Marine Crabs: The Soldiers of the Ocean

Eating baby turtles got crabs a bad reputation, but there is much more to these ten-legged living armors—and more to their relationship with turtles, too. So, let’s take a deep breath and dive in.

What to Expect in this Article

  • What Kind of Animals are Crabs?
  • The Biology of a Crab
  • Spider Crabs:
  • Pea Crabs:
  • Soldier Crabs:
  • Ghost Crabs:
  • Oceanic Crabs
  • Crabs on our Crabby Planet
  • What’s Next?

What Kind of Animals are Crabs?

Crabs are crustaceans, so closely related to the shrimp, lobsters, and crayfish, we already talked about.

To get from the crustaceans to the crabs, we again follow pretty much the same path: Multicrustacea, Malacostraca, Eumalacostraca, Eucarida, Decapoda, Pleocyemata. This time, we climb on to the Brachyura, the short-tailed crabs or true crabs.

Crustacean Tree of Life designed by Kate Hildenbrand based on the NCBI database

Within the Brachyura, there are two extant groups: the Dromicea and the Eubrachyura. Most of the crabs you might know belong to the Eubrachyura—and all of the ones I’ll talk about today. Though I definitely encourage you look into the sleepy or sponge crabs, as they are just plain adorable.

There are other things humans call crabs—some of them for good reason—but even the more closely related Anomura with the hermit crabs, porcelain crabs, and the like, would just make this article far too long to get anything done properly. I’m sure we’ll return to the hermits, at least, at some point.

And while animals like horseshoe crabs are among the most popular edible “crabs,” they aren’t even remotely crabs, so we won’t get into them today either. Horseshoe crabs might get their own segment at some point, as they are pretty ducking impressive—and also interesting from an ecology standpoint.

Crab shape is apparently popular in evolution—popular enough to warrant a term: carcinization.

The Biology of a Crab

As crustaceans, crabs, no matter if real or imposter, have an exoskeleton and need to molt to grow. In the case of crabs, this exoskeleton is pretty damn strong thanks to mineralized chitin. Well, it’s still pretty damn hard, as ocean acidification is ducking crabs over pretty badly. Crab, I’m breaking the doom-and-gloom rule even earlier than usual, aren’t I.

When it comes to their anatomy, crabs are essentially folded lobsters. They have five limbs on either side, so ten total—as decapods, that makes a lot of sense. Their cephalothorax, so the region that includes the head and chest, makes up essentially their entire carapace. If you look at a crab from the top, that’s all you see. The abdomen or pleon is folded under. The swimming legs are in there, too.

The underside of a male (top) and a female (bottom) individual of Pachygrapsus marmoratus, showing the difference in shape of the abdomen. Photo credit: Wikimedia / Pachygrapsus_marmoratus_2008_G2

To make things a bit more confusing, the telson, so the actual tail, stayed at the butt-end of the crab.

On the head-end of things, there are stalked eyes. Fun fact: these stalks play a role in molting, as they secrete hormones. Weird, right?

In addition to the cute stalked eyes, crabs have two sets of short antennae and one set of long antennae, though long might be a bit misleading here. Compared to our lobster friends, crabs have tiny antennae, sometimes barely visible at all.

Underneath their head, there’s the mouth, mandibles, and a few accessories for easier eating.

Like lobsters—well, the true ones—crabs have claws. Also, like lobsters, these aren’t equal. How unequal they are depends on the type of crab. Fiddler crabs (Astruca mjoebergi) with their vividly colored primary claw are a stunningly pretty example of more unequal claws.

Crabs can be found all over the world in the ocean, freshwater habitats, and on land. They can be a few millimeters small or larger than a human. They can look pretty boring or be vividly colored. There is a lot of variety, even if we just look at the crabs in the ocean.

And don’t worry, we’ll get around to the turtles—and how moving from the butt crack of turtles turns crabs LGBTQ+. Seriously.

Spider Crab: Molting in Giant Mountains

Spider crabs are serious nightmare material for most people: the Japanese spider crab (Macrocheira kaempferi) is thought to be the crustacean with the largest leg span. These armored spidery crab can reach up to 1 meter (3 ft) carapace size, which doesn’t sound too bad, but then you hear about the 4 meters (13 ft) leg span, and any arachnophob’s worst nightmare has become reality.

