Crustaceans: All You Need to Know in a Nutshell

folder_openDecapoda, Malacostraca
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Crustaceans are fascinating invertebrate animals belonging to the phylum Arthropoda, subphylum Crustacea, and include a wide range of species such as crabs, lobsters, shrimp, and barnacles. They are mostly aquatic creatures but can also be found in some terrestrial habitats. Many crustaceans are well-known for their hard exoskeletons made of chitin, which provide protection and support for their bodies.

These creatures are incredibly diverse, with over 40,000 identified aquatic species, and display unique characteristics, such as multiple pairs of legs and jointed appendages. They’re also equipped with feelers for touch, smell, and sensing their environment. The world of crustaceans offers a vast range of shapes, sizes, and lifestyles, from small water fleas to sizable lobsters and crabs.

Some examples of crustaceans found in various habitats include:

  • Crayfish: Found in freshwater environments and known for their large, strong claws.
  • Barnacles: Marine arthropods that often attach themselves to rocks, ships, and even other animals.
  • Pill bugs: Terrestrial crustaceans, commonly found in gardens and various damp habitats.

Crustaceans play a significant role in aquatic ecosystems and have several adaptations that allow them to thrive in their specific environments. These fascinating creatures have much to offer in the world of science, cuisine, and general curiosity, making them an essential part of our planet’s biodiversity.

What Are Crustaceans

Classification and Diversity

Crustaceans are a group of arthropods that are primarily aquatic, with some found in terrestrial habitats. Some common examples of crustaceans include:

  • Crabs
  • Lobsters
  • Shrimp
  • Barnacles

They are a diverse group with over 40,000 to 50,000 living species.

Physical Features

Crustaceans have several unique features such as:

  • An exoskeleton made of chitin
  • Gills for breathing
  • Two pairs of antennae
  • Jointed legs

Their hard exoskeleton provides protection and support, but must be shed periodically in order to grow.

Segmented Body

The body of crustaceans is divided into two main parts:

  • Cephalothorax: This is the fused head and thorax region
  • Abdomen: The rear part of the body

This distinct segmentation helps in their mobility and performing various tasks.


Crustaceans have a variety of specialized limbs, each serving a unique function. Some common types include:

  • Walking legs: Used for locomotion
  • Swimming legs: Help in swimming and maintaining balance
  • Feeding appendages: Aid in capturing and handling food

Overall, crustaceans are a fascinating and diverse group of arthropods with a wide range of physical features, segmented bodies, and specialized limbs. Understanding their diversity, anatomy, and lifestyle can provide insights into the broader world of invertebrates.

Major Groups of Crustaceans


Decapods are a large group of crustaceans that include familiar species such as shrimp, crabs, lobsters, and crayfish. They are characterized by the following features:

  • Ten legs (hence the name decapod)
  • Segmented body divided into a cephalothorax and an abdomen

Decapods are an essential part of the marine ecosystem and are often consumed by humans and fish. Some examples of decapods include:

  • Dungeness crabs
  • Maine lobsters
  • Black tiger shrimp


Barnacles are sessile crustaceans that attach themselves to hard surfaces like rocks or the hulls of ships. They belong to the class Maxillopoda and share these key traits:

  • Body encased in a protective, calcareous shell
  • Feather-like appendages for filter-feeding
  • Directly releases larvae into the water

Examples of barnacle species are acorn barnacles and goose barnacles. Barnacles are often considered a nuisance by sailors as they grow on ship hulls and can cause significant drag.


Krill represent a different group of crustaceans belonging to the class Malacostraca. They are small, shrimp-like creatures, and are an essential food source in the marine food chain. Key aspects of krill include:

  • Small size (usually 1 to 2 inches long)
  • Swarming behavior in large groups
  • Filter-feed on phytoplankton

Krill species include Antarctic krill and Northern krill. They serve as a primary food source for many marine animals like whales and fish.

Feature Decapods Barnacles Krill
Legs Ten N/A Six
Body Regions Cephalothorax and abdomen N/A N/A
Appearance Shrimp, crabs, lobsters Encased in shells Shrimp-like
Feeding Habits Omnivorous Filter-feeders Filter-feeders

Habitats and Adaptations

Marine Life

Crustaceans are an important part of marine life, dominating the seas with over 40-50,000 living species. They inhabit various habitats like the arctic and ocean waters.


