Crabs are one of the most iconic creatures found in oceans and beaches around the world. With their hard outer shells, sideways walk, and formidable pincers, they are instantly recognizable. But are these charismatic crustaceans actually a type of fish?
This is a question that has puzzled both experts and amateur nature lovers alike.
If you’re short on time, here’s a quick answer to your question: No, crabs are not fish. Crabs belong to the subphylum Crustacea, which includes lobsters, shrimp, barnacles and many other invertebrates. Fish belong to an entirely different phylum called Chordata.
While crabs and fish share an aquatic habitat, they have very different anatomies, life cycles and evolutionary lineages.
In this comprehensive article, we will examine the key differences between crabs and fish in depth. We’ll look at their anatomy, reproduction, habitat, evolutionary history and taxonomic classification.
With over 6,500 crab species on the planet, we’ll focus on some of the most common types of true crabs as case studies. By the end, you’ll have a clear understanding of why zoologists definitively categorize crabs and fish as very different creatures.
The Anatomy and Physical Features of Crabs
The Crustacean Body Plan
Crabs belong to the subphylum Crustacea, which includes lobsters, shrimp, and barnacles. All crustaceans share a similar basic body plan. They have a hard exoskeleton, pairs of jointed appendages, and two pairs of sensory antennae.
The exoskeleton must be shed periodically in order for the crab to grow in a process called molting. Molting leaves crabs vulnerable until their new exoskeleton hardens.
Crabs Have a Hard Exoskeleton
The exoskeleton of a crab is made up of calcium carbonate and provides protection for the internal organs. It is divided into two main parts – the cephalothorax (which is covered by the carapace) and the abdomen. The carapace is a protective shield that covers the head and chest.
Under the carapace, crabs have five pairs of legs for walking and claws for capturing prey and self-defense.
Crabs Walk Sideways
Crabs have legs that extend outward and then forward, allowing them to walk sideways or diagonally. Their last set of legs is usually modified to help propel them forward and backward. The sideways walking gives crabs a wide field of view to spot predators and prey.
Their legs have pointed tips that allow them to grip surfaces as they move.
Crabs Have Powerful Pincers
The frontmost pair of legs has developed into large, powerful pincers or claws. The claws are used for defense, combat with other crabs, and catching and tearing food. Each claw has a different function – one for cutting or biting and one for crushing.
The size and shape of the pincers can differ significantly between crab species depending on their main food sources.
Crabs Have Stalked Eyes
Crabs have eyes located on short stalks that extend from the carapace. This allows their eyes to extend beyond the shell so they can spot predators and prey. Having eyes on stalks also gives crabs a nearly 360 degree field of vision.
They have two compound eyes that are made up of many smaller units called ommatidia. Some crabs even have light-sensitive spots near their mouthparts.
Crabs Breathe Through Gills
Crabs respire through gills that are located under the carapace near the base of their legs. Oxygenated water is drawn over the gills, allowing gas exchange – oxygen is absorbed from the water while carbon dioxide is released.
Gills need to stay moist in order for crabs to breathe, so they cannot spend much time out of water. Land crabs have adapted gills that can function in air.
The Anatomy and Physical Features of Fish
Fish Have a Vertebral Column
Unlike crabs, true fish have a vertebral column or backbone made up of vertebrae. This provides structure and support for their bodies and allows them to swim more efficiently in the water (Blue Planet Aquarium, 2021).
The vertebral column runs from the head to the tail, protecting the spinal cord and allowing fish to move with greater maneuverability.
Fish Are Covered in Scales
Most species of bony fish are covered in overlapping scales that protect their skin and reduce friction as they glide through the water. According to National Geographic (https://www.nationalgeographic.org/), these scales grow from the deeper layer of skin and are made up of the same collagen protein found in human fingernails.
The scales come in a variety of shapes and give fish a shimmery, silver appearance.
