The humble sea cucumber is much more than a harmless marine creature, although it may look like an oddly shaped cucumber or fat sausage it may come as a surprise to many that this animal has a few defensive strategies of its own to combat predators. Below we explore some of the ways in which a sea cucumber defends itself from hungry predators such as stingrays, sharks and other fish.
The first and most common way a sea cucumber defends itself is by ejecting a kind of sticky net from its anus. This net composed of eggs and sticky threads of a protein-like substance which can entangle and immobilize a predator. A second common defensive strategy involves the sea cucumber expelling its organs through their anus. This process is known as evisceration and it can be used as a defense mechanism when a predator attempts to grab onto the cucumber. The lost organs will eventually grow back over time however the predator often takes the bait and in its haste to consume the organs it furthers itself away.
Experts believe that sea cucumbers may also sense electrical signals through their skin in order to detect threats before they even occur. When the cucumber senses the impending presence of a predator it can quickly bury itself in the sand or mud and to hide from its aggressor. Additionally, some species have developed tough, leathery skin and harder exteriors which can help to protect them from predators.
Finally, a study by scientists from the University of California in 2019 have shown that some species of sea cucumbers may also brandish retractable spikes called Cuvierian tubules from their skin. These spikes have a multitude of defensive purposes such as scaring away smaller fish as well as larger predators. The spikes are capped with specialized venom cells and they can be fired in half a second releasing a toxic substance which can immobilize a predator in an instant.
Physical Defence Mechanisms
In addition to the above mentioned tactics sea cucumbers also exhibit physical defensive behaviours such as rolling up into a tight ball and burrowing into the sand to avoid sight. This behaviour is commonly referred to as ‘balling up’ and it helps to keep the cucumber hidden from hungry predators.
Moreover, sea cucumbers may also remain still and play dead in situations when evasive manoeuvres are not practical. This behaviour can also aid in masking the cucumber’s scent and it often works as predators typically feed off moving prey. In a study conducted by researchers from Okinawa Institute of Science and Technology it was concluded that sea cucumbers have the ability to sense light and dark and by quickly transitioning their pigmentation they can engage in ‘cryptic behaviours’.
While the examples analysed above are mostly applicable to species which impose physical harm there are also species which feed off of cucumbers such as the Amphipholis sp. This species is uniquely adapted to disrupt the pedofilament network of the sea cucumber and thus, it is effective at quickly disabling them. To combat this threat cucumbers have developed a unique method in which they will emit a special shockwave along the pedofilament network which the predator uses as a conduit.
Chemical Defence Mechanisms
Finally, sea cucumbers have their own unique defence mechanisms which are preprogrammed into their DNA such as those which trigger the production of chemicals to repel predators. Through a series of chemical compounds produced by the cucumber they can emit a noxious odour which is specialised to deter species which may feed off of them.
Furthermore, there are also species of sea cucumber which emit a light which can repel certain species of predator. This light is emitted in the form of luminescent fluids and studies have suggested that its primary purpose is to scare away smaller predatorial fish. The luminescent compounds are composed of powerful proteins which are found in organs located near the anus and they can be switched off by the cucumber just as quickly as they are switched on.
Defensive Behaviour Tactics
In a study conducted by marine biologists from COI it was found that sea cucumbers often resort to defensive behaviour such as kin recognition and social aggregation. Through searching for an individual or group of sea cucumbers which are the same species they create a stronger collective force which can effectively fend off larger predators. Additionally, kin recognition is beneficial since it has been asserted that the sea cucumbers have the capacity to sense a family member and thus act as a collective defence.
Moreover, some species of sea cucumbers also have a specialised secretory system which can be used to detect predators and then to send out a signal to alert the other cucumbers of the impending danger. By emitting a specialised gas such as ammonia or hydrogen sulphide they send out a warning to their kin which is effective at preventing larger predators from approaching.
Furthermore, some species of sea cucumbers may also emit a sound which can scare away predators as well as competing species of cucumber. This sound is emitted in the form of pops or clicks and can travel up to several metres. Scientists believe that the sound issued is used as a warning and it may also function as a sort of communication between the cucumbers.
Mimicry Strategies
In summary, it can be seen that the sea cucumber has a diverse range of strategies which it can employ in order to defend itself against potential threats. A further method which is employed is that of mimicry where the cucumber takes on the guise of another species in order to deter predators. Through the adoption of patterns on its skin the cucumber can effectively mimic the colouring and behaviour of a species which predators tend to avoid. This strategy is commonly seen in multiple species of sea cucumber and is effective in deterring predators.
In addition to this, the cucumber may also use its shape shifting abilities to its advantage. In a study conducted by scientists from the Biodiversity Institute of Ontario it was discovered that some species of sea cucumber have the capacity to change their shape upon the approach of a predator. This is beneficial as it gives the cucumber more accessible surfaces to burrow its body into and thus, increase its chances of survival.
Finally, some species of sea cucumber have even developed defensive tentacles which they may use as a last resort when other defence mechanisms are not possible. In an article published by SCIENCE Magazine in 2018 it was demonstrated how some species of sea cucumber have the capacity to deploy long tentacles which can wrap around predators in an attempt to immobilise them. The tentacle is composed of a slime like material which is difficult to remove and it can ensnare predators for extended periods of time.
Defensive Tactics In Summary
In conclusion, it can be seen that the sea cucumber is a resourceful and ingenious creature as it has the capacity to defend itself against predators from a range of sources. This includes its ability to eject sticky substances, emit foul smelling gases, discharge spikes, play dead, hide in the sand, mimic the colouring and behaviour of other creatures and even deploy defensive tentacles. It is through these defence mechanisms that sea cucumbers are able to survive and prosper in turbulent marine environments.