  • Japanese spider crab: one of the largest crustaceans, living in deep ocean waters
  • Barnacles: found on shorelines, attached to rocks and other surfaces


Crustaceans can also thrive in freshwater habitats. These include lakes, rivers, ponds, and streams. About 10% of the nearly 40,000 aquatic species exist within freshwater environments.


  • Crayfish: found in rivers and lakes across the world
  • Water flea: tiny crustacean, essential to the freshwater food chain

Terrestrial Crustaceans

Some crustaceans have adapted to live on land, typically in moist environments. The rare land-dwelling species showcase the diversity of crustacean habitats.


  • Pill bugs (also known as woodlice or roly-polies) are terrestrial, living in leaf litter and under rocks.

Comparison table:

Habitat Characteristics Examples
Marine Saltwater, ocean waters Spider crab, barnacle
Freshwater Lakes, rivers, streams Crayfish, water flea
Terrestrial Moist land areas Pill bugs

Diet and Feeding

Common Prey and Predators

Crustaceans have a diverse diet, ranging from:

  • Phytoplankton
  • Algae
  • Small fish
  • Other small crustaceans

For instance, the American lobster is an omnivore, consuming a mix of plants and meat. In the wild, their prey includes:

  • Fish
  • Mollusks
  • Other crustaceans

They use their strong mandibles to crush shells and tear apart their prey. Some crustaceans like crabs can also be scavengers, feeding on dead organisms or detritus.

Predators of crustaceans vary depending on the species. Common predators include:

  • Larger fish
  • Marine mammals
  • Birds
  • Humans

Impact on Ecosystem

Crustaceans play a crucial role in maintaining the balance of aquatic ecosystems. As a vital part of the food web, they:

  • Serve as a food source for other marine animals
  • Act as filter feeders, recycling nutrients in the water
  • Maintain the population of their prey through predation

However, crustaceans can face challenges due to environmental changes, such as ocean acidification and pollution. Implementing conservation efforts, such as sustainable fishing practices, can help preserve their populations and maintain the ecosystem’s health.

Reproduction and Life Cycle

Mating and Reproduction

Crustaceans, like many invertebrates, reproduce sexually. Most have separate sexes, but some species are hermaphrodites. Mating usually involves the transfer of sperm from the male to the female. After fertilization, females typically produce eggs which they may carry on their bodies or release into the environment.

Examples of crustacean reproduction:

  • Lobsters: Female lobsters produce eggs that are fertilized by the male’s sperm and then carried on her swimmerets (small, leg-like appendages on the tail) until they hatch.
  • Crabs: Similar to lobsters, female crabs carry fertilized eggs on their abdomens until the larvae emerge.

Larval Stages and Growth

Crustacean larvae go through several distinct developmental stages, with many undergoing a number of molts to grow and develop their segmented body. Crustacean growth can vary widely depending on the species. Here are some basic characteristics of crustacean larvae:

  • Often have different forms from their adult counterparts
  • Adapted for different environments and feeding habits

Examples of crustacean larval stages:

  • Zoea: First larval stage for many crustaceans, featuring a small size, spines, and swimming appendages
  • Megalopa: Transitional stage for certain species (e.g., crabs) where they start to resemble their adult form

Crustaceans display various growth rates. For example, lobsters molt around 25 times in their first 5-7 years, reaching approximately one pound. After this, males may molt once a year and females once every two years, increasing in size with each molt.

Comparison of Lobster and Crab Growth:

Lobster Crab
Approx. 25 molts in first 5-7 years Growth rate varies depending on species
Males molt once a year; females once every two years Molting frequency depends on species and age
Lobsters increase 15% in length and 40% in weight with each molt Weight and length increases vary among crab species

Overall, the reproduction and life cycle of crustaceans is diverse and fascinating, with each species adapting its own strategies for survival and growth.

Special Cases and Unique Crustaceans

Land-Dwelling and Semi-Aquatic Species

There are several land-dwelling and semi-aquatic crustaceans. Two examples are pillbugs and crayfish:

  • Pillbugs, also known as woodlice, are terrestrial isopods that belong to the subphylum Crustacea.
  • Crayfish, which are freshwater crustaceans, have semi-aquatic habitats and breathe through gills, extracting oxygen from water.

Both of these species have adapted to their environments in unique ways. For example:

  • Pillbugs have evolved biramous legs that allow them to walk on land.
  • Crayfish possess specialized maxillae that help them filter water for oxygen.

Notable Crustaceans

Some unique crustaceans include the Japanese spider crab and the tadpole shrimp:

  • The Japanese spider crab is the largest crustacean, with a leg span of up to 3.8 meters.
  • Tadpole shrimp are ancient crustaceans known for their ability to live in temporary pools.

