How Do You Distinguish a Male Lobster From a Female Lobster?

The Swimmerets Vary for Lobster Genders

Distinguishing a male lobster from a female
Distinguishing a male lobster from a female. On the left, you can see the male's distinctive first pair of swimmerets. On the right is a female carrying eggs. Jennifer Kennedy, Licensed to

Want to know the sex of a lobster you've caught or are about to eat? Here's how to tell.

How to Distinguish Sexes in American Lobster

Lobsters have feathery appendages called swimmerets, or pleopods, underneath their tail. 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 into the sex of a lobster. The first pair of swimmerets (the pair closest to the head) just behind the walking legs point up towards 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 get fertilized. The female stores the eggs under her abdomen (tail) for 10-11 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 may find unfertilized eggs (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 may carry up to 80,000 eggs at one time. 

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. As her molting time draws near, females visit 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 "boxing" match and the female enters the den. When it's time to molt, the male protects her during this vulnerable time.  A molted lobster is very soft and takes at least half an hour to be able to stand. It is at this point the male rolls the female over onto her back and the male transfers the sperm packet to the female's seminal receptacle. The female holds the eggs until she is ready to fertilize them. 

Spiny Lobster Sexing

Spiny lobsters (rock lobster) are usually sold as tails, rather than live, so you may not get your 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 tail. 

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

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