Arthropods: The Creatures with the Jointed Feet

Up next in our Marine Biodiversity labs was the phylum Arthropoda! (As a forewarning, besides blogging to share my  experiences, I use my lab blogs as a way to study and review what we learned for upcoming quizzes. There’s a lot of terminology, but I hope I explained it in a way that makes sense! If you do have any questions, don’t hesitate to ask me in the comments!)

Arthropods cover a wide range of species that can be found anywhere from the bottom of the seas to flying though our skies. Arthropoda means “jointed foot” and, as with the other phyla,  these creatures come in various shapes and forms but have certain characteristics in common. Insects and crustaceans belong to this massively diverse group. Even my tarantula, Archie, is an arthropod.

Archie makes a rare appearance

Archie makes a rare appearance

Though most people prefer crustaceans…

Image from

Poor Sebastian…
Image from

One of the most noticeable Arthropod characteristics is their metameric body segmentation. In other words, they have sections of their body that seem quite similar in structure and repeat themselves. This can most clearly be seen when examining their legs, since they have a single pair of jointed appendages that go along with each segment. The body of an Arthropod is covered by an exoskeleton. They have no bones inside their body, but are instead covered by a hard, chitinous shell that serves largely for protection. Because of the rigidity of the exoskeleton, when an Arthropod grows, it needs to completely shed and replace its covering, called molting. Basically, they crack their old exoskeleton, push it off, and allow their new exoskeleton underneath their bodies to harden. It’s quite the interesting process. Here’s a video of a molting horseshoe crab to give you more of a visual:

When the animal first molts, its new exoskelleton has not yet hardened, and it is extremely vulnerable. For example, soft-shell crabs can be quite popular for consumption, allowing diners to enjoy their dinner without the time-consuming task of removing the shell. These crabs are actually blue crabs that have freshly molted. They’re shells simply havent had time to harden yet.

Arthropods are coelomates. (They have internal body cavities separated from the exterior of the animal by a fluid and connected by tissue. Humans, for example, are also coelomates. Check out my Annelids: A Rather Squirmy Lab blog for a bit more information on the subject.) They also have an open circulatory system. This means that instead of blood vessels running through their bodies, their blood is pushed through a hemocel, a blood containing body-cavity where the blood is diffused through cell walls.

closed circulatory system Image from

closed circulatory system
Image from

open circulatory system Image from

open circulatory system
Image from

Once we established the main characteristics of an Arthropod, it was time to get more detailed.

Within the major phylum Arthropoda, there are 3 subphylum: Trilobitomorpha, Chelicerata, and Crustacea.

Trilobites are all extinct. Their 3-segmented bodies (head, thorax, and telson) and seemingly unspecialized, decorative appendages can only be seen today in their fossilized form. While I was working at the Houston Zoo, I took a Saturday to explore the Houston Natural Science Musume’s Hall of Paleontology. They had quite a few of these awesome little critters on display.






They’re pretty fascinating.

The next subphylum, Chelicerata, is extant (still around today). They have a body that is divided into 2 distinct sections: the cephalothorax (front) and abdomen (rear). Chelicerae is the term given to their first pair of appendages. These are the claws they use for feeding. The second pair of appendages are the pedipalps, which are modified for different functions depending on which animal you’re looking at. Members of this subphylum also lack antennae.

Class Merostomata

The living representation of this group is the horseshoe crab. They’re pretty neat tank-like animals that have an interesting clotting agent in their blue blood. When their blood comes in contact with bacteria, it clots. The blue color comes from the copper it contains. Because of its special properties, these crabs are “milked” for the biomedical community, removing about 1/3 of their blood before returning them to the water. The practice is somewhat controversial. It is still largely unknown how they are affected by the stress and physical demands of the process.

If their unusual body and shell shape isn’t enough of an identifying marker, you can always count on their gills and  telson. They have 6 pairs of normal appendages, and 5-6 pairs of appendages towards the rear of their bodies (abdomen) that are modified as gills. At the very end of their body, you can see the spike-like telson. The telson is used for steering and to right themselves if they become flipped upside down.

