The Crittery Biology of Crustaceans

Shrimp are underwater bugs? Or are bugs flying shrimp? In this one, we’re talking about crustaceans. From land crabs the size of dinner plates to tiny krill that feed the whales, this group holds some cool members. Don’t know what crustaceans are? From lobster and crabs to shrimp and prawns, there are a lot of things humans eat in the crustacean group, so even if you don’t know the group, you’ve likely encountered a few members.

Orientation on the Tree of Life

Before we get into cool crabs and animals named “What the whale eats,” I’ll quickly orient us on the Tree of Life. If you don’t care and just want to hear me get too excited about shrimp and the like, there are chapter headings, so you can skip the orientation.

To get to the crustaceans, we have to walk back the Tree of Life a little. For the last few weeks, we’ve been talking about the mollusks. Instead of starting at the bottom, let’s retrace our steps from there. If you need a refresher on the Tree of Life base, make sure to watch the episodes already published. I’m also working on creating a guide where you can look up all the groups we talk about.

Okay, we only need to take two steps back from the mollusks. The mollusks are part of the Lophotrochozoa, which in turn belong to the Spiralia. And with that, we reach the Protostomia. A long time ago, we talked about Protostomia and Deuterostomia, groups divided by where their mouth and ass end up in early development. The Protostomia were the ones where the primordial mouth does not become the after.

To reach our crustaceans, we walk to the Ecdysozoa, which we’ve talked about before. They are the group that contains a bunch of worm stuff and the so-called Panarthropoda. In this group, there are more worms, namely the velvet worms, as well as the adorable water bears, the microscopic tardigrade. And while velvet worms are actually super cute and look like soft velvety slugs in all kinds of colors with stubby little feet like those of a tardigrade, they aren’t our topic today. Instead, we’ll follow the branches to the Arthropoda.

Within the arthropods, there are two groups that are interesting enough to talk about: the Chelicerata, the group that contains spiders, but also the weird horseshoe crabs, and the creepily cool sea spiders. We’ll get into all those at a later point.

The other group is the group called Mandibulata, which contains the millipedes (well, and centipedes and the like), but also the Pancrustacea. Within the pancrustaceans, we find the crustaceans and Hexapoda, so the insects and their underwater friends.

The Pancrustacea are a rather new group in this entire mess of the tree of life. Jerome Regier and a few co-authors did a major phylogenetic study in 2010 where they analyzed a giant amount of DNA sequences from 75 arthropods. They end their study in the fun sentence that their results “represent a step towards ending the often-heated, century-long debate on arthropod relationships.” They really believed in their stuff, didn’t they? But considering that it’s now the commonly accepted framework, I guess they had good reason.

Technically, that already covers the question I posed earlier: both the crustaceans and the insects are Pancrustaceans, so they are very related and part of the same clade. Of course, that doesn’t make insects flying shrimp or shrimp into swimming bugs, but I still like the notion. It’s like calling corals upside-down jellies. Of course, it doesn’t work that way, but they are still very related and different versions of the same thing.

But, obviously, I won’t just stop talking now. That would be disappointing—or a relief, of course. But if it’d be a relief, I really don’t know why you are listening to this.

Photo: Unsplash

The Pancrustacea

So, the simplest division of the pancrustaceans is into the land and the water things. The crustaceans are the water ones and the hexapoda, the insects, are the land and air ones—well, kinda, as we’ll see.

As there is so much diversity here, I decided to cover the crustaceans in this episode, while moving the insects to the next one. But even with that, we have much to get through, so let’s dive in.

Since we brushed over the Ecdysozoa in general, let me quickly wrap up the characteristics we’ve gained on the way: first, there was the multi-layered cuticle made from organic material that makes the Ecdysozoa what they are: molting animals. The Panarthropoda branch added clawed feet and a segmented body with paired appendages (that’s legs and the like). When we reached the actual arthropods, those appendages gained joints. That’s actually what arthropod means: jointed foot.

Very unsurprisingly, as the Pancrustacea belong to the Mandibulata, they have mandibles. The Mandibulata include just the Myriapods in addition to the Pancrustacea, and the same scientists we talked about earlier confirm that this is a sensible group division. We’ll talk about the myriapods, so the group of many-legged animals that the millipedes and centipedes belong to with the terrestrial stuff—in as little detail as I can get away with.

Okay, the final structure that both crustaceans and insects have in common is complex vision. Arthropods rely on a complex compound eye and/or eye spots called ocelli. The complex compound eyes are made up of fifteen to several thousand separate hexagonal columns called ommatidia. Eye spots are single lenses without retinas. I’ll tell you more about this when we cover the insects and spiders, as spider eyes are actually made up of just these eye spots.