There is also the krill:

  • Krill, a small shrimp-like species, play an essential role in marine food chains.

Evolution and Geological History

Crustaceans have a diverse lineage as part of phylum Arthropoda and subphylum Crustacea. Their fossil records date back to the Jurassic and Cretaceous periods. Some key evolutionary features include:

  • A segmented body divided into cephalothorax and abdomen
  • A hard, protective carapace covering the cephalothorax

Comparison Table

Crustacean Type Habitat Main Features
Pillbug Terrestrial Biramous legs, woodlouse
Crayfish Freshwater Semi-aquatic, gills, maxillae
Japanese Spider Crab Marine Largest crustacean, leg span up to 3.8 meters
Tadpole Shrimp Temporary pools Ancient crustaceans
Krill Marine Essential in marine food chains

Understanding special cases and unique crustaceans helps provide insight into the tremendous diversity within the subphylum Crustacea.

Crustaceans and Humans

Economic Importance

Crustaceans like crabs, shrimp, and lobsters play a significant role in the global economy. They are:

  • Widely consumed as seafood
  • Sources of income for fishermen and related industries

These animals are known for their delicious taste and are often considered luxurious food items. Some species, like hermit crabs, also serve as popular pets and aquarium inhabitants.

Conservation and Threats

Crustaceans face various conservation challenges that ultimately affect their populations and ecosystems. Some primary threats include:

  • Habitat loss: Coastal development and pollution reduce suitable living spaces
  • Overfishing: High demand for seafood leads to overexploitation
  • Climate change: Warmer ocean temperatures and ocean acidification impact reproduction and survival rates

Furthermore, additional factors such as predation and invasive species can influence the stability of crustacean populations.

Now, let’s compare crustaceans to arachnids based on their features and characteristics.

Aspect Crustaceans Arachnids
Phyla Subphylum Crustacea Subphylum Chelicerata
Habitat Mainly aquatic environments Terrestrial environments
Appearance * Jointed bodies
* External skeletons
* Many pairs of legs
* Compound eyes
* Two main body parts
* Four pairs of legs
* Simple eyes
Examples Crabs, shrimp, lobsters Spiders, scorpions, ticks

Understanding the importance of crustaceans and the threats they face helps us make informed decisions about how to protect them and their ecosystems for future generations.

Reader Emails

Over the years, our website, has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Sand Crab or Mole Crab


Brazilian coast bug
This is the picture of a very common bug in the coast of brazil. It lives in the sand and it’s size ranges from 3 or 4 mm to 3 cm. It’s like some kind of betlee that buries it self in the sand. Thank you very much

Hi Javier,
We have very fond memories of capturing Sand Crabs in the interstitial zone of the beaches near Topanga Canyon, CA in the 70s. Thanks for sending in a truly awesome photo of a Sand Crab.

Hi guys,
Ah, one of my favorite kind of cute little critters, Mole Crabs! This is Emerita brasiliensis, the Brazilian Sand Crab or Mole Crab. It’s one of the species shown at: Here on the East Coast of the US we have lots of really nice Atlantic Mole Crabs, Emerita talpoida. From what I read, there are in total four different species of sand crab on the Atlantic coast of the Americas, and two species on the Pacific coast, including your Emerita analoga (the other Pacific species is more tropical.)

Letter 2 – Unknown "Bug" in Aquarium


What is this bug?
Location: In a home aquarium Florida
July 4, 2011 10:35 pm
Hey bugman, I live in FL and I have recently found these bugs in my home aquarium. They are little, about the size of a little round ball on the end of a sewing pin. They are scurrying around my tank and if I mess with them they get under the substrate or hide in the wood that I have in there. The tank has been set up for a couple of years and these things just appeared overnight it seems. I can’t seem to find an image of them anywhere on the internet. Could you help me. I don’t know if they are harmful or not. Thanks Lori
Signature: Thanks Lori

Aquarium Bug

Hi Lori,
We haven’t a clue what this might be, but the behavior you describe indicates it is most likely some type of Crustacean.  As you know, any time you introduce something new to an aquarium, including a plant or rock or live food, you run the risk of hitchhikers arriving as well.  Perhaps one of our readers will be able to provide some assistance.  It appears your aquarium is fresh water.  Please confirm.

