean current, and predators since eggs and young are more vulnerable to these factors.
What’s more, as the eggs develop, they require a stable environment for the embryo to successfully grow and a nest provides this. But even before eggs are laid, some creatures will make a nest as a way of attracting a mate.
Each marine creature has its own special way of constructing a nest using a variety of materials, including sand, rocks, shells, and sometimes, even their own bodies. For example, some aquatic species, like frogs and fish might make a nest from bubbles. While others, like the stickleback, secrete a special substance from their kidneys to hold together sticks and plant stems, creating a rounded nest.
Moreover, the methods in which a particular species constructs a nest can vary greatly. With so many different types of nests, they form the very structure of the habitat in which they are found and can even influence things like sediment distribution and nutrient cycling.
Fish such as mudskippers and plecos are known for building burrows in which to take shelter and protect their young from predators and strong ocean currents.
These burrows are often sound within the sediment and may be a simple tunnel or a more complex system of tunnels and chambers. Within the burrow, the controlled conditions are perfect for raising young, but many creatures also use them as somewhere to hide out while they wait to ambush passing prey.
After a burrow has initially been built, many marine creatures will continue to modify them according to their changing needs and to maintain the burrow.
Even more interesting is that some marine animals form mutualistic relationships with others that burrow. A great example of this is the snapping shrimp and the gobi, whose combined efforts ensure protection for both species.
Some fish, like cichlids, don’t create a burrow but rather a pit in the substrate, usually sand, in which to lay their eggs.
Structures Using Debris
The marine environment is filled with natural debris and there are lots of creatures that use this to their advantage. The type of debris can vary greatly but may include things like seaweed, rocks, and discarded shells. Doesn’t this just show how resourceful and intelligent some of these creatures really are!
The wonderful thing about using debris like this is that it’s often the right color for camouflage, offering further protection to the creature. For example, lamprologus fish will inhabit old snail shells which they bury in the sand, keeping them safe from predators.
Many crab species, including the spider crab are well known for using debris to construct a nest, and some even use special secretions to adhere the shell to their own bodies.
Utilization of Existing Structures
As well as making use of the debris found in the ocean, some creatures will use existing structures to create a nest. They don’t always just move in and get on with their lives; there are some animals that will modify the structure to meet their own unique needs.
The benefit of using a structure that already exists is that a creature can ramp up its protection from predators. Many of these structures include caves and crevices where several species of fish will lay their eggs on the cave ceiling.
Amazingly, there are even some animals that use other lifeforms for protection, such as certain species of shrimp which live within sponges. Not only does this offer protection, but it gives them easy access to food particles and a place to mate. However, while this is a common phenomenon, scientists aren’t sure whether this is a parasitic or mutualistic relationship, uncertain as to whether the sponge benefits from it.
Marine Species with Nesting & Shelter-Building Behaviors
Many creatures in the ocean have unique and exciting ways of building a shelter and protecting themselves. Let’s take a look at some of the most interesting.
1. Damselfish (Pomacentridae)
There are around 250 species of damselfish (Pomacentridae) which are mainly tropical, marine creatures. One of the things that they’re well known for is the way they create their nests in the sandy seabed. The damselfish will clear out an area of the sand, near rocks or coral reefs, in which to lay their eggs. Not only does this method ensure protection, but it also camouflages the eggs from predators.
There are even some species of damselfish that will build their nests in close proximity to one another for protection; as they say, there’s strength in numbers. In any case, it’s usually the males that guard the nest and can be very territorial in order that their young survive. After females lay their eggs, the males will oxygenate them through fanning and keep the nest clean and defended. However, once the eggs hatch, both parents are involved in caring for the young. There are even some, like the Altrychthis damselfish that will care for young laid by brood parasites.
Other species, such as the Hypsypops rubicundus will carefully clean the nest site of any organic matter but will leave behind certain things, like red algae, which serves as a nutritious food source for the juveniles once they hatch.
2. Gobies (Gobiidae)
Found hidden among the coral reefs all over the world, gobies from the family Gobiidae, consist of more than 2000 species. One of the things that is common among these fish is their habit of burrow building in the sandy reefs which they inhabit. These burrows are normally found close to crevices and other structures and act as a form of protection from extreme conditions and against predators.
One such example is the burrowing goby (Trypauchen vagina) which is found in the Indo-Pacific region and is known for digging burrows that are irrigated by water to keep them oxygenated. But once the burrow is constructed, the work doesn’t end for the goby as it’ll keep clearing debris and reinforcing the burrow walls to ensure its structural stability.
