Determining the sex of a lobster is common in commercial fishing and biological research. The physical differences between male and female lobsters, known as sexual dimorphism, are subtle but distinct, centered on reproductive adaptations. Accurate identification is important for protecting breeding females. A careful inspection of the lobster’s underside reveals the most reliable indicators, requiring safe handling to examine the specialized appendages.
The Key Distinction: Swimmerets
The most definitive method for sexing a lobster involves examining the first pair of pleopods, or swimmerets. These small, fin-like appendages are located on the underside of the abdomen, closest to the body, and show a clear distinction based on shape and texture. To perform this check, safely handle the lobster by grasping the carapace while keeping the claws secured to prevent injury.
In male lobsters, the first pair of swimmerets is stiff and spear-like. These specialized appendages are used to transfer sperm packets to the female during mating. Conversely, the first set of swimmerets on a female lobster is soft, flexible, and feathery, resembling the remaining pleopods further down the tail. This brush-like structure helps the female clean, aerate, and attach her developing eggs. The difference between the male’s stiff swimmerets and the female’s soft ones provides immediate and accurate identification.
Visual Cues on the Body
While swimmerets offer the most conclusive evidence, visual cues can also distinguish between the sexes. One difference is the relative width of the abdomen, or tail. Female lobsters possess a broader, more flared abdomen compared to males. This adaptation creates a larger surface area on the underside of the tail, necessary to accommodate and protect the large cluster of eggs a female carries when brooding.
Male lobsters generally exhibit larger and more robust claws than females of the same body size. This increased claw mass is thought to be an adaptation for male-male competition. Males often engage in aggressive displays and physical fights to establish dominance and secure mating rights. These secondary traits are generalizations and should always be confirmed by checking the structure of the first set of swimmerets.
Identifying Egg-Bearing Females
The presence of eggs attached externally to the female’s body provides an unmistakable identification, classifying the animal as a “berried” lobster. A berried female carries thousands of fertilized eggs, known as roe, glued to the fine hairs of her swimmerets. The eggs are typically dark reddish-brown or black when first laid. A female will carry this cluster for up to a year until the larvae are ready to hatch.
The practice of carrying eggs externally is a protective strategy, and removing these females from the population can significantly impact the species’ reproductive success. Due to their importance in maintaining sustainable populations, it is a legal requirement in many fishing regions that any egg-bearing female must be immediately returned to the water. This regulation is enforced to ensure the survival of the next generation and is a cornerstone of modern lobster conservation efforts.