How to Distinguish Male Lobsters From Females

Want to know the sex of a lobster you have caught or are about to eat? Here are several ways to tell:

Lobster Anatomy

Lobsters have feathery appendages called swimmerets, or pleopods, underneath their tails. These swimmerets help a lobster swim and are also where a female lobster (sometimes called a hen) carries her eggs. Swimmerets also can clue you in to the sex of a lobster. The first pair of swimmerets (the pair closest to the head) just behind the walking legs point up toward the head. They are thin, feathery, and soft on a female but hard and bony on a male.

Also, the female has a rectangular shield between her second pair of walking legs, which she uses to store sperm after mating with a male. This is where the male inserts those hard swimmerets during mating, releasing sperm that the female stores. When it's time to release her eggs, they flow past the sperm and are fertilized. The female stores these eggs under her abdomen (tail) for 10 to 11 months. 

Because they carry eggs, females tend to have a wider tail than males. Females carrying fertilized eggs aren't usually harvested, but inside a female lobster you might find unfertilized eggs, or roe. They are green when fresh and bright red after the lobster is cooked. (They are also called "coral" because of the color.) These can be eaten. Females can carry up to 80,000 eggs at one time. 

Courting Ritual

Despite their ferocious appearance, lobsters have a complex courtship ritual that is often described as "touching." Males and females mate after the female molts. The males live in caves or dens, and as her molting time draws near, a female visits the dens and wafts a pheromone toward the male via her urine, which is released from openings near her antennae. The male energetically beats his swimmerets.

Over a few days, the female approaches the den and checks out the male. They eventually initiate a mock "boxing match" and the female enters the den. During molting the female is vulnerable—she is very soft and takes at least half an hour to be able to stand—so the male protects her. At this point the male rolls the female over onto her back and transfers the sperm packet, or spermatophore, to the female's seminal receptacle. The female holds her eggs until she is ready to fertilize them. 

Spiny Lobster Sexing

Spiny lobsters (rock lobsters) are usually sold as tails, rather than live, so you might not get a chance to try out your lobster sexing skills at a market that sells spiny lobsters. However, these lobsters also can be sexed using the swimmerets on the underside of their tails. 

In females, the swimmerets on one side might overlap those on the other. You might also see a dark patch, where the spermatophore is located at the base of her last pair of walking legs. They might also have claw-shaped pincers at the end of their fifth pair of walking legs that help hold the eggs. Roe mmight be found inside whole spiny lobsters.

Sources:

• Lobsters, Gulf of Maine Research Institute
• Lobster 101: Reproduction and Life Cycle, Maine Lobstermen's Community Alliance
• How to Determine the Sex of Regulated Invertebrates, State of Hawaii Division of Aquatic Resources
• Lobster Biology, The Lobster Conservancy