Burrows are used for shelter and for protecting eggs and fry and the gobies will protect their burrows using tactile and visual communication to establish their territory. Depending on the species, these burrows can be very intricate with a system of tunnels, chambers, and entrances.
Some goby species will move into the nests of mud shrimps (Austinagebla edulis), where they form a symbiotic relationship. The gobies have protection from the burrow while their droppings serve as food for their hosts.
3. Mantis Shrimps (Stomatopoda)
There are around 450 species of mantis shrimp (Stomatopoda) that grow between 3.9 and 11.8 inches (10 and 30 cm) in length. Each species has its own behaviors, but one thing that common among them all is the intricate burrows they build in the sea floor, which comprise a series of complex tunnels, chambers, and entrances. They have special appendages which are perfectly designed for digging and shaping the burrows.
Not only this, but mantis shrimps will also gather debris, like rocks and sand, to strengthen their burrows which they use as both a protective home and somewhere from which to ambush their prey. They have powerful claws which allow them to effectively grab their prey in one swift movement.
The type and shape of the nest varies by species. For example, the peacock mantis shrimp (Odontodactylus scyllarus) builds a U-shaped nest near coral reefs. There are even some species that can’t be bothered to build their own nests and will fight their neighbors to take over an existing nest.
Interestingly, mantis shrimps will build their burrows close to other individuals and will use a series of vibrations to communicate. What’s more, when the mantis shrimp is away from home, it can easily find its way back using polarized light, the sun, and specialized senses.
The mantis shrimp move sediment around which supports nutrient cycling within its environment, making these creatures essential to their ecosystems.
4. Mudskippers (Oxudercinae)
The mudskipper (Oxudercinae) is an unusual creature that lives in the water as well as on land. It is known to form small social groups with which it builds interconnected burrows to enhance protection against predators. However, these burrows, which are typically built along the edge of the water in mud or sand, also provide the mudskippers with a place to digest their food.
Most mudskipper burrows consist of one entrance that leads to branching tunnels and, during low tide, the chambers may fill with water. This ensures hydration, however some species, like the Periophthalmodon schlosseri, store air in the burrow. You’d think that the mudskipper digs out its burrow but on the contrary, these slender fish with legs, use their mouths to pick up and sling mud out of the way.
When on land, mudskippers have a range of adaptations including the ability to keep their skin moist which allows them to continue absorbing oxygen out of the water. However, in order to retain this moisture, the mudskipper must hide out in its burrow which allows it to stay moist and breathe.
5. Parrotfish (Scaridae)
Parrotfish, belonging to the family Scaridae, are a group of around 95 species that are known to be incredibly healthy for the coral reefs where they live thanks to their algae grazing habits. But these fish are also interesting in terms of their nesting behaviors.
Instead of creating a permanent nest, the parrotfish is able to secrete mucous bubbles which cocoon it as it sleeps and keep it protected from would-be predators and other threats. It’s important to note that parrotfish don’t sleep in the way that you or I do but instead enter a period of reduced activity, leaving them more vulnerable.
However, this isn’t something we see in all parrotfish, but some species, like the queen parrotfish (Scarus vetula), secrete these mucus bubbles from the mouth. It’s thought that the mucus masks the fish’s scent and provides it with a warning if a predator tries to break the membrane. Plus, that slime doesn’t taste too good!
While it is thought that the cocoon provides some form of camouflage, there are certain parrotfish species such as swathy parrotfish (Scarus niger) and iridescent parrotfish (Calotomus viridescens) that are naturally colored for blending in.
As well as their algae grazing, parrotfish contribute to reef health because their cocoons aid in nutrient cycling. What’s more, these cocoons are often inhabited by other organisms that form a symbiotic relationship with the parrotfish.
6. Hermit Crabs (Paguroidea)
If there’s one species that’s incredibly well known for its use of discarded shells as a home, it’s the hermit crab (Paguroidea). But did you know that these crabs have to choose a suitably sized shell as they’ll only grow to within its limits? Like other crustaceans, the hermit crabs molt their exoskeletons as they grow. When this happens, they’ll also drop their shell and find a new one to grow into. However, while their exoskeleton reforms, obtaining a new, suitable shell is crucial for them, as it provides protection, and they can retreat inside it when threatened.
More often than not, hermit crabs will use sea snail shells and these provide them not only with protection but a portable home which allows them to forage while still being protected. What’s more, many of their chosen shells provide them with camouflage from predators. There are even some species that decorate their shells with anemones and algae in order to look more attractive and boost their ability to blend in. There’s even one species called the blanket hermit crab (Paguropsis Henderson) that skips the shell altogether in favor of a sea anemone which protects it with its stingers.