Photo: Unsplash

The Crustacean Body Plan

So, now, crustaceans are mostly aquatic animals, though technically, there are three terrestrial groups: the amphipods, the isopods, and the decapods all have some truly terrestrial members. More on that later.

But, as I said, most crustaceans actually live in water. Most of them are free-living, so moving around, too, though there are some like the barnacles that are sessile.

For the next bit, just think of a shrimp to make your life a little easier. If you have never seen shrimp unpeeled, maybe do yourself a favor and look at a picture. Unless your driving. If you are driving, definitely don’t look up a picture.

So, crustaceans have segmented bodies. We’ve established that. There’s the cephalon, so the head, the pereon or thorax, and the pleon or abdomen.

Cephalon: head; pereon: thorax; pleon: abdomen. Yeah, right, why just make it easy? But, as we know from the cephalopods, cephalon makes sense for head. The head and body, so the cephalon and the pereon, might be fused into a cephalothorax. Cephalon plus thorax makes cephalothorax. At least they are consistent. Thanks!

All of this stuff is surrounded by a hard exoskeleton, and just like insects, they need to shed those to grow. The shell is made up of a dorsal tergum, ventral sternum, and a lateral pleuron. Pardon my Latin. Let’s try that again: they have a backplate called a tergum, a breastplate called a sternum, and side plates called pleurons.

To make things more complicated, various parts of the exoskeleton can fuse, so it’s hard to figure out where what plate starts and ends, and such. Also, the side plates, the pleurons, might be missing. Yeah, I know. Oh, and crabs, for example, have the entire abdomen essentially folded underneath them.

Okay, each of those body segments can have appendages. So, there are antennae, mandibles, and maxillae. Maxillae are feeding legs used for tasting and manipulating food. All of those are attached to the head part of the crustaceans. Remember, the head is called a cephalon. On the thorax, the pereon, there are legs. The legs can be either walking legs, pereiodpods, or feeding legs, maxillipeds. The abdomen, the pleon, bears pleopods. Pleopods are weird swimming legs used for anything from catching food to brooding eggs. They can even wear their own gills and help with oxygenation. In some species, the males have specialized ones for sperm transfer called gonopods. Phew, so many pods. How do they keep track of so many legs and things?

The abdomen then ends in something called a telson, which carries the anus, but can also have two more leg things called uropods that form a tail fan. So many -pods.

Crustacean Biodiversity

As I said, there’s a lot of biodiversity in this taxon, so while they technically share these features, they might look different or be reduced. After all, the Crustaceans cover everything from teeny-tiny shrimps to giant land crabs the size of dinner plates.

I said earlier that there are land-based crustaceans that throw off that entire division into hexapods and crustaceans. Well, to be fair, they still return to the sea to release their eggs. The coconut crab, the largest terrestrial crab, is one example. We’ll talk more about them later. On a much smaller scale, the woodlice, tiny crustaceans that look like cute little pills of millipede, actually cheat. They don’t return to the sea, but instead put their eggs into damp moss and other wet terrains.

Oh, that reminds me of those frogs that use elephant footprints for their young. Can’t wait to tell you about that weirdness. So, let’s keep climbing.

Okay, so the crustaceans. As always, no one seems to agree on the branches, so we’ll go with the NCBI database again. I just trust them the most. They go with the phylogenetic approach we talked about in the previous episode, and that just makes more sense than the mere biological approach.

Okay, so groups, right. There are the Branchiopoda. They include small organisms with cute names like fairy shrimp, sea monkey, or much less cute names like water fleas. You might know some of this group from those toy store sets where you get some eggs to put in water. Because of how easy it is to culture them, some Branchiopoda are also popular as aquarium food. Most of them live in freshwater, many in temporary ponds. That’s why some of them are so excellent for toys: their eggs survive dried for very long times. But, otherwise, I just don’t care enough to tell you more. If I find fun facts about them beyond their use as “aquasaurus.”

Even less interesting to me are the Cephalocarida,  tiny benthic marine crustaceans that mostly feed on detritus, so dead stuff flakes. Despite cool-named members like mustache shrimp and fish lice. Fish lice aren’t the same as the salmon louse, which is a big issue in aquaculture, so I don’t even think we’ll return to them ever again. Moving on. The Remipedia are a class of blind crustaceans that can be found all over the world in aquifers. Their body is further segmented, and they look more like worms than anything else to me. If the myriapods didn’t branch off before the Pancrustaceans, I’d think they were underwater myriapods.

And that leaves only the Multicrustacea, the group that contains all the crustaceans people usually know about. Well, as this group includes four-fifth of all the crustacean species, it’s not that surprising.

The Crustaceans People Know

There, we’ve rushed through the less exciting stuff and finally get to talk about the Hexanauplia, the Malacostraca, and the Thecostraca.

