In 2017, biologists described in the journal PeerJ a behavior they had watched in aquariums, in boxer crabs collected in the Gulf of Eilat, in the Red Sea. When one of its two anemones was taken away, the crab tore the remaining one in half. Each half grew back.
That kind of detail was missing from the old version of this top 10. It mixed bees, ants and lorises in with marine life, and it gave pufferfish and blue-ringed octopuses weapons they simply do not have. So we sorted it out.
Here are ten documented marine defenses, grouped by strategy, along with what they mean for you if you dive or snorkel near these animals.
Which marine animals sacrifice part of their body to survive?
Sea cucumbers and sea stars use autotomy: they deliberately give up part of their body to escape danger. The sea cucumber ejects its internal organs. The sea star drops an arm. Both then regrow what they lost, at a pace that varies a lot from one species to the next.
In sea cucumbers, this is called evisceration. A review published in 2022 in the journal Biology lists several observed triggers: an encounter with a predator, but also crowding, water that is too warm or a lack of oxygen. Depending on the order the animal belongs to, the organs come out through the front of the body or through the cloaca. The digestive tract almost always goes. The respiratory trees often follow, and sometimes the gonads.
Regrowth follows no single timetable. The same review reports about one week in Holothuria scabra, roughly a month in Holothuria glaberrima, and 45 days before a return to normal in Apostichopus japonicus.
The old article’s figure of “six weeks” for every species did not hold up. Some sea cucumbers in the family Holothuriidae have another weapon too, Cuvierian tubules: sticky threads they shoot at an attacker. They stick.
The sea star, for its part, lets go of an arm that a predator has grabbed or injured. In a few species, a detached arm that carries part of the central disc can even grow into a complete new animal. That ability does not make it untouchable, though. The giant triton, a sea snail that preys on the crown-of-thorns starfish on the Great Barrier Reef, is proof of that.
Boxer crabs and hermit crabs: what do they use anemones for?
Boxer crabs of the genus Lybia hold a small anemone in each claw. They wave it at an attacker and also use it to catch food. Hermit crabs live in empty shells, and some species place an anemone on the shell, whose stinging cells add to their protection.
The word “mutualism” that this page used to apply here needs a caveat.
The authors of the 2017 PeerJ study recall a finding from their earlier work: the crab controls how much food its anemone gets, and so how much it grows. It stays small. When a crab without an anemone meets one that has them, they fight, and the first crab almost always leaves with a whole anemone or a piece of one. Genetic analysis shows that the two anemones held by a single crab are clones. The researchers describe it as a unique case of one animal inducing asexual reproduction in another.
But does the anemone get anything out of it? We cannot settle that here.
The hermit crab plays a different game. Its borrowed shell is far from “impenetrable,” despite what the old text claimed, and it has to swap shells as it grows, which leaves it exposed during the move. In the Mediterranean, you will often see the anemone Calliactis parasitica attached to the shell of certain hermit crabs.
Why do cuttlefish and octopuses vanish so fast?
Cephalopods combine two defenses: camouflage and ink. A cuttlefish changes the color and texture of its skin in an instant, thanks to pigment cells controlled by its nervous system. Octopuses, cuttlefish and squid can also release a cloud of ink before they shoot off.
Those cells are called chromatophores. Each one holds a pigment sac surrounded by tiny muscles, and the nervous system controls when they open, which explains how fast the change happens. Small bumps called papillae rise or flatten, and the skin goes from smooth to grainy. The animal’s outline blurs against rock or algae.
Still, camouflage sometimes fails.
Ink is the backup plan. It contains melanin, and in some cephalopods a denser cloud mixed with mucus keeps a rough version of the animal’s shape while the animal itself moves away, pulling the predator’s attention elsewhere. In short, the decoy stays put. The real one leaves. That is often where a successful escape happens.
One more thing for you as a diver: a cloud of ink means an animal has been disturbed, which is a good reason to back off.
Which marine animals borrow or produce a poison?
Three groups on this list rely on chemistry. Some nudibranchs recycle the defenses of their prey, pufferfish store tetrodotoxin in their tissues and inflate with water, and the blue-ringed octopus delivers that same toxin through a bite. Its rings act as a warning.
| Species | Defense | What the old article said | Correction |
|---|---|---|---|
| Aeolid nudibranchs | Stinging cells stolen from their prey | Toxic bubbles | No bubbles; stored in the cerata |
| Pufferfish | Inflation and toxin in its tissues | Releases its toxin into the water | Toxin stays in the body, dangerous if eaten |
| Blue-ringed octopus | Venomous bite | Sprays venom | No spraying; envenomation by bite |
| Archerfish | Water jet | Long-range defense | Hunting technique, removed from this list |
Aeolid nudibranchs feed on hydroids or anemones without setting off their stinging cells. They move those cells to the tips of their cerata, the finger-like growths that cover their back, where the cells stay ready for use. A fish that bites a ceras gets the sting. Other nudibranchs take chemical compounds from the sponges they graze on, and their bright colors often work as a warning signal. It is recycling.
The old version talked about “toxic bubbles,” an image that matches no mechanism ever described in these mollusks.
The pufferfish swallows water until it becomes a ball that is hard to swallow. Its toxin is concentrated in certain organs, which vary by species, and it does not spread into the water around it. The blue-ringed octopus lives in the Indo-Pacific. Its bite can kill a human.
What should you do if you meet these animals while diving or snorkeling?
Look, but do not touch. Several defenses on this list are triggered by contact: a nudibranch’s stinging cells, a blue-ringed octopus bite, the stress of a pufferfish that inflates. A sea cucumber that gets handled can also eviscerate. If your dive center gives a briefing on local marine life, follow its instructions first.
Touching marine life has a cost, for the animal and for you. We cannot promise you will meet any of these species, since their presence depends on the site, the season and luck. Nothing is guaranteed. Good visibility does not change the safety picture either. It just makes it easier to get close to an animal you should not have brushed against.
The last defense in this top 10 belongs to the pistol shrimp. It snaps its large claw shut so fast that it creates a bubble, and when that bubble collapses it produces a loud crack and a shock wave that can stun small prey or drive off an intruder.
So the choice is quick to make.
Keeping your hand away from a nudibranch costs you a slightly wider photo. Touching it can cost you a sting, and cost the animal needless stress, or even an injury. You can weigh that in two seconds.
FAQ
Does the archerfish use its water jet for defense?
No, it uses its water jet to hunt. It aims at insects resting above the surface, knocks them into the water and eats them. That is why we removed it from this list.
How long does it take a sea cucumber to regrow its organs?
It depends on the species. A review published in 2022 in Biology gives about one week for Holothuria scabra, one month for Holothuria glaberrima and 45 days before a return to normal for Apostichopus japonicus.
Do boxer crabs live in mutualism with their anemones?
The crab clearly benefits from the anemones, which it uses for defense and for feeding. A 2017 study shows that it limits their food, steals them from other crabs and tears them in half to get two. Whether the anemone benefits is still debated.
Is the blue-ringed octopus dangerous to divers?
Its bite delivers tetrodotoxin and can be fatal to a human. It does not attack without a reason. The risk comes mostly from handling it, and if someone is bitten, call the local emergency number (112 in the EU) right away.
Sources
- Boxer crabs induce asexual reproduction of their associated sea anemones by splitting and intraspecific theft (Schnytzer et al., PeerJ, 2017)
- A Review of Histocytological Events and Molecular Mechanisms Involved in Intestine Regeneration in Holothurians (Su, Yang and Sun, Biology, 2022)
Photo: Diego Delso, Wikimedia Commons, CC BY-SA 4.0
