Molt increment, molt cycle duration, and tissue growth in Palaemonetes pugio Holthuis larvae

Abstract

Growth in larvae of the shrimp Palaemonetes pugio Holthuis was examined to find if growth of the dorsal carapace (molt increment, MI), epidermis and muscle growth was dependent on the molt cycle duration (MCD). The dorsal carapace grew by 9–25 % at each molt through eight larval and one postlarval instars. Variation in carapace length (CL) decreased with larval age. Larvae reared individually through the first three instars demonstrated different trends in molt increment for each instar. Although half of the larvae underwent a continuous increase or decrease in MI (compared to the preceding MI), other larvae demonstrated an increase followed by a decrease, or vice versa, in MI. There was an inverse relationship between CL and MI for instar I, but not for instar II and III larvae. Growth of the epidermis and muscle was studied in instar II larvae. The epidermis in the posterior dorsal carapace increased in cell density during the 1st day of the molt cycle. At ecdysis to instar III, the cell density returned to a level equivalent to that of Day 1, instar II. The increase in cell density during instar II was proportional to the increase in area of the cuticle at ecdysis (= square of MI for CL) and could be used to predict the size of instar III larvae. The lateral thoraco-abdominal extensor muscle grew in length and width throughout the larval period and muscle size was highly correlated with CL. The MCD was inversely related to temperature in larvae reared at 15, 20, 25 or 30 °C. The MI was greatest at 20 and 25 °C. The MI, epidermal growth, and muscle growth, however, were independent of MCD. Exposure to 20-hydroxyecdysone (20HE) significantly shortened the intermolt period (A-C) in instar II larvae but did not decrease the MCD. There was no difference in MI or growth of the epidermis or muscle in 20HE-treated larvae. The results show that, during the first three larval instars, MI is independent of MCD and that tissue growth is not affected by the duration of the molt cycle. Growth and molting appear to be regulated in a coordinated but separate manner.