Fish Swim Upright
Fish swim upright with their bellies facing down and the majority of muscles concentrated near their tail fin. The University of California (https://www.universityofcalifornia.edu/) explains that this anatomical configuration allows them to propel themselves forward by oscillating their tail and body from side to side.
This is markedly different from the anatomy of crabs that walk horizontally along the seafloor on bent legs.
Fish Have Paired Fins
According to the Australian Museum’s Ichthyology Department (https://australian.museum/), most species of bony fish have pairs of pectoral and pelvic fins that aid with steering, braking, and balancing. They differ considerably from the paddle-shaped limbs of crabs.
The paired fins give fish greater maneuverability and control as they swim through currents or navigate tight spaces.
Fish Have Protruding Eyes
Fish eyes protrude slightly from their heads and provide nearly 360 degrees of vision so they can easily detect predators or prey (Monterey Bay Aquarium, 2022). Their slightly bulging eye placement increases visual coverage to compensate for the fact that sound and smell are limited underwater.
In contrast, crabs have eyes that recede into sockets in their shell-covered heads.
Fish Breathe Through Gills
Fish have gills that extract dissolved oxygen from the surrounding water so they can breathe underwater. According to the Smithsonian Science Education Center (https://ssec.si.edu/), gills are delicate tissues filled with blood vessels and capillaries that provide a large surface area for gas exchange.
They are covered by a protective flap called the operculum. Bony fish have four pairs of highly-vascularized gills, while crabs use featherlike structures called gill bailers to breathe.
The Life Cycle and Reproduction of Crabs
Crabs Mate Through Copulation
Crabs reproduce sexually through a process called copulation, whereby the male crab uses its modified abdomen to pass sperm into the female’s abdomen (Ref: Encyclopedia Britannica). After copulation, the female stores the sperm internally until her eggs are mature and ready to be fertilized.
Interesting fact – some crabs exhibit mate guarding where the male crab stays close to the female after mating and carries her in his pincers to protect her until she is ready to spawn!
Crabs Carry Eggs on the Abdomen
Once the female crab’s eggs are fertilized internally, she extrudes them and attaches them to the tiny hair-like projections on her abdomen. The egg mass or “sponge” contains up to 2 or 3 million eggs!
Female crabs are highly protective of their egg sponges and will defend them vigorously against predators. According to marine biologists, only 1 in 1,000 to 1 in 10,000 crab eggs on average survives to become an adult crab (UF Fisheries and Aquatic Sciences).
Crabs Have a Larval Stage
After incubating the fertilized eggs attached to her abdomen for a couple of weeks, the female crab releases her eggs into the ocean water, where they hatch into tiny larval crabs called zoea. These minute crab larvae drift and swim in the ocean’s water column, developing through several zoea stages over weeks to months before molting into a post-larval stage called a megalopa.
According to research, the extended larval stage allows young crabs to disperse to new habitats for better survival (Journal of Crustacean Biology).
Crabs Molt Their Shell as They Grow
In order to grow larger, crabs have to shed their rigid exoskeleton in a process called molting. As a soft-bodied crab emerges from its old shell, it expands larger before its new shell hardens. To molt successfully, crabs need exposure to oxygenated water and adequate nutrition and minerals.
According to studies, temperature and salinity also impact the frequency and timing of molts (Ref: Aquaculture). Depending on conditions and species, crabs may molt frequently when young, then annually or biennially as adults.
The Life Cycle and Reproduction of Fish
Most Fish Reproduce by Spawning
The vast majority of fish species reproduce through a process called spawning. This involves the female fish releasing eggs and the male fish releasing sperm into the water, allowing the eggs to be fertilized externally.
Spawning behavior varies dramatically between fish species – some species construct nests or exhibit mating rituals, while others simply release their gametes into open water.
Popular food fish like salmon, trout and bass build nests, called redds, in streambeds where the females lay their eggs. The males then swim over the nest to fertilize the eggs. Other fish like eels and sharks don’t build nests at all, and fertilization happens randomly in the environment.