Letter 3 – Mating Warf Roaches


Subject: Centipede?
Location: Beaufort, SC
June 7, 2017 11:52 am
Saw these mating….what ever they are…. along the inlet area of Beaufort, S.C.
I have looked in all my bug books and I can’t find anything like them. Can you tell me what they are?
Signature: Janet Fox

Mating Warf Roaches

Dear Janet,
Your image is awesome.  Though they are sometimes called Warf Roaches, these are actually Marine Isopods, a type of Crustacean related to the terrestrial Wood Louse.  We are confident that we have correctly identified your mating Warf Roaches as
Ligia exotica thanks to BugGuide where the range is listed as:  “Atlantic coast NJ south to Florida.”

Letter 4 – Freshwater Crustaceans found in Spring Water


Subject:  bug id
Geographic location of the bug:  Santa Cruz Mountains
Date: 08/08/2019
Time: 08:45 PM EDT
Your letter to the bugman:  Found these bugs in my spring water filter, and a few in my tank.  Any ideas?
How you want your letter signed:  lesley obermayer

Freshwater Crustacean

Dear Lesley,
We cannot provide you with a species identification, but this is some type of Freshwater Crustacean, probably an Amphipod or Isopod.  Most Amphipods are found in saltwater, but they can also be found in freshwater, including in caves.  According to BugGuide:  “A clear view of the antennae is needed to identify freshwater amphipods beyond order level.”  The only freshwater species pictured on the Natural History of Orange County is
Gammarus, and we would eliminate that as a possibility.  California Fish and Game has an online paper entitled Checklist of inland aquatic Isopoda (Crustacea: Malacostraca) of California, but there are no images.  BugGuide has an image of an Isopod in the family Asellidae and the genus Lirceolus that looks very similar to your individual, and we would attempt additional research by searching for members of the family Asellidae found in California.  The previously noted paper by California Fish and Game includes many family members, but again, there are no images.  This could be a rare endemic species, or it might be an exotic introduction.   Though we cannot provide you with anything more specific, we do feel confident that their presence will not adversely affect the quality of your spring water.

Freshwater Crustaceans
Dear Daniel,
Thank you so much. You appear to have identified our bugs.
Now we have to figure out how to eliminate them. Appreciate you taking the time and trouble to help us out.
Enjoy your days,
Lesley Obermayer
Freshwater Crustaceans

You are welcome.  You might want to check with your local natural history museum for a more definitive identification.

Letter 5 – Leafcutting Bee lives with Hermit Crabs!!!


Hello Mr.Bugman.
WOW! What a Treasure trove of Bug Info! I am a Land Hermit Crab owner and I have recently found a Bee or Wasp, Dont quite know. She is Mostly Black with some thin White Stripes around her Abdomen and Seems to have Some Fuzz on the Thorax. I am in Western Massachusetts and Since it is Early February, Releasing it out into the Environment is not an Option at this time. Can ya Please Help me Identify her And if known, What May I add to my Crab Habitat for food? The Pictures of her is on a Piece of Cholla Wood. I am guessing that she is about 1/8 to 1/4 inch in Length. Thank you for any assistance.

Hi Mike,
This looks like a Leaf-Cutting Bee in the genus Megachile. Female bees like cells in rotting wood or soil with circles of leaves that they cut with their mandibles. The cells are then filled with pollen and nectar and an egg is laid. The best food source you can provide for your tenant consists of fresh flowers, so you might be amassing a substantial florist bill.

Letter 6 – Water Fleas


Subject:  Pond Pupa?
Geographic location of the bug:  Connecticut
Date: 04/24/2019
Time: 03:05 PM EDT
Your letter to the bugman:  Good afternooon,
These little guys move with a jerking movement, are about 2mm in length, inhabit the water about 6-18 inches below the surface and are so numerous that it’s hard to believe.
How you want your letter signed:  Dylan

Water Fleas

Dear Dylan,
These are freshwater Crustaceans in the genus
Daphnia, commonly called Water Fleas because of the way they move through the water in a “jerking movement.”  Daphnia are a common live food used by many enthusiasts to feed aquarium fish.  You can find matching images on An Image-Based Key to the Zooplankton of North America and there is a nice drawing on Researchgate.  According to the National Center for Biotechnology Information:  “The genus Daphnia includes more than 100 known species of freshwater plankton organisms found around the world … They inhabit most types of standing freshwater except for extreme habitats, such as hot springs. All age classes are good swimmers and are mostly pelagic, i.e., found in the open water. They live as filter feeders, but some species may frequently be seen clinging to substrates such as water plants or even browsing over the bottom sediments of shallow ponds. Adults range from less than 1 mm to 5 mm in size, with the smaller species typically found in ponds or lakes with fish predation. The ecology of the genus Daphnia may be better known than the ecology of any other group of organisms.”