Japanese spider crab. Photo credit: Wikimedia / Hans Hillewaert (Lycaon)

But rest assured, they aren’t aggressive. National Geographic even calls them gentle giants. I mean, they are detritivores, so they only feed on dead things with the occasional live fish or invertebrate. You aren’t on their menu.

The Japanese spider crab lives in the Pacific around Japan and Taiwan at deeper depths of 50 to 500 meters (160 to 1,600 feet), so way outside the range of most people—and they usually only come up to the shallower end of their range for mating.

And, of course, they start out tiny. Like our lobster friends, crabs lay a lot of eggs. Japanese spider crabs can lay a million eggs per mating season. That’s a literal crabload of eggs.

The little babies are then completely on their own. After a couple of days, they molt for the first time. Remember, crabs are crustaceans, so they need to molt to grow. Their hard mineralized exoskeleton can’t expand, so they grow in increments. They grow a squishy shell inside the old one, then shed the outer one, and harden the new one. They can grow more than 20% with one molt. But, molting is a big risk, even for giant spider crabs, as they are soft and squishy with wobbly legs for a while after molting.

In captivity, the molting process alone takes about an hour and a half. And that’s just the time it takes to shed the old skin.

I couldn’t find out if Japanese spider crabs also exhibit this behavior but other spider crabs do something pretty damn weird when they need to molt: the mountain of molt.

The Giant Spider Crab—which weirdly isn’t the largest one—congregates off the Melbourne coast every year for their winter molt. These mottled orange crabs like it cold, so they spend most of the year in deeper waters, feed there, and enjoy the chilly waters. During one of the first full moons of winter, they set out toward the shallower end of their range. Hundreds upon hundreds of them march together like a nightmare army.

When they reach their destination, they form a giant mountain by crawling on top of each other. And then all of them molt together, giving them a better chance of survival during this very vulnerable time.

Cod, I’m glad I don’t have any phobias. Being fascinated by these things is so much more fun than getting nightmares. When can I go dive during one of these migrations? Please, thank you.

Pea Crab: A Tiny Parasite

Pea crabs are one of the many reasons I wanna go back to New Zealand at some point: tiny parasitic crabs that live in mussels, sea urchins, and other animal host.

Pea crabs are the smallest crabs, about a hundred times smaller than the Japanese Spider crabs. Their carapace reaches about a centimeter (less than half an inch). The male is a lot smaller and thinner than the female and their color is much harder to spot, because the male actually has to weather the ocean.

But let’s start at the beginning: Pea crabs produce even more eggs than non-parasitic crabs. Jessica Feickert, a researcher at the Leigh Marine Laboratory in New Zealand figured that this is because they don’t need to move around and find food and such, so they have some extra energy for more eggs. And while most crabs only reproduce once per year, these little pals are much more fecund. They reproduce year round.

These larvae then have to deal with the open ocean for a bit until they find a host to live in. While non-parasitic larvae use acoustic signals to find a place to settle, Jessica found out that pea crabs use chemical cues instead—smelling out their home.

The female stays in her mussel forever after choosing a host. She doesn’t really have a choice, to be frank: she’s too large to leave. The females are rounder, larger, less mobile than the males—and when they hold eggs under their abdomens, those egg masses can double the size of their body, making them even less mobile.

The male, on the other hand, has to leave their mussel to find a female. That’s why they are smaller, slimmer, and camouflaged much better. They then set out to find a female—again apparently through chemical signals—and strokes the mussel until they open up and the little dude can join the female inside.

Nature is ducking weird!

Light-Blue Soldier Crab: Moving Forward!

If you’ve ever watched crabs on the beach, you’ll likely have noticed that they move sideways. Somehow, their ten legs are best adapted to this weird sideways shuffle. But that’s not true for all crabs. Some crabs have actually adapted to moving forward instead. I think it’s what gave the light-blue soldier (Mictyris longicarpus) crab their name.

Light-Blue Soldier Crab. Photo credit: Wikimedia / LiquidGhoul

These soldier Crabs live on beaches in Australia and some parts of Asia. They aren’t super big with a carapace of only 2.5 cm (1 inch), but what they lack in size, they make up in numbers.