Fish Eggs Are Fertilized Externally
Since most fish release their sperm and eggs into the water, fertilization takes place externally. This is in contrast to most land animals, which usually undergo internal fertilization through copulation.
Once the eggs are fertilized, they continue to develop while floating freely in the water column or resting on the substrate.
External fertilization gives fish eggs exposure to predators and requires the release of enormous numbers of eggs to ensure survival. A single female fish may produce tens of thousands to millions of eggs during a spawning cycle to compensate for the low survival rate.
Fish Go Through Distinct Larval Phases
Fish eggs typically hatch into larvae instead of fully-formed juvenile fish. These larvae look nothing like adult fish – they are usually transparent with rudimentary eyes and organs. Different fish larvae include:
- Alevins – newly hatched salmon larvae still attached to a yolk sac for nourishment.
- Fry – free-swimming larvae that feed on plankton after absorbing their yolk.
- Fingerlings – more developed larvae with fins and scales but no mature sex organs.
Depending on the species, the larval phase may last weeks to years before the fish transitions into its juvenile form. Larval mortality rates are extremely high due to their vulnerability to predators.
Fish Grow Continuously Without Molting
Most fish species grow continuously throughout their lives and do not molt or shed their exoskeleton like crustaceans. Their scales, skin and bones expand as the fish grows. Growth rate depends on habitat, temperature and food availability.
Coldwater fish like trout may only grow a few inches per year, while warmwater fish like bluegill can grow several inches in a single season if conditions are optimal.
Indeterminate growth allows some fish species to reach impressive sizes. For example, massive snook over 50 inches long can be found in Florida waters. The Pacific halibut has been known to exceed 500 pounds!
The Habitats and Ecology of Crabs
Crabs are Primarily Marine Animals
The vast majority of crab species live in saltwater habitats like oceans, seas, and estuaries. Only a small percentage of crab species, such as the terrestrial coconut crab and semi-terrestrial fiddler crabs, inhabit freshwater environments or land.
As primarily marine creatures, most crabs are superbly adapted to life underwater.
With their streamlined exoskeletons and paddle-like legs, crabs can nimbly swim and walk along the seafloor. They also have gills that allow them to breathe dissolved oxygen from the water. While some crabs do come ashore for breeding or in search of food, they still require sufficient moisture to breathe through specialized gill chambers called branchiostegal lungs.
Crabs Live in Intertidal Zones
Many crab species thrive in the intertidal zone, the area of shore that gets submerged during high tides and exposed during low tides. By living here, crabs have access to food sources both in the sea and on land.
Fiddler crabs, hermit crabs, and rock crabs are some of the most common intertidal crab species.
Intertidal crabs hide in burrows or rocky crevices when the tide goes out to retain moisture. They emerge during high tide to feed on algae, organic debris, mollusks, and other small creatures. The fluctuating tides bring crabs an ever-changing buffet!
Crabs as Scavengers and Predators
Most crabs are opportunistic feeders, meaning they will eat just about anything they can find. They commonly scavenge dead plant and animal matter. However, many crabs are also skilled predators that hunt live prey.
Crabs use their pincers and claws to capture, kill, and feed on animals like clams, oysters, mussels, shrimp, barnacles, snails, and even other crabs. Larger crab species may eat fish, sea stars, and octopuses as well. Their diverse diets allow crabs to inhabit a wide range of aquatic biomes.
Crabs Play Key Ecological Roles
As abundant bottom-dwellers, crabs are critical components of marine and estuarine food webs. Green and blue crabs, in particular, are considered keystone species in their respective ecosystems along the Atlantic and Gulf coasts.
Their populations significantly impact community structure and biodiversity.
Crabs also recycle nutrients by consuming dead organisms and become food themselves for many larger predators like seabirds, sea otters, octopuses, fish, and even humans. Beyond food web dynamics, some crab species have ecosystem engineering effects through their burrowing activities.
