A quick guide to the differences between males and females
Mature male and female crayfish exhibit obvious sexual dimorphism. Most of the distinguishing features of females (e.g. wide tails, larger body size) are related to particular adaptations for rearing offspring, while those for males are related to male to male competition (e.g. larger claws) and to reproduction (paired gonopods, hooks on walking legs). Descriptions of the different life histories of males and females follow, along with a quick guide to the differences and a gallery of the different anatomical features of males and females.
|Abdomen||Narrower, with larger swimmerets||Wider, with smaller swimmerets (for holding eggs)|
|Reproductive organ||Elongate paired gonopods||Single round gonopore|
|Color||Lighter, less covered in algae (because they molt 2x/year)||Darker, more covered in algae (because they only molt 1x/year)|
|Hooks on 2nd set of walking legs||Present in F1||Absent|
Most of the differences between male and female crayfish are relative differences: females are smaller overall, with smaller chelipeds (claws), wider tails, and larger swimmerets. Females also lack hooks on their 2nd pair of walking legs and have a single circular orifice at the base of the cephalothorax, called a gonopore. Mating occurs in the fall, but the eggs are not immediately fertilized. Instead, after mating with one or more males, the female will store the sperm through the winter in a spermatheca. When she releases the unfertilized eggs in the spring, she’ll simultaneously secrete a substance, called glair, that breaks down the sperm plug that has encased and protected the sperm throughout the long winter. The now fertilized eggs are held in place on her tail.
After hatching, the larvae also stay attached to her tail. Her wider tail and larger, stronger swimmerets help create a safe and secure environment for the young crayfish, protecting for the next few weeks before dispersing. But there’s always a risk that the young will get knocked free of their mother’s tail. Fortunately, adult females secret a hormone that larvae can detect. If displaced the larvae will swim back towards their mom.
Males are larger, have narrower tails, and larger chelipeds (for fighting off other males and pinning females during mating). After hatching, both male and female larvae will molt 8-10 times as they grow. Shedding their exoskeleton allows them to get bigger with each molt. It also allows them to regenerate lost limbs. Crayfish typically mature in the second fall after hatching, after which females will molt only once per year, males twice. Molting twice allows for what’s called cyclic dimorphism, where males take on different morphologies for their breeding (F1) and non-breeding (F2) forms. The difference is subtle, but necessary. Their mating form (F1) has a pair of spiny hooks on the second pair of walking legs that are used for clasping onto the female during mating. They also have gonopods, small legs adapted for depositing sperm. In the F1 form these become rigid and stronger. Males maintain this form for just a few weeks or months before reverting back to their non-breeding (F2) form.