Water Fleas

Thank you!
You all are amazing, I really appreciate it!

Letter 7 – What’s That Blurry Thing????


Subject: looks like a crawfish
Location: reidsville nc
March 22, 2014 10:24 am
Found this crawfish looking creature in a puddle in the woods, what is it?
Signature: Amanda and Billy

What's That Blur???
What’s That Blur???

Dear Amanda and Billy,
We got a real chuckle out of your request.  Your images are all too blurry to identify.  We have posted the clearest (this is so relative) of the three.  For the sake of classification, we are categorizing it as a Crustacean.  Perhaps it is an Amphipod.  See the Missouri State Biology website.

Letter 8 – Spider Egg Case


Subject: Unidentified, Unusual Egg Case
Location: Southern Michigan
April 27, 2013 7:39 pm
Dear Bugman:
I found this unusual looking egg case, while hunting for fossils. It was in a crumbly, sedimentary boulder, along with dozens of sow bugs, which were exposed when I split the rock open. The eggs are visible as round bumps through the papery/silky covering. Was wondering if some type of spider made this, or another kind of arthropod such as the sow bugs?
Signature: Chris O

What's That with the Sow Bugs???
What’s That with the Sow Bugs???  A Spider Egg Case.

Dear Chris,
We do not recognize this thing, but we would not rule out a fungus.  We are posting this as unidentified and we hope that either we or our readership might find and answer for you.

Dear Daniel:  After looking all over the internet for similar photos & an answer, I found a link on Bug Guide which has an almost identical photo of this type of egg sac.  It appears to be the creation of a type of ground spider.  Species mentioned during my searching are gnaphosid, zelotes and corrinidae.  Here’s a link to the pic on Bug Guide:

Dear Chris,
Thanks so much for following up on this posting.


  • Bugman

    Bugman aka Daniel Marlos has been identifying bugs since 1999. is his passion project and it has helped millions of readers identify the bug that has been bugging them for over two decades. You can reach out to him through our Contact Page.

    View all posts
  • Piyushi Dhir

    Piyushi is a nature lover, blogger and traveler at heart. She lives in beautiful Canada with her family. Piyushi is an animal lover and loves to write about all creatures.

    View all posts
Tags: Crustaceans

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11 Comments. Leave new

  • Those are so much fun to look for at the beach!

  • If it swims through the water with a sort of “hopping” motion near the bottom of the tank, it could possibly be an Ostracod, or “seed shrimp.”

    If it scurries across the gravel, plants, and tank ornaments, and has visible legs, it’s probably a Copepod of some sort.

    Both crustaceans are rather common, and usually come in on aquarium plants. They’re completely harmless, and feed upon algae, detritus, and other fine organic particles suspended in the water column.

  • From the behaviour description, I’d agree with the seed shrimp ID, as daphnia don’t hang out in/on the substrate.
    Not a lot of fish eat seed shrimp because they’re so well encased, but they are completely harmless. I think they’re kinda cute:-)
    Take a look at this video clip and see if it’s a match (
    cheers, Storm

  • sure looks like a spider egg sac, according to Track and Sign of Insect by Charley Eiseman.

  • Hello,

    I received in the mail a book that I had purchased. Inside was an egg case like this one and a black spider with a white circle around his bottom. It looked as though he had no hair on him and the slightest bit of white on his joints on his legs.

  • Hello,

    I received in the mail a book that I had purchased. Inside was an egg case like this one and a black spider with a white circle around his bottom. It looked as though he had no hair on him and the slightest bit of white on his joints on his legs.

  • Plus… The package came from NJ and the egg sack/spider are attached to cardboard.

  • Found a green leaf looking beatle when i emptied out my fish tank. anybody know more less what it could be?

  • Went surf fishing in Oceanside, with no bait. When I got to the beach I set my pole up and went and dug out a sand crab (Mole crab). Put it on my hook and cast out into the breakers. Within a minute I caught a fish and reeled it in. I like to fish but I don’t like eating fish so I unhooked it and put it back in the water. Just as I was getting ready to repeat a man came up to me and told me that, what I had done, was the greatest thing he had ever seen, with the digging of the bait, catching a fish and putting it back. I was going to keep fishing but, after the guy said that, I left. I figured that he probably told all his friends about the great surf fisherman of Oceanside. I didn’t want to stay and disappoint him. 🙂


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