The Habitats and Ecology of Fish
Most Fish Species Live in Marine Environments
The vast majority of fish species, around 70%, inhabit marine ecosystems like coral reefs, kelp forests, seagrass meadows, and the open ocean. The saltwater habitats provide ample food sources, protection, breeding grounds, and nursery areas for many fish to thrive.
Herring, pollock, cod, mackerel, tuna, and seahorses are some common examples of marine fish. Though some may venture into brackish or fresh waters at times, they ultimately rely on the ocean environment.
Fish Inhabit Diverse Aquatic Zones
Fish occupy diverse aquatic areas including coastal wetlands, estuaries, seas, and oceans around the world. Species distribution varies based on factors like temperature, depth, salinity, food availability and habitat structures.
For instance, fast swimming predatory fish like barracudas and marlin frequent warmer surface waters, while demersal species like halibut and flounder live near the sea floor. Reef habitats also host rich and complex fish assemblages.
The Feeding Habits of Fish Vary Widely
Most fish eat a diverse array of prey items from tiny plankton to small fish and invertebrates. Feeding strategies encompass carnivorous fish like tuna, herbivorous fish like parrotfish that consume algae, detritivores like catfish that scavenge decaying organic material, and omnivorous fish like trout that eat both plant and animal matter.
The jaw structure and mouth placement of fish help determine their unique feeding ecologies and diets within aquatic food chains.
Fish Play Critical Roles in Aquatic Ecosystems
As key components of marine and freshwater habitats globally, fish provide vital ecosystem services. They cycle nutrients throughout the water column, act as bioindicators of habitat health, control prey populations as top predators, support fisheries, and more.
Protecting biodiverse fish assemblages allows balanced, productive, and resilient aquatic systems to prosper. However, issues like overfishing and pollution threaten the survival of many fragile fish populations.
The Evolutionary History and Taxonomy of Crabs
Crabs Evolved from Other Crustaceans
Crabs belong to the crustacean family and share a common evolutionary ancestor with lobsters, shrimp, and other decapod crustaceans. The earliest crab-like fossils date back to the Jurassic period over 175 million years ago.
These primitive crab ancestors likely evolved from a shrimp or lobster-like creature through a process called carcinization, where the abdomen evolved to be folded under the thorax.
Over millions of years of evolution, crabs developed key adaptions like a broad carapace (shell) to protect their bodies and strong pincers for defense and catching food. The crab body shape provides many advantages, which explains why the crab form has evolved independently several times in different crustacean lineages.
There are Thousands of Crab Species Worldwide
Today, there are over 6,700 known species of crabs living in all of the world’s oceans. The incredible diversity of crabs is a testament to their evolutionary success. Some key crab families with wide representation include:
- Spider crabs – Found in seas worldwide; known for their long, spindly legs and rounded carapace; e.g. Japanese spider crab has a leg span over 12 feet wide!
- Fiddler crabs – Live in burrows in intertidal zones; only one claw is enlarged (males especially); over 100 species identified.
- Swimming crabs – Their last pair of legs are flattened for swimming; includes blue crabs and stone crabs.
- Hermit crabs – Use abandoned shells to protect their soft abdomens; examples are Hawaiian red crabs and Australian land hermit crabs.
There are even tiny pea crabs that live symbiotically inside the shells of mollusks like clams and oysters!
Crabs Belong to the Subphylum Crustacea
The scientific subphylum name for crabs and other crustaceans is Crustacea. True crabs belong to the order Decapoda, which includes around 15,000 species including lobsters, crayfish, and shrimp. The defining feature of decapods is their five pairs of thoracic legs.
Within Decapoda, crabs belong to the infraorder Brachyura, which contains over 6,000 crab species. All crabs share key identifying features like a short, broad carapace, a small abdomen folded under the body, stalked eyes, and a pair of pincers.