But choosing a new shell isn’t without its challenges, as many hermit crabs may fight for the best shells. It’s not uncommon to see many crabs in a group, looking for a new shell that fits them perfectly.
7. Triggerfish (Balistidae)
Triggerfish, from the Balistidae family, are a group of around 40 fish that are known for their bright colors and unique markings. When it comes to nesting, they’re just as unique, with individuals creating nests in the sandy seabed. These fish have powerful teeth and these are used, along with blowing air from the mouth, to clear away sand.
These nests tend to be circular, and females will lay their eggs here with the males standing guard over both eggs and mom as well as maintaining the nest and keeping it clean.
Males get very territorial during mating season and also use their strong teeth to ward off intruders, including humans. However, it’s mainly the Titan triggerfish (Balistoides viridescens) that is the most aggressive. Interestingly, the triggerfish territory extends upwards as opposed to outwards so swimmers are encouraged to make a horizontal getaway if they’re confronted by a territorial daddy trigger.
Usually, triggerfish create their nests in sandy areas, and they’re often found close to coral reefs where this very behavior is vital to the biodiversity of their habitat as well as for things like nutrient cycling.
8. Decorator Crabs (Majidae)
Making a nest doesn’t always mean building a physical structure. In fact, the decorator crabs from the Majidae family use their own bodies to make extraordinary structures. They do this through the use of debris like shells, sponges, algae and other items with which they adorn their exoskeletons.
This special adaptation is aided by the crabs’ bristles and hooks that allow them to select, grasp, and attach their chosen debris. Items are typically chosen according to the surroundings as a way of helping the crab camouflage and keep them safe from predators. In some cases, even living things like anemones are used.
But how does decorating oneself draw attention away? It seems counterproductive until you learn that these crabs do this to make their outline less obvious. As the environment changes, the crab will modify its decorations to continue blending in. One example of this was seen in the great spider crab (Hyas araneus). A group of individuals was initially decorated with seaweed but when they were relocated, the crabs switched their adornments in favor of local materials.
While this is an ingenious way to ensure survival, decorator crabs are known to affect the predator-prey dynamics within their ecosystems.
9. Pufferfish (Tetraodontidae)
There are around 120 species of pufferfish (Tetraodontidae), and as well as being known for their ability to inflate their bodies as a form of self-defense, they’re also known for creating intricate designs in the sandy seabed which they make with their bodies and fins.
It’s the males that perform this behavior in an effort to attract a mate. Once they attract a female, these designs are then used as a potential nesting site. But there’s hot competition as females will scour the ocean floor looking for the best designs, which may be made up of intricate shapes, ridges, circles, and much more. What’s super amazing is that males can spend up to 6 weeks making their designs, all for the sake of mating for a few seconds!
In order to further draw attention, the male will also perform elaborate displays. The type of design often depends on the pufferfish species, and they can also serve as territorial markers. The habitat and specific mating ritual can also affect the type of design each fish may produce. For example, the white-spotted pufferfish (Torquigener albomaculosus) makes beautiful concentric circles in the sand.
10. Spider Crabs (Majidae)
Spider crabs (Majidae), so named because of their long arachnid-like legs, are masters of camouflage. But that isn’t because of their natural color or shape; these amazing crabs have the ability to adorn themselves with organic debris, such as small creatures and seaweed to help them blend into their environment.
In order to do this, spider crabs have special structures on their bodies and hooked hairs which allow them to attach debris similar to that found in their habitat, therefore helping them to conceal themselves. This is a great defense mechanism against predators like rays and octopus. If the crab’s surroundings change, then it will modify its decorations to better fit in.
There are even some species, such as the Notomithrax ursus that covers itself in algae and uses this not only for decoration but as a food source. The common spider crab (Maja bracydactyla) also uses algae and is so efficient in its camouflage that even professionals have trouble spotting them among the seaweed.
Just like other crab species, spider crabs undergo regular molts where they shed their exoskeleton. This also means getting rid of their current adornments and replacing them with new ones once the molt and regrowth is complete.
11. Marine Worms (Polychaeta)
The Polychaeta family is made up of more than 10,000 species of marine worms, and many of them are known to build tubular shelters using materials found in their habitats, such as fragments of shells and sand. These tubes are designed to keep the worms safe from predators and other threats within the environment.