Class Pycnogonida

More commonly known as sea spiders, this class is entirely marine. They have 4-6 pairs of legs and no antennae.

Their trunk, or main body has lateral processes that join with the legs (see the diagram below). They also have 4 eyes, a short abdomen, and a proboscis. Males and females can be identified by their ovigerous (or egg-bearing) legs. Interestingly, males are actually responsible for carrying the eggs in this class of animals. The ovigerous legs in females are often much smaller.

The final subphylum, Crustacea, is distinctively different from Chelicerata. For instance, crustaceans have 2 pairs of antennae. They have 3 pairs of mouthparts, consisting of mandibles (used for biting, cutting, and holding food), and the 1st and 2nd maxillae (used for tasting and manipulating food). Their limbs are biramous, meaning they are branched in two (which you’ll be able to see more clearly in the prawn dissection below). These appendages are usually specialized according to the function required by the animal. Their trunk is divided into a thorax (between the head and abdomen) and abdomen. Their thorax may also bear a carapace, the shell or exoskeleton on their back. This exoskeleton is made from chiton, as with other arthropods, but may also contain calcium salts and proteins.

Class Ostracoda

This class of organisms is more commonly known as seed or mussel shrimp. They are very round and tiny, but they can range in size from smaller than a poppy seed  to as large as a meatball.

Under the microscope, it was a bit difficult to see their individual parts. Besides the antennae, legs, and the black dot of a naupliar eye (a simple eye that first appears in the larval stage), you can’t see much. They have a bivalve carapace (two shells covering their body like a mussel) that does a great job of concealing their insides. These images, however, help with that:

The body is largely composed of the head with a very reduced trunk, segmentation, and few appendages. They usually have less than 2 pairs of legs. The 2 pairs of antennae are used for movement, while the legs and body are covered in  sensory hairs.

Class Copepoda

The word “copepoda” originated from Greek terminology translating to “oar-foot.” When examining the swimming behavior of a copepod, their name makes complete sense. They have a pair of appendages on the same body segment that they move together, thus using their legs like an oar to “row” through the water.

The body is larger at the front than at the back, and the head has one eye. They have 6 pairs of legs, with the first (maxillipeds) being modified for feeding. The abdomen consists of the 5 body segments and 5 pairs of swimming legs. Their enormous antennae extend from the body at a 90˚ angle. These antennae can actually be used to distinguish male from female.

If you examine the antennae right copepod, the male, you notice that the right antennae is slightly bent towards the end. That is because it is slightly modified to help him grasp the female. He also has a slightly longer 5th pair of legs, helpful for transferring his spermatophore to his mate. Females are often much larger than males, and sometimes trailing egg sacs give them away.

Class Cirripedia

Barnacles! These guys are another creature you might not immediately classify as an animal. You’ll only find them in the ocean, sessile on the ocean bottom or living on larger animals.

Their antennae are smaller and they have specialized mouthparts. When they attach to something, they settle down headfirst, suspending their 6 pairs of legs in the air. These legs have setae (little bristles) used to bring in food. They tend to colonize together, waving their little legs in the air to create miniature currents and bring in organic particles. Here they are, hard at work:

Interestingly, they are also hermaphrodites, containing both male and female reproductive organs. In order to maintain genetic diversity, they can sexually reproduce with their close neighbors. This video isn’t the highest quality, but they explain the process well:

Recently, I came across an article, “Finally, the truth about barnacle sex is revealed: Genetic analysis shows that the sessile crustaceans can broadcast sperm in water,” that  described an alternative method of reproduction. They discovered that their sperm can also travel through the water to fertilize more distant barnacles. It’s not a very reliable method, but it works! (Click the link above to check out the article.)

About 1/3 of barnacle species are parasitic, showing no features expected of crustaceans, but having the cyprid larval stage of other barnacles (a non-feeding stage in which the larvae look for a place to settle).

Class Malacostraca

This is a the largest class of crustaceans, all consisting of an 8-segmented thorax, 6-segmented abdomen, and telson (tail). On all of these body segments you will find appendages. Their first few legs (usually the first 1-3 paris) are maxillipeds (legs used for feeding). The rest are called pleopods and are used for swimming, burrowing, carrying eggs, etc. Because the class is so large, we looked at further subdivisions.