So while crabs closely resemble other decapod crustaceans, their unique anatomy warrants their special classification as Brachyuran crabs.
Common Crab Species and Families
Some of the most common crabs harvested for human consumption come from the Cancridae, Portunidae, and Varunidae families, including:
Dungeness crab | Cancridae family |
Blue crab | Portunidae family |
Snow crab | Portunidae family |
King crab | Lithodidae family |
Coconut crab | Gecarcinidae family |
There are also over 1,300 species of hermit crabs spanning multiple families like Diogenidae and Coenobitidae. While most crabs live in the ocean, terrestrial species like coconut crabs and robber crabs have adapted to live on land.
The amazing diversity of crabs is a sign of their successful evolution!
The Evolutionary History and Taxonomy of Fish
Fish Evolved Over 500 Million Years Ago
Fish have a long evolutionary history dating back over 500 million years ago to the Cambrian period. The first fish likely evolved from primitive chordates with basic backbones. Over the course of millions of years, fish diversified into the major groups we see today.
The earliest fossil evidence of fish comes from jawless fish called ostracoderms that lived during the Ordovician period about 450 million years ago. Jawed fish appeared around 420 million years ago during the Silurian period.
Ray-finned fish, which make up about 96% of all fish species living today, evolved around 380 million years ago during the Devonian period. Lobe-finned fish, which gave rise to the tetrapods and ultimately mammals, birds, reptiles and amphibians, also emerged during the Devonian.
There are Around 33,000 Fish Species Worldwide
According to FishBase, the online database cataloging all finfish species worldwide, there are currently 33,100 documented fish species globally as of March 2024. This number is constantly growing as new species are discovered.
The vast majority of fish species are ray-finned fish in the class Actinopterygii. According to FishBase, there are about 31,500 ray-finned fish species, making up over 95% of all fish. Other major fish groups include sharks and rays (class Chondrichthyes) with around 1,200 species, lobe-finned fish (class Sarcopterygii) with around 8 species, and jawless fish (class Agnatha) with around 115 species.
Fish Belong to the Phylum Chordata
All fish belong to the phylum Chordata, which is characterized by having a backbone or notochord. Within the phylum Chordata, fish belong to the subphylum Vertebrata, since they have backbones. More specifically, fish are members of the superclass Pisces, which distinguishes them from other vertebrates like amphibians, reptiles, birds and mammals.
There are three main classes of fish in the superclass Pisces:
Bony fish are further divided into ray-finned fish (class Actinopterygii), lobe-finned fish (class Sarcopterygii) and lungfish (class Dipnoi). So while fish may seem like a simple category, they represent an evolutionarily diverse group spanning millions of years!
Common Fish Orders and Families
There are over 160 different orders and 500 families of bony and cartilaginous fish. Some of the largest and most diverse orders include:
Some well-known fish families include cichlids, guppies, bettas, tetras, seahorses, tuna, mackerel, butterflyfish, triggerfish, pufferfish, angelfish, discus and gobies. This immense diversity reflects the long evolutionary history and adaptations of fish to aquatic environments around the world.
Conclusion
In this comprehensive overview, we’ve explored all the key differences between crabs and fish – from their anatomy, reproduction and ecology to their evolutionary history and scientific classification.
While they share some superficial similarities, it’s clear that crabs and fish are fundamentally different creatures that belong to completely separate branches of the animal kingdom.
Crabs are crustaceans, a group of invertebrates including lobsters and shrimp that possess a hard exoskeleton, stalked eyes, powerful pincers and a distinctive larval development. Fish are vertebrates with backbones, paired fins, protruding eyes and a life cycle of continuous growth without molting.
When viewed in detail, crabs have far more in common with their fellow crustaceans than they do with fish.
So the next time you see a crab scuttling across the beach, you can say definitively: no, that quirky marine animal is not a fish at all. With its armored shell, sideways gait and formidable claws, the crab stands alone as one of the most iconic – and decidedly fishless – creatures of the sea.