THE MOLTING CYCLE OF THE SPINY LOBSTER, PANULIRUS ARGUS LATREILLE. I. MOLTING AND GROWTH IN LABORATORY-MAINTAINED INDIVIDUALS

Abstract

1. In Panulirus argus, premolt animals may be detected by the appearance of a resorptive or ecdysial line along the branchiostegites.2. One of the most obvious external changes in Panulirus a few hours before ecdysis is the swollen condition of the intersegmental membranes.3. Five to fifteen minutes preceding ecdysis, animals of 80-89 mm. carapace length increase their body weight by approximately 10%. This weight increase begins four to six hours preceding ecdysis and is entirely due to water absorption. There is a loss of weight following ecdysis due to loss of the exuviae. Within a few hours following molt, the weight of the individual begins to rise and continues to do so until a maximum is reached at 28-35 days. The water content begins to rise the day preceding molt, reaches its highest peak (11% above the intermolt value of 71%) one day following molt, slowly declines but does not return to its intermolt value before 28-35 days. At this period the animal has fully calcified its new skeleton, and apparently all growth of soft tissues is completed. Weight stability is not attained in 80-89 mm. carapace length animals until 28-35 days following molt.4. Due to resorption from the ecdysial line along the branchiostegites and resorption from an articulating condyle connecting the branchial chamber to the posterior edge of the branchiostegites, progressive swelling and lateral expansion of the thorax is allowed.5. Turgidity within the new stomach causes considerable pressure to be exerted on the surrounding regions and results in the progressive swelling and lateral expansion of the thorax preceding ecdysis. This is due to a gas which appears between the old stomach lining and the new.6. Photographs and captions depict ecdysis in Panulirus.7. The molting process appears to be directly correlated with water temperature. In Bermuda average monthly water temperatures range from 17° in February to 29° C. in August. Little molting occurs in the months of December through April, during which time the water temperatures are lowest. Water temperatures of 22° C. in May are sufficient to initiate molting. The number of molting individuals in the laboratory population increases throughout the summer months with a maximum in June, July, August, and September, at which time the highest water temperatures of 28-30° C. are reached. This means that the intermolt periods are shortest during the summer months. The number of molting individuals during late fall declines, as temperatures decrease. Water temperatures of 19° C. or below, however, are required to stop the molting process in the winter. Decrease in molting frequency in the fall seems to be a consequence of decreased metabolism.8. The growing period of the Bermuda lobster appears to be limited effectively to approximately seven months in the laboratory (May-November).9. Smaller individuals grow more rapidly in the summer months than do larger individuals. Although the summer intermolt periods are shorter in smaller than in larger animals, the actual number of molts observed during a period of one year in four of the size groups maintained in the laboratory (50-89 mm. carapace length) proved to be approximately the same. An average of four molts per year was observed in these groups. As winter months approach, the smaller as well as the larger animals cease to molt and grow.10. The smaller the animals, the greater the per cent growth by weight. Larger animals, however, gain more actual weight than smaller animals per molt.11. There is a greater per cent growth by length per molt in smaller than in larger animals. The difference observed between larger and smaller animals is an apparent one because a smaller animal increasing its carapace length the same amount as a larger one shows a greater per cent growth by length.12. Growth by length does not necessarily accompany molting. This was evidenced in some laboratory-maintained animals. In virtually all of these cases the environmental conditions of the individuals were poor.