The Hexanauplia are a critical group that I wish I cared more about. They include the copepods and a parasitic group called the Tantulocarida. Parasites will be their own topic at some point, but we’ll skip them for now. We’ll also ignore the copepods now, as they’ll be part of an episode on zooplankton in the future. They are one of the most important plankton groups. For now, it’s enough to know that they are planktonic shrimpy things—not shrimp, though.

The Thecostraca include mostly the barnacles. Barnacles aren’t the best at looking like crustaceans. Seriously, I’d have thrown them in with the clams and other shelled stuff. But, no matter how much they look like pimples on every ocean surface, they are crustaceans. They have all the developmental stages including the nauplius larva, and inside their strange pimple shell, they look like crustaceans, kind of. As barnacles will likely be the bane of my existence when I move onto a sailboat, I will definitely tell you more about them in the future. For now, let’s move on.

Last but absolutely not least are the Malacostraca, which includes all that stuff humans eat: crabs, lobsters, and the like. Malacostraca means soft-shelled, which might seem a little ironic when applied to things like crabs and lobsters. There’s roughly 40,000 species in the Malacostraca. What they all share is their general body organization, though the body form can differ quite a bit. We already talked about the three parts of the crustacean body. In the Malacostraca, those segments called tagmata are further divided. The head, cephalon, is divided into five segments, the thorax, pereon, is divided into eight segments, and the abdomen, pleon, into six segments plus that telson anal segment.

This group contains a wide range of organisms from Krill, which means “What the whale eats” and build the majority of food for the kinds of whales that scoop up big mouthfuls of ocean to feed, to much larger ones like the impressive coconut crab or the California Spiny Lobster.

We’ll do an entire episode on crabs, as well as one on lobsters and crayfish. Oh, and shrimp and the like, of course. That’s three more episodes on crustaceans. I’m surprisingly excited. Just like with the mollusks, I think a more detailed look into different example species and our human impact on them is in order. The amount of shrimp and lobster humans eat is incredible.

Photo: Brocken Inaglory (

Coconut Crabs

So, where does the coconut crab I promised you fit in? The coconut crab, Birgus latro, is a terrestrial hermit crab. But, these things are huge! They are the largest terrestrial crabs we know about. An adult can grow to a meter (about 3 feet) in size and weigh 4 kilograms. Not bad for a hermit crab, right?

I already told you that these coconut crabs need to lay their eggs in water because their young live a planktonic life in the beginning. The problem is: coconut crabs can’t swim or breathe underwater, so their migration to the coast is always a hazardous gauntlet. When they are young, coconut crabs still have gills, but those get replaced by a special kind of lungs. So, like us, they can no longer breathe underwater and will die if submerged for too long.

But, before you get scared of the giants and their equally giant claws called chelae—especially that left one, which is much larger than the right, let me tell you that these things are not poisonous—well, unless they eat too much sea mango and build up the toxin cardenolides.

Though considering the fact that their pincers can be really painful to humans, and they don’t like to let go, this might be small consolidation. Locals seem to suggest tickling the crab in the soft bits, though I am not keen to find out if that works.

My main saving grace for these really cool crabs is their size. They are big enough to keep track off, very unlike that thumbnail-sized jelly we talked about a few episodes ago. I definitely prefer my dangers large enough to see. And as the coconut crab likes to mostly eat fresh fruit and dead things, I am pretty sure it doesn’t want to eat me—unless I’m dead, of course.

So, why is the coconut crab called thus? First, it’s also called a robber crab, probably because it likes to steal food from humans, but I think coconut crab has the nicer ring to it. Can we really call them robbers if they likely have no clue that the food they steal belongs to someone else?

Coconut crabs actually know how to open coconuts. They take the coconuts, rip off the strands with their claws, and then ram their large claw into one of those three germination holes (you know, those spots that kinda look like bowling ball finger holes?) until it opens. Once they have an opening, the rest isn’t too hard. But, it can still take a coconut crab a few days to open one.

I hope my travels bring me past any of the islands they inhabit all over the Pacific Island region. They can be found in Polynesia, Melanesia, Micronesia, but also more West toward Madagascar.

And with those giant cool crabs that are so not representative of the crustaceans, but far too cool to ignore, we’ll leave it for today. Next time, we’ll explore the land-based insects—well, and the few that are actually aquatic. Damn it, this whole classification thing will drive me insane at some point.

And you know what? By the time I’m done telling you all this, there’s probably changes everywhere again. But, no matter how much the branches change, I love learning about how everything is connected, and what the basics of all that biodiversity are.

This series explores those branches, what they have in common, and how everything is connected. It will be the basis for our explorations into species, environmental impact, and all things nature and biodiversity. I’m excited.



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