Depending on the species of the worm and its unique anatomy, the tubes can vary in size, shape, and overall design. What’s interesting is that traction between the tube and the body of the worm dictates how it can move around inside and can even prevent it from being pulled out by a hungry predator. There are even some species that are able to secrete a special mucus to hold the tube together. At times, worms will mix the mucus with stone and sand which allows them to attach the tube to a surface.
Some deep sea species will also produce mucus that acts as a food source for bacteria. In this symbiotic relationship, the worms benefit as the bacteria provides a shield from the heat of deep sea vents.
Other species have specialized bristles that enable them to burrow into the substrate. In doing so, they not only gain protection but also establish a base for feeding.
12. Coconut Crabs (Coenobitidae)
Coconut crabs, from the Coenobitidae family, are among some of the largest crabs, growing to around 3.3 feet (1 meter) and are found in the coastal regions of the Indian and Central Pacific Oceans. Here, they make use of naturally occurring shelters like fallen trees and crevices within the rock or even existing burrows made by other creatures.
These crabs might not build their own nests, but they are well known for their innovative use of discarded shells which they use as a form of protection. Like other crabs, such as the hermit crab (Paguroidea), coconut crabs will switch their shells as they grow, choosing something that perfectly fits their size. What’s interesting is that it’s only the juveniles that will use shells for protection. Once they reach adulthood, they develop a very thick, hard skin that affords them the protection they need.
Not only this, but the coconut crab is an adept climber, often seen scaling trees in search of shells and food such as fruit and nuts. Their opportunistic and scavenging feeding makes them important nutrient cyclers and shapers of their ecosystems. However, when a predator, like a lizard or feral pig, comes close, they will retreat into their shell.
13. Snapping Shrimp (Alpheidae)
The Alpheidae family is made up of around 1119 species of shrimp, each with their own behaviors. But what’s common among them is their ability to make burrows consisting of intricate tunnels made both in muddy and sandy seabeds as well as rocks. In order to make these burrows, snapping shrimp have special claws for digging. What’s fascinating is that these snapping claws make an incredibly loud sound, up to 210 decibels!
Snapping shrimp use their burrows for protection and somewhere to feed, waiting to ambush passing prey and snatch it with their powerful claws, and, at times, they may live in colonies with a queen. Each tunnel system will have various escape routes, should a predator try to attack.
However, it isn’t only the shrimp that live inside for these species are known for their mutualistic relationship with gobies. These fish will often inhabit the burrows for protection, but they also use their vision (snapping shrimp can’t see very well) to guard the nest and watch for danger.
Because of how these burrows are created, snapping shrimp play an important role in shaping the sediment and have an effect on the water flow and nutrient cycling.
14. Octopuses (Octopoda)
Did you know that there are around 300 species of octopus (Octopoda)? These are some of the most intelligent creatures in the ocean and are often found near the sea floor. Here, they’re known to create dens in various areas like rocks, crevices, and even burrows within the sediment.
By creating these shelters, the octopus is afforded protection from predators and somewhere to rest. If it is traveling then an octopus may even build a temporary shelter, but this is usually a more basic structure. While octopuses are usually solitary creatures, scientists have discovered several octopus ‘cities’ where several dens have been built in close proximity to one another.
In terms of their main den, octopuses typically use their long arms to dig or manipulate the substrate as well as using materials like shells and rocks for reinforcement. Normally an octopus will carefully select a location for its den based on prey availability, but they also use their dens to lay their eggs. But after females have laid and cared for their eggs, they die shortly thereafter.
Most octopus species are nocturnal and will leave their dens at night to hunt for things like small fish, crabs, and clams. When they’re threatened by a predator, they’ll quickly retreat to their hideout. However, owing to their coloration, these animals are excellent camouflage experts and can even adjust their texture and color to fit in with their surroundings.
15. Paper Nautilus (Argonauta)
With a name like paper nautilus (Argonauta), it’ll come as no surprise that these cephalopod species make a shell that has a similar texture to paper. A species of octopus, the paper nautilus has a special ability to secrete the material needed to make this shell, although scientists agree that it’s not a true shell but more of a natural structure used to protect eggs.
Within the shell, there are gas-filled chambers that ensure the structure floats on the water’s surface. It’s within these chambers that the eggs are kept but the paper nautilus itself will also spend most of its life within the confines of the shell. However, these shells, which are spun out of glands within the arms of the paper nautilus, are extremely delicate and prone to damage, being constructed of two fibrous layers.
The term nautilus comes from an old world meaning sailor which was given because of how the shells are designed to float. Once the eggs hatch and leave the shell, it will often float to shore where they’re commonly found.
It is worth noting that only the females are able to produce these shells, the much smaller males do not have this ability.