The basic physical features of the Western rock lobster are: compound eyes on mobile stalks, antennae, six pairs of small limbs around the mouth, and five pairs of walking legs.
Panulirus cygnus also have a strong and muscular abdomen with an exoskeleton that is segmented on their backs. They have swimmerets underneath the tail which ends in a tail fan. Their body color is pale to dark purple-brown with pale spots on the abdomen. The antennulae and legs are uniform in color with pale streaks.
Western rock lobsters have a maximum mass of 5 kilograms, and the largest ever reported was 5.5 kilograms, or about 12 pounds. The average length of their carapace is 8-10 centimeters, but their body is 9-11 centimeters long in ovigerous females and lobsters with spermatophores. The main distinction between different species of lobster can be made from their abdominal somites. In Panulirus cygnus there are pubescent grooves on the dorsal surface of the somites and there is a pubescent area along the posterior region.
As for sexual dimorphism, there are three main distinguishing characters between males and females. The first is the fifth pair of walking legs. In females the pair ends in a small claw, and in males, the pair ends in a point without a claw. The second difference is the location of the genital pore. At the base of the third pair of legs is the genital pore for the females, whereas in males, the genital pore is at the base of the fifth pair of legs. The third difference is between the pleopods/swimmers on the Rock lobsters' tails. In a female there are inner and outer pleopods used to carry her eggs; a male simply has four pairs of single pleopods.
Another related issue to sexual dimorphism in Panulirus cygnus is the hormones that are secreted by androgenic glands to induce the development of male sex characters. Also called the glands that make the male, the androgenic glands not only have an effect on males, they also have an effect on females. If they are artificially implanted into a female Western rock lobster, her ovaries will begin to turn into testes. As a result, not only will she start to produce sperm, but during the next molt her appendages will be reformed with male characteristics (such as are mentioned above).
Also, lobsters lower their metabolic rate when they are cooled, and as a result, their responses are altered.
Range mass: 5 .5 (high) kg.
Average mass: 5 kg.
Range length: 9-11 (high) cm.