Male vs. Female Crabs: How to Spot the Differences

Who doesn’t love a good old-fashioned day at the beach? You can walk in the sand, see the rocky shores, and observe the glory of the natural wildlife. Beaches can have some of the most incredible animals in the world, most of which are extremely diverse in nature. 

One common creature that can be found on the beach are crabs. They are so common in fact that they can be found in every ocean in the world, and have been documented on every continent except Antarctica. With that said, crabs also boast a wide variety of body plans, meaning that each species that you encounter can have an entirely different collection of characteristics. 

Like many animals worldwide, crabs generally look very similar regardless of sex. In fact, without the proper information, you may not even be able to tell the difference between a male and a female crab. Knowing the sex of crabs is incredibly important though. For example, if you are hunting crabs, you must know their sex because there are strict rules on what you can keep. Regardless, even if you aren’t planning on going crabbing soon, knowing how to tell male and female crabs apart can be a fun bit of information to have under your belt! 

How to Tell the Difference Between Male and Female Crabs

Understanding the underlying biology of animals can be incredibly useful in understanding why they look and act the way they do. Science is chalked full of repeating patterns that can help us in activities such as grouping animals together, or in the case of crabs, determining their sex. With that said, let’s take a look at a few patterns and rules that you should be aware of when trying to understand the differences between male and female crabs.

Be Cautious of the Species You’re Working With 

One very important thing to keep in mind is the species that you are trying to determine the sex of. As mentioned previously, crabs are extremely common around the world, which can both help and hinder the process. For instance, you may be able to find upwards of 10+ species on a single beach alone, which can complicate differences between sex and species. 

Even though crab species have similar biology, there may be underlying differences between males and females of a certain species that are not universal among all crabs. In other words, it may be helpful to first identify what species of crab you are looking at before you determine if it is male or female. 

By understanding an animal on a species level, you can consult outside resources such as field guides for more specific information that may pertain to their morphology and biology. Again, if you are trying to determine the sex of a crab, most species follow the same patterns. Nevertheless, it is always a good rule of thumb to double-check first! 

Consider the Size of the Crab You’re Looking at 

Similarly, crab size plays an important role in the morphology of all species. As juveniles, male and female crabs can look incredibly similar. Most of the differences that occur between the sexes are only developed after they reach full sexual maturity. Thus, you may mistake all small crabs to be of one sex, even if they are a mix of young males and females. This is because they haven’t had time to develop the significant differences in their morphology. So, take in mind the size and species of the crab you are working with before you begin to make any definite guesses on the sex. 

Abdomen Width

The most prominent characteristic that can help distinguish between males and females is abdomen width. It can sometimes be difficult to find, but the abdomen is the main portion of the underside of the crab. It is divided as a separation plate of the shell. Specifically, it lies directly on the backside, usually where people pick crabs up from. For most species, females boast larger abdominal plates. Most adult female plates are both taller and wider than those that can be found on males. In contrast, male crabs typically have a less pronounced plate, that is extremely narrow. 

“Internal” Anatomy 

Don’t worry if the side and width of the abdominal plate aren’t enough information for you to correctly sex a crab. If you want a deeper look at the crab, you can also gently pull back on the abdominal flat. Underneath the flat, male and female crabs have incredibly different anatomy. Although it is not technically internal anatomy, these structures are typically hidden from view when the crab is going about its daily activities. Males will typically have two large spikes called gonopods, whereas females will have numerous filamentous legs known as pleiopods. 

If you are going to try this method on a live crab, be sure to be extremely gentle. Only pull back on the abdominal plate if it is easily movable, and refrain from doing anything that will stress the crab out any further.

Claw Size 

One last characteristic that may differ between male and female crabs is claw size. Take note though, that claws are the most variable trait between species. Some species have no sexual dimorphism with claws, whereas it is the most visible dimorphism in others. Dimorphisms in this sense refer to the presence of traits that differ between males and females within a species. 

For the majority of crab species, males tend to have larger claws. This will look different depending on the species. Some may be long, wider, or even bulbous at the base. In any case, the vast majority of species follow this pattern. 

With that said, you shouldn’t sex female crabs based on how small their claws are in comparison. This is because some males don’t develop large claws, due to a lack of genetic need or another underlying cause. With specific species, males may even have claws that look conspicuously different than female claws. For instance, male fiddler crabs grow one large claw, which is used for defense and reproductive allure. Likewise, other species’ males may have claws that have entirely different coloration or serration on the ends. 

Why Do Male and Female Crabs Have Such Small Differences?

Crabs fall within a rather unique group of animals that look relatively similar between sexes while also retaining some notable morphological differences. In other words, all crabs will have the same basic features: a hard exoskeleton, 10 legs with two specialized claws, and eyestalks.

In contrast to these very vital similarities, they also have a few minute differences between the sexes. As discussed earlier, the presence of internal gonads, different abdomen shapes, and different claw sizes can all help distinguish between males and females But why is this? What exactly drives this differentiation process in crabs? 

The small differences between males and females can all be attributed to their roles in reproduction. Females typically have smaller claws and larger abdomens. This reflects their need to house, protect, and develop eggs, which requires a lot of space and energy. Likewise, male crabs don’t exhibit an enlarged abdomen as they will never need to carry eggs.

Additionally, male crabs also have larger claws than females. They use these to fight off predators. Furthermore, they also use them to fight off competing males, and to attract females in the pursuit of breeding!

The photo featured at the top of this post is © Mike Raube-Boyer/Shutterstock.com