Quick refresher on taxonomy– organisms are classified in each of the following categories: domain, kingdom, phylum, class, order, family, genus, species. Domain is the least specific and broadest classification, while species is the smallest and most specific.

To complicate things a bit further, you can also have super- or sub- classifications within the categories. Super- would refer to a ranking above that categories, while sub- refers to a ranking below that category. Confused yet? Hopefully that made a bit of sense, or maybe this diagram will help out a bit. It does not contain super- classifications, but it at least gives you an idea to the overall flow.

So… within the Class Malacostraca, we looked  at 2 different superorders, Eucardia and Peracardai, and orders within each of those groups.

Superorder Eucardia

These organisms all have compound eyes that are stalked, and the carapace is fused with the body segments of the thorax.

  • Order Decopoda

Decapoda, meaning 10 feet, all have 10 or more legs. In addition, they have 3 pairs of maxillipeds (feeding legs). For this part of the lab, we partially dissected a prawn, plucking off each of its individual legs on one side of the body  and antennae and identifying each.

prawn and the legs

prawn and the legs

I a bit morbid, like a kid with ants and a magnifying glass, individually pulling off the legs, but it was interesting to see how they were designed differently according to their use.

Crabs also fit into this order, and we were able to examine specimens on ice.



The abdomen in these guys is reduced and folded up under the thorax (it’s the triangular part you see in the center of the body.) The shape of the abdomen can be used to identify males versus females.

The female (on the left) has a much wider abdomen, while the male (on the right) is much more narrow and triangular. The wider shape allows females to carry and brood their fertilised eggs here.

Chelipeds are the large claws you see. They can be used for feeding, defense, grabbing things, or for attracting a mate. The fiddler crab has taken this enlargement to the extreme.

The pleopods (other legs) are not used for swimming but mainly for reproduction and scuttling. Their back legs, however, may be modified to help with swimming.

Superorder Peracardia

The 2nd superorder we looked at is characterized by their small size. They’re almost never larger than 2cm. A naupilar eye (simple, larval eye) never develops, and the females have a brood pouch.

  • Order Isopoda

Growing up, I knew these guys as roly-polys! We would find them all the time looking for bugs when we rolled over the logs my mother used to border her flower beds. For some reason, she never appreciated it much… Sorry, Mom! But that  where all that bug-exploring experience has gotten me!

Image from

Image from

These guys are dorso-ventrally compressed. This means that they’re flattened in appearance, with their length being much longer than their height. Their pleopods (legs) have been modified into abdominal gills, and their eyes are not stalked (like those of a crab). The thorax and abdomen do not have a distinctive start and stop point, making it look like one connected body, and it doesn’t have the hardened carapace we see with other crustaceans. For appendages, they have one pair of grabby mouthparts (maxillipeds) and 7 paris of legs.

This is the largest order of crustaceans, and there are even some parasitic forms, like this guy:

Say hello to Cymothoa exigua, the tongue eating parasite. This video helps to explain them a bit:

  • Order Amphipoda

Amphipods are, as their name suggests, distinguished from isopods by their legs. Amphi- meaning 2, and poda- meaning legs or feet, refers to the different types of legs they have. Iso-, meaning one or equal, refers to the identical appearance of the isopods legs. (I feel like if I spoke Greek or Latin I would be so much better at identifying these animals!) Though they look different they have the same overall number of legs (1 pair of maxilipeds and 7 pairs of regular legs).

Their bodies are laterally compressed (flattened side-to-side) and their gills are found on their thorax. Like isopods, they have non-stalked eyes, and the thorax and abdomen are not clearly defined.

Amphipods are a very diverse group too, with some surviving even in Antarctica. Check out the video below to get a close up of these guys… they’re kinda cute:

Okay, phew! That’s it! That’s what we covered for Arthropods! Now who’s ready for the next  lab: the phylum Echinodermata! Get ready for starfish, sea urchins, and sea cucumbers!

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