They spend most of their time buried in the sand, but at low tide, they emerge in large armies to feed on wet sand—well, the life inside the wet sand.

Watching them march in this absolutely not crab-like fashion is more than fascinating, and I hope to be able to capture this spectacle in the future. It’s getting grayer and colder outside, and I’m already longing for warmer temperatures and any place that isn’t Germany. But, crabby conditions aside, let’s move on to our baby turtles.

Ghost Crabs: More Than Baby Turtle Enemies

When baby turtles emerge from the sand, they are adorable and ducking vulnerable. Between light pollution sending them the wrong way, sea gulls, and our crabby friends, few of them reach the open ocean, let alone maturity.

Footage of ghost crabs munching on baby turtles circulated the internet a while back. BBC released a clip of a baby turtle wriggling their way free from such an attack—a clip with more than four million views on Youtube. Clearly, humans give a crap about baby turtles.

Though I’ll never fully be able to understand why humans always root for the cuter animal—completely ignoring the need for predators to keep ecosystems healthy—I must admit seeing a healthy turtle swim away into the surf makes my heart sing, too.

We’ll get back to turtle-crab interaction in a moment, but first, let’s have a quick look at these ghost crabs.

Ghost crabs like it warmer than some of the other crabs we’ve talked about—and dryer. They inhabit the intertidal zones of and supralittoral zones (that’s the area just above the high tide line) of tropical and subtropical areas all over the world. They even make it into estuaries with their brackish waters. Adapted to terrestrial life, they can mate year round.

As the top carnivore with “essentially no terrestrial competitors or predators” at the top of a filter-feeding food chain, ghost crabs are a sign of a healthy beach, often seen as an indicator species, but they don’t have the easiest life either. Humans have changed beaches in just about every aspect, and ghost crabs numbers are declining. While research suggests burrowing crabs are among the “most ecologically important invertebrates of intertidal beaches,” little is done to protect them.

If we get over the turtle thing, these things are cute enough to use them. Humans protect what’s cute, and you can’t tell me these things with their stalked eyes aren’t cute. Might be worth a try.

Oceanic Crabs: Abandoning Turtle Butts and Monogamy

The oceanic crab (Planes minutus) might have been the most fun discovery of my crab research. These crabs are special in many ways: they are pelagic, so living in the open ocean instead of on the bottom or at the shore, and they have a very special relationship with both turtles and plastic.

Oceanic crabs can swim—though only forward and for a limited time. Research found that they can swim for 35-45 minutes before sinking (at a water temperature of 28 degrees). They swim with all ten of their legs, by the way.

But, they prefer not to swim. Floating is much less work, so these oceanic crabs inhabit anything that can be considered an island. They might live on Sargassum seaweed, wood, floating plastic debris, or the butt crack of a loggerhead turtle.

The gap in the shell around a turtle’s tail seems to be a very happy place for these crabs. The gap is large enough for a couple to stay there together in happy monogamy.

But with more and more plastic debris in the ocean, there are more large islands available for our ten-legged friends. And it’s turning them away from monogamy. Instead of a single couple, they now have relationships with multiple partners.

I don’t know what I find cooler: that these turtles inhabit the butt crack of turtles or that plastic pollution changes their sexual preferences.

A Columbus crab living on a loggerhead sea turtle. Photo credit: Wikimedia / BMC Ecology 2013

What’s Next?

Either way, crabs play a vital role in ecosystems. And we’re ducking with the balance. Due to the rising ocean temperatures, crabs migrate further north, devastating ecosystems that are just not ready for them. Due to humans shipping things all over the world, invasive crab species are devastating ecosystems all over the world—and climate change with milder temperatures means they don’t even die in winter.

Some of the most invasive crabs like the European green crab can survive long times without water, so they are very hard to get rid of.

Crabs—in their own ecosystems—are essential for keeping everything in balance, so protecting them, their habitat, and the planet we share with them, is essential for all of us.

To leave you in a more cheerful mood: here’s a video of a baby turtle escaping from a crab. Poor crab, but ducking cute turtle!

Want to keep learning? Read my article about